Hurricane Katrina impacts the breeding bird community in a bottomland hardwood forest of the Pearl River basin, Louisiana

Abstract

We monitored breeding bird communities and vegetation both before and after Hurricane Katrina category 2 winds severely damaged extensive bottomland hardwood forest of the Pearl River basin, south Louisiana. Many trees were felled by wind, most others were stripped of leaves and branches, and the canopy opened considerably (57%). Blackberry thickets sprouted and expanded to cover almost all of what was previously a patchily open forest understory. The bird community changed distinctively following the hurricane, driven primarily by increased density of species that prefer dense understory (regenerating) habitat. Individual species that increased significantly in density included one year-round resident, Carolina wren, and five breeding migrants, white-eyed vireo, Swainson's warbler, Kentucky warbler, hooded warbler, and yellow-breasted chat. These patterns were predictable responses to the opened canopy and increased density of understory vegetation. However, over three years following the storm, most species, especially canopy breeders, showed no distinct numerical response to the hurricane, which suggests that the initial bird community was resistant to hurricane disturbance. Only one species, Acadian flycatcher, declined significantly after the hurricane, presumably because of loss of its preferred open understory breeding and feeding habitat. Our results thus document and reinforce the important role hurricanes play along the Gulf coast in structuring forest bird communities by altering understory habitat. We expect habitat changes will continue as invasive plant species further change forest community structure, and as large storms increase in frequency in relation to global climate change. Thus, we also expect continued changes to the bird community, which may include additional future declines.

Introduction

Hurricanes have relatively large annual impact areas and economic costs relative to most other types of natural disturbance (Dale et al., 2001). As a result of habitat disturbance, hurricanes also have strong effects on animal communities in forested ecosystems (reviewed by Michener et al., 1997). For example, in the Caribbean, frequent hurricanes are an important force structuring forest bird communities (Wunderle et al., 1992, Wunderle, 1995). The impact of hurricanes on animal communities depends on geographic location, and severity and extent of storm damage, among other factors. Post-hurricane conditions, such as drought or fire events, and the timing of storms in relation to breeding and migration also influence community response (Lynch, 1991, Waide, 1991). As with any disturbance, the response of individual species to hurricanes is not uniform (Rittenhouse et al., 2010a), and disturbance-dependent species can even benefit (Greenberg and Lanham, 2001, Tejeda-Cruz and Sutherland, 2005). In some cases, bird communities are only minimally affected or rebound quickly following hurricanes (Waide, 1991, Murphy et al., 1998, Tejeda-Cruz and Sutherland, 2005, Yaukey, 2008). Surprisingly, little recent research addresses the effects of hurricanes on forest bird communities along the northern Gulf of Mexico, where hurricanes are frequent and have strong influences on coastal forest composition and structure (Williams et al., 1999). However, following Hurricane Katrina, radar tracking of habitat use indicated that for several weeks migratory songbirds avoided the most severely impacted forests as well as high quality stopover habitat (bottomland hardwood forest) in favor of not only less disturbed, but also typically less preferred habitat (intensively managed pine), before densities among habitats returned to typical patterns (Barrow et al., 2007). Following Hurricane Rita in southwest Louisiana, arthropod abundance had increased after one year, but food availability for birds was lower in severely damaged forest than in lightly damaged forest because of reduced foraging substrate (Dobbs et al., 2009). In urban and suburban New Orleans, all of the most common bird species experienced severe declines, some for just months and others lasting for more than a year (Yaukey, 2008). At a regional scale, Rittenhouse et al. (2010a) compared community similarity before and after four hurricanes that made landfall along the Gulf and Atlantic coasts of the United States. They found that similarity of bird communities in the first year following hurricanes was lower for all species combined as well as for species nesting in the mid-story or canopy, and changes to community diversity persisted for at least five years. Nonetheless, few before–after reports document how these recent Gulf Coast hurricanes impact forest bird breeding community composition, especially in relation to plant community changes.

We studied the response of a bottomland hardwood bird community to Hurricane Katrina, which passed directly over our study site in the Pearl River basin with category 2 wind speeds (Fig. 1). We compared habitat and avian communities in the Pearl River before and after the hurricane. We expected that the severe habitat changes caused by the hurricane (Chambers et al., 2007) would lead to (1) increased density of avian species that prefer dense understory habitat created by loss of canopy cover, and (2) decreased density of avian species that prefer open understory, closed canopy forest. Because the hurricane occurred in the middle of the autumn migration period, we also expected the hurricane to have strongest negative effects on resident species, which would have been more exposed to the short-term habitat changes such as defoliation.