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Evolutionary Ecology ISSN 0269-7653Volume 25Number 6 Evol Ecol (2011) 25:1335-1355DOI 10.1007/s10682-011-9473-y
Rapid experimental shift in host use traitsof a polyphagous marine herbivore revealsfitness costs on alternative hosts
Erik E. Sotka & Pamela L. Reynolds
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ORI GIN AL PA PER
Rapid experimental shift in host use traitsof a polyphagous marine herbivore revealsfitness costs on alternative hosts
Erik E. Sotka • Pamela L. Reynolds
Received: 11 November 2010 / Accepted: 12 March 2011 / Published online: 27 March 2011� Springer Science+Business Media B.V. 2011
Abstract The host breadth of any particular herbivore reflects a compromise between
evolutionary forces that promote specialism and those that promote polyphagy. Because
most terrestrial herbivorous insects specialize, explorations of this evolutionary balance
have focused largely on specialist than on polyphagous herbivores. Here, we experimen-
tally tested whether fitness-based tradeoffs in utilizing alternative hosts can be detected
within a polyphagous marine herbivore. The marine amphipod Ampithoe longimana occurs
on multiple seaweeds year-round (especially the genera Sargassum, Ulva and Hypnea), but
is particularly abundant on the diterpene-rich genus Dictyota during warmer summer
months. If fitness-based tradeoffs in using these alternative hosts are present, A. longimanamay experience fluctuating selection across seasons. To test this possibility, we performed
a controlled natural-selection experiment in which amphipods were isolated on Dictyota or
a mixed seaweed assemblage that did not include Dictyota. Within 15 weeks (less than five
overlapping generations), Dictyota-lines had greater feeding tolerance for Dictyota and its
secondary metabolites than did mixed-seaweed-lines. Dictyota-line females reproduced
more quickly than did mixed-seaweed-line females on Dictyota, but mixed-seaweed-line
juveniles had greater growth on Sargassum and Ulva and higher fecundity on all hosts than
did Dictyota-line juveniles. While experimental shifts in preference and performance are
likely genetically-mediated, our experimental protocol does not preclude a role for phe-
notypic plasticity. The presence of a fitness cost to evolving greater preference for Dictyotasuggests that fluctuating selection may operate on feeding preference across seasons, but
E. E. Sotka (&)Department of Biology and Grice Marine Laboratory, College of Charleston, 205 Fort Johnson Road,Charleston, SC 29412, USAe-mail: [email protected]
P. L. ReynoldsDepartment of Biology, University of North Carolina, Chapel Hill, NC, USAe-mail: [email protected]
Present Address:P. L. ReynoldsThe College of William and Mary, Virginia Institute of Marine Science, 1208 Greate Road, GloucesterPoint, VA 23062-1346, USA
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our test of this hypothesis was equivocal. We suggest that one reason that polyphagy
persists within A. longimana and potentially other marine grazers is because polyphagy
broadens resource use across seasons, and this benefit outweighs the fitness-based costs
that can favor specialism. Our results also reinforce the notion that timescales of ecological
and evolutionary dynamics can overlap.
Keywords Controlled natural selection experiment � Marine plant-herbivore
interactions � Secondary metabolite � Trade-offs � Herbivore tolerance
Introduction
The evolutionary mechanisms that sculpt the host breadth of herbivores are complex and
have proven vexing to disentangle (Futuyma and Moreno 1988; Jaenike 1990; Schoo-
nhoven et al. 2005; Strauss and Zangerl 2002; Tilmon 2008), but one parsimonious view is
that the breadth of any particular herbivore reflects a ‘compromise’ (Rausher 1992)
between evolutionary forces that promote specialism versus generalism. If true, then a
complete understanding of host breadth requires biologists to simultaneously evaluate the
strength of multiple forces (e.g., host availability, food quality, and predator or abiotic
refuge) across the tremendous diversity of herbivores and their feeding strategies.
Because most terrestrial herbivorous insects utilize a restricted number of plant families
as hosts (Bernays and Graham 1988; Strong et al. 1984), it is not surprising that this
literature is dominated by studies on specialists. In contrast, the evolutionary ecology of
small polyphagous herbivores has been relatively neglected, despite the sizable number of
known polyphagous insects (root-feeding insects, grasshoppers and some lepidopterans:
Bernays and Minkenberg 1997; Novotny and Basset 2005; Novotny et al. 2002; Singer
2008; Wiklund and Friberg 2009) and the profound impacts of some of these generalists on
the population dynamics and fitness of their hosts (Price 2003).
One of the important differences between generalist and specialist insects is their response
to seasonal variation in the quantity and quality of favored host plants. Specialists tend to
emerge when the young, more nutritious foliage of favored host plants become available
(Cates 1980; Novotny and Basset 1998; Wolda 1988). In contrast, many (but not all)
polyphagous herbivores reproduce year-round and are thus forced to shift hosts across
seasons either because favored host plants are unavailable or of poor food quality. These host
shifts are largely assumed to be phenotypic, but there remains the untested possibility that the
shift may reflect fluctuating selection for alternative hosts. This could occur when popula-
tions of a short-lived herbivore adapted to summer hosts during the summer, to winter hosts
during the winter, or both. To our knowledge, the possibility of seasonal genetic response has
not been addressed for any herbivore. The distinction between a plastic and temporal genetic
response is important, because both would yield generalism at the population level (Kassen
2002; Levins 1968), but through very different evolutionary mechanisms.
Marine plant-herbivore interactions allow biologists to test ecological and evolutionary
hypotheses that were developed from terrestrial plant-insect interactions and using taxa
that are phylogenetically independent (e.g., Amsler and Fairhead 2006; Hay and Steinberg
1992; Jormalainen and Honkanen 2008; Paul et al. 2001; Sotka et al. 2009). One prominent
group of marine herbivores is amphipods (Crustacea), which are proposed by Hay et al.
(1987) as ‘‘insect equivalents’’ due to a number of shared traits: (1) insects and amphipods
are small relative to the physical size of their host plants, (2) both taxa utilize their host
plants as food and habitat, and (3) both can have large impacts on host community
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structure when released from consumption by enemies (Davenport and Anderson 2007;
Duffy and Hay 2000; but see Poore et al. 2009; Tegner and Dayton 1987).
Most herbivorous amphipods are polyphagous. Approximately 60% of amphipod spe-
cies in the herbivorous family Ampithoidae (hereafter, amphipods) can be found on two or
more orders of seaweeds (Poore et al. 2008), a rate of specialization that is comparable to
that for tropical root-feeding insects (Novotny and Basset 2005). Amphipods are unable to
diapause and cannot emerge when preferred hosts are available (as many specialist insects
do). This has led to the hypothesis that amphipod polyphagy is favored evolutionarily
because preferred hosts are unavailable during some seasons, thus forcing amphipods onto
other hosts (Hay and Steinberg 1992; Steneck and Watling 1982). While it is clear that
marine herbivores respond to seasonal shifts in host availability by altering their diets in a
phenotypically-plastic manner (Cannicci et al. 2007; Clements and Choat 1993; Kotta et al.
2006), to our knowledge, there are no published studies that looked for a seasonal genetic
response to the seasonality of algal availability (i.e., fluctuating selection) or the evolu-
tionary potential for a genetic response.
