Document: ROB-3-65-17

A habitat specialist in a naturally fragmented landscape: Thresholds and constraints.

SCHOOLEY, R.L.* and J.A.WIENS

Colorado State University, Fort Collins, CO 80523 USA ¹

Abstract:
A central issue in both basic and applied ecology regards the response of organisms to habitat patchiness. In particular, how does patch area interact with other landscape variables at multiple spatial scales to influence the distribution and abundance of animals? We asked this question for a habitat specialist with limited mobility that lives in naturally fragmented landscapes: A cactus bug (Chelinidea vittiger) that occurs within patches of its host plant (Opuntia polyacantha). Within landscapes that represented gradients of patch density and matrix structure, we mapped and measured attributes of all cactus patches (>1,200 total) and of intervening unsuitable habitat. We surveyed each patch and counted the number of nymph and adult cactus bugs. The proportion of patches occupied by bugs ranged from 0.45 to 0.68. Incidence functions based on logistic regression analysis indicated a nonlinear relationship between patch area and the probability of occupancy. Below a threshold patch size, occupancy was unlikely irrespective of isolation or other landscape features. For occupied patches, the relationship between patch area and number of cactus bugs was triangular within log-log space. We used limiting-factor models based on quantile regression to estimate the upper bound of this relationship. Residuals from this patch-area constraint line were correlated with patch quality and may be related to other landscape variables. Systems that operate on small spatial scales, such as ours, can provide general insights into animal responses to environmental heterogeneity.

Keywords: constraints, fragmented habitat, mosaics, patch size

This abstract is being presented at: 1:15 PM in session:
Oral Session #53: Terrestrial Invertebrate Ecology.