Effects of Habitat Enrichment on Patterns of Diet Selection

Effects of Habitat Enrichment on Patterns of Diet SelectionAuthor(s): Steven B. Murden and Ken L. RisenhooverSource: Ecological Applications, Vol. 3, No. 3 (Aug., 1993), pp. 497-505Published by: Ecological Society of AmericaStable URL: http://www.jstor.org/stable/1941918 .

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Ecological Applications, 3(3), 1993, pp. 497-505 i) 1993 by the Ecological Society of America

EFFECTS OF HABITAT ENRICHMENT ON PATTERNS OF DIET SELECTION'

STEVEN B. MURDEN AND KEN L. RISENHOOVER2 Department of WildliJe and Fisheries Sciences, 7exas A&M University,

College Station, Texas 77843-2258 USA

,Abstract. Foraging theory predicts that animals should feed more selectively in response to an increased abundance of high-quality foods. We tested these predictions by examining changes in patterns of diet selection by white-tailed deer (Odocoileus virginianus) and Angora goats (Capri hircus) in response to habitat enrichment. When a high-quality supplement was provided ad libitum, both species responded by increasing foraging effort and by feeding more selectively on natural forages. Supplemented animals consumed a greater proportion of plants containing high concentrations of crude protein (CP) and digestible energy (DE), which resulted in an increase in average diet quality. Responses of individual deer and goats varied and appeared to be related to differences in habitat heterogeneity between trial enclosures. Our results suggest that supplementation may be disruptive to normal behavioral processes affecting the distribution of free-ranging herbivores on the landscape. These processes may be important in reducing the probability of excessive utilization of palatable forage species, and to the competitive relationships between range plants.

Key words. Angora goats; Capri hircus; diet selection;foraging etJbrt; habitat heterogeneity; Odo- coileus virginianus; selectivity; supplemnentation; white-tailed deer.

INTRODUCTION

Current theory suggests that patterns of diet selection by ruminants are influenced by the nutritional status of the animal (Westoby 1974), avoidance of toxic plant secondary compounds (Freeland and Janzen 1974, Owen-Smith and Cooper 1987, Robbins et al. 1987), experience and learning (Bartmann and Carpenter 1982, Provenza and Balph 1987, 1988, Gillingham and Bun- nell 1989), and the physical and logistical constraints on daily intake (Hudson and White 1985, Spalinger 1985, Risenhoover 1987). Theoretically, the decision to consume a food item is determined by its value relative to other available food items and the costs associated with search and handling time (Spalinger et al. 1986, Stephens and Krebs 1986). In this model (i.e., simultaneous encounters), foods are ranked according to their net yield of some currency per unit handling time, and are included in the diet until net returns from foraging decline (Stephens and Krebs 1986). As the availability of more profitable food items declines, the search time needed per prey item captured increases to a point where it becomes profitable to also include less rewarding forages in the diet.

Few studies have tested diet selection theory and its predictions as they relate to free-ranging ruminants. Progress in understanding diet selection has been hin- dered by the complexity of foraging decisions confront- ing ruminants (Owen-Smith and Novellic 1982), and

by the controversy surrounding the "currencies" (e.g., nutrient selection vs. avoidance of toxins) influencing animal decisions (reviewed by Stephens and Krebs 1986). These problems make it difficult to assess food abundance and quality accurately, and to evaluate animal responses to changes in habitat quality.

An alternative approach, which avoids the problems associated with measuring resource availability and quality, involves measuring animal responses to habitat enrichment (Boutin 1990). The addition of a known high-quality food supplement provides the opportu- nity to assess the effects of forage quantity and quality on patterns of resource use.

In this study, we examined changes in patterns of diet selection and foraging effort by white-tailed deer and Angora goats in response to the addition of a high- quality food source (i.e., pelleted ration). Where resources were abundant, we predicted both ruminants would forage more selectively than where resources were scarce (i.e., where resources were scarce, deer and goats were expected to use a wider range of forages, including more items of lower quality). Thus, we predicted that the addition of an ad libitum supply of high- quality food to the environment would reduce the influence of time constraints on diet selection and would allow animals to increase foraging effort and to feed more selectively. Such an increase in selectivity should produce a more specialized diet of higher quality. Deer and goats were chosen for study because they represent distinct feeding styles (Hofmann 1989) that may re- spond differently to food types and their availabilities. White-tailed deer are "concentrate selectors" and pos-

I Manuscript received 10 January 1992; revised and ac- cepted 2 September 1992.

