Thursday, October 14, 2010

Not All Invasions Are Created Equal

Researchers from the Institute of Botany, Academy of Sciences of the Czech Republic, have brought some clarity to the tricky business of conservation management; at least for the possible behaviour and impact of thirteen invasive species in meadow ecosystems in Central-Europe. As with the rest of science, there are no ultimate truths just many complicated equations and inferences which allow us to suggest what may be.

In their extensive study the researchers had two main questions; do introduced species affect the biodiversity (i.e. variety, abundance and evenness of species) within a native plant community? And, if there is an effect, what factors contribute to their success?
To answer these questions they selected sites with meadow ecosystems, throughout the Czech Republic, in which the thirteen chosen invasive species were present. For each site, two 16 m2 areas, adjacent to one another, were sectioned off with one displaying heavy invasion and the other displaying little to no invasion. Over a period of four years a total of 260 vegetation plots were monitored.

With all of the data collected they calculated, using statistical analysis, the similarity between each pair of plots and each plots community characteristics including species richness, impact of invasion on diversity, invader’s height and percent cover, and differences between the invading and native dominant species.

As usual the results are complicated. No invasion will be identical every time so it was important to gather data from many different environments to compare the variable responses of plant communities with and without invasions. The short answer is that yes invasive species affect biodiversity and there are roughly five main factors that contribute to the success of invasive species. The longer answer is that not all invasive species actually had a negative impact on the plant communities but for those invasive species that did have an impact there was a decrease in the variety and abundance of species but the factors contributing to this were species specific. Each of the thirteen species impacted the communities differently. Some invaders reduced species numbers by up to 90%, whereas, other invaders had little to no impact on species numbers.

There are five factors for successful invasion and depending on the species these factors differ in importance. 1) Grow tall and fast, 2) Possess large cover to obtain the most light and block out the sun from other small species, 3) Develop extensive root systems underground to take-up water and nutrients and block competitors, 4) Form a homogeneous stand, and 5) Invade community where you can outcompete. There are always exceptions and sometimes an invasion can be successful without competing with the dominant native species or creating a monoculture. For example, Impatiens glandulifera invade riparian zones where they cannot compete with the dominant native species. They may not decrease species abundance but they can decrease species diversity.

Sometimes how we gauge an invasion is based on how valuable the area is to us. Fallopia species and Solidago gigantea tend to invade areas with lots of weeds, whereas Heracleum mantegazzianum and Lupinus polyphyllus can invade nutrient poor sites where many rare species can be found. From a conservation perspective, the weed infested area may be less valuable than the area supporting rare species.

Fallopia sachalinensis
This study was one of the largest in scope for the collection of data and as these species are invasive across many countries in Central-Europe the results can be used to understand and predict invasions in similar regions to those observed. For conservation purposes the researchers highlighted key things to look at and understand when considering an action plan in an ecosystem. Each invasive species can have a different effect on a community and this is largely based on competition with the native dominant species, impact of invasion should be measured based on what species remain and what qualities they possess rather than what was lost. Some invading species have a very clear result in their impact at a community level while others may not be so direct.

Although studies like this can provide guidelines for evaluating similar habitats and their risk for invasion, the intricate dynamics of these natural systems and each organism’s uniqueness may continue to keep us guessing.

Hejda, M., Pyšek, P. and Jarošik, V. 2009. Impact of invasive plants on the species richness, diversity and composition of invaded communities. Journal of Ecology. 97:393-403.

Aurora Patchett

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