Comment on Titcomb et al.'s 'Interacting effects of wildlife loss and climate on ticks and tick-borne disease'.

Abstract

Understanding the impact of anthropogenic disturbances such as defaunation and climate change on vector-borne disease risk is critically important. Titcomb et al. [1] experimentally tested the interactive effects of these perturbations on tick-borne disease risk within long-term, size-selective, large-herbivore exclosures, replicated across a precipitation gradient in East Africa. They found that the abundance of adult ticks increased with increasing degrees of wildlife exclusion (from exclusion of only mega-herbivores to exclusion of all herbivores greater than or equal to 5 kg) and that this effect was stronger in more arid sites. Tick-borne pathogen prevalence remained unchanged. Based on these results, the authors conclude that loss of large wildlife species and climate change increase tick-borne disease risk by increasing the densities of adult ticks. However, given that adult ticks of the collected species all depend on large wildlife as final host, this conclusion is both counterintuitive and in contrast to earlier studies that conclude that ticks disappear when their specific, final host species are lost [2–4]. Here, we offer an alternative interpretation of their data. We argue that the apparent increase in tick abundance in these exclosures may actually reflect prolonged questing activity of adult ticks that fail to find a host. Given the absence of a final host, reproduction and hence local recruitment of ticks should be minimal at best. When large wildlife are progressively excluded, the continued presence of ticks inside the exclosures will therefore increasingly depend on rodents and other small mammals that move across fences and import immature ticks from the surrounding area [5]. Once inside, these immature ticks moult into adults, which require larger wildlife as final hosts. When the latter are absent, the adult ticks will continue …