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ARTICLE IN PRESS Basic and Applied Ecology 10 (2009) 97–102 www.elsevier.de/baae INVITED VIEWS IN BASIC AND APPLIED ECOLOGY Biodiversity change and ecosystem function in tropical forests Owen T. Lewis Department of Zoology, University of Oxford, South Parks Road, Oxford OX1 3PS, UK Abstract Recent debate about the fate of tropical forests has focused attention on the consequences of forest degradation and fragmentation for their diversity and composition, and the likely functional consequences of these changes. Existing E-mail address: [email protected]. 1439-1791/$ - see front matter r 2008 Gesellschaft für Ökologie. Published by Elsevier GmbH. All rights reserved. doi:10.1016/j.baae.2008.08.010 ARTICLE IN PRESS 98 O.T. Lewis / Basic and Applied Ecology 10 (2009) 97–102 data suggest that the responses of tropical forest plant and animal communities to habitat change are idiosyncratic, although a few consistent patterns are emerging. In particular, it is apparent that conventional diversity and richness metrics may not adequately represent anthropogenic changes to community structure and organisation. A widespread trend is towards ‘biotic homogenisation’: while disturbed forests may often have an equal or even a greater number of species than undisturbed forests, these species are typically drawn from a restricted pool; and endemic, restricted-range or habitat-specialist species are most likely to decline or go extinct. Similarly, studies have documented marked changes in the structure of food webs, even where the richness and diversity of component species remains little altered. What are the likely consequences of such changes for the important ecosystem functions performed by biodiversity, such as pollination and decomposition? Much of the extensive literature on the relationship between biodiversity and ecosystem function is of limited utility for answering this question, because experimental designs do not consider species-specific contributions to ecosystem function, abundance, degree of redundancy, or extinction-proneness; and few such studies have been carried out under realistic levels of diversity under field conditions, particularly in highdiversity ecosystems such as tropical forests. Furthermore, the focus has almost always been on richness as the explanatory variable, rather than the composition or structural attributes of communities. I briefly review recent papers that have begun to tackle these important issues, and consider how future research might help us understand the functional consequences of realistic changes to species composition and food-web ‘biostructure’ in tropical forests. r 2008 Gesellschaft für Ökologie. Published by Elsevier GmbH. All rights reserved. Zusammenfassung Die Aufmerksamkeit in der aktuellen Diskussion um das Schicksal der tropischen Wälder konzentrierte sich auf die Folgen der Degradierung und Fragmentierung der Wälder für die Vielfalt und Zusammensetzung und auf die wahrscheinlichen funktionalen Konsequenzen dieser Veränderungen. Die vorhandenen Daten lassen vermuten, dass die Reaktionen der tropischen Pflanzen- und Tiergemeinschaften des Waldes auf die Habitatveränderungen idiosynkratisch sind, auch wenn ein paar konsistente Muster auftauchen. Es ist insbesondere offensichtlich, dass die konventionellen Maße für Diversität und Artenreichtum die anthropogenen Veränderungen in der Lebensgemeinschafts-Struktur und -Organisation möglicherweise nicht adäquat repräsentieren. Ein weitverbreiteter Trend geht zu einer ‘‘biotischen Homogenisierung’’. Während gestörte Wälder im Vergleich zu ungestörten Wäldern häufig eine ebenso hohe oder gar höhere Anzahl an Arten haben, stammen diese Arten typischerweise aus einem beschränkten Artenpool. Für endemische Arten, Arten mit beschränkter Verbreitung und Habitatspezialisten besteht die größte Wahrscheinlichkeit seltener zu werden oder auszusterben. In ähnlicher Weise haben Untersuchungen deutliche Veränderungen in der Struktur der Nahrungsnetze gezeigt, selbst wenn der Artenreichtum und die Diversität der vorhandenen Arten nahezu unverändert war. Was sind die wahrscheinlichen Konsequenzen dieser Veränderungen für wichtige Ökosystemfunktionen die aufgrund der Diversität erfüllt werden, wie beispielsweise die Zersetzung und Bestäubung? Eine Vielzahl der umfangreichen Literatur über die Beziehung zwischen der Biodiversität und den Ökosystemfunktionen ist von begrenztem Nutzen bei der Antwort auf diese Frage weil die experimentellen Ansätze nicht die artspezifischen Beiträge zur Ökosystemfunktion, wie Abundanz, Grad der Redundanz und Austerbeneigung berücksichtigen. Und wenige dieser Untersuchungen wurden bei realistischen Leveln von Diversität unter Freilandbedingungen durchgeführt, insbesondere nicht in hochdiversen Ökosystemen wie tropischen Wäldern. Darüber hinaus war der Fokus fast immer auf den Artenreichtum als erklärende Variable gerichtet, anstatt auf die Zusammensetzung oder strukturellen Eigenschaften der Lebensgemeinschaften. Ich gebe einen kurzen Review über die neueren Veröffentlichungen, die begonnen haben, diesen wichtigen Punkt in Angriff zu nehmen und schätze ab, inwieweit die zukünftige Forschung uns helfen kann, die funktionalen Konsequenzen von realistischen Veränderungen in der Artenzusammensetzung und Nahrungsnetz- Biostruktur" in tropischen Wäldern zu verstehen. ’’ r 2008 Gesellschaft für Ökologie. Published by Elsevier GmbH. All rights reserved. Keywords: Diversity; Ecosystem functioning; Extinction; Food-webs; Interactions; Rainforest; Secondary forests Introduction Human pressures modifying the diversity and composition of biological communities are most intense where human populations are growing and developing most rapidly: in the tropics. Approximately half of the earth’s closed-canopy tropical forest has already been converted to other uses, and the population of tropical countries, having almost trebled since 1950, is projected to grow by a further 2 billion by 2030 (Wright, 2005). Rates of tropical forest habitat degradation and destruction are higher than in almost any other biome ARTICLE IN PRESS O.T. Lewis / Basic and Applied Ecology 10 (2009) 97–102 (Pimm, 2001; Sala et al., 2000). Tropical forest habitats have the highest biodiversity on the planet, and so the potential loss of species and the likely scale of alterations to community structure and organisation are enormous. Here, I briefly discuss some of the consequences of forest degradation and fragmentation for the diversity, composition and structure of tropical forest communities, and the functional consequences of these changes. My focus is particularly on insects and their interactions with other species. This is partly because these are the systems most familiar to me, and partly because their enormous diversity, biomass and wide range of functional roles in tropical forest ecosystems mean that they are likely to be sensitive in responding to habitat changes, and also key players in the functioning of these systems. Habitat destruction and disturbance Forest clearance for intensive agriculture or timber is a major component of forest loss, with approximately 5.8 million hectares of tropical forests converted to pasture and plantation globally (Mayaux et al., 2005). However, habitat ‘loss’ in tropical forests is only one part of the picture. The outcome of logging or agriculture is rarely outright forest destruction, but rather an altered, degraded, but still largely forested habitat; and the tropical landscape increasingly comprises a fragmented network of relatively intact patches, separated by a matrix that may vary from ‘recovering’ secondary forest, to intensively cultivated land. Wright and Muller-Landau (2006) argue strongly that ‘secondary’ forests, of various sorts, represent the future of tropical forests, and it will only be possible to maintain a tiny fraction of the tropical forested area within parks or reserves, entirely free from direct human impacts. In some parts of the tropical world the forested area is actually increasing as cleared areas are re-colonised by secondary forests (Wright & Muller-Landau, 2006; but see Sodhi, Brook, & Bradshaw, 2007). Some of Wright and Muller-Landau’s projections of the potential for secondary forests to rescue us from a tropical forest extinction crisis are highly controversial (Barlow et al., 2007; Brook, Bradshaw, Koh, & Sodhi, 2006; Gardner et al., 2007; Laurance, 2007) and this debate highlights the urgent need to gather data on the extent to which the diversity and composition of ‘old-growth’ tropical forest faunas and floras can be maintained in ‘disturbed’ tropical forest habitats. 99 and at varying intensities, is affecting the species richness or diversity of particular plant and animal groups (e.g., Barlow et al., 2007; Greiser Johns, 1997; Pinard, 2005; Putz, Blate, Redford, Fimbel, & Robinson, 2001). Existing data suggest that the responses of tropical forest plant and animal communities to habitat change are highly idiosyncratic (Barlow et al., 2007; Lawton et al., 1998). There are several reasons why this may be the case, including genuine taxon-specific differences in sensitivity, and across-study variations in the spatial scale of the study or the degree and form of ‘disturbance’ (Lewis & Basset, 2007). Careful metaanalyses (e.g., Gray, Baldauf, Mayhew, & Hill, 2007) will be needed to separate artefacts from genuine trends. Even if individual studies suffer from pseudoreplication (and often they do), they should individually represent replicates for synthetic meta-analyses (Cottenie & De Meester, 2003). However, it is growing increasingly clear that focusing on summary metrics such as diversity and richness will often provide a misleading picture of the effects of tropical forest disturbance on ecological communities. This becomes apparent when we look at the species composition in ‘disturbed’ and ‘less-disturbed’ habitats, and the abundance and persistence of individual species rather than summary statistics that average across assemblages. A particular trend is towards biotic homogenisation (McKinney & Lockwood, 1999): while disturbed forests may often have an equal or even a greater number of species than undisturbed forests, these species are typically drawn from a restricted pool; and restricted-range or habitat-specialist species are most likely to decline or go extinct. In many butterfly assemblages, for example, forest disturbance allows a suite of mobile, widespread and generalist taxa to colonize and coexist with much of the existing fauna (Hamer, Hill, Lace, & Langan, 1997; Lewis, Wilson, & Harper, 1998; Spitzer, Jaros, Havelka, & Leps, 1997; Spitzer, Novotny, Tonner, & Leps, 1993; Thomas, 1991). In this case, a high richness or diversity forest is not necessarily one of high conservation ‘value’. Biologists assessing the effects of humans on tropical forest biodiversity need to rely less on summary statistics, and more on the biological characteristics of the taxa they study, which will influence both their sensitivity to disturbance (‘response traits’) and their contributions to ecosystem function (‘effect traits’: Lavorel & Garnier, 2002). Changes in function Changes in diversity and composition A growing set of studies throughout the tropics has investigated how human disturbance, in various forms So what are the likely consequences of these sorts of biodiversity changes for the important ecosystem functions performed by biodiversity, such as pollination and ARTICLE IN PRESS 100 O.T. Lewis / Basic and Applied Ecology 10 (2009) 97–102 decomposition? Much of the extensive literature on the relationship between species richness and ecosystem function is of limited utility for answering this question (Leps, 2004; Schmid & Hector, 2004). There are several reasons for this. First, the literature is dominated by studies of the relationship between diversity and productivity in temperate plants, and studies of functionally important organisms at higher trophic levels (e.g., insects, which Wilson (1987) described as the ‘‘little things that run the earth’’) are scarce. Second, most experimental studies are concerned with documenting the ‘mean’ effect of adding or removing species, regardless of their individual contribution to ecosystem function, abundance, degree of redundancy, or extinction-proneness. The abundance and traits of component species are likely to be at least as important as counts of numbers of species (Dı́az, Symstad, Chapin, Wardle, & Huenneke, 2003; Lepš, 2004). For example, loss of more abundant species, or non-random extinction patterns leading to loss of an entire functional group is likely to have more marked functional consequences, compared to random patterns of species loss (Dı́az et al., 2003; Larsen, Williams, & Kremen, 2005); loss of species is rarely a random process. Third, few such studies have been carried out under realistic levels of diversity under field conditions, particularly in high-diversity ecosystems such as tropical forests. I will highlight two recent studies involving dung beetle assemblages which have begun to tackle these important issues in tropical forests. Dung beetles move and bury animal faeces, and can enhance forest regeneration by improving soil fertilization, aeration and nutrient cycling, and by reducing seed predation and interspecific competition or other sources of density-dependent mortality (Andresen, 2002; Davis et al., 2001; Estrada, Anzuras, & Coastes-Estrada, 1999). The rate at which dung is buried can be measured in the field, and the correspondence between dung burial rates and species richness or composition can be calculated (Klein, 1989). Larsen et al. (2005) studied dung beetle assemblages on recently isolated islands of forest in Venezuela, and were able to relate the process of dung burial to real patterns of species loss. They found that large-bodied dung beetles were both the most extinction-prone and the most efficient dung-buriers. Thus, loss of ecosystem function was marked and much larger than would be predicted by a random order of species extinction. Slade, Mann, Villanueva, and Lewis (2007) also studied the functional effects of dung beetles, this time in Malaysian Borneo, using simple field experiments to manipulate the access of different functional groups of dung beetles to patches of resource. Again, large beetles were the most important functionally, but the experimental approach allowed the contributions of different sets of species to be quantified in isolation and in different combinations. Overall, both dung and seed removal increased with dung beetle functional group richness. However, levels of ecosystem functioning were idiosyncratic depending on the identity of the functional groups present, indicating an important role for functional group composition; and a full complement of functional groups was required to maximize ecosystem functioning. These studies demonstrate that ecologists will need to move beyond the focus on species richness to understand the functional consequences of biodiversity changes. Changes in structure Another aspect of biodiversity that is overlooked if we restrict ourselves to species richness as the explanatory variable of interest is ‘biostructure’ (McCann, 2007). All species are embedded in complex webs of mutualistic and antagonistic interactions, and nowhere are these webs more complex and diverse than in tropical forests (Janzen, 1983). The effects of species loss within food webs can be unpredictable, and may propagate some distance through interlinked chains of trophic linkages. For example, removal of a single species from a tropical forest food web can have widespread indirect effects through apparent competition (Morris, Lewis & Godfray, 2004; Morris, Lewis, & Godfray, 2005). Similarly, alterations in herbivore abundance can lead to trophic cascades (Dyer & Letourneau, 1999; Letourneau & Dyer, 1998). Given the major effects that insects can have on plant fitness (Marquis, 2005; Marquis & Braker, 1993) and potentially plant diversity (Connell, 1971; Janzen, 1970), alterations in insect assemblages may have major repercussions for wider tropical ecosystems. It seems likely that altered interaction structure represents a functionally important but largely overlooked consequence of human habitat modification. McCann (2007) makes the analogy with animal physiology: it is the underlying architecture of organisms and not just the individual body parts that maintain the bodily functions necessary for life. Disturbance is likely to lead to ‘re-wiring’ of ecological networks, altering the patterns and intensities of the direct and indirect interactions linking species. Although a wide array of food web statistics can be described and calculated, we currently lack an understanding of which of these food web attributes are likely to be most sensitive to anthropogenic changes. A key task for ecologists will be to understand the sensitivity of food web ‘biostructure’ to various forms and intensities of human disturbance. In a recent study, we found that species identity and conventional community descriptors (such as species richness and diversity indices) differed little across a tropical gradient of habitat disturbance; but these same communities showed markedly different ARTICLE IN PRESS O.T. Lewis / Basic and Applied Ecology 10 (2009) 97–102 patterns of trophic interaction (Tylianakis, Tscharntke, & Lewis, 2007). Thus, once again, using conventional measures of species richness or diversity may mean that we are overlooking ecologically important changes to communities. Theory suggests that food web structure may be critically linked to stability and function (McCann, 2007; Rooney, McCann, Gellner, & Moore, 2007), but empirical data are lacking, particularly for diverse tropical systems. Collecting data to investigate this further will be far more challenging than documenting changes in richness and diversity, because documenting networks of interactions is inevitably a tedious and time-consuming task. New approaches such as DNA barcoding (which has the potential to speed the process of taxonomic sorting) and stable isotope analyses (which can help reconstruct trophic pathways) may provide shortcuts to documenting biostructure. Such efforts will be essential if we are to progress beyond describing the consequences of human actions for biodiversity patterns, to understanding the consequences of these changes for ecological processes. Acknowledgements I thank Teja Tscharntke for his kind invitation to write this perspective, Eleanor Slade for many discussions on the functional consequences of biodiversity loss in tropical forests, and Navjot Sodhi and an anonymous reviewer for their helpful comments on the manuscript. 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