Invasion Biology: The way forward

We’re following up on our previous post in which we reported that empirical studies do not support the hypotheses of invasion biology.  In that case, six hypotheses of invasion biology were tested by empirical studies and largely failed.  Furthermore, more recent studies are less supportive than older studies, indicating declining support for the assumptions of invasion biology.

Now we are going to tell you about a new publication by another team of scientists who challenged other assumptions about invasive plants and also conducted their own original research of one of the most basic assumptions of invasion biology:  that invasions are facilitated by disturbance.

We introduced our readers to the leader of this research team, Professor Angela Moles, in a recent post about the mounting evidence that attempts to eradicate non-native species are futile.  Professor Moles (University of New South Wales, Australia ) gave a TED (Technology, Entertainment, Design) presentation in which she reported that introduced species have changed significantly since their introduction and that if they weren’t yet new species, they soon would be.  She proposed that non-native plants in Australia be granted citizenship.

Professor Moles collaborated with 21 scientists all over the world (Uganda, Indonesia, Mexico, USA, Australia, New Zealand, Japan, Argentina, Estonia, New Zealand) in the study that resulted in a recently published article entitled, “Invasions:  The trail behind, the path ahead, and a test of a disturbing idea.”  *

The trail of invasion biology

As the title suggests, the article begins by reporting that after 30 years and 10,000 publications, invasion biology has tested many assumptions and found inconsistent evidence to support them:

• The search for traits of introduced plants that predict invasiveness has been a dead end:  “…it is not currently possible and will probably never be possible to predict which species are likely to become problem invaders on the basis of traits alone.  We therefore suggest that this is one area of invasion biology that merits less attention in the future.”
• Invasion biology predicted that lack of genetic variability would hinder evolutionary adaptation in introduced species.  This assumption has not been supported by empirical studies:  “…rapid evolution has been repeatedly demonstrated in introduced populations, and the predicted reduction in genetic variance has not been observed.” 
• Rapid evolution of introduced species has been well established by empirical studies:  “We have reached the point where additional case studies demonstrating rapid evolutionary change in introduced species are unlikely to have a major impact on our understanding of invasions.”  New research questions are needed.
• There is little evidence to support the assumption that introduced plant species will cause extinction In native communities:  “…there are astonishingly few documented cases of native plants being driven to extinction by competition from introduced plants.  There is no evidence for any native species in the United States being driven to extinction even within a state, by competition from an introduced plant species.”

The way forward in invasion biology

Professor Moles and her team then tell us why invasion biology has not been able to prove the assumptions on which the theory is based.  The theory of invasion biology was based on untested assumptions that have been accepted as true although there is no empirical evidence to support them.  The goal for the future of invasion biology should be to identify these assumptions that have been accepted as dogma, test them, and abandon those that are not consistent with empirical facts. 

The authors of this study also, “…join a growing chorus, suggesting that our approach to invasion biology has been too simplistic.”  Studies have tended to focus on the features of introduced plants in isolation.  A more fruitful line of inquiry will consider the complex interactions between newly introduced species and their new environment:

“Rather than focusing on one factor at a time, we need to find ways (including multivariate analysis) to synthesize information about the recipient habitats/ communities, the characteristics of both resident species and the invaders, demographic processes, propagule pressure [measure of the number of species released into a region in which they are not native], the differences between current conditions and those with which the resident species evolved, evolutionary change to both native and introduced species, plasticity and feedbacks and interactions between different species and processes.”

You might say, “Phew! That sounds like a daunting task.”  And so it is, but this team of scientists takes it on with an elaborate and complex study of one of the most basic assumptions of invasion biology:  that disturbance facilitates plant invasions.

Does disturbance facilitate plant invasions?

“Disturbance is thought to facilitate invasion by simultaneously opening new ground for colonization, decreasing the competition from resident native species and releasing pulses of resources.”  The definition of “disturbance” has varied in different studies, but generally includes fire, grazing, agriculture, erosion, wind, and flood.  Empirical tests of this theory have produced mixed results.  Even when the results have been positive, they have not persisted over the long-term.

Because disturbance is a natural feature of all ecosystems, native species have adaptive features that enable them to respond to natural disturbances.  Therefore, the research team theorized that it is not disturbance per se which creates opportunity for invasions by introduced species, but rather changes in the disturbance regime.  Their research study was therefore designed to distinguish between the level of disturbance and changes in the level of disturbance.

Given the international composition of their research team, they were able to select 200 sites in eight countries.  They selected only those sites for which the natural patterns of disturbance were known.  Their research methods were statistically complex and a detailed description of them is beyond our comprehension and probably many of our readers, but we encourage those with the necessary scientific knowledge to read the article which is available on the internet.

Their analysis of these 200 sites led them to the conclusion that the change in disturbance regimes was far more predictive of the success of invasions than the level of disturbance but that both variables explained only 7% of the variation in the percent of cover or species richness contributed by introduced species.

In other words, one of the most basic assumptions of invasion biology did not pass an empirical test of its validity.  Invasions by introduced plants are largely unexplained by disturbance.

The future of invasion biology

*Moles, Angela, et. al., “Invasions:  The trail behind, the path ahead, and a test of a disturbing idea,” Journal of Ecology, British Ecological Society, 2012, 100, 116-`127.  All quotes are from this article