Behavioral signature of intraspecific competition and density dependence in colony‐breeding marine predators

Abstract

In populations of colony-breeding marine animals, foraging around colonies can lead to intraspecific competition. This competition affects individual foraging behavior and can cause density-dependent population growth. Where behavioral data are available, it may be possible to infer the mechanism of intraspecific competition. If these mechanics are understood, they can be used to predict the population-level functional response resulting from the competition. Using satellite relocation and dive data, we studied the use of space and foraging behavior of juvenile and adult gray seals (Halichoerus grypus) from a large (over 200,000) and growing population breeding at Sable Island, Nova Scotia (44.0 oN 60.0 oW). These data were first analyzed using a behaviorally switching state-space model to infer foraging areas followed by randomization analysis of foraging region overlap of competing age classes. Patterns of habitat use and behavioral time budgets indicate that young-of-year juveniles (YOY) were likely displaced from foraging areas near (<10 km) the breeding colony by adult females. This displacement was most pronounced in the summer. Additionally, our data suggest that YOY are less capable divers than adults and this limits the habitat available to them. However, other segregating mechanisms cannot be ruled out, and we discuss several alternate hypotheses. Mark–resight data indicate juveniles born between 1998 and 2002 have much reduced survivorship compared with cohorts born in the late 1980s, while adult survivorship has remained steady. Combined with behavioral observations, our data suggest YOY are losing an intraspecific competition between adults and juveniles, resulting in the currently observed decelerating logistic population growth. Competition theory predicts that intraspecific competition resulting in a clear losing competitor should cause compensatory population regulation. This functional response produces a smooth logistic growth curve as carrying capacity is approached, and is consistent with census data collected from this population over the past 50 years. The competitive mechanism causing compensatory regulation likely stems from the capital-breeding life-history strategy employed by gray seals. This strategy decouples reproductive success from resources available around breeding colonies and prevents females from competing with each other while young are dependent.

Invasive lionfish increase activity and foraging movements at greater local densities

“…If this is the case, then only certain individuals may change their foraging locations and behavior at higher levels of competition (Webster 2004, Breed et al 2013. In the present study, the behaviors of randomly selected individual lionfish were fairly consistent with the initial behaviors of all lionfish on the reefs, which suggests that the effects of competition may be equally experienced by all individuals.…”

“…Hunting at more periods of the day could in turn cause prey species with various diurnal patterns to be differentially susceptible to predation. In addition, predators at higher densities may expand their foraging range, which could enable them to exploit resources that have not yet been depleted and/or escape aggressive interactions in the areas with high densities of conspecifics (Micheli 1997, Forrester et al 2006, Breed et al 2013. If predators forage over broader distances, then prey species that inhabit the newly exploited habitats may be consumed.…”

Individual variation in seasonal movements and foraging strategies of a land-locked, ice-breeding pinniped

“…between Galapagos fur seals Arctocephalus galapagoensis and sea lions Zalophus wollebaeki (Villegas-Amtmann et al 2013), grey seals and harbour seals (Svensson 2012) and, within species, between juveniles and adults (Crawford et al 2012, Breed et al 2013) and mature males and females (Andersen et al 2013). However, it is less commonly reported among adults of the same sex within species (Kernaléguen et al 2015), as here.…”

Taking animal tracking to new depths: synthesizing horizontal–vertical movement relationships for four marine predators

“…Direct measures of feeding (e.g., via stomach temperature sensors, jaw gapes, head jerks, and other methods) are possible, yet long-term data sets are relatively rare (Bestley et al 2010, Naito et al 2013, so inference of feeding usually relies on ascribing the foraging components from either or both of the diving and track data. Telemetry-based studies to investigate how changes in movement behavior are related in the horizontal and vertical dimensions typically follow a three-step process: (1) filtering the horizontal locations, which can contain significant inherent error; (2) discriminating the foraging components using either process-based (e.g., hidden Markov models; Patterson et al 2009) or more heuristic methods (e.g., first passage time; Fauchald andTveraa 2003, Gurarie et al 2009); (3) using statistical inference to link the vertical and horizontal dimensions; Breed et al 2013). However, the complex individual-based time series which telemetry data comprise are driving ever more sophisticated analytical efforts that unify these steps .…”