Habitat Heterogeneity and Foraging
Efficiency: an Individual-Based Model
Mitigating the Biotic Effects of
Overabundant Deer Populations in National Parks
Quantification of Foraging Movements
A Scale-Sensitive Evaluation of
Landscape Heterogeneity and its Effects on Plant-Animal Interactions
Modelling Tick-Host-Landscape
Interactions: an Individual-Based Approach
Effects of Habitat Enrichment on Patterns
of Diet Selection
Assessing Landscape Heterogeneity at
Different Spatial and Temporal Scales
The Impacts of Mining on Landscape
Composition and Structure
Group Foraging and Individual
Decisions: The Distribution of White-Tailed Deer in Patchy Environments
Assessing Short-Term Effects of
Intra- and Inter-Specific Competitors on Large Generalist Herbivores
Effects of Plant Spinescence on
Forage Intake by Browsing Ungulates
Foraging by Desert Rodents: Effects of
Spatial Scale and Predation
Spatial and Behavioral Responses of
White-Tailed Deer to Forage Depletion
Intake, Diet Utilization, and Digestive
Kinetics of White-Tailed Deer and Angora Goats
Giving-Up Density as an Indicator of
Habitat Quality for Northern Bobwhites
Influence of Forage Resources on the
Allocation of Foraging Effort by White-Tailed Deer and Angora Goats
Utilization of Internal and External
Markers to Estimate Digestibility
Terpene Concentrations in Balsam Fir:
Influence of Growth Form and Foliage Age
Analyzing the Effects of Moose Herbivory
on Balsam Fir Dynamics Using a Simulation Model
Habitat
Heterogeneity and Foraging Efficiency: an Individual-Based ModelJohn H. Roese, Ken L. Risenhoover, and L. Joe Folse
Foraging behavior is an individual rather than a population phenomenon, and is the result of the complex interaction between an animal and its environment. However, differences between individuals within a population are often ignored in ecological theory and models. When individual variation is trivialized, foraging is implicitly treated as a population level phenomenon. Each animal within a population must develop strategies and tactics that allow it to become a successful forager. Conventional approaches to studying foraging strategies are based on evolutionary assumptions and are of limited utility for exploring tactical questions of interest to biologists and managers. We developed a heuristic model which avoided broad-based strategic assumptions. We focused on the influence of temporal and spatial heterogeneity on the foraging tactics and efficiency of an individual moose (Alces alces). Although the model is quite simple, it includes components such as plant size and bite size that are often overlooked in foraging models. The combination of a temporally and spatially heterogeneous habitat, a conservative representation of the information available to the forager and simple foraging rules produced complex behavior. Our model demonstrated that short-term foraging efficiency is inherently variable. Intake rate fluctuated dramatically, even in homogeneous habitats. Spatial and temporal variability in the habitat increased the variability in foraging performance. The magnitude of this variability could be critical to the survival of time-limited foragers such as moose. Our model was designed to operate at a time step short enough to capture the short-term variability in intake rate. It is clear from our simulations that the structure and variability of resources in the environment may be critical to foraging performance. The significance of habitat variability is especially important from a modeling viewpoint as it emphasizes the advantages of building foraging models at the level of the individual. Because of its unique position in the environment, each individual in a population will be faced with a spatially and temporally unique distribution of resources. Models which incorporate these differences may result in significantly different population level responses than would be predicted by a population level model in which each individual is presented with identical resources.
