browse impacts of introduced mule deer to island … introduced ungulates, mule deer, quercus...

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Oak ecosystem restoration on Santa Catalina Island, California: Proceedings of an on-island workshop, February 2-4, 2007. Edited by D.A. Knapp. 2010. Catalina Island Conservancy, Avalon, CA.

BROWSE IMPACTS OF INTRODUCED MULE DEER TO ISLAND SCRUB OAK HABITATS

ON SANTA CATALINA ISLAND, CALIFORNIA

Thad Manuwal and Rick Sweitzer

Department of Biology, University of North Dakota

Grand Forks, ND 58202 [email protected], phone 701-777-2164

[email protected], phone 701-777-4676

ABSTRACT: Islands commonly harbor unique species that are particularly susceptible to damage by

introduced organisms. Historically, no large ungulates existed on Santa Catalina Island, but several were

introduced including mule deer (Odocoileus hemionus). Previously, there were no data for assessing impacts of mule deer to endemic trees and shrubs on the island. Our objectives here are to investigate the

impacts of introduced mule deer on island scrub oak (Quercus pacifica) habitats; more specifically, to (1)

determine seasonal mule deer diets, (2) estimate browse use of two rare endemic trees/shrubs, (3) identify

factors impinging on island scrub oak seedling survival, and (4) differentiate impacts of multiple introduced large ungulates on oak regeneration. Our results suggest deer select for forbs and grasses

during the annual wet season and rely on woody browse during the annual dry season. Mule deer

dramatically reduce available current annual growth twigs of rare island endemic shrubs. However, physical damage to oak seedlings by bison (Bos bison) and competition from non-native grasses appears

more important for reduced seedling survival than browsing by mule deer. Further, island scrub oak

seedling densities were higher in areas of the island where deer are present but other feral ungulates have been removed longest.

KEYWORDS: Introduced ungulates, mule deer, Quercus pacifica, regeneration, Santa Catalina Island,

seedlings

INTRODUCTION

The introduction and spread of nonnative species is an important conservation problem in general but

especially on islands (Savidge 1987, Cree et al. 1995), which typically have higher proportions of

endemic species and are more prone to invasion (Lodge 1993, Simberloff 1994). Further, insular endemic

plant species often lack adequate chemical and structural defenses and are more susceptible to damage from introduced herbivores (Bowen and VanVuren 1997). Island ecosystem composition and function

may be drastically altered by the introduction of large ungulates (Husheer et al. 2003). As an example,

Sitka black-tailed deer (Odocoileus hemionus sitkensis) were introduced to Haida Gwaii, Canada in the late 1800s. Once on the island, the black-tailed deer foraged preferentially on the seedlings and lower

branches of western red cedar (Thuja plicata), which, as an island isolate population, had reduced

chemical defenses against herbivores (Vourc’h et al. 2000). Over a 50 year time period, foraging by nonnative Sitka black-tailed deer on Haida Gwaii caused major reductions in overall plant cover

(Stockton et al. 2005), songbird populations (55-70% reduction), and abundance and density of

invertebrates compared to those same organisms on nearby islands without deer (Allombert et al. 2005a).

Santa Catalina Island (hereafter Catalina Island), part of California’s Channel Islands (a group of islands

off the coast of southern California), has no native large ungulates, and the largest native herbivore is the

beechey ground squirrel (Spermophilus beecheyi nesioticus). However, the island has a long history of introduced ungulates including mule deer (Odocoileus hemionus). Catalina Island was occupied by feral

goats (Capra hircus) before mule deer were introduced in the early 1930s (Coblentz 1977), whereas bison

(Bos bison) and feral pigs (Sus scrofa) were introduced around the same time as mule deer (Sweitzer et al.

2005). By the 1980s conservationists were more aware of the myriad ecological

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problems with introduced species, and in the 1990s the Santa Catalina Island Conservancy (hereafter the

Conservancy), a non-profit organization which owns and manages 88% of Catalina Island, implemented an active restoration program to eradicate feral goats and feral pigs (Schuyler et al. 2002). By early 2005

feral goats were eradicated and nearly all feral pigs had been removed (Schuyler et al. 2002, Santa

Catalina Island Conservancy unpublished records). Recent research on the ecological effects of the

introduced bison population (Sweitzer et al. 2003, Constible et al. 2005) supported a decision by the Conservancy to reduce but not entirely remove the bison herd. Mule deer remain widespread and

abundant at an unknown level, and very little is known of their foraging impacts and population ecology

useful for management.

