Members of family Cervidae, commonly known as deer, are found throughout the globe. Deer were introduced to many locations but even in locations where deer are native, a lack of predators and warming climate often leads to deer overabundance. When deer become overabundant they act as ecosystem engineers altering landscapes by consuming plants, competing with native animals and spreading disease. One of the best-studied ways that deer alter their habitat is by consuming vegetation and changing understory plant communities. Since the 1980s, researchers have asked whether deer browsing has particularly large effects on birds that may need understory plants for nesting substrate. This effect has been occasionally documented in the literature, but to date, no research syntheses have provided clear evidence for impacts across studies. To this end, we conducted a meta-analysis that synthesized existing evidence for the impacts of native and introduced deer on bird abundance.
Results/Conclusions
Our initial literature search found 1,553 potentially relevant research articles. We pre-defined inclusion and exclusion criteria that we used to screen articles for the meta-analysis. The inclusion criteria provided a total of 31 articles that contained the quantitative data needed for a meta-analysis. A mixed effects meta-analytic model showed that higher deer abundance correlated with lower bird abundance when compared to areas without deer or where deer were excluded. The underlying mechanism may be that deer browsing reduces nesting and perching substrate for bird species. In addition to overall effect size, we explored moderating variables such as length and location of study. Interestingly, most studies took place over an average of 1-3 years. We found a strong geographic bias in where deer impacts on birds are studied. Most research occurred in north America and Europe. We also explored whether native bird abundance decreased more in island or mainland studies. We found that the greatest impacts on native bird abundance occurred on islands compared to mainland. By synthesizing evidence from over 30 studies on deer populations from across the globe, we have a strong foundation to inform land management and policy that aim to manage deer populations and increase bird abundance.