Ampithoe longimana study system
The amphipod Ampithoe longimana is an abundant herbivore within high-salinity estu-
aries from Nova Scotia to Florida. It utilizes seagrasses (Nelson 1980) and at least 25
genera within 13 orders from all macroalgal phyla as hosts (Brooks and Bell 2001; Duffy
1989; McCarty 2008). Dispersal among hosts in A. longimana has not been assessed, but
given that other ampithoid amphipods are not isolated to a single host plant (Poore 2004),
it is likely that A. longimana are individually polyphagous. In North Carolina, the
amphipod is particularly abundant on seaweeds in the tropical genus Dictyota during the
warmer summer months (Duffy and Hay 1994). This host provides refuge from predation
by omnivorous pinfish, which voraciously consume most non-Dictyota seaweeds but
avoid Dictyota and its diterpene alcohols (Duffy and Hay (1991, 1994). However,
Dictyota is unavailable during the colder winter and spring months (Richardson 1979)
(Fig. 1). Although Ampithoe longimana is found on other seaweeds year-round (Sar-gassum, Ulva, Hypnea, Gracilaria), these hosts are likely to be particularly important
during the winter and spring when Dictyota is unavailable. Because amphipods produce
offspring year-round (time to 1st reproduction is 14–21 days; Cruz-Rivera and Hay 2001;
Nelson 1980; Sotka and Hay 2002), it is possible that A. longimana is subject to fluc-
tuating selection across seasons for alternative hosts (i.e., use of Dictyota during warmer
months and of other seaweeds during colder months) if there are tradeoffs in using these
alternative hosts.
We used a controlled natural-selection experiment (Fry 2003) to test whether fluctuating
selection could potentially shape A. longimana host use. We mimicked seasonal field
distributions by isolating amphipods on Dictyota species (Dictyota-line) or a mix of
common seaweeds without Dictyota (mixed-seaweed-line) for 15 weeks. Feeding prefer-
ence and juvenile performance assays addressed the following questions: (1) Does
amphipod feeding preference for Dictyota differ between Dictyota-lines and mixed-sea-
weed-lines? (2) Are these differences mediated by the lipophilic secondary metabolites
produced by Dictyota? (3) Does fitness (survivorship, growth, reproduction) of juveniles
differ across experimental lines when confronted with Dictyota versus other foods? The
tradeoff hypothesis predicts that the reaction norms of fitness when reared on alternative
foods will cross across experimental lineages (Fry 1996). (4) Among field-collected
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amphipods, does the feeding preference for Dictyota vary among seasons? The fluctuating
selection hypothesis predicts that A. longimana populations will have greater affinity for
Dictyota at the end of summer compared to populations collected at the end of winter.
Materials and methods
Collection of organisms
All seaweeds (Sargassum filipendula, Dictyota menstrualis, D. ciliolata, Gracilariaverrucosa, Hypnea musciformis, Ulva lactuca and U. intestinalis) were collected from
shallower than 2.0 m below MLLW within seagrass, jetty and mudflat habitats in Bogue
Sound, North Carolina (34�410 N, 76�410 W). We returned seaweeds within 2 h of col-
lection to running seawater tables at the University of North Carolina’s Institute of Marine
Sciences in Morehead City, NC. Within 24 h, seaweeds were rinsed, cleaned of all epi-
biota and were either flash frozen on dry ice or added fresh to experiments. Frozen
seaweed tissue was returned to the College of Charleston’s Grice Marine Laboratory in
Charleston, South Carolina, lyophilized, ground into a fine powder using a Wiley mini-
mill, and stored at -20�C until use. Previous work indicates that lyophilization maintains
concentrations of some lipophilic metabolites such as diterpenes (Cronin et al. 1995) and
does not substantially lessen the ability of seaweeds with known lipophilic deterrents to
reduce consumer feeding rates (Bolser and Hay 1996; Cruz-Rivera and Hay 2001). Using
lyophilized seaweeds ensures that the same tissues are offered to all A. longimana at all
time points and there are no temporal changes in seaweed quality. Moreover, for A.longimana and the seaweeds used here (Sargassum, Dictyota and Ulva), the ranking of
feeding preference is similar regardless of whether amphipods are offered fresh or
lyophilized seaweed tissue (Duffy and Hay 1991; Sotka, unpublished data), suggesting that
morphological aspects are less important than nutritional or chemical defenses in deter-
mining seaweed palatability.
To seed the selection experiment, female Ampithoe longimana were collected in June
2008 from the seaweeds listed above plus Ectocarpus spp. We did not assay the feeding
preferences of these females prior to their introduction into the experiment although overall
initial preferences across cultures is likely to be similar as amphipods from multiple
sources were mixed thoroughly before allocation to the replicate lines.
Fig. 1 Seawater temperatures(max to min) within BogueSound, North Carolina fromJanuary to December. Among themost important host plants for theamphipod Ampithoe longimanaare tropical seaweeds in thegenus Dictyota, which is foundabundantly (black bar) andintermittently (grey bars) duringwarmer months. Subtidal datarecorded at Rachel CarsonNational Estuarine ResearchReserve during 2004 (34.708N;76.628W; http://nerrs.noaa.gov/)
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Controlled natural selection experiment
Three replicate cultures were isolated with mostly Dictyota menstrualis and some D.ciliolata (=Dictyota-line), while six replicate cultures were isolated with a mix of seaweeds
from several divisions (= mixed-seaweed-line; S. filipendula, G. verrucosa, H. musciformisand U. lactuca). Approximately 50 pregnant females were placed into each of nine plastic
tubs that contained * 4L of aerated seawater (30–32 ppt; 25�–28�C) and exposed to full
spectrum aquarium lights (T-5 fluorescent lighting bulbs; Ushio, Tokyo, Japan) at a 12:12
(day: night) cycle and to natural light from nearby windows. Field-collected females tend
to produce between 5 and 20 juveniles, and we estimate that the initial starting density was
between 250 and 1,000 amphipods per replicate tub. Females plus their juveniles were
allowed to consume U. intestinalis for 7 days before the treatment seaweeds were added.
Ulva intestinalis was chosen as an initial base diet when establishing the experimental
cultures because it is a readily-available, high-quality food that has no known secondary
metabolites. Prior to addition to the cultures, all seaweed was dipped twice in freshwater
for 30s to remove amphipods and robustly shaken to remove snails and other epibionts.
Seawater was replaced every 1–2 weeks, and seaweeds were replenished as needed (once
or twice per week).
We did not quantify the population sizes within these cultures, although the numbers
were likely on the order of 100s rather than 1,000s per replicate tub. Thus, we cannot
eliminate the possibility that genetic drift played a role in the evolution of host use traits,
although this possibility seems unlikely given the consistency of the feeding preferences
across independent lines (see Results). We did not create discrete generations (i.e., remove
adults after they had reproduced), and therefore our experimental selection is constrained
by the presence of overlapping generations. We do not have good estimates of longevity of
A. longimana, but we have generally seen 6–8 weeks in the laboratory (personal obser-
vation). Field-based estimates of longevity in other ampithoids is on the order of months
(Sainte-Marie 1991). Because amphipods become fecund after approximately 2–3 weeks
on high quality diets at 20–25�C (Cruz-Rivera and Hay 2001; Sotka et al. 2003), our
15-week experiment likely represents between two and five generations.