Catalina Island harbors six species of plants found only on this island and 15 that are endemic to the

Channel Islands. All of these plants are at risk from foraging and other activities of introduced ungulates. The island scrub oak (Quercus pacifica) is an especially important endemic tree and a dominant

component of the island scrub oak woodland/chaparral habitat type or community of the Channel Islands

(Junak et al. 1995). Oaks and their associated woodland habitats are of great ecological importance

throughout the world and especially in California. Oak woodland communities in California are especially diverse with more than 1400 species of flowering plants, over 300 species of vertebrates, and thousands

of invertebrate species (Pavlik et al. 1991, Tyler et al. 2006). Several vertebrate species depend on oak

woodland habitats on Catalina Island, including the endemic island fox (Urocyon littoralis catalinae; Moore and Collins 1995) and the orange crowned warbler (Vermivora celata sordida, Sillett pers.

comm.). However, recent observations of island scrub oak have indicated low regeneration rates (Stratton

2001). This phenomenon is not specific to Catalina Island; a relative lack of regeneration among several species of oaks has been reported in mainland California (White 1966, Griffin 1971), however see Tyler

et al. (2006). Reported causes for limited regeneration include physical and/or foraging damage by deer,

feral pigs, and cattle (White 1966, Borchert et al. 1989, Sweitzer and Van Vuren 2002), predation by

rodents (Borchert et al. 1989), water stress (Matzner et al. 2003), and competition with non-native grasses (Gordon and Rice 2000).

Because of the known impacts of nonnative ungulates on insular plants, there is concern that mule deer are damaging oak woodlands and endemic trees and shrubs on Catalina Island by browsing. A major

focus of our research was investigating the impacts of introduced ungulates (deer and bison) on oak

woodland habitats and oak seedling recruitment. Our primary objectives were to: (1) determine seasonal

mule deer diets, (2) estimate browse use of two rare endemic trees/shrubs, (3) identify factors impinging on island scrub oak seedling survival, and (4) differentiate impacts of several introduced large ungulates

on oak regeneration.

STUDY AREA

Catalina Island is a 194-km2 island located 40 km south of coastal Los Angeles, in Los Angeles County,

California. Elevation on the island ranges from sea level to 640 m, with a topography dominated by a

northwest-southeast mountain range containing a series of lateral canyons (Schuyler et al. 2002). The

climate is Mediterranean with relatively mild temperatures throughout the year and a long term mean

annual precipitation of 290 ± (S.D.) 155 mm, mostly occurring between November and April (Schoenherr et al. 1999). There are four common habitat types on the island: (1) coastal sage scrub, characterized by

coastal sage (Artemisia californica) and prickly pear cactus (Opuntia littoralis); (2) grassland, dominated

by exotic annual grasses and forbs, such as bromes (Bromus spp.) and storksbill (Erodium spp.), and interspersed with native bunch grasses (Nasella spp.); (3) island chaparral represented by evergreen and

drought-resistant shrubs and low trees such as island scrub oak and lemonade berry (Rhus integrifolia);

and (4) riparian habitats, limited to a few permanent or ephemeral streams in relatively deep canyons and marshy wetland areas, represented by cottonwood (Populus trichocarpa), willow (Salix spp.) various

sedges and rushes, and mule fat (Baccharis pilularis) (Knapp 2002).

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Three cross-island fences were established on Catalina Island in the 1990s to facilitate the eradication of feral pigs and goats (Schuyler et al. 2002), effectively dividing the island into four zones (Figure 1).

These two feral ungulates were then eradicated in each zone sequentially (zone 1 – 1998, zone 2 – 2000,

zone 3 – 2003, zone 4 – all goats and most pigs by January 2005). Because feral pigs are known to

negatively impact tree seedling survival by rooting (Sweitzer and Van Vuren 2002) and feral goats seriously damage trees and shrubs by browsing (Coblentz 1977), we anticipated the presence of greater

densities of oak seedlings in zone 1 compared to zones 2 and 3. Further, because bison on Catalina Island

rarely ventured across the isthmus into zone 1 historically, and were prevented from crossing into zone 1 by fence after the early 1990s (Sweitzer et al. 2005), any possible bison-related impacts to tree seedlings

would be focused in zones 2 and 3. Our research was focused within island zones 1-3; no research was

conducted within zone 4 because of the lack of extensive scrub oak woodland habitat in the area and because of the relatively high level of human activity associated with the city of Avalon.

