Scientific approaches to literature review, including quantitative methods for research synthesis (meta-analysis), have become more established in ecology over the past 15 years. Formal systematic review protocols, now well established in medical research synthesis, have not yet been widely implemented in ecology. Systematic reviews can be narrowly focused (the general approach in medicine) or broad, but always depend on reproducible and transparent methodology. We carried out a systematic review of the experimental and theoretical literature on biological invasions (to 06/2010) and created a database of our results. Our goal was to gather and categorize this large and diverse body of information to assess what has been done and identify areas where research needs are most pressing. We investigated a) major invasion hypotheses tested, b) habitats and locations of the studies, c) target species, and d) the type of study (e.g. field experiment or observations, greenhouse experiments, theoretical work). We did not include studies focused on invasion impacts, control methods, reports on incidence of invaders, or agricultural studies. We followed systematic review methodology used in medical research synthesis.
Results/Conclusions
Our initial search produced 8,330 studies, and of these, we identified 2,533 relevant papers. We categorized 19 major hypotheses/explanations for invasion in this literature. The most commonly addressed hypotheses were a) inherent superiority of invaders and b) disturbance; 80% of the studies addressed or tested only a single explanation or hypothesis. The effects of climate change on invasions were the focus of fewer than 2% of the studies. Approximately three times more studies were carried out in terrestrial than in aquatic habitats. About 80% of the studies concerned plants. Fewer than 7% focused on predators. This identifies a serious shortcoming in the literature, as invasive predators are arguably the greatest threat to native species. Almost half the studies lasted more than one year. Fewer than 10% of the studies were conducted in tropical regions. “Research bias” favoring certain species and systems may be producing a subtly distorted picture of biological invasions, rather than necessarily reflecting the most important problems or even the most common cases of invasion. Our database will provide opportunities for searching many issues in the biological invasion literature, and perhaps in guiding future research directions.