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Understanding local and global extinction is a fundamental objective of both basic and applied ecology. Island biogeography theory (IBT) and succession theory provide frameworks for understanding extinction in changing landscapes. We explore the relative contribution of fragment size vs. succession on species' declines by examining distributions of abundances for 18 plant species declining over time in an experimentally fragmented landscape in northeast Kansas, USA. If patch size effects dominate, early-successional species should persist longer on large patches, but if successional processes dominate, the reverse should hold, because in our system woody plant colonization is accelerated on large patches. To compare the patterns in abundance among patch sizes, we characterize joint shifts in local abundance and occupancy with a new metric: rank occupancy-abundance profiles (ROAPs). As succession progressed, statistically significant patch size effects emerged for 11 of 18 species. More early-successional species persisted longer on large patches, despite the fact that woody encroachment (succession) progressed faster in these patches. Clonal perennial species persisted longer on large patches compared to small patches. All species that persisted longer on small patches were annuals that recruit from the seed bank each year. The degree to which species declined in occupancy vs. abundance varied dramatically among species: some species declined first in occupancy, others remained widespread or even expanded their distribution, even as they declined in local abundance. Consequently, species exhibited various types of rarity as succession progressed. Understanding the effect of fragmentation on extinction trajectories requires a species-by-species approach encompassing both occupancy and local abundance. We propose that ROAPs provide a useful tool for comparing the distribution of local abundances among landscape types, years, and species. Key words: abundance; extinction; grassland; habitat fragmentation; island biogeography theory (IBT); occupancy; ROAP; species abundance distribution; succession.

Over the last several decades, there has been a growing appreciation of the importance of nonequilibrial phenomena and transient dynamics in explaining the structure of ecological communities. This paper provides an overview of theoretical themes related to resource pulses. Theoretical models suggest short-term responses to a single pulse can qualitatively differ from longer-term responses. Recurrent resource pulses can alter community structure, permitting coexistence that otherwise would not occur, or hamper coexistence mechanisms effective in stable environments. For a given resource input, system responses can be more dramatic with short pulses. Resource pulses can cause transitions between alternative states. Dispersal permits species to exploit locally sporadic resource pulses and persist in environments that on average are unsuitable. All these issues are ripe for further theoretical explorations. Key words: reactivity; resource pulses; spatiotemporal variation; transient dynamics.

Although the canonical concept of intraguild predation evokes images of predators and prey, several subdisciplines within ecology have developed theory not specifically framed in terms of predation and competition and often using system-specific terminology, yet functionally quite similar. Here, we formulate models combining exploitation and competition in predator prey, host-parasitoid, and host-pathogen communities to compare dynamics, food web structure, and coexistence criteria for these disparate communities. Although dynamic stability in the coexistence region varies strongly among systems, in all cases coexistence of two consumers on a single resource occurs only if the intraguild prey species is more efficient than the intraguild predator at suppressing the abundance of the basal resource, and if the intraguild predator accrues a sufficient gain from attacking the intraguild prey. In addition, equilibrial abundances of all species in all three formulations respond similarly to increases in productivity of the basal resource. Our understanding of predator-prey and parasitoid-host communities has benefited from explicit examination of intraguild predation (IGP) theory, and we suggest that future research examining pathogen communities, in particular, will benefit substantially from explicit recognition of predictions from IGP theory. Key words: coexistence; community module; competition; disease transmission; exploitation; food web; heuristic model; trophic interactions.

A rich body of theoretical literature now exists focused on the three-species module of intraguild predation (IGP), in which a top predator both attacks and competes with an intermediate predator. Simple models of intraguild predation are often unstable, either because one consumer is excluded, or because sustained oscillations emerge from long feedback loops. Yet, many natural IGP systems robustly persist. Standard models of intraguild predation simplify natural systems in crucial ways that could influence persistence; in particular, many empirical IGP systems are embedded in communities with alternative prey species. We briefly review the key conclusions of standard three-species IGP theory, and then present results of theoretical explorations of how alternative prey can influence the persistence and stability of a focal intraguild predation interaction. Key words: alternative prey; asymmetry; community modules; intraguild predation.