We generated twice as many mixed-seaweed replicates than Dictyota-line replicates
because we initially intended to compare the evolutionary response of amphipods in the
presence and absence of cues from predatory pinfish cues. However, we failed to detect
consistent differences in feeding preference between amphipods from fish-cue vs. no-fish-
cue treatments (data not shown) and therefore analyzed both sets of lines as mixed-
seaweed-lines. We assayed between two and five cultures per treatment type each week
and made adjustment for unequal sample sizes in subgroups using a Satterwaite approx-
imation (following Sokal and Rohlf 1981).
Feeding choice assays
We assessed changes in feeding preference using laboratory-based feeding choice assays in
which amphipods were offered a choice between freeze-dried tissue from a single Dictyotaspecies and Ulva intestinalis. Freeze-dried seaweeds were embedded within agar and
bound onto window screen (for recipe see Sotka and Giddens 2009). This procedure
created two grids (i.e., 6 9 5 squares of each type of food) to quantify consumption.
Individual replicates were terminated and measured after consumption of at least 10 total
squares to ensure amphipods had fed enough to demonstrate feeding preferences. We
disregarded replicates in which all of one grid and at least half of another grid were
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consumed to prevent skewing of preferences due to reduced availability of one food type.
Assays generally lasted less than 2 days, but no longer than 5 days. The mean replicate
size per feeding assay per replicate line was 16 (standard error = 1.1) and the number of
total squares consumed averaged 20.0 (standard error = 0.4).
For each replicate amphipod assay, we calculated the proportion of Dictyota tissue
consumed using the ratio: (Number of squares of Dictyota consumed) 9 (Total # of
squares consumed)-1. These values were then assessed statistically using nonparametric
ANOVAs, where statistical significance was evaluated by comparing F-ratios with an
expected distribution generated from 1,000 permutations of the dataset (Anderson 2001)
using R (http://cran.r-project.org). The non-parametric approach is appropriate because the
data are not normally distributed and could not be transformed to yield normality, and
because all replicates are independent and exchangeable. We pursued a one-way non-
parametric ANOVA that nested the effect of replicate culture within treatment. We also
pursued a one-way nonparametric ANOVA in order to assess the effect of season on
feeding preference, and used a series of pairwise nonparametric ANOVAs for post hoc
analysis. We also assessed for the final week of assays of D. menstrualis (Week 19) the
relative consumption of Ulva and D. menstrualis using a paired t-test.
We offered amphipods a feeding choice between U. intestinalis tissues coated with or
without lipophilic metabolites from Dictyota ciliolata. Plants were collected from Radio
Island Jetty in June 2008, flash-frozen on dry ice, lyophilized, milled and stored at -20�C.
In October 2008, six grams of dried tissue were extracted thrice for 30 min in a total of
180-mL of 1:1 ethyl acetate:methanol. The extract was Whatman-disc filtered and rotary-
evaporated to remove solvents. The extract was then dissolved in 20-mL of ethyl acetate,
added to freeze-dried U. intestinalis, and rotary-evaporated. To create control foods,
20-mL of ethyl acetate was added to freeze-dried U. intestinalis and rotary-evaporated.This assay was designed and analysed as a nested ANOVA, with replicate (n = 8–11) tubs
within each treatment type.
Performance assays
We assessed the fitness of amphipods by rearing juveniles collected from experimental
females at week 15. Females were collected from each of the two sets of lines, and offered
fresh Ulva intestinalis for 1 week. Females were collected haphazardly across all cultures
(tubs) but we did not record tub origin for each female. Four emerged juveniles from each
of thirty females were isolated with one of four food treatments (no food, fresh tissue of
Dictyota menstrualis, Sargassum filipendula or Ulva intestinalis) within individual 40-mL
Petri dishes. We maintained the experiment at room temperature, changed seawater and
foods every 3–4 days, and checked for mortality and female maturity every 1–2 days.
Antibiotics (100 mg/mL each of streptomycin and penicillin) were dissolved into seawater
collected from Bogue Sound, NC in order to minimize bacterial infection in our assay. At
the end of 25 days, surviving amphipods were digitally photographed and body length
determined using Image-J.
All animals isolated with no food died within 4 days. For the survivors, we used
parametric survival curve analysis to assess survivorship curves and age to maturity (which
combines days to reproduction with the overall proportion of females that became
reproductive). Because all juveniles from replicate lines were grouped together, we cannot
estimate variation in these parameters. Growth rates and female fecundity were assessed
via a series of nonparametric ANOVA similar to those described for feeding assays above.
Because the results indicated that the selected lines shifted their life-history strategies
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(see Results), we directly compared the relative contribution of survivorship, number of
eggs produced, and age at maturity in a single estimate of population rate of intrinsic
capacity (rC) following Southwood (1978), which is a proxy for intrinsic rate of population
increase. The estimate of population growth rate does not account for multiple broods nor
longevity of the amphipods. To assess whether rC estimates were significantly different, we
performed bootstrap analysis following Meyer et al. (1986). Individuals were sampled
(with replacment) 1,000 times per treatment combination (e.g., Dictyota-line on Dictyota),
and estimates of mean rC and their 95% confidence intervals were calculated via Equa-
tion 3 of Meyer et al. (1986).
Temporal and spatial variation in feeding preference
In order to assess temporal variation in feeding preference, A. longimana were collected in
November 2008 and May and October 2009, and May and November 2010 from Sar-gassum filipendula in Bogue Sound. In order to minimize the effect of recent feeding
history on feeding preferences (Poore and Hill 2006), individuals were initially fed for
3 days on only Ulva intestinalis before we initiated the feeding choice experiment. We
also collected amphipods from alternative hosts (Dictyota vs. Hypena, and Dictyota vs.
Sargassum) to test for the presence of sympatric host races. To compare the feeding
preferences of A. longimana collected on alternative host sources (e.g., Dicytota vs.
Hypnea), we used a paired t-test to compare feeding rates on the two seaweeds and an
unpaired t-test to compare the relative consumption of D. menstrualis between amphipod
sources.
Results
Experimental shift in feeding preference
Experimental shift of adult feeding preferences occurred within 15 weeks (Fig. 2a). During
the early stages of the experiment (Week 7), adults from Dictyota-lines and mixed-sea-
weed-lines did not significantly differ in their relative preference for Dictyota because of
significant variance among replicate lines within treatments (Table 1). By the end of Week
12 (estimated at between two to five overlapping generations), adults from Dictyota-lines
consumed significantly more D. menstrualis than did adults from mixed-seaweed-lines,
and within-treatment variance was nonsignificant.
To minimize the role of recent feeding history, we placed all lines onto a mixed
seaweed diet for 2 weeks and then re-evaluated feeding preference for Dictyota men-strualis. A set of paired t-tests found that Dictyota-lines consumed slightly more D.menstrualis than Ulva (Fig. 2a; t = 2.059, n = 46, P = 0.043) while mixed-seaweed-lines
consumed significantly more Ulva than D. menstrualis. (t = -2.76, n = 40, P = 0.007).
