It's time to take a more optimistic approach, and look at the positive
effects that invasive species can potentially have on ecosystems. I’ll
start by proposing counter-arguments for the 3 problems described in ‘Why the bad press’.
1. PredationLet’s take a look at the European Green Crab, which was a successful invader to America at locations where the native predator (Blue Crab) had been removed as a result of human activity. Due to the local extinction of Blue Crabs, the population of an herbivorous crab species lower down the food chain dramatically increased. This herbivore was able to strip salt marshes of hundreds of hectors of vegetation, namely cordgrass, which resulted in salt marsh die-off and impacted negatively on the ecosystem. The invasion of the Green Crab is thought to have significantly reduced the population of the herbivorous crab via predation. Optimistically there was found to be a positive correlation with Green crab abundance and cordgrass recovery. (Bertness and Coverdale, 2013)
2. Competition
When an invasive species expands its geographic range, it competes for resources with pre-existing species. If
some native individuals are able to survive, then invasives may drive the evolution of these native species and select for traits which increase fitness. Ferrero-Serrano et
al., (2009) found this to be true for native grass populations. The authors
demonstrated that as a consequence of previous competition imposed by invaders, evolved species were more likely to be able to tolerate new
invaders. Importantly the authors argue that this knowledge may help advance
the understanding and practice of restoration ecology.
3. Disease
Ok so this one was a little bit harder….even after trawling
the internet I have failed to come up with a valid argument!
To compensate I have found a case study to share with you that
I thought was particularly interesting. Pejchar and Mooney (2009) reviewed whether different ecosystem services in South Africa were positively or negatively
impacted on by the invasion of woody plant species.
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| (Pejchar and Mooney, 2009) |
This review by Pejchar and Mooney (2009) highlights just how many ecosystem services invasive species can affect. Moreover, there is only one ecosystem service where invasive species had a strictly positive influence. Whilst reading the literature in preparation for this blog post I found that this was often the case. As a result I am personally still not swayed; to me it seems evident that invasive species have a net negative effect.
Let me know what you think!
Elena
Hi Elena,
ReplyDeleteThanks for the interesting post! I would also tend to say that invasive species have a net negative effect. Although there are numerous examples where the introduction of invasive species can be positive, the fact remains that we can't really know ahead of time what the ecosystem response will be because ecosystems are so complex. The precautionary principle often used in environmental science tells us to take preventive action in the face of uncertainty - I think this seems sensible for invasive species management.
Cheers,
Katherine
Hi Katherine,
DeleteI agree, the fact that we cannot determine the future response to invasives is key. The precautionary principle is an excellent point and is almost certainly the basis as to why a key focus on invasive species management is often to prevent them entering the ecosystem in the first place. In my next post i'm going to be looking at whether it's getting harder to prevent invasions as a consequence of climate change.
Elena