Density dependence of seabirds

Published: 15 October 2015

Managing the effects of environmental change and anthropogenic pressures on marine ecosystems requires an understanding of how the population dynamics of species are influenced by different types of environmental variation and disturbance. This is particularly important for species that exert considerable influence on system stability and resilience, such as top predators like seabirds, cetaceans and seals. 

Population processes, such as density dependence, particularly for long-lived species, are key regulators of population abundance. Negative density-dependence occurs in populations when the growth rate of a population is affected by its size, with lower population growth rates occurring at higher population densities. The strength of density dependence in populations drives important feedbacks on age-specific survival and fecundity; key determinants of population size and change in long-lived species.

Density dependence in population growth rates is known to respond to environmental conditions in diverse taxa and systems. Moreover, density dependence often increases in strength under times of poor environmental conditions (resources or climate) to adversely affect both survival and recruitment. Therefore, an understanding of how population processes and abundance in top marine predators are influenced by spatial and temporal variation in environmental variables, such as climate and heterogeneity in lower trophic levels, is critical to understanding the dynamics of marine ecosystems subject to environmental change.

Within MERP a collaborative team of marine top predator ecologists and mathematical modellers are working to understand how spatio-temporal variation in resources and climate have interacted to shape the strength of density dependence and the abundance of key seabird species in UK waters over the last four decades.

This work is organised around an emerging tenet from terrestrial macroecological studies that has demonstrated both positive (temporal variation in climate) and negative (spatial variation in food resources) relationships between environmental stochasticity and the strength of density dependence in populations. To date, analyses linking environmental heterogeneity and population processes in this way have been restricted to terrestrial systems or experimental mesocosms, and have tended to focus on species from intermediate trophic levels. Within MERP we are exploring these linkages in multiple seabird species, adding to the current state of knowledge by drawing inference from species of higher trophic levels in a marine environment.

Our research draws upon the fantastic long-term monitoring of seabirds across the UK by many different organisations. Many seabird species have been studied intensively at colonies going back over four decades, providing a great opportunity to look for effects of environmental change on population abundance and dynamics. A particularly important component of this work involves linking the analyses of long-term population trends to mathematical models that simulate interactions between multiple species and trophic levels within the marine ecosystem. These models will help MERP scientists to understand how environmental change affects the abundance and dynamics of marine top predators, and how this in turn may drive top-down processes in marine food webs. Mathematical and empirical models have demonstrated that the way in which density dependence acts on groups at the top of the food web (via growth or mortality) has a notable effect on the scale and penetration of top-down cascades. Therefore, it is critical to understand exactly how density dependence operates in these top-predator populations to enable more effective prediction and management of marine ecosystems under pressure.