At the outset of the study in December 2004, we identified five general study areas where habitats were

dominated by island scrub oak (Figure 2). The five areas were Bulrush Canyon, the Empire Landing area north of the Airport-In-The-Sky between Twin Rocks and Valley of the Ollas, Swain’s Canyon near the

Volunteer Camp, and the west end of the island (Figure 2). We had originally planned to include upper-

Cottonwood canyon as one of the five focus areas, but near-record rainfall during January to April 2005 resulted in the area being inaccessible when key aspects of the research were in development. These five

research focus areas were targeted for oak seedling plot survival plots and assessing oak regeneration

potential.

Figure 1. Map of Catalina Island illustrating locations of three cross-island fences ( )

established in the 1990’s, which partitioned the island into four feral animal removal zones.

Inset in the upper right shows the location of Catalina Island in relation to mainland southern

California and several nearby Channel Islands.

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METHODS

Browse Use of Endemic Shrubs/Trees

Diets

We used microhistological analyses of fecal samples to determine the proportion of endemic trees and shrubs consumed by mule deer on Catalina Island during different seasons. Microhistological analysis is a

technique that uses epidermal fragments of plant tissue to determine the relative frequency of plant

species in ungulate feces (Sparks and Malechek 1968, Reynolds et al. 1978, VanVuren 1984). We collected a minimum of five fresh mule deer fecal pellet groups from within each of the five study areas

of the island from December 2004 to November 2006. Samples were collected from each of the study

areas to effectively spread sampling across the entire island. Fecal pellet groups were considered fresh if

they had an almost shiny exterior and a green interior. Pellets that were dried out and brown in the interior were considered old and were not collected. In addition, we collected fecal samples from hunter harvested

deer in cooperation with hunting guides (Wildlife West Inc.; http://www.wildlifewestinc.com, accessed

July 23, 2007) during three September to December hunting seasons from 2004 to 2006.

Fecal samples from both opportunistic collection and hunter harvested deer were stored frozen (-20˚ C)

prior to being processed and shipped to the Diet Analysis Laboratory at Washington State University in Pullman, Washington for microhistological identification of plant cell fragments and diet estimates to the

level of genus and species. Processing consisted of sub-sampling each individual sample to create 13

bimonthly “composite” samples (Dec-Jan, Feb-Mar, Apr-May, etc.) from Dec 2004 to November 2006.

We were unable to collect samples in August 2005 and February 2006. Plants identified as being consumed by deer were grouped into five forage classes (grasses, endemic forbs, non-endemic forbs,

endemic shrubs/trees, non-endemic shrubs/trees, and other) for analyses of potential dietary differences

between years and among two month periods or seasons.

Endemic Shrub Exclosure

In 1999 a large wildfire spread across the Goat Harbor area of Catalina Island. Shortly after the “Goat Harbor Burn,” the Conservancy established several exclosures in the area to protect several endemic

woody plants [Catalina ironwood (Lyonothamnus floribundus ssp. floribundus), island bush poppy

(Dendromecon harfordii), and felt leaf ceanothus (Ceanothus arboreus)] from browse damage by deer and bison. In August 2005, we altered the fences at three of the exclosures to experimentally assess

browse impacts of mule deer on two species of endemic shrubs. Portions of the exclosures were altered so

that several individuals each of felt leaf ceanothus (N = 3) and island bush poppy (N = 4) were exposed to mule deer (experimentals), and the same number of each remained protected inside the fence (controls).

These individual shrubs were marked with a stake and all unbrowsed (available) and browsed current

annual growth (CAG) twigs within a one meter radius from the stake center and from 0 to 2.0m in height

were enumerated. These experimental and control shrubs in the Goat Harbor Burn area were monitored for numbers of available and browsed CAG twigs in August 2005 and then monthly from January to

November 2006. We used repeated-measures ANOVA to compare numbers of available CAG twigs for

experimental and control D. harfordii and C. arboreus trees by sample period.

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Ungulate Impacts on Scrub Oak Seedling Regeneration

Oak Seedling Survival/Mortality Experiment

We used experiments with nursery grown oak seedlings to examine the effects of ungulates and other

environmental factors on seedling survival. Approximately 1700 oak tree seedlings from the J. H. Ackerman Native Plant Nursery on Catalina Island were made available for the study by the

Conservancy. We designed an experiment using a subset of 1600 of the healthiest of these seedlings in

which 100 seedlings each were planted in 16 different 30-m X 30-m plots (hereafter seedling plots) in island scrub oak chaparral habitats (3 seedling plots in each of the 5 research focus areas except Twin

Rocks, where 4 plots were established; Figure 2). Details on the design of the oak seedling survival

experiments follow.

