Michael S. Gaines

Analysis of adaptation in heterogeneous landscapes: Implications for the evolution of fundamental niches

SummaryThe fundamental niche is a description of the range of environmental conditions in which the mean fitness of a population exceeds or equals unity, and outside of which its mean fitness is less than one. The fundamental niche is a mean phenotype of a population, a trait that can evolve by natural selection. In the analysis of the evolution of adaptations by natural selection one must specify the range of environments within which the relative fitnesses of alternative phenotypes are compared. Population dynamics automatically biases the environments experienced by an evolutionary lineage, simply because more individuals tend to be found within the fundamental niche than outside it (unless the population as a whole is going extinct). We argue that this basic asymmetry biases adaptive evolution toward further improvement to conditions inside the fundamental niche, even at the expense of fitness outside it. This suggests that natural selection may act principally as a conservative force on fundamental niches. We place the particular problem of the evolution of fundamental niches into the general framework of specifying the spatiotemporal scale for the analysis of adaptation in heterogeneous environments and introduce the notion of a ‘phylogenetic envelope’, a heuristic representation of this scaling. Because all of microevolution necessarily occurs within the constraint of the evolutionary dynamics of the fundamental niche, we conclude that understanding such dynamics should be of central concern to evolutionary ecologists.

Diverse and Contrasting Effects of Habitat Fragmentation

Different components of an ecosystem can respond in very different ways to habitat fragmentation. An archipelago of patches, representing different levels of fragmentation, was arrayed within a successional field and studied over a period of 6 years. Ecosystem processes (soil mineralization and plant succession) did not vary with the degree of subdivision, nor did most measures of plant and animal community diversity. However, fragmentation affected vertebrate population dynamics and distributional patterns as well as the population persistence of clonal plant species. The results highlight the dangers of relying on broad community measures in lieu of detailed population analyses in studies of fragmented habitats.

Habitat fragmentation and movements of three small mammals (Sigmodon, Microtus, and Peromyscus)

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 M2), clusters of medium patches (288 M2), and clusters of small patches (32 M2). 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 frag- mentation 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.

Vegetation Dynamics in an Experimentally Fragmented Landscape

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 m2, 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 land- scape. 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 diver- gence 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.

GENETIC CHANGES IN FLUCTUATING VOLE POPULATIONS

Microtine rodents undergo periodic fluctuations in population density (Krebs et al., 1969). Ecologists have searched unsuccessfully for the driving force behind these fluctuations, invoking environmental factors such as weather (Elton, 1929), nutrition (Pitelka, 1964), disease (Elton, 1925), and predation (Pearson, 1966). What has been conspicuously ignored in most studies on the regulation of rodent numbers is the genetic composition of the population. This study is a continuation of previous efforts (Krebs et al., 1969; Tamarin and Krebs, 1969) combining population genetics and ecology to provide predictive insight into population fluctuations of small rodents. The purpose of the study was threefold: (1) to demonstrate how the genetic architecture of a population changes with different phases of a population fluctuation, (2) to correlate genetic changes with demographic events occurring in the population, and (3) to establish the temporal and spatial replicability of these genetic changes. Several attempts have been made to monitor genetic changes in fluctuating rodent populations. Canham (1969), studying transferrin and albumin polymorphisms in fluctuating populations of Peromyscus maniculatus, Cleithrionomys rutilus, and C. gapperi, found that heterozygote fitness