Secondary succession reflects, at least in part, community assembly--the sequences of colonizations and extinctions. These processes in turn are expected to be sensitive to the size of the site undergoing assembly and its location relative to source pools. In this paper we describe patterns of succession over 18 years in an experimentally fragmented landscape created in eastern Kansas, USA, in 1984. The design of the experiment permits one to assess the influence of patch size and landscape position on successional dynamics. The general trajectory of succession follows that typical of succession in much of the eastern United States. In the initial years of the study, there was relatively little effect of patch size or distance to sources. Here we show that spatial effects in this system have become increasingly evident with time, as gauged both by repeated-measures ANOVA and ordination techniques. Woody plants have colonized more rapidly (per unit area) on large and nearby patches. Species richness at a local (within-quadrat) scale in general has increased, with slightly greater richness in large than in small patches later in the study. Temporal stability in community composition has generally been greater in large patches. Spatial heterogeneity in community composition has increased during succession, but with different patterns in large and small patches. This long-term experiment suggests that landscape structure influences many aspects of community structure and dynamics during succession, and that such effects become more pronounced with the passage of time. Key words: community assembly; habitat fragmentation; island theory; old fields; patch size and distance effects; prairie-forest ecotone; spatial heterogeneity; species richness; succession; turnover.

Disturbance can directly affect the interactions among species at different trophic levels. Because disturbances are typically localized, and many consumers are constrained in their mobility, disturbances can generate spatial variability in trophic interactions. Here, we consider how fire alters plant-insect interactions in a longleaf pine ecosystem in central Florida, USA. We hypothesized that fire in the short term would directly depress herbivore abundance, and that the rate of re-colonization by herbivores would depend upon distance from the edge of the recently burned area. Thus, fire should generate spatially varying herbivory. Orthopterans are dominant insect consumers in our system, and many species are flightless, either as juveniles or adults; these species are particularly likely to show constrained post-fire mobility. We quantified insect abundance and herbivory levels on eight common understory plants in the edge and interior of three recently burned sites. As predicted, insect abundance at the edge of burned areas was much higher than in the interior. In addition, all plant species experienced at least twice the level of herbivory in the edge than in the interior of burned sites. This demonstrates that disturbance can create strong spatial variation in the magnitude of trophic interactions. We suggest that larger burns may aid in plant management by reducing herbivory, whereas the presence of unburned refugia may be critical to insect conservation. Key words: disturbance ecology: fire; Florida (USA) pine ecosystem; herbivory; Orthoptera; prescribed burn; sandhill ecosystem; spatial gradient; trophic interactions.

We investigate how coexistence between competitors may be influenced by habitat selection when habitats represent either sources or sinks, and given that dispersal is free to evolve. Evolutionary stable dispersal between source and sink habitats can occur if local fitnesses vary temporally, either due to intrinsic factors (e.g., chaotic dynamics) or extrinsic factors (e.g., environmental stochasticity). The model assumes locally linear Lotka-Volterra competition between two species. Given sufficiently low density-independent mortality in the sink, dispersal between habitats is an evolutionary stable strategy (ESS). Given a trade-off between competitive ability in the source and mortality in the sink, a sink habitat can promote species coexistence in the source habitat if the inferior competitor species experiences lower mortality in the sink. This highlights how sink habitats may provide mechanisms of coexistence in heterogeneous landscapes. In a second scenario, the competitors have distinct habitat preferences, resulting in the 'Ghost of Competition Past' (i.e., complete habitat partitioning) with stable population dynamics. With unstable population dynamics, dispersal between habitats becomes the ESS, and the Ghost vanishes leading either to coexistence of the competitors in both habitats or global exclusion of one species. Our results highlight the importance of jointly considering the effects of spatial heterogeneity and temporal variability when analyzing the coexistence of competing, mobile organisms. Key words: evolutionary stable strategy (ESS); Ghost of Competition Past; habitat selection; mechanisms of coexistence; source-sink dynamics; spatial stochasticity; temporal stochasticity.

