Effects of habitat fragmentation on plant guild structure in the montane Atlantic forest of southeastern Brazil

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Abstract

Habitat fragmentation is a major cause of biodiversity erosion in tropical forests. The Brazilian Atlantic forest has both high species richness and a long history of anthropogenic disturbance, beginning with colonial agriculture in the sixteenth century. Here we examine the species composition and guild structure of woody plants within five montane Atlantic forest fragments of the Tiet River basin, State of São Paulo, southeastern Brazil, ranging from 5 to 7900 ha in area. We found a negative relationship between fragment size and the relative importance of tree and shrub species that (1) depend on abiotic modes of seed dispersal, (2) are shade-intolerant, and (3) occupy the forest canopy. As fragment size decreased, there was a marked rise in the relative importance of ruderal species, primarily in the Compositae, Euphorbiaceae, Solanaceae, and Leguminosae. There also was a 9% average decline in smaller fragments in relative importance of Myrtaceae, Lauraceae, Sapotaceae, and Rubiaceae, which are the main sources of fleshy fruits for vertebrate frugivores in these forests. Our results suggest that predictable shifts in plant guild structure occur as tropical forest fragments are reduced in size, and that small fragments may become dominated by edges and the surrounding habitat matrix. We suggest that small forest fragments will be unlikely to preserve intact plant and animal assemblages of Brazil's Atlantic coastal forest

Introduction

Forest fragmentation causes many physical and biological changes as a result of habitat loss and insularization (Lovejoy, Bierregaard, Rylands, Malcolm, Quintela, Harper, Brown, Powell, Powell, Schubart and Hays, 1986, Laurance, 1990). As forest landscapes become increasingly fragmented, populations of forest species are reduced, dispersal and migration patterns are interrupted, ecosystem inputs and outputs are altered, and previously isolated core habitats become exposed to external conditions, all of which result in a progressive erosion of biological diversity (Terborgh and Winter, 1980, Tilman, May, Lehman and Nowak, 1994). The creation of abrupt forest edges exposed to open habitats can severely modify local microclimatic conditions, increase tree mortality, and promote the establishment of non-forest species (Lovejoy, Bierregaard, Rylands, Malcolm, Quintela, Harper, Brown, Powell, Powell, Schubart and Hays, 1986, Kapos, 1989, Laurance, Ferreira, Rankin-de-Merona and Laurance, 1998a). Predictable shifts in the abundance and composition of plant species should also occur, in large part because of greater recruitment or lower mortality of light-dependent gap specialists along forest edges (Murcia, 1995, Laurance et al. 1998b).

Many consequences of forest fragmentation may have prolonged lag effects and require a sufficient “relaxation” period, with communities passing through a series of transitional states before final equilibrial conditions are reached (Simberloff, 1976, Tilman, May, Lehman and Nowak, 1994, Terborgh, Lopez, Tello, Yu and Bruni, 1997). Responses to insularization and habitat loss are thus best assessed in older fragments that have had adequate time to equilibrate (cf. Turner, Chua, Ong, Soong and Tan, 1996, Kellman, Tackaberry and Meave, 1997). The Brazilian Atlantic forest contains mainly older fragments (>50 years) and has one of the highest levels of plant and animal endemism of any continental tropical forest (Klein, 1980, Prance, 1982a, Prance, 1982a, Brown, 1987, Leitao Filho, 1994, Thomas, Carvalho, Garrison and Arbelaez, 1998). Of 127 woody species described for this region in the Flora Neotropica monographs, 68 (56%) are endemic (Mori et al., 1981), including 39 species in the Chrysobalanaceae alone (Prance, 1987). In addition to a large number of endemic plants, the Atlantic forest exhibits remarkably high plant diversity which may rival or exceed that of Amazonian forests (Silva and Leitao Filho, 1982, Mori, Boom, Carvalino and Santos, 1983, Martins, 1989, Peixoto and Gentry, 1990, Joly, Leitao Filho and Silva, 1991, Brown and Brown, 1992, Leitao Filho, 1994).

Following a history of severe and repeated anthropogenic disturbance since the sixteenth century, the Atlantic forest has been reduced to several thousand fragments accounting for only 8% of its pre-Columbian extent, which once encompassed some 12% of the Brazilian territory (SOS Mata Atlântica and INPE, 1993). Although this relentless fragmentation process has been curbed in recent decades, it has by no means been halted. For example, some 11% of the surviving Atlantic forest cover was lost between 1985 and 1990, leaving only 285,000 ha of forest remaining (SOS Mata Atlântica and INPE, 1993). While alarming from a conservation perspective, this intensely fragmented landscape with both old and new isolates provides an excellent opportunity for fragmentation studies (Brown and Brown, 1992, Viana and Tabanez, 1995, Tabarelli and Mantovani, 1997b).

This study evaluates the effects of habitat fragmentation on the structure and composition of woody plant guilds in Atlantic forest fragments that vary considerably in size (5–7900 ha). Guild structure is defined in terms of functional groups that relate to seed-dispersal mode, regeneration requirements, and height of mature individuals in the forest (sensu Giller, 1984). In particular, we determine whether and how the composition of different plant guilds varies with fragment size. We also consider the long-term viability of fragments remaining in the Brazilian Atlantic forest.

Section snippets

The forest fragments

This study examines the plant communities in five fairly well studied Atlantic montane forest fragments of the Atlantic Plateau within the Tietê River basin, State of São Paulo, southeastern Brazil. The fragments are located on the outskirts or just outside the urban perimeter of the city of São Paulo, ca. 100 km inland from the Atlantic coast (Fig. 1), at 700–850 m elevation (Table 1). All five fragments are relatively old (isolated for >50 years) and range from 5 to 7900 ha. They have

Effects of fragment size

The five fragments clearly differed in the proportions of species in guilds with varying light requirements at the seedling and sapling stage (Gadj=24.4, d.f.=4, P<0.001), growth form (Gadj=17.3, d.f.=4, P=0.001), and dispersal syndrome (Gadj=21.5, d.f.=4, P<0.001; G-tests for independence). The two smallest fragments had the highest proportions of shade-intolerant and canopy species dispersed by abiotic means (Fig. 2), and these guilds declined in importance as fragment size increased.

Floristic effects of fragment size

Our findings demonstrate that fragments of the montane Atlantic forest of southeastern Brazil differ in the guild structure of their plant assemblages, and that these changes are related to fragment size. Given the limited available data, we were unable to compare relative abundances of species in different guilds, but earlier studies (Aragaki and Mantovani, 1993, Tabarelli, 1994) suggest that our conclusions would have been strengthened had patterns of abundance been taken into account. Our

Acknowledgments

We thank the State of São Paulo Research Council (Fundação de Amparo á Pesquisa do Estado de São Paulo, FAPESP) for funding this study. We are enormously grateful to the Brazilian botanists who have contributed to our present understanding of the Atlantic forest flora. Herbarium staff at the São Paulo Botany Institute identified all plant vouchers. Phil Judge kindly prepared Fig. 1. W. F. Laurance, A. Carkeek, and an anonymous reviewer offered constructive criticisms on the manuscript.

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