By Jesse Passman
It is a general concept that has ruled both the wilderness of Africa and playgrounds of human civilization – the winners come out on top and push the losers to extradition (or back into the classroom). Much of ecology has looked at a concept similar to this, known as “winner-loser replacement,” in the face of anthropogenic habitat destruction and invasive species. The “winners” have generally been viewed as the “exotics” – generalist species that have been imported and thus have no natural predators. These invasive species come to dominate the ecosystem and can drive native species to extinction through competition. It has been seen as a huge issue in many ecosystems as humans increasingly globalize and move about the world – bringing many species to new places and pushing other endemic species, the “losers,” out. This all leads to “biotic homogenization,” or the slow transformation of an ecosystem rich in different species into a species-poor one.
However, an article written by Marcelo Tabarelli (Universidade Federal de Pernambuco) et al, has been looking into a new, unexpected source of “winner-loser replacement” – native species themselves. It seems that invasive species are not the only culprit in the breakdown of species richness. According to Tabarelli et al, in tropical rainforest ecosystems; such as those found in the Brazilian Amazon, the Atlantic forest, and Mesoamerica; deforestation has led to “hyperfragmentation” of habitat – essentially, the habitat is split into many small pieces, thus limiting the effective range of species. With this hyperfragmentation comes a greater proportion of “edges.” Just as a puzzle has more edges than a similar sized piece of cardboard, so does the habitat.
The edges of the habitat have a much different set of abiotic factors affecting them – they receive greater amount of sunlight and wind disturbance (as the wind is not blocked by any trees). Native species from the interior of the forest have difficult withstanding these new conditions. They are “specialists” and are not adapted to a wide range of conditions. “Pioneer species” – species that can withstand a greater range of conditions – love it on the edges. According to Tabarelli et al, this contrast leads to a change in the species make-up. The secondary “pioneer species” grow in abundance while the specialists (such as old growth forest) lose abundance. The researchers assert that this is not a competitive process. It is not necessarily that the old-growth woody plants are being forced out – but rather they cannot survive on the edges and thus slowly replaced by the sun-demanding disturbance-loving pioneers. So, a metamorphosis occurs and the edges lose the diversity of the specialists as the pioneers grow in abundance.
One can see how this presents an issue. As more edges are created by deforestation and other anthropogenic habitat disturbance, more biotic homogenization occurs. The number of specialist species decreases as they cannot survive in the new edges. According to the researchers, this leads to sort of trophic breakdown. As the specialist, old-growth species disappear, so too do the species that are dependent on them. Essentially, over time, species are lost – they go extinct from the area.
But how did Tabarelli and his co-scientists come to this conclusion? How did they rule out the generally accepted theory that invasive species are at the root of the issue? They compiled the data from many past experiments on forest edge flora composition, including some of Tarabelli’s own in 2008, and compared it to data available on invasive species and their impacts on tropical rainforests. Although invasive species are well-known for their ecological impacts, there is apparently very little data on their impact on species in rainforests – the researchers state that there is little evidence that many of the species decreasing in their presence are being out-competed by exotic species. Rather, evidence based on patterns of tree assemblage shows that the native old growth species are disappearing as the pioneer species are proliferating. The old growth specialists are the “losers” being replaced by the few ubiquitous “winner” species.
So what does this mean for the world of conservation biology? It just reaffirms that anthropogenic habitat destruction, whether it be by deforestation, burning, or other means, is leading to less species diversity – “biotic homogeneity.” More habitat fragmentation is leading to more edge habitats which is leading to the proliferation of a certain choice of “winner species.” The “losers” are then left battered and broken in the dust, dragging down those that depend on them with them.
Fig. 1 – Comparison of the events of an invasion by an outside exotic species (right) to those of the proliferation of an endemic species (the pioneer “winner” species in this study – left)
Marcelo Tabarelli, Carlos A. Peres, Felipe P.L. Melo, The ‘few winners and many losers’ paradigm revisited: Emerging prospects for tropical forest biodiversity, Biological Conservation, Volume 155, October 2012, Pages 136-140, ISSN 0006-3207, 10.1016/j.biocon.2012.06.020.
“World of Change: Amazon Deforestation : Feature Articles.” World of Change: Amazon Deforestation : Feature Articles. NASA, n.d. Web. 18 Sept. 2012. <http://earthobservatory.nasa.gov/Features/WorldOfChange/deforestation.php?all=y>.