John M. Fryxell

Are there general mechanisms of animal home range behaviour? A review and prospects for future research

Home range behaviour is a common pattern of space use, having fundamental consequences for ecological processes. However, a general mechanistic explanation is still lacking. Research is split into three separate areas of inquiry - movement models based on random walks, individual-based models based on optimal foraging theory, and a statistical modelling approach - which have developed without much productive contact. Here we review recent advances in modelling home range behaviour, focusing particularly on the problem of identifying mechanisms that lead to the emergence of stable home ranges from unbounded movement paths. We discuss the issue of spatiotemporal scale, which is rarely considered in modelling studies, as well as highlighting the need to consider more closely the dynamical nature of home ranges. Recent methodological and theoretical advances may soon lead to a unified approach, however, conceptually unifying our understanding of linkages among home range behaviour and ecological or evolutionary processes.

Causes and consequences of migration by large herbivores

Many populations of large herbivores migrate seasonally between discrete home ranges. Current evidence suggests that migration is generally selected for as a means of enhancing access to high quality food and/or reducing the risk of predation. The relative importance of these alternative selection pressures should depend on the demographic circumstances facing a given population. Seasonal migration also has important implications for the structure and dynamics of large herbivore communities. Migrants should tend to be regulated by food availability, while residents should tend to be regulated by predators As a result, migrants should often outnumber residents by a considerable margin - a pattern seen in several tropical and temperate ecosystems. Differences in the mode of regulation could also imply that competition for resources will be weak in purely resident assemblages, but strong in communities dominated by migrants. Continual grazing by resident herbivores can sometimes lead to degeneration of vegetation, while systems supporting migrants are apparently more resilient. This implies that migration can have an important impact on the long-term persistence of plant-herbivore systems, particularly in areas with slow rates of vegetation regeneration.

Forage Quality and Aggregation by Large Herbivores

Three general hypotheses have been proposed to explain why many large herbivores have highly aggregated patterns of distribution: dilution of predation risk, maintenance of forage in an immature but highly nutritious growth stage, and ideal free distribution in relation to spatial variation in either forage quality or primary productivity, Evaluation of the latter two hypotheses has been hampered by lack of a formal theoretical framework for assessing the effects of forage quality on resource acquisition by herbivores. A model is proposed that demonstrates how trade-offs between instantaneous intake and forage digestibility might lead to optimal rates of energy intake at iow to intermediate levels of forage abundance. Simulations based on the energy-intake model suggest a number of key environmental factors that should determine the impact of forage maturation and spatial variation on herbivore distribution patterns. All of the proposed advantages of aggregation may have general applicability, but the relative importance of each probably varies according to the specific circumstances facing a given herbivore population

HUMAN ACTIVITY MEDIATES A TROPHIC CASCADE CAUSED BY WOLVES

Experimental evidence of trophic cascades initiated by large vertebrate predators is rare in terrestrial ecosystems. A serendipitous natural experiment provided an opportunity to test the trophic cascade hypothesis for wolves (Canis lupus) in Banff National Park, Canada. The first wolf pack recolonized the Bow Valley of Banff National Park in 1986. High human activity partially excluded wolves from one area of the Bow Valley (low-wolf area), whereas wolves made full use of an adjacent area (high-wolf area). We investigated the effects of differential wolf predation between these two areas on elk (Cervus elaphus) population density, adult female survival, and calf recruitment; aspen (Populus tremuloides) recruitment and browse intensity; willow (Salix spp.) production, browsing intensity, and net growth; beaver (Castor canadensis) density; and riparian songbird diversity, evenness, and abundance. We compared effects of recolonizing wolves on these response variables using the log response ratio between the low-wolf and high-wolf treatments. Elk population density diverged over time in the two treatments, such that elk were an order of magnitude more numerous in the low-wolf area compared to the high-wolf area at the end of the study. Annual survival of adult female elk was 62% in the high-wolf area vs. 89% in the low-wolf area. Annual recruitment of calves was 15% in the high-wolf area vs. 27% without wolves. Wolf exclusion decreased aspen recruitment, willow production, and increased willow and aspen browsing intensity. Beaver lodge density was negatively correlated to elk density, and elk herbivory had an indirect negative effect on riparian songbird diversity and abundance. These alternating patterns across trophic levels support the wolf-caused trophic cascade hypothesis. Human activity strongly mediated these cascade effects, through a depressing effect on habitat use by wolves. Thus, conservation strategies based on the trophic importance of large carnivores have increased support in terrestrial ecosystems.

