Debating the Tropical Extinction Crisis
Chaired by:
 William F. Laurance, ATBC Past-President, Smithsonian Tropical Research
Institute, Balboa, Panama
 S. Joseph Wright, Smithsonian Tropical Research Institute, Balboa, Panama
A strident debate has erupted in the field of conservation biology. On the one hand is the
traditional view, espoused by scientists such as Peter Raven, Stuart Pimm, William
Laurance, and others, that suggests that rampant tropical deforestation is driving a megaextinction crisis that, if unabated, might ultimately rival mass extinction events in earth’s
geological history. One the other hand is the recent view, proposed by S. Joseph Wright
and Helene Muller-Landau, that net losses of tropical forest cover are likely to be far
more modest than previously suggested, at least over the coming century, so that the
number of extinctions that might be caused by habitat loss should be re-evaluated
downwards. This latter argument is largely based on analyses that suggest that (1) human
population growth is slowing, especially in tropical rural areas; (2) large areas of
secondary forest are becoming established naturally so that net deforestation is slowing;
and (3) many tropical species will be able to persist in secondary and logged forests
(Wright & Muller-Landau 2006a, 2006b). As might be expected, an emerging debate of
this nature, with its critical and far-ranging implications, has precipitated vigorous
rejoinders and counter-arguments (Brook et al. 2006; Gardner et al. 2007; Sloan 2007;
Laurance 2007). This dispute revolves around two general questions. First, what factors
actually drive tropical deforestation and prevent natural secondary sucession from reestablishing forest cover? Second, what are the biological consequences of the ongoing
loss of primary, old-growth forests and of increases in secondary and logged forests?
Both questions have many layers of complexity. For example, the proximate and
ultimate factors that drive net changes in tropical forest cover involve disparate social,
economic, human-demographic, and policy considerations that originate within and differ
among tropical countries as well as extrinsic factors related to trade and global economic
development. We propose to convene a symposium at the ATBC-Morelia meeting that
focuses on this debate. The six speakers would be evenly divided between those who
generally support the Wright and Muller-Landau perspective, versus those who oppose it.
The goal would be to provide the audience with an overview of the emerging issues and
research questions at the center of this important and timely debate.
Introduction: William F. Laurance
Land use change, climate change and the extinction of tropical forest
species
S. Joseph Wright1 and Helene Muller-Landau2
1
Smithsonian Tropical Research Institute
2
University of Minnesota
Email: wrightj@si.edu
It is widely predicted that the conversion of tropical forests to agriculture and other
human uses will lead to a mega-extinction event similar in severity to the megaextinction events that define geological epochs. We believe this prediction is unlikely to
be realized, and that conservation strategies should be adjusted to allocate more resources
to combat other threats. Three factors combine to mitigate the extinction threat posed by
land use change. First, though old-growth forest continues to be lost, secondary forests
are increasing in area. Thus most recent land cover models, including those in the
Millenium Ecosystem Assessment, predict that total tropical forest cover will not fall
below 30% of its pre-industrial extent. Second, many tropical species do not require oldgrowth forests. We will use the birds of Panama to illustrate how tolerance to land cover
change varies among species. Third, tropical biodiversity is greatest in the Americas
where land use conversion is relatively limited. For example, of 155,000 described
tropical plant species, 90,000 are restricted to the Americas. Climate change might
already have overtaken land use change as the leading cause of documented extinctions
on tropical continents. This includes large numbers of tropical montane amphibians that
have been driven extinct by pathogens that have benefited from altered climates over the
past 20 years. The potential for climate change to cause tropical species extinctions has
important conservation implications. Each nation is responsible for governing land use
within its sovereign territory, and thus, tropical nations bear responsibility for addressing
extinction threats due to land use change. In contrast, it is largely wealthy non-tropical
nations that have precipitated the climate crisis and should shoulder primary
responsibility for mitigating the resulting threats to biodiversity. Future climate change
conventions must mobilize resources to prevent tropical species extinctions.
Keywords: biodiversity, conservation, climate change, land use change, deforestation
And now a word from Chicken Little: reflections on the tropical
extinction crisis
William Laurance
Smithsonian Tropical Research Institute, Balboa, Panama
Email: laurancew@si.edu
Joseph Wright and Helene Muller-Landau (WML) have recently argued that the future
loss of tropical biodiversity is likely to be considerably lower than is widely anticipated. I
critique several assumptions of their analysis, and suggest that it is premature to adopt
such an optimistic perspective. Among their most fundamental assumptions is that future
forest cover in developing nations will be largely determined by local population density.
I assert, however, that increasing globalization and industrialization in developing nations
will progressively weaken this relationship over time. WML further assume that future
pressures on forests will decline because of slowing population growth and increasing
urbanization, but I suggest that land conversion for both biofuels and food will accelerate
markedly, exacerbating pressures on forests. In addition, the WML analysis focuses
mainly on large continental regions, whereas tropical extinctions are likely to be greatest
in smaller biodiversity hotspots that have high species endemism and already-severe
forest loss. Finally, the WML projections of species extinctions focus solely on forest
conversion, but a range of other further threatsincluding climatic change, exotic species
and pathogens, and a range of environmental synergismsare also likely to have serious
impacts on future tropical biodiversity.
Keywords: Environmental synergisms, extinction, forest conversion, human population
growth
Is there a global extinction crisis for biodiversity?
