Ecosystem Decay of Amazonian Forest Fragments: a 22-Year Investigation

We synthesized key findings from the Biological Dynamics of Forest Fragments Project, the world’s largest and longest-running experimental study of habitat fragmentation. Although initially designed to assess the influence of fragment area on Amazonian biotas, the project has yielded insights that go far beyond the original scope of the study. Results suggest that edge effects play a key role in fragment dynamics, that the matrix has a major influence on fragment connectivity and functioning, and that many Amazonian species avoid even small ( 100-m–wide) clearings. The effects of fragmentation are highly eclectic, altering species richness and abundances, species invasions, forest dynamics, the trophic structure of communities, and a variety of ecological and ecosystem processes. Moreover, forest fragmentation appears to interact synergistically with ecological changes such as hunting, fires, and logging, collectively posing an even greater threat to the rainforest biota. Descomposición del Ecosistema en Fragmentos de Bosque Amazónico, Una Investigación de 22 Años Resumen: Sintetizamos resultados clave del proyecto sobre Dinámicas Biológicas de Fragmentos de bosque, el estudio experimental sobre fragmentación del hábitat más largo y de mayor trayectoria del mundo. A pesar de que inicialmente el proyecto se diseñó para evaluar la influencia del área de fragmentos en biotas del Amazonas, ha proporcionado un entendimiento que va mas allá del propósito original del estudio. Los resultados sugieren que los efectos de borde juegan un papel clave en las dinámicas de los fragmentos, que la matriz tiene una influencia mayor sobre la conectividad y el funcionamiento del fragmento y que muchas de las especies del Amazonas evitan áreas taladas pequeñas (de hasta 100 m de ancho). Los efectos de la fragmentación son altamente eclécticos, alterando la riqueza y abundancia de especies, las invasiones de especies, las dinámicas del bosque, la estructura trófica comunitaria y una variedad de procesos ecológicos y del ecosistema. Mas aún, la fragmentación del bosque aparentemente interactúa sinergísticamente con cambios ecológicos como lo son la caza, los incendios y la tala, representando colectivamente una gran amenaza sobre la biota del bosque lluvioso. Paper submitted January 18, 2001; revised manuscript accepted June 20, 2001. 606 Ecosystem Decay of Amazonian Fragments Laurance et al. Conservation Biology Volume 16, No. 3, June 2002 Introduction The Amazon Basin contains over half the Earth’s remaining tropical rainforests and is experiencing the world’s highest absolute rate of deforestation (Instituto Nacional de Pesquisas Espaciais [INPE] 2000; Laurance et al. 2001 a ). Because rapid forest conversion is causing widespread habitat fragmentation (Skole & Tucker 1993; Laurance 1998), the fate of many Amazonian species ultimately will depend on their capacity to persist in fragmented landscapes or isolated nature reserves. The Biological Dynamics of Forest Fragments Project (BDFFP) was initiated in 1979 as a large-scale experiment to assess the effects of fragmentation on Amazonian biotas (Lovejoy et al. 1983, 1986; Bierregaard et al. 1992). It is the world’s largest and longest-running experimental study of habitat fragmentation (cf. Debinski & Holt 2000). Originally, the project’s main goals were to assess the influence of fragment size on Amazonian animal and plant communities, to identify a minimum critical size for rainforest reserves, and to help resolve the heated SLOSS (single large versus several small reserves) debate (e.g., Simberloff & Abele 1976; Wilcox & Murphy 1985). Over time, however, many additional research aims have been added as new insights have developed. A key feature of the BDFFP is that standardized abundance data were collected for trees, understory birds, mammals, amphibians, and various invertebrate groups prior to experimental isolation of the forest fragments. This permitted a far more rigorous assessment of fragmentation effects than would have been possible with only comparisons of fragmented versus intact forest. In addition, the long-term nature of the BDFFP and its synthetic approach, integrating studies of many taxa and numerous ecological and ecosystem processes, have provided insights impossible in most other fragmentation studies. We synthesized key BDFFP findings from the past 22 years based on a survey of over 340 publications and theses and herein highlight their implications for forest conservation. The first part of our review focuses on extrinsic factors that influence fragment biotas—particularly area, edge, matrix, isolation, and sample effects. The second part identifies key communityand ecosystem-level effects of fragmentation on tropical forests.