- Send reprint requests to this author.



498 STEVEN B. MURDEN AND KEN L. RISENHOOVER Ecological Applications Vol. 3, No. 3

sess morphological adaptations for utilizing plants containing rapidly fermentable cell solubles (i.e., forbs and woody plant foliage). In contrast, goats are "interme- diate" feeders and are better adapted for utilizing fi- brous plant material (i.e., graminoids and browse).

METHODS

This study was conducted at the Texas Agricultural Experiment Station (TAES) 45 km southeast of Sonora, Texas (300 N, 1000 W) during the fall of 1990. Vege- tation in this area, a semiarid oak woodland savanna, is typical of the Edwards Plateau ecological region (Hatch et al. 1990). Plant species occurring on the study area have been described by Huston et al. (I198 1). Rain- fall at the station averages 580 mm annually with peaks in May and September.

To test our hypotheses, four hand-reared white-tailed deer and four Angora goats were randomly assigned to eight temporary 1 44-i2 enclosures constructed of 2.1 m high nylon fencing (Tensar "Polygrid RF," Tensar Corporation, Morrow, Georgia). Enclosures provided control of study conditions and facilitated observation of animal feeding. Hand-reared animals were used to minimize potential biases created by enclosures and human observers. Prior to trials, deer and goats were housed in separate 0.4-ha enclosures where they were maintained on native vegetation, a pelleted ration, and alfalfa hay.

During phase one of the feeding trial (days 1-7), deer and goats were allowed to forage on native vegetation while patterns of diet selection and feeding behavior were observed. On day 8, all individuals were moved to eight adjacent 144-iM2 enclosures and provided ad libitum access to a high-quality pelleted ration (16% CP and 16.0 kJ/g DE), and diet selection patterns were observed for an additional 7 d. During the supplementation phase of the experiment, two deer were removed due to health concerns.

Observations totaling 60 min of continuous foraging activity were used to determine feeding patterns and foraging effort of each deer and goat during each trial phase. Diets selected (number of bites by species) were recorded using a portable cassette recorder. Bite sizes (in grams) were estimated by hand-plucking forage samples representing consumed plants and plant parts (Baker and Hobbs 1982). Diet richness was estimated as the number of plant species in the diet. Movement rates (in metres per minute) of foraging animals were determined by counting steps during 5-min intervals and multiplying by an average step length (in metres) determined independently for each animal by measuring the distance traversed during a known number of steps.

Diet selectivity (percent) was calculated by measuring the proportion of rare plant species in the diet as described by Hobbs et al. (1983). However, to permit detection of subtle changes in diet selection, we defined rare species as those forages contributing <55% of the

estimated standing crop (in kilograms per hectare) in enclosures. For individual deer and goats, diet com- parisons were limited to plant species common to pairs of enclosures in both trial phases. To facilitate analysis of changes in use of native vegetation in response to supplementation, the portions of the diets contributed by the pelleted ration were omitted.

Prior to the feeding trials, vegetation in each enclosure was sampled by clipping 10 randomly distributed 0.0625-iM2 plots to determine species composition, relative availability (in kilograms per square metre), and nutritive quality. Standing crop (in kilograms per square metre) was estimated on a species and plant part basis and separated into live and dead categories. Forage samples were oven-dried at 50?C for 48 h, weighed, and ground through a Wiley mill with a 1-mm mesh screen. Prior to chemical analysis, samples were ho- mogenized using a Cyclotec 1093 mill (Tecator, Ho- ganas, Sweden) with a 1-mm mesh screen to obtain a uniform particle size. Concentrations of CP (percent nitrogen x 6.25) in forage samples were determined using micro-Kjeldahl techniques (Horwitz 1980). Gross energy (GE, in kilojoules per gram) in forages was determined by bomb calorimetry. In vitro digestible organic matter (IVDOM, percent) was determined according to Goering and Van Soest (1970) as modified by Huston et al. (1981) using rumen inocula from a fistulated steer fed alfalfa hay. An index of forage DE content (in kilojoules per gram) was calculated as the product of IVDOM and GE.