All oak seedlings used in this experiment were produced from acorns that were collected on the island

and grown in torpedo style greenhouse pots by Conservancy personnel 1-3 years prior to use in the study. In January 2004 we measured each of the 1704 seedlings in the nursery for total height (± 2 mm), and

ranked them by health status. Fewer than 1700 of the seedlings were ranked as being in good to excellent

Figure 2. Location of sixteen experimental seedling plots (black diamonds) established in five

study areas (black ovals) of Catalina Island, California. Shaded areas represent island scrub oak

woodland habitats.

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health and therefore suitable for use in the experiments. Data on seedling height were compiled and used

to partition seedlings into four different size classes (size class 1 <100mm, size class 2- 101 to 200mm, size class 3- 201 to 300mm, size class 4 >300mm). The distribution of oak seedlings among size classes

constrained the study design such that each seedling plot could include 33 seedlings within size class 1,

34 seedlings within size class 2, 20 seedlings within size class 3, and 13 seedlings within size class 4. We

used a randomization procedure to identify and select 100 individual seedlings among the four size classes for each of 16 experimental seedling plots.

ArcGIS 9.0 (ESRI, Redlands, CA) was used to identify seedling plot locations based on the goal of planting a minimum of three seedling plots within each of the five focus areas. As noted above, four

seedling plots were positioned in the Twin Rocks focus area (Figure 2). A 100-m X 100-m grid was

placed over island scrub oak habitats that fell within a 400 m buffer strip around drivable roads on the island (Figure 2), rendering plot locations reasonably accessible for hand-carrying seedlings and planting

equipment to the area. We utilized a randomization method to select grid cells for planting; the first three

random grids in each focus area that encompassed a 30-m X 30-m area suitable for planting were used for

seedling plots. Grid areas were considered unsuitable for planting when soils were very rocky, or not scrub oak woodland.

Each 30-m X 30-m seedling plot was subdivided into 100 3-m X 3-m cells for planting (planting positions for each individual seedling were assigned by randomization). Individual tree seedlings were planted in

the approximate center of each plot cell unless trees, rocks or other obstacles prevented planting access to

that area of the cell with a two person gas-powered soil auger. A 20 cm diameter auger bit was used to excavate planting holes to a depth of 40-45 cm. After the planting holes were cleared of loose soil,

seedlings were carefully extracted from their nursery tubes with root balls intact and placed into the hole

with stems at approximate ground level. Excavated soil was used to backfill around each seedling and

tamped firmly into place using a hand trowel. A metal washer was inserted into the loose soil adjacent to the seedling in order to locate seedlings with a metal detector for monitoring. Seedlings at two seedling

plots were “watered in” with 7.5-9.5 l of water per seedling slowly drained into the soil from plastic water

containers placed on the edges of the refilled holes. Watering in was not done at the other 14 seedling plots because soils were very moist from winter rains that occurred before and during the planting period

(late January to early March 2005).

We visited and evaluated all seedling plots approximately every six weeks from March to August in 2005, and from January to November in 2006, for a total of eleven monitoring periods. During each plot visit

and for each individual seedling we recorded data on seedling status (alive/dead), general health (poor,

good, or very good), seedling height (tallest apical bud standing in a natural position ± 2 mm), evidence of deer browse of seedlings (any branchlets that were browsed were hand clipped perpendicular to axial

growth to preclude recounting on subsequent visits), direct (excluding browse) and indirect ungulate

disturbances within 1-m radius of each seedling, surrounding cover type (beneath tree canopy, open, within cover such as woody debris or prickly pear cactus), and vegetative cover (height of herbaceous

vegetation immediately surrounding the seedling).

We investigated factors contributing to survival and/or mortality of island scrub oak seedlings using a logistic regression model. Predictor variables included in the model were browse disturbance [BR_DIST

(expressed as the number of times an individual seedling was browsed/the number of times that seedling

was recorded as alive)], ungulate disturbance [UNG_DIST (total number of times an individual seedling was disturbed by deer or bison activity with the exception of browse/ total number of times seedling was

alive)], other disturbances [OTHER_DIST (number of times an individual seedling was disturbed by

rodents, insects, or other/number of times seedling was alive)], indirect disturbance [IND_DIST (number of times deer and bison activity was observed within 1-m radius of seedling/ total number of times

seedling was alive)], cover [COVER (whether seedling was positioned in protective cover or not)],

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canopy position [CNPY (whether seedling was positioned under canopy or not)], and herbaceous cover

[HERB_COV (expressed as the mean of herbaceous height directly surrounding the seedling minus seedling height over all monitoring periods)]. All backward-stepwise logistic regression (α ≤ 0.05)

analyses were performed using the statistical software R (R Development Core Team 2005).

