Monday, 13 February 2012

Effects of humans on plant biodiversity

It is easy to say that humans have influenced the world's biodiversity, but much harder to actually quantify our impacts. For one thing, we rarely (if ever) know the condition that ecosystems were in before we altered them; for another, biodiversity is impacted by a number of other factors besides humans, including latitude, climate, and topography.

However, using a relatively simple equation, collaborators from the University of Maryland and the Georg-August University of Gottingen have made a first stab at calculating anthropogenic species richness (ASR), or species richness patterns that results from human alteration of native habitats. ASR reflects native species richness minus all species lost (ASL) and all species introduced (ASI) as a result of anthropogenic influence. Local extinctions are usually a result of habitat loss and outcompetition by introduced species, while species gains generally reflect the presence of agricultural crops and ornamental species. In the current research, each of these components of ASR was calculated using previously-published models that, on the whole, are more likely to underestimate the influence of humans than overestimate it. Thus, ASR is only an estimate but one that probably gives us a decent best guess of the minimum effect that humans have probably had on vascular plant richness around the globe.

The researchers calculated ASR for each of 16,805 focal regions generated by dividing all of the Earth's ice-free land surface into equally-sized (7,800 square km) chunks. Although ASR was estimated separately for each region, the scientists could simultaneously evaluate patterns across all cells in order to investigate the effects of anthropogenic disturbance at a global scale. This revealed that species richness has been impacted by humans across a whopping 93% of the planet's ice-free land area.

Depending on the region being investigated, this pattern was a result of local extinctions, local introductions, or a combination of the two. Just over half of terrestrial landscapes have lost more than 5% of their native species, while over a quarter have lost more than 10%. At the same time, 89% of habitats have experienced invasion by exotic species; more than a quarter of the biosphere has in excess of 10% introduced species. So many plants have been introduced, in fact, that gains from invasives equal or outweigh losses of natives in many locations, with the result that overall species richness patterns are not drastically altered. To put that in numerical terms, while 93% of the planet has been impacted by human disturbance, only 61% has actually experienced a net change in richness.

(A 2005 map of biodiversity hotspots around the world. The authors of the current study examined 34 hotspots identified by Mittermeier et al. in 2004.)

Only rarely did the researchers find locations where richness was actually declining; when they did, it was a result of extremely high native losses that far outpaced "exotic gains." Habitats that were identified as particularly likely to suffer from richness depletions richness were grasslands, savannas, shrublands, and deserts. Over half of 32 "biodiversity hotspots" exceeded the global median for human-caused diversity fluctuations, indicating that "the most biodiverse regions on Earth also tend to be among the most challenged by anthropogenic transformation." Overall, the most anthropogenically altered hotspot was Japan, while the least impacted was the East Melanesian Islands (however, both of these results need to be interpreted with caution since island habitats are intrinsically different from their mainland counterparts when it comes to biodiversity levels and susceptibility to anthropogenic influences).

While some of the patterns reported here might be disheartening to some, the authors suggest that their results aren't all bad news. They are quick to point out that even in the most disturbed regions, many native plant species are thriving--despite the loss of some endemic species and the gain of many nonnatives. Further, it is likely that native species are quick to re-establish themselves in areas where humans once lived but can no longer be found. Thus, as many people abandon rural areas and flock to urban and urbanizing regions, the habitats they leave behind may be reclaimed by indigenous species.  Finally, the researchers suggest that we may be able to develop methods of "redirecting" ecological succession such that native plant species, rather than new exotics, can be encouraged to replace dying invasives.

Of course, that may require the development of new management techniques, plus it only addresses the problem of varying richness. Evenness and diversity--both in general, and in relation to phylogeny and function--are two other important characteristics of species assemblages, and future studies will be needed to see how these traits have been impacted by anthropogenic activities worldwide. It would also be useful to link these patterns to specific aspects of human existence--such as transportation networks, land use, and economics; those data could help explain particular biodiversity patterns and suggest potential methods of management. Should such analyses prove fruitful, one next step might be applying them to other groups of organisms as well, including animals, bacteria, and fungi.

Ellis, E.C., Antill, E.C., and Kreft, H. 2012. All is not loss: plant biodiversity in the Anthropocene. PLoS ONE 7(1):e30535.

Thanks to the following websites for providing the images used in this post:

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