Density‐dependent increase in superpredation linked to food limitation in a recovering population of northern goshawks Accipiter gentilis

Abstract

16A better understanding of the mechanisms driving superpredation, the killing of smaller 17 mesopredators by larger apex predators, is important because of the crucial role superpredation 18 can play in structuring communities and because it often involves species of conservation 19 concern. Here we document how the extent of superpredation changed over time, and assessed 20 the impact of such temporal variation on local mesopredator populations using 40 years of 21 dietary data collected from a recovering population of northern goshawks (Accipiter gentilis), 22 an archetypical avian superpredator. We then assessed which mechanisms were driving 23 variation in superpredation, e.g., was it opportunistic, a response to food becoming limited (due 24 to declines in preferred prey) or to reduce competition. Raptors comprised 8% of goshawk diet 25 on average in years when goshawk abundance was high, which is higher than reported

Why do top predators engage in superpredation? From an empirical scenario to a theoretical framework

“…Indeed, superpredation levels increased with the decreasing abundance of main prey, indicating that food stress should be a relevant determinant. This is in fact the most frequently suggested cause of superpredation reported in birds of prey and mammalian carnivores (Korpimäki and Norrdahl 1989, Palomares and Caro 1999, Sunde et al 1999, Serrano 2000, Rutz and Bijlsma 2006, Lourenço et al 2011a, Hoy et al 2017). This relationship is consistent with the well-established prediction of optimal diet theory, i.e.…”

“…However, the causes of lethal interactions are still poorly understood, which is possibly linked to the great difficulty in assessing the frequency in which top predators engage in these interactions. Four main causes have been proposed to explain lethal interactions: 1) food stress, 2) opportunistic superpredation, 3) removal of a competitor, and 4) removal of a potential threat (Polis et al 1989, Palomares and Caro 1999, Sergio and Hiraldo 2008, Ritchie and Johnson 2009, Lourenço et al 2014, Hoy et al 2017; Table 1). These different causes can have different implications for mesopredators and influence trophic cascades, i.e.…”

“…by varying in their potential to generate mesopredator suppression/release phenomena and indirect effects at lower trophic levels (Mueller et al 2016, Terraube andBretagnolle 2018). The food-stress hypothesis has been the most frequently cited explanation: top predators will include a higher proportion of mesopredators in their diet when facing a decrease in main-prey availability (Korpimäki and Norrdahl 1989, Tella and Mañosa 1993, Serrano 2000, Rutz and Bijlsma 2006, Lourenço et al 2011a, Hoy et al 2017. The opportunistic-superpredation hypothesis is applicable when a top predator includes mesopredators in its diet, only driven by chance, and mostly because of its superiority, without being under food stress (Lourenço et al 2014, Hoy et al 2017.…”

“…The food-stress hypothesis has been the most frequently cited explanation: top predators will include a higher proportion of mesopredators in their diet when facing a decrease in main-prey availability (Korpimäki and Norrdahl 1989, Tella and Mañosa 1993, Serrano 2000, Rutz and Bijlsma 2006, Lourenço et al 2011a, Hoy et al 2017. The opportunistic-superpredation hypothesis is applicable when a top predator includes mesopredators in its diet, only driven by chance, and mostly because of its superiority, without being under food stress (Lourenço et al 2014, Hoy et al 2017. The competitor-removal hypothesis has been suggested since killing a competitor may free up resources for a top predator (Polis et al 1989, Palomares and Caro 1999, Sunde et al 1999, Helldin et al 2006.…”

“…A practical approach to unravelling the causes of lethal interactions among top predators involves analysing the relationships between the percentage of superpredation in individuals' diet and the abundance of both main prey and mesopredators in nature (Lourenço et al 2011a, Hoy et al 2017). In addition, to determine the competitive degree of the interaction, it is important to compare the frequency of superpredation on mesopredators that compete with the top predator and the frequency of superpredation on mesopredators that do not compete or pose a serious threat (Sunde et al 1999, Serrano 2000, Lourenço et al 2014, Morosinotto et al 2017.…”