Differential Impacts of Large Ungulates

Introduced ungulates may impact seedling survival and long-term persistence of scrub oak woodlands on

Catalina Island by rooting (feral pigs), browsing (mule deer, feral goats), and trampling or wallowing (bison). We positioned tree seedling belt transects (4-m X 40-m; Bruinderink and Hazebroek 1996,

Sweitzer and Van Vuren 2002) within scrub oak woodland habitats in Zones 1, 2, and 3 (the zones were

demarcated by cross-island fences; Figure 1) to examine these potential impacts. We expected more seedlings in zone 1 (feral pigs and goats removed longest) compared to zones 2 and 3 if feral goats and

pigs differentially limited the survival of oak seedlings. Alternatively, if nonnative mule deer

differentially limit the survival of seedlings we expected similar densities of seedlings among the three

zones (the range of mule deer encompasses all of Catalina Island). Locations of starting positions for 9 (zone 1), 24 (zone 2), and 28 (zone 3) seedling belt transects were randomly generated in scrub oak

woodland habitats using ArcGIS. Fewer transects were located in zone 1 because the overall area of scrub

oak woodland is less in this area of the island than in zones 2 and 3. From the starting positions and in a random azimuth from 1-360º, all tree seedlings (< 1200 mm in total height) within 2 m of each side of a

40 m transect line were identified and measured from ground level to the terminal bud (± 5 mm) during

the summers of 2005 and 2006. Enumerating and measuring seedlings was facilitated by use of a rope frame wherein a series of rectangular 2 X 4 m quadrats along the length of each 40 m belt transect line

were searched. We used two-way ANOVA to compare density of seedlings (rank-transformed) among

zones during different years (Quinn and Keough 2002).

RESULTS

Browse Use of Endemic Shrubs/Trees

Diets

Forage class composition varied seasonally throughout the study. Browse (endemic and other shrubs) comprised between 50.7 and 86.1% of deer diets throughout the year, and contributed a major portion of

deer diets in the driest months (Jun-Sept.). Mule deer consumed endemic shrubs during all sampling

periods; however, the majority of total browse per sampling period consisted of non-endemic shrubs/trees, which are far more available than endemics (Figure 3). Forbs contributed a large portion of

deer diets in the spring months (Apr – May) when they were observed to be most abundant.

Quercus species made up a large portion of the shrub/tree component of deer diets, however, most

Quercus material consumed consisted of leaf material (Figure 4). Most of the leaf consumption occurred

in the spring months when new leaves would be flushing and presumably contain fewer chemical

defenses against herbivory. See Appendix 1 for a list of all plants consumed by mule deer.

Endemic Shrubs in Goat Harbor Burn Exclosures.

There was one known incursion by an individual mule deer into one of the Goat Harbor Burn exclosures

after they were modified in August 2005. In November 2006 a mule deer was able to enter one of the

exclosure plots and browsed on several C. arboreus twigs before departing. The fence was reinforced to prevent a second incursion.

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Figure 4. Seasonal Quercus species leaf and stem tissue consumption of mule deer

on Santa Catalina Island, California.

Figure 3. Seasonal diet composition of mule deer on Santa Catalina Island, California from

December 2004 through December 2006.

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In August 2005, before treatment plots were exposed to deer, the number of available CAG twigs was

similar in control and treatment plots for both C. arboreus (Mann-Whitney test, U = 6, p-value = 0.7) and D. harfordii (Mann-Whitney test, U = 6, p-value = 0.7). Subsequent to being exposed to browsing by

mule deer, total available CAG twigs were significantly and rapidly reduced in treatment compared to

control plots for both C. arboreus and D. harfordii (F1, 115 = 138.63, p-value <0.0001; Figure 5). In

addition, D. harfordii had significantly more available CAG twigs than C. arboreus (F1, 115 = 4.65, p-value = 0.0332), which was likely due to the “brushy” growth form of D. harfordii.

Ungulate Impacts on Scrub Oak Seedling Regeneration

Oak Seedling Mortality Factors

Sixty-four percent (1017) of 1600 island scrub oak seedlings planted in the experimental seedling plots

remained alive as of November 2006. On average, 7.0% ± SE 0.5 of the 1600 planted seedlings

experienced some type of disturbance between monitoring periods. The most common type of disturbance

was browsing by deer: twenty-six percent (426) of the seedlings were browsed over the duration of the study. An average of 4.7% ± SE 0.4 of the 1600 planted seedlings had evidence of browsing during each

monitoring period (Figure 6). In general, the large majority of disturbance to planted oak seedlings was by

nonnative ungulates (Figure 6).

