/Ve i3~l)5;kI

FOOD HABITS OF STONEFLIES (PLECOPTERA) IN THEGUNNISON AND DOLORES RIVERS, COLORADO

THESIS

Presented to the Graduate Council of the

North Texas State University in Partial

Fulfillment of the Requirements

For the Degree of

MASTER OF SCIENCE

BY

Randall L. Fuller, B. S.

Denton, Texas

August, 1976

Fuller, Randall L., Food Habits of Stoneflies

(Plecoptera) in the Gunnison and Dolores Rivers, Colorado.

Master of Science (Biology), August, 1976, 79 pp., 4 tables,

16 illustrations, bibliography, 32 titles.

Gut contents of 2,500 stonefly nymphs, comprising

10 species, from the Gunnison and Dolores Rivers, Colorado

were examined from Dec., 1974-Oct., 1975. Perlidae species

were carnivorous feeding primarily on chironomids, mayflies

and caddisflies. Seasonal patterns of ingestion and pre-

ference varied among species and predator sizes and between

rivers. Early instar polyphagous species utilized detritus

in the fall, eventually shifting to carnivorous habits

as they grew through winter-spring. Pteronarcids fed

predominantly on detritus. Dietary overlap of predators

was greatest in the Gunnison River, with subtle mechanisms

such as prey species and size selectivity, temporal

succession and seasonal shifts to detritus-plant material

in some, providing reduction of competition. A more complete

partitioning of prey resources was evident in the Dolores

River.

TABLE OF CONTENTS

PageLIST OF TABLES.i.v.. ............ lv

LIST OF ILLUSTRATIONS......... . . . . .* .. . v

Chapter

I. INTRODUCTION..............

II. MATERIALS AND METHODS . ...

III. RESULTS AND DISCUSSION.- -

. . 0 0 * .*

Pteronarcellabadia (Hagen).-.-.-.-......Pteronarcys californica (Newport) . ... *Cultus aestivalis (Needham and Claassen) .Isogenoides zionensis (Hanson)....*.....Skwalla parallela (Frison).............Isoperla fulva (Claassen) ...........Claassenia sabulosa (Banks)...........Hesperoperla pacifica (Banks)...0......Chloroperlidae...-.-............... ....Feeding Periodicity..-.-.-.-... . .....Dietary Overlap.........-.-...-........

BIBLIOGRAPHY. ......... ... . . . . .*.0.0. . .

4

8

* . . . 1621253030344051

. . . . 586568

. . . . 7;

iii

. . . .

LIST OF TABLES

Table Page

1. Relative abundance of food complexorganisms (availability) in the DoloresRiver, Colo., May-Oct.,1975....................9

2. Relative abundance of food complexorganisms (availability) in the GunnisonRiver, Colo., May-Oct.,1975...... . . ...... 12

3. Coefficients of Dietary Overlap forcarnivorous stoneflies in the Gunnisonand Dolores Rivers, Colo...................69

4. Chironomidae prey thresholds representedin guts of stonefly predators, Gunnison andDolores Rivers, Colo., Dec.,1974-Oct.,1975. . . . 71

iv

LIST OF ILLUSTRATIONS

Figure Page

1. Seasonal food habits of 145 Pteronarcellabadia nymphs, Dolores River, Colo., Dec.,1974-Oct.,1975.................. . 18

2. Seasonal food habits of 146 Pteronarcellabadia nymphs, Gunnison River, Colo., Dec.,1974-Oct.,1975..........................*. 20

3. Seasonal food habits of 194 Pteronarcyscalifornica nymphs, Dolores River, Colo.,Dec.,1974-Oct.,1975. . . . . . . . . . . . . . . .24

4. Food habits of 124 late instar Cultusaestivalis nymphs, Dolores River, Colo.,May-July,1975..-...-.- . . . . . . . .......... 27

5. Food habits of 15 late instar Cultusaestivalis nymphs, Gunnison River, Colo.,June,1975...-................. . 29

6. Seasonal food habits of 87 Skwalla parallelanymphs, Gunnison River, Colo., May-Oct.,1975. . . 33

7. Seasonal food habits of 134 Isoperla fulvanymphs, Gunnison River, Colo., Dec.,1974-Oct.,1975............ . . . . . . . . . . . 36

8. Food habits of 71 late instar Isoperla fulvanymphs, Dolores River, Colo., May-July,1975. . . 39

9. Seasonal food habits of 173 Claasseniasabulosa nymphs,

LIST OF ILLUSTRATIONS--Continued

Figure Page

10. Seasonal food habits of 120 Claasseniasabulosa nymphs, 2.5mm, Dolores ,River,Colo., Dec.,1974-Oct.,1975. ................ 47

12. Seasonal food habits of 362 Claasseniasabulosa nymphs, >2.5mm, Gunnison River,Colo., Dec.,1974-Oct.,1975.. . ... ..... 49

13. Seasonal food habits of 118 Hesperoperlapacifica nymphs, 2.5mm, Gunnison River,Colo., May-Oct.,1975............ ...... 56

15. Food habits of 56 late instar Chloroperlidae(Suwallia sp., Sweltsa sp., Triznaka sp.)nymphs, Dolores River, Colo., May-June,1975. . . 60

16. Seasonal food habits of 238 Chloroperlidae(Suwallia sp., Sweltsa sp., Triznaka sp.)nymphs, Gunnison River, Colo., May-Oct.,1975. . .62

vi

CHAPTER I

INTRODUCTION

Traditional riverine ecological studies have emphasized

taxonomic inventories and community structure of the impor-

tant aquatic insects. There is a current surge of interest

in gathering knowledge concerning functional roles of

organisms and process-oriented mechanisms within lotic

insect communities (Cummins 1974). A significant, and often

the major energy input in streams is allochthanous, con-

sisting primarily of leaf fall and dissolved organic matter

from the adjacent watershed (Chapman and Demory 1963;

Egglishaw 1964; Chapman 1966; Minshall 1967; Tilly 1968;

Vannote 1969; Cummins 1971, 1974; Fisher and Likens 1972,

1973; Hall 1972).

