Biological Invasions and Deletions: Community Change in South Florida
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BIOLOGICAL CONSERVATION Biological Conservation 87 (1999) 341±347 Biological invasions and deletions: community change in south Florida Elizabeth A. Forys a,*, Craig R. Allen b aEckerd College, Environmental Science, St Petersburg, FL 33711, USA bDepartment of Zoology, University of Florida, Gainesville, FL 32611, USA Received 28 September 1995; received in revised form 27 May 1998; accepted 2 June 1998 Abstract This study used the endangered and exotic fauna of south Florida, USA, to test three non-exclusive hypotheses about community change. Over one third of the vertebrate fauna of south Florida is either endangered or exotic. We assumed that in the future, many of the currently endangered species will become extinct, while established exotics become more widespread and abundant. Using species' distributions, body mass data, and niche classi®cations, we compare the past (without exotics) and our predicted future (with exotics, without endangered species) vertebrate communities to determine if the future fauna would be on average smaller, more generalized feeders, or if there would be direct niche replacement. The results of the comparisons indicate that none of the hypotheses explained all of the expected changes in the vertebrate fauna of south Florida, and that the future vertebrate fauna of south Florida is likely to be very dierent from that of the recent past. These changes are symptomatic of the profound ecosystem- level changes occurring here. Hypotheses generated by systemic-level investigations are more likely to increase our understanding of invasions and extinctions. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: South Florida; Endangered species; Exotic species; Invasive species; Community structure 1. Introduction exist in small, fragmented populations from which recovery is unlikely. The state of Florida ranks only after California in the Despite extinctions, the number of vertebrate species number of federally listed endangered and threatened in Florida has been increasing, due to the establishment vertebrate species present (United States Fish and of non-indigenous species. Florida presently has the Wildlife Service, 1991), the majority of these listed spe- largest number of breeding exotic vertebrate species in cies occur within the southern third of the state. This is the continental United States (Oce of Technology particularly signi®cant because of the relative paucity of Assessment, 1993). South Florida's seaports and air- native vertebrate species in this region. Vertebrate spe- ports handle large amounts of cargo and human immi- cies richness declines from north to south in Florida grants, and also a diversity of vertebrate species. Some with decreasing habitat diversity (Means and Simberl- of these animals are shipped purposely for pet-trade, o, 1987; Forys, unpubl. data). Some of the listed spe- tourist attractions, or livestock; others are inadvertent cies (e.g. bald eagle Haliaeetus leucocephalus) may be imports (Loftus and Hernon, 1984). Most of these spe- capable of recovery and eventual delisting; others, such cies fail to reproduce in south Florida (Wilson and as the Carolina parakeet Conuropsis carolinensis are Porras, 1983), but some are able to establish breeding already extinct. The decline in the majority of endan- populations. Their chance for survival may be enhanced gered vertebrate species in south Florida is linked to by south Florida's subtropical climate; many estab- habitat destruction and fragmentation (Humphrey, lished exotic species may be from similar climates and 1992; Moler, 1992). Most currently endangered species may be `preadapted' to south Florida (Newsome and Noble, 1986). Some of these exotic species are having negative * Corresponding author. Tel.: +1-813-864-7880; fax: +1-813-864- impacts on native species. The feral cat Felis catus has 8382; e-mail: [email protected] increased predation pressure on many small mammals, 0006-3207/98/$Ðsee front matter # 1998 Elsevier Science Ltd. All rights reserved PII: S0006-3207(98)00073-1 342 E.A. Forys, C.R. Allen/Biological Conservation 87 (1999) 341±347 birds, and reptiles (Churcher and Lawton, 1987). The comprised of proportionally more herbivorous and Cuban treefrog Osteopilus septentrionalis has been omnivorous species and fewer carnivorous species than linked with the decline of some native hylids (Wilson the historic community. and Porras, 1983). If habitat alteration continues, south Florida will 1.3. Niche replacement hypothesis experience more extinctions. Until dramatic changes in policy and enforcement occur, more exotics will become Some researchers have predicted that the exotic spe- established. The eect these changes will have on the cies that succeed are not necessarily the largest or most composition and structure of the future vertebrate generalized feeders, but are simply the ones that can use fauna in south Florida is not known. However, there the same resources as that of the extinct, or declining are a number of hypotheses concerning which native species (Lawton, 