We evaluate the goals of meta-analysis, critique its recent application in ecology, and highlight an approach that more explicitly links meta-analysis and ecological theory. One goal of meta-analysis is testing null hypotheses of no response to experimental manipulations. Many ecologists, however, are more interested in quantitatively measuring processes and examining their systematic variation across systems and conditions. This latter goal requires a suite of diverse, ecologically based metrics of effect size, with each appropriately matched to an ecological question of interest. By specifying ecological models, we can develop metrics of effect size that quantify the underlying process or response of interest and are insensitive to extraneous factors irrelevant to the focal question. A model will also help to delineate the set of studies that fit the question addressed by the meta-analysis. We discuss factors that can give rise to heterogeneity in effect sizes (e.g., due to differences in experimental protocol, parameter values, or the structure of the models that describe system dynamics) and illustrate this variation using some simple models of plant competition. Variation in time scale will be one of the most common factors affecting a meta-analysis, by introducing heterogeneity in effect sizes. Different metrics will apply to different time scales, and time-series data will be vital in evaluating the appropriateness of different metrics to different collections of studies. We then illustrate the application of ecological models, and associated metrics of effect size, in meta-analysis by discussing and/or synthesizing data on species interactions, mutual interference between consumers, and individual physiology. We also examine the use of metrics when no single, specific model applies to the synthesized studies. These examples illustrate that the diversity of ecological questions demands a diversity of ecologically meaningful metrics of effect size. The successful application of meta-analysis in ecology will benefit by clear and explicit linkages among ecological theory, the questions being addressed, and the metrics used to summarize the available information. Key words: ecological models and meta-analysis; effect size in meta-analysis; interaction strength; meta-analysis goals, metrics, and models; metrics, ecological relevance; predation; ratio dependence; time scale., INTRODUCTION For the past forty years, experiments have been heralded as the most powerful tool in the ecologist's field kit. Indeed, since the seminal work of Connell (1961), field experiments [...]

We use simple mathematical models to explore the indirect interactions between two prey species that share a predator when all three species undergo population cycles. The results are compared to analogous findings for systems that reach a stable equilibrium point. It is common for removal of one prey from a cycling system to result in a decrease in the mean density of the remaining prey species, contrary to the usual logic of apparent competition. Even when apparent competition between prey exists, its magnitude is usually reduced by population cycles. This effect occurs when the predator has a concave-down relationship between prey abundance and its own per capita growth rate. Such relationships can occur because of a saturating functional or numerical response. We investigate how prey density dependence and the shape of the predator's functional and numerical responses affect the sign and magnitude of this indirect interaction between prey species. There may also be (+, -) interactions between prey that differ significantly in their susceptibility to the common predator. Key words: apparent competition; apparent mutualism; environmental variation; indirect effects; limit cycles; shared predation., INTRODUCTION Few, if any natural populations are constantly at equilibrium densities. Variation in the weather and other environmental parameters results in what we will term exogenous variability in populations. Endogenous [...]

The evolution of resistance by insect and weed pests to chemical pesticides is a problem of increasing importance in applied ecology. It is striking that the evolution of resistance by target pest species in biological control is much less frequently reported, particularly in control involving parasitoids and predators, rather than pathogens. Although it is conceivable that this reflects biases in reporting or frequency of application, we suggest that there is a puzzle here worthy of scrutiny, and we outline several potential underlying causes. In order of discussion (not necessarily of importance), these are: (1) lack of genetic variation; (2) genetic constraints on selection; (3) weak selection; (4) temporally varying selection; and (5) coevolutionary dynamics. We, in particular, focus on the potential for weak selection on the host for increased resistance, despite effective control. The very spatial mechanisms (e.g., refuges, metapopulation dynamics) believed to facilitate the persistence of many natural enemy-victim systems with strong biological control may also incidentally provide an environment where selection is weak on target pests to evolve improved resistance to control agents, thereby biasing coevolution toward the enemy. The basic insight is that in a spatially heterogeneous environment, a strong limiting factor on a population can be a weak selective factor. The hypotheses presented here provide ingredients needed to predict which biological control systems might be evolutionarily stable, and which not. Our aim in this thought piece is to stimulate more attention to the evolutionary dimension of biological control systems. Key words: coevolution; evolution of pest species; genetic constraints on selection; niche conservatism; resistance to biological control., INTRODUCTION One of the most enduring and societally important enterprises in applied ecology throughout this century has been the control of populations of economically damaging species, particularly of agricultural weed [...]