A direct test of relative consumption of Dictyota indicates that although adults from
Dictyota-lines did not feed on Dictyota as readily after this ‘relaxation’ period, the adults
maintained a greater feeding preference for D. menstrualis relative to adults from the
mixed-seaweed lines (Fig. 2a; Table 1).
We also document an experimental shift of feeding preference for Dictyota ciliolata.
Dictyota-lines consumed more D. ciliolata than did mixed-seaweed-lines at weeks 12 and
15 (Fig. 2a; Table 1). As has been seen repeatedly with these amphipods (Cruz-Rivera and
Hay 2003; Duffy and Hay 1991; Sotka and Hay 2002), feeding preferences for Dictyota
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species are mediated by lipophilic metabolites, including diterpene alcohols. When offered
a feeding choice between control and extract-coated foods at Week 15, Dictyota-lines
consumed twice as much extract-coated tissue than did the mixed-seaweed-lines (Fig. 2c;
Table 1).
Fitness tradeoffs in using alternative seaweeds
The experimental shift in host use revealed the presence of fitness-based tradeoffs when
utilizing alternative seaweeds. At the end of week 15, we removed females from experi-
mental lines and isolated their juveniles on three seaweed diets (Dictyota menstrualis,
Sargassum filipendula, and Ulva intestinals) and measured diet- and lineage-specific rates
of survivorship, growth and reproduction. Survivorship did not significantly differ among
amphipod lines, neither when compared on a single diet (analyses not shown) nor when all
diets were combined (v2 = 1.99; P = 0.160; Fig. 3a). The growth rate of juveniles from
mixed-seaweed-lines was faster than those of Dictyota-lines when all diets were combined
Fig. 2 Changes in feeding preferences during a controlled natural-selection experiment. Relative feedingpreference (mean ± S.E.) for (a) Dictyota menstrualis (b) D. ciliolata and (c) the lipophilic extract of D.ciliolata within paired-choice feeding assays is plotted for each amphipod line. Amphipod lines wereisolated on Dictyota species (‘Dictyota-lines’) or a mix of other seaweeds (‘Mixed-seaweed-lines’) for15 weeks. All cultures were placed onto a mixed seaweed diet on week 16 (see arrow). Points connected bylines indicate results from individual selection lines. See Table 1 for statistical details
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(Fig. 3b). Growth also varied across diet (Ulva [ Dictyota [ Sargassum), but an inter-
action between diet and lineage was not significant. Mixed-seaweed-lines produced a
greater number of eggs than did Dictyota-lines when animals from all diets were combined
(Fig. 3c; Table 2). When reared on Dictyota, Dictyota-line females became reproductive
more quickly compared to the mixed-seaweed females (Fig. 3d; v2 = 4.87; P = 0.027).
There was no difference among lines in time to female reproduction when reared on
Sargassum (v2 = 0.64; P = 0.420) or Ulva (v2 = 0.21; P = 0.650; Fig. 3d).
Table 1 ANOVAs from feeding preference assays in Fig. 2
D.f. M.S. F P
Relative consumption of Dictyota menstrualis
Week 7
Treatment 1 0.912 1.708 0.273
Culture {Treatment} 3.4 0.534 2.872 0.019
Error 85 0.111
Week 12
Treatment 1 3.064 26.859 0.008
Culture {Treatment} 3.8 0.114 1.288 0.277
Error 85 0.085
Week 15
Treatment 1 7.938 47.617 0.002
Culture {Treatment} 4 0.167 1.984 0.100
Error 137 0.084
Relaxation
Treatment 1 0.878 199.015 \0.001
Culture {Treatment} 3.7 0.004 0.067 0.992
Error 80 0.068
Relative consumption of D. ciliolata
Week 12
Treatment 1 2.417 50.331 0.003
Culture {Treatment} 3.6 0.048 1.03 0.405
Error 87 0.046
Week 15
Treatment 1 3.341 37.742 0.005
Culture {Treatment} 3.7 0.089 0.652 0.626
Error 139 0.138
Relative consumption of D. ciliolata extract
Week 15
Treatment 1 2.089 19.311 0.016
Culture {Treatment} 3.5 0.108 1.020 0.407
Error 47 0.106
Treatment indicates two types of selection lines (Dictyota-lines vs. Mixed seaweed-lines). Between two andfive cultures are nested within each treatment type
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Thus, the mixed-seaweed-lines tended to grow more quickly (Fig. 3b) and their females
produced more eggs (Fig. 3c) relative to the Dictyota-lines, but the Dictyota-lines became
fecund more quickly on Dictyota (an average of 3.4 days earlier; 15.0 vs. 18.4 days;
Fig. 3d). In order to reconcile these life-history patterns, we summarized survivorship, age
to maturity, and female fecundity into a simplified metric, the intrinsic rate of population
increase (or rC). There is a clear qualitative shift among lineages in the hierarchical rank of
diets that provide the greatest population growth: Dictyota provided Dictyota-lines with the
greatest rC, while Ulva intestinalis provided the greatest rC for the mixed-seaweed lines.Both sets of experimental lineages had their lowest rC on Sargassum filipendula (Fig. 3e).
Testing predictions using field-collected amphipods
The rapid evolution of host use traits for alternative seaweeds (Fig. 2), in combination with
the presence of an apparent tradeoff in using alternative hosts (especially for female time to
maturity; Fig. 3d) yields two predictions. The first is that individuals collected from
Dictyota or alternative hosts differ substantially in host use traits by either genetic or
phenotypic mechanisms. There was no significant difference in the willingness of Dict-yota- or Sargassum-collected amphipods to consume these seaweeds (Fig. 4a; unpaired
t-test P = 0.803). There was also no significant difference among Dictyota- or Hypnea-
collected amphipods to consume these seaweeds (Fig. 4b; unpaired t-test P = 0.113).
The second prediction is that feeding preference in field-collected populations may shift
seasonally due to fluctuating selection. That is, amphipods collected when Dictyota has
been abundant (i.e., fall) should be more willing to consume Dictyota compared to am-
phipods collected when Dictyota is absent (i.e., spring). When offered a feeding choice
between freeze-dried tissue from Dictyota menstrualis and Ulva intestinalis, A. longimanacollected in November-2008 (at the end of fall) consumed nearly twice as much Dictyotatissue as A. longimana collected in May-2009 (at the end of winter) (Fig. 4c). However,
amphipods collected in October-2009 did not differ from amphipods collected in May-
2009, June-2010 nor October 2010. Thus, if there is fluctuating selection on Dictyotausage, then its effectiveness varies across years. It is unlikely these patterns are driven by
their recent history, as all amphipods were collected from a mix of non-Dictyota seaweeds
(e.g., Sargassum and Hypnea) and fed on Ulva intestinalis for at least 3 days prior to
feeding assays, although there remains the possibility of effects of previous diet that linger
for more than 3 days.
