Michael A. Bowers

Body Size and Coexistnce in Desert Rodents: Chance or Community Structure?

We use a simple test to evaluate the null hypotheses that desert rodent communities are composed of species assembled at random with respect to body size. This hypothesis is unequivocally rejected for species in the granivore guild: species of similar size (body mass ratios <1.5) coexist less frequently in local communities and overlap less in their geographic distributions than expected on the basis of chance, suggesting that their co—occurrence is precluded by interspecific competition. When granivore species and members of other guilds are combined in the same analysis, the patterns apparent in granivores diminish or disappear. These results indicate that ability to detect community structure depends to a large extent on thorough knowledge of the organisms and choice of appropriate statistical tests. We suggest that recent claims that many communities are indistinguishable from random associations of species are premature.

Landscape Ecology of Mammals: Relationships between Density and Patch Size

A much discussed issue in landscape ecology is how processes that operate within spatially subdivided subpopulations scale-up to create a larger, landscape-level dynamic. A first step in answering this question is to ask to what degree subpopulations within a landscape vary in performance. Here we test the null hypothesis that densities of mammalian populations are constant over patches of varied size, i.e., that performance, as estimated via density, does not covary with patch area. Using a composite database from published studies, we found that densities of 20 of 32 species did not vary with patch area, while five showed increasing and seven decreasing density-area relationships. Studies reporting significant density-area relationships tended to include a greater number of patches of a greater range of sizes than those that reported no relationship, suggesting that statistical power may be an issue. Landscapes comprised of smaller, less-isolated patches tended to have negative density-area relationships and landscapes with larger, more isolated patches tended to have positive density-area relationships. Our results suggest that no consistent density-area relationship operates over all systems of patches. Instead, the patterns appear to be scale-dependent : frequent movement of individuals in the process of selecting habitats (patches) over smaller-scaled landscapes produced negative density-area relationships; movement of individuals among more isolated patches appeared to involve larger- and longer-scale population processes involving colonization and extinction and positive density-area relationships. Despite the fact that patches represent a central focus in landscape ecology, they appear to be a construct of human convenience rather than biological entities with a set number and kind of processes.

Interpatch movements in spatially structured populations: a literature review

We used published data of individuals moving among habitat patches to answer questions pertaining to frequency of interpatch movements and subsequent effects on population dynamics. A review of 415 published articles produced data for 89 species-system combinations where movements were recorded in sufficient detail to include in our analysis. The percentage of individuals in a population that moved among habitat patches ranged from 0.00 to 93.00%, with a mean of 16.84%. Scaling this statistic by generation time yielded a mean movement rate of 15.45 ± 3.27% per generation. The relatively low movement rates suggest that subpopulations, except those of invertebrates, should not be highly integrated. Less than half of the empirical studies reported on the population effects of interpatch movement. Of these, thirty-three studies yielded population effects on 34 individual species in 45 species-systems. They reported movement having a positive effect 28 times, a negative effect twice and a neutral effect 14 times. Despite its importance, relatively few studies document rates of interpatch movement and far fewer determine population level consequences of these movements. This deficiency limits our ability to understand the dynamics of spatially structured populations and apply that knowledge to conservation efforts.

A test of the ‘hot’ mustard extraction method of sampling earthworms

Abstract Earthworm densities in soil are difficult to quantify. Part of the problem is that they are incorporated closely into the soil structure, which makes their extraction tedious and time consuming ( Edwards, 1991 ). In addition, earthworms have widely varying patterns of activity and occupy a range of soil depths depending upon season, species, and life history stage ( Bouche and Gardner, 1984 ). Behavioral methods, which involve stimulating earthworms with a chemical or other expellant (e.g. an electrical shock) and collecting them as they emerge at the surface, have been proposed as efficient extraction methods that do not require physical disruption of the soil system. We examined the effectiveness of one such technique: the non-toxic ‘hot’ mustard extraction method. We compare hot mustard extraction with digging and hand-sorting, and conclude that it provides a consistent index of earthworm abundance across a range of soil and land-use types.