Although experimental trials were conducted in a pasture under a uniform management regime (burned on a 4-yr rotation cycle and livestock grazing excluded), conspicuous differences existed in the distribution patterns of forage classes, especially browse. Because separate enclosures were used during trial phases, it was necessary to test the assumption that resources available to animals in enclosures did not differ between trial phases before changes in foraging patterns could be attributed to habitat enrichment. The relative abundance (in kilograms per hectare) of forbs, grasses, and browse in pairs of enclosures used during trial phases was compared using a G test (R x C) of independence (Sokal and Rohlf 1981:745). Relationships between forage abundance and animal foraging patterns were examined using regression analysis. Kolmogorov- Smirnov two-sample tests (Sokal and Rohlf 1981:440) were used to test for homogeneity between distributions of CP and DE concentrations contained in native plants in the diets of supplemented and nonsupplemented deer and goats. G tests (Sokal and Rohlf 1981: 724) were used to identify sources of heterogeneity between distributions of nutrients in diets of supplemented and nonsupplemented animals. A two-factor analysis of variance (ANOVA) with repeated measures (Sokal and Rohlf 1981:348) was used to assess differences in foraging effort and diet selectively between supplemented and nonsupplemented deer and goats.



August 1993 EFFECTS OF ENRICHMENT ON DIET SELECTION 499

RESULTS

The relative availabilities of grasses, forbs, and browse in experimental enclosures differed between treatment phases for 4 of the 6 experimental animals used in trials (P < .05). However, animal foraging behavior (i.e., movement rate, selectivity) and diet quality (DE, CP) could not be predicted based on the composition and relative abundance of forages in enclosures (Table 1). The average CP concentration in the diets of deer and goats was negatively correlated (P < .05) with forb abundance. However, because dietary CP concentration was unaffected by supplementation, we considered this relationship unimportant to our evaluation of the effects of supplementation on diet selection. Thus, we considered our treatment (i.e., supplementation) to be the primary factor responsible for changes in animal foraging behavior and diet selection between trial phases.

During the supplementation phase of the experiment, deer and goats continued to utilize native forages (30 and 210% of ingested dry matter, respectively) despite ad libitum access to the high-quality pelleted ration. Dry matter intake (native forages + supplement) during 60-min observation periods increased 8 and 5 1 % for deer and goats, respectively.

Deer and goat diet richness (i.e., number of plant species in the diet) did not differ between supplemented

and nonsupplemented trial phases (Table 2). However, significant changes occurred in the relative composition of deer and goat diets as a result of enrichment (P < .001). When a high-quality supplement was provided, both ruminants increased their use of rare forages and consumed proportionately less of species common in the environment (Fig. 1). Although average selectivity (i.e., use of rare forages) increased 17% for deer and 31 % for goats in response to supplementation (Ta- ble 2), selectivity did not differ between trial phases for deer (P = .96) or goats (P = . 19). Average movement rates of foraging deer and goats increased with supplementation, but differences between trial phases were not significant (P = .51 and P = .69, respectively; Table 2).

Changes in diet composition following supplementation produced significant shifts (P < .005) in the distributions of nutrients consumed by deer and goats (Figs. 2 and 3). Deer consumed greater amounts of forages containing CP concentrations in the 8-9.9 and 16-17.9% ranges, and DE in the 6.0-7.9 and 10.0-1 1.9 kJ/g ranges (P < .005). Goats made greater use of forages containing 12-15.9 and 20-21 .9% CP and 9.0- 9.9 and 1 1.0-1 1.9 kJ/g DE (P < .00 5). Despite changes in diet composition, and associated shifts in the nutritional characteristics of diet components, average concentrations of CP and DE in the diet (native forages only) did not change significantly as a result of enrich-

TABLE 1. Linear relationships between forage availability, behavior, and diet quality of white-tailed deer and Angora goats based on least squares regression analysis.