Many lotic aquatic insects appear to have evolved

toward facultative food habits, enabling utilization of

'both autochthanous and allochthanous energy inputs. This

facultative nature may be manifested through diel, seasonal,

or size adjustments in feeding, in order to obtain maximum

efficiency, avoidance of predation and decreased competi-

tion. Major studies that have elucidated such variations

1

2

in stoneflies are few (Sheldon 1969, 1972: Tarter and

Krumholz 1971; Vaught and Stewart 1974; Cather and Gaufin

1975).

This facultative expression makes it difficult to

even define some aquatic insect species as a detritivore,

herbivore, omnivore, or carnivore (Coffman et al. 1971),

and certainly precludes the stereotyping of genera or

families as to their food habits and feeding behavior.

Notwithstanding, such generalizations are common in the

literature.

From a functional viewpoint, the lack of knowledge

of the facultative food habits of important species in

communities has led to the introduction of a "paraspecies"

concept (Boling et al. 1974) enabling modeling of stream

detritus utilization (Cummins 1971, 1974) and energy flow.

Paraspecies is a grouping of immatures of various insect

species that at least seem to have common feeding habits.

It is obvious that gross error in grouping can result

unless detailed study of feeding in grouped species has

been done.

Stonefly food habits have been studied by Brinck (1949),

Hynes (1941), Jones (1950), Mackereth (1957), Muttkowski and

Smith (1929), Richardson and Gaufin (1971) and Sheldon (1969,

3

1972) yet, little is known of prey selection relating to

possible diel, seasonal, and size class variations which

are of obvious importance when assessing community energetics.

Life history studies have contributed much information on

seasonal shifts (Cather and Gaufin 1975; Vaught and Stewart

1974), and Sheldon (1972) described interactions between

four closely related stoneflies in the same stream. I have

attempted in this study to identify the trophic interactions

of an entire stonefly community, concurrently in two major

rivers, and what effect the seasonal availability of food

organisms and/or organic detritus has on their food habit.

CHAPTER II

MATERIALS AND METHODS

The Gunnison River originates in Almont, Colorado,

at the junction of the East and Taylor Rivers. It is a

large, fast flowing river with peak discharge occurring from

the latter part of May to mid-July. The substrate consists

primarily of large boulders and rock-rubble with a few

sandy pools. The sample site was located on private property

of the Lost Canyon Resort, ca. 2.0 miles south of Almont.

The Dolores River has its headwaters on the south

slope of the San Miguel Mountains and flows southwest to

the city of Dolores. There it changes directions flowing

north and west until it enters the Colorado River System.

The upper Dolores is of moderate size with swift currents

and large riffles, and the substrata is generally similar

to that of the Gunnison River. During the study period,

peak discharge occurred from early May to late June, being

more spontaneous and slightly preceeding that on the Gunnison.

Collections were made near the city limits of Dolores.

Field sampling began in early Dec., 1974, when afternoon

collective samples were made on both rivers. To assess any

4

5

possible diel, as well as seasonal periodicity, monthly

samples were taken from May-Oct., 1975, at mid-afternoon

and about midway between sunset and midnight. Sampling

on both rivers was always accomplished within a two-day

period of each month. Kick nets (1,000u mesh opening),

seines (471u mesh opening), and drift nets (271u mesh net,

271u mesh plankton bucket) were utilized for collecting

stoneflies for gut analyses. An attempt was made to collect

at least 25 individuals per species for both the afternoon

and evening samples/sampling date. Specimens were preserved

in 60-70% isopropanol.

A combination of sampling techniques was used on each

sampling date to determine availability of potential food

items in habitats occupied by stoneflies: (1) random rock

scrapings were taken adjacent to where stoneflies had been

collected and frozen for microscopic examination in the

laboratory. Numbers of algae and diatoms were estimated

in a Palmer counting cell from counts of five random Whipple

fields obtained from each of five aliquots. (2) Macro-

invertebrate availability was determined using a two-stage

kick net of my own design, having 1,000u and 125u mesh

openings in the first and second stages, respectively.

Quantitative samples from random 50 X 50cm bottom areas were

6

taken with a 27Tu mesh opening seine. The two-stage kick

net samples were collectively taken from representative types

of bottom substrate, in at least three areas adjacent to

stonefly habitat.

For each species, where possible, the stomach contents

of at least 25 individuals/afternoon and evening samples

for each sampling date were dissected by the procedure

outlined by Hynes (1941) and then examined microscopically

for relative fullness and composition. Head capsule width

was measured for all stoneflies and only contents in the

foregut were examined. In animals where the foregut was

empty, any presence of material in the rest of the gut was

noted to supplement assessment of diel feeding periodicity.

Reference slides were made of all representative,

distinctive sclerotized animal parts for later identification.

Head capsules of prey organisms were measured and in the

case of chironomids, complete head capsules were saved

for generic identification.

Gut contents of nymphs containing plant material and/or

detritus were squeezed into a Palmer counting cell, and five

randomly selected Whipple fields were counted. For early

instars, especially with members in the family Chloroperlidae,

the gut contents from more than one individual were dispersed

7

in the Palmer counting cell for enumeration to obtain a

X/individual.