Scale and heterogeneity in habitat selection by elk in Yellowstone National Park

Abstract Resource selection functions (RSF) can be used to explore the role of scale in determining patterns of habitat use. We estimated RSFs for 93 radiocollared adult female elk (Cervus canadensis) with resource availability defined at four spatial scales and two seasons in Yellowstone National Park. Habitat selection differed markedly among scales and seasonal ranges. During winter elk moved to ranges at lower elevations where snow water equivalents were low and selected landscapes with a mix of forest and open vegetation at all spatial scales. Areas of high vegetation diversity were selected at large spatial scales during summer, whereas elk selected less diverse areas on winter range. During summer elk selected forests that burned 12-14 y earlier, but they used these burns less than expected by chance during winter. Habitat selection by elk occurred at multiple spatial scales; thus, we cannot prescribe a single scale as being best for modelling habitat use by elk. Instead, selection of an appropriate scale will vary depending on the research question or management issue at hand.

Why are Migratory Ungulates So Abundant

Migratory ungulates outnumber residents by an order of magnitude in several savanna ecosystems in Africa, as was apparently the case in other grasslands around the world before the intervention of modern man. Migrants may be more numerous than residents because (1) they use a much larger area, (2) they make more-efficient use of resources, or (3) they are less vulnerable to regulation by predators. These hypotheses were examined using simulation models of migratory and sedentary wildebeest in the Serengeti ecosystem. The larger area used by migrants would not lead inevitably to higher numbers. In seasonal environments, herbivore abundance is probably determined by food availability during periods of resource scarcity. Even though migrants may have access to greater food supplies for most of the year, this would not lead to increased abundance if both morphs have similar food supplies during the leanest period of the year. Rotational grazing could lead to increased numbers of migrants relative to residents only if migrants are able to use mature vegetation that has accumulated while they are foraging elsewhere. This is unlikely for savanna ecosystems in Africa because tropical grasses decline rapidly in quality as they mature. Moreover, our simulations suggest that in the Serengeti such a process would at most produce a twofold difference in abundances of migrants and residents. We conclude that increased efficiency in resource use by migrants is insufficient to explain the order-of-magnitude disparities in abundance seen in some African ecosystems. Our simulations suggest that realistic numbers of predators could regulate resident herbivores at low population densities, whereas such regulation is probably rare for migratory herds. When residents and migrants have overlapping ranges, migrants should always outcompete residents, reducing them to low numbers. These results suggest that differences in the modes of regulation explain the predominance of migratory herbivores in some grassland ecosystems.

Individual behavior and community dynamics

1 Introduction.- 1.1 Objectives.- 1.2 Topics to be Covered.- 1.3 Predator-Prey Dynamics.- 1.4 Competition.- 1.5 Summary.- 2 Diet Selection.- 2.1 Nutrient-Maximizing Diets.- 2.1.1 Experimental Evidence for Optimal Diets.- 2.1.2 Partial Preferences.- 2.1.3 Diet Selection and the Functional Response.- 2.1.4 Experimental Tests of the Functional Response.- 2.1.5 Diet Choice and Population Dynamics.- 2.2 Evolutionary Dynamics of Diet Selection.- 2.3 Balanced Nutrient Diets.- 2.3.1 Experimental Evidence for Balanced Diets.- 2.3.2 Balanced Diets and Population Dynamics.- 2.4 Summary.- 3 Prey Defense.- 3.1 Types of Defenses.- 3.2 Defense Effects and Population Parameters.- 3.3 Parameter Effects on Dynamics.- 3.4 Time Allocation.- 3.4.1 Risk-Sensitive Prey.- 3.4.2 3-Link System.- 3.5 Optimal Defense.- 3.5.1 Perfectly Timed Induced Defense.- 3.5.2 Lagged Induced Defense.- 3.6 Summary.- 4 Habitat Use and Spatial Structure.- 4.1 Habitat Variation.- 4.2 Energy-Maximizing Habitat Use.- 4.2.1 Experimental Evidence for Optimal Habitat Choice.- 4.2.2 Habitat Choice and Predator-Prey Dynamics.- 4.2.3 Habitat Choice and Competitive Dynamics.- 4.3 Evasion of Predators by Prey.- 4.3.1 Experimental Evidence of Predator Evasion.- 4.3.2 Food Chain Dynamics.- 4.4 Spatial Structure.- 4.4.1 Experimental Evidence of Optimal Patch Use.- 4.4.2 Patchy Predator-Prey Dynamics.- 4.4.3 Experimental Evidence of Patchy Predator-Prey Dynamics.- 4.5 Summary.- 5 Size-Selective Predation.- 5.1 Diet Selection Model.- 5.1.1 Self-Thinning.- 5.1.2 Size-Dependent Consumption.- 5.1.3 Size-Selection and Population Dynamics.- 5.2 Partial Predation Model.- 5.2.1 Consumption Model.- 5.2.2 Partial Predation and Population Dynamics.- 5.3 The Size Structure Challenge.- 5.4 Summary.- 6 Interference and Territoriality.- 6.1 Interference.- 6.1.1 Interference and Population Dynamics.- 6.1.2 Social Structure and Interference Levels.- 6.1.3 Spatial Structure and Interference.- 6.2 Territoriality.- 6.2.1 Optimal Territory Size.- 6.2.2 Evidence for Optimal Territory Size.- 6.2.3 Optimal Territory Size and Population Dynamics.- 6.2.4 Systematic Foraging.- 6.2.5 Central-Place Foraging and Prey Spatial Refugia.- 6.2.6 Central-Place Foraging and Population Dynamics.- 6.3 Summary.- 7 Epilogue.- References.

