Monday, June 10, 2013

Quantifying the damage of invasive Species

Quantifying the damage of Invasive Species
Jeffrey Kresse

Himalayan Blackberry, one of the most nefarious invasives
Beginning with the Colombian exchange and continuing through globalization, humans have purposefully and accidentally introduced species to new ranges through intercontinental transportation. We have introduced species to control pests or for hunting purposes but these schemes usually have unintended negative consequences for the ecosystem. Many people consider invasive species a threat to biodiversity because they can outcompete native species for resources, and because no predators or diseases exist to keep the invasive species in check. Predators and diseases often coevolve with their prey and hosts, so when a new exotic species suddenly appears it can have a large advantage over native organisms. Or perhaps the invasive species is simply more efficient than native species in the area ever had to be. For example, here is a picture of Himalayan blackberry, a very common invasive species of the Pacific Northwest that often will overgrow its competition from the inside out.

For stationary plant species, which is the focus of this blog, this often manifests itself as one species shading or crowding out another species. An aggressive invasive species can disrupt an ecosystem and cause local extirpations of native species in a relatively short amount of time compared to those caused by say climate change, which is happening gradually and gives organisms a chance to adapt. Despite the grave threat posed by invasive species to global biodiversity, our knowledge of the effects they cause is still quite superficial and limited. Only in the past 30 years or so have scientific studies and experiments that actually quantify the damage of invasive species begun to appear in the literature. I will discuss two of these which I believe provide valuable insights into how we should begin to manage invasive species. Both focus on plant species, which are easier to measure because they are sessile. The first concerns the threshold at which an invasive shrubby cactus begins to impact native plant species richness. The second investigates the differences in community composition that arise when an invasive grass species is planted before the native species and vice versa.
P. aculeata, the invasive used in the study

First, Paterson et al. describe how the invasive shrubby cactus Pereskia aculeata, pictured here, impacts biodiversity. Pereskia aculeata, known as Barbados gooseberry or leaf cactus, is native to Central America and is considered an invasive species in the country of South Africa. Since this is a fun blog post, as an aside, the authors never mentioned the common names or even the native range, which underscores the crippling inability of most scientists to convey information in layman’s terms. Moving on, five sites of 2500 square meters were selected based on P. aculeata vigorously invading pristine, tropical-like areas, where it is considered the biggest threat. 120 half meter quadrats were sampled at each site along five transects (lines spanning the length of the area) using random numbers to determine where to sample. Species richness (the overall number of species) as well as Simpson’s D and Shannon-Weaver’s H indexes of biodiversity were plotted against the density of P. aculeata. All three graphs show a decline until about 30% P. aculeata density, after which they all leveled off. The authors conclude that keeping the invasive species P. aculeata below 30% would grant improved biodiversity (Paterson et al 2011). Beyond just informing the management of this particular invasive species in South Africa, more studies like this one should be done to determine just how deadly a given invasive species is. Comparing these values would obviously help us prioritize which invasive species need to be removed yesterday, but like I said our knowledge is still insipient.

The second article relating to invasive species written by Dickson et al. deals with priority effects, a new perspective on invasive species. Unfortunately this particular article prohibits copying graphs out so there will be none. Priority effects concern what happens when an invasive plant begins to grow before a native species, which can ultimately alter the late successional community. The authors hypothesize that invasive plants (they studied grasses) would benefit more from priority effects than native species due to their already faster rate of growth and greater fecundity. Interestingly, they steam sterilized the soil and then added soil microbes back in to make each pot uniform and create a controlled experiment. Although they had 56 pots to factor in seed density and replicates, the basic idea was planting one native or invasive species three weeks before planting five other types of grasses in the same pot and assessing the impact on biodiversity. Shockingly, planting a native grass first led to an 81% increase in biodiversity as measured by Simpson’s D compared to invasive first pots. Conversely, invasive species that were planted first had increased biomass and reduced the biomass of the species that were planted in the pot later (Dickson et al. 2012). The take away message is that giving invasive species a temporal advantage is a very bad idea, because their faster rate of growth makes it difficult for native species to survive in their wake. To me priority effects are particularly relevant to disturbance ecology, we must be ready to step in and remove overpowering ruderal invasives lest biodiversity and native plant biomass should become unable to recover.

Nutria, an invasive semi-aquatic rodent from Japan
Invasive species thresholds and priority effects are our initial attempts on our way to understanding invasive species mechanics in any real detail. I labored mightily to find the two articles that I did, and both are about plants which are easy to measure because they are stationary. Nevertheless invasive animals from pest insects to large mammals are no less important than invasive plants and ought to be studied despite their confounding ability to move. Invasive species are detrimental to their native counterparts, the loss of which ripples through the ecosystem’s established food web. But beyond that, many native and endemic species have a special significance to the people living in the region, not just aesthetically, but because those species make the place unique. More and more people are beginning to realize the dangers of invasive species and take action like here in England ( Although we live in a largely capitalist world, when it comes to plant communities, no one wants to see a world where the most efficient, fastest growing and reproducing species dominate.

Works Cited
Paterson, Iain, Julie Coetzee, Martin Hill, and Douglas Downie. "A Pre-release Assessment of the Relationship Between the Invasive Alien Plant, Pereskia Aculeata Miller (Cactaceae), and Native Plant Biodiversity in South Africa." Biological Control, 57.1 (2011): 59-65.

Dickson, Timothy, Jennifer Hopwood, and Brian Wilsey. "Do Priority Effects Benefit Invasive Plants More Than Native Plants? an Experiment with Six Grassland Species." Biological Invasions, 14.12 (2012): 2617-2624.


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