Ivlev's Electivity Index (Ivlev, 1961) was utilized

to identify any selection of prey organisms; intra- and

inter-specific competition was compared using Horn's (1966)

Coefficient of Dietary Overlap. Although such measurements

originally applied to fishes, they have since been utilized

in comparing food habits of stream insects (Stewart et al.

1973; Vaught and Stewart 1974; Rhame and Stewart 1976).

CHAPTER III

RESULTS AND DISCUSSION

The rivers studied had similar benthos species

composition, yet differed in the particular taxa that were

dominant. The Dolores River was generally dominated by

mayflies (22-56%), and the Gunnison River by chironomid

larvae (47-68%) (Tables 1, 2). Other taxa exhibited moderate

to small populations that vary seasonally according to their

respective life cycles. The three dominant prey groups,

mayflies, caddisflies and chironomid larvae exhibited peak

populations in late summer and early fall as recruitment

appeared (Table 1, 2). Large numbers of oligochactes occurred

in the Gunnison River in Aug. Progressive amounts of

filamentous algae became available beginning in June, and

extending into late fall. Particulate organic matter input

from riparian deciduous vegetation (Salix sp., Populus sp)

occurred in the fall, and on the Dolores River an interesting

"flocculent detrital mat", containing large amounts of the

diatoms Cymbella sp., Fracilaria sp., Gomphonema sp.,

Navicula sp., and Synedra sp. formed over the bottom in Sept.

Large numbers of hydropsychid caddisflies and mayflies were

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observed feeding in this detrital mat. These rivers,, there-

fore, generally fit the description by Cummins (1974) in

that they are geared to major input of energy and nutrients

in the fall and are temperature compensated.

Following is a brief explanation of the graphic data

(Figs. 1-16), referred to in the discussion of food habits

and preference of each stonefly species studied. Bars for

each month express the percent by volume of each indicated

dietary component. Calculations were based only on stomachs

that contained food (indicated as the proportion of total

stomachs examined, at top of bar for each month)', Numbers

within the bars indicate actual total number of prey taken

by the total number of predators examined and therefore,

do not necessarily correlate with the height of the bar.

Data are expressed in this way to facilitate comparisons

with studies using only numbers or volumes: Ivlev's Index

(Ivlev, 1961) appearing to the right of each bar component

representing indentifiable prey, was calculated as follows.-

E = (ri - pi) / (ri + pi)

where:

ri= relative content of any ingredient inthe ration (as a percentage of the wholeration).

pi= relative value of the same ingredient inthe food complex of the environment

16

Negative electivity or avoidance is indicated by -l to 0,

random feeding by values of 0, and preference or selection

by 0 to +1. In the absence of detailed knowledge of feeding

behavior, microhabitats and diel movements by predators and

prey, avoidance and selection might also be interpreted as

not available or more available for feeding, respectively.

Pteronarcella badia (Hagen)

P. badia inhabits primarily debris in slower areas

of the Dolores and Gunnison Rivers. Its life cycle is

univoltine and emergence peaks occur in mid-July and late

June, respectively, on these two streams. Richardson and

Gaufin (1971) characterize the species as polyphagous,

feeding predominantly on plant matter and detritus (78.5%).

They found some animal matter, primarily mayfly nymphs

and chironomid larvae in 40% of the animals opened.

My study revealed a diet high in detritus throughout

the year, with significant quantities of moss tissue on both

rivers in winter and spring months, except in Dec. on the

Dolores (Figs. 1, 2). Moss tissue made up 57.5% and 75% of

the diets of mature nymphs in June in the Dolores and

Gunnison Rivers, respectively. Early instars first

appeared in Sept. on both rivers, at which time guts contained

over 75% detritus. The desmid, Cosmarium sp. was abundant

17

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in both rivers and in the gut contents in Sept. (Gunnison)

and Oct. (both rivers) (Figs. 1, 2). The diatoms

Cymbella sp., Synedra sp., Fragilaria sp., Gomphonema sp.,

Navicula sp., and Diatoma sp. composed 7.5-15% of these

small nymphs diet. By Oct., the flocculent detrital mat

on the Dolores had broken down and surfaces of rocks

supported luxurient growths of Cladophora sp., although

growing P. badia nymphs continued to feed predominantly

on detritus. On the Gunnison River in Oct. the nymphs

continued to ingest detritus.

The pattern of this species, in these rivers seems

geared to processing and utilization of fall leaf debris

input for growth, and utilization of increasing amounts of

moss tissue prior to emergence.

Pteronarcys californica (Newport)

This species inhabits the swiftest areas of the stream

(Richardson and Gaufin, 1971) and exhibits a semivoltine

life cycle requiring 3 to 4 years for completion (Hynes,

1970). They were only present on the Dolores River and

emerged in late May and June. P. californica has been

reported to be primarily herbivorous by Muttkowski and

Smith (1929) and Chapman and Demory (1963). Richardson

and Gaufin's (1971) study indicated this species to feed

22

predominantly on detritus (79.7%) which corresponds with

results from this study (Fig. 3).

Since no marked differences between larger and

smaller instars were noted, the results were combined. The

algae availability samples assisted in the assessment of

selection for particular algal or diatom species, yet did

not enable quantification of the detrital component or

amounts of moss tissue. This made it difficult to determine

selection or possible shifts. Diatoms that occurred in

guts in Sept. were representative of the detrital mat that

covered the entire river bottom. In Oct. the largest

percentages of the diet consisted of detritus and the

filamentous algae, Cladophora sp., (Fig. 3). This branched

filamentous form was dominant in the river and could be

found on most rocks along the stream. In Dec. and May

animal material consisting chiefly of Simuliidae larvae

was enountered. The slow, non-aggressive, and almost

cumbersome movement and behavior of P. californica probably

precludes it from ever becoming an efficient predator,

although this species did occasionally ingest animals such

as simuliids. Moss tissue was an important component of

the diet, except in the fall where there was a shift to

diatoms and algae associated with the flocculent mat.

