Berggren, P. | bioRxiv Science

bioRxiv2024

Changing feeding levels reveal plasticity in elasmobranch life history strategies

Life history strategies are shaped by phylogeny, environmental conditions and individual energy budgets, and have implications for conservation biology. We summarised life history traits of 151 elasmobranch species into life history strategies for two contrasting feeding levels in a principal components analysis. Two axes, reproductive output and generation turnover, structure elasmobranch life history strategies, contrasting with results from similar studies. Species positions in this life history space were not fixed, but shifted to higher reproductive output when feeding level increased. We also found that both axes predicted population performance, but that population growth rate does not necessarily inform on a species demographic resilience. Finally, neither axis predicted IUCN conservation status. Our analyses reveal plasticity in species life history strategies and warn against extrapolating the life history strategy framework from one environment to another when predicting a species response to (climate) change, perturbations, and (over)exploitation.

ecology↗

bioRxiv2024

Offspring size resolves a latitudinal population growth rate paradox in rays and skates

The maximum intrinsic population growth rate, rmax, is a key determinant of the limits for sustainable fishing and is increasingly used in risk assessments. Metabolic theory suggests that rmax scales with adult body size, temperature (and hence depth) such that smaller-bodied species and those in warmer, shallower waters have greater rmax and, therefore, will be less sensitive to overexploitation. However, warm shallow-water tropical rays have lower rmax than cold deep-water temperate skates contra to the metabolic expectation. To resolve this paradox, we build from recent advances that suggest that offspring size may be key to understanding rmax. Specifically, we examine how rmax is related to adult size, offspring size, temperature, and depth across 85 ray and skate species. Our results show that offspring size mediates relationships between rmax, adult body size, temperature, and depth. Indeed, tropical rays had, on average, larger offspring and lower rmax compared to the temperate skates, despite living in warmer, shallower waters. Thus, despite the expectation from theory that tropical species should have faster life histories compared to temperate species, our result explains why tropical rays are actually less resilient. It remains unclear as to why tropical rays have such large offspring but we speculate that this is due to greater predation risk in shallow tropical waters driving the additional maternal investment in offspring size via the evolution of viviparity and matrotrophy. Our work highlights the complex relationships among life histories and the environment and may help explain global biogeographic patterns of intrinsic sensitivity to overexploitation.

ecology↗