Multiple movement modes by large herbivores at multiple spatiotemporal scales

Recent theory suggests that animals should switch facultatively among canonical movement modes as a complex function of internal state, landscape characteristics, motion capacity, and navigational capacity. We tested the generality of this paradigm for free-ranging elk (Cervus elaphus) over 5 orders of magnitude in time (minutes to years) and space (meters to 100 km). At the coarsest spatiotemporal scale, elk shifted from a dispersive to a home-ranging phase over the course of 1–3 years after introduction into a novel environment. At intermediate spatiotemporal scales, elk continued to alternate between movement modes. During the dispersive phase, elk alternated between encamped and exploratory modes, possibly linked to changes in motivational goals from foraging to social bonding. During the home-ranging phase, elk movements were characterized by a complex interplay between attraction to preferred habitat types and memory of previous movements across the home-range. At the finest temporal and spatial scale, elk used area-restricted search while browsing, interspersed with less sinuous paths when not browsing. Encountering a patch of high-quality food plants triggered the switch from one mode to the next, creating biphasic movement dynamics that were reinforced by local resource heterogeneity. These patterns suggest that multiphasic structure is fundamental to the movement patterns of elk at all temporal and spatial scales tested.

Can parks protect migratory ungulates? The case of the Serengeti wildebeest

The conservation of migratory species can be problematic because of their requirements for large protected areas. We investigated this issue by examining the annual movements of the migratory wildebeest, Connochaetes taurinus, in the 25000 km2 Serengeti-Mara Ecosystem of Tanzania and Kenya. We used Global Positioning System telemetry to track eight wildebeest during 1999–2000 in relation to protected area status in different parts of the ecosystem. The collared wildebeest spent 90% of their time within well-protected core areas. However, two sections of the wildebeest migration route – the Ikoma Open Area and the Mara Group Ranches – currently receive limited protection and are threatened by poaching or agriculture. Comparison of current wildebeest migration routes to those recorded during 1971–73 indicates that the western buffer zones appear to be used more extensively than in the past. This tentative conclusion has important repercussions for management and needs further study. The current development of community-run Wildlife Management Areas as additional buffer zones around the Serengeti represents an important step in the conservation of this UNESCO World Heritage Site. This study demonstrates that detailed knowledge of movement of migratory species is required to plan effective conservation action.

Foraging theory upscaled: the behavioural ecology of herbivore movement.

We outline how principles of optimal foraging developed for diet and food patch selection might be applied to movement behaviour expressed over larger spatial and temporal scales. Our focus is on large mammalian herbivores, capable of carrying global positioning system (GPS) collars operating through the seasonal cycle and dependent on vegetation resources that are fixed in space but seasonally variable in availability and nutritional value. The concept of intermittent movement leads to the recognition of distinct movement modes over a hierarchy of spatio-temporal scales. Over larger scales, periods with relatively low displacement may indicate settlement within foraging areas, habitat units or seasonal ranges. Directed movements connect these patches or places used for other activities. Selection is expressed by switches in movement mode and the intensity of utilization by the settlement period relative to the area covered. The type of benefit obtained during settlement periods may be inferred from movement patterns, local environmental features, or the diel activity schedule. Rates of movement indicate changing costs in time and energy over the seasonal cycle, between years and among regions. GPS telemetry potentially enables large-scale movement responses to changing environmental conditions to be linked to population performance.