External morphology explains the success of biological invasions

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Azzurro et al. 2014 External morphology explains the success of biological invasions. Ecology Letters 17: 1455-1463.


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Zarah Pattison

There is rarely a shortage of papers attempting to explain why particular species are more or less invasive than others. Since Charles Elton’s seminar work in 1958 (The Ecology of Invasions by Animals and Plants) there has been a rapid increase, particularly in the last 20 years, in publications surrounding the topic of invasion ecology. The silver bullet to help prevent invasions would be to determine which characteristics contribute most to invasion success and therefore enable us to predict the seriousness of an invasion for prevention and management. Azzuro et al. (2014) offered something a bit unique in their attempt to explain invasiveness by using the external morphology of species, fish species in this instance.

The aim of this study was to explore whether morphological traits could explain the abundance of introduced fish species entering the Mediterranean Sea via the Suez Canal. The Mediterranean Basin is suggested to have a monopoly of vacant niches, which may be contributing to the successful establishment of invasive species therein. Therefore the use of species morphology as a proxy for its ecological status in a community, could explain niche availability and the potential population increase post establishment.

Our initial thoughts were positive. The paper was written well, succinct and enjoyable. A large data set was used in an analysis which none of us had expertise in, but it was still clear what the authors were trying to achieve. (Very) basically a polygon encompassing the morphological space of the native fish community was used and the traits of non-native fish species were plotted across the native morphospace. The results showed that invasive non-native fish species were more abundant either outside or on the outer perimeter of the native morphospace where niche occupancy was low. Non-native species morphologically similar to native species, were less abundant and less likely to establish.

The paper definitely added to the breadth of our invasion ecology knowledge. However, like most studies in invasion ecology, the results are difficult to generalise. Negative caveats of many invasion ecology papers focused on specific species are just that: species specific and not amenable to generalisation. This can be frustrating from a conservation point of view. The authors themselves discussed the limitations of this study particularly in the case of invasive non-native lionfish (Pterois spp.) which has a rather unique morphology. Another point raised was that environmental conditions were not taken into account in this study, particularly fishing quotas which could lead to fluctuating populations regardless of native status. Additionally, life history/functional traits, which are used in many plant invasion studies, were not considered.

Overall the paper delivered its aim, but the title is very confident. Perhaps “External morphology can additionally explain the success of species specific biological invasions” would be more appropriate. However, we can’t test for everything in a study (we all know this!) and we all agreed this was a good piece of interesting science.

Join us next week where Eilidh McNab will lead a discussion of a paper recently published in Biological Conservation entitled: Field work ethics in biological research.

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