Table 2 ANOVAs from amphi-pod size and number of eggs(Fig. 3b–c). Treatment indicatestwo types of selection lines(Dictyota-lines vs. Mixed sea-weed-lines). Diet was eitherSargassum, Dictyota or Ulva
D.f. M.S. F P
Final size of amphipods
Treatment 1 2.393 4.730 0.035
Diet 2 10.932 20.546 \0.001
Treatment 9 Diet 2 0.832 1.644 0.177
Error 86 0.506
Number of eggs
Treatment 1 8.787 4.193 0.047
Diet 2 2.063 0.984 0.407
Treatment 9 Diet 2 0.146 0.0695 0.930
Error 33 2.096
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Fig. 3 Fitness of amphipods from the controlled natural-selection experiment. Amphipods from two typesof selection lines (Dictyota- or mixed-seaweed lines) were reared for 25 days on three diets (n = 29;Dictyota menstrualis, Ulva intestinalis and Sargassum filipendula). a Survivorship, b body size (n = 13–17;see Table 2 for statistical details), c female fecundity (n = 3–9; see Table 2 for statistical details) andd female age to maturity are indicated. The asterisk in d refers to a significantly faster maturity rate ofDictyota- than of mixed-seaweed females when reared on Dictyota. e Survivorship, age to maturity, andfecundity were used to estimate the capacity of increase (rC) and its 99% confidence intervals. Lettersindicate significant differences as detected by confidence intervals (Meyer et al. 1986)
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Discussion
The host breadth of any particular herbivore reflects a ‘compromise’ (Rausher 1992)
between evolutionary forces that promote specialism and those that promote generalism.
Because most terrestrial herbivorous insects specialize (Bernays and Graham 1988; Strong
et al. 1984), we have a clearer sense of this evolutionary balance for specialist than for
polyphagous herbivores, either marine or terrestrial (see Introduction). Here, we experi-
mentally addressed the evolution of feeding preference in the polyphagous herbivore
Ampithoe longimana. One principal finding is that within a few months of a controlled
natural-selection experiment, lines isolated on Dictyota displayed greater feeding tolerance
for Dictyota and its secondary metabolites compared to lines isolated on alternative hosts
(Sargassum, Hypnea, and Ulva; Fig. 2).
Fig. 4 The feeding preferences of amphipods collected from field-collected amphipods. a Dictyota-collected and Sargassum-collected amphipods were offered a choice of freeze-dried Dictyota menstrualisand Ulva intestinalis during August 2009. Mean (±S.E.) consumption of tissues during the assay arepresented. b Dictyota-collected and Hypnea-collected amphipods were offered a choice of fresh tissue ofDicytota menstrualis and Hypnea musciformis during July 1999. An asterisk indicates a significant(P \ 0.05) paired t test when comparing consumption of the alternative seaweeds. Mean (±S.E.)consumption of tissues during the assay are presented. c Relative consumption (Mean % Dicyota ± S.E.) offreeze-dried Dictyota menstrualis of amphipods collected in the fall and spring. Treatments that share aletter are statistically indistinguishable by Tukey–Kramer posthoc
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The shift in feeding preferences likely reflects evolution because non-genetic expla-
nations are less compelling for several reasons. First, feeding preference for Dictyota is a
heritable, autosomal trait (Sotka 2003; Sotka et al. 2003). Second, we forced individuals
to all consume the same food (Ulva intestinalis) for several days before assays began,
minimizing the effects of recent feeding history. Third, even after we allowed all lines to
consume the mixed-seaweed diet for 2 weeks, Dictyota-lines continued to consume
relatively more D. menstrualis than did the mixed-seaweed-lines (See week 19; Fig. 2a).Fourth, when amphipods were collected in the field from Dictyota versus alternative
hosts (Sargassum or Hypnea), host type did not significantly predict the willingness to
consume Dictyota (Fig. 4a, b). Finally, the patterns of feeding preference by adults for
Dictyota versus Ulva mirrored the relative performance of offspring on those foods.
Specifically, Dictyota-line adults only weakly preferred Dictyota to Ulva in feeding
choice assays (Fig. 2a at week 19), and their performance was equivalent on the two
seaweeds (as measured by the intrinisic rate of growth; Fig. 3e). In contrast, mixed-
seaweed-line adults significantly preferred Ulva over Dictyota (Fig. 2a) and their off-
spring also had greater performance on Ulva than Dictyota. Given that these offspring
were reared outside the experimental treatments and thus were naıve to either food, these
patterns suggest that both performance and preference differences across lineages have a
genetic component.
While these lines of evidence indicate that shifts in feeding preference are likely
genetic, our experimental protocol does not preclude a role for phenotypic plasticity. To
ensure that feeding preferences and performance patterns were genetically-mediated,
amphipods would need to be reared on a uniform food source for at least two generations
after selection occurred. Thus, our observed feeding patterns (Figs. 2 and 4) and perfor-
mance tradeoffs (Fig. 3) could be have been mediated by feeding experiences of young
juveniles or epigenetic cues inherited from mothers. Genetically-mediated differences
among individuals in feeding preference and plastic responses would both yield gener-
alism at the population level (Kassen 2002; Levins 1968), but through different
mechanisms.
Presence of a performance cost
Our experiment also suggests fitness consequences of adapting to alternative hosts.
Dictyota-line females become reproductive more quickly when on Dictyota than did
mixed-seaweed-line females (Fig. 3d). In contrast, when isolated on Sargassum and Ulva,
mixed-seaweed-lines grew more quickly and produced more eggs than did Dictyota-line
individuals (Fig. 3a–c). These patterns suggest there is a cost to adapting to Dictyotawhen isolated on non-Dictyota seaweeds. The observed cost does not appear to be
symmetrical; that is, a greater cost is incurred when Dictyota-adapted individuals are
isolated on non-Dictyota species, compared to when mixed-seaweed-lines are isolated on
Dictyota. While fitness costs do not necessarily indicate a formal tradeoff because our
reaction norms do not cross, the presence of fitness costs to evolving greater preference
for Dictyota are strong enough in theory to drive the evolution of specialist feeding
preferences for Dictyota (Fry 1996). One caveat to these conclusions is that because we
haphazardly sampled females from across experimental cultures, we cannot statistically
assess the relative strength of between-culture and between-treatment variation in
performance.
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These laboratory-based estimates of fitness are mediated by food quality and as such, do
not address whether amphipod fitness is altered by extrinsic plant traits (i.e., abiotic stress,
mate encounter success, or predation refuge). In this system, there is reason to suspect
that field conditions will amplify observed tradeoffs. Specifically, earlier maturation of
females from Dictyota-lines compared to females from mixed-seaweed-lineages when on
Dictyota (a 3 day difference) (Fig. 3d) may be highly and ecologically relevant. We have
no empirical estimates of predation rates of A. longimana on Dictyota in the field, but
presumably the delayed maturity of mixed-seaweed-lines on Dictyota would make those
genotypes more susceptible to predation than Dictyota-lines (Duffy and Hay 1994). If true,
then the cost of adapting to a mixed-seaweed assemblage may be greater in the field than is
apparent in our laboratory-based assays. Whether fitness tradeoffs are mediated by genetic
or phenotypic mechanisms, the ecological interactions with predators would be function-
ally similar.