Predation hazard and seed removal by small mammals : microhabitat versus patch scale effects

Predator avoidance may involve response strategies of prey species that are time and space specific. Many studies have shown that foraging individuals avoid predators by altering microhabitat usage; alternatively, sites may be selected according to larger-scale features of the habitat mosaic. We measured seed removal by two small mammal species (Peromyscus leucopus, and Microtus pennsylvanicus) at 474 stations over an experimentally created landscape of 12 patches, and under conditions of relatively high (full moon) and low (new moon) predatory hazard. Our objective was to determine whether predator avoidance involved the selection of small-, medium-, or large-scale features of the landscape (i.e., at the scale of microhabitats, habitats, or habitat patches). We found rates of seed removal to vary more with features of whole patches than according to variation in structural microhabitats within patches. Specific responses included: under-utilization of patch edge habitats during full moon periods, and microhabitat effects that were only significant when considered in conjunction with larger-scale features of the landscape. Individuals residing on large patches altered use of microhabitats/habitats to a greater extent than those on smaller patches. Studies just focusing on patterns of microhabitat use will miss responses at the larger scales, and may underestimate the importance of predation to animal foraging behavior.

Bumble bee colonization, extinction, and reproduction in subalpine meadows in northeastern Utah

The distributional patterns of six bumble bee (Bombus) species in discrete subalpine meadows in Utah were studied over the course of a summer. Data on the movements of individuals were obtained through mark—recapture studies of queens during nest initiation and of workers later in the summer. Censuses of the species and abundance of bumble bees and flowers in sample quadrats of 20 meadows revealed patterns of diversity, colony extinction (inferred from the absence of a species after it had been previously recorded in the meadow), and production of new reproductives. The results suggest that the number and diversity of queens that colonize a meadow in early summer is a function of meadow area. In mid to late season, after nest sites have been chosen and colonies established, meadow floristics (flower composition) governs within—meadow Bombus species composition and the persistence, ontogeny, and reproduction of bumble bee colonies. Workers appear not to utilize flowers outside the meadow where their colony is located. Specific results were that (1) mid— to late—summer bumble bee diversity was best correlated with meadow floristics, (2) flower compositions in meadows where local extinctions of Bombus occurred differed markedly from those where all species persisted, (3) the distribution of new reproductives among the meadows was independent of bombus species composition, and (4) late—season reproductive timing of Bombus flavifrons appeared to be a function of floristic composition and, particularly, nectar availability. These results underscore the importance of flower resources while downplaying the role of interactions among Bombus colonies and species in accounting for patterns of species persistence and colony reproduction.

Seed removal experiments on desert rodents: the microhabitat by moonlight effect

. 1980. Food habits of the guanaco (Lama guanicoe) of Tierra de Fuego, Chile. Turrialba, 30:177-181. REDON, J. 1982. Lichens of arid South America. J. Hattori Bot. Lab., 53:337-339. ROCHELLE, J. 1980. Mature forests, litterfall and patterns of forage quality as factors in the nutrition of black-tailed deer on northern Vancouver Island. Unpubl. Ph.D. dissert., Univ. British Columbia, Vancouver, 295 pp. RUNDEL, P. 1978. Ecological relationships of desert fog zone lichens. Bryologist, 81:277-293. RUNDEL, P. W., AND M. MAHU. 1976. Community structure in a coastal fog desert in northern Chile. Flora, 165:493-505. SIMONETTI, J. A., AND E. R. FUENTES. 1985. Guanacos versus cabras y conejos como moduladores del matorral de Chile central. Pp. 76-87, in Actas de la IV Convencion Internacional sobre Camelidos Sudamericanos, Punta Arenas, Chile (C. Venegas and C. Cunazza, eds.). 285 pp.

