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.
Highlights
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
The Chagos Archipelago is home to eighteen species of breeding seabirds, many of which are present in globally-significant numbers, leading to the designation of several Important Bird and Biodiversity Areas (IBAs) within the archipelago[30]
Female C. sordidus around islands with abundant seabird populations grew an estimated 34.91% faster than those around rat-infested islands with few seabirds (Fig. 1a, b; estimated k = 0.38 vs. 0.27; estimated difference in k = 0.10, 95% highest posterior density interval [highest posterior distribution intervals (HPDI)] = 0.02 to 0.20)
Summary
Cross-ecosystem nutrient subsidies may boost demographic rates of consumers in recipient ecosystems, which in turn can affect population and community dynamics. 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. These results demonstrate additional benefits, and potential trade-offs, of restoring natural nutrient pathways for recipient ecosystems. The importance of breeding seabirds in providing natural nitrogen inputs, and bolstering fish biomass, productivity, and ecosystem functioning, has been recently recognized[17,18] This includes evidence that a small species of herbivorous damselfish grows faster around islands with abundant seabird populations compared to islands with invasive rats and few s eabirds[17]. The functional significance of parrotfishes scales nonlinearly with body size[22,23,24,25], providing a direct link between individual growth rates and ecosystem function
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