Forage availability (kg/ha)

Forbs Grasses Browse Total*

Variable F r.2 p F r.2 P F r2 p F r2 p

Behavior: Movement rate (m/min) 1.13 0.10 .31 0.11 0.01 .74 0.41 0.04 .54 0.26 0.03 .62 Selectivity (%) 0.08 1.01 .79 0.07 0.01 .79 0.08 0.01 .79 0.22 0.02 .65 Diet richnesst 1.12 0.11 .29 0.00 0.00 .95 8.22 0.45 .02 3.92 0.28 .08

Diet quality: Crude protein (/) 6.77 0.40 .03 0.28 0.03 .61 1.38 0.12 .27 0.68 0.06 .43 Digestible energy (kJ/g) 0.09 0.08 .36 0.37 0.04 .56 0.38 0.04 .55 1.37 0.12 .27 * Total forage biomass available to deer and goats in enclosures. t Diet richness is defined as the number of plant species in the diet.

TABLE 2. Foraging behaviors of supplemented and nonsupplemented (control) white-tailed deer and Angora goats in response to habitat enrichment at the Texas Agricultural Experiment Station, Sonora, Texas.

Deer* Goats

Control Supplemented Control Supplemented

n X ?1 SE n X S1SE n X S1SE n X?1SE

Movement rates (m/min) 4 3.8 ? 1.1 2 5.1 ? 1.8 4 1.4 ? 0.2 4 2.4 ? 0.5 Diet selectivity (%)t 4 75.0 ? 17.0 2 92.0 ? 6.0 4 47.0 ? 18.0 4 78.0 ? 11.0 Diet richnesst 4 12.0 ? 2.7 2 11.5 ? 0.5 4 13.8 ? 1.2 4 11.5 ? 0.9

* Two deer were removed during the supplementation phase due to health concerns. t The proportion of rare plants (< 5% of the standing crop) in the diet. t Diet richness is defined as the number of plant species in the diet.




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ment (Table 3). However, when the contributions of the pelleted ration are included with those of native forages, average CP and DE concentrations in the diet increased significantly for deer (P < .05) and goats (P < .01).

DISCUSSION

The simultaneous encounter model predicts that animals should feed more selectively (i.e., specialize) in

response to an increased abundance of high-quality foods (Waddington and Holden 1979, Stephens and Krebs 1986). Although results from this study were not statistically definitive, trends in the data supported predictions from theory. When native forages were supplemented with a high-quality pelleted ration, both deer and goats responded by increasing foraging effort (i.e., movement rates while foraging) and by feeding more selectively. Supplemented animals increased their

IDeef CGoats Forage Availability

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use of high-quality native forages, and as a result, average diet quality increased.

As predicted, both deer and goats responded to habitat enrichment by increasing their use of rare, high- quality forages. Although diet richness remained relatively unchanged, the overall contributions of rare forages to deer and goat diets increased significantly following habitat enrichment (Fig. 1). Apparently, the availability of the pelleted supplement altered the

"constraint assumptions" affecting forage selection (Stephens and Krebs 1986:38). By consuming pellets, animals obtained a larger portion of their nutritional needs in a shorter period due to the high quality of the pelleted ration (DE and CP) and reductions in the time required for food searching (the pelleted ration was offered ad libitum at one location in each enclosure). Thus, consumption of the high-quality pelleted ration may have reduced time constraints and their influence

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Digestible Energy Concentration (kJ/g) FIG. 3. Distributions of digestible energy concentrations in native forages available and consumed by supplemented and

nonsupplemented white-tailed deer and Angora goats.




on diet selection, and allowed animals to invest more time selecting each gram of forage ingested. If time constraints on search time resulted in animals including less desirable food items in the diet, we anticipated that the relaxation of this constraint would permit animals to forage more efficiently (i.e., increase nutrient capture rates). Conversely, if diet selection was not limited by search time constraints, we predicted diet composition would not be affected by the removal of time constraints. The observed responses of deer and goats to habitat enrichment (i.e., increased mobility, increased selectivity, and dietary shifts) support the hypothesis that time constraints were affecting diet selection and diet quality.