23

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25

Whether this species selected the moss tissue or fed

randomly on the plant material that was abundant was not

determined. Since the peak discharge during May and June

caused a scouring of the substrate, periphytic vegetative

growth was almost non-existent at that time. Rock

scrapings from July and Aug. contained little filamentous

algae, diatoms, or moss tissue.

This stonefly, therefore, appears to be a generalist,

feeding on detritus or living plant forms as they are

encountered.

Cultus aestivalis (Needham and Claassen)

This univoltine species emerges by late June to early

July on the Gunnison River, and mid-July on the Dolores.

No detailed food studies have been made of this species.

Only later instar nymphs were successfully collected prior

to emergence on both rivers (Figs. 4, 5). Large numbers

of the stoneflies collected in Aug.-Oct. in the fine mesh

availability net (Table 1) were probably of this species,

but the present state of taxomic knowledge did not allow

identification.

In both rivers, nymphs fed exclusively on chironomid

and other dipteran larvae (primarily simuliids) (Figs. 4, 5).

The presence of chironomid larvae of varied sizes,

26

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30

.05-.35mm h.c.w., suggests there is no prey size (threshold)

preference, and that C. aestivalis utilizes those smaller

larvae that perlids such as C. sabulosa would leave.

Groups such as mayflies, stoneflies, and caddisflies,

important prey for other carnivores and polyphagous

species, were totally avoided.

Isogenoides zionenises (Hanson)

This species is univoltine, emerging in late May on

the Gunnison River and early to mid-June on the Dolores

River. Populations were not large enough to adequately

ascertain monthly variations. The gut contents of 30

individuals from both rivers indicates it to be primarily

carnivorous feeding predominantly on Ephemeroptera nymphs

(26%), Chironomidae (32%), Simuliidae (20%), and Trichoptera

larvae (22%). Of the 16 nymphs examined in Oct., no shift

occurred to detritus or plant material as seen in other

perlodid species.

Skwalla parallela (Frison)

Numbers of fed S. parallela nymphs, sufficient to

gain some insight into its food habits were obtained only

from the Gunnison River (4, 36 and 47 in May, Sept. and

Oct., 1975, respectively), although the species did occur

on the Dolores. This species has a univoltine cycle with

31

emergence occurring in late April and early May.

Food habits of large nymphs collected in Mar.-Apr.

in Colorado and Utah were characterized as polyphagous by

Richardson and Gaufin (1971). Overall, they found that

guts contained 58.7% animal matter, primarily mayfly

nymphs (25.3%) and chironomid larvae (21.3%). Plant material

composed 13-68% of the diet. Larger percentages of filamentous

algae were found in the 41 fed guts from Utah (13%) than

from the 21 fed guts from Colorado (3%). Diatoms made up

only 1.5% of the diet.

Animal matter comprised over 75% of the diets of four

mature nymphs collected from the Gunnison River in May, 1975

(Fig. 6). Approximately equal volumes of mayflies and

stoneflies (34% and 32%, respectively) predominated as prey;

detritus made up 24%, while chironomids and simuliids were

relatively insignificant in the diet (Fig. 6). Small nymphs

appeared in samples again in Sept., when they were highly

carnivorous, feeding predominantly on and showing positive

electivity for chironomid larvae, which made up 65% of the

diet (Fig. 6). Interestingly, as in the perlids C. sabulosa

and H. pacifica, a shift in the diet and preference occurred

between Sept. and Oct. samples; in S. parallel the shift

represented a change from a carnivorous to a polyphagous food

32

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34

habit (Fig. 6). Filamentous algae and diatoms made up

30% and 18%, respectively, of the diets of 47 fed nymphs

in Oct. Filamentous algae consisted almost entirely of

Ulothrix and dominant diatoms were Synedra sp., Fragilaria

sp., and Cymbella sp. These were also the most abundant

in the environment.

Isoperla fulva (Claassen)

Sufficient numbers of I. fulva for analysis were

collected in Dec., 1974 and Oct., 1975 on the Gunnison,

and during spring months just prior to emergence on both

rivers (Figs. 7, 8). The species exhibits a univoltive

cycle, emerging from mid-June to early July on the Gunnison,

and early to mid-July on the Dolores.

Richardson and Gaufin (1971) examined 202 fed guts

from eight streams in Colorado and Utah, collected in the

months Mar.-June. Diets were composed of ca. equal

proportions of chironomid larvae (27.7%) and unidentified

animal matter (18.3%). They characterized the species as

primarily carnivorous, with animals making up 70.4% of the

diet. They noted that specimens from the Taylor River

and West Elk Creek fed very heavily on Simuliidae larvae,

and to a lesser degree on Trichoptera larvae.

35

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37

Small nymphs appearing in Oct. on the Gunnison were

phytophagous, feeding primarily on filamentous algae (47.5%),

diatoms (15.5%) and detritus (28%) (Fig. 7). The major

algae fed upon was Ulothrix sp., and most common diatoms

were Fragilaria sp., Synedra sp., and Cymbella sp. Only

9% of the diets were composed of animal matter. A shift

was evident in Dec., when animal matter comprised 72.5%

of the diet, primarily chironomid larvae and mayflies;

filamentous algae disappeared from the guts (Fig. 7).

