Abstract
Predators are an important ecological and evolutionary force shaping prey population dynamics. Ecologists have extensively assessed the lethal effects of invasive predators on prey populations. However, the role of non-lethal effects, such as physiological stress or behavioural responses like dispersal, has been comparatively overlooked and their potential population effects remain obscure. Over the last 23 years, we developed a mark-recapture program for the Audouin’s gull and an intensive carnivore monitoring program to assess how the appearance and invasion of the study site by carnivores affects population dynamics. We evaluate changes in turnover of discrete breeding patches within the colony, age structure and breeding performance. Once carnivores entered the colony, the number of occupied patches increased, indicating a higher patch turnover. Breeders responded by moving to areas less accessible to carnivores. More importantly, the presence of carnivores caused differential (and density-independent) breeding dispersal: experienced, better-performing breeders were more likely to leave the colony than younger breeders. This differential dispersal modified the age structure and reduced the reproductive performance of the population. Our results confirm the importance experience in the study of populations. The role of differential dispersal for animal population dynamics might be more important than previously thought, especially under scenarios of global change.
Highlights
Biological invasions are one of the most important drivers of ecological change globally[1,2]
Breeders responded to carnivore presence by moving eastwards (~180 m/y, R2 = 0.79, p-value < 0.001) towards patches surrounded by water, and less accessible to carnivores (Fig. 1 in the main text and Tables 1, 2 in electronic Supplementary Material S1 and video in electronic Supplementary Material S2)
Clutch size and breeding success in the different patches in 2012 was negatively correlated with the proportion of young and inexperienced breeders in each of the patches, this relationship was only significant for breeding success (Fig. 2f and see electronic Supplementary Material S1, Tables 1–2)
Summary
Biological invasions are one of the most important drivers of ecological change globally[1,2]. We explore potential non-lethal effects of invasive native mesocarnivores at a previously predator-free colony of Audouin’s gull (Ichthyaetus audouinii)[26]. This species usually lays 3 egg-clutches, it is endemic to the Mediterranean and until the 1980s its population was regarded as threatened[27]. We hypothesize that the occurrence of carnivores stimulates experienced, but not inexperienced, birds to undergo breeding dispersal, which causes an increase in the proportion of younger and inexperienced breeders occupying habitats with carnivores This increase in the proportion of younger individuals should have effects on breeding performance, potentially influencing egg volume, clutch size and/or breeding success
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