Natural nutrient subsidies alter demographic rates in a functionally important coral-reef fish

Abstract

By improving resource quality, cross-ecosystem nutrient subsidies may boost demographic rates of consumers in recipient ecosystems, which in turn can affect population and community dynamics. However, empirical studies on how nutrient subsidies simultaneously affect multiple demographic rates are lacking, in part because humans have disrupted the majority of these natural flows. Here, we compare the demographics of a sex-changing parrotfish (Chlorurus sordidus) between reefs where cross-ecosystem nutrients provided by seabirds are available versus nearby reefs where invasive, predatory rats have removed seabird populations. For this functionally important species, we found evidence for a trade-off between investing in growth and fecundity, with parrotfish around rat-free islands with many seabirds exhibiting 35% faster growth, but 21% lower size-based fecundity, than those around rat-infested islands with few seabirds. Although there were no concurrent differences in population-level density or biomass, overall mean body size was 16% larger around rat-free islands. Because the functional significance of parrotfish as grazers and bioeroders increases non-linearly with size, the increased growth rates and body sizes around rat-free islands likely contributes to higher ecosystem function on coral reefs that receive natural nutrient subsidies. More broadly, these results demonstrate additional benefits, and potential trade-offs, of restoring natural nutrient pathways for recipient ecosystems.

Seabird diversity and biomass enhance cross-ecosystem nutrient subsidies

“…For example, plankton is an important source of energy and nutrition on many coral reefs, and planktivores can drive patterns in overall fish assemblages due to their high relative abundance [55,56]. Overall, the strong positive effect of seabird biomass on fish biomass is consistent with previous work demonstrating higher fish growth, productivity and biomass around rat-free islands with abundant seabirds compared to rat-infested islands with few seabirds [20,21,57,58].…”

The movement ecology of fishes

“…Population‐level processes can be exaggerated with important implications for species distribution, ecosystem dynamics, habitat availability and species conservation, to name a few. The redistribution of large numbers of individuals in space and en masse can lead to stark shifts or pulses in nutrient supply (Allgeier et al ., 2017), changing the surrounding environment by increasing abiotic effects, in turn leading to substantial repercussions for ecosystem functioning (Benkwitt et al ., 2021). Schooling behaviour can also influence population demographics through the generation of inter‐ and intraspecific interactions.…”

Terrestrial invasive species alter marine vertebrate behaviour

“…Invasive species, including black rats (Rattus rattus), disrupt this nutrient pathway by driving declines in seabird densities via predation 23 . Nutrient subsidies from seabirds flowing onto coral reef ecosystems result in higher nitrogen stable isotope quantities (δ 15 N) in algae and fish 24,25 , enhanced coral growth 26 , higher reef fish biomass 27 and faster growth rate in herbivorous fishes 22,28 . Furthermore, the presence of invasive rats negatively impacts reef fish diversity and ecosystem function on adjacent coral reefs 29 .…”