Testing major invasion hypotheses with the Hierarchy-of-Hypotheses approach

Background/Question/Methods

Major ecological hypotheses are typically formulated to be broadly applicable, e.g. across taxonomic groups and habitats. This is also true for hypotheses about invasive alien species. For example, the biotic resistance hypothesis (also known as diversity-invasibility hypothesis) posits that ecosystems with high biodiversity are more resistant against invaders than ecosystems with low biodiversity. Another example is the enemy release hypothesis which posits that the absence of predators or parasites in the exotic range is a cause of invasion success. These and other major invasion hypotheses cannot be tested by a single empirical study. Instead, each empirical study typically tests a certain specific formulation of a major hypothesis, under specific conditions and for one (or a limited number of) focal species. This situation raises the questions how results of specific case studies can be related to broad hypotheses, and in general, how specific empirical evidence can be used to assess major hypotheses.

Results/Conclusions

We suggest the hierarchy-of-hypotheses (HoH) approach as a tool to organize research, and as a basis for the assessment of hypotheses. In a HoH, a broad, overarching hypothesis branches into increasingly narrow and specific formulations of this hypothesis. The most specific formulations are empirically testable.

In our presentation, we demonstrate the functionality of a HoH for a well-known and much discussed hypothesis of invasion ecology: the enemy release hypothesis. We show that our approach can be used to organize empirical evidence by specifying sub-hypotheses. Based on a newly developed weighting procedure, we assess empirical evidence for each sub-hypothesis. Our results show that overall, there is nearly as much evidence in favor as against the enemy release hypothesis; hence, the overall picture is quite blurry. However, a closer look at the sub-hypotheses reveals that specific formulations of the enemy release hypothesis are clearly empirically supported, whereas other formulations receive hardly any support.