Late instar nymphs in May fed primarily on chironomid larvae,

and randomly ingested caddisfly larvae. In June, the animal

material was almost exclusively chironomids, with a positive

electivity of .23. As in C. aestivalis, there was no indi-

cation of selection for larger instar chironomids. Therefore,

the diet of I. fulva on the Gunnison River appeared to change

as nymphal development proceeded from phytophagous (fall)

to polyphagous (winter) to carnivorous (spring) (Fig. 7).

Large nymphs in the Dolores River in the spring also

showed a highly carnivorous diet (Fig. 8). They fed

primarily on chironomid and simuliid larvae, displaying

consistent positive electivity for them. Negative electivity

was consistently shown for mayflies, which were ingested in

small numbers and volumes, except in June and they made up

38

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42.5% by volume of the diet, at a time when chironomids

were very low in abundance.

Claassenia sabulosa (Banks)

This species has a semivoltine life cycle, and

emergence occurred from early Aug. to Sept. Nymphs

inhabited large rock areas of riffles (Richardson and Gaufin

1971) in both the Dolores and Gunnison Rivers.

Small nymphs,

41

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period July-Sept., suggests that the small C. sabulosa

nymphs in the Dolores River are opportunistic. However,

in the Gunnison River, nymphs of the same size display

a definite pattern of ingestion and preference for chironomid

larvae during the summer period, shifting to an

opportunistic pattern in Oct. (Fig. 10). May, 1975 data

suggests that this fall shift might carry through the winter

into the following spring.

Larger nymphs ( >2.5mm) in the Dolores River, fed

primarily on mayflies and caddisflies throughout the year

(Fig. 11), although ca. 27% of guts contained chironomid

larvae during Dec. Random feeding or positive electivity

was shown for these prey in most months. A different pattern,

however, was displayed by later instar C. sabulosa nymphs in

the Gunnison River. They fed primarily on chironomid larvae

during the summer, June-Sept., and shifted to a diet of

mayflies, stoneflies, and caddisflies in Oct. (Fig. 12).

Chironomids, although still abundant, were completely avoided.

Detritus was present in stomachs from larger Gunnison River

nymphs throughout the study (Fig. 12); this material was

probably ingested accidentally or was incorporated in prey

guts at time of capture.

44

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These data and this species represent a good

example of why stereotyping food habits on trophic position

(niche) of insect carnivores, based on casual sampling of

limited season, river system or size class, may be mis-

leading. In this species food habits of small and large

nymphs differ, and differences in seasonal patterns and

between river systems within a size are manifested. It is

interesting that the feeding patterns basically follow

a seasonal cycle e.g., the June-Sept. "summer" season and

Oct., Dec., May "winter" season. Of course, winter sampling

was inadequate during this study to form a basis for serious

conclusions as to trophic dynamics during that period.

However, drift sampling (unpublished data) on the Dolores

River in Dec., 1974 showed high rates of exuvial drift

of insects suggesting a high rate of growth of some taxa.

Energy for such growth could come directly from active

feeding, or from fat reserves stored for overwintering, to

be discussed later.

There is some indication that prey size thresholds

may be an important releaser, in addition to specificity, at

least during seasons when opportunism is displayed, in

C. sabulosa.

51

Richardson and Gaufin (1971) found that 67 large

C. sabulosa nymphs, X lengths 16 and 19mm, from Lake Fork

of the Gunnison River and Soap Creek, collected in Aug.,

1963, fed primarily on Ephemeroptera nymphs and

"unidentified animal matter". Few chironomids, caddisflies,

and no stoneflies were found in stomachs. These data do

not correspond with percentages I found in larger nymphs

from either the Gunnison or Dolores Rivers in Aug. 1975.

Large percentages and numbers of chironomids were ingested

on the Gunnison, and caddisflies on the Dolores in Aug., in

addition to mayflies. It can only be assumed that differences

relate to variations between river locations and/or food

complex variations between these study periods of 1963 and

1975. Richardson and Gaufin (1971) also reported that four

large nymphs collected in Mar., 1963, on the upper Provo

River in Utah, contained 95% detritus and undetermined

matter, and that plant material was the dominant food. They

characterized the species as carnivorous, but did not deal

with preference since food availability was not quantified.

Hesperoperla pacifica (Banks)

This species usually inhabits the swiftest areas of

rivers (Richardson and Gaufin 1971) and occurred only on the

Gunnison River site during this study. It is semivoltine,

52

with emergence occurring from mid-June to early July.

Its food habits have been characterized as primarily

carnivorous by Muttkowski and Smith (1929) and Richardson

and Gaufin (1971).

Diets of nymphs less than 2.5mm h.c.w. consisted of

over 85% by vol. animal matter in all months (Fig. 13).

Chironomidae larvae represented over 60% in all months

except May, 1975, and electivity for them was consistently

positive. In May, 1975, there was a partial shift to

mayflies, with positive electivity. Relatively minor

numbers and percentages of mayflies (except May), stonefly

nymphs, caddisfly larvae, other diptera larvae and

miscellaneous items occurred in the guts, and electivity

for them was usually negative (Fig. 13).

