Impacts of poor food availability on positive density dependence in a highly colonial seabird

Abstract

For species with positive density dependence, costs and benefits of increasing density may depend on environmental conditions, but this has seldom been tested. By examining a colonial seabird (common guillemot) over a period of unprecedented poor food availability, we test two contrasting hypotheses suggesting that birds breeding at high density have: (i) greater leeway to increase foraging effort owing to more effective defence of unattended chicks against predators; and (ii) less leeway, owing to more attacks on unattended chicks by neighbouring adults. Supporting hypothesis 1, birds at high density increased provisioning rates and hence survival of chicks by foraging simultaneously with their partners, whereas at low density, unattended chicks were liable to be killed by predatory gulls and, unexpectedly, razorbills. Simultaneously, supporting hypothesis 2, heightened aggression towards unattended chicks at high density frequently resulted in infanticide, undermining benefits from collective defence against predators. Consequently, over 25 years, the magnitude of positive density dependence was independent of mean breeding success. These data indicate previously unsuspected trade-offs between costs and benefits of increasing density under changing environments. Previous generalizations about the importance of high density for reproductive success have so far remained robust, but such trade-offs could have unpredictable consequences for future population dynamics.

Skipped breeding in common guillemots in a changing climate: restraint or constraint?

“…A string of years of poor breeding success occurred at this colony from 2004 to 2008 (Figure S3), associated with poor feeding conditions around the breeding colony (Ashbrook et al, 2010). Skipping frequency was also noticeably higher during this period (Figure 2A), which would be consistent with the restraint hypothesis, i.e., birds "chose" to skip when breeding conditions were perceived in advance to be poor.…”

Contrasting effects of food quality and quantity on a marine top predator

“…The number of prey items (or sample size) is given in parentheses on fledging success for Canadian murres despite 10-fold changes in prey abundance and density. Flexible time budgets allowed parents to buffer prey variability to sustain feeding rates, provided that prey availability did not fall below threshold levels (Burger & Piatt 1990, see also Uttley et al 1994, Ashbrook et al 2010. Such parental buffering appears to be a plausible explanation for our observed uniform feeding rates despite differences in sprat abundance.…”

“…Decreased weight-at-age of sprat seems to be a probable cause of the observed lower fledging success in Baltic murres, analogous to previously published observations suggesting such a link also to chick fledging mass (Österblom et al 2001, 2006). Breeding success in North Sea murres is strongly associated with length of preferred prey, at least when prey availability exceeds threshold levels (Wanless et al 2005, Frederiksen et al 2006, Ashbrook et al 2010. Sprat was relatively abundant during our study period and the relationships detected in the Baltic Sea may be different in situations when food availability is limited.…”

“…A breeding failure of North Sea murres was also partly explained by diet shifts and low quality of prey (Wanless et al 2005). However, at the same time, food availability decreased, and the combined impact resulted in unprecedented low breeding success during several consecutive breeding seasons (Ashbrook et al 2008(Ashbrook et al , 2010. Top predators may thus be influenced simultaneously by changes in both food quantity and quality, and this may affect different components of reproduction and survival.…”

Fluctuations in food supply drive recruitment variation in a marine fish

“…A large body of evidence suggests that food supply can influence population dynamics by, for example, altering reproductive performance of adults and/or survival of young. This has been documented for a range of vertebrates, including terrestrial mammalian herbivores [5,6], marine mammals [7], passerine birds [8], seabirds [7,9], and freshwater [10] and marine fish [11][12][13]. Although fluctuations in food supply are ubiquitous in nature, conservation and management strategies frequently assume that populations have constant mean vital rates through time.…”