Fire History and the Structure of Pine-Rockland Bird Assemblages
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R E S E A R C H A R T I C L E ABSTRACT: Managers of natural areas often employ controlled disturbances as a tool to manage plant and animal populations. This approach assumes that disturbances are responsible for the structure of biological communities and that appropriate application of the disturbance will ensure the persistence of native plants and animals. If species in a community do not respond predictably to variation in disturbance regime, then management strategies based on emulating disturbance may fail to ensure the persistence of all species. In this study, we examined the efficacy of using prescribed fire as a tool • for managing populations of breeding and wintering birds in the pine rocklands of southern Florida. We found that variation in fire history had little effect on vegetation structure and no effect on bird abundance. Instead, vegetation structure was more closely associated with water-table elevation and soil type, whereas most of the observed variation in the structure of bird assemblages appeared to be a function of degree of urbanization in the landscape. That the structure and composition of bird as- semblages was independent of variation in fire history suggests that manipulating the fire regime, at least within the range of variability observed in this study, is unlikely to prove effective as a means to manage bird populations. In general, our results argue for caution in assuming that a single process can Fire History and be used to control the structure of biological communities, especially in systems where landscapes have the Structure of been substantially altered by human activity. Index terms: Florida, hydrology, pine rocklands, prescribed fire, slash pine Pine-rockland Bird INTRODUCTION not without its critics, however; concerns Assemblages include whether scientists and managers Many of the concepts central to conser- have sufficient knowledge of how natural vation biology have received relatively disturbances act to adequately replicate little empirical evaluation (e.g., Simber- them (Landres et al. 1999) or whether John D. Lloyd 1,3 loff 2001), yet they form the basis for we can identify which disturbances to managing most populations of plants and manipulate or replicate (Simberloff 2001). 1Ecostudies Institute animals (Schulte et al. 2006). For example, As such, it remains unclear under what set P.O. Box 106 the concept of coarse filters (Noss 1987; of conditions a process-oriented, coarse- South Strafford VT, 05070 Hunter et al. 1988; Hunter 2005) argues filter approach to conservation will reliably that populations of most species in a com- ensure the persistence of the majority of munity can be controlled by manipulating species in a community. Additional case key structuring processes, especially dis- studies that evaluate the efficacy of this 2 Gary L. Slater turbances that shape the abundance and approach are sorely needed (Bestelmeyer 2Ecostudies Institute distribution of important habitat elements. et al. 2003; Hunter 2005). PO Box 703 This approach is commonly applied in a Mount Vernon WA, 98273 variety of ecosystems to manage plant and In this paper, we present the results of a animal populations (Leopold et al. 1963; case study in which we examined how Hunter 1993; Haufler et al. 1996; Long one taxonomic group responded to the and Smith 2000). Coarse-filter manage- application of a coarse-filter management ment strategies that focus on restoring strategy. In particular, we examined how • normative disturbance regimes assume breeding- and wintering-bird assemblages that the abundance of each species in a responded to variation in prescribed-fire community is a predictable function of the regimes in the pine-rockland ecosystem intensity and frequency of the disturbance. of south Florida. Pine rocklands are fire- Maintaining a characteristic community dependent, found only on limestone out- structure requires only that disturbance is crops in south Florida, the Bahamas, and 3 Corresponding author: applied at an intensity and frequency that Cuba. In the absence of fire, hardwoods [email protected] approximates the historic range of vari- come to dominate the canopy, inhibiting ability. Intuitively appealing, coarse-filter growth and reproduction of pines, and management strategies allow managers to eventually the pine rockland is replaced address the needs of multiple species with by a dry, broad-leafed tropical forest. In a single action and do not require a detailed the presence of natural fire regimes, pine understanding of the habitat requirements rocklands are characterized by relatively of each species in a community. open-canopied stands of south Florida Natural Areas Journal 32:247–259 slash pine (Pinus elliottii var. densa Little The use of disturbances as a coarse filter is and Dorman) rising above understories Volume 32 (3), 2012 Natural Areas Journal 247 of grasses, hardwood shrubs, and palms tion in the fire regime should produce varia- includes approximately 4600 ha of pine (Serenoa repens Bartram, Sabal palmetto tion in vegetation structure, which