Mitigating
the Biotic Effects of Overabundant Deer Populations in National ParksKen L. Risenhoover, H. Brian Underwood, and Jerry L. Cooke
Over-abundant white-tailed deer popu-lations can adversely impact features of landscapes being managed for esthetic or historical values. Under such circumstances active intervention may be required to preserve unique attributes of these areas, and to achieve existing management objectives and policies. However, success at managing deer populations is likely to vary among Parks depending upon their size, position in the landscape matrix, and their attractiveness to deer in neighboring areas. Given the significance of these variables, it is difficult to predict which of the potential treatment options (i.e., harvest, habitat modification, fencing, sterilization, etc.) is most likely to accomplish management objectives (i.e., reduce deer densities). Justification for deer management programs are especially important in National Parks due to the increased attention afforded them by the public and special interest groups. We present a simulation environment which combines spatially-explicit habitat features from a Park's existing geographic information system (GIS) with an individual-based deer population model to evaluate the likelihood that alternative management strategies will achieve Park management goals. The system imposes user-defined treatments on the deer population and/or landscape, and monitors the movements and spatial responses of the population over a five-year period, and performs multiple iterations to assess the probability of possible outcomes. Deer responses to different treatment scenarios can be viewed as numerical changes in population size and composition over time, or they can be viewed visually by contrasting changes in deer spatial distribution using spectral maps generated by the simulation. Additional features of the simulation include identifying potential travel corridors and areas of seasonally high use by deer. Treatments can be designed and applied to populations or a specific sex-age class, or to specific areas delineated by the user. Results of treatment scenarios can be contrasted by users and provide strategic input which can be incorporated into Park management and planning activities.
Quantification
of Foraging MovementsJohn H. Roese and Ken L. Risenhoover
Herbivores, especially those in poor quality habitats, are often time-constrained due to their digestive physiology. Random movement through a habitat is rarely a productive method of finding or exploiting resources, so the need to search for high quality forage is critical. Consequently, movement patterns are of interest. Conventional foraging models often ignore or trivialize movement patterns by assuming that the animal has complete knowledge of the habitat. It is more likely, however, that behavioral mechanisms (tactics) have evolved to improve search efficiency without requiring omniscience. Our goal was to develop a method of analyzing foraging movements which could be used as an index of animal condition and habitat quality. We developed a simulation model of foraging in which movement was represented stochastically as a sequence of steps in response to perceived resources. A series of simulations were conducted in which various characteristics of the forager (e.g., lateral reach, perceptual ability) and the habitat (e. g., plant density, plant size/ variability, bite size/variability) were systematically varied. A step-by-step record was kept of the simulated foraging movements under each set of conditions. Univariate, multivariate, and iconic exploratory data analysis techniques were used to compare foraging paths. Iconic data analysis techniques proved most useful for recognizing trends in complex movement patterns. Icons are graphical representations of complex data. Because all of the data are represented in the icon, it is often possible to highlight subtle patterns undetectable by traditional data reduction methods. Chernoff faces are iconic representations of human faces. Each variable of interest is assigned to a particular feature of the face such as the curvature of the mouth, slant of the eyebrows, or length of the nose. Humans can often detect subtle patterns in these facial features that elude a quantitative description. We used Chernoff faces to help identify the variable(s) responsible for trends in the foraging paths. Replicate paths within each treatment were averaged and each path variable assigned to a particular facial feature. Chernoff faces revealed consistent patterns in foraging paths between treatments. Increasing the lateral reach produced consistent changes in seven features of the Chernoff faces. An increase in the size of the forager's perceptual field produced consistent trends in six facial features. Consistent changes in these facial features correspond to trends in foraging movements patterns. These trends indicated that increasing the lateral reach of the animal resulted in a more directed path, while increases in the perceptual field led to increasingly complex movement patterns. Trends identified by iconic analysis indicated relationships between measurable components of the foraging paths and characteristics of the foraging animal. Repeated measures linear regression analysis revealed significant linear correlations between characteristics of the forager and foraging movement patterns.
