Scale Of Habitat Patches Research Articles

ELEPHANTS AS AGENTS OF HABITAT CREATION FOR SMALL VERTEBRATES AT THE PATCH SCALE

Ecologists increasingly recognize the ability of certain species to influence ecological processes by engineering the physical environment, but efforts to develop a predictive understanding of this phenomenon are in their early stages. While many believe that the landscape-scale effects of ecosystem engineers will be to increase habitat diversity and therefore the abundance and richness of other species, few generalities exist about the effects of engineering at the scale of the engineered patch. According to one hypothesis, activities that increase structural habitat complexity within engineered patches will have positive effects on the abundance or diversity of other organisms. Here I show that, by damaging trees and increasing their structural complexity, browsing elephants create refuges used by a common arboreal lizard. Observational surveys and a lizard transplant experiment revealed that lizards preferentially occupy trees with real or simulated elephant damage. A second experiment showed that lizards vacate trees when elephant-engineered refuges are removed. Furthermore, local lizard densities increased with (and may be constrained by) local densities of elephant-damaged trees. This facilitative effect of elephants upon lizards via patch-scale habitat modification runs contrary to previously documented negative effects of the entire ungulate guild on lizards at the landscape scale, suggesting that net indirect effects of large herbivores comprise opposing trophic and engineering interactions operating at different spatial scales. Such powerful megaherbivore-initiated interactions suggest that anthropogenic changes in large-mammal densities will have important cascading consequences for ecological communities.

Breeding dispersal in a heterogeneous landscape: the influence of habitat and nesting success in greater snow geese

Despite numerous studies on breeding dispersal, it is still unclear how habitat heterogeneity and previous nesting success interact to determine nest-site fidelity at various spatial scales. In this context, we investigated factors affecting breeding dispersal in greater snow geese (Anser caerulescens atlanticus), an Arctic breeding species nesting in two contrasting habitats (wetlands and mesic tundra) with variable pattern of snowmelt at the time of settlement in spring. From 1994 to 2005, we monitored the nesting success and breeding dispersal of individually marked females. We found that snow geese showed a moderate amount of nest-site fidelity and considerable individual variability in dispersal distance over consecutive nesting attempts. This variability can be partly accounted for by the annual timing of snowmelt. Despite this environmental constraint, habitat differences at the colony level consistently affected nesting success and settlement patterns. Females nesting in wetlands had higher nesting success than those nesting in mesic tundra. Moreover, geese responded adaptively to spatial heterogeneity by showing fidelity to their nesting habitat, independently of snowmelt pattern. From year to year, geese were more likely to move from mesic to high-quality wetland habitat, regardless of previous nesting success and without cost on their subsequent nesting performance. The unpredictability of snowmelt and the low cost of changing site apparently favour breeding-site dispersal although habitat quality promotes fidelity at the scale of habitat patches.

Habitat use and group size of pied cormorants (Phalacrocorax varius) in a seagrass ecosystem: possible effects of food abundance and predation risk

Both food abundance and predation risk may influence habitat use decisions. However, studies of habitat use by birds in marine environments have focused only on food abundance. I investigated the possible influences of food abundance and predation risk from tiger sharks (Galeocerdo cuvier) on habitat use by pied cormorants (Phalacrocorax varius) over two spatial scales and on cormorant group size. Cormorants were usually solitary, but group size was highest in shallow habitats during months when shark density was low. Regardless of season, cormorant density within shallow habitats was higher over seagrass than sand, and cormorants were distributed between these two microhabitats proportional to prey density. Therefore, cormorants appear to respond to prey abundance at a relatively narrow spatial scale (i.e., tens of meters). At the habitat-patch scale (~1 km), the density of cormorants and their prey (teleosts) was higher in shallow habitats than in deep ones, but the density of cormorants was influenced by an interaction between water temperature (i.e., season) and habitat. There was decreased use of shallow habitats as water temperature, and the density of tiger sharks, increased. When shark density was low, cormorants were distributed across habitats roughly in proportion to the abundance of fish, suggesting that cormorants respond to food abundance at the scale of habitat patches. However, as shark abundance increased, the relative density of cormorants dropped in the dangerous shallow habitats such that there was a greater density of cormorants relative to their food in deep habitats when sharks were abundant. This suggests that pied cormorants trade-off food and risk by accepting lower energetic returns to forage in safer habitats. This study provides the first evidence that marine habitat selection by birds may be influenced by such a trade-off, and provides further evidence that tiger sharks are important in determining habitat use of their prey and mediating indirect interactions within Shark Bay.