Use of the simultaneous encounter model to explic- itly predict deer and goat diet selection requires observers to have complete knowledge of the factors influencing each foraging decision (i.e., net rewards). This model assumes both foraging rewards (i.e., energy, protein) and search and handling time influence animal feeding behavior and diet selection. Thus, the simultaneous encounter model may be inappropriate for an- alyzing large herbivore foraging decisions due to the complexity of the food acquisition problems confront- ing large generalist herbivores (Owen-Smith and No- vellie 1982, Hudson and White 1985, Spalinger and Hobbs 1992). The forages consumed by ruminants contain relatively dilute concentrations of most nutrients. Furthermore, because plants vary in their nutritional qualities, ruminants must invest considerable time and effort foraging each day in order to locate and consume enough acceptable plant material to meet their nutritional requirements (Risenhoover 1987). Conse- quently, large herbivores are required to make consid- erably more foraging decisions per day compared to other consumers (Chacon and Stobbs 1976, Senft et al. 1987).

In our trials it was not possible to accurately assess the instantaneous availability and quality of all prey items (forages) being considered by individual deer and goats. We estimated relative forage availability in enclosures based on conditions prior to animal exposure

to resources. However, because animals had previous experience with available forages, enclosures were relatively small, and trials spanned 7 d, we assumed animals were adequately aware of the feeding opportunities in enclosures. Regardless, this assumption was consistent between trial phases.

For several variables (i.e., movement rate, selectivity, diet quality), differences between trial phases were large but not statistically significant. The lack of significance between trial phases can be attributed to variability in the behavior of individual deer and goats, and to small sample size. For this study, the availability of animal subjects was limited, and was further reduced during the supplementation phase when two deer were taken off trial due to health concerns. Variability in the behavior of individuals may be related to genetics (Ar- nold 198 1, Ritchie 199 1), experience, and learning (Provenza and Balph 1987), differences in physiolog- ical state (Hudson and White 1985), and perhaps, differences in resources and their spatial distributions in trial enclosures. In this study, the availabilities of rare plant species varied among enclosures. Although changes in foraging behavior could not be explained by the availability of forage classes (Table 1), animal responses to these rare plants and their spatial distributions may have contributed to the observed variation in behavioral responses.

Overall, four goats were more responsive to supplementation than were two deer. Goat movement rates while foraging increased 70% in response to enrichment compared to only 34% for deer. Similarly, supplementation produced larger increases in diet selectivity (31 vs. 17%) and dry matter intake (51 vs. 8%) for goats than for deer. Differences between deer and goat responses to habitat enrichment may be explained by the high degree of selectivity exhibited by deer prior to supplementation. Compared to goats, deer selectivity and foraging effort was already high prior to supplementation (Table 3). Thus, although deer selectivity increased following supplementation, the magnitude of the increase may have been constrained by the limited opportunities to improve performance. The large dif-

TABLE 3. Diet quality of supplemented and nonsupplemented (control) white-tailed deer and Angora goats at the Texas Agricultural Experiment Station, Sonora, Texas. Values represent crude protein and digestible energy of native forages only.

Crude protein (%) Digestible energy (kJ/g)*

Control Supplemented Control Supplemented

n X S1SE n X S+1SE n X S1SE n X+1SE

Deer:t Available 4 7.9 ? 0.5 2 7.7 ? 0.4 4 1.9 ? 0.2 2 2.1 ? 0.1 Consumed 4 10.6 ? 0.8 2 11.7 ? 0.2 4 2.1 ? 0.1 2 2.1 ? 0.1

Goats: Available 4 8.0 ? 0.5 4 8.8 ? 0.2 4 1.9 ? 0.1 4 2.0 ? 0.1 Consumed 4 10.1 ? 0.8 4 10.9 ? 0.5 4 2.1 ? 0.1 4 2.4 ? 0.1 * Digestible energy was calculated as the product of in vitro digestible organic matter (%) and gross energy (kJ/g). t Two deer were removed during the supplementation phase due to health concerns.




ferences in diet selectivity and movement rates between deer and goats observed in this study are consistent with results from comparative studies of deer and goat feeding behavior at the Sonora Research Cen- ter (R. A. Jacobson, unpublished data).