A
Scale-Sensitive Evaluation of Landscape Heterogeneity and its Effects on Plant-Animal
InteractionsMarkus G. Walsh, Ken L. Risenhoover, Steven B. Murden, Christine M. Doucet, and Barbara A. Walsh
Heterogeneous landscapes alter ecological interactions by modifying fluxes of organisms, material and energy. Most constituent ecological processes and patterns are regulated by dynamics occurring simultaneously at several spatial or temporal scales. However, it remains unclear how animals perceive and integrate these patterns. Consequently, a given landscape has functionally different geometries that depend in part on the scale of perception by animals. However, resource geometry is also rapidly modified in response to seasonality or through disturbance such as fire. It is unknown how these factors influence animal perception of resource distributions. The objectives of this study are to evaluate landscape level plant-animal interactions at multiple spatial scales and to develop a series of predictive spatial models that characterize specific plant and animal responses to landscape resource heterogeneity. Landscape heterogeneity will be evaluated using conditional spatial simulation of ground data in conjunction with digital imagery obtained from a newly developed tethered blimp remote sensing platform. The project will be carried out as an interdisciplinary research effort involving the Departments of Rangeland Ecology and Management, Wildlife and Fisheries Sciences, and Veterinary Parasitology at the Texas Agricultural Experiment Station in Sonora, Texas. Funding is provided through the Research Enhancement Program of the Texas Agricultural Experiment Station.
Modelling
Tick-Host-Landscape Interactions: an Individual-Based ApproachKen L. Risenhoover, Wen Yan, John H. Roese, P. D. Teel, and Jerry L. Cooke
We developed a rule-based, event-driven model of ruminant foraging behavior and used it to evaluate how resource abundance and heterogeneity influence deer and cattle movements and other processes responsible for the spread and maintenance of "cattle fever" tick populations on the landscape. Forage resources were modelled as individual plant objects and subdivided into 5 forage classes (1 forb, 1 grass, and 3 browse). Each plant was characterized in terms of its consumable biomass, bite size, and fiber and energy content. Individual forager objects were characterized in terms of the physical, physiological and cognitive attributes of either white-tailed deer (Odocoileus virginianus) or cattle. Animal movements were modelled as a sequence of steps in response to perceived resources. Tick dispersion was a function of host-mediated dispersal on the landscape. As predicted, forage abundance and heterogeneity strongly influenced deer and cattle movements and residence patterns on the landscape. Areas providing forage resources attractive to both deer and cattle are "hot spots" for tick attachment and disease transmission. Results demonstrate that temporal and spatial attributes of tick detachment from one host and the likelihood of new host encounter at the at location determine rates of tick dispersal and population size. This project was funded by the Texas Agricultural Experiment Station.
Effects of
Habitat Enrichment on Patterns of Diet SelectionSteven B. Murden and Ken L. Risenhoover
Foraging theory suggests that patterns of diet selection by ruminants are determined largely by animal nutritional status, avoidance of toxic plant secondary compounds, and the physical and logistical constraints on daily intake. Theoretically, the decision to consume a food item is a function of its value relative to alternative food items and the costs associated with search and handling time. In this model, foods are ranked according to their net yield of some currency per unit handling time, and are included in the diet provided they do not reduce foraging efficiency. This study tested diet selection theory and its predictions as they relate to ruminants. It examined changes in patterns of diet selection in response to an increased abundance of high-qualityfoods. When a high-quality supplement was provided ad libitum, both white-tailed deer and Angora goats 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 and digestible energy, which resulted in an increase in average diet quality. Our results suggest that supplemental feeding 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. Supplementation may disrupt this process by allowing animals to continue to concentrate in areas where resources have been heavily utilized.
Assessing
Landscape Heterogeneity at Different Spatial and Temporal ScalesSteven B. Murden, Markus G. Walsh, and Ken L. Risenhoover
The spatial and temporal heterogeneity of rangeland resources greatly influence the distribution and performance of herbivores occurring on the landscape. Indeed, the success of grazing systems is largely dependent on the resource manager's understanding of landscape heterogeneity and its influences on animal dispersion and performance, and of how the accumulation of grazing pressure affects the qualities of the individual plants being consumed. Although the importance of landscape complexity is widely acknowledged, current techniques for assessing vegetative resources and their nutritional characteristics ignore the importance of spatial and temporal heterogeneity, and the influence of scale on the accuracy of measurements. This project will develop and evaluate new approaches for using standard photography and videography to spectrally-define rangeland vegetative components, and will provide a basis for quantifying the spatial and temporal heterogeneity of forages and their characteristics at the Texas Agricultural Experiment Station in Sonora, Texas. Our research objectives are: to develop and test approaches for utilizing a hierarchical series of spectrally- classified aerial photographs to map and monitor spatial and temporal changes in vegetative resources on the landscape; to test the validity of spectral image classifications by ground- truthing identified sites and assessing true differences in their physical, vegetative, and nutritional characteristics; and to examine the short-term and long-term effects that grazing and prescribed burning have on the spatial heterogeneity and temporal dynamics of vegetative resources typical of oak-woodland savanna habitats in the Edward's Plateau. This study is being funded by the Texas Agricultural Experiment Station Research Enhancement Program.