NICHE SHIFT BY VIRGINIA OPOSSUM FOLLOWING REDUCTION OF A PUTATIVE COMPETITOR, THE RACCOON

Increases in mesopredator populations due to habitat fragmentation may have cascading effects on prey (e.g., songbirds) and may increase competitive interactions within the guild. We compared micro- and macrohabitat selection of the Virginia opossum (Didelphis virginiana) between areas with and without removal of the raccoon (Procyon lotor), a putative competitor, in a fragmented habitat in north-central Oklahoma. Live trapping conducted in 1998–2001 resulted in 482 total captures of opossums. Vegetation was sampled in summer (April-August) around trapsites associated with 235 of those captures. Ordination analyses indicated that opossums shifted microhabitat use toward eastern red cedar (Juniperus virginiand) forest when density of raccoons was reduced. Geographic Information Systems analyses at the habitat-patch scale revealed no differences in habitat selection after raccoon reduction. Niche breadths were high for both species, but niche overlap varied by time and treatment. Overlap of habitat patches between raccoons captured preremoval and opossums captured postremoval was high, suggesting a shift in habitat selection. The niche shift by opossums and change in niche overlap between raccoons and opossums supported our competition hypothesis. We acknowledge our lack of replication, but note that our work represents the 1st report of experimental manipulation coupled with work at multiple scales to examine competitive relationships between these key mesopredators.

Influences of Patch Size and Microhabitat on the Demography of Two Old-Field Rodents

We used capture/recapture methods to test the responses of two small mammal species (Peromyscus leucopus and Microtus pennsylvanicus) to small- (microhabitat) and large- (patch) scale habitat variation. Analyses examined the responses of individuals to microhabitat variation among trap stations as well as differences in the density and persistence time of adults and juveniles, and the proportion of reproductive females on experimentally created patches of three sizes (0.0625, 0.25, and 1.0 ha). With the exception of transient P. leucopus, all groups shared significant correlations with microhabitat at the scale of trap stations. By contrast, only juvenile P. leucopus exhibited a response to patch-size (i.e., higher densities on small relative to larger patches). Microhabitat differences among patches also accounted for variation in M. pennsylvanicus densities (but not P. leucopus) in analyses of covariance. Our results suggest that both individuals and populations of M. pennsylvanicus responded to habitat variation at the microhabitat scale, while P. leucopus appeared to respond to both microhabitat and patch scale habitat variation. We note that species characteristics (particularly relative dispersal ability) may prove critical in predicting the scale of habitat responses. We conclude by noting that current theory that assumes uniform responses of population to homogeneous patches is too simplistic to be of much predictive value in field tests.

Spatial scale, patchiness and population dynamics on land

The most striking difference between land and open sea is the greater structural heterogeneity of terrestrial environments. I make a distinction between two principal kinds of patches at two spatial scales, defined by the relative contributions of behaviour and demography to variation in density. At the scale of resource patches, movements of individuals among the patches influence the frequencies of ecological interaction among the mobile individuals and their offspring. Many studies have demonstrated how independently aggregated spatial distributions generally enhance stability of single-species and many-species dynamics. At the scale of habitat patches, assemblages of local populations connected by migration constitute metapopulations.