Tropical rainforests and the need forcross-continental comparisonsRichard T. Corlett1 and Richard B. Primack2

1Department of Ecology & Biodiversity, The University of Hong Kong, Pokfulam Road, Hong Kong, China2Biology Department, Boston University, Boston, MA 02215, USA

Ecologists have tended to overemphasize the common

features of tropical rainforests on different continents. In

reality, the five major tropical rainforest regions (tropical

America, Africa, Southeast Asia, Madagascar and New

Guinea) are distinct ecological and biogeographical

entities. Although it is easy to find examples of at least

superficial convergence between unrelated organisms

in these different regions, there are many other cases

where convergence is incomplete or there are no

obvious ecological equivalents. Pantropical compari-

sons with standardized methods are needed for the

insights that they can provide into rainforest ecology

and the help that they can offer in identifying conserva-

tion strategies that are appropriate to regional con-

ditions. Here, we suggest ways in which the practical

difficulties of such pantropical comparisons can be

minimized.

One boreal forest, but many tropical rainforests

Biogeographers divide up the terrestrial ecosystems of theworld in two fundamentally different ways: (i) into biomes,defined by the dominance of particular plant functionaltypes; or (ii) into biogeographical regions, based on thedistribution of plant and animal taxa. In the northerntemperate zone, where most ecologists live and work,these two classifications are confounded, because themajor temperate biomes occur within a single biogeogra-phical region, the Holarctic (divided for some purposesinto two, the Nearctic and Palearctic). This reflects theoverall similarity in the taxonomic composition of eachbiome across the northern temperate region, as a result ofa long history of intermittent connections.

The humid tropics are a different matter. Although thetropical rainforest biome can be recognized by itsdistinctive structure and physiognomy wherever it occurs,biogeographers ever since the great 19th-century natur-alist Wallace have consistently divided it into three to fivebiogeographical regions, depending on which group oforganisms they are interested in. We have argued else-where that there are indeed five major regions (in tropicalAmerica, Africa, Southeast Asia, Madagascar and NewGuinea, with smaller outliers in Australia, South Asia andon many tropical islands) and that each of these regions isa distinct ecological and biogeographical entity [1].

Corresponding author: Corlett, R.T. (corlett@hku.hk).Available online 10 January 2006

www.sciencedirect.com 0169-5347/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved

Unfortunately, ecologists, with a few exceptions [2], havetended to focus on the similarities and underplay thedifferences among these regions. In doing so they have notonly missed many opportunities for comparative studies,but might have also given the false impression thatrainforest conservation is a single problem with a singlesolution. Here, we discuss why pantropical comparisonsare needed and how the practical difficulties of suchcomparisons could be minimized.

Rainforests as natural experiments

Given that most tropical rainforest today is on fragmentsof the Mesozoic supercontinent of Gondwana, it has beentempting to attribute similarities, such as shared plantfamilies, to this common geological origin. However, thefossil record and molecular estimates of divergence timesfor rainforest-associated plant lineages both suggest thatangiosperm-dominated tropical rainforest appeared noearlier than the mid Cretaceous [c. 100 million years ago(Mya)] [3,4], when the break-up of Gondwana was alreadywell underway (Box 1). Because most major lineages ofrainforest-associated plants and animals are youngerthan this [1], Gondwana-era connections arethus irrelevant.

The use of calibrated molecular phylogenies has alsodemonstrated the importance of long-distance trans-oceanic dispersal in some plant families [5], but currentdistribution patterns (such as the numerous range limitscoincident with Wallace’s Line, the eastern faunalboundary of the Asian region) show that this is lesscommon in animals (Box 2). The five major rainforestregions are isolated today by marine or desert barriers,but, in most cases, were even more isolated during much ofthe evolutionary history of their dominant plant andanimal lineages (Box 1). In effect, they represent five,more or less independent, evolutionary responses to thehumid tropical environment.

New Guinea, for example, supports a rainforest with alargely Asian flora, but no primates, ungulates oreutherian carnivores [1]. Madagascan rainforests havecarnivores and primates, each of which is the result of asingle colonization event [6], but no ungulates and,indeed, no members of most of the vertebrate familiesfound in African rainforests. African and Asian rainforestshave many squirrels and few parrots, whereas therainforests of the Neotropics and New Guinea havemany parrots, and few and no squirrels, respectively [1].

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Box 1. Tropical rainforest histories

Most tropical rainforests today are on fragments of the ancient

southern supercontinent of Gondwana. However, most modern

groups of plants and animals originated after the break-up was well

underway and radiated during the Tertiary (65–2 Mya) in the period

of maximum isolation among the five major rainforest regions

(Figure I). The isolation of Madagascar has lasted for almost 90

million years, so vertebrate recolonization after the Cretaceous–

Tertiary extinctions (65 Mya) was entirely transoceanic [6]. The

isolation of Africa during the late Cretaceous and early Tertiary (90–

24 Mya) produced a spectacular mammalian radiation in an

endemic clade, the Afrotheria, represented today by the elephants,

hyraxes, elephant-shrews, aardvark and golden moles [46]. Africa

has been physically connected to Asia for 24 million years, but

aridity has limited the exchange of rainforest taxa for much of that

period [47]. South America was isolated for most of the past 70

million years, giving rise to many endemic radiations; it became

connected to North America through the Isthmus of Panama a

mere 3–4 Mya, enabling a dramatic intermingling of biotas known

as the Great American Interchange [48]. Australia and New Guinea

moved north towards Asia throughout the Tertiary, resulting in a

major influx of Asian rainforest plants into lowland New Guinea

[47]. The dry land connection needed to enable a ‘Great

Australasian Interchange’ for the vertebrate fauna is still 40 million

years in the future.

Box 2. Are plants and animals different?

In striking contrast to the birds, mammals and other vertebrates,

where no genera and few families occur in all the major rainforest

regions, all continental rainforests share many of the same plant

families (e.g. Annonaceae, Fabaceae, Lauraceae and Rubiaceae) and a

few genera (e.g. Ficus) [18,38]. One possible explanation for this

apparent contrast between plant and animal distributions is a

difference in the way that plant and animal systematists use the

taxonomic hierarchy. The use of dated molecular phylogenies

removes this potential bias, leaving just two possibilities: either the

major rainforest plant lineages are older than the vertebrate lineages

and could thus achieve pantropical distributions while the major

rainforest land masses were still connected (Box 1), or long-distance

dispersal over water barriers is easier for plants than it is for

vertebrates. Although there is some evidence in support of both

views, an increasing number of dated molecular phylogenies suggest

that plant disjunctions are often younger than animal disjunctions and

can often only have arisen from transoceanic dispersal [5]. The greater

ability of rainforest plants than rainforest vertebrates to disperse

across water is also illustrated by the biotas of rainforests on oceanic

islands, where floristic diversity at all taxonomic levels falls off more

slowly with increasing isolation than it does in even the best-dispersed

vertebrate groups [50].

Getting there is only part of the problem and this apparent difference

between the biogeography of rainforest plants and animals raises

other interesting questions. The ability of the same plant families to

persist in rainforests throughout the tropics for tens of millions of

years suggests that each family occupies a distinct volume of niche

space [51]. Equally intriguing is the implication that the resident

animal lineages in tropical rainforests have had to adapt to a

continually changing flora over evolutionary time, as new plant

genera and families arrived (R. Toby Pennington, personal communi-

cation). The paradoxical rainforests of the New Guinea lowlands are

an extreme example of this phenomenon, with a largely endemic

vertebrate fauna occupying a forest dominated by post-Miocene Asian

plant immigrants [47]. The ecological consequences of this could be

investigated by comparing plant–vertebrate relationships in lowland

New Guinea with those in Southeast Asia, with its very different fauna,

and Australia, where the Asian plant influx is less prominent.

NA PA

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PA, PalearcticNA, Nearctic

NT, NeotropicalAF, AfricanI, IndianAN, AntarcticAU, AustralianM, Madagascar

Mountain ranges

Lowlands

(d) Middle Miocene (14 mya) Modern world(e)

Figure I. The break-up of Gondwana from the late Jurassic (152 Mya) (a) to the present day (e). Angiosperm-dominated tropical rainforests might have originated !

100 Mya, but most rainforest lineages are younger than this. The isolation of the five major rainforest regions on separate continental fragments between the late

Cretaceous (100 Mya) (b) and Miocene (20 Mya) (d) laid the foundations for the distinctiveness of their modern biotas. Modified, with permission, from [49].

Review TRENDS in Ecology and Evolution Vol.21 No.2 February 2006 105

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Figure 1. Examples of apparent convergence. Ecologists have traditionally

emphasized examples of morphological and behavioral convergence between

unrelated rainforest organisms. This example shows pairs of rainforest mammals

from tropical America (a) and Africa (b). From top to bottom, left to right, the pairs

are: capybara Hydrochaeris hydrochaeris and pygmy hippopotamus Hexaprotodon

liberiensis; paca Agouti paca and water chevrotain Hyemoschus aquaticus; agouti

Dasyprocta sp. and royal antelope Neotragus pygmaeus; grey brocket deer

Mazama gouazoubira and yellow-backed duiker Cephalophus silvicultor; giant

armadillo Priodontes maximus and giant pangolin Manis gigantea. Reproduced,

with permission, from [54].

Review TRENDS in Ecology and Evolution Vol.21 No.2 February 2006106

Only the Neotropics has leaf-cutting, fungus-growingants, whereas only the Neotropics and New Guinea lackfungus-growing termites. There are dipterocarps in allfive regions (attesting to a probable Gondwanic origin ofthe family), but they occur in lowland rainforests only inAsia and New Guinea, and they dominate the forestcanopy only in Asia.

It is not a perfect ‘experiment’ in rainforest assembly:such natural experiments never are. The ‘treatments’ areunreplicated, are not assigned randomly, and are notcompletely independent. Differences in, for example, theherbivore community, are confounded by differences inclimate, flora and predators. There has also been somemixing of floras and faunas as first Africa, then SouthAmerica became attached to the northern continents, andas the water gaps between Southeast Asia and NewGuinea narrowed. Overall, however, this ‘experiment’ isprobably as good as it gets when there is only one planet towork with.

Convergence, partial convergence and non-convergence

Ecologists might be justified in downplaying differencesamong regions if convergent evolution had producedsimilar adaptations in unrelated groups of organisms ineach region. If marsupials had the same ecological roles in

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New Guinea as primates and Carnivora do elsewhere, andparrots and squirrels were interchangeable as muscular-jawed seed predators, then perhaps all rainforests couldbe seen as at least functionally similar. Ecologists havetraditionally emphasized examples of apparent conver-gence, although typically at a superficial level (Figure 1),whereas there are many other cases where convergence isincomplete or where there are apparently no ecologicalequivalents in other regions.

Hummingbirds (Neotropics), sunbirds (Africa and Asia)and honeyeaters (New Guinea) have convergent morpho-logical and physiological adaptations to a watery, sugary,low-protein diet [7], but most hummingbirds hover whilevisiting flowers, whereas sunbirds and honeyeaters perch,with consequences for the way in which bird-pollinatedflowers and subsequent fruits are displayed in each region[8]. Phyllostomid (Neotropics) and pteropodid (all otherrainforests) fruit bats have evolved their frugivorous andnectarivorous habits independently [9]. Most phyllosto-mids are tiny echolocating bats that can navigatecluttered rainforest understoreys and hover to takesmall fruits or to sip nectar from relatively delicateflowers. Most pteropodids, by contrast, are larger strong-flying bats that locate food by vision and smell, andtypically perch while feeding on larger fruits and more-robust flowers. Fruit-eating Old World monkeys (Africaand Asia) are relatively large primates with trichromaticcolor vision [10] and well-developed cheek pouches thatenable them to spit, rather than swallow, seeds [11]. Fruit-eating New World monkeys (Neotropics) are smallerprimates, with most individuals being dichromatic. Theylack cheek pouches and typically swallow and defecateeven large seeds. The monkey and fruit bat examples alsoillustrate a more general pattern: both fruits and fruit-eating animals are consistently larger in Africa and Asiathan they are in the Neotropics [12].

The need for cross-continental comparisons

There are two basic reasons, beyond simple curiosity, whywe need more comparative studies that look at two or morerainforest regions. The first is the opportunity that suchcomparisons provide for insights into rainforest ecologyand the second is the insights that they provide into theconservation of these regions.

Insights into rainforest ecology

A good example of a problem best approached throughpantropical comparisons is the relationship betweendipterocarp dominance in Southeast Asian rainforestsand the supra-annual patterns of mass flowering and mastfruiting that have major consequences for the feeding andranging behavior of animals [13,14]. Although thesynchronized reproduction of numerous species overhuge areas is unique to Southeast Asia, the samecombination of mast-fruiting ectomycorrhizal trees, poorlydispersed fruits and a tendency to dominate forests isshared by several African and at least one Neotropicalspecies of rainforest Caesalpiniaceae plants [15]. Thereare also low-diversity dipterocarp-dominated forests inNew Guinea and forests dominated by the ectomycor-rhizal dipterocarp sister group, Sarcolaenaceae, in

Box 3. A pioneering comparison of rainforest bird communities across continents

During the 1970s, ornithologist David L. Pearson compared rainforest

communities at six sites, three in Amazonia and one each in Gabon

(Central Africa), Indonesian Borneo and Papua New Guinea [26]. At

each site, he spent 225–714 h recording the foraging height, substrate

and foraging technique (e.g. gleaning, sallying, pecking and probing at

bark or fruit-eating) for every individual bird seen in w15 ha of typical

lowland rainforest. In spite of the limitations of the methodology (a

single season in a small sample area at each site), the results of this

pioneering study have stood up well to subsequent local research.

Overall bird species richness was highest in the Amazonian plots

and lowest in New Guinea (Figure I). The most variable foraging

technique was pecking and probing, which was used by many

species of woodpeckers and woodcreepers in the Amazon, largely

by woodpeckers in Asia and Africa, and only occasionally by a

parrot in New Guinea. The importance of frugivory also

varied considerably, with the New Guinea plot having both the

highest density of fruit-eating birds and the highest proportion of

obligate frugivores. Ant-following birds were found only in the

Amazonian and African plots, where swarm-raiding army

ants occur.

Pearson discussed the possible influences of various factors,

including history, current climatic seasonality and non-avian compe-

titors, on the patterns he recorded in his study plots. The words

‘preliminary’ and ‘initial’ appear throughout this discussion, and it is a

pity that subsequent studies have not followed up on his pioneering

approach. We know a great deal more about the ecology of rainforest

bird communities than we did 30 years ago, but comparative studies

are now more often done in the library than the field. There is room for

both approaches, but there is no substitute for replicated field studies

using standard techniques.

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Figure I. A pioneer cross-continental comparison: the cumulative numbers of

bird species found in David Pearson’s rainforest field sites after 225–714 h of

observation. Data from [26].

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Madagascar [16]. Comparative multi-year studies offlowering, seed output, seed predation, nutrient uptakeand insect abundance in these forests would be the bestway to identify the factors that favor supra-annualreproduction in some forests and not others.

The network of large-scale forest plots set up by theCenter for Tropical Forest Science (CTFS: http://www.ctfs.si.edu) provides an excellent opportunity for pantropicalbotanical comparisons, albeit with gaps in Madagascarand New Guinea [17]. However, this opportunity has beenexploited little, with most studies focusing on a single plotor on comparisons within a region. One of the moststriking differences between the Southeast Asian andNeotropical rainforest plots is that the understorey in theAsian plots is dominated by sterile saplings of canopytrees, whereas that of the Neotropical plots is rich infruiting shrubs and small trees [18]. This difference hasnot only had a large impact on the density and diversity ofunderstorey frugivore communities [1,19], but alsosuggests there is a fundamental divergence in the wayin which canopy trees regenerate [18]. A cross-continentalcomparison of understorey dynamics, based on the CTFSplots, is the best way to investigate this further. Equallyinteresting would be studies of the functional conse-quences of the differences between rainforest regions inthe plant families that dominate climber and epiphytecommunities [18,20], although complete census data arerare for both.

In contrast to the situation with rainforest botany,cross-continental comparisons have had a long andfruitful history in primate studies [21,22]. In recentyears, comparisons that take into account differences in

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the visual systems of the major primate clades haveproved particularly interesting, and have resulted in newinsights into the ecology of primates and their forests[10,23]. However, the narrow taxonomic focus of thesecomparisons has also been a limitation. Except inMadagascar, primates radiated into forests that werealready occupied by arboreal non-primate groups, some ofwhich have apparently pre-empted potential primateniches [22]. In the Neotropics, sloths occupy the arborealleaf-eater niche and marsupials dominate the night, whilein Asian rainforests, the diversity of squirrels seems tohave limited the opportunities for small primates.Pantropical community-level comparisons that includedprimates and putative ‘primate equivalents’ would beparticularly valuable and could include rainforests in NewGuinea, where primates are absent and marsupialsoccupy all arboreal niches.

Other mammalian groups that have been the subject ofrecent cross-continental comparisons include fruit bats[24] and large herbivores [25], although, in both cases, thelack of comparable field data from standardized methodswas a problem. Birds are in many ways ideal for cross-continental comparisons, because it is relatively easy toassign them to guilds for comparisons between commu-nities that have no taxa in common. Thirty years ago,David Pearson made a pioneering comparison between thebird communities in all the major rainforest regionsexcept Madagascar (Box 3) [26], but this approach hasnever been used subsequently and most ornithologicalstudies are now confined to one region.

Few groups of invertebrates are known well enough ateven one site for cross-continental comparisons. The major

Review TRENDS in Ecology and Evolution Vol.21 No.2 February 2006108

exceptions are social insects, particularly termites, antsand bees. The absence of fungus-growing Macrotermitinaefrom the Neotropics and New Guinea is only one of severalfundamental differences among regions in the taxonomicmake-up of rainforest termite communities [27]. Do thesetaxonomic differences translate into differences in theirrole in decomposition, energy cycling and nutrientcycling? Do non-termite taxa compensate for absenttermite clades? A pantropical comparison of the structureand function of termite communities is the best wayto identify the unique roles, if any, of particulartermite clades. Rainforest ant communities showequally striking differences [1], with equally unknownfunctional consequences.

Cross-continental comparisons suffer from the sameproblem with potential confounding variables, as do allsuch ‘natural experiments’. African rainforests are notsimply Asian rainforests minus the dipterocarps, and theabsence of primates is only one of many ways in whichNew Guinea is distinctive. However, combining cross-continental comparisons with manipulative experiment(such as excluding particular taxa from an area ofrainforest [28]) can increase confidence in any conclusions.Large-scale experiments are beyond the resources of mostrainforest ecologists, but ‘accidental experiments’ result-ing from moving rainforest taxa across biogeographicalboundaries (such as is the introduction of Apis melliferahoneybees to the Neotropics [29]) can provide furtherinsights into rainforest ecology.

Differing strategies for conservation

The example of honeybee introductions illustrates thesecond major reason why cross-continental comparativestudies are needed. The introduction of Apis is of far morethan just intellectual interest, because the relatively lowdiversities of other bees in forests where Apis is nativesuggest that its introduction to new areas would havedisastrous consequences for native bee faunas and theplants that depend on them [29,30]. Cross-continentalcomparisons could help predict the impact of present andfuture honeybee introductions and suggest ways in whichthis impact could be limited. More generally, comparativestudies can help identify conservation strategies appro-priate to regional conditions, rather than relying on a ‘onesize fits all’ approach to saving ‘the rainforest’. Thesestrategies can then be fine-tuned to local ecological andcultural conditions.

Differing strategies are needed for several reasons.Most obviously, because the major immediate threats areoften different: logging and cash-crops in Southeast Asia,the bushmeat crisis and political instability in Africa,subsistence farming and cattle grazing in Madagascar,and all these and more in the Neotropics [1]. Differentrainforest regions might also differ in their vulnerabilitiesto particular impacts as a result of their uniquebiogeographical histories. Southeast Asian rainforestshave suffered massive logging damage because of theexceptionally high density of commercially valuabledipterocarps [31]. Elsewhere, rainforest loggers oftentarget one or a few species, so the impact depends ontheir abundance. In comparison with loggers, bushmeat

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hunters show relatively little discrimination, except bysize and ease of capture, but the impacts on ecosystemfunctions will vary between regions because of thedifferent ecological roles of the major game species.Frugivorous ungulates dominate kills in Africa [32],whereas large rodents are often the preferred prey in theNeotropics [33]. These two groups of animals are some-times viewed as ecological equivalents, but process fruitsand seeds in different ways [1], suggesting that theimpacts of their extirpation on forest regeneration willalso differ substantially. Fragmentation threatens rain-forests in all regions, but the vulnerability of SoutheastAsian forests could be increased by the responses ofnomadic seedeaters adapted to supra-annual cycles offeast and famine [13].

We can predict that the most damaging invasive specieswill be from major lineages that were previously absentfrom a region. The most worrying recent example is theestablishment of long-tailed macaques Macaca fascicu-laris on the hitherto primate-free island of New Guinea,near Jayapura [34]. We do not know to what extentarboreal marsupials already occupy potential macaqueniches in New Guinea, but a catastrophic impact onnesting birds can be predicted if these primates spread.Other examples of introductions that threaten regionaluniqueness are bush pigs Potamochoerus larvatus inMadagascar [35], feral pigs Sus scrofa in New Guineaand Australia [36], and a variety of Neotropical bat-dispersed pioneer tree species (e.g. Cecropia, Muntingiaand Piper) in the Old World tropics (e.g. [37]).

The inherently most vulnerable rainforests are likely tobe those on the islands of New Guinea and Madagascar.Treating these rainforests as ‘continental’ can be justifiedby their geological histories and botanical diversities(greater on a per-hectare basis than African rainforests[38]), but they resemble the rainforests of oceanic islandsin the dominance of their vertebrate faunas by a fewendemic radiations. This not only makes them peculiarlyvulnerable to introduced species from ‘missing’ groups,but also means that key ecological processes depend onone or a few related species. Examples of this dependenceare the key roles in seed dispersal of the endemic lemurradiation on Madagascar [39] and endemic cassowaries inAustralia and New Guinea [1,40].

At the other extreme, it has been suggested that somebiotas are more resilient to current threats as a result ofpast episodes that removed the more vulnerable species[41]. If this is true for rainforests, then we would predictgreatest resilience in Africa, where the modern biota hassurvived not only a massive contraction in rainforest areaduring the Pleistocene glaciations [42], but also a longerperiod of human impact than elsewhere. The apparentlylower sensitivity of African rainforest primates tofragmentation [43] could be evidence for this resilience,but it must be emphasized that survival through past‘extinction filters’ cannot predict resilience in the face ofcompletely new threats.

In Madagascar, inherent vulnerability met modernhumans 2000 years ago, resulting in the extinction ofmany large vertebrates that have no ecological equiva-lents today [44]. Deforestation now threatens many of

Box 4. Differing routes to recovery?

The re-establishment of rainforest in deforested areas is largely

dependent on seed dispersal by vertebrates. There is little overlap

between the vertebrate families responsible for seed dispersal in the

Neotropics and those in tropical Asia and Africa, and relatively little

between these regions and Madagascar and New Guinea [1,52]. The

early stages of woody succession in the Neotropics are dominated

by tiny-seeded pioneers dispersed by bats and ‘fruit-mashing’

emberizid passerines (tanagers and their relatives), whereas in the

rainforest regions of Africa and Asia, larger-seeded pioneers are

dispersed by bulbuls and other ‘fruit-gulping’ birds [1,53]. Seed

dispersal by birds, but not bats, depends on the availability of

perches, so we would predict different patterns of woody invasion at

open sites in each region.

The dispersal of larger seeds (O15 mm diameter) is beyond the

capability of most open-country passerines throughout the tropics, but

the regions also differ in the willingness of larger forest frugivores (O

2 kg) to cross open areas between fragments and to enter pioneer

vegetation. In Africa and, to a lesser extent, Asia, large-bodied

rainforest frugivores up to the size of apes and elephants regularly

enter non-forest habitats, where they often cause severe crop damage,

whereas similar reports are rare in the Neotropics [53].

The consequences for landscape connectivity and woody succession

of these dichotomies in the behavior of the major frugivore families in

the Old and New Worlds have not yet been investigated in detail, but

anecdotal support is sufficient to warn against extrapolating the results

of studies of forest recovery and restoration from one region to another.

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the species that survived this initial catastrophe. Evi-dence for similar megafaunal extinctions in New Guinea isless clear, at least for the rainforest, and satellitephotographs give the impression that the third largestcontiguous block of rainforest in the world is stillrelatively intact. However, high birth rates, a loggingboom and poor governance threaten the future of therainforest on both sides of the political divide of the island[1]. The rainforests of Madagascar and New Guinea thusdeserve more attention from ecologists and conservation-ists than they currently receive.

As attention shifts in the coming decades from savingrelatively intact rainforest ecosystems to restoringdepleted, degraded and deforested areas, the differencesbetween rainforest regions are likely to become more,rather than less, important. This is because of the key rolethat vertebrate seed-dispersal agents have in the re-establishment of plant species at sites from which theyhave been eliminated (Box 4).

How can we do cross-continental comparisons?

There are good reasons why pantropical comparisons arerare. Coming to terms with tropical rainforest diversity atone site is difficult enough, even without the addedchallenges of an unfamiliar language and culture.Repeating this at five or more sites is beyond mostresearchers’ capabilities. Funding is also a problem, withthe costs of travel multiplied by the lack of direct flightsbetween tropical regions. The CTFS plot network is themost ambitious existing example of a comparative studycoordinated by a single organization [17]. One way toexpand the comparative approach that we advocate herewould be to extend this network, filling in the moreobvious gaps, and encouraging its use for a broader rangeof studies. Large plots require a massive investment,however, so the coverage will always be limited and even50 ha is far too small for the study of many key ecologicalprocesses. A useful complement to the large plots, whichwould overcome some of their limitations, would be apantropical network of long-term elevational transects(Oliver Phillips, personal communication). Small forestplots, which are within the capabilities of local researchgroups, have had a major role in identifying patternswithin the Neotropics [38,45], and an expanded pantropi-cal data set would facilitate cross-continental comparisonsthat took these differences within regions into account.

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Most researchers are confined by time and money to asingle site, so the comparative element can only come fromthe use of standardized methods by different researchersin different rainforest regions. A variety of groups, such asthe International Canopy Network (ICAN: http://www.evergreen.edu/ican/), now encourage internationalresearch collaborations, and a day was devoted tofostering international collaboration at the 2005 confer-ence on Frugivores and Seed Dispersal in Brisbane,Australia (http://www.learnaboutwildlife.com/Frugiv-ory2005.htm). These types of international collaborationamong independent research groups will probably be themost effective way to provide the necessary comparisonsbetween tropical rainforests on different continents.

AcknowledgementsThis paper builds on our recent book, Tropical Rainforests: An Ecologicaland Biogeographical Comparison, and all the many people who helped uswith the book have indirectly contributed to this article. In addition, wethank Colin Chapman, David Dudgeon, Les Kaufman, Bill Laurance,Abraham Miller-Rushing, Oliver Phillips and Navjot Sodhi, who read andcommented on earlier drafts of the article.

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