Diets of nymphs greater than 2.5mm also consisted of

over 85% animal matter in most months (Fig. 14). More

opportunism was exhibited in these larger nymphs, with

no one prey group usually predominating. Chironomids

comprised over 50% of the diet only in June and July when

they were in later instars and were more abundant than

mayflies and caddisflies. Electivity for chironomids was

positive throughout the summer months, June-Sept. In May

and Oct., caddisfly larvae made up the largest dietary

53

LA

C4

v

ra

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L

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57

component, with positive electivity, representing a

shift from chironomids, which exhibited negative preference

(Fig. 14). In both months, the shift to caddisflies occurred

when chironomid larvae were the most abundant food item,

yet were in very early instars. This may indicate a

preference for only larger chironomid larvae (or caddisflies)

which Sheldon (1969) also observed. Relatively small

percentages of mayflies were ingested, and electivity

was negative for this category in all months. Small

percentages of other dietary components, including stonefly

nymphs were ingested throughout the study. Random feeding

on stonefly nymphs was indicated in May (+.07), positive

selection in Oct. (+.5) and negative electivity in other

months.

Richardson and Gaufin (1971) reported that 69, 20-22mm

long specimens, pooled from three locations in Colo. and

Utah, ate primarily Ephemeroptera nymphs, Chironomidae

larvae, unidentifiable animal matter and Trichopera larvae

in that order, beginning with greatest percent ingested.

Their data are only generally comparable with my May samples

of large nymphs ( >2.5mm) since all their specimens for gut

analysis were collected from Apr. (Utah) and May (Colo.),

and consisted entirely of 20-22mm long individuals. Such

58

pooling of localities, examining only one size class and

looking at the food habits of only one seasonal time period

(month), would tend to obscure the periodicity and

variations in food habits of a species. Also, Richardson

and Gaufin (1971) did not quantify availability, in order

to obtain information on preference, and help explain reasons

for variations and seasonal shifts in diets. They indicated

that "there is little selection of animal food; this species

feeds voraciously upon any animal it finds". My data

indicated that smaller nymphs are selective for chironomids

and mayflies, and that larger ones selectively feed on

chironomids and Trichoptera larvae on a seasonal basis

(Fig. 14). This species undoubtedly either avoids feeding

on oligochaetes that were abundant on the Gunnison River

from July-Sept. (Table 2) or they were not as available for

feeding due to a behavioral peculiarity of the predator

or the prey. Small amounts of detritus encountered in guts

could have been present in the guts of ingested chironomid,

mayfly, caddisfly, and other prey.

Chloroperlidae

It was necessary to pool the analysis of the food

habits of the three genera, Sweltsa sp., Suwallia sp. and

Triznaka sp., since early nymphs of all three, and even

59

later instar nymphs of the latter two genera cannot

presently be distinguished (R.W. Baumann, personal

correspondence). Of course, such pooling of taxa

obscures species-specific relationships. They all appear

univoltine in the study rivers, with Sweltsa sp. and Triznaka

sp. emerging in mid-to late June and Suwallia sp. in late

July to early Aug. Stanford (1975) demonstrated that

Sweltsa sp. and Suwallia sp. were univoltine in the Flathead

River, Montana. Harper (1973) has demonstrated that two

years are required for the life cycle of Alloperla (Sweltsa)

onkos in southern Ontario.

A polyphagous diet was indicated by Stanford (1975)

for these genera. My data substantiate this; however,

considerable shifting of ingestion and dietary preference

of these pooled species was displayed (Figs. 15, 16).

In May, on the Dolores River, chloroperlid nymphs

fed exclusively on animal matter, primarily composed of

chironomid larvae, with a +.57 electivity (Fig. 15).

Relatively large volumes of simuliids were also ingested

but because of high availability, an electivity of -.32 was

shown. Only 4 of 26 pre-emergent nymphs examined in June had

fed, and they indicated a shift in diet to polyphagy with

over 50% composed of detritus (Fig. 15). Though a few

60

U)

4 .

ro

(0

(0 1

0 0

O S

0 I

(0 .

o) U

LH a0 4

0

cflH

4-J 0Sfl4

ro 0-(0

.r4

14 ,

..C 0

ON-H

LH

mi

62

Suwallia sp. adults were found in Aug., no nymphs were

collected.

Chloroperlid nymphs on the Gunnison, unlike those

on the Dolores, were polyphagous in May. Chironomid larvae

were preferred, with smaller amounts of mayflies, algae,

diatoms, and miscellaneous organisms (ostracods) also

ingested (Fig. 16). Detritus made up 34% of the May diet.

As on the Dolores River, these nymphs ceased feeding just

prior to emergence in June. The diet of only three individuals

that had fed was composed mostly of chironomids (Fig. 16).

Lateinstar Suwallia sp. nymphs examined in July were

primarily phytophagous, with Ulothrix sp. making up over 50%

of the diet, and the diatoms Fragilaria sp., Synedra sp.,

and Cymbella sp. composing 12.5%. Algae availability samples

revealed that Stigeoclonium sp. made up 75% of available

filamentous algae. This indicates that nymphs were selecting

Ulothrix sp. Stanford (1975) reported similar results in the

species Skwalla parallela in Montana. Larger pre-emergent

Suwallia sp. nymphs in Aug. ingested primarily chironomids,

and showed preference for the smaller size classes. Very

small nymphs in Oct., recruited from the summer emergence,

displayed a completely different pattern; ingesting detritus

(48%), diatoms (35.7%), filamentous algae (4.5%) and the

63

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65

remainder of small quantitites of animals. When we are able

to discern species or even genera of these chloroperlids,

we may find that they have partitioned the food pool to

avoid competition.

Feeding Periodicity

Types and volumes of organisms or detritus ingested

did not differ markedly between mid-afternoon and post-

sunset for most species in both rivers. Large C. sabulosa

nymphs in Aug. on the Gunnison selected more mayflies after

dark than in the afternoon (32 and 8, respectively). I

believe that diel feeding periodicity is probably manifested

in many stonefly species, but more samples at frequent day

intervals, within seasonal time frames would be necessary

for elucidation.