In spatially heterogeneous habitats, plant community change may reflect spatially localized population-level processes that are sensitive to the size of an average habitat patch. However, local species turnover can also be determined by initial conditions and large-scale processes, in which case patch size effects may be overridden. To examine the role of patch size in directing secondary succession, we subdivided a newly abandoned agricultural field into an array of experimental patches (32, 288, and 5000 [m.sup.2], grouped to sample equivalent portions of the field), and have thereafter censused the resident plant and animal communities at regular intervals. Here we report results from the first 6 yr of studies on the changing vascular plant community in an experimentally fragmented landscape. The general course of change in all patches followed a trajectory typical of old-field succession, toward increasing dominance by longer lived and larger plant species. The same group of species that dominated at the start of the study continued to dominate after 6 yr, although in very different proportional abundances. Larger patches were more species rich than their smaller counterparts, and had a higher proportion of nonshared species, but the additional species were transient and low in abundance. Spatial heterogeneity in vegetation, measured as local community dissimilarity, increased in all patches but to a lesser extent in the largest patches, where censuses of nearby permanent quadrats indicated less divergence over time. At a population level, the strongest effect of patch size was that local populations of clonal species were more prone to disappear from the smallest patches. Nevertheless, summary measures of temporal community change did not reflect significant differences in localized species turnover. We conclude that patch size does not markedly affect the rate or pattern of early secondary succession, at the scales imposed in our experiment. Key words: colonization; habitat fragmentation; old field; patch size; population persistence; secondary succession; spatial dynamics; spatial heterogeneity; species-area relations., INTRODUCTION The spatial context of plant succession Succession, directional temporal change in species composition or relative abundances, is a central theme in plant community ecology (Glenn-Lewin et al. 1992, Miles [...]

We studied the effects of habitat fragmentation on the movements of cotton rats (Sigmodon hispidus), deer mice (Peromyscus maniculatus), and prairie voles (Microtus ochrogaster) living in a fragmented landscape using 7.7 yr (1984-1992) of mark - recapture data. The study area included three kinds of 0.5-ha blocks: single large patches (5000 [m.sup.2]), clusters of medium patches (288 [m.sup.2]), and clusters of small patches (32 [m.sup.2]). We tested three predictions regarding animal movements. First, animals should move longer distances as fragmentation increases. Second, the proportion of animals moving will decrease as fragmentation increases. Third, species will show more movement from putative sources to putative sinks. In support of our first two predictions, all species (except male cotton rats) moved farther, and lower proportions of animals moved, as fragmentation increased. In testing our third prediction, we found no trends, for all species, between patch size and the net number of animals a block either imported or exported, indicating source-sink dynamics were probably not occurring on our study site. Furthermore, animals of all species (except female deer mice) switched more frequently to blocks of larger patches. For prairie voles in the spring and deer mice in the summer, relative abundances among blocks predicted from a Markov matrix model of switching probabilities showed high congruence with the actual abundances, indicating movement and abundances were related. In both cotton rats and prairie voles but not in deer mice, more juveniles and nonreproductive animals switched between blocks than did adults or reproductive animals. Deer mice switched more frequently than did either cotton rats or prairie voles; the latter species switched in similar frequencies. We discuss the implications of our data for modeling and conservation. Key words: dispersal; habitat fragmentation; Markov modeling; Microtus ochrogaster; movement; patchiness; Peromyscus maniculatus; scale; Sigmodon hispidus; space use; spatial heterogeneity; source-sink dynamics., INTRODUCTION Ecologists are becoming increasingly aware of the role of spatial heterogeneity in population and community dynamics (e.g., Ricklefs 1987, Karieva and Anderson 1988, Karieva 1990, Levin 1992). Movement by [...]

E. WILLIAM SCHWEIGER [1,4] JAMES E. DIFFENDORFER [2] ROBERT D. HOLT [1] RAYMOND PIEROTTI [1] MICHAEL S. GAINES [3] Abstract. We compared the density and spatial distribution of four small [...]

The University of Kansas Department of Ecology & Evolutionary Biology Natural History Museum and Center for Biodiversity Research Lawrence, Kansas 66044 Dettner, Konrad, Gerhard Bauer, and Wolfgang Volkl, editors, 1997. [...]

We live on the threshold of the birth of a new discipline, one which might be christened 'Applied Evolutionary Biology.' This emerging discipline consists of the rigorous application of the [...]