DEMOGRAPHIC RESPONSES TO HABITAT FRAGMENTATION: EXPERIMENTAL TESTS AT THE LANDSCAPE AND PATCH SCALE

We tested the ecological consequences of habitat fragmentation by comparing the density, population growth rate, survivorship, and recruitment of Microtus pennsylvanicus populations within a 20-ha fragmented landscape with those of populations in a 20-ha unfragmented landscape. We also tested for fragment-size effects by comparing the same measures of demographic performance across three fragment sizes (0.06, 0.25, and 1.0 ha). During 17 censuses between June 1993 and October 1994, we recorded 10020 captures of 3946 individuals and found strong landscape differences but weak fragment-size effects. Although fragmentation reduced the habitable area by 72%, density and adult recruitment were significantly higher on the fragmented landscape relative to the control. With the exception of adult recruitment (higher on small relative to medium and large patches), no significant demographic differences existed among patches of different size. Low rates of between-population movement and differential juvenile growth rates suggested that higher recruitment rates on the fragmented landscape likely resulted from enhanced local reproduction rather than from immigration. Thus, despite the fact that populations in the fragmented landscape experienced severe habitat loss, some individuals on fragments accrued important reproductive advantages (possibly as a result of diminished social costs or enhanced food resources). That population and individual responses to fragmentation could differ so dramatically provides a novel result that illustrates the importance of using hierarchical field designs in tests of population responses to large-scale habitat alteration. We conclude that controlled, large-scale field tests can serve as an important intermediary between the inherent abstraction of simulation modeling and what is observed in the real world.

Use of space and habitats by meadow voles at the home range, patch and landscape scales

Using capture/recapture methods, we examined the spatial usage patterns of Microtus pennsylvanicus within and between experimentally created habitat patches of three sizes (1.0, 0.25 and 0.0625 ha) and between a 20-ha fragmented and a 20-ha continuous habitat landscape. We tested the prediction that home ranges near patch edges would be qualitatively different from those in patch interiors, and that the edge:interior habitat ratio could be used to make predictions concerning the dispersion and spatial use of individuals occupying different sized patches and between landscapes with different habitat structure. We found adult females on patch edges to have larger and more exclusive home ranges, larger body sizes, longer residence times, and to reproduce at a higher frequency than those in patch interiors. These “edge effects” also appeared to be largely responsible for the greater proportion of larger, reproductive females we found in small than larger patches and in the fragmented than in the continuous habitat (control) landscape. The selection of higher quality edge habitats by dominant females and the relegation of sub-dominants to patch interiors provides an explanation for the observed differences in the distribution and performance of females over patches and between landscapes.

COMMUNITY ORGANIZATION IN HUMMINGBIRDS: RELATIONSHIPS BETWEEN MORPHOLOGY AND ECOLOGY

AssTRACT.--Hummingbird species inhabiting restricted geographic regions exhibit morphological patterns that differ significantly from those predicted by null models in which species are selected at random from appropriate species pools. Temperate North American hummingbirds are convergent: more similar in bill length, body weight, and wing length than predicted by several null models. These temperate species also are more similar to each other than they are to more closely related (congeneric) species from subtropical and tropical habitats. Hummingbirds of the Greater and Lesser Antilles show a nonrandom distribution of species among islands: all islands inhabited by hummingbirds have at least two species, and these fall into two distinctly different size categories. Allometric scaling of bill length with respect to body mass is distinctive in Antillean hummingbirds; bill length increases more rapidly with increasing body weight in West Indian hummingbirds than in random samples of hummingbirds of the world or in other birds. These morphological patterns appear to reflect two ecological processes: interspecific competition among hummingbirds and mutualistic coevolution with flowers. Hummingbird species of similar morphology use similar floral resources but rarely coexist in the same local areas. Species of divergent morphology exploit different food resources and frequently coexist locally. Length of the bill, which influences access to different kinds of flowers, is particularly important in the organization of these simple hummingbird associations. Received 28 June 1983, accepted 5 July 1984.