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Basic and Applied Ecology 10 (2009) 97–102
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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
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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
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(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.
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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
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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
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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.
The author is supported by a Royal Society University
Research Fellowship.
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