Invasion biology

Biological invasions represent one of the great current threats to global biodiversity. However, in this adversity rests opportunity as species introductions represent large-scale natural experiments to investigate fundamental ecological and evolutionary questions. In a meta-analysis of over 5,500 rates of phenotypic change from 90 species published in the American Naturalist (Westley 2011), I showed an important role of invasive species in illuminating how quickly phenotypic traits can change in populations of plants and animals.

This paper emerged out of my Ph.D. comprehensive exam at Memorial University of Newfoundland where I was asked to discuss what insights have been gleaned by the study of non-native invasive species. I wanted to add some ‘teeth’ to my exam response and wondered what invasive species have contributed to our fundamental understanding of how fast traits, like body size and shape, can change in wild populations. I also wanted to know whether the use of invasive species rather than native species to examine this question was biasing our interpretation of how quickly change can occur in nature, especially because the conditions of biological invasions seem ripe for revealing rapid change.

The results of this study show that invasive species are frequently used as model organisms to investigate evolutionary processes but represent only a small number of individual species. Perhaps more surprisingly, both invasive and native species show generally similar rates and patterns of contemporary trait changes through time. The results also suggest a potentially important role of the environment to drive trait change in wild populations but it remains unclear whether many of these populations have genetically diverged. I concluded the paper by suggesting that future work to understand how both genetic and environmental factors influence trait expression is especially important as subtle changes in trait values can influence the long-term sustainability and persistence of exploited or invading species.

How does a novel landscape shape the spatial distribution of an invading species? Can we identify habitats prone to successful invasion to better prioritize conservation efforts?  The abstract of Westley & Fleming (2011) is copied below, which encapsulates such an effort to address these questions.

Aim:  We investigated watershed-scale abiotic environmental factors associated with population establishment of one of the ‘world’s 100 worst alien invaders’ on a temperate Atlantic island. Within the context of the conservation implications, we aimed to quantify 1) the early history and demographics (numbers and origins) of human-mediated brown trout (Salmo trutta) introductions, 2) the current distribution of established populations, and 3) the watershed-scale environmental factors that may resist or facilitate trout establishment.

Location:Island of Newfoundland,Canada.

Methods: We combined field sampling with historical and contemporary records from literature to assemble a presence-absence and physical habitat database for 312 watersheds on Newfoundland. Probability of watershed establishment was modelled with general additive ANCOVA models to control for non-linear effects of propagule pressure (i.e. the distance to and number of invasion foci within a biologically relevant range) and model performance based on AIC.

Results: Between 1883 and 1906, 16 watersheds were introduced with brown trout from the Howietoun Hatchery, near Sterling, Scotland. Since that time, populations have established in 51 additional watersheds at an estimated rate of spread of 4 km yr-1. We did not detect any obvious abiotic barriers to resist trout establishment, but showed that for a given amount of propagule pressure that relatively large and productive watersheds were most likely to be established.

Main conclusions: Brown trout have successfully invaded and established populations in watersheds of Newfoundland and are currently slowly expanding on the island. Populations are more likely to establish in relatively large and productive watersheds, thereby supporting predictions of island biogeography theory. However, we suggest that all watersheds inNewfoundlandare potentially susceptible to successful brown trout invasion and that abiotic factors alone are unlikely to act sufficiently as barriers to population establishment.

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