Large instar C. sabulosa and H. pacifica nymphs, in the

Gunnison River, followed similar patterns of ingestion

(Figs. 12, 14). They fed predominantly on chironomid larvae

in summer and early fall, and ingested caddisflies, mayflies

and stoneflies in late fall and spring, when most chironomids

were in early instars. This seasonal periodicity probably

reflected selection of preferred prey organisms within

threshold sizes. Smaller C. sabulosa and H. pacifica nymphs,

in the Gunnison River, fed primarily on chironomid larvae

66

throughout the study, with C. sabulosa ingesting more

mayflies and caddisflies in Oct. (Figs. 10, 13). Dolores

River C. sabulosa nymphs preferred mayflies and caddisflies

throughout the study (Figs. 9, 11). Chironomid larvae

were of moderate abundance in the river (Table 1), and

were ingested at higher rates by smaller instars (Fig. 9).

Only in Dec. did chironomids contribute a major portion to

the larger nymphs diet.

These perlid stoneflies either decreased their rate

of ingestion or spread it out over a longer day cycle in

fall-winter. I very seldom encountered a full-fed nymph

in either river during these periods, and during peak

discharge in June there was a marked increase in occurrence

of empty stomachs. Especially large quantities of fat

reserves were observed in the fall and after active spring

feeding in May. There is probably lowered food availability

for ingestion in June and ingestion rates and fat storage

capability are probably adjusted to get nymphs through such

stress periods. Similar adjustments in food intake have been

reported by Sheldon (1969) and Stanford (1975).

Mature C. aestivalis nymphs, examined from both rivers

were strictly carnivorous, utilizing primarily chironomid

larvae (Figs. 4, 5).

67

All polyphagous species, S. parallel, I. fulva and

the chloroperlids, exhibited similar seasonal ingestion

patterns in the Gunnison River. During the recruitment

period in the fall, small nymphs utilized detrital components

from allochthanous leaf-fall input (Figs. 6, 8, 16). They

eventually shifted to a more carnivorous diet as they grew.

As in the perlids, these species decreased their ingestion

rate just prior to emergence in June, but not in fall-winter.

They perhaps had stored sufficient calories for emergence

well ahead of its timing, and were more temperature

compensated for winter feeding than perlids, i.e. perlids

stored fat reserves with lowered winter feeding, resumed

by May, whereas perlodids fed continuously during colder

periods.

P. badia and P. californica fed predominantly on

detritus throughout their development in the Dolores River

(Figs. 1, 3). Both fed on diatoms in fall-winter; P.

californica ingested more algae and small numbers of diptera

larvae in Oct. and Dec., respectively, and P. badia ingested

animal matter (3 Heptageniidae nymphs) only in May

(Figs. 1, 3). Moss tissue made up a relatively large

dietary component in both rivers for P. badia especially

in June (Figs. 1, 2) and for P. californica in the Dolores

68

River in all months except Sept. (Fig. 3).

Dietary Overlap

To get some idea of potential competition between

and among species, the Coefficient of Dietary Overlap

as described by Horn (1966) was utilized:

SC = 2Y XXi Yi

i=ls 2 s 2SXi+i=l

where:

C = overlap coefficients = food categoriesXi= that proportion of the total diet of

species X from a given category of food i.

Yi= that proportion of the total diet of speciesY from a given category of food i.

C varies from 0, when samples are completely distinct

(no food categories in common), to 1 when the samples are

identical with respect to proportional food category compo-

sition. Values greater than 0.60 indicate significant over-

lap.

Comparison of food categories (taxa) ingested by

mature and early instar C. sabulosa and H. pacifica nymphs

in the Gunnison River showed significant overlap with all

other carnivorous and polyphagous species (Table 3). In

all cases, comparisons in Table 3 were made only for major

69

TABLE 3. Coefficients of Dietary Overlap for carnivorousstoneflies in the Gunnison and Dolores

Rivers, Colo.

Lh Ln N N* *c4 V A

V A ai ILH H

U) a) 4- ) 4

0 A::5 (o a -0

M fa o o o a.H a A En U) (0 ro

4-) -U) (TJ 44 M Ha) 04 tq rq c

Cults aestivalis .99 .96 .90 .98 .95 .90

Skwalla parallel .73 .82 .79 .77 .69 .79

Isoperla fulva .92 .77 .62 .70 .68 .93Wr

H

Claassenia sabulosa 2.5mm .19 .31 .81 .85 .89 .74 Hzz

Hesperoperla pacifica 2.5mm .63

Chloroperlidae .75 .77 .38 .30DOLORES RIVER

70

food categories between nymphs of different species

(and/or sizes indicated) that coexisted during the same

time frame. Such a comparison obscures differences that

reduce sompetition due to preference for specific taxa

within a food group, i.e. significant overlap might be

indicated when comparing two predators feeding on the

general category "mayflies", but subtle preferences for

particular genera or species would actually reduce overlap

to non-significance. Problems of identification of

especially early instar prey organisms make valid comparisons

very difficult. However, both differences in timing of

ingestion and prey sizes ingested probably provide sufficient

separation for reduction of competition (overlap) in these

predators. Sheldon (1969) noted that semivoltine species

(such as C. sabulosa) are able to separate their sizes and

therefore, take advantage of a wider range .of prey sizes,

providing less competition. In this study, early instar

C. sabulosa and H. pacifica nymphs, as well as perlodids and

chloroperlids, fed on smaller instar chironomid larvae

(Table 4). Seasonal shifts, such as the fall shift of

smaller perlodids (potentially competitors of smaller

perlids here) to detritus and plant matter would also reduce

total competition between species on a yearly basis.