1984; Herbold and Moyle, 1986). species are most vulnerable to extinction, and which Because resources may be partitioned among species on exotic species make successful invaders. the basis of body size (Brown, 1975; Brown and For this paper, we assumed that in the future, many Maurer, 1987), they will be replacing species of similar of the currently endangered species will go extinct, while size, diet, and foraging strata. This hypothesis assumes the established exotics will become more widespread that most exotic species have become established after and abundant. We test three non-exclusive hypotheses the decline of the endangered native species or have about community change related to endangered and replaced natives through competitive exclusion. Thus, exotic fauna to predict how the future vertebrate fauna our third prediction is that individual exotic species will of south Florida (with exotics, without endangered spe- replace individual extinct or declining species in cies) will compare to the past fauna. resource use (diet and foraging strata) and body mass. 1.1. Body-mass dierence hypothesis 2. Methods Species that are the largest members in their guilds (Leck, 1979; Willis, 1980; Pimm et al., 1989), or that We used three terrestrial vertebrate groups in our have high body mass in the community (Laurance, analysis: herpetofauna (reptiles and amphibians), birds 1991), are hypothesized to be extinction-prone because and mammals. Species' distributions were obtained they require large areas for foraging and are more likely from museum records (Allen, unpubl. data), published to be rare (Arita et al., 1990). Species that have smaller accounts, and the Florida Breeding Bird Atlas (Kale et body size may be more likely to be successful invaders al., in press). Only species that had established breeding because they are generally r-selected (Kitching, 1986; populations in at least some natural areas in south Lodge, 1993), more likely to be inadvertently intro- Florida were included in the analysis. We used the duced, and more likely to be human commensals boundaries of the Everglades sub-ecoregion (Bailey, (Myers, 1986; Newsome and Noble, 1986). The predic- 1983), that included the counties of Broward, Collier, tion, therefore is that the average body size of the future Dade, Hendry, Lee, Monroe, and Palm Beach. vertebrate communities of south Florida will be lower A species was considered to be an exotic either if it than the average body size of the historic past or the was introduced to south Florida by humans, or if it was present. a non-indigenous species that had invaded south Flor- ida since European colonization. A species was con- 1.2. Diet dierence hypothesis sidered to be endangered if it was listed by the state of Florida as being extinct, endangered, threatened, or a Invading species that have the capacity to occupy a species of special concern (Florida Game and Fresh- wide potential niche are predicted to be more likely to water Fish Commission, 1994). A listed subspecies was succeed in their new environment (Arthington and included only if it was the sole subspecies of a species Mitchell, 1986; di Castri, 1990). Species that are highly present in the Everglades sub-ecoregion. specialized in their diet, foraging strategy, or behavior In most cases, vertebrate body masses were collected are more likely to face extinction when habitat is frag- from published accounts or unpublished ®eld data. For mented or altered (Faaborg, 1979; Ambuel and Temple, some herpetofauna, no published records of body mas- 1963). Carnivorous species generally require larger ses were available and body mass was determined by home ranges to meet their metabolic needs (McNab, weighing a sample of alcohol-preserved whole-animal 1983), and are more likely to be aected by pesticides museum specimens. Although some weight changes and other contaminants than herbivorous or omnivor- occur during the preservation process, these changes ous species (Douthwaite, 1992; Outridge and Scheu- tend to be <10% (Haighton, 1956; Mount, 1963). hammer, 1993). The second prediction, then, is that the In all cases, adult male and female weights were future vertebrate community of south Florida will be averaged. E.A. Forys, C.R. Allen/Biological Conservation 87 (1999) 341±347 343 To test the body-mass dierence hypothesis, we used Table 1 Number and percentage of non-endangered native, endangered native, median values rather than means because the data was and exotic vertebrate species in south Florida and the average body not normally distributed. Using a Mann-Whitney-U mass and standard error of each group test, we ®rst compared endangered species with native, non-endangered species for each vertebrate group and Taxonomic group N Percentage Median body SE of species mass (g) then exotic species with native species for each group. The diet dierences and species replacement hypoth- Bird eses both concerned niche use. Niche classi®cation was Non-endangered 98 65 351.1 72.6 Endangered 21 14 890.9 316.8 based on diet and foraging strata based on Eisenberg Exotic 32 21 395.9 166.1 (1981, Appendix 1) for mammals and herpetofauna and Herpetofauna Ehrlich et al.