Figure 5. Total available current annual growth (CAG) twigs for Ceanothus arboreus (n = 3) and Dendromecon harfordii (n = 4) in exclosures (Control) and exposed to deer browsing (Treatment)

beginning Aug. 2005 through Nov. 2006. Differences between control and treatment groups are

significant (F1, 115 = 138.63, p-value <0.0001). Note the difference in the Y-axis scale.

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Logistic regression analyses of data on scrub oak seedlings identified ungulate disturbance, ungulate

activity within 1m radius of the seedling, and herbaceous cover taller than the seedling as important predictors of survival (Table 1). Data indicate seedlings experiencing ungulate disturbance were located

in areas with repeated ungulate activity, and those within herbaceous vegetation taller than the seeding

were less likely to survive than seedlings that either: 1) experienced no ungulate disturbance, 2) were not

in areas of ungulate use, or 3) were taller than the surrounding herbaceous vegetation. Seedlings that had some evidence of browsing by mule deer did not experience lower probabilities of survival than non-

browsed seedlings.

Figure 6. Identified sources of disturbance (browsing, trampling, chewing, etc.) by

monitoring period for scrub oak seedlings planted among 16 experimental seedling plots on

Catalina Island, California.

Differential Impacts of Large Ungulates

Density of scrub oak seedlings varied among zones (F2,57 = 5.34, P = 0.0075). Post hoc tests were not

powerful enough to detect which zones differed because of unbalanced sample sizes, however, there was

a general trend for more seedlings in zone 1 than in zones 2 and 3 (Figure 7). There were more seedlings along belt transects in 2005 than in 2006 (F1,57 = 35.53, p-value <0.0001). Interactions of year and zone

were considered in the original model but were not significant (F2, 55 = 2.35, p-value = 0.1046) and were

removed for the final model.

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Table 1. Summary of coefficient estimates and their associated statistics for the

reduced logistic regression model with response variable – oak seedling survival.

Variable Coefficient

Estimate S.E. z-statistic p-value

CONSTANT 2.246 0.115 19.605 <0.0001

UNG_DIST -0.874 0.382 -2.288 0.0222

IND_DIST -1.477 0.214 -6.899 <0.0001

HERB_COV -0.006 0.001 -9.170 <0.0001

Figure 7. Mean density Quercus pacifica seedlings per island zone for 2005 and 2006. Error bars represent one standard error.

DISCUSSION

Diet composition of mule deer on Catalina Island varied seasonally as they altered their foraging patterns

to include proportionally more browse from trees and shrubs during seasonally dry periods. Grasses and forbs were abundant in diets when they were abundant and nutritious (seasonal wet periods). This pattern

was consistent with mule deer being classified as intermediate feeders (Hofmann 1989). Browse was the

dominant forage class consumed by mule deer on Catalina Island in all months, which was generally consistent with diets reported for mule deer in other areas of California (Schauss and Coletto 1986, Gogan

and Barrett 1995, Marshal et al. 2004), southeastern Oregon (Main and Coblentz 1996), and Arizona

(Krausman et al. 1997).

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Among the most compelling and dramatic results of this study was the heavy browsing on two species of

Catalina Island endemic shrubs (D. harfordii, C. arboreus) after we altered several exclosures in the Goat Harbor area to expose a fixed number of them to deer browsing. These two endemic shrubs experienced

heavy foraging pressure, which resulted in mortality of one C. arboreus tree 15 months after exposure to

mule deer. All five of the other C. arboreus and D. harfordii trees in treatment plots were entirely devoid

of leaves below 2.0 m. These results support the idea that insular endemics are more prone to damage by introduced herbivores (Bowen and VanVuren 1997). Further, although several endemic shrubs were

detected in the diets of mule deer, relatively few of these plants remain on the island because of historic

intensive browsing by mule deer and feral goats. Based on the results from the Goat Harbor experiment, it is unlikely that any of the remaining endemic shrub individuals on Catalina Island will recover from

overuse by feral goats as long as a significant population of mule deer remains on the island.