00

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72

Larger C. asabulosa and H. pacifica nymphs fed on larger

chironomid larvae (Table 4), and differed in families of

mayflies ingested. C. sabulosa utilized large numbers of

ephemerellids (53%), with baetids (25%) and heptageniids (22%)

of secondary importance. H. pacifica ingested more baetids

(57%), and fewer ephemerellids (35%) and heptageniids (8%).

This provides a good indication of why I feel that valid

analysis of competition among benthic predators, in the

future, will require detailed studies with prey identified

to species. Richardson and Gaufin (1971) suggest that

differences in microhabitats exist between these species,

with H. pacifica being more common on algal mats where

chironomids are more abundant and C. sabulosa occurring

in swifter, more unstable areas where little algal growth

is present. This would decrease the availability of

chironomids and force C. sabulosa to rely on mayflies.

My results seem to support this contention. Some temporal

separation of these species may also be accountable to the

June and Aug. emergence, respectively, of H. pacifica and

C. sabulosa.

Small perlodid and chloroperlid species, and smaller

instars of larger perlodid species, exhibited similar feeding

periodicity patterns and showed significant overlap with

73

other carnivorous and polyphagous species in the Gunnison

River (Table 3). However, temporal separation, fall dietary

shifts previously mentioned, more subtle month-to-month

shifts and prey threshold differences reduced competition.

S. parallel emerges in May, and its nymphs are first to

appear in large numbers in Sept. I. fulva emerges in June;

nymphal reproduction (recruitment) first appeared in Oct.

C. aestivalis emerged in July and its nymphs were first

collected in May. The chloroperlids Sweltsa sp. and

Triznaka sp. emerged in June and Suwallia sp. in Aug.

Chironomid head capsule sizes in guts suggested that

S. paralela reached a higher minimum prey threshold in

Oct., before appearance by I. fulva nymphs (Table 4).

Comparison of mean prey thresholds in Sept. (or other months)

(Table 4) showed differences that might have reduced

competition for at least that major dietary component among

perlid and perlodid species. The chloroperlids fed on smaller

chironomids than I. fulva in Oct. (Table 4). C. aestivalis

is less abundant than I. fulva on the Gunnison River,

perhaps due to the higher degree of dietary overlap.

Dolores River carnivores and omnivores exhibited less

overlap than on the Gunnison River. Large C. sabulosa nymphs

did not significantly overlap any other species; exploitation

74

of their prey pool is perhaps responsible at least in part,

for its very abundant populations on the Dolores, and

relatively low populations of I. zionesis and S. parallel.

Another possible explanation for differences in overlap

between the Dolores and Gunnison Rivers might be the high

competition with, and even predation on carnivorous insects

in the Dolores by abundant numbers of sculpins (Cottus sp.).

Except in Dec., larger C. sabulosa, when feeding on

chironomids, usually selected larger instar larvae (Table 4).

Smaller C. sabulosa nymphs in the Dolores River showed signi-

ficant overlap only with later instar I. fulva nymphs.

Comparison of chironomid head capsule sizes did not indicate

any prey size separation, of course, only June, July results

were compared (time when these sizes of the two species

coexisted) and during other months C. sabulosa avoided

overlap (or dominated because of) by selecting mayflies. I

believe that taking larger sample sizes and concentrating

more on winter-spring sampling would show that these species

do not overlap to the extent suggested by an analysis such

as reported in Table 3.

Perlodids and chloroperlids in the Dolores exhibited

significant overlap among the species with I. fulva and

C. aestivalis indicating the greatest competition (Table 3).

75

All fed on similar sized chironomids (Table 4) and did not

show any major temporal separation. Differential growth

rates possibly separate them temporally, but it would be

difficult to identify with only late instar nymphs to compare,

and until life cycles are elucidated. Chloroperlids are

separated in time from C. aestivalis and I. fulva and all

appear to feed only on very small chironomids (Table 4).

Suwallia sp. populations were low in the Dolores, possibly

due to competition with I. fulva and C. aestivalis during

June-July, and unstudied peculiarities of insect feeding

Cottus sp. populations.

These results suggest a relatively high degree of

overlap in diets of dominant Gunnison River stonefly

predators, with subtle mechanisms such as specific prey

selectivity, temporal succession and prey threshold dif-

ferences separating them. High species diversity and large

populations of prey also would tend to obscure expected

exclusion or adjustments in abundance due to competition

for food. A more complete partitioning of prey resources

is evident in the Dolores River, possibly due to greater

physical-chemical perturbation and large Cottus sp. pop-

ulations leading to smaller prey populations. Without

such divisions or partitioning, species would compete for

76

similar resources to the exclusion of the less efficient

predators (Zaret and Rand 1971). The Gunnison River

possesses a high degree of overlap, yet its prey pool is

much larger and more diverse. Thus, no one resource is

limiting and all species are free to exploit whatever food

resource they encounter. In both rivers, detrital, algae

and moss resources are utilized in part by the pteronarcids,

P. badia and P. californica.

Predation on Chironomidae Larvae

The four genera of chironomids present on both rivers

were AblabisMyia sp., Cricotopus sp., Prodiamesa sp.,

and Rheotanytarsus sp., with Cricotopus sp. being the most

abundant. Specificity of predation for any particular genus

was not exhibited by any stonefly species. Similar percents

of chironomid taxa were present in guts of predatory stone-

flies as compared to abundance in the riverine habits.

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