A recent collaboration, spearheaded by Cornell University’s Department of Ecology and Evolutionary Biology, has yielded some important new insights into the role which human cultures may be having on the progression of certain invasive species on the island of Trinidad in the West Indies.  Specifically, the research team focused on a type of freshwater snail which has been previously identified as a non-native specimen to the region; the ability of this snail to proliferate on the island in freshwater environments seems to be accelerated in areas where habitat destruction has occurred, and the researchers ultimately arrive at a realistic, concise explanation for why this seems to be the case.  Their results, which provide an interesting investigation of the overlap between invasive species introduction and habitat destruction (both of which promote environmental change) were published on June 25th in PLoS One’s online journal.

Tarebia granifera, a snail which originated in southeast Asia, has been recognized throughout the last two decades as an invasive species which has far exceeded the boundaries of its original local habitat.  Though the snail originated in several Asian countries, it has now been identified on three different continents worldwide, and seems to thrive and proliferate very easily in both tropical and sub-tropical areas.  There are several theories for how the snail may have achieved such a ubiquitous distribution, though it is widely agreed that the most important factors relate to accidental introduction by humans through the aquarium trade.  Additionally, the snail has been introduced to Caribbean regions intentionally, as a method for containing outbreaks of schistosomaisis (a tropical parasitic disease) outbreaks.  Aside from both of these factors relating to Tarebia’s now-widespread distribution, other potential human-related drivers of the snail’s habitat expansion were explored in the current study on the island of Trinidad, where land conversion and deforestation by the island’s inhabitants has dramatically altered the local landscape in recent years.

To understand how humans may be affecting the snail’s distribution, it was first observed that significantly higher populations of the organism seemed to exist in areas where riparian vegetation had been removed from the areas surrounding freshwater streams to make way for farmland.  Without a canopy cover to provide shade for these harvested plots, it appears that an abnormally high amount of ground cover (caused by increased sunlight availability) provides an optimal food source for the invasive snails to thrive in.  Additionally, after the snails’ distribution zone spreads into these harvested plots, it becomes much easier for the organism to then further invade into adjacent areas where habitat destruction has not yet taken place.  Thus, it appears that even land conversion practices on a small scale can inordinately affect the distribution of the invasive species throughout nearby areas as well. 

While this conclusion was a relatively straightforward one to make, increasing the scope of the investigation to determine whether an increase in snail populations was having a detrimental effect on the surrounding ecosystem required a different line of research.  To accomplish this, the research team collected snail samples from the environments under consideration (open-, closed-, and partially-closed canopies), and gauged the amount of NH4 being excreted by the organisms.  Based on the densities of snail populations in each environment, the degree of change with regard to environmental nitrogen flow was extrapolated; it was found that the amount of nitrogen being provided by the snails into the environment was indeed significantly higher in the areas where riparian cover had been lost, suggesting that the increased prevalence of Tarebia granifera in these areas could represent a significant impact on the surrounding ecosystem. 

Ultimately, the results from the study provide information which is useful for several reasons with regard to how increases in invasive snail population can affect ecosystem dynamics; most notable among these is the concept that changes to normal nitrogen cycles can result in a “trophic dead end” for energy production in the surrounding area.  Since the snails do not seem to have a common predator which has co-evolved to see them as a food source, the growing populations of these snails could serve to sequester the energy fixed by plants, rather than passing that energy up to higher levels of the food chain.  Further, since these increases in snail populations seem to be exacerbated by the man-made impacts of habitat degradation and land development, this research could hopefully provide alternative methods for the way in which riparian landscapes on the island of Trinidad are converted into commercially useful landscapes.  As direct links between human impact, invasive species proliferation and far-reaching consequences to natural ecosystems are difficult to make, it will be interesting to see whether more dramatic changes to these riparian landscapes are on the horizon as a result of the Tarebia populations on the island of Trinidad.

For more information, see the original research article in PLoS One:

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0038806