The bad side of island life

It’s not news that island populations are vulnerable to invasion by alien competitors and predators. A new study by Helena Berglund and colleagues in American Naturalist reviews the literature on the topic and presents an analysis of the effects of invasive species on islands with different levels of endemicity (that is, proportions of species found only on the island).

Here’s a useful paragraph of context from the paper’s introduction:

Isolation from the nearest mainland appears to be one of the most consistent predictors of the number of extinctions of bird species on oceanic islands (Blackburn and Gaston 2005), and many cases of depauperate flora and fauna on remote and isolated islands are known (Elton 1958). Isolation and remoteness is a combination that also qualifies as a vulnerability predictor, for several reasons. First, islands are often small territories with small populations that historically have faced a higher risk of extinction than mainland territories (Manne et al. 1999; Trevino et al. 2007), and more isolated island communities are recolonized at a slower rate than the less isolated ones (Brown and Kodric?Brown 1977). Second, less isolated and adjacent islands have a higher frequency of introductions of new species than more isolated and remote islands (MacArthur and Wilson 1963). If extinctions and replacements of native species follow from introductions of new ones, communities on less remote islands will face a higher risk of replacement of the most susceptible species, so that their communities become more resilient to new introductions in the future (i.e., extinction filters; Rodriguez 2001). The same reasoning may lend support to insularity as an explanation for the relative rarity of high?impact invaders in marine as opposed to freshwater systems (Cox and Lima 2006).

After the paper’s analysis, there is substantial discussion of the relation of invasive species to human disturbance. For one thing, invasive species on islands tend to occupy different niches from the endemics they replace. The paper cites other work (Case 1996) suggesting that this is because the invaders are exploiting human-created habitats. Also, there is a positive relation between island area and proportion of threatened or extinct endemics. The authors observe that island biogeography predicts a negative relation between species number and island size, which might suggest greater ecosystem robustness on larger islands. But larger islands are “more intense” subjects of human resource extraction, which may enable a greater number of invasive species.

On the subject of robustness to change:

This interpretation of endemism in the light of isolation goes hand in hand with the application of the term stability, defined by Cronk (1997) as the tendency of biota, their interactions and community processes to persist in a given geographical area for long periods of time (p. 483). He used it to suggest that continents have high robustness stability, or short?term stability to extrinsic change, whereas isolated islands tend to have high long?term stability with relict species but lower short?term stability to extrinsic change (Cronk 1997), such as the introduction of NIS. The high vulnerability to NIS by species on isolated islands may also come from traits evolved in the absence of interactions with immigrants, for example, flightlessness in birds in the absence of predators (McNab 1994), exaggerated enemy release (Cronk 1997), and specialization (Wijesinghe and Brooke 2004). The vulnerability of endemic species may then arise from their geographically restricted distributions and isolation from many types of competitors, predators, and diseases.

I tend to think that the evolution of small brains in the absence of continental human competitors probably qualifies.

For my own notes, this is a useful reference-full sentence on the topic of rapid evolution:

The present call for more attention to the long?term modulating effects of invasive species is supported by evidence accumulated during the last decade for rapid adaptation to a recipient community in the invader species and selection for new traits and novel genotypes in native species in response to invaders (Mooney and Cleland 2001; Lambrinos 2004; Strauss et al. 2006; Strayer et al. 2006; Whitney and Gabler 2008).

As concerns native species, these are examples of rapid environmental changes causing selection. For invader species, the ecological change is coupled with massive population growth. In that sense, humans are themselves an invasive population in many habitats.

References:

Berglund H, Järemo J, Bengtsson G. 2009. Endemism predicts intrinsic vulnerability to nonindigenous species on islands. Am Nat (in press) doi:10.1086/598501