The effects of a successional habitat mosaic on a small mammal community

Small herbivorous mammals living in successional patchy environments may be affected by both patch size variations and successional changes. If patch size affects densities of residents then larger patches should have higher densities compared to smaller patches because larger patches often have greater habitat diversity and lesser chances of stochastic events destroying the population. If patch size affects length of time animals spend on patches (persistence rates) then smaller patches should have lower persistence rates compared to larger patches because smaller patches contain fewer resources. Larger animals may be constrained by body size, as well as population size, to reside only on the larger patches because smaller patches do not contain enough resources. If successional changes affect resident animals that are host specific on colonizing plant species, then the effects may be noticeable immediately following field abandonment because plant species replacement rates are most rapid then. Four small mammal species were included: Sigmodon hispidus, Reithrodontomys mega- lotis, Microtus ochrogaster, and Peromyscus maniculatus. The field was divided into patches that varied in area by two orders of magnitude with patches of each size treatment placed in each section of the field. Patch size effects on densities and persistence rates of the four mammal species were tested with analyses of variance. Plant species were grouped according to seven life history traits so as to include relative percent cover of most plant species. Multiple linear regression analyses were used to identify which variables (patch size, field sections, and percent relative cover) significantly explained variations in small mammal densities. Effects of patch size on animal densities and persistence rates varied among animal species. The largest species, Sigmodon hispidus, was never found on the smallest patches, but densities of Reithrodontomys megalotis, Microtus ochrogaster, and Peromyscus manicu- latus were often highest on the smallest patches. Animal density variations were often significantly explained by patch size using multiple regression analyses (P ? .001). However, lack of consistent trends with respect to the other independent variables included in the equations and variable R2 values suggested that other, unmeasured, variables may have influenced the results. Persistence rates of the three species were often highest on the largest patches. Results suggested that animals visited small patches more often than large patches without staying, and used large patches to create territories. Small and medium patches may have been used as archipelagos of preferred habitat interspersed with less desirable habitat. The first stage of old-field succession was completed by the end of the 3rd yr: perennials replaced annuals as the dominant plant species. Neither patch size nor field sections sig- nificantly affected percent cover.

An Experimental Analysis of Dispersal in Fluctuating Vole Populations: Demographic Parameters

Demographic attributes of dispersers from Microtus ochrogaster populations in eastern Kansas, USA, were characterized by three experimental methods: (1) voles were trapped and removed from two grids every 2 wk from 1973—1977 and voles that colonized these vacuums were compared to residents on neighboring control grids; (2) two grids were burned in the springs of 1976 and 1977; voles colonizing grids during a 6—wk period after the fire were compared to resident voles before the fire; (3) ten virgin male and female voles from a laboratory colony were introduced in 1975—1976 and again in 1976—1977 into three fenced enclosures equipped with exit doors. After breeding populations were established, voles emigrating through exit doors were compared with voles remaining inside the enclosures. In the removal—grid and fenced—enclosure experiments, more dispersal occurred when resident populations were increasing or at peak densities than during phases of declining density. The loss from control grids attributable t...

Population Dynamics of Microtus Ochrogaster in Eastern Kansas

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THE INTERACTION OF HABITAT FRAGMENTATION, PLANT, AND SMALL MAMMAL SUCCESSION IN AN OLD FIELD

We compared the density and spatial distribution of four small mammal species (Microtus ochrogaster, Peromyscus maniculatus, Sigmodon hispidus, and P. leucopus) along with general measures of an old field plant community across two successional phases (1984–1986 and 1994–1996) of an experimental study of fragmentation in eastern Kansas. During the early phase the plant community was characterized by little spatial or temporal variance across patch size, consistent with spatially neutral models of succession. In contrast, there was a strong, species-specific effect of patch size on small mammal species distribution and abundance. The lack of variance in vegetation structure across patch size during the early seres suggests that small mammal distributions were responding in large part to features of the system other than variance in vegetation structure and composition across patch size. As succession proceeded, the colonization of the system by woody plant species precipitated a series of patch size effect...

TEMPORAL PATTERNS OF ALLOZYMIC VARIATION IN FLUCTUATING POPULATIONS OF MICROTUS OCHROGASTER

Microtine rodents undergo 2-4 year fluctuations in population density known as population cycles. A genetic-behavioral mechanism that is based on an rand Kselection argument has been proposed by Chitty (1960, 1967) for controlling these cycles. During the phase of increasing numbers when mutual interference is minimal, genotypes with a high reproductive effort have a selective advantage, whereas during the peak phase when mutual interference is intensive, there is selection for aggressive behavior. Although aggressive genotypes have an advantage with respect to intraspecific competition for food and space, their fitness is reduced in other ways which sets the stage for the decline phase. Thus, Chittys hypothesis assumes that natural selection coupled with changes in the genetic composition of the population is the driving force behind population cycles. The results from studies monitoring allozymic variation in fluctuating vole populations are compatible with Chittys hypothesis. Semeonoff and Robertson (1968) observed changes in gene frequency at a plasma esterase locus in a declining population of Microtus agrestis. Canham (1969) found that heterozygote fitness at transferrin and albumin loci was correlated with density in Peromyscus maniculatus and Clethrionomys gapperi populations. Changes in gene frequency at transferrin (Tamarin and Krebs, 1969) and leucine aminopeptidase loci (Gaines and Krebs, 1971) were correlated with