Herbivore population regulation and resource heterogeneity in a stochastic environment.

Large-mammal herbivore populations are subject to the interaction of internal density-dependent processes and external environmental stochasticity. We disentangle these processes by linking consumer population dynamics, in a highly stochastic environment, to the availability of their key forage resource via effects on body condition and subsequent fecundity and mortality rates. Body condition and demographic rate data were obtained by monitoring 500 tagged female goats in the Richtersveld National Park, South Africa, over a three-year period. Identifying the key resource and pathway to density dependence for a population allows environmental stochasticity to be partitioned into that which has strong feedbacks to population stability, and that which does not. Our data reveal a densitydependent seasonal decline in goat body condition in response to concomitant densitydependent depletion of the dry-season forage resource. The loss in body condition reduced density-dependent pregnancy rates, litter sizes, and pre-weaning survival. Survival was lowest following the most severe dry season and for juveniles. Adult survival in the late-dry season depended on body condition in the mid-dry season. Population growth was determined by the length of the dry season and the population size in the previous year. The RNP goat population is thereby dynamically coupled primarily to its dry-season forage resource. Extreme environmental variability thus does not decouple consumer resource dynamics, in contrast to the views of nonequilibrium protagonists

Prediction of intake and digestion in ruminants by a model of rumen kinetics integrating animal size and plant characteristics

Simulation modelling was used to investigate interactions between forage degradation characteristics, rumen processes and body weight, and to predict the voluntary food intake and digestion of a range of forages. Predicted voluntary intake and digestion agreed well with empirical data, explaining 61 and 70%, respectively, of variance in observed values. Since the data covered a wide range of animal weights and forage qualities, these results suggest that the model is a useful means of integrating the effects of animal and forage variables. Interactions were examined between animal weight and diet quality, as defined by the proportion of potentially digestible cell contents and cell walls and their rates of digestion. Retention time of food in the digestive tract was shown by regression to scale with W⁰·²⁷. The time taken to comminute large fibre particles also scaled with W⁰·²⁷. Longer retention of digesta by large ruminants increases digestive efficiency compared with small animals and would permit them to survive on lower-quality foods. The model showed that maximum intake of metabolizable energy scales with c. W⁰·⁸⁷, greater than the scaling of maintenance with W⁰·⁷

Resource partitioning by ungulates on the Isle of Rhum

This paper describes the seasonal changes in vegetation community use by red deer, cattle, goats and ponies on the Isle of Rhum, Scotland. During the winter, when food resources were of low abundance and digestibility, the ungulates showed extensive resource partitioning. During the summer, when resource availability and digestibility was high, the grazing species, red deer, cattle and ponies congregated on the vegetation communities which contained high biomasses of a high quality resource, mesotrophic graminoids and forbs. Goats, with a digestive system adapted to dealing with browse, foraged primarily on the communities dominated by dwarf shrubs. The patterns of resource use in this group of ungulates are discussed in relation to competition; species had relatively exclusive esource use during periods of low food availability during tye winter and had a high degree of resource use overlap when food was abundant during the summer. This suggests that there was little competition for food during the summer and that exploitative competition for the high quality foods led to resource partitioning during the winter. Senarios are described which predict the pattern of resource use between two species (one competitively superior to the other on the preferred resource) utilizing mutually or exclusively preferred resources. A model developed by Illius and Gordon (1987), based on the allometry of metabolic requirements and bite size, is used to provide a mechanistic explanation for the observation that the red deer were able to exploit the high quality plant communities during the winter, whereas the cattle moved off to feed on poorer quality communities at this time

Modelling the nutritional ecology of ungulate herbivores: evolution of body size and competitive interactions

A simulation model is used to quantify relationships between diet quality, digestive processes and body weight in ungulate herbivores. Retention time of food in the digestive tract is shown by regression to scale with W⁰·²⁷, and to be longer in ruminants than in hindgut fermenters. Allometric relationships between whole gut mean retention time (MRT, h) and weight (W) were: MRT = 9.4 W⁰·²⁵⁵ (r² = 0.80) for hindgut fermenters and MRT = 15.3 W⁰·²⁵¹ (r² = 0.76) in ruminants. Longer retention of ingesta by large-bodied ruminants and hindgut fermenters increases digestive efficiency relative to small animals and permits them to survive on lower-quality foods. Compared with ruminants, hindgut fermenters' faster throughput is an advantage which outweighs their lower digestive efficiency, particularly on poor quality foods, provided that food resources are not limiting. This suggests that the predominance of ruminants in the middle range of body weights results from their more efficient use of scarce resources under conditions of resource depletion. Considering only physical limitations on intake, the model shows that the allometric coefficient which scales energy intake to body mass is 0.88 in ruminants and 0.82 in hindgut fermenters. The advantages of large body size are countered by disadvantages where food quantity is limited, and we suggest that the upper limit to ungulate body size is determined by the ability to extract nutrients from feeding niches during the nadir of the seasonal cycle of resource quality and abundance

Constraints on diet selection and foraging behaviour in mammalian herbivores

In a recent critique of optimality theory, Gray (1986) questioned the validity and usefulness of the optimality approach to the empirical study of foraging behaviour. He called instead for an epigenetic approach integrating morphological, physiological and behavioural processes. Without denying the role of optimality theory as a quantification of the premise that natural selection leads to adaptation (see Krebs & McCleery 1984), it must be recognised that optimality is an obscure condition to the extent that constraints are poorly defined. In this paper, we attempt to clarify and quantify the morphological, physiological and behavioural constraints on foraging in mammalian herbivores