Numbers of Bird Species on the California Islands

Power, Dennis

A model in the form of a path diagram is presented that attempts to explain statistically the numbers of land bird species breeding on the islands off the coast of southern California and northwestern Baja California, Mexico. Interrelationships among selected variables are investigated by step-wise multiple regression analysis. With the present scheme numbers of bird species is largely accounted for by numbers of native plant species (67%). Secondarily, the degree of isolation, as measured by average inter-island distance (including in the average the distance to the nearest mainland point), and unknown factors account for the residual variation in numbers (14% and 19%, respectively). In turn, numbers of plant species is explained largely by island area (58% with untransformed data and 69% with log transformed data). Latitude also explains independently a significant portion of residual variation in plant species numbers (25% with untransformed data and 15% with log transformed data). The remaining variation is accounted for by unknown factors (17%). Island area is a relatively poor predictor of bird species diversity. In terms of causal pathways, the present study indicates that coastal islands with floras that are more rich and possibly structurally more complex tend to support a larger avifauna. Furthermore, islands that are more isolated tend to have a lower equilibrial species number, which in accord with the equilibrium theory is due to reduced immigration rates and increased extinction rates. In addition, larger islands support a greater number of plant species, presumably because they provide a larger target for propagules, a greater diversity of sites for plant growth, and with larger populations have lowered extinction rates. Independent of area, a latitude effect suggests a relationship such that numbers of plant species increase with rainfall along the coast.

Numbers of Bird Species on the California Islands

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