The species - area relationship may be the strongest empirical generalization in community ecology. We explore the effect of trophic rank upon the 'strength' of the species - area relationship, as measured by z, the slope of a log(species) vs. log(area) plot. We present a simple model for communities closed to immigration, composed of 'stacked specialist' food chains (where each plant species supports a specialist herbivore, which in turn sustains a specialist carnivore, etc.), that predicts z should increase with trophic rank; the model brings out some of the spatial implications of sequential dependencies among species. We discuss empirical examples in which the z values of taxa differing in trophic rank were reported and lament the shortage of well-documented examples in the ecological literature. Several examples fit the expected pattern, but others do not. We outline several additional reasons why z values might increase with trophic rank, even for generalists. If the qualitative assumptions of the model are relaxed, the predicted effect of trophic rank on z should weaken or even be reversed. Trophic rank may not have a systematic effect on the species - area relationship if (1) there are strong top-down interactions leading to prey extinctions; (2) communities are open, with recurrent immigration, particularly at higher trophic ranks; (3) consumers are facultative generalists, able to exist on a wide range of resource species; or (4) systems are far from equilibrium. Our aim in this thought piece is to stimulate community ecologists to link theoretical and empirical studies of food web structure with analyses of spatial dynamics and landscape ecology, and to encourage empirical studies of the species - area relationship focused on comparisons across taxa varying in trophic rank. Key words: distribution; generalist; island; patch; specialist; specialist vs. generalist food webs; species - area; trophic rank and species - area relationship., INTRODUCTION The tendency for species richness to increase with area (the 'species - area relationship') is one of the most robust empirical generalizations in ecology (May 1975, Rosenzweig 1995). Most [...]

Habitat loss is often considered the greatest near-term threat to biodiversity, while the impact of habitat fragmentation remains intensely debated. A key issue of this debate centers on the problem of scale-landscape or patch-at which to assess the consequences of fragmentation. Yet patterns are often confounded across scales, and experimental designs that could solve this scaling problem remain scarce. We conducted two field experiments in 30 experimental landscapes in which we manipulated habitat loss, fragmentation, and patch size for a community of four insect herbivores that specialize on the cactus Opuntia. In the first experiment, we destroyed 2088 Opuntia patches in either aggregated or random patterns and compared the relative effects of landscape-scale loss and fragmentation to those of local patch size on species occurrence. This experiment focused on manipulating the relative separation of remaining patches, where we hypothesized that aggregated loss would disrupt dispersal more than random loss, leading to lower occurrence. In the second experiment, we destroyed 759 Opuntia patches to generate landscapes that varied in patch number and size for a given amount of habitat loss and assessed species occurrence. This experiment focused on manipulating the subdivision of remaining habitat, where we hypothesized that an increase in the number of patches for a given amount of loss would lead to negative effects on occurrence. For both, we expected that occurrence would increase with patch size. We find strong evidence for landscape-scale effects of habitat fragmentation, with aggregated loss and a larger number of patches for a given amount of habitat loss leading to a lower frequency of patches occupied in landscapes. In both experiments, occurrence increased with patch size, yet interactions of patch size and landscape-scale loss and fragmentation drove species occurrence in patches. Importantly, the direction of effects were consistent across scales and effects of patch size were sufficient to predict the effects of habitat loss and fragmentation across entire landscapes. Our experimental results suggest that changes at both the patch and landscape scales can impact populations, but that a long-standing pattern-the patch-size effect-captures much of the key variation shaping patterns of species occurrence., (© 2023 The Ecological Society of America.)

Emigration propensity (i.e., the tendency to leave undisturbed patches) is a key life-history trait of organisms in metapopulations with local extinctions and colonizations. Metapopulation models of dispersal evolution typically assume that patch disturbance kills all individuals within the patch, thus causing local extinction. However, individuals may instead be able to leave a patch when it is disturbed, either by fleeing before being killed or simply because the disturbance destroys the patch without causing mortality. This scenario may pertain to a wide range of organisms from horizontally transmitted symbionts, to aquatic insects inhabiting temporary ponds, to vertebrates living in fragmented forests. We generalized a Levins-type metapopulation model of dispersal evolution by adding a new parameter of disturbance escape probability, which incorporates a second source of dispersal into the model: disturbance-induced emigration. We show that disturbance escape expands the domain of metapopulation viability and selects for lower rates of emigration propensity when disturbance rates are high. The fitness gains from disturbance-induced emigration are generally moderate, suggesting that disturbance escape might act more as a complementary dispersal strategy rather than a replacement to emigration propensity, at least for metapopulations that meet the assumptions of the Levins-type model. Yet disturbance-induced emigration may in some circumstances rescue a metapopulation from long-term extinction when the combination of high disturbance rates and low local population growth rates compromises its viability. Further, a metapopulation could persist exclusively by disturbance escape if local carrying capacities are large enough to counterbalance two sources of mortality: mortality driven by disturbance and mortality during dispersal. This study opens two promising research lines: (1) the investigation of disturbance escape in metapopulations of ephemeral habitats with unsaturated populations and non-equilibrium dynamics and (2) the incorporation of information costs to investigate the joint evolution of disturbance escape and emigration propensity., (© 2021 The Authors. Ecology published by Wiley Periodicals LLC on behalf of Ecological Society of America.)

