Measuring individuality in habitat use across complex landscapes: approaches, constraints, and implications for assessing resource specialization

Abstract

Many mobile marine species are presumed to utilize a broad spectrum of habitats, but this seemingly generalist life history may arise from conspecifics specializing on distinct habitat alternatives to exploit foraging, resting/refuge, or reproductive opportunities. We acoustically tagged 34 red drum, and mapped sand, seagrass, marsh, or oyster (across discrete landscape contexts) use by each uniquely coded individual. Using 144,000 acoustic detections, we recorded differences in habitat use among red drum: proportional use of seagrass habitat ranged from 0 to 100%, and use of oyster-bottom types also varied among fish. WIC/TNW and IS metrics (previously applied vis-à-vis diet specialization) consistently indicated that a typical red drum overlapped >70% with population-level niche exploitation. Monte Carlo permutations showed these values were lower than expected had fish drawn from a common habitat-use distribution, but longitudinal comparisons did not provide evidence of temporally consistent individuality, suggesting that differences among individuals were plastic and not reflective of true specialization. Given the range of acoustic detections we captured (from tens to 1,000s per individual), which are substantially larger sample sizes than in many diet studies, we extended our findings by serially reducing or expanding our data in simulations to evaluate sample-size effects. We found that the results of null hypothesis testing for specialization were highly dependent on sample size, with thresholds in the relationship between sample size and associated P-values. These results highlight opportunities and potential caveats in exploring individuality in habitat use. More broadly, exploring individual specialization in fine-scale habitat use suggests that, for mobile marine species, movement behaviors over shorter (≤weeks), but not longer (≥months), timescales may serve as an underlying mechanism for other forms of resource specialization.

Shifting individual habitat specialization of a successful predator living in anthropogenic landscapes

“…This result may be related to the low availability of urban dumps in the area covered by GPS-tracked individuals (Navarro et al 2016). Although all GPS-tracked yellow-legged gulls could potentially exploit all available habitats, based on PSi values -a metric to test individual specialization (Bolnick et al 2002, Fodrie et al 2015) -we found clear differences in habitat use among individuals within the population. These results are coherent with the prediction that some generalist or opportunistic species are composed of ecologically heterogeneous individuals that repeatedly differ in behaviour and use of different subsets of available resources (Bolnick et al 2002, Bearhop et al 2004, Bell et al 2009).…”

“…Individual specialization in habitat use by each tracked yellow-legged gull was quantified following Bolnick et al (2002) and Fodrie et al (2015). Specifically, we calculated the proportional habitat use by each yellow-legged gull as the number of habitatspecific positions divided by the total number of GPS positions recorded during the entire tracking period for a particular individual.…”

“…Specifically, we predicted that individuals within the population would show niche segregation with respect to habitat use. To test for such individual strategies, we applied metrics previously used to identify diet specialization (Bolnick et al 2002, Fodrie et al 2015. These metrics were applied to spatial data obtained from 18 yellowlegged gulls that were GPS-tracked simultaneously during 4 wk of the breeding season (Bouten et al 2013).…”

Intra- and inter-individual variation in the foraging ecology of a generalist subantarctic seabird, the gentoo penguin

“…We used complementary bio-logging and stable isotope analysis, coupled with morphometric measurements to: (1) describe their inter-individual variation in morphology, spatial use and dive behaviour; (2) quantify their intra-individual variation in foraging behaviour; (3) investigate the links between consistency in foraging behaviour, distances travelled and body condition; and (4) describe their interindividual variation in feeding ecology, and determine if dietary specialisations exist and are maintained outside of the breeding season. We predicted that: (1) individuals would differ greatly in foraging metrics, as gentoo penguin diet and behaviour are known to vary among colonies and between individuals of the same colonies, and that such variation would be attributed to differences in body mass, which influences dive depth (Lescroël et al 2004, Lescroël & Bost 2005, Cook et al 2013, Polito et al 2015, Camprasse et al 2017; (2) dietary and behavioural consistency would be detected, as populations usually considered generalists are increasingly shown to be composed of individual specialists (Woo et al 2008, Araújo et al 2011, Loxdale et al 2011, Layman & Allgeier 2012, Fodrie et al 2015; and (3) individuals displaying higher consistency in foraging behaviour would travel shorter distances and have higher body condition, as such consistency is thought to allow individuals to forage more efficiently (Bolnick et al 2003, Estes et al 2003.…”

“…Individual specialists have been defined as 'individuals whose niche is substantially narrower than their population's niche for reasons not attributable to their sex, age or discrete morphological group' (Bolnick et al 2003, p. 3). Even populations usually thought to be generalists can actually be composed of individual specialists, referred to as Type 'B' generalists (individuals each specialising on a different but narrow range of food types) as op posed to Type 'A' generalists (individuals all taking a wide range of food types) (Araújo et al 2011, Loxdale et al 2011, Layman & Allgeier 2012, Fodrie et al 2015.…”