The presence of fitness costs in A. longimana mirrors a growing list of fitness-tradeoffs
in other seaweed-herbivore interactions. Molluscan herbivores (snails and slugs) produce
radular types that differ in their effectiveness when grazing particular seaweeds (Steneck
and Watling 1982), and tradeoffs arise when a mismatch between radular type and the
available seaweed occurs (Padilla 1985; Trowbridge 1991). Other mollusks are apparently
susceptible to analogous mismatches in other herbivore traits (e.g., ‘suction pressure,
salivary enzyme or feeding methods’ (Trowbridge and Todd 2001)). The isopod Idoteabalthica also displays tradeoffs: individuals collected from the brown seaweed Fucusvesiculosus grew faster on Fucus than did individuals collected from seagrass Zosteramarina, and vice versa (Vesakoski et al. 2009).
The growing number of tradeoffs found within polyphagous herbivores (including
terrestrial insects; Singer 2008) remains paradoxical given that tradeoffs tend to favor the
evolution of specialization. One resolution of this conflict is that the benefit of using
multiple hosts (i.e., broadening the resource base) may outweigh its fitness cost (Poore
et al. 2008). Polyphagy in A. longimana appears to be favored because it broadens resource
use across seasons, and this benefit outweighs the fitness-based costs that favor the evo-
lution of specialism on Dictyota. As a contrasting example, the isopod Idotea balthicaspecializes on the brown seaweed genus Fucus (Jormalainen et al. 2001) despite the fact
that Fucus is a relatively poor quality food for Idotea juveniles relative to alternative hosts.
However, Fucus can be found year-round while those alternative foods largely disappear
during colder winter months. Thus, A. longimana is a generalist that uses the seasonally
available Dictyota as one of a suite of species, while Idotea is a specialist on the con-
sistently-available Fucus. For these herbivores, the temporal variability of host plants and
host breadth are positively related, a hypothesis that has broad theoretical (Levins 1968)
and empirical support (Cates 1981; Jaenike 1978, 1990; Kassen 2002; Wiklund and Friberg
2009; but see Futuyma 1976).
Equivocal evidence for fluctuating selection
Because this amphipod can evolve host use traits rapidly (within a few months), it is
possible that feeding preference for Dictyota may evolve seasonally via fluctuating
selection. Indeed, individuals collected in November-2008 (after the warm summer)
showed greater feeding preference for Dictyota than did individuals collected in May-2009
(after the cold winter; Fig. 4c). However, feeding preference did not differ between spring
and fall collections in 2009 nor 2010, across which we would have expected an increase in
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feeding preference for Dictyota. There are several possible explanations for the lack of a
field response during the summer of 2009 and 2010. First, any signal of fluctuating
selection would require that Dictyota-adapted individuals would disperse from Dictyotaplants onto Sargassum and such effective dispersal may differ between years. Second,
selection to utilize Dictyota may not have been as effective during the summer of 2009
relative to the summer of 2008. Third, the field pattern may reflect genetic drift, inade-
quately-sampled populations, or both. Resolving these alternatives will require assaying
amphipod feeding preferences over a number of years, and rearing those amphipods within
a common garden environment for a full generation in order to minimize environmental
and maternal effects.
Implications of rapid evolution
If the shift we document here has a genetic component, then the pace of evolution of
A. longimana toward their autrophic prey is rapid (i.e., less than 10 rather than 100s or
1,000s of generations) and comparable to the pace seen among terrestrial herbivorous
insects (Fry 2003), and freshwater (Hairston et al. 2001) and marine copepods (Colin
and Dam 2004). Rapid evolution has implications for studies that utilize closed-system
mesocosms and small herbivorous grazers with short generation times (e.g., A. longi-mana becomes mature within 3–4 weeks). A preliminary survey of these studies, which
include the expanding literature on grazer biodiversity and ecosystem function, reveals
a mean (±S.E.) time of duration of 5.5 ± 1.2 weeks (Range = 1–22 weeks; Table 4 in
Appendix). Given that we detected shifts (genetic or both genetic and phenotypic) in
feeding traits within time spans encompassed by some of these studies suggests pre-
vious studies in these mesocosms may partly reflect evolutionary interactions, rather
than wholly ecological phenomena as is implicitly assumed (Hairston et al. 2005). Our
results also indicate that when small short-lived herbivores are exposed to alternative
and seasonally-available hosts that generate fitness-based tradeoffs, the possibility of
fluctuating selection in host use traits across seasons should be explored (Thompson
1998).
Acknowledgments We thank Artur Veloso, John Bruno and the University of North Carolina’s Institute ofMarine Sciences for logistical support, Tina Bell, Mark Hay and Bob Podolsky for thoughtful discussions,and the National Science Foundation for funding (OCE-0550245; DEB-0919064). This is Grice PublicationNumber 364.
Appendix
See Tables 3 and 4.
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Table 3 Raw lifetable data (summarized in Fig. 3e)
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Table 3 continued
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Table 4 A survey of published studies that use closed-system mesocosms and small marine herbivores
Herbivore Herbivore sp. Habitat Timespan(days)
Reference
Amphipods Mix dominated by: Hyale spp.,Elasmopus levi, Corophium spp.
Macroalgae (mix) 22 Bruno andO’Connor(2005)Isopod Paracerceis caudata
Amphipod Ampithoe longimana Macroalgae (mix) 28; 21 Bruno et al.(2008)Urchin Arbacia punctulata
Fish Lagodon rhomboides
Amphipods Gammarus mucronatus, Cymadusacompta, Ampithoe longimana
Seagrass (Zosteramarina)
42 Canuel et al.(2007)
Isopods Erichsonella attenuata, Idotea baltica
Amphipods Most abundant: Ampithoe longimanaDulichiella appendiculata
Macroalgae (mix) 154 Duffy and Hay(2000)
Isopod Paracerceis caudata
Gastropod Diastoma varium
Amphipods Gammarus mucronatus, Ampithoelongimana, Cymadusa compta
Seagrass (Zosteramarina)
28 Duffy andHarvilicz(2001)
Amphipods Cymadusa compta, Dulichiellaappendiculata, Gammarus mucronatus
Seagrass (Zosteramarina)
42 Duffy et al.(2003)
Isopods Erichsonella attenuata, Idotea baltica
Amphipods Cymadusa compta, Ampithoe longimana,Gammarus mucronatus
Seagrass (Zosteramarina)
42 Duffy et al.(2005)
Isopods Erichsonella attenuata, Idotea baltica
Amphipods Ampithoe longimana, Cymadusa compta Seagrass (Zosteramarina)
56 France and Duffy(2006a)Isopods Erchsonella attenuata, Idotea baltica
Amphipods Gammarus mucronatus, Ampithoevalida, Cymadusa compta, Dulichiellaappendiculata, Elasmopus levis
Seagrass (Zosteramarina)
47 France and Duffy(2006b)
Isopods Erchsonella attenuata, Idotea baltica,Paracerceis caudata
Ciliates Euplotes sp., Diophrys sp.,Eutintinnus inquilinum