Browsing by mule deer was not identified as a major mortality factor for oaks in seedling plots, at least at

this young stage. However, results from diet analyses indicated that we underestimated mule deer

foraging on island scrub oak based on their focus on leaves of oaks instead of stems. We had no reliable

way of tracking consumption of leaves at seedling plots, and it would have been logistically impossible to have done so. Loss of many new leaves on relatively young seedlings or saplings would very likely

reduce the rate of growth for the seedling/sapling, thereby extending the period the seedling/sapling

remains relatively small (< 2 m) and exposed to deer browsing. It is also likely that browsing by deer would increase as oak seedlings emerge from the grass/forb matrix and become more visible to deer.

The most important mortality factor we identified for oak seedlings on Catalina Island was direct physical disturbance (trampling and wallowing) by bison. In accordance with this result, oak seedling densities

along seedling transects were higher in zone 1 (bison not present) than in zones 2 and 3 (bison present).

Several studies have addressed physical disturbances to oak seedlings (Bruinderink and Hazebroek 1996,

Sweitzer and Van Vuren 2002) but few have specifically addressed trampling activities by large ungulates. Coppedge and Shaw (1997) reported that bison on the Tallgrass Prairie Preserve in Oklahoma

significantly impacted saplings and shrubs by horning and rubbing, and concluded that abundant bison in

pre-European North America likely limited the distribution of woody vegetation in the Great Plains. Evidence of ungulate activity near seedlings was another key mortality factor for oak seedlings. However,

we are unsure of the mechanism by which deer presence may have negatively influenced seedling

survival other than direct browsing.

Tall herbaceous cover around oak seedlings also contributed to lower seedling survival at the seedling

scale. When recording data at the seedling plots we noticed that many of the seedlings growing in areas

dominated by tall and dense non-native grasses (Avena spp., Brachypodium distachyon, Bromus spp) appeared less vigorous than others. Although our study did not address the mechanism (s) by which tall

vegetation caused higher seedling mortality, it is likely that herbaceous vegetation reduced soil moisture

and soil nutrients by uptake. For example, Danielsen and Halvorson (1991) found that growth and survivorship of seedlings of valley oak (Quercus lobata) was reduced by nonnative grasses. Similarly,

Gordon and Rice (2000) reported direct competition for water in the soil between nonnative grasses and

seedlings of blue oak (Quercus douglasii) in northern California. In Minnesota, nonnative grasses caused

a reduction in survival of northern pin oak (Quercus ellipsoidalis; Davis et al. 2005). At the seedling plot scale, seedling survival was reduced in areas with deep thatch due to the slow decomposition of deep

layers of dried grasses and their accumulation over many years.

Data from seedling belt transects suggest important variation in numbers of oak seedlings growing in

different areas of Catalina Island. Oak seedling densities appeared more abundant on the west end of the

island compared to two regions encompassing the central portion of the island. Feral pigs and feral goats were eradicated from the zone 1 area of Catalina Island nearly 4 years and over 6 years before these

ungulates were eradicated from zones 2 and 3, respectively (Schuyler et al. 2002). Bison have also been

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excluded from zone 1 historically. Together these preliminary findings suggest seedling numbers are

recovering in zone 1 by release from rooting and other activities of feral pigs and goats. The large number of seedlings found in Zone 1 may also reflect the absence of bison trampling activities in this area.

Higher densities of seedlings found along transects in summer of 2005 highlights the importance of inter-

annual precipitation to oak recruitment. Germination of white oaks such as island scrub oak occurs in late fall and is often triggered by fall or winter rains (Pavlik et al. 1991, Tyler et al. 2006). Environmental

conditions are critical at this stage for the survival of oak seedlings; more seedlings survive the year after

germination in wet years compared to drought years. Catalina Island’s seasonal precipitation for July 1, 2004 to June 30, 2005 was 680.72 ± SD 110.0 mm, the second highest in recorded history. This is

approximately 400 mm above long term average precipitation. In contrast, the seasonal precipitation for

the same time period in 2005-2006 was 241.55 ± 24.64 mm (Santa Catalina Island Conservancy, unpublished data). Further, we observed relatively high levels of mast in the field during the fall of 2004.

The combination of near record rainfall and high masting dynamics during the winter of 2004-2005 likely

contributed to the higher densities in summer of 2005 compared to 2006.

The results we have presented from this study are important for identifying the effects of introduced

ungulates on scrub oak woodland habitats on Catalina Island. Deer browsing activities on island scrub

oak seedlings did not have detrimental effects on survival in the first years following planting, however, the trampling activities of bison and deer were identified as negative influences to oak seedlings through

several different methods and may be at least partly responsible for the relatively low numbers of oak

seedlings detected along seedling belt transects in zones 2 and 3 compared to zone 1. Consequently the combination of introduced grasses and large ungulates may be having detrimental effects on the scrub oak

communities of Catalina Island.

