Functional attributes of two subtropical shrubs and implications for the distribution and management of endangered bird habitat

Abstract

Aims The fruits of Erithalis fruticosa L. and Lantana involucrata L. are important in the diet of US federally endangered Kirtland’s Warblers (Setophaga kirtlandii) wintering in the Bahamas archipelago. These two shrubs occur in tropical and subtropical dry forests, including forests that have been subjected to recent disturbance. Despite their importance to the endangered warbler, the disturbance ecology of these shrubs is poorly understood. We sought to determine, based on functional characteristics of the plants, whether their presence is favored by a particular type or regime of disturbance. Methods We used data from field experiments (seed broadcasting and shrub cutting) conducted on the island of Eleuthera, The Bahamas to determine mechanisms of and conditions favoring establishment and persistence (‘vital attributes’) of E. fruticosa and L. involucrata, which enabled categorization according to the plant functional types defined by Noble and Slatyer (1980). We then compared hypothesized distributions of these plant functional types among different anthropogenic disturbance regimes to observed distributions of E. fruticosa and L. involucrata in order to identify distur bance regimes most likely to produce habitat used by Kirtland’s Warblers. Important Findings E. fruticosa and L. involucrata were functionally categorized as widely dispersed but largely shade intolerant species capable of establishing or regenerating individuals after disturbance via both seeds and vegetative mechanisms. Both hypothesized and observed distribution patterns indicated the shrubs were favored by a regime of frequent disturbance producing open canopy and ground layers. Among the anthropogenic disturbances we examined, areas of large-scale land clearing combined with subsequent goat grazing most often supported E. fruticosa and L. involucrata, while the shrubs were relatively rare in burned areas. Utilizing the plant functional type framework in combination with field data to evaluate predictions of species occurrence among different disturbances regimes provides a strong theoretical basis for conservation strategies. Understanding which disturbance types favor a habitat of concern and the mechanisms by which they do so can aid the prior