Sex‐specific effects of fisheries and climate on the demography of sexually dimorphic seabirds

Abstract

Many animal taxa exhibit sex‐specific variation in ecological traits, such as foraging and distribution. These differences could result in sex‐specific responses to change, but such demographic effects are poorly understood. Here, we test for sex‐specific differences in the demography of northern (NGP, Macronectes halli) and southern (SGP, M. giganteus) giant petrels – strongly sexually size‐dimorphic birds that breed sympatrically at South Georgia, South Atlantic Ocean. Both species feed at sea or on carrion on land, but larger males (30% heavier) are more reliant on terrestrial foraging than the more pelagic females. Using multi‐event mark‐recapture models, we examine the impacts of long‐term changes in environmental conditions and commercial fishing on annual adult survival and use two‐sex matrix population models to forecast future trends. As expected, survival of male NGP was positively affected by carrion availability, but negatively affected by zonal winds. Female survival was positively affected by meridional winds and El Niño–Southern Oscillation (ENSO), and negatively affected by sea ice concentration and pelagic longline effort. Survival of SGPs did not differ between sexes; however, survival of males only was positively correlated with the Southern Annular Mode (SAM). Two‐sex population projections indicate that future environmental conditions are likely to benefit giant petrels. However, any potential increase in pelagic longline fisheries could reduce female survival and population growth. Our study reveals that sex‐specific ecological differences can lead to divergent responses to environmental drivers (i.e. climate and fisheries). Moreover, because such effects may not be apparent when all individuals are considered together, ignoring sex differences could underestimate the relative influence of a changing environment on demography.

What lies beneath? Population dynamics conceal pace‐of‐life and sex ratio variation, with implications for resilience to environmental change

“…Our study exemplifies why it is important to consider the demographic composition of populations rather than simply population‐wide metrics when estimating population resilience (Isaac, 2009; Keith et al, 2008). Various studies report that climate change can affect the sexes differently (Gianuca et al, 2019; Petry et al, 2016), in ways likely to impact the plasticity and resilience of populations (Le Galliard, Fitze, Ferrière, & Clobert, 2005; Miller & Inouye, 2013). Particularly, where climate affects the distribution of fixed individual heterogeneity in populations, it may carry unforeseen consequences for population resilience and persistence (Jenouvrier, Péron, & Weimerskirch, 2015).…”

Breeding stage, not sex, affects foraging characteristics in masked boobies at Rapa Nui

“…By tracking simultaneously incubating and chick-rearing individuals, one can rule out that changes in temporal prey availability created the differences in foraging parameters between birds, and thus, test for sexual segregation in foraging while facing the constraints due to chick provisioning. Understanding the differences between sexes during different stages of breeding is important, because dissimilarities in fitness or survival rates caused by differential exposure of males and females to different conditions or threats may lead to an imbalanced sex ratio, with population dynamic consequences (Phillips et al 2005;Harrison et al 2011;García-Tarrasón et al 2015;Gianuca et al 2019). Furthermore, understanding the persistence of foraging behaviors among members of a population improves the ability to manage areas in relation to seabird life stages (Oppel et al 2018).…”

Foraging behaviour and habitat-use drives niche segregation in sibling seabird species

“…Variation in foraging patterns among giant petrels breeding at Marion Island exposes species and sexes to different threats, which should be investigated in more detail (e.g. [ 40 , 86 ]) and potentially considered in their conservation and management. Future work should also address the role of interannual variation in resource availability on segregation patterns, particularly in males ashore.…”