No abstract

No abstract

Both physical and biological processes shape species assemblages (communities). For birds, vegetation structure has long been assumed to be the dominant factor in habitat selection, especially along successional gradients. While vegetation may be important as a proximate factor, detailed knowledge of ultimate factors governing habitat selection is required. Gradients of microclimate, especially temperature and moisture, may be such an ultimate factor through direct physiological pressures on birds or indirectly through distribution and availability of food resources. We documented the existence of gradients of both vegetation structure and microclimate in the undergrowth of seasonally humid forest in central Panama. To assess the relative importance of these gradients in shaping local avian distribution, birds were netted in undergrowth (up to 3 m above ground) during 2—wk periods in dry (March) and wet (July) season for 4 yr (1979—1982). A total of 3037 captures of 95 species was recorded during that period. Although patterns of species richness and capture rate are relatively simple, species composition and other assemblage attributes are complex and difficult to interpret without careful evaluation of the dynamics of individual species. The variety of activity patterns exhibited by birds on moisture and vegetation gradients clearly demonstrates the complexity of dynamics that affect assemblage attributes. Overall, birds are more active at dry sites and at sites with intermediate shrub density. However, activity levels change with time, suggesting that avian activity reflects a synamic process of habitat selection. Though no food resource data are available at present, the general pattern of habitat use on diurnal time scales for several guilds suggests that birds track microclimatic optima for physiological reasons. Habitat selection processes are both extremely complex and variable in time and space for the avifauna of tropical forest undergrowth. Indeed, the scale of study in space and time is important in determining the conclusion of a study. The recent perception of a dichotomy between equilibrium (deterministic) and nonequilibrium (stochastic) assemblages is undermined by these results. Habitat selection in birds produces dynamic (nonequilibrium) assemblages in space and time. However, these are not stochastic assemblages. Each species seeks habitat optima in the context of current environmental conditions on diurnal, seasonal, and between—year time scales. Thus, instead of nonequilibrium and stochastic assemblages, the avifaunas of tropical forest undergrowth are nonequilibrium but relatively predictable from knowledge of current environmental conditions.

No abstract

No abstract

We examined the impacts of possible future land development patterns on the biodiversity of a landscape. Our landscape data included a remote sensing derived map of the current habitat of the study area and six maps of future habitat distributions resulting from different land development scenarios. Our species data included lists of all bird, mammal, reptile, and amphibian species in the study area, their habitat associations, and area requirements for each. We estimated the area requirements using home ranges, sampled population densities, or genetic area requirements that incorporate dispersal distances. Our measures of biodiversity were species richness and habitat abundance. We calculated habitat abundance in two ways. First, we computed the total habitat area for each species in each landscape. Second, we calculated the number of habitat units for each species in each landscape by dividing the size of each habitat patch in the landscape by the area requirement and summing over all patches. Species richness was based on presence of habitat. Species became extinct in the landscape if they had no habitat area or no habitat units, respectively. We then computed ratios of habitat abundance in each future landscape to habitat abundance in the present for each species. We also computed the ratio of future to present species richness. We then calculated summary statistics across all species. Species richness changed little from present to future. There were distinctly greater risks to habitat abundance in landscapes that extrapolated from present trends or zoning patterns, however, as opposed to landscapes in which land development activities followed more constrained patterns. These results were stable when tested using Monte Carlo simulations and sensitivity tests on the area requirements. We conclude that this methodology can begin to discriminate the effects of potential changes in land development on vertebrate biodiversity.

Artificial nest experiments have been used in an attempt to understand patterns of predation affecting natural nests. A growing body of literature suggests that neither relative rates nor patterns of predation are the same for artificial and natural nests. We studied nest predation and daily mortality rates and patterns at real and artificial ground and shrub nests to test the validity of artificial nest experiments. We monitored 1667 artificial and 344 natural nests, over seven trials, in three regions, across 58 sites in Ontario. We controlled for many of the factors thought to be responsible for previously reported differences between predation rates on natural and artificial nests. Although artificial nests in our study resembled natural nests, contained eggs of appropriate size, shape, and color of target bird species, and were placed in similar microhabitats as natural nests, the rates of predation on these nests did not parallel rates on natural nests for any region in terms of absolute rate or pattern. Predation rates on artificial nests did not vary between years, as they tended to for natural nests, and the magnitude of predation pressure on artificial ground nests compared with shrub nests did not show the same pattern as that on natural nests. In general, rates of predation on artificial nests were significantly higher than on natural nests. Our results suggest that conclusions derived from artificial nest studies may be unfounded. Given that many influential ideas in predation theory are based on results of artificial nest experiments, it may be time to redo these experiments with natural nests.Resumen: Se han utilizado experimentos con nidos artificiales con la intención de entender los patrones de depredación que afectan a los nidos naturales. La bibliografía sugiere que ni las tasas relativas ni los patrones de depredación son iguales para nidos artificiales y naturales. Estudiamos las tasas y patrones de depredación de nidos y de mortalidad diaria en nidos reales y artificiales sobre el suelo y en matorrales para probar la validez de los experimentos con nidos artificiales. Monitoreamos 1667 nidos artificiales y 344 nidos naturales, en siete pruebas, en tres regiones, en 58 sitios en Notario. Controlamos muchos de los factores que se piensa son responsables de diferencias entre tasas de depredación en nidos naturales y artificiales reportadas previamente. Aunque los nidos artificiales en nuestro estudio se asemejaron a nidos naturales, contenían huevos de tamaño, forma y color adecuados para la especie de ave y fueron colocados en microhábitats 382 Predation Patterns on Artificial and Real NestsBurke et al.similares a los de nidos naturales, las tasas de depredación en estos nidos no fueron similares a las tasas en nidos naturales en ninguna región en términos de tasa o patrón absoluto. Las tasas de depredación en nidos artificiales no variaron de un año a otro, como fue la tendencia en nidos naturales, y la magnitud de la presión de depredación en nidos sobre el suelo comparada con nidos en arbu...

Conservation actions could be more efficient if there is congruence among taxa in the distribution of species. Patterns in the geographical distribution of five taxa were used to identify nationally important regions for conservation in Canada. Two measures of surrogacy were significantly and positively correlated among taxa, and conservation areas selected for one taxon represented other taxa significantly better than random selections. However, few large protected areas exist in the sites of highest conservation value in southern Canada; these regions are therefore a priority for future conservation regard. By focusing this effort on threatened and endangered species, which are a national priority in Canada, most other species could also benefit.