Review
The fate of Amazonian forest fragments: A 32-year investigation

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Abstract

We synthesize findings to date from the world’s largest and longest-running experimental study of habitat fragmentation, located in central Amazonia. Over the past 32 years, Amazonian forest fragments ranging from 1 to 100 ha have experienced a wide array of ecological changes. Edge effects have been a dominant driver of fragment dynamics, strongly affecting forest microclimate, tree mortality, carbon storage, fauna, and other aspects of fragment ecology. However, edge-effect intensity varies markedly in space and time, and is influenced by factors such as edge age, the number of nearby edges, and the adjoining matrix of modified vegetation surrounding fragments. In our study area, the matrix has changed markedly over the course of the study (evolving from large cattle pastures to mosaics of abandoned pasture and regrowth forest) and this in turn has strongly influenced fragment dynamics and faunal persistence. Rare weather events, especially windstorms and droughts, have further altered fragment ecology. In general, populations and communities of species in fragments are hyperdynamic relative to nearby intact forest. Some edge and fragment-isolation effects have declined with a partial recovery of secondary forests around fragments, but other changes, such as altered patterns of tree recruitment, are ongoing. Fragments are highly sensitive to external vicissitudes, and even small changes in local land-management practices may drive fragmented ecosystems in markedly different directions. The effects of fragmentation are likely to interact synergistically with other anthropogenic threats such as logging, hunting, and especially fire, creating an even greater peril for the Amazonian biota.

Introduction

The rapid loss and fragmentation of old-growth forests are among the greatest threats to tropical biodiversity (Lovejoy et al., 1986, Sodhi et al., 2004, Laurance and Peres, 2006). More than half of all surviving tropical forest occurs in the Amazon Basin, which is being seriously altered by large-scale agriculture (Fearnside, 2001, Gibbs et al., 2010), industrial logging (Asner et al., 2005), proliferating roads (Laurance et al., 2001a, Killeen, 2007), and oil and gas developments (Finer et al., 2008).

The exploitation of Amazonia is driving forest fragmentation on a vast spatial scale. By the early 1990s, the area of Amazonian forest that was fragmented (<100 km2) or vulnerable to edge effects (<1 km from edge) was over 150% greater than the area that had been deforested (Skole and Tucker, 1993). From 1999 to 2002, deforestation and logging in Brazilian Amazonia respectively created ∼32,000 and ∼38,000 km of new forest edge annually (Broadbent et al., 2008). Prevailing land uses in Amazonia, such as cattle ranching and small-scale farming, produce landscapes dominated by small (<400 ha) and irregularly shaped forest fragments (Cochrane and Laurance, 2002, Broadbent et al., 2008). Such fragments are highly vulnerable to edge effects, fires, and other deleterious consequences of forest fragmentation (Laurance et al., 2002, Barlow et al., 2006, Cochrane and Laurance, 2008).

Starting in 1979, the Biological Dynamics of Forest Fragments Project (BDFFP) has been assessing the impacts of fragmentation on the Amazon rainforest and biota (Lovejoy et al., 1986, Bierregaard et al., 1992, Pimm, 1998, Laurance et al., 2002). Today, 32 years later, it is the world’s largest and longest-running experimental study of habitat fragmentation, as well as one of the most highly cited ecological investigations ever conducted (Gardner et al., 2009, Peres et al., 2010). As of October 2010, BDFFP researchers had produced 562 publications and 143 completed graduate theses (http://pdbff.inpa.gov.br), focusing on the responses of a wide array of animal and plant taxa to fragmentation as well as research on secondary forests, global-change phenomena, and basic forest ecology.

The last general review of forest fragmentation research at the BDFFP was nearly a decade ago (Laurance et al., 2002), and we present here an updated synthesis. We highlight several key conclusions from our last review but emphasize new findings and their implications for forest conservation, including recent works by BDFFP investigators that encompass large expanses of the Amazon basin.

Section snippets

Project background

The BDFFP is located 80 km north of Manaus, Brazil and spans ∼1000 km2 (Fig. 1). The topography is relatively flat (80–160 m elevation) but dissected by numerous stream gullies. The heavily weathered, nutrient-poor soils of the study area are typical of large expanses of the Amazon Basin. Rainfall ranges from 1900 to 3500 mm annually with a moderately strong dry season from June to October. The forest canopy is 30–37 m tall, with emergents to 55 m. Species richness of trees (⩾10 cm

Sample effects are important in Amazonia

Many species in Amazonian forests are rare or patchily distributed. This phenomenon is especially pronounced in the large expanses of the basin that overlay heavily weathered, nutrient-poor soils (e.g. Radtke et al., 2008), where resources such as fruits, flowers, and nectar are scarce and plants are heavily defended against herbivore attack (Laurance, 2001). This has a key implication for understanding forest fragmentation: given their rarity, many species may be absent from fragments not

Forest hydrology is disrupted

The hydrological regimes of fragmented landscapes differ markedly from those of intact forest (Kapos, 1989). Pastures or crops surrounding fragments have much lower rates of evapotranspiration than do forests because they have far lower leaf area and thus less rooting depth. Additionally, such clearings are hotter and drier than forests. Field observations and heat-flux simulations suggest that desiccating conditions can penetrate up to 100–200 m into fragments from adjoining clearings (Malcolm,

Matrix structure and composition affect fragments

Secondary forests have gradually overtaken most pastures in the BDFFP landscape. This lessens the effects of fragmentation for some taxa as the matrix becomes less hostile to faunal use and movements. Several species of insectivorous birds that had formerly disappeared have recolonized fragments as the surrounding secondary forest grew back (Stouffer and Bierregaard, 1995b). The rate of bird extinction has also declined (Stouffer et al., 2008). A number of other species, including certain

Rare disturbances can leave lasting legacies

Rare events such as windstorms and droughts have strongly influenced the ecology of fragments. Rates of tree mortality rose abruptly in fragmented (Laurance et al., 2001c) and intact (Williamson et al., 2000; S. Laurance et al., 2009a) forests in the year after the intense 1997 El Niño drought. Such pulses of tree death help drive changes in the floristic composition and carbon storage of fragments (Laurance et al., 2007). Leaf-shedding by drought-stressed trees also increases markedly during

Ecological distortions are common

Many ecological interactions are altered in fragmented forests. Fragmented communities can pass through unstable transitional states that may not otherwise occur in nature (Terborgh et al., 2001). Moreover, species at higher trophic levels, such as predators and parasites, are often more vulnerable to fragmentation than are herbivores, thereby altering the structure and functioning of food webs (Didham et al., 1998b, Terborgh et al., 2001).

BDFFP findings suggest that even unhunted forest

Species losses are highly nonrandom

Species extinctions in the BDFFP fragments have occurred in a largely predictable sequence, with certain species being consistently more vulnerable than others. Among birds, a number of understory insectivores, including army ant-followers, solitary species, terrestrial foragers, and obligate mixed-flock members, are most susceptible to fragmentation. Others, including edge/gap species, insectivores that use mixed flocks facultatively, hummingbirds, and many frugivores, are far less vulnerable (

Long-term research is crucial

Many insights from the BDFFP would have been impossible in a shorter-term study. The exceptional vulnerability of large trees to fragmentation (Laurance et al., 2000) only became apparent after two decades of fragment isolation. Likewise, the importance of ephemeral events such as El Niño droughts (Williamson et al., 2000, Laurance et al., 2001c) and major windstorms (Laurance et al., 2007) would not have been captured in a less-enduring project. Many other key phenomena, such as the kinetics

Amazonian reserves should be large and numerous

A key conclusion from BDFFP research is that nature reserves in Amazonia should ideally be very large—on the order of thousands to tens of thousands of square kilometers (Laurance, 2005, Peres, 2005). Only at this size will they be likely to maintain natural ecological processes and sustain viable populations of the many rare and patchily distributed species in the region (Ferraz et al., 2007, Radtke et al., 2008); provide resilience from rare calamities such as droughts and intense storms (

The future of the BDFFP

The BDFFP is one of the most enduring and influential ecological research projects in existence today (Gardner et al., 2009, Peres et al., 2010). From the prism of understanding habitat fragmentation, there are vital justifications for continuing it. The project, moreover, is engaged in far more than fragmentation research: it plays a leading role in training Amazonian scientists and decision-makers, and sustains long-term research on global-change phenomena, forest regeneration, and basic

Acknowledgements

We thank Gonçalo Ferraz, Robert Ewers, Reuben Clements, Will Edwards, and four anonymous referees for helpful comments on the manuscript. The National Institute for Amazonian Research (INPA), Smithsonian Institution, US National Science Foundation, Brazilian Science Foundation (CNPq), NASA-LBA program, US-AID, Mellon Foundation, Blue Moon Fund, Marisla Foundation, and other organizations generously supported the BDFFP. This is publication number 562 in the BDFFP technical series.

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