Given the high quality of the pelleted ration, it re- mains unclear why deer and goats continued to utilize native forages. The concentration of DE in the pelleted ration (16.0 kJ/g) far exceeded the highest concentration found in native forages (best species, 13.6 kJ/g). One possible explanation is that deer and goat forage selection was influenced more by the concentration of CP in plants. If this hypothesis is true, deer and goats should have consumed only native forages containing CP concentrations - 16%. The distributions of CP in native forages consumed (Fig. 2) indicated that the diets of both ruminants contained forages with lower concentrations of CP. It is possible that, during the supplementation phase of the experiment, deer and goat use of native forages was not related to forage quality due to the ad libitum availability of the pelleted ration. The nutritional qualities of this ration far ex- ceed those required by either ruminant (NRC 1981, Verme and Ullrey 1984). This apparent paradox has been reported previously by others examining the influence of supplemental feeding on ungulate diet selection (Verme and Ullrey 1984, Schmitz 1990).

Supplementation is commonly used in range and wildlife management to maintain animal condition during periods of nutritional stress (Baker and Hobbs 1985, Holechek et al. 1989). Our results suggest supplementation may be disruptive to normal behavioral processes that reduce overgrazing of rangeland resources by wild and domestic herbivores. Under free- ranging conditions, animals normally disperse from habitats where forage resources have become depleted (Arnold and Dudzinski 1967). Supplementation may disrupt this process by allowing animals to continue to concentrate in areas where resources have been heavily utilized. When supplemented, animals can avoid low-quality forages and selectively consume re- maining palatable plant species. The longer the period of supplementation, the greater the likelihood of excessive utilization of preferred plants. Extended grazing pressure may lead to the loss of palatable plant species and eventually to simplification of vegetative communities (Holechek et al. 1989, Briske and Heit- schmidt 1991).

Short- and long-term responses of animal popula- tions to supplemental feeding are largely unknown (Boutin 1990). The supplemental feeding of wild and domestic ungulates may alter patterns of animal dis- persion on the landscape and may lead to the development of distinctly different plant communities (reviewed by Crawley 1983). These changes may affect the distribution of other animal species on the landscape, and eventually, impact ecosystem processes such

as energy flow and nutrient cycling (Briske and Heit- schmidt 1991).

Advocates of the use of supplements argue that the availability of a high-quality ration reduces animal de- pendence on native forages during times of nutritional stress (Vallentine 1990). Their basic premise is that animals will prefer to consume the supplement, thereby reducing grazing impacts on native plants. In our study, deer and goats continued to utilize native forages (30 and 21% of ingested dry matter, respectively) when provided ad libitum access to a high-quality pelleted ration. Thus, our results do not support this hypothesis.

Changes in deer and goat feeding behavior due to enrichment were variable and statistical significance was not detected for all variables. Our inability to reject null hypotheses (i.e., no significant change due to supplementation) was mostly a function of variation in individual behavior. Additional studies using larger sample sizes across a range of habitat quality are needed to delineate animal dietary responses to habitat enrichment. Experiments in controlled environments will be necessary to understand the effects of habitat heterogeneity and plant spatial distributions on individual animal foraging decisions.

ACKNOWLEDGMENTS

We wish to thank J. A. Bailey, W. E. Grant, R. J. Hudson, K. J. Jenkins, L. A. Renecker, N. J. Silvy, and C. A. Taylor for their helpful comments on an earlier draft of this manuscript. We also thank C. Jackson and R. A. Jacobson for field assistance. This research was supported by the Texas Agri- cultural Experiment Station and is contribution TA-30335.

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Effects of Habitat Enrichment on Patterns of Diet Selection