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34


Appendix 1. Percent composition of mule deer diets on Santa Catalina Island, California from September

2004 to November 2006 as determined by fecal microhistological analysis.

Sept-Nov

2004 2004/05 2005/06 2005 2006 2005 2006 2005 2006 5-Sep 2006 2005 2006

GRASSES

Avena spp. 2.1 1.3 0.8

Bromus spp. 5.6 5.0 13.1 3.3 1.5 2.0 1.4

Leymus spp. 0.8 1.3

Melica imperfecta 2.9 3.2

Nassella (Stipa) 3.6 0.8 2.5 2.1 1.0

Poa spp. 5.6 2.5 1.6 1.4

Unk. Grasses 2.1 0.8 1.3 2.1 0.6 0.9

Total Grasses 16.9 12.0 21.1 9.6 2.5 0.6 3.6 6.9

FORBS

Achillea millefolium 0.2 0.6 1.4

Astragalus trichopodus 2.6 2.1 5.5 3.6 2.8 2.4 0.4 8.3

Calystegia macrostegia 0.2

Chenopodium spp. 1.4

Claytonia perfoliata 2.4

Eriogonum leaf 4.9 1.7 2.8

Eriogonum stem 2.3 4.2 2.1

Foeniculum vulgare 1.5

Galium spp. 0.4 0.4

Keckiella cordifolia 1.0

Lonicera spp. stem 1.3

Lotus/Lathyrus 0.8 1.3 4.8 3.6 6.2 6.1 0.4 4.8

Lupinus spp. 3.1 8.0 3.8 18.5 10.1 4.8 10.8

Mirabilis californica 1.3

Phacelia cicutaria 0.4 1.7

Rumex spp. 0.9

Sanicula spp. 0.8

Trifolium/Medicago/Melilotus 11.3 4.6 7.5 9.5 2.7 0.2

Vicia ludoviciana 0.5 0.4 0.8

Unk. Forbs 4.9 2.9 5.9 2.1 5.6 7.3 6.0 5.1

Total Forbs 27.3 21.5 28.2 26.0 44.1 31.6 11.6 35.7

SHRUBS

Adenostoma fasciculatum 11.8 24.9 21.5 6.5 13.9 17.1 29.1 16.6

Arctostaphylos catalinae leaf 2.1 1.7 1.2

Artemisia spp. leaf 12.8 2.9 7.0 0.8 6.0 5.3

Atriplex semibaccata 0.5 0.8

Baccharis spp. 2.5 4.6 0.4 2.1 1.8 9.9 3.7

Ceanothus spp. leaf 0.8 2.1

Cercocarpus betuloides leaf 3.6 2.1 5.4 5.1 0.6 2.0

Malacothamnus fasciculatus 6.2 4.9 0.4 6.4

Opuntia littoralis 1.0 1.7 1.8 3.2

Populus spp. leaf 3.3 2.6 1.2 2.8

Populus spp. stem 0.8 3.7 1.2 3.2

Quercus spp. leaf 10.8 13.8 2.5 36.7 19.8 24.4 11.5 2.8

Quercus spp. stem 1.3

Rhamnus pirifolia leaf 0.8 4.2 2.5 2.0 1.4

Rhamnus pirifolia stem 0.9

Rhus spp. leaf 1.5 2.9 0.4 2.1 2.3

Salix leaf 0.8 1.7 3.1 2.8

Salix stem 4.6 3.4 2.1 2.1 2.6 4.3 7.1 5.1

Symphoricarpos mollis leaf 1.2

Symphoricarpos mollis stem 0.8

Unk. Shrub leaf 1.0 2.1 1.3 2.9 2.6 3.7 1.2 3.7

Unk. Shrub stem 1.0 2.1 0.8 0.4 2.6 2.4 3.2 2.3

Total Shrubs 51.2 64.4 50.7 64.4 53.4 67.8 84.8 56.5

Nut 3.6 1.3 0.0 0.0 0.0 0.0 0.0 0.0

Seed 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.9

Thorn 0.0 0.8 0.0 0.0 0.0 0.0 0.0 0.0

Insect 1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

TOTAL 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0

No. Species 17 26 19 23 14 14 18 20

Aug-Sept Oct-NovSpecies

Dec-Jan Feb-Mar Apr-May Jun-Jul

browse impacts of introduced mule deer to island … introduced ungulates, mule deer, quercus...

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