The Impacts
of Mining on Landscape Composition and StructureKen L. Risenhoover, Jason L. Price, Jerry L. Cooke, and Markus G. Walsh
Strip mining is a large-scale disturbance that alters landscape structure and complexity and greatly impacts wildlife habitat. Impacts frequently associated with mining include reductions in habitat availability, fragmentation, and/or changes in the spatial pattern of habitat resources on the landscape. These landscape changes have an impact on species richness, population dynamics and viability and the biological diversity of communities (Forman and Godron 1986, Morrison et. al. 1992). This study documents changes in habitat composition and structure resulting from 20 years of strip mining for lignite coal at the Big Brown Mine in east Texas. Habitat composition of the mine site and surrounding area was delineated using aerial photographs taken in 1969 and 1989. Patch size and shape, connectivity and spatial arrangement were analyzed using a geographic information system (GIS). Following 20 years of strip mining, forest cover has declined 26 percent in the mine lease area compared to only 5 percent in the surrounding area. Reclamation activities have attempted to restore woody cover on 5 percent of the mined landscape. Mining has fragmented forest cover and eliminated patches of more than 200 hectares. The resulting landscape contains fewer forest patches that on average are smaller and more isolated in their distribution. Funding was provided by the Texas Utilities Mining Company, Fairfield, Texas.
Group
Foraging and Individual Decisions: The Distribution of White-Tailed Deer in Patchy
EnvironmentsStephan G. Kohlmann and Ken L. Risenhoover
Foragers in heterogeneous (patchy) environements must select a patch based on many factors. Competition among animals exploiting the same set of patches simultaneously will cause forage intake by the individual to decline, and may force some animals to utilize patches of lesser value but with fewer competitors. Theoretically, this process is expected to produce an equilibrium distribution of foragers among patches in relationship to patch profitability. A special case of this equilibrium distribution is the "ideal free distribution" (IFD), where all individuals receive equal foraging gains. It assumes that all animals accurately assess instantaneous changes in patch profitability and that foragers are "free" to exploit any patch at any time (i.e. no resource defense exists). We evaluated the validity of the ideal free distribution hypothesis (IFD) for a group of seven captive white-tailed deer (Odocoileus virginianus) feeding at two artificial patches having different reward opportunities for individuals. According to the IFD model, we expected the deer distribution to match the distribution of the foraging rewards (i.e., richer patches should contain more deer than poor patches). Although in our experiments observed more deer using the poorer quality patch than predicted from the quantitative distribution of resources. Consequently, deer foraging in the rich patch received higher benefits than those feeding in the poor patch. Because all deer used both patch types with similar frequency and intake rates did not differ among individuals, we concluded that individual deer did not differ in their competitive abilities. Switching rates were highest immediately following initiation of a new reward ratio, but declined rapidly over time. We conclude that foraging decisions and patch use of deer are the result of a highly dynamic process constrained by individual perception and information gathering through sampling. This project was funded by the Texas Agricultural Experiment Station.
Assessing
Short-Term Effects of Intra- and Inter-Specific Competitors on Large Generalist HerbivoresSteven B. Murden, and Ken L. Risenhoover
Forage competition among large generalist herbivores is difficult to assess because of problems associated with identifying "acceptable" food abundance for different herbivores and the dynamic nature of the diet selection process. We present a conceptual model of forage competition and identify potential short- and long- term processes affecting animals competing for forage. We evaluated model predictions, experimentally, by exposing two different generalist herbivores, white-tailed deer (Odocoileus virginianus) and Angora goats, to increasing numbers of intra- and interspecific competitors and recording animal foraging behavior and feeding efficiency over sequential 7-day periods. Experiments were conducted in portable enclosures on native rangeland, and animals were not supplementally fed. Enclosure size was adjusted to maintain a constant herbivore density (69 animals/ha). Increasing numbers of competitors had negligible effects on patterns of forage selection, movement, intake rates, and feeding efficiency of individuals of either herbivore species. Foraging behaviors varied among individuals, but, in general, were similar across treatments. Although forage availability varied among enclosures, variance in resource abundance and heterogeneity could not explain animal feeding behavior and performance during trials. Diet overlap among deer and goats was relatively high (56%), but did not produce the changes in animal movement rates, feeding rates, and nutrient ingestion rates predicting if animals were competing for forages. Our results suggest that large generalist herbivores are capable of compensating for the short-term influences other herbivores have on forage availability; even when animals are confined and at very high densities. Our results demonstrate that diet overlap, by itself, does not always produce the short-term negative effects on animal foraging efficiency anticipated if animals are competing for forage. We recommend that caution be observed when making inferences regarding competitive interactions among large generalist herbivores when inferences are based solely on measures of dietary overlap.
Effects
of Plant Spinescence on Forage Intake by Browsing UngulatesSteven B. Murden, Ken L. Risenhoover, and Joseph K. Treybig
Thorns and spines are considered anti-herbivore defense mechanisms, but their influence on feeding processes affecting ungulate foraging efficiency is not well understood. Spines and thorns can potentially retard or hinder herbivory in several ways: (1) thorns and spines may increase the time required to grasp and harvest leaves; (2) they may cause herbivores to select and ingest smaller bites; or (3) they may cause herbivores to utilize alternative foods. We tested these hypotheses by observing the foraging responses of Angora goats presented various woody plant species occurring at the Texas Agricultural Experiment Station near Sonora, Texas. Bumelia lanuginosa and Ziziphys obtusifolia, both with long, straight, rigid thorns, were used. Acacia greggii, which has short, curved spines, was also used. Preliminary results indicated that woody plant species having short, curved spines did not significantly alter Angora goat feeding behaviors. Goats browsing on species having straight, rigid thorns, however, had significantly reduced bite rates, bite sizes and intake rates. Our results suggest that woody plants with long, rigid thorns are better defended against herbivory by browsing ungulates. This anti-herbivore defense strategy may strongly influence ungulate diet selection. Funding for this project was provided by the Texas Agricultural Experiment Station.
Foraging by
Desert Rodents: Effects of Spatial Scale and PredationStephan G. Kohlmann, Jeff Peacock, and Ken L. Risenhoover
Deserts are spatially and temporally dynamic systems in which resources change unpredictably depending on climatic factors and microsite characteristics. Foraging in such environments is constrained by the spatial distribution of food patches and their predictability. Foraging theory commonly assumes omniscient foragers that have perfect knowledge of the resource distribution; this hypothesis clearly does not apply well to desert communities. This study investigated foraging decisions of nocturnally active rodents in the Black Gap Wildlife Management Area in west Texas. We established six artificial food patches at three sites in scrub vegetation. The total amount of resources present in each set of patches was held constant, but individual patches within each set varied along a resource gradient. We manipulated the distance between food patches and their relationship to cover and monitored their relative use by rodents. Results indicated that foraging by desert rodents is density-independent (i.e., at random), but patches having higher predation risk (open habitat) were used less often than those offering better concealment from predators. These results support our original hypothesis that foraging strategies of desert rodents have evolved under constraints imposed by heavy predation and unpredictable dynamics of food patches rather than under food limitation and the need to assess food resources of patches. This project was supported by the Rob and Bessie Welder Wildlife Foundation and the Texas Parks and Wildlife Department.
Spatial
and Behavioral Responses of White-Tailed Deer to Forage DepletionStephan G. Kohlmann and Ken L. Risenhoover
Natural populations of herbivores often face nutritional bottlenecks during droughts, winter or under heavy competition with livestock. Because all ungulates feed selectively to some degree, forage availability and quality will be reduced under these conditions and animals must increase foraging efforts to survive. Behavioral responses to forage depletion may consist of reduced selectivity, increased bite size and movement to less depleted areas. Hence, spatial distribution of wild herbivores should reflect available foraging conditions. We tested predictions regarding the effects of forage depletion on foraging effort, diet selection and quality by observing the behavior of captive white-tailed deer foraging within four 150-square-meter enclosures with grazing exposure varying from 16 to 64 days. Deer foraged mostly on forbs in intermediately depleted patches, but used browse heavily in the most and least depleted patches. As forage became sparse, deer took larger bites and foraged less selectively. Patch use by deer was heavily skewed toward the heavily and lightly grazed environments, while moderately grazed patches were used the least. Deer residence time in patches was predicted by crude protein intake rates and not by biomass availability. The high use of the heavily grazed patch was not anticipated and was the result of the production of protein-rich regrowth. We conclude that vegetative biomass alone is a poor indicator of foraging rewards for selectively feeding herbivores. This project was funded by the Texas Agricultural Experiment Station.
Intake, Diet
Utilization, and Digestive Kinetics of White-Tailed Deer and Angora GoatsGlenn W. Stout, Ken L. Risenhoover, and Roy A. Jacobson, Jr.
The ability of domestic and wild ruminants to utilize forage in the environment is constrained by forage aquisition and assimilation. Forage assimilation involves a dynamic relationship between intake, digestion, and digestive kinetics, and has been poorly described for wild ruminants. The purpose of this study was to compare parameters of intake and digestion for white-tailed deer (Odocoileus virginianus) and angora goats, and explain their strategies of diet utilization from estimates of digesta flow and volume of the gastrointestinal tract. Two diets, carelessweed (Amaranthus palmeri) and live oak (Quercus virginiana) were fed fresh in two separate digestion trials. Strategies of diet utilization were contrasted from estimates of digesta flow and volume of the gastrointestinal tract. We predicted intakes were higher for deer on both diets, while goats digested both diets more completely. Both deer and goats failed to meet their daily energy requirements on either diet, and protein requirements were met only on the carelessweed diet. For both diets, deer exhibited a higher weight-specific compartmental mass compared to goats. Bulk fill appeared to limit further intake by deer on the carelessweed diet. Potential factors limiting goat intake of carelessweed remain unclear. Our results suggest the nutritional value of live oak as a forage for deer and goats has been over emphasized.
Giving-Up
Density as an Indicator of Habitat Quality for Northern BobwhitesStephan G. Kohlmann and Ken L. Risenhoover
Food, water, and cover are often heterogeneously distributed in natural environments. Thus, foragers should select patches offering the highest rewards and should stay in such areas only as long as their net gain is higher than that of comparable alternatives. The resource density at which a forager abandons a given patch is termed the giving-up density (GUD) and this measure may be a powerful predictor of overall habitat quality and therefore may be useful for management. According to this theory, when resources are sparse, animals should exploit patches to a lower resource density than when resources are abundant and richer patches can be found easily. We tested the GUD model using captive northern bobwhites in controlled environments using trays filled with pre-determined amounts of seed mixed into sand. We predicted birds would utilize patches containing the highest seed density, ultimately leading to the equalization of patch quality. However, selection may depend on the prior knowledge the forager has about the resource distribution and the spatial arrangement of patches. When the placement of patches was predictable, birds selected richer patches over poorer ones. Birds also foraged randomly when the distance between patches was negligible. However, as patches became more spatially distinct, GUDs equalized, indicating that birds were able to select the more profitable patches. Overall, our experiments suggest that GUD may have utility as a predictor of habitat quality for wildlife. Funding for this project was provided by the Rob and Bessie Welder Wildlife Foundation.
Influence
of Forage Resources on the Allocation of Foraging Effort by White-Tailed Deer and Angora
GoatsRoy A. Jacobson, Jr., Ken L. Risenhoover, and Charles A. Taylor, Jr.
We compared seasonal patterns of foraging effort by white-tailed deer (Odocoileus virginianus) and Angora goats feeding in two pastures under different management regimes (burned, grazed) to investigate how forage availability and quality affected feeding behavior. On average, deer movements while foraging were two to three times more rapid compared to goats during both fall and summer in both pastures. Deer movements remained relatively constant in both pastures and seasons, whereas goat movement rates increased during fall in response to declining forage availability. Deer and goat movement rates declined significantly during the first fifteen to twenty minutes of foraging bouts as animals adjusted to available resources in trial enclosures. Diel patterns of feeding activity differed between ruminants and varied among individuals. Deer exhibited a polyphasic pattern of activity and averaged thirteen to fourteen hours of feeding activity per twenty-four-hour period. Goats were more diurnal and spent less time (11.3 hours) foraging per day. White-tailed deer were highly mobile, selective feeders, and invested more effort in forage acquisition compared to goats during all season-pasture combinations. Compared to deer, Angora goats were less selective, localized feeders and appeared to depend more upon their ability to digest a wider array of forages. These differences in feeding strategy suggest that deer and goats may respond differently to forage availability and to habitat modification. Landscape pattern and structure should be considered when predicting impacts of range management on these ungulates.
Utilization
of Internal and External Markers to Estimate DigestibilityG. W. Stout, Ken L. Risenhoover, W. C. Ellis, and R. A. Jacobson, Jr.
Particulate markers commonly are used to study the dynamics of digestion in the gastro-intestinal tract of large herbivores. Unfortunately, none of the commonly used markers fully satisfy the assumptions and criteria necessary to be considered an ideal marker. Ideal markers must not be absorbable or metabolizable, they must not affect or be affected by the digestive tract or its microbial populations, they must be intimately associated with the materials of interest, and the method of estimation in digesta samples must be specific and sensitive, and not interfere with other analyses. Theoretically, markers which are intrinsic to the particles of interest would best meet these criteria. The objectives of this study were to describe natural concentrations of internal rare earths occurring in range forage plants, and to evaluate their potential value as markers of digestibility and intake in comparison to other markers. Eleven forages were analyzed to determine naturally occurring concentrations of internal rare earths. Coefficients of dry matter digestibility (DMD) estimated using internal rare earths were compared to coefficients estimated from three other markers and in vivo estimates from four white-tailed deer (Odocoileus virginianus) and four Angora goats (on two diets, liveoak and carelessweed). The internal rare earths, scandium (Sc) and europium (Eu), consistently occurred in all forages. Marker occurrence was highly variable but not correlated with season, rainfall, or forage class. Internal rare earths over-estimated DMD in all but the deer-carelessweed trial. In all cases, the internal organic marker indigestible neutral detergent fiber (INDF) underestimated DMD. The external markers ytterbium (Yb) and samarium (Sm) accurately estimated DMD in all but the deer-carelessweed trial. Precise detection of internal rare earths, and thus poor estimation of DMD may have been influenced by high concentrations of the external rare earths, Yb and Sm.
Terpene
Concentrations in Balsam Fir: Influence of Growth Form and Foliage AgeMary Terra-Berns, Ken L. Risenhoover, and Howard J. Williams, Jr.
Empirical studies suggest that many plants have to herbivory by evolving chemical compounds that deter feeding by herbivores. The type of chemical defense employed by plants is influenced by the availability of resources in the environment. Deciduous plants growing in resource rich environments generally have adequate carbon and nutrients for growth and commonly utilize nitrogen-based defenses (i.e., phenolics). In contrast, coniferous plants occurring in relatively nutrient poor boreal forests normally have inherently slow growth rates, persistent foliage, and synthesize carbon-based defenses (i.e., terpenes). Although considered a generalist herbivore, the browsing patterns of moose at Isle Royale National Park indicate that these animals are highly selective when feeding on woody plants. It has been hypothesized by Risenhoover that moose (Alces alces) browsing patterns on balsam fir, an important winter food source, may be influenced by the browsing history on individual plants and by the resulting impact this herbivory has on the chemical defenses present in needles and twigs. This hypothesis is supported by the following observations: 1) When available, the foliage of windfallen balsam fir trees is heavily utilized; 2) New foliage on individual plants "releasing" during intervals of low moose browsing pressure appear to be less palatable; 3) Moose often crater through snow to feed on the lower twigs of releasing saplings and ignore lush new foliage on the same stem; and 4) Saplings in certain areas and exhibiting suppressed vertical growth are unbrowsed whereas saplings in other areas are heavily utilized by moose. Collectively, these observed patterns suggest possible differences may exist in the types and quantities of resin terpenes associated with foliage age and growth form. To evaluate this hypothesis we analyzed terpene concentrations in various growth forms and foliage ages of balsam fir and contrasted these values to patterns of herbivory by moose by Risenhoover at Isle Royale. As predicted, terpene concentrations were highest in "releasing" saplings and in young foliage. Concentrations of terpenes were negatively correlated to foliage age. Tissues collected from windfallen trees and from heavily browsed saplings had the lowest terpene concentrations. B-Pinene was the predominant terpene compound (62-74%) occurring in needles and twigs. A pinene, camphene, limonene and bornyl acetate also were present in relatively high concentrations. Ten other compounds previously reported to occur in balsam fir resin were either absent or present in small concentrations. Our results were generally consistent with the predictions of the "resource availability hypothesis." These observations lend support to Risenhoover's hypotheses that terpene concentrations influence patterns of moose herbivory on balsam fir, and help to explain moose foraging patterns at Isle Royale.
Analyzing the
Effects of Moose Herbivory on Balsam Fir Dynamics Using a Simulation ModelKen L. Risenhoover, M. Lynn Risenhoover, and Rolf O. Peterson
In most ecosystems, trophic dynamics are poorly understood because of the complexity of interactions between species (herbivory, predation, parasitism, etc.), and the paucity of long-term studies describing the role of these interactions. Long-term studies are useful for exposing the full range of interactions, and therefore, helping to clarify ecosystem processes. For example, large herbivores have a tremendous impact on ecosystem processes such as energy flow, nutrient cycling, and succession. Long time intervals may be needed to document herbivore impacts on these processes. Isle Royale National Park, located in Lake Superior, is well known for its ongoing studies (1958 to present) of moose-wolf predator-prey relationships. Despite the valuable contributions of this long-term research, the underlying mechanisms driving ecosystem processes in this island system remain poorly understood. Studies have focused on upper level trophic interactions (wolves and moose), and have ignored the importance of moose herbivory. Browsing by moose has had a major impact on the composition and structure of the island's vegetation. Recent studies have clearly demonstrated that long-term changes in forest structure have resulted as a result of herbivory by moose. These effects are especially evident in balsam fir, a dominant species in the forest canopy and a primary winter food for moose. An increasing number of balsam fir stems are repeatedly browsed by moose, preventing their vertical growth. Eventually, the current seed producing trees will die and there will be few replacements which could cause a shift in the composition of the forest. Ultimately, these changes in the forest may alter the food supply for moose and may increase their vulnerability to predation and increase their susceptibility to starvation during the harsh winters on Isle Royale. We are developing a spatially explicit simulation model to examine how moose herbivory will influence forest dynamics and trophic interactions at Isle Royale. This model will integrate available GIS databases with population models for moose, wolves and balsam fir. We anticipate that a simulation model of this type will provide insights into the mechanisms that drive the trophic interactions of this island system.