PARENT SESSION
Contributed Oral Session 118: Biogeography: Community Composition
Thursday, August 11, 8:00 AM - 11:30 AM, Meeting Room 519 B, Level 5, Palais des congrès de Montréal

Spatial scaling effects on nested subset structure within Oregon fish assemblages.

McGarvey, Daniel^*,1, ¹ The University of Alabama, Tuscaloosa, AL, U.S.A.

ABSTRACT- Nested subsets occur when the species found at depauperate sites are also found at successively richer sites, but rare species occur only at the richest sites, such that a stair-step presence-absence matrix is formed. Nested structure has been ascribed primarily to differential extinction of species within a regional species pool, and involves three prerequisites: 1. a common biogeographic history; 2. comparable contemporary environments; and 3. a lack of interactive, interspecific exclusion (e.g., competition and predation) among sympatric congeners. Accordingly, the detection of nested subsets can provide important insight to the mechanisms that govern community structure. But nested subset analyses are highly scale dependent, requiring ecologists to explicitly address the spatial relevance of their studies. I tested the hypothesis that nested subset structure exists within Oregon fish assemblages, at both within- and among-basin scales. When spatially explicit point samples (presence-absence data) were superimposed upon longitudinal profiles, at approximately equidistant intervals, I failed to detect significantly nested fish assemblage structure (p > 0.01) at the complete-basin scale. Considerable longitudinal zonation exists within Oregon river basins, however, due to species turnover along corresponding environmental gradients. This zonation masked within-basin nestedness; when distinct, within-basin assemblages were independently analyzed, nested subset structure proved to be significant (p < 0.01). Also, when among basin comparisons were made, using complete-basin taxa lists (i.e., regional species pools), significantly nested structure was detected for the Umpqua, Kilchis, Willamette, Deschutes, and John Day Rivers. These results suggest that Oregon fishes may comprise non-equilibrium communities, in which regional-scale extinction and colonization dynamics are more instrumental than local-scale, interspecific niche partitioning.

Key words: nested subset, Oregon fishes, spatial scale, community structure