Experimentally induced reversal of a predator risk effect under changing environmental conditions and its implications for population responses
It has long been established that direct mortality due to predation can be a major force in population ecology. More recently it has been shown that the indirect or risk effects of predators and predation (also known as non-lethal or non-consumptive effects) can have as great, or even greater, impacts on individual fitness and population dynamics. This study focuses on a well known avian response to predation risk, body mass change, to ask how this indirect predator risk effect may change in response to changing environmental conditions. The use of innovative RFID monitoring technology allowed an experimental approach to be applied in nature to test how a wild bird species (the Great Tit Parus major ) responds to perceived changes in predation risk and allowed the following questions to be answered: 1) can identical experimental heightening of perceived predation risk produce opposite anti-predator responses under different environmental conditions?, 2) can predator response in a single local population change over a small geographic scale (less than a km)?, 3) over what time scale can a change in response to predator risk occur (years to weeks)?
Under harsh mid- winter conditions consisting of cold days, long overnight fasting and short daily foraging periods, great tits responded to experimental heightening of perceived predation by adopting a mass-dependent predation risk type response ( a loss of body mass) when their perceived predation risk was increased by flying model sparrowhawks as an experimental treatment. In response to exactly the same experimental treatment in milder conditions of warmer temperatures, shorter fasting period and longer foraging periods great tits responded by adopting an interrupted foraging response (an increase in body mass). The results show that great tits can strategically reverse their mass response to predators depending on environmental conditions and that they can do so over short temporal and spatial scales. Starvation-predation risk trade-off theory is used to interpret the results and predict that climate change will make such changes in this individual predation risk response widespread and this is likely to significantly change population dynamics of both this species and many others through changing winter survival rates.