Why do some species become invasive? Research on many invasive species tells us that some particular characteristics help increase “invasability”, such as producing large numbers of offspring, being able to self-pollinate, or being able to utilize a wide variety of resources. Another proposed reason is that changes in the local environment (temperature, food availability) may temporarily give an exotic species a large advantage, helping a species to become well-established and then invasive, whereas it was merely benign previously. A recent study in Ecology Letters (here) utilized a thirty-year dataset to examine the question of how a change in availability of local resources or a change in the “enemies” (competitors, predators) of an invader might trigger an invasion. This study illustrates several very important points about invasions, including: exotic species can quickly become invasive when limiting environmental factors change, and native ecosystems that are currently able to resist invasion may be less robust in the future.
Dr. Ginger Allington and colleagues studied a desert community in which an exotic annual plant called Pinweed (Erodium cicutarium) existed at low levels for twenty years, and then over just a few years, became so abundant it now dominates the ecosystem. To determine the cause of this abrupt change, the authors needed to collect two points of evidence: they needed to demonstrate that some environmental factor limits growth of the invader (e.g. an ecological mechanism), and they needed to demonstrate that the ecological factor of interest declined or increased at the same time that the invader’s population increased. The study took place at the Portal Project (Arizona, USA), a long term ecological experiment on (among other things) the effects of small rodents as seed predators.
The authors first showed that the invader became abundant on plots from which rodents were excluded. Meanwhile, the invader was very rare in plots where rodents were allowed, suggesting that rodents eat the seeds of the invader, limiting their population size. In the authors’ words, “by 1984, [Erodium] had been observed on all four rodent removal plots and all four [large-bodied rodent] removal plots. In contrast, the invader did not appear on a single control plot until 1983… [and] accounted for only a small fraction of the annual plant community on these plots; E. cicutarium comprised <5% of the community on control plots for almost 20 years.” However, in 1997, the invader population “rose rapidly and by 2007, it had become the dominant species in the annual plant community and was present on nearly every quadrat. The increase in Erodium cicutariumon on control plots was preceded by a decline in [rodent] abundance in the early 1990s,” which occurred after several years of low precipitation. The authors inferred that little rainfall meant that rodent population decreased (due to less food availability), and fewer rodents meant low seed predation. This in turn allowed the invader to produce more seeds, which built up in the soil. Subsequent return of precipitation led to a Erodium population explosion, and a shift towards an invader-dominated system. In sum, the study thus demonstrated both a mechanism for limitation of the invader (seed predation), and an increase in the invader’s numbers when that limitation was changed.
Few long terms studies such as this one exist, and thus the mechanisms controlling invasive populations are often hypothetical. The authors emphasize that long term study of populations is crucial to determining when, where, and how invasions will occur.
The study also suggests that a key species interaction (granivory- the consumption of plant seeds by animals) controlled the invading plant. A management approach for invasive species might be to focus on maintaining keystone interactions in a community, to help maintain ecosystem stability and resistance to invasion. The authors suggest that these interactions may be a more important focus than a focus on promoting high species diversity in the ecosystem (a large number of diverse species is another factor that is hypothesized to help resist invasions).
The authors close by putting their study in the context of ongoing global warming and climate changes. In their words, “Climate models predict more frequent extreme weather events in the coming decades. The resulting increased environmental stochasticity will likely create new niche opportunities for introduced species via fluctuations in both resources and important consumers. Exotic species currently regulated by strong biotic control today may come to dominate many communities in the future. This case study highlights the kinds of complex interactions that are difficult to predict, but that may be the root cause of the dominance of exotics.” To predict future invasions, the authors conclude, we need better understanding of current ecological interactions, and long term monitoring.