Doctoral Dissertations

Dean L. Urban

Date of Award

12-1986

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Ecology and Evolutionary Biology

Major Professor

Herman H. Shugart

Committee Members

Don DeAngelis, Bob O' Neill, Sandy Echternacht, Sue Riechert

Abstract

A tandem approach of field studies and simulation modeling was used to examine avian demography in a landscape mosaic of habitat patches. A particular goal was to attempt to account for the regional decline in abundance of a subset of bird species sensitive to forest fragmentation. Species abundance patterns in forest patches were framed as the consequence of individual birds' demographics, constrained by their landscape context; this context was partitioned to emphasize habitat availability (or area), accessibility (or isolation), and localized factors affecting reproductive success (nest predation and brood parasitism). Each of these constraints was examined in turn, to assess their relative contribution to species abundance patterns observed in landscape mosaics.

A forest simulation model was used to develop a theoretical basis for the importance of microhabitat pattern in forest bird communities. Simulated patterns in microhabitat availability could provide for successional trends in bird species diversity, a relation between niche position and species abundance, the occurrence of more rare species than common ones, and a species/area effect. Hypotheses about microhabitat variety and bird species distribution in landscapes were not supported by data from woodlots in Cadiz Township, southern Wisconsin. But a mechanistic understanding of the agents affecting microhabitat pattern suggested scenarios under which empirical estimates of microhabitat diversity might contribute substantially to predictions of birds species abundance patterns.

Habitat accessibility did not seem to be an effective constraint on species distribution in the Cadiz mosaic. Neither patterns of spatial dispersion of common species, nor species tallies per woodlot offered compelling evidence that any woodlots were so isolated as to affect avian vagility. Simulation experiments suggested that patch isolation becomes increasingly important as the mean distance between patches increases, until this distance is similar to the dispersal range of the bird species; in mosaics with still greater distances between patches, isolation decreases in importance as an explanatory variable.

Sensitivity analysis of natality terms in the simulation model implicated factors affecting reproductive success as potentially powerful constraints on the distribution of species susceptible to nest predation and brood parasitism. Analysis of model uncertainty suggested that predictions of population trends at the landscape scale cannot rely on detailed demographic mechanisms, but should proceed instead to derive coarser-resolution models that might be implemented at the landscape scale. The hierarchical conceptual model developed in this study represents a synthetic general model, a framework that can be simplified under specified scenarios to provide predictions about bird species abundance patterns in landscape mosaics.

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