Habitat-Use Patterns of Avian Seed Dispersers in the Central Everglades

Abstract

Fruit characteristics of tree island tree species and the ecology of many tree island bird species suggests that avian seed dispersers may be particularly important for maintaining plant communities on tree islands in the Everglades. Identifying habitat-use patterns of the disperser is one of the keys to understanding the mechanisms of seed movement, which is a basis of vegetation establishment and succession. Knowledge of avian seed dispersal will aid the Everglades restoration process by increasing the predictability of vegetation changes. In this study we focused on habitat-use patterns of potential seed-dispersing birds and how they vary seasonally. Our objectives were to describe the avian community on tree islands and in the surrounding marsh, to identify, based on the literature, which bird species were potential seed dispersers, and to quantify patterns of abundance of avian frugivores among seasons and between marsh and tree island vegetation types. We conducted field surveys to determine bird abundance and species richness. The red-winged blackbird (Agelaius phoeniceus), common yellowthroat (Geothlypis trichas), white-eyed vireo (Vireo griseus) and boat-tailed grackle (Quiscalus major) were the most abundant species overall. Total bird abundance and species richness were higher at tree island than marsh sites. There was a tendency for total bird abundance to be higher in winter than spring but no such pattern was evident with species richness. We used existing literature on the percentage of fruits or seeds in diets to classify 49 bird species we observed in the central Everglades into one of three guilds (i.e., high fruit-use, low fruit-use, no fruit-use). Forty-seven percent belonged to the no fruit-use guild, 41% belonged to the low fruit-use guild, and 12%belonged to the high fruit-use guild. The high fruit-use guild consisted of American robin (Turdus migratorius), blue jay (Cyanocitta cristata), cedar waxwing (Bombycilla cedrorum), gray catbird (Dumetella carolinensis), northern cardinal (Cardinalis cardinalis) and red-bellied woodpecker (Melanerpes carolinus). Members of this guild were detected only on tree islands. The low fruit-use guild was equally abundant between habitats and among seasons. The no fruit-use guild exhibited strong seasonal differences in abundance, particularly in the winter when an influx of birds occurred in the marsh. Because of differences in habitat-use patterns, members of the high fruit-use guild would likely disperse seeds only within and among extant tree islands. In contrast, birds in the low fruit-use guild, which occurred in both marsh and tree island habitats, would be most likely to disperse seeds to newly formed tree islands. Tree islands form naturally where peat soils accumulate or from processes associated with the Everglades restoration. In such a dynamic ecosystem, it is likely that species from both the high and low fruit-use guilds are necessary for maintaining existing tree island vegetation and colonizing newly formed tree islands with vegetation. We suggest that there are at least two ways that tree islands contribute to the function and health of the Everglades. First, tree islands contribute a large portion of the overall macro-species diversity in the Everglades. Second, they function as “keystone habitats“ for many Everglades animals. Tree islands support terrestrial species of plants and animals, but they also provide critical habitat for many aquatic animal species that require elevated substrate for some period in their annual life cycle. In doing so, tree islands provide benefits to the ecosystem greater than their proportional occurrence. We believe healthy tree islands should be a high priority for conservation and a requirement for successful restoration of the ecosystem.