Review: Laurance WF (2006). Have we overstated the tropical biodiversity crisis? Trends in Ecology and Evolution, 22(2):65-70. doi:10.1016/j.tree.2006.09.014

Feature Paper: Laurance WF (2006). Have we overstated the tropical biodiversity crisis? Trends in Ecology and Evolution, 22(2):65-70. doi:10.1016/j.tree.2006.09.014

Author Abstract: Tropical forests are the most biologically diverse and ecologically complex of terrestrial ecosystems, and are disappearing at alarming rates. It has long been suggested that rapid forest loss and degradation in the tropics, if unabated, could ultimately precipitate a wave of species extinctions, perhaps comparable to mass extinction events in the geological history of the Earth. However, a vigorous debate has erupted following a study by Wright and Muller-Landau that challenges the notion of large-scale tropical extinctions, at least over the next century. Here, I summarize this controversy and describe how the debate is stimulating a serious examination of the causes and biological consequences of future tropical deforestation.

Note to Readers: Follow links above for author email, full article text, or the publishing scientific journal. Author notes in my review are in quotes.

Review: This week we address a topic best elucidated by the author's topic sentence: "Are we on the verge of a massive die-off of tropical species?" While the author uses the examples of tropical forests through various means, the ramifications of the article are significant for coral reefs, another threatened habitat restricted mostly to tropical regions. And as mentioned multiple times in this weekly column, the 1998 El Niño-Southern Oscillation Event killed a full 16+ % of corals worldwide (but up to 100% in some locations, most noticeably in the Maldives and other central Indian Ocean islands). This meant that 1 out of 8 shallow-water, reef-building (hermatypic) corals worldwide died over the course of a few weeks, including some corals determined to be between 600 and 1,000 years old. But before we continue on the thread of marine conservation, we'll get back to the points presented by the author for tropical forests.

The key hypothesis tested in this paper is whether the idea of ecosystem collapse following "rapid forest loss and degradation" is possible for the tropics in the form of mass extinctions.

The author argues that there are scientists both whom believe that mass extinctions are imminent at current rates of degradation and those whom believe that such claims are exaggerated. Certainly, predicting future events (in this case, extinctions) from current trends, especially when data for such trends is less than uniform or under-reported (in the case of illegal logging) is always difficult.

The author's stated purpose in this paper, from the last sentence of the abstract, is to "summarize this controversy and describe how the debate is stimulating a serious examination of the causes and biological consequences of future tropical deforestation."

The first circumstantial evidence presented to claim rapid extinction is occuring is that we know: "most tropical forests are extraordinarily rich in species, remarkably complex ecologically and disappearing at truly alarming rates." Therefore, since scientists cannot ever know the complete biodiversity of the world, it is suspected that many species, even before they are recorded scientifically, go extinct through anthropogenic means. And IF tropical forests are diverse (known through extensive but not exhaustive surveys), and IF such forests are being cut down quicker than they can be surveyed, then it follows logically that a lot of biodiversity is being lost in the process. 

The author then looks at whether such biodiversity lost is consistent with extinctions or with culling (as in "sustainable harvest" of resources) and whether current exploitation rates justify claims of future ecosystem collapse.

The key dissenting position is summarized by the ecologist Dr. S. Joseph Wright, who concluded through extensive research that future "deforestation will slow, regeneration
will accelerate, and mass extinction of tropical forest species will be avoided."

In evaluating the two sides, the author discusses the key assumptions of the Wright and Muller-Landau position of a predicted avoidance of mass extinctions.

Those authors state that most deforestation of an unsustainable kind comes from rural populations (so-called "slash and burn" agriculture, whereby all trees and plants are cut down and then the open area is burned to encourage the regrowth of grasses, which are among the first resettlers after a fire, and which are used to feed cattle and other animals for human consumption). Urban populations are not as linked to deforestation as poor, rural populations. 

One could also say the same about over-fishing, where traditional fishermen tend to sustainably harvest catch for generations while rural immigrants to urban coastal areas tend to take up fishing as a quick way to earn income and therefore don't care about sustainable harvesting techniques.

Wright and Muller-Landau claim that because the trend in population growth favors increasing population density in urban areas over rural areas, that as a result of the "expected minor growth in rural populations, net forest cover [from today] will not change dramatically, especially in Asia and the Americas." The authors claim that as a result, extinction rates in tropical forest should not increase more than 15 - 35% of current levels over the next 20+ years, and while this amount is a significant chunk of biodiversity, the authors claim that it does not justify claims of predicted ecosystem collapse and mass extinctions.

Of particular note though is that nothing is mentioned of keystone species (individual, usually enigmatic or large species), with whose extinction multitude other species also die out because their fates are intertwined. Thus, in the Caribbean, a die-off of several species of sea urchins, coupled with mass coral bleaching and overfishing of herbivorous fish caused phase-shifts of reefs from coral dominated to algal dominated through a cascading "domino" effect. Whether the "more important" species will die off in the predicted 15 - 35% noted above is not known. Still, an eighth to a third of species loss is better than the reverse, where only an eighth to a third of species survive!

Wright and Muller-Landau further predict that with increasing urbanization, many rural slash-and-burn agricultural lands will be abandoned, leaving time to buffer against extinctions and help encourage forest regeneration.

Wright and Muller-Landau also point out that between 2 million to 12,000 years ago, "especially [in] Equatorial Africa, forests repeatedly contracted into small refugia during cooling and drying phases of the Pleistocene." Coupled with the long history of human hunting in the "Amazon, Congo Basin and New Guinea," the authors feel that "such areas would have already lost [before today] many of their extinction-prone species, rendering them less vulnerable to future species losses from deforestation." However, such peoples rarely had a history of extensive agriculture and were instead traditionally hunters and gatherers. Therefore, those forests are unlikely to have experienced the scale of slash-and-burn agriculture they experience today.

However, Wright and Muller-Landau do concede that extinction rates are "likely to be higher in biodiversity hotspots, such as Madagascar, the Philippines and the Brazilian Atlantic forests." Laurance points out that "The 25 biodiversity hotspots identified by Myers et al.
include the entire known ranges of nearly half (44%) of all known vascular plants and over a third (35%) of all vertebrates, and the 16 hotspots that sustain tropical forest have already lost, on average, 90% of their forest cover. Because many local endemics in these hotspots are outside of protected areas, species extinctions are likely to be substantially higher than those projected by Wright and Muller-Landau for the major continental regions."

Laurance points out other uncertainties with the Wright and Muller-Landau paper:

1. Local endemics can be found even within areas where the majority of species have large geographical ranges.

2. Old-growth forests house more species than just net forest cover of similar amounts.

3. Uncertain population trends have variable global projections, from "7 to 15 billion people" by 2100.

4. Growing industrialization and globalization is not uniform (Equatorial Africa is remaining largely rural despite population growth and time) and industrial farming companies and cattle ranches are becoming the norm, leading to larger tracts of deforestation in a given area (rather than patchiness from small-scale farming) than historically.

5. The link between rural populations and forests may not be as clear as Wright and Muller-Landau conclude.

6. The magnitude of extinctions is greatly contested and debated.

7. The precautionary principle "maintains that one should err on the side of caution in conservation matters," which Wright and Muller-Landau don't appear to do.

Nevertheless, despite continuing ongoing debate and obvious work for Wright and Muller-Landau to continue, it would be nice if the future weren't so grim as many predict. 

As a result, I believe that it IS better to proceed with caution and to conserve as much as possible because we never know what the future may hold and it would be a shame if a lot would be gone before it's even discovered!