August 13, 2015
The Other Side of the Biodiversity Crisis
Species turnover caused by humans. Green areas have seen relatively little change; in red areas, the turnover has been 50% or more. Yellow areas are intermediate. From Ellis, E.C., E.C. Antill, and H. Kreft (2012) "All Is Not Loss: Plant Biodiversity In the Anthropocene." PLoS ONE 7:e30535
Visit almost any city in the US or elsewhere today, and you are likely to find restaurants from all corners of the world: Indian, Thai, Italian, American, you name it. Clearly, gastronomical diversity within cities has increased hugely over the past couple of centuries. Now go to a city in another country -- and the range of cuisines on offer is likely to be nearly identical. This is a hallmark of globalization: increased diversity locally, decreased diversity globally. As Breakthrough Institute Senior Fellow Erle Ellis and colleagues show in a recent paper, the same phenomenon also applies to plants.
For it is not only goods and people that are increasingly criss-crossing the world today: the last few hundred years have seen unprecedented levels of human-induced movement of plant and animal species. This resulting mixing of "native" and "exotic" species has created a global hodgepodge of species assemblages, some of which form entirely novel anthropogenic ecosystems. Introductions of exotic species -- witting or unwitting -- have in a minority of cases been associated with the extinction of native species, and the phenomenon has therefore mostly been seen in negative terms. Indeed, non-native species have been held in such low regard by many conservationists that it has even been suggested they be excluded from the definition of biodiversity. This view is still prevalent: witness this short piece from yesterday's Huffington Post describing a variety of attempts to eradicate the "dirty, loud and downright invasive" starling from the US. Apparently, some animals really are more equal than others.
This long-held neglect of species deemed to be out of their place has become increasingly untenable in the last decade. In 2003, Dov Sax and Steven Gaines showed that, contrary to conventional wisdom, species richness had in fact increased in many regions -- that is, if you count the immigrants too. The reason is simple: extinctions of native species is in most cases less significant than the number of new arrivals, leading to a net increase in species richness. On many oceanic islands, for example, the number of plant species has doubled as a result of species introductions. In California, the diversity of reptiles, amphibians and freshwater fish is higher today than it was before European colonization.
Getting a good picture of global patterns of species immigrations and extinctions has, however, proved elusive. Erle Ellis thus writes on his blog that "What we don't know about the global patterns of plant biodiversity exceeds what we do know." To address this paucity of knowledge, Ellis and his colleagues constructed a ground-breaking model -- the "the first spatially explicit integrated assessment of the anthropogenic global patterns of vascular plant species richness" -- which gives us some rough indications of the global trends. The results surprised them. Writes Ellis:
The big story of plant biodiversity in the Anthropocene is not about loss at all. Our model predictions indicate that human systems have caused a net increase in plant species richness across more than two thirds of the terrestrial biosphere, mostly by facilitating exotic species invasions.
The result is
increasingly globalized and homogenized anthropogenic plant communities characterized by reduced native richness but enriched in species at the regional landscape scale by exotics drawn globally from the relatively small pool of species that either tolerate or benefit from the novel anthropogenic habitats created by human residence and use of land.
Just as with restaurants, then, biotic globalization is a double-edged sword. But since most ecosystem processes operate on a local or regional level, the increased species richness should in principle be good news for ecosystem functioning and resilience, both of which are related to diversity (although species richness is a blunt measure of it). Empirical support for this thesis is provided in an upcoming paper by Joseph Mascaro and others, who show that in lowland Hawaiian rainforests,
aboveground biomass, productivity, nutrient turnover...and belowground carbon storage either did not differ significantly or were significantly greater in novel relative to native forests.
This leads them to conclude that
Ecosystem processes will continue even after dramatic losses of native species diversity if simple functional roles are provided by introduced species. Because large portions of the Earth's surface are undergoing similar transitions from native to novel ecosystems, our results are likely to be broadly applicable.
The implications of these new findings turn the dominant thinking about human impacts on biodiversity and ecosystems upside down, as -- according to Sax and Gaines -- the "assumption of much current research is that diversity is declining at all spatial scales."
Ellis's et al's new piece adds to the weight of reports challenging this apparent misconception. As the title of their paper indicates, in the Anthropocene, all is not loss. Perhaps it is time to follow the advice of ecologist James Brown (cited here):
It has become imperative that [as] ecologists, evolutionary biologists, and biogeographers ... we use our expertise as scientists not for the futile effort to hold back the clock and preserve some romantic idealized version of a pristine natural world, but for a rational attempt to understand the disturbed ecosystems that we have created and to manage them to support both humans and wildlife.