Fish extinctions alter nutrient recycling in tropical freshwaters

Abstract

There is increasing evidence that species extinctions jeopardize the functioning of ecosystems. Overfishing and other human influences are reducing the diversity and abundance of fish worldwide, but the ecosystem-level consequences of these changes have not been assessