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This book is about ‘resource ecology', which we defined in our introductory chapter as "the ecology of trophic interactions between consumers and their resources". We have chosen to focus on a particular class of consumers, namely, large mammalian placental herbivores. All chapters in this book deal actually with ungulates (in the broad sense, thus including elephant), whether free-ranging or domestic, but we are certain that every chapter is of much use to understand other classes of herbivores, such as marsupials, herbivorous birds or even herbivorous reptiles. In the comment on Chapter 4, the definition of ‘resource' is given as "usable energy or any biotic or abiotic substance directly exploited by an organism, which includes food, nutrients, water, atmospheric gas compounds, as well as light, and the use of which can lead to the (temporary) exhaustion of that resource". The essence of the concept of ‘resource' is that organisms can compete for a resource and that it can be limiting the growth of individual organisms or of populations. In herbivores, the resource that is most interesting from a conceptual point of view, is herbage, because the feedback relations that exist between consumers and this type of resource (see for instance Van de Koppel et al. 2002; Rietkerk et al. 2002a; Van Langevelde et al. 2003). This of course does not deny the fact that other resources, such as water, or environmental conditions, such as temperature, can be very important factors to understand the distribution of herbivores (Bailey and Provenza, Chapter 2; Stein and Georgiadis, Chapter 3). From the consumer's perspective, acquiring sufficient resources, such as energy, nutrients and water, are conditions for life and reproduction. In resource ecology, foraging is the central process because it leads to growth, survival and reproduction of the animal. This book deals with foraging, and it ignores predation or disease and highlights only a restricted set of fitness parameters of the consumer. [ABSTRACT FROM AUTHOR]

The question Van Wieren and Van Langevelde (Chapter 11) are trying to find an answer to, namely "Why are there so many species?", and especially "Why are there so many herbivore species at some location?" is an intriguing one, but not a simple one. To solve this question, one must first look into the exact articulation of the question. The word "Why" is particularly knotty. The question could be reformulated as "What causes the existence of so many species?", but also as "What is the function of so many species?" or even "How did so many species evolve?". At first sight, Van Wieren and Van Langevelde deal with the first question, about the cause. However, a closer look at the text reveals that they try to find an answer to another question than the one they pose, namely, "What allows so many different species to co-exist?". This is a pity, because if they had tried to find an answer to the question about causality, then they would have taken, hopefully, an evolutionary and dynamic approach. Now their approach is static, and focuses on the conditionality instead of the causality. [ABSTRACT FROM AUTHOR]

This chapter tries to address the question "Why are there so many species?" with a focus on the diversity of herbivore species. We review several mechanisms of resource specialisation between herbivore species that allow coexistence, ranging from diet specialisation, habitat selection to spatial heterogeneity in resources. We use the ungulate community in Kruger National Park to illustrate approaches in niche differentiation. The habitat overlap of the ungulate species is analysed, continued with the overlap in diet and the spatial heterogeneity in resources. This focus on the constraints on species' exclusive resources is a useful tool for understanding how competitive interactions structure communities and limit species diversity. In explaining community structure of mobile animals, we argue that the existence of exclusive resources governed by spatial heterogeneity plays an important role. Tradeoffs between food availability and quality, food availability and predation risk, or food and abiotic conditions (different habitat types) may constrain competitive interactions among mobile animals and allow the existence of exclusive resources. We propose that body mass of the animals considered is crucial here as animals with different body mass use different resources and perceive spatial heterogeneity in resources differently. A functional explanation of the role of body mass in the structuring of communities is still lacking while the study of how much dissimilarity is minimally needed to permit coexistence between strongly overlapping species is still in its infancy. Nevertheless, a theoretical framework is emerging from which testable hypotheses can be generated. [ABSTRACT FROM AUTHOR]

This chapter deals with the issue of so-named ‘key resource areas'. These particular localities within a landscape are endowed with resources that allow many more animals to live there than would have been expected on the basis of the ‘general' features of that landscape or ecosystem. Scholte and Brouwer (Chapter 10) advocate the point that the resource under scrutiny in the ‘key resource area' is not by necessity herbage; it can also be water. In deserts, oases have in fact been considered as such for millennia and people found them even worth defending at quite great cost. This is confusing, though, because indeed water is a resource for the vegetation and indeed water is a conditional necessity for most animal species; however, it is not a key resource in the sense of Illius and O'Connor (1999), because once the conditionality of the presence of water is sufficiently met there will be no further increase in herbivores. Yet, Scholte and Brouwer rightfully concentrate on floodplains and wetlands. Wetlands and especially their associated grassy floodplains have for hundreds of years played a key role in the economies of Fulani (Peul) and other cattle-herding societies. The same holds for those in southern Africa along, for example, the Zambezi, where Barotse have herded their cattle for generations, or along the Nile, where Nuer and Dinka have done the same (see, for instance, the work of Evans-Pritchard 1940). In East Africa, key resource areas have also been identified by anthropologists already in the 1940s: in areas where floodplains did not fulfil this function, mountains were catching higher amounts of monsoonal rainfall (Huntingford 1933, 1953a, b; Homewood and Rodgers 1991; McCabe 1994; Prins and Loth 1988; Sperling and Galaty 1990; Ruttan and [ABSTRACT FROM AUTHOR]

Semi-arid rangelands show much spatial heterogeneity, with some parts producing more and better quality food for herbivores. The concepts of ‘Key Resource' and ‘Key Resource Area' have been developed to describe a resource that ‘provides good-quality forage' and that ‘reduces (inter-)annual variation in forage supply'. Illius and O'Connor (1999) formalised these concepts, arguing that in key resource areas herbivores experience a density-dependency relation with food resources, generally during the dry season. In other areas, generally during the wet season, non-equilibrium conditions govern the relation between herbivores and their food resources. They further argued that it is implicit that key resources show lower inter-annual variability than occurs on the (alternative) dry-season range, buffering livestock densities from climatic conditions. Key resource and outlying areas must further operate in a source-sink manner. In this chapter, we discuss the various assumptions and conclusions regarding key resources and key resource areas, using the floodplains of the Sahel, especially those of Waza-Logone in Cameroon, as examples. Sahelian floodplain grasslands are intensively exploited during the dry season, with cattle densities on a year-round basis about five times as high as in surrounding drylands. We come to the conclusion that the inter-annual variability in the quantity of the forage production of the Sahelian floodplains is not less, but often greater than that of surrounding areas. Forage quantity, however, may be more constant. [ABSTRACT FROM AUTHOR]

Mobility is indeed a perfect tool to optimise exploitation through tracking changing resources, as shown in Boone et al. (Chapter 9) by the example of transhumance systems in various parts of the world. Fragmentation and private access can limit mobility of herds so that key resources can no longer be used, decreasing the overall productivity of these livestock systems. The chapter recommends therefore the reinstalments of mobility wherever possible, and indicates the risks associated with a reduced mobility of herds. [ABSTRACT FROM AUTHOR]

Large-scale movements allow large herbivores to cope with changes in seasonal forage supply. Pastoralists use mobility to convert low-value ephemeral forage into high-value livestock. Transhumant pastoralists may move livestock less than ten to hundreds of kilometres. In semi-arid tropical sites, water and forage shortages in the dry season cause pastoral livestock to move to water or key resource areas. In temperate summers, livestock may be moved to higher-elevation snow-free meadows. In winters, animals may be moved lower to warmer sites, or to mountain valleys protected from steppe winds. Despite the recognised value of mobility, pastoral mobility is being reduced around the world. Changes in the mobility of three pastoral groups are reviewed, the Aymara of the South-American highlands, Mongolians, and the Maasai of Kenya and Tanzania, for which quantitative results are given. The Maasai of Kajiado District, Kenya are subdividing some group ranches into individually owned parcels. In subdivided Osilalei Group Ranch, herders moved an average of 5.6 km per day, whereas in undivided northern Imbirikani, herders moved 12.5 km per day. [ABSTRACT FROM AUTHOR]

Trait plasticity in physiology, behaviour, morphology and life history enables organisms to survive and populations to persist despite temporal variability in environmental conditions and resource availability. Through non-linear responses, the effect of adverse periods outweighs that of benign conditions, following Jensen's inequality. This chapter considers how large mammalian herbivores adjust broader aspects of their foraging behaviour to cope with variability over different temporal frames: within a day, day versus night, between days, over seasonal cycles and between years. It outlines the conceptual foundation for ‘adaptive resource ecology', covering changes in diet composition, daily time allocation, foraging movements, metabolic rate, digestive capacity and fat stores. The functional response relating food intake rate to food availability changes its form depending on the temporal scale. To link resource variability in time and space to population dynamics, the intake response needs to be transformed into a biomass or energy gain response over a seasonal time frame. Foraging models based on rate averaging can be misleading, while challenges in applying dynamic optimisation models need to be surmounted. Models assuming equilibrium relationships between resource supplies and population growth are inappropriate for coupling resource gains to population dynamics. [ABSTRACT FROM AUTHOR]

Resources are unequally distributed over the landscapes and it is only seldom that food of a herbivore at a given spot exactly matches its requirements. However, because non-sessile animals can move, they can assemble a diet from different patches that, in its total, does meet the intake requirements. Because herbivores of different sizes have different requirements for energy and nutrients, a linearprogramming model that takes into account the different satisficing requirements of herbivores of a range of body masses (or of reproductive status) yields new insights into the causality of the differential way that these animals use the same landscape. Depending on landscape configuration and extent, and especially grain size of the distribution of resources, our model predicts that lactating females are much more constrained than other animals of the same species vis-à-vis the array of patches in the landscape. We also predict that small ruminants should be much rarer than large ruminants, and conclude that small ruminants can only survive under most circumstances if they are specialised feeders or if they live in a fine-grained landscape. We further conclude that natural selection favours ruminants with a large body mass to those with a small body mass if nutrient acquirement is the dominant selection force. [ABSTRACT FROM AUTHOR]

All animals are faced with substantial variation in resource abundance over time and space. Patch-use theory, often based on optimality principles, can be useful in gaining insight into possible evolutionary solutions to this puzzle. A key consideration in applying patch-use theory to large terrestrial herbivores is that local variation in the nutritional quality of food is often inversely related to local resource abundance. Trade-offs between resource quality and abundance can change traditional models of patch use in important ways, some of which are explored in this chapter. I consider two aspects of patchuse decisions: which patches to visit and how long to stay in a patch, once visited? Empirical data for large herbivores often suggest that optimality principles are useful in explaining which patches are used in a landscape, but are less successful at explaining how long herbivores choose to stay in a particular patch. I end the chapter by exploring emerging challenges in applying patch-use principles to landscape ecology of large herbivores. [ABSTRACT FROM AUTHOR]

Resource heterogeneity and its effects on consumers are crucial in the dynamics of landscapes with large herbivores. Although all elements necessary for a general quantitative theory of resource heterogeneity and foraging behaviour across spatial scales are available, such a theory has not been put forth yet. We need to learn what scales, what resources and what types of heterogeneity are relevant to conserve and manage landscapes with large herbivores. More specifically, what scales, variables and heterogeneity are important in determining intake and diet selection by large herbivores? Large herbivores interact with their resources through a series of nested processes such as ingestion, searching, digestion and resting, which define relevant scales. Empirical relationships between animal performance and average resource abundance are scale-specific. Extrapolations should be based on explicit models to change scale, and will benefit by using concepts and techniques from geostatistics. Heterogeneity and average herbage mass are frequently related, so that measured effects on intake cannot be unequivocally attributed to total herbage mass. Resource heterogeneity can affect intake and behaviour through nonlinearity of responses to local conditions, selectivity and changes of local functional response due to global conditions. In general, coarser resolution of heterogeneity allows a greater selectivity. These points are illustrated with examples from the literature and reinterpretation of published and unpublished data. [ABSTRACT FROM AUTHOR]

The distribution of food is an important predictor for the distribution and density of herbivores in an ecosystem. Determining the distribution and densities of resource quantity and quality in space and time is therefore a crucial step towards understanding the spatial arrangement of herbivores. In recent years remote sensing has become the tool of choice for producing high spatial-resolution impressions of the variability of the landscape, and in particular land cover. Remote sensing is slowly moving away from mapping the surface into discrete land-cover classes. More and more, it is now used to produce highly accurate probability maps of presence, depicting the percentage of individual pixels covered with a certain surface element. This more closely represents the continuous nature of natural phenomena. Recent studies have indicated that it is possible to measure the chemical composition of foliage too. Recently a case study in Kruger National Park confirmed that it is possible to measure nitrogen concentration and phenolic compound levels in grass and trees accurately, with a spatial resolution of 4 meters. This opens doors for new lines of research, where the distribution of herbivores can be linked to the actual resource distribution. [ABSTRACT FROM AUTHOR]

The famous statement "Lies, damned lies, and statistics" is attributed to Benjamin Disraeli (1804 - 1881), British Prime Minister for the Conservative Party. When he made that pronouncement, he possibly referred to one of the original meanings of statistics, namely, the (quantitative) description of nation states. In the 19th century, the three developmental lines merged of what we now call ‘statistics', that is to say, the quantitative description of societies, the study of sets of objects and the analysis of probabilities. Statistics became the quantitative investigation of equal elements or objects belonging to one set. At that time, two major ways to test hypotheses emerged; one was the experimental way (which became dominant in physics and chemistry) and the other was the statistical way, where a theory or hypothesis was confronted with observations (typical for biology and medicine). Finally, statistics developed into a powerful tool to discover underlying mechanisms that explain variation in patterns or processes. [ABSTRACT FROM AUTHOR]

Understanding the spatial distribution of species is a fundamental issue in ecology, yet quantitative descriptions of animal species' distributions are rare. In this chapter, we use a spatialstatistics approach to describe the spatial distribution of herds of large herbivores in Laikipia, central Kenya. We used Global Positioning System technology and spatial point pattern analysis (F-, G- and J-functions) to characterise herd distributions of the 9 most abundant species comprising large herbivore communities in African savannas. F-function analysis is based on estimating the probability of a herd occurring within radius r of randomly selected focal points. G-function analysis is similar, but based on randomly selected focal herds. The J-function is derived from the ratio of G- and F-functions. Comparing results from the different functions was instructive about possible causes of spatial patterning at the landscape level. All species displayed consistently aggregated distributions under F- and J-function analyses, partly because wildlife has been displaced by humans and livestock from sections of the study area. By contrast, the G-function provides a description of dispersion under more natural conditions because areas lacking herds are excluded from the analysis. G-function results showed 5 species to display random or nearly random dispersion patterns (zebra, impala, Grant's gazelle, eland and hartebeest), while the remainder were aggregated (African elephant, giraffe, African buffalo and Thomson's gazelle). [ABSTRACT FROM AUTHOR]

Grazing distribution is an important component of the foraging ecology of large herbivores.Recognising the differences in foraging behaviours that occur along spatial and temporal scales is critical for understanding the mechanisms that result in grazing distribution patterns. Abiotic factors such as topography, water availability and weather and biotic factors such as forage quantity and quality affect the distribution of large herbivores. Numerous empirical studies have shown that large herbivores typically match the time spent in an area with the quantity and quality of forage found there. Although the observed grazing patterns have been documented, the underlying behavioural processes are still being elucidated. Cognitive foraging mechanisms assume that animals can use spatial memory to remember the levels of forage resources in various locations, while non-cognitive mechanisms require that behaviours such as intake rate, movement rate and turning frequency vary in response to forage resource levels. The ability of animals to use spatial memory during foraging has been demonstrated in several species including livestock, which suggests cognitive mechanisms are possible. Optimal-foraging theory can also be used to help explain behavioural processes. Giving-up rules based on marginal-value theorem appear to work well for large herbivores when a patch or feeding site can be noticeably depleted within an appropriate temporal scale such as a grazing bout or when forage availability is limited. However, givingup rules do not always explain movements among feeding sites when forage is plentiful. [ABSTRACT FROM AUTHOR]

"The question is often raised: why not stay with the real world and generalize our experiences in some succinctly descriptive form? The only answer is that such an approach never proves adequate. In evolutionary biology, it produces inductive generalizations that are encapsulated in tendencies or ‘rules' (e.g. Bergmann's rule). Causal explanations, the heart of any science, are hard to reach and often impossible to prove by means of such concepts. The descriptive, natural-history stage of science is eventually replaced by a deductive theoretical stage, basically mathematical in nature, which creates the abstractions and measurements necessary to deepen causal analysis. (…) The purpose of mathematical theory is to deal with "all possible worlds". The purpose of experiments and field [work] is to deal with the real world: To measure the parameters, to search for new parameters, and to improve the theory which is ultimately our most effective way of viewing the real world." (Wilson and Bossert 1971, pp. 40 - 41) [ABSTRACT FROM AUTHOR]

Owen-Smith (Chapter 8) describes the current state-of-the-art of the foraging models in relation to different temporal scales. He argues that the static, equilibrium approach of the traditional optimal foraging models does not seem to hold for reallife foraging studies, as trait plasticity is needed to cope with changes in foraging conditions over time. Addressing different temporal scales, from the variability on a certain day to day-to-day changes, and seasonal fluctuations of foraging conditions, it becomes clear that present-day models must be improved in order to be able to accommodate the variability in environmental conditions. Apparently, current models cannot yet fully cope with the importance of scale in foraging models. Indeed, the effects of both temporal and spatial scales on foraging behaviour need to be incorporated in the available models. An important gap is that few studies have been carried out that implicitly study the impact of these scale issues on foraging theory, let alone the hierarchy of different scales. There is an urgent need for studies that address the effect of scale on foraging behaviour. [ABSTRACT FROM AUTHOR]

Fryxell's aim (Chapter 6) is to evaluate the current understanding of forage intake and patch selection by herbivores across temporal and spatial scales, where resource (food) heterogeneity is large. His approach starts at a description of the functional response, i.e., the food intake response of consumers to quantitative changes in the resource supply. Re-developing several models of food and energy intake applicable at the detailed level of feeding station, he places bite size or bite processing central to short-term food procurement. This is then developed into longer-term (daily) food or energy intake functions, where digestive rather than bite-size or bite-processing constraints may operate. [ABSTRACT FROM AUTHOR]

In Chapter 5, Laca poses that currently there is no general quantitative theory of large-herbivore foraging behaviour in landscapes with heterogeneous resources. Though his stated goal here is not to put forward such a theory, he aims to present a number of relevant concepts and theories, and to place them in a coherent framework. Based on these, he poses some questions and hypotheses in an attempt to fill apparent knowledge gaps. [ABSTRACT FROM AUTHOR]

How many prey to take of a certain type or how long to stay in a patch are key questions of a foraging animal according to the optimal foraging theory (OFT) (Krebs and Davis 1986). Within the OFT, the goal for herbivores generally is some form of energy maximisation within the limits of certain constraints. Although the application of energy as single currency has had some success, it is widely recognised that focusing on energy alone is not sufficient to explain the foraging behaviour of herbivores. Especially the complex, and ever changing, nature of their diet, together with the many constraints to be taken into account, poses problems (Krebs and Davies 1986; Simpson et al. 2004; Illius et al. 2002; Bailey and Provenza, Chapter 2). Essential here is that herbivores tend not to stay in a patch as long as predicted, and/or do not select a diet which provides maximal energy gain (Van Wieren 1996; Bailey and Provenza, Chapter 2). Because of this, alternative models have been developed, among them the sufficing principle (defined by Ward (1992) as choosing between different options when information-processing limits the ability of an animal to make optimal decisions), and the satiety hypothesis. The question here is if and/or how the satiety hypothesis fits into the OFT. [ABSTRACT FROM AUTHOR]

Prins and Van Langevelde (Chapter 7) use a linear-programming model to assess the extent to which ruminants of different size are able to satisfy their nutrient, energy and protein requirements from a landscape composed of two ‘food' patches that differ in their relative densities of these important nutritional variables. Amongst their findings they conclude that, overall, small species are less able to balance their nutritional requirements when patches are widely dispersed than are large species, and are, therefore, more likely to be found in fine-grained (i.e., more closely dispersed food patches) than in course-grained ecosystems. Whilst this is an interesting and testable hypothesis, I will argue that it is the ‘foodscape', not the landscape, that foraging animals respond to. My view is that the conclusions are an artefact of the model description rather than an actuality of the real world in which ruminants forage for a living. I posit that the dispersion of food in the landscape is a species-specific construct with the result that the foodscape of two species foraging in the same landscape will differ because of their differing views of food and their differing ability to select that food from the array of non-food on offer (see also Underwood 1983). [ABSTRACT FROM AUTHOR]

What is a resource, how are resources distributed, and how do they change over time, together with the possibilities of mapping these resources through remote sensing, are the subjects of this chapter. Skidmore and Ferwerda (Chapter 4) follow Morrison and Hall (2002) in their definition of what a resource is, namely "a resource is any biotic or abiotic factor directly used by an organism, and includes food, nutrients, water, atmospheric gas concentrations, light, soil, weather (i.e., precipitation, temperature, evapotranspiration, etc.), terrain, and so on". The central notion of a resource is that it is used. However, Morrison and Hall and also Skidmore and Ferwerda confuse ‘use' in the sense of ‘exploit' or ‘consume as material' with ‘use' in the sense of ‘benefit from'. As a matter of fact, the Oxford English Dictionary (OED) defines ‘resource' as "stock that can be drawn on, available assets, or means of supplying what is needed". Assets and stock can dwindle if they are used faster than their replenishment rate, and if that happens they are used up. We think that the term ‘resource' should be limited to this meaning, and thus disagree with Skidmore and Ferwerda the way they apply this key term: ‘weather' cannot be used, ‘temperature' is a state variable, and ‘terrain' cannot increase or decrease. Where light is a non-depletable resource, weather and temperature are environmental conditions. These are variables that describe an organism's habitat, and are therefore sometimes classified as one of the species' niche dimensions, but not its resource. [ABSTRACT FROM AUTHOR]