Douglas W. Morris

Toward an ecological synthesis: a case for habitat selection

Habitat selection, and its associated density and frequency-dependent evolution, has a profound influence on such vital phenomena as population regulation, species interactions, the assembly of ecological communities, and the origin and maintenance of biodiversity. Different strategies of habitat selection, and their importance in ecology and evolution, can often be revealed simply by plots of density in adjacent habitats. For individual species, the strategies are closely intertwined with mechanisms of population regulation, and with the persistence of populations through time. For interacting species, strategies of habitat selection are not only responsible for species coexistence, but provide one of the most convenient mechanisms for measuring competition, and the various community structures caused by competitive interactions. Other kinds of interactions, such as those between predators and prey, demonstrate that an understanding of the coevolution of habitat-selection strategies among strongly interacting species is essential to properly interpret their spatial and temporal dynamics. At the evolutionary scale, the frequency dependence associated with habitat selection may often allow populations to diverge and diversify into separate species. Habitat selection thereby demonstrates how we can map microevolutionary strategies in behavior onto their population and community consequences, and from there, onto macroevolutionary patterns of speciation and adaptive radiation. We can anticipate that future studies of habitat selection will not only help us complete those maps, but that they will also continue to enrich the panoply of ideas that shape evolutionary ecology.

Ecological scale and habitat use

Population density can respond to habitat at different scales. If habitat selection occurs as a consequence of resource exploitation, the density of fine—grained consumers should reflect microhabitat variation. But if habitat use is controlled by a variety of selective pressures, it is no longer apparent whether density should respond to micro— or macrohabitat. These two alternatives can be tested simultaneously by multiple regression models where macrohabitats are represented by dummy variables. When the local densities of two Temperate Zone rodents were analyzed in this way, macrohabitat and temporal effects were consistent significant predictors of rodent density; microhabitat was not. This analysis suggests species whose patterns of resource and habitat use probably depart from classical interpretations of species coexistence. It is probably premature to assess the role of habitat selection in the structure of ecological systems until the results of further tests of habitat scaling are reported. See f...

Habitat-dependent population regulation and community structure

SummaryDensity-dependence provides a causal link between processes acting at different levels of ecological organization. The linkage between density-dependent habitat use, population regulation and community organization is examined on the basis of qualitative and quantitative differences between habitats. These differences are expressed as characteristic shapes on isodars which are lines of equal fitness, and are plotted in density space as lines at every point of which the fitness of individuals in one habitat is equal to that of individuals in another. Isodars can be constructed for single species or modified to include the effects of interacting species. Isodars are easily analyzed by linear regression to differentiate between alternative modes of population regulation and to suggest patterns of community structure. Different isodars are causally related to different kinds of community structure, and suggest the existence of four new forms of community organization; equal, differential, switched and mixed preferences. A preliminary isodar analysis on a common rodent species demonstrates that population regulation depends upon habitat, and that mixed preferences probably organize the rodent community. Habitat-dependent population regulation has farreaching implications to studies of temporal and spatial scale, and to all ecological processes that are density-dependent.

Scales and costs of habitat selection in heterogeneous landscapes

SummaryTwo scales of habitat selection are likely to influence patterns of animal density in heterogeneous landscapes. At one scale, habitat selection is determined by the differential use of foraging locations within a home range. At a larger scale, habitat selection is determined by dispersal and the ability to relocate the home range. The limits of both scales must be known for accurate assessments of habitat selection and its role in effecting spatial patterns in abundance. Isodars, which specify the relationships between population density in two habitats such that the expected reproductive success of an individual is the same in both, allow us to distinguish the two scales of habitat selection because each scale has different costs. In a two-habitat environment, the cost of rejecting one of the habitats within a home range can be expressed as a devaluation of the other, because, for example, fine-grained foragers must travel through both. At the dispersal scale, the cost of accepting a new home range in a different habitat has the opposite effect of inflating the value of the original habitat to compensate for lost evolutionary potential associated with relocating the home range. These costs produce isodars at the foraging scale with a lower intercept and slope than those at the dispersal scale.Empirical data on deer mice occupying prairie and badland habitats in southern Alberta confirm the ability of isodar analysis to differentiate between foraging and dispersal scales. The data suggest a foraging range of approximately 60 m, and an effective dispersal distance near 140 m. The relatively short dispersal distance implies that recent theories may have over-emphasized the role of habitat selection on local population dynamics. But the exchange of individuals between habitats sharing irregular borders may be substantial. Dispersal distance may thus give a false impression of the inability of habitat selection to help regulate population density.

OPTIMALLY FORAGING MICE MATCH PATCH USE WITH HABITAT DIFFERENCES IN FITNESS

We tested the fundamental assumption of the “optimality paradigm” that the foraging behavior of individual organisms corresponds to what we would expect if it had been honed by natural selection to match habitat differences in reproductive success. First, we used long-term studies of life history and habitat selection in white-footed mice to illustrate that the fitness of females living in the forest is greater than that of females living in forest-edge habitat. Second, we used short-term foraging studies to evaluate whether food patches located in the forest provided more value to foragers than did those in the edge. Third, we used foraging studies and data on the occurrence of predators to demonstrate that animals foraging in areas with little cover face higher risks than when they forage in areas with more cover. We confirmed three a priori predictions: (1) Individual mice abandoned foraging patches at higher harvest rates in edge habitat than they did in forest. (2) Individuals harvested resource patches to lower quitting harvest rates under cover than they did when patches were located in the open. (3) The difference in quitting-harvest rate between “open” and “covered” patches was less in the safe forest habitat than it was in the risky edge habitat. Our results yield an impressive fit with our previous knowledge of habitat differences in reproductive success and substantiate the premise that short-duration strategic decisions by individuals match habitat differences in fitness.