Nigel Stork
School of Resource Management, University of Melbourne
Many leading biologists have stated that the current extinction phase is more extensive
and rapid than anything seen before, including the five well recorded mass extinction
events that occurred in the last 500 million years when up to 70% of marine organisms
previously known from fossil records disappeared. In 1993 Smith and co-workers
assessed the rate of increase in extinctions for various groups of organisms by looking at
the rate of change of the Red Data Book status of species as over time they move from
rare possibly right through to extinction. Their assessment was that 50% of groups such
as birds, mammals and palms might be extinct in the next couple of hundred of years or
sooner. However, the vast majority of biodiversity are neither mammals or plants but are
insects and other invertebrates. In an earlier study Nick Mawdsley and I found that for the
British fauna, birds and mammals were much more endangered or rare than insects and
other invertebrates. In this paper evidence is presented that supports the view that most
invertebrates are much less likely to become extinct than vertebrates. I also examine
current evidence that suggests that climate change may be a more insidious threat to
global insect diversity than habitat loss. Previously I have also posed the view that many
insects who have a strong host-specificity with either plant or animal hosts may suffer the
same ‘co-extinction’ fate as their hosts. I review how much such co-extinctions may
contribute to current extinction rates. In conclusion, I propose that biologists concerned
by current extinction rates will get greater political traction and change by focussing
their arguments for macroorganisms rather than biodiversity as a whole.
Size matters: the functional extinction of large vertebrates in tropical
forests
Richard Corlett
University of Hong Kong
Email: corlett@hku.hk
Anthropogenic global extinctions of tropical Asian forest vertebrates are still rare, but
local and regional extinctions are widespread, although widely underreported because of
wishful thinking or official embarrassment. Even where a species is still locally extant it
may persist at such a low density as to be functionally extinct. To a first approximation,
the vulnerability of tropical forest vertebrates to hunting and other human impacts
increases with body size. The ecological roles of vertebrates are also strongly influenced
by body size, and the largest species of carnivores, frugivores, browsers, and grazers have
unique ecological roles that cannot be substituted by smaller species. These roles are now
highly threatened, particularly in tropical Asia. Most tropical Asian forests have lost most
or all of their megafauna (> 45 kg) and many other large vertebrates (> 5 kg) now occupy
only fractions of their former ranges. Hunting is the major threat to large forest
vertebrates, so species-area relationships are likely to be a poor predictor of extinctions.
Correlations between hunting, logging and fragmentation in tropical Asia make it
difficult to assess their impacts separately, but current evidence suggests that many large
vertebrates could persist in mosaics of secondary and logged forest if protected from
hunting.
Keywords: hunting; extinction; Asia; forests
The potential of rural-urban migration and agriculture modernization to promote
neotropical biodiversity conservation
H. Ricardo Grau1, T. Mitchell Aide2 & N. Ignacio Gasparri1
1
CONICET-Universidad Nacional de Tucumán, Argentina
2
Dept. of Biology, University of Puerto Rico.
The growing human population and the increase in per capita food consumption are
increasing food demand. To balance increasing agriculture production and nature
conservation, we must assess the efficiency of land-use strategies. In the Neotropics
rural-urban migration is contributing to land-use efficiency by favoring the abandonment
of marginal areas for agriculture, and promoting the consumption of agricultural products
from modern systems of higher productivity. This process is favoring the conservation
and recovery of tropical montane and dry ecosystems in different regions of the
Caribbean, Central and South America. In contrast, the growing global demand for food
has increased deforestation particularly in lowland ecosystems that are more appropriate
for modern agriculture. For example, soybean production, mainly exported to China and
Europe, has become the major driver of deforestation in dry forest/savanna ecosystems of
South America. We compared land cover/use patterns and human population trends
since the early 1970s in two regions with contrasting development pathways in the Chaco
dry forests of northern Argentina. The area (c. 13 million hectares) includes one of the
largest continuous patches of tropical dry forests and has experienced rapid land use
change. In regions where land use has been driven by government-sponsored colonization
programs, the expansion of extensive grazing has led to a growing rural population, low
food production, and widespread environmental degradation. In contrast, in regions
dominated by market-driven soybean expansion, the rural population has decreased, food
production is between 300 and 800% greater, and low-density extensive cattle production
has declined over extensive forested areas, resulting in a land-use strategy that better
balances food production and biodiversity conservation. This study provides examples of
how socioeconomic modernization can influence land-use change and have positive
effects for biodiversity conservation.
Keywords: biodiversity, Chaco, land use change, modern agriculture, rural-urban
migration.
Will urbanization cause deforested areas to be abandoned in Brazilian
Amazonia? An examination of the Wright/Muller-Landau assumptions
for the future of the world’s largest tropical forest.
Philip M. Fearnside
Instituto Nacional de Pesquisas da Amazonia (INPA)
Email: pmfearn@inpa.gov.br
Rural-Urban migration has been hypothesized by Wright and Muller-Landau as a process
that could reduce tropical forest loss worldwide and allow secondary forest to occupy
abandoned clearings. However, the usefulness of this global generalization is highly
dependent on its being correct for Brazilian Amazonia, which has the worlds largest
remaining area of tropical forest. Among the features of the deforestation process that
make urbanization an unlikely mechanism for land abandonment in Brazil is the fact that
most deforestation is done by ranchers rather than by shifting cultivators. A family of
shifting cultivators is only capable of farming a limited area and cannot greatly expand
the area it cultivates if additional land becomes available. Ranchers, by contrast, can
occupy huge areas and expand their holdings when the opportunity arises. Were part of
the population to leave for the cities their land would usually be bought by neighboring
ranchers who would maintain and expand the deforested landscape.
Keywords: Deforestation, Population, Migration, Secondary Forest, Rainforest