Recent controversy over whether biodiversity reduces disease risk (dilution effect) has focused on the ecology of Lyme disease, a tick-borne zoonosis. A criticism of the dilution effect is that increasing host species richness might amplify disease risk, assuming that total host abundance, and therefore feeding opportunities for ticks, increase with species richness. In contrast, a dilution effect is expected when poor quality hosts for ticks and pathogens (dilution hosts) divert tick blood meals away from competent hosts. Even if host densities are additive, the relationship between host density and tick encounters can be nonlinear if the number of ticks that encounter a host is a saturating function of host density, which occurs if ticks aggregate on the remaining hosts rather than failing to find a host before death. Both dilution and amplification are theoretical possibilities, and assessing which is more prevalent required detailed analyses of empirical systems. We used field data to explore the degree of tick redistribution onto fewer individuals with variation in intraspecific host density and novel data-driven models for tick dynamics to determine how changes in vertebrate community composition influence the density of nymphs infected with the Lyme bacterium. To be conservative, we allowed total host density to increase additively with species richness. Our long-term field studies found that larval and nymphal ticks redistribute onto fewer individuals as host densities decline, that a large proportion of nymphs and adults find hosts, and that mice and chipmunks feed a large proportion of nymphs. White-footed mice, eastern chipmunks, short-tailed shrews, and masked shrews were important amplification hosts that greatly increased the density of infected nymphs. Gray squirrels and Virginia opossums were important dilution hosts. Removing these two species increased the maximum number of larvae attached to amplification hosts by 57%. Raccoons and birds were minor dilution hosts under some conditions. Even under the assumption of additive community assembly, some species are likely to reduce the density of infected nymphs as diversity increases. If the assumption of additivity is relaxed, then species that reduce the density of small mammals through predation or competition might substantially reduce disease risk.

Vertebrate herbivores and fire are known to be important drivers of vegetation dynamics in African savannas. It is of particular importance to understand how changes in herbivore population density, especially of elephants, and fire frequency will affect the amount of tree cover in savanna ecosystems, given the critical importance of tree cover for biodiversity, ecosystem function, and human welfare. We developed a spatially realistic simulation model of vegetation, fire, and dominant herbivore dynamics, tailored to the Serengeti ecosystem of east Africa. The model includes key processes such as tree-grass competition, fire, and resource-based density dependence and adaptive movement by herbivores. We used the model to project the ecosystem 100 years into the future from its present state under different fire, browsing (determined by elephant population density), and grazing (with and without wildebeest present) regimes. The model produced the following key results: (1) elephants and fire exert synergistic negative effects on woody cover; when grazers are excluded, the impact of fire and the strength of the elephant-fire interaction increase; (2) at present population densities of 0.15 elephants/km2, the total amount of woody cover is predicted to remain stable in the absence of fire, but the mature tree population is predicted to decline regardless of the fire regime; without grazers present to mitigate the effects of fire, the size structure of the tree population will become dominated by seedlings and mature trees; (3) spatial heterogeneity in tree cover varies unimodally with elephant population density; fire increases heterogeneity in the presence of grazers and decreases it in their absence; (4) the marked rainfall gradient in the Serengeti directly affects the pattern of tree cover in the absence of fire; with fire, the woody cover is determined by the grazing patterns of the migratory wildebeest, which are partly rainfall driven. Our results show that, in open migratory ecosystems such as the Serengeti, grazers can modulate the impact of fire and the strength of the interaction between fire and browsers by altering fuel loads and responding to the distribution of grass across the landscape, and thus exert strong effects on spatial patterns of tree cover.