TESTS OF DENSITY-DEPENDENT HABITAT SELECTION IN A PATCHY ENVIRONMENT'

A simple regression analysis can be used to assess the response of animal density to differences in habitat quality. The same test can evaluate general predictions of habitat selection theory as well as search for differences in the shapes of habitat suitability- density functions, something previous tests have been unable to do. Combined with de- mographic or other estimates of fitness, regression tests can provide new insights into the evolution of habitat selection. Regression and fitness tests were used to explore the pattern of density-dependent habitat use in two temperate-zone rodents. The intensity of population regulation appeared to be inversely related to a habitats carrying capacity. Variation in density-dependent habitat choice suggests new and unexpected dispersal strategies that vary with habitat heterogeneity. The predictions of the theory are complicated when habitat quality varies independently of population density. Sweepstake fitness rewards may be reaped by animals that would previously have been assumed to have made a suboptimal habitat choice. At the level of microhabitat, habitat selection models appeared to be much less capable of predicting variation in population density. Such a relation could be due to complex and inverse relationships between microhabitat quality and carrying capacity, or it could simply reflect scaling patterns in habitat selection.

Considering ecological dynamics in resource selection functions.

1. Describing distribution and abundance is requisite to exploring interactions between organisms and their environment. Recently, the resource selection function (RSF) has emerged to replace many of the statistical procedures used to quantify resource selection by animals. 2. A RSF is defined by characteristics measured on resource units such that its value for a unit is proportional to the probability of that unit being used by an organism. It is solved using a variety of techniques, particularly the binomial generalized linear model. 3. Observing dynamics in a RSF - obtaining substantially different functions at different times or places for the same species - alerts us to the varying ecological processes that underlie resource selection. 4. We believe that there is a need for us to reacquaint ourselves with ecological theory when interpreting RSF models. We outline a suite of factors likely to govern ecologically based variation in a RSF. In particular, we draw attention to competition and density-dependent habitat selection, the role of predation, longitudinal changes in resource availability and functional responses in resource use. 5. How best to incorporate governing factors in a RSF is currently in a state of development; however, we see promise in the inclusion of random as well as fixed effects in resource selection models, and matched case-control logistic regression. 6. Investigating the basis of ecological dynamics in a RSF will allow us to develop more robust models when applied to forecasting the spatial distribution of animals. It may also further our understanding of the relative importance of ecological interactions on the distribution and abundance of species.

Density-dependent habitat selection: testing the theory with fitness data

SummaryAccording to density-dependent habitat selection theory, reproductive success should decline with increased density. Fitness should be similar between habitats if habitat selection follows an ideal free distribution; fitness should be dissimilar between habitats if habitat selection is modified by territorial behavior. I tested these assumptions by examining a variety of fitness estimates obtained from white-footed mice living in nest boxes in forest, forest edge and fencerow habitats in southwestern Ontario. As expected, mean litter size declined with increased population density. Litter sizes, adult longevity and the proportion of adult animals in breeding condition were not significantly different among the three habitats. The success at recruiting at least one offspring to the adult population and the number of recruits per litter were much greater in the forest than in either of the other two habitats. Fitness was thus unequal among habitats and the results confirm both assumptions of density-dependent habitat selection theory for territorial white-footed mice.

Spatial scale and the cost of density-dependent habitat selection

SummaryHabitat selection costs depend upon the scale of habitat. At the fine-grained microhabitat scale, cost is linked to optimal foraging, and habitat selection should be abandoned even though fitness is greater in one microhabitat than in another. At the coarse-grained macrohabitat scale, cost is linked to emigration, and habitat selection should often be maintained even though fitness may be less in the ‘preferred’ macrohabitat than in others. Macrohabitat selection cost is easily incorporated into habitat selection theory and can be tested by linear regression techniques on isodars (lines of every point at which the fitness of individuals in one habitat equals that of individuals in another). The results of one recent survey of white-footed mice living in different macrohabitats are consistent with the predictions of emigration cost.

Habitat matching: Alternatives and implications to populations and communities

SummaryI evaluate habitat matching rules based on ideal distribution models of density-dependent habitat use. Recent approaches and the ideal free continuous input matching rule on which they depend, are restricted to only those habitats that are jointly occupied across the full range of population sizes. These assumptions may often be inappropriate to field applications of habitat matching. I develop alternatives that can be applied to a wide array of ideal forms of habitat selection, including the ideal free, continuous input example. Input matching can be distinguished from assumptions of consumer-resource models and preemptive habitat use by regressions of density between paired habitats (isodars). Isodars for continuous input models should be linear on a logarithmic scale, while those for consumer-resource models should be linear on an arithmetic scale. Pre-emptive isodars can be distinguished from the others by dramatic non-linearities at both low and high densities. Field data on white-footed mice support the consumer-resource theory. Implications of the rules for population regulation and community organization are highlighted by new models that specify how the fitness of pre-emptive habitat selectors should decline with increasing density. Strong non-linearities produced by comparisons between variable and homogeneous habitats produce reversing source-sink population regulation and a new form of cyclical community dynamics. Variable habitats act as a source of emigrants at low density and a sink for immigrants at high density. Subordinate species may occupy only the variable habitat at both low and high density.