Enemy release promotes range expansion in a host plant

Climate is considered to be the predominant driver shaping species distributions at macroecological scales, yet the importance of incorporating biotic interactions in predicting future range margins under climate change scenarios is increasingly being recognised. We used translocation studies to investigate how survival and growth patterns of an understory shrub planted at latitudes within its range, at its range limit and beyond its polewards boundary (in areas it may colonise as a result of shifting climate envelopes) are affected by the presence of a primary herbivore. Specifically, we tested the null hypotheses that: (1) biotic interactions do not exert a significant role in limiting survival and growth rates across the limits of a host plant's latitudinal range, and (2) at smaller spatial scales biotic interactions do not exert a significant role in determining survival and growth rates at edge versus interior position within a forest fragment. We found that the understory shrub Macropiper excelsum is able to survive polewards of its current latitudinal limit within the first year after transplant; in fact, growth is higher outside the plant's current natural range than within its present-day distribution. This trend is particularly pronounced in forest core environments and corresponds closely to patterns of reduced herbivory outside the plant's range. The absence of the primary herbivore, Cleora scriptaria, and concomitant reduction in the suppressive effects of herbivory outside of the plant's range appear to be supporting enhanced growth and survival. If host plants are able to successfully track their climatic niche and disperse into novel areas prior to the arrival of their natural predators, it is possible that 'enemy release' may facilitate the establishment of plant species. These findings highlight the importance of considering biotic interactions alongside abiotic variables when predicting future species' ranges under climate change.