Support for hypotheses in invasion biology is uneven and declining

“Support for major hypotheses in invasion biology is uneven and declining” is the title of a paper recently published in NeoBiota* co-authored by seven scientists from all over the world (Germany, USA, Spain, Canada, Czech Republic).  The title captured our attention because it is consistent with our viewpoint.

The international team of scientists analyzed 371 empirical studies which tested six major hypotheses in invasion biology.  They found that empirical evidence for these hypotheses is uneven and declining.  The hypotheses that were tested by the studies were:

• Invasional meltdown:  the presence of invasive species facilitates invasion and survival of additional new species.
• Novel weapons:   invasive species with traits new to an exotic habitat have a competitive advantage over native species.
• Enemy release:  introduced species have a competitive advantage in the exotic range because they are released from their enemies in the new environment.
• Biotic resistance:  More biologically diverse ecosystems are more resistant to invasion.
• Tens rule:  10% of newly introduced species escape to the wild; 10% of those naturalize in the wild; 10% of those become invasive.
• Island susceptibility:  Invasive species are more likely to become established and have major ecological impacts on islands than on continents.

The scientists counted the number of studies that support, question/oppose, or are undecided/inconclusive about each hypothesis.  They also compared the number of supporting studies when the hypothesis was new with the number of supporting studies published recently to determine the decline in support for the hypothesis.  Here’s what they found:

Although support is strongest for the invasional meltdown hypothesis, recent studies are less supportive than early studies, indicating substantial decline in support.  Declining evidence of invasional meltdown is consistent with the fact that exotic species are eventually integrated into the food web which reduces their populations, stabilizing their spread. There is apparently little evidence that islands are more susceptible to invasion than continents and few studies have been done to test the hypothesis.

Declining support for scientific hypotheses has been observed in many disciplines, particularly medicine, ecology, and psychology.  The scientists who study this phenomenon theorize that the decline is attributable to some combination of these factors:

• Over time the amount of available long-term data increases.
• The best examples which are the strongest cases for the hypothesis are most likely to be studied first.
• Publication bias favors new hypotheses and those for which the results are conclusive.

The NeoBiota paper also observes that the empirical evidence supporting each hypothesis varies by taxonomic group (plants, invertebrates, vertebrates) and habitat type (terrestrial, freshwater, marine).  For example:

• The novel weapons hypothesis has been tested only for plants in terrestrial habitats.
• Support for the invasional meltdown hypothesis is even across taxa and habitats.
• Support for biotic resistance is strongest in marine habitats.

Where is invasion biology headed?

The authors of the NeoBiota paper are not suggesting that invasion biology be abandoned.  Rather their goal is to redirect scientists in the field to more productive efforts, such as:

• Where a hypothesis cannot be generalized to all taxa and habitats specify exactly where it is applicable.
• Rather than focusing on newly introduced species, focus on the interaction of a those species with their new environment. 
• Discard those hypotheses that don’t work.

Based on our fifteen years of experience studying the native plant movement and its theoretical underpinnings in invasion biology we wholeheartedly support the advice of the authors to focus scientific efforts on the interaction of new species with their new environment.  We strongly believe that the success of newly introduced species is based largely on changes in the environment into which they are introduced.  In other words, invasions are more a result of changes in the environment than on the characteristics of the introduced species.

We also endorse the advice that scientists be more specific about the applicability of the assumptions of invasion biology.  We have seen the damage done by sweeping generalizations about how ecosystems operate in the hands of hobbyists.  Nature is complex and we do not necessarily understand all the factors operating in a given environment.  We hope that scientists will lead the way to the public’s more nuanced understanding of ecosystems. 

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*Jeschke, Jonathan, et. al., “Support for major hypotheses in invasion biology is uneven and declining,” NeoBiota, 14: 1-20 (2012)