Ecological release and the impact of urbanization on bird communities in mainland and island avifaunas in the Caribbean Basin

Background/Question/Methods

Ecological release occurs when species expand their habitat use in low diversity regions, such has been demonstrated in comparisons of habitat occupancy by birds in islands in the Caribbean and comparable mainland areas.   Because the avifaunas of Caribbean islands are dominated, relatively, by generalist species (omnivores vs. species which are principally granivores, frugivores, nectivores, insectivores, carnivores, etc.), we would expect less impact of urbanization on the composition of their avifaunas compared to mainland avifaunas.  We reviewed the literature on mainland and island avifaunas in the Caribbean region and accessed data for five regions, two mainland regions from coastal Caribbean and three islands (Puerto Rico, Grenada, and Tobago) where species lists allowed us to distinguish between specialist and generalist species across a range of habitats.  We then identified the most rural habitats (primary or secondary forest) and then compared the guilds found there to urban habitats (“residential” or “downtown gardens”).

Results/Conclusions

As expected from previous studies of ecological release in the Caribbean, similarity of avian communities in rural and urban areas was highest on islands and lowest on the mainland.  Among avifaunas, ratios of generalist to specialist species (G/S) were ≤ 0.5 in mainland forests communities and nearly identical ratios were found in urban areas, indicating that generalists and specialist guilds were equally affected by urbanization.   This result was surprising because we expected greater dominance of generalists in urban habitats, no matter the region. For island avifaunas, G/S was usually > 1 (range = 0.72 – 2. 73), indicating a greater dominance of generalist species.  Moreover, G/S were always higher in urban areas (1.25 – 2.73) in comparison to forest habitats nearby (0.72 – 1.48) indicating a greater relative occupancy of generalist species in urban areas on islands.  Thus, effects of urban environments on consumer communities are clearly dependent on regional history and biogeography.