should rockland. The climate within the study (Walter) Lodd. ex Schult. & Schult. f., in turn produce variation in bird species area is subtropical, with a pronounced and Coccothrinax argentata (Jacq.) L.H. composition and abundance of birds. If dry season from November to April and a Bailey). Most (> 90%) of the area in south bird assemblages in pine rocklands do not wet season from May to October. About Florida once covered by pine rocklands, predictably vary as a function of variation 75% of rainfall occurs during the wet known as the Miami Rock Ridge, has in the fire regime, then fire, at least within season, with most of the wet-season rains been replaced by agricultural fields and the range of variation observed, does not act falling during convective storms early in residential or commercial developments as a coarse filter for birds in this system and the season or during tropical cyclones in (USFWS 1999). alternative management strategies may be the late wet-season; July and August are needed to maintain or restore bird popula- relatively dry (Snyder et al. 1990). Annual Accepting that fire is necessary in checking tions in the pine rocklands. We tested these temperature variation is relatively modest, successional pathways that lead inevita- predictions by examining the relationship with an average July temperature of 29 bly to the replacement of pine rocklands between fire history (the number of fires °C and an average January temperature by broad-leafed forests, a critical and and length of the fire-free period), vegeta- of 19 °C. outstanding question remains: within the tion structure, and bird abundance at 156 range of fire-return intervals that prevent locations in south Florida, while at the same Historically, fire-return intervals in the pine succession to hardwood hammocks, does time controlling for a suite of potentially rocklands are thought to have averaged variation in fire regime explain extant varia- confounding variables that might mask the between three and seven years (Wade et tion in the composition and structure of effect of fire history. al. 1980; Snyder 1986; Snyder et al. 1990; plant and animal assemblages at different Liu et al. 2005; Sah et al. 2006). Most sites within the pine rocklands? In other We focused on birds because they have been fires probably occurred during the wet words, is any fire regime that prevents the particularly vulnerable to the degradation season, with the greatest acreage burning encroachment of hardwoods adequate in and destruction of the pine rocklands and, in June, when lightning is common but promoting the mix of plants and animals as such, are of concern to land managers. fuel moisture remains low (Taylor 1981). characteristic of pine rocklands, or is some Seven species of breeding birds have been Ignitions by humans have greatly extended level of fine-tuning required? The primary extirpated from the pine rocklands since the fire season; most human-caused fires in management objective in pine rocklands the middle of the last century: wild turkey south Florida occur during the dry season, has been to restore putatively natural fire (Meleagris gallopavo Linneaus), summer although, as with natural fires, the largest regimes (Maguire 1995; USFWS 1999), the tanager (Piranga rubra Linneaus), brown- fires occur at the transition from dry to assumption being that doing so will create headed nuthatch (Sitta pusilla Latham), wet season (Gunderson 1994; Gunderson and maintain the environmental conditions Eastern bluebird (Sialia sialis Linneaus), and Snyder 1994). necessary to support the characteristic as- red-cockaded woodpecker (Picoides bo- semblage of plants and animals. However, realis Vieillot), Southeastern American Establishing survey points this assumption has rarely been tested. kestrel (Falco sparverius paulus Linneaus), As a coarse filter, fire at any reasonable and hairy woodpecker (Picoides villosus We collected data at 104 points in Long interval may be adequate in maintaining Linneaus). Three species, including wild Pine Key and 52 points in pine-rockland forests of slash pine. Within the broad turkey, brown-headed nuthatch, and East- preserves managed by Miami-Dade County range of fire-return intervals that prevent ern bluebird, have been reintroduced to (Figure 1). We established points at Long encroachment of hardwoods, however, sites Everglades National Park, although the Pine Key in 2005 and in Miami-Dade burned at different intervals may support long-term viability of these populations County parks in 2006. For Long Pine very different assemblages of plants and is uncertain (Lloyd et al. 2009). Key, we placed survey points at randomly animals. If so, this information is of great chosen locations, with the restriction that importance in fine-tuning fire as a coarse METHODS survey points were > 350 m apart and filter. Alternatively, variation in biological were surrounded by > 100 m of contigu- assemblages among different sites within ous pine forest. We had fewer potential the pine rocklands may be driven by Study area sites to choose from in the Miami-Dade factors other than fires – hydrology, for County park system, so our selection of example.