Microalgae (mix) 21 Gamfeldt et al.(2005)
Gastropods Littorina littorea, L. saxatilis,L. fabalis
Periphyton 35; 56 Hillebrand et al.(2009)
Amphipod Gammarus duebenii
Isopod Idotea garnulosa
Shrimp Palaemon elegans
Isopod Idotea baltica Seagrass (Zosteramarina), epiphytes
10 Jaschinski andSommer(2008)
Amphipod Gammarus oceanicus
Gastropods Littorina littorea,Rissoa membranacea
Isopod Idotea baltica Seagrass (Zosteramarina)
7, 21 Jaschinski et al.(2009)Amphipod Gammarus salinus
Gastropod Littorina littorea
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References
Amsler CD, Fairhead VA (2006) Defensive and sensory chemical ecology of brown algae. Adv BotanicalRes 43:1–91
Anderson M (2001) Permutation tests for univariate or multivariate analysis of variance and regression. CanJ Fish Aquat Sci 58:626–639
Bernays EA, Graham M (1988) On the evolution of host specificity in phytophagous arthropods. Ecology69:886–892
Bernays EA, Minkenberg O (1997) Insect herbivores: different reasons for being a generalist. Ecology78:1157–1169
Bolser RC, Hay ME (1996) Are tropical plants better defended? Palatability and defenses of temperate vstropical seaweeds. Ecology 77:2269–2286
Brooks RA, Bell SS (2001) Mobile corridors in marine landscapes: enhancement of faunal exchange atseagrass/sand ecotones. J Exp Mar Biol Ecol 264:67–84
Bruno JF, O’Connor MI (2005) Cascading effects of predator diversity and omnivory in a marine food web.Ecol Lett 8:1048–1056
Bruno JF, Boyer KE, Duffy JE, Lee SC (2008) Relative and interactive effects of plant and grazer richnessin a benthic marine community. Ecology 89:2518–2528
Cannicci S, Gomei M, Dahdouh-Guebas F, Rorandelli R, Terlizzi A (2007) Influence of seasonalfood abundance and quality on the feeding habits of an opportunistic feeder, the intertidal crabPachygrapsus marmoratus. Mar Biol 151:1331–1342
Canuel EA, Spivak AC, Waterson EJ, Duffy JE (2007) Biodiversity and food web structure influence short-term accumulation of sediment organic matter in an experimental seagrass system. Limnol Oceanogr52:590–602
Cates RG (1980) Feeding patterns of monophagous, oligophagous, and polyphagous insect herbivores: theeffect of resource abundance and plant chemistry. Oecologia 46:22–31
Cates RG (1981) Host plant predictability and the feeding patterns of monophagous, oligophagous, andpolyphagous insect herbivores. Oecologia 48:319–326
Clements KD, Choat JH (1993) Influence of season, ontogeny and tide on the diet of the temperate marineherbivorous fish Odax-pullus (Odacidae). Mar Biol 117:213–220
Colin SP, Dam HG (2004) Testing for resistance of pelagic marine copepods to a toxic dinoflagellate. EvolEcol 18:355–377
Cronin G, Lindquist N, Hay ME, Fenical W (1995) Effects of storage and extraction procedures on yields oflipophilic metabolites from the brown seaweeds Dictyota ciliolata and D. menstrualis. Mar Ecol ProgSer 119:265–273
Cruz-Rivera E, Hay ME (2001) Macroalgal traits and the feeding and fitness of an herbivorous amphipod:the roles of selectivity, mixing, and compensation. Mar Ecol Prog Ser 218:249–266
Cruz-Rivera E, Hay ME (2003) Prey nutritional quality interacts with chemical defenses to affect consumerfeeding and fitness. Ecol Monogr 73:483–506
Davenport AC, Anderson TW (2007) Positive indirect effects of reef fishes on kelp performance: theimportance of mesograzers. Ecology 88:1548–1561
Duffy JE (1989) Ecology and evolution of herbivory by marine amphipods. University of North Carolina atChapel Hill, Chapel Hill
Duffy JE, Harvilicz AM (2001) Species-specific impacts of grazing amphipods in an eelgrass-bed com-munity. Mar Ecol Prog Ser 223:201–211
Duffy JE, Hay ME (1991) Food and shelter as determinants of food choice by an herbivorous marineamphipod. Ecology 72:1286–1298
Duffy JE, Hay ME (1994) Herbivore resistance to seaweed chemical defense - the roles of mobility andpredation risk. Ecology 75:1304–1319
Duffy JE, Hay ME (2000) Strong impacts of grazing amphipods on the organization of a benthic com-munity. Ecol Monogr 70:237–263
Duffy JE, Richardson JP, Canuel EA (2003) Grazer diversity effects on ecosystem functioning in seagrassbeds. Ecol Lett 6:637–645
Duffy JE, Richardson JP, France KE (2005) Ecosystem consequences of diversity depend on food chainlength in estuarine vegetation. Ecol Lett 8:301–309
France KE, Duffy JE (2006a) Consumer diversity mediates invasion dynamics at multiple trophic levels.Oikos 113:515–529
France KE, Duffy JE (2006b) Diversity and dispersal interactively affect predictability of ecosystemfunction. Nature 441:1139–1143
Fry J (1996) The evolution of host specialization: are trade-offs overrated? Am Natur 148:S84–S107
Evol Ecol (2011) 25:1335–1355 1353
123
Author's personal copy
Fry JD (2003) Detecting ecological trade-offs using selection experiments. Ecology 84:1672–1678Futuyma DJ (1976) Food plant specialization and environmental predictability in Lepidoptera. Am Nat
110:285–292Futuyma D, Moreno G (1988) The evolution of ecological specialization. Ann Rev Ecol Syst 19:207–233Gamfeldt L, Hillebrand H, Jonsson PR, Chase J (2005) Species richness changes across two trophic levels
simultaneously affect prey and consumer biomass. Ecol Lett 8:696–703Hairston NG, Holtmeier CL, Lampert W, Weider LJ, Post DM, Fischer JM, Caceres CE, Fox JA, Gaedke U
(2001) Natural selection for grazer resistance to toxic cyanobacteria: Evolution of phenotypic plas-ticity? Evolution 55:2203–2214
Hairston NG, Ellner SP, Geber MA, Yoshida T, Fox JA (2005) Rapid evolution and the convergence ofecological and evolutionary time. Ecol Lett 8:1114–1127
Hay ME, Steinberg PD (1992) The chemical ecology of plant-herbivore interactions in marine versusterrestrial communities. In: Rosenthal G, Berenbaum M (eds) Herbivores: their interactions withsecondary plant metabolites: ecological and evolutionary processes. Academic Press, San Diego,pp 371–413
Hay ME, Duffy JE, Pfister CA, Fenical W (1987) Chemical defense against different marine herbivores: areamphipods insect equivalents? Ecology 68:1567–1580
Hillebrand H, Gamfeldt L, Jonsson PR, Matthiessen B (2009) Consumer diversity indirectly changes preynutrient content. Marine Ecol Prog Ser 380:33–41
Jaenike J (1978) Resource predictability and niche breadth in the Drosophila quinaria species group.Evolution 32:676–678
Jaenike J (1990) Host specialization in phyophagous insects. Annu Rev Ecol Syst 21:243–273Jaschinski S, Sommer U (2008) Functional diversity of mesograzers in an eelgrass-epiphyte system. Mar
Biol 154:475–482Jaschinski S, Aberle N, Gohse-Reimann S, Brendelberger H, Wiltshire K, Sommer U (2009) Grazer
diversity effects in an eelgrass–epiphyte–microphytobenthos system. Oecologia 159:607–615Jormalainen V, Honkanen T (2008) Macroalgal chemical defenses and their roles in structuring temperate
marine communities. In: Amsler CD (ed) Algal chemical ecology. Springer, Heidelberg, pp 57–89Jormalainen V, Honkanen T, Heikkila N (2001) Feeding preferences and performance of a marine isopod on
seaweed hosts: cost of habitat specialization. Marine Ecol Prog Ser 220:219–230Kassen R (2002) The experimental evolution of specialists, generalists, and the maintenance of diversity.
J Evol Biol 15:173–190Kotta J, Orav-Kotta H, Paalme T, Kotta I, Kukk H (2006) Seasonal changes in situ grazing of the meso-
herbivores Idotea baltica and Gammarus oceanicus on the brown algae Fucus vesiculosus andPylaiella littoralis in the Central Gulf of Finland, Baltic Sea. Hydrobiologia 554:117–125
Levins R (1968) Evolution in changing environments; some theoretical explorations. Princeton UniversityPress, Princeton, p 132
McCarty AT (2008) Regional variation in feeding preferences of the marine herbivore Ampithoe longimanaBiology. College of Charleston, Charleston, p 89
Meyer J, Ingersoll C, McDonald L, Boyce M (1986) Estimating uncertainty in population growth rates:jackknife vs. bootstrap techniques. Ecology 67:1156–1166
Nelson WG (1980) The biology of eelgrass (Zostera marina L.) amphipods. Crustaceana 39:59–89Novotny V, Basset Y (1998) Seasonality of sap-sucking insects (Auchenorrhyncha, Hemiptera) feeding on
Ficus (Moraceae) in a lowland rain forest in New Guinea. Oecologia 115:514–522Novotny V, Basset Y (2005) Review: Host specificity of insect herbivores in tropical forests. Proc R Soc Ser
B 272:1083–1090Novotny V, Basset Y, Miller SE, Weiblen GD, Bremer B, Cizek L, Drozd P (2002) Low host specificity of
herbivorous insects in a tropical forest. Nature 416:841–844Padilla D (1985) Structural resistance of algae to herbivores. Mar Biol 90:103–109Paul VJ, Cruz-Rivera E, Thacker RW (2001) Chemical mediation of seaweed-herbivore interactions:
ecological and evolutionary perspectives. In: McClintock JB, Baker B (eds) Marine chemical ecology.CRC Press, Boca Raton, pp 227–266
Poore AGB (2004) Spatial associations among algae affect host use in a herbivorous marine amphipod.Oecologia 140:104–112
Poore AGB, Hill NA (2006) Sources of variation in herbivore preference: among-individual and past dieteffects on amphipod host choice. Mar Biol 149:1403–1410
Poore AG, Hill NA, Sotka EE (2008) Phylogenetic and geographic variation in host breadth and compo-sition by herbivorous amphipods in the family ampithoidae. Evolution 62:21–38
Poore A, Campbell A, Steinberg PD (2009) Natural densities of mesograzers fail to limit growth ofmacroalgae or their epiphytes in a temperate algal bed. J Ecol 97:164–175
1354 Evol Ecol (2011) 25:1335–1355
123
Author's personal copy
Price PW (2003) Macroevolutionary theory on macroecological patterns. Cambridge University Press,Cambridge, p 302
Rausher MD (1992) Natural selection and the evolution of plant-insect interactions. In: Roitberg K, IsmanMB (eds) Insect chemical ecology: an evolutionary approach. Chapman and Hall, New York, pp 20–88
Richardson JP (1979) Overwintering of Dictyota dichotoma (Phaeophyceae) near its northern distributionlimit on the east coast of North America. J Phycol 15:22–26
Sainte-Marie B (1991) A review of the reproductive bionomics of aquatic gammaridean amphipods—variation of life-history traits with latitude, depth, salinity and superfamily. Hydrobiologia223:189–227
Schoonhoven LM, van Loon JJA, Dicke M (2005) Insect-plant biology. Oxford University Press, New York,p 440
Singer MS (2008) Evolutionary ecology of generalism. In: Tilmon KJ (ed) Specialization, speciation, andradiation: the evolutionary biology of herbivorous insects. University of California Press, Berkeley,pp 29–42
Sokal RR, Rohlf FJ (1981) Biometry, 2nd edn. W.H. Freeman and Company, New YorkSotka EE (2003) Genetic control of feeding preference in the herbivorous amphipod Ampithoe longimana.
Mar Ecol Prog Ser 256:305–310Sotka EE, Giddens H (2009) Seawater temperature alters feeding discrimination by cold-temperate but not
subtropical individuals of an ectothermic herbivore. Biol Bull 216:75–84Sotka EE, Hay ME (2002) Geographic variation among herbivore populations in tolerance for a chemically-
rich seaweed. Ecology 83:2721–2735Sotka EE, Wares JP, Hay ME (2003) Geographic and genetic variation in feeding preference for chemically
defended seaweeds. Evolution 57:2262–2276Sotka EE, Forbey J, Horn M, Poore AGB, Raubenheimer D, Whalen KE (2009) The emerging role of
pharmacology in understanding consumer-prey interactions in marine and freshwater systems. IntegrComp Biol 49:291–313
Southwood TRE (1978) Ecological methods, with particular reference to the study of insect populations.Chapman and Hall, London
Steneck RS, Watling L (1982) Feeding capabilities and limitation of herbivorous molluscs: a functionalgroup approach. Mar Biol 68:299–319
Strauss SY, Zangerl AR (2002) Plant-insect interactions in terrestrial ecosystems. In: Pellmyr O, HerreraCM (eds) Plant-animal interactions: an evolutionary approach. Blackwell Science Ltd, Oxford,pp 77–106
Strong DR, Lawton JH, Southwood SR (1984) Insects on plants: community patterns and mechanisms.Harvard University Press, Cambridge
Tegner MJ, Dayton PK (1987) El Nino effects on southern California kelp forest communities. Adv EcolRes 17:243–279
Thompson J (1998) Rapid evolution as an ecological process. Trends Ecol Evol 13:329–332Tilmon KJ (2008) Specialization, speciation, and radiation: the evolutionary biology of herbivorous insects.
University of California Press, Berkeley, p 360Trowbridge CD (1991) Diet specialization limits herbivorous sea slug’s capacity to switch among food
species. Ecology 72:1880–1888Trowbridge CD, Todd CD (2001) Host-plant change in marine specialist herbivores: ascoglossan sea slugs
on introduced macroalgae. Ecol Monogr 71:219–243Vesakoski O, Rautanen J, Jormalainen V, Ramsay T (2009) Divergence in host use ability of a marine
herbivore from two habitat types. J Evol Biol 22:1545–1555Wiklund C, Friberg M (2009) The evolutionary ecology of generalization: among-year variation in host
plant use and offspring survival in a butterfly. Ecology 90:3406–3417Wolda H (1988) Insect seasonality: why? Annu Rev Ecol Syst 19:1–18
Evol Ecol (2011) 25:1335–1355 1355
123
Author's personal copy