Most headlines about biodiversity follow the same storylines: Species are going extinct at an incredible rate. We are in the middle of the Earth’s sixth extinction event. The number of endangered species keeps growing because of habitat loss and invasive species.
But Erle Ellis and his colleagues offer a slightly different perspective on the way in which our Earth’s biodiversity is changing. In an article published by PLoS ONE, they look closely at an enormous amount of data on global plant biodiversity, separated into regions of the Earth. Ellis describes one of their most surprising findings on his blog:
“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.”
How can this be? How can we be in the middle of the Earth’s sixth extinction event while the number of species (species richness) is increasing across most of the planet? The answer lies in looking at how regional biodiversity is changing differently from global biodiversity as a whole.
Globalization has certainly led to a mixing of Earth’s biodiversity. As we move ourselves and the stuff we want around the world at greater rates, we move more species around the world too. We’ve been doing this for thousands of years, of course, but the pace has picked up dramatically over the past few decades. Just think about the number of container ships unknowingly carrying plants, insects, fungi, bacteria and other organisms with them.
Before people started moving species around the planet in substantial numbers, organisms interacted with just a subset of the Earth’s inhabitant and evolved in ways that reflected their relatively isolated surroundings. A different set of organisms inhabited each continent, island, and mountaintop, reflecting how these species interacted with each other and their environment.
Mixing together these different sets of species through our actions on a global scale is a huge centuries-long experiment unfolding before our eyes. Most regions of the world are seeing a rise in the total species richness because the number of introduced species added is greater than the number of native species that have gone extinct so far. Introduced and native species are coexisting for now, so the number of species in many regions has gone up.
However, this is very likely to be temporary as these new sets of interacting species “duke it out” and a subset of them go extinct. This temporary stage is often referred to as our “extinction debt” — we’ll be paying the debt down in the coming decades.
As for global biodiversity, it is still declining. There’s nothing new that this article says about that.
The bigger — and more interesting — argument that the authors make is that ecosystems today are vastly different from what we’ve seen in the past and that human activities rival geologic forces. As Ellis writes, again on his blog:
“We set out on this work more than 3 years ago with the goal of testing a key hypothesis … that current global patterns of biodiversity are better explained by global patterns in human systems (populations, land use) than by the “natural” biophysical patterns of the Earth system (climate, geology).”
And they seem to do a good job of it, at least for plants.
In other words, one of the Big Changes in the Anthropocene that we are seeing is that human activities are becoming a major force shaping the composition of ecosystems. The changes in biodiversity that we are observing are now so large that our current era will be recorded in the geologic record.
Ellis EC, Antill EC, Kreft H (2012) All Is Not Loss: Plant Biodiversity in the Anthropocene. PLoS ONE 7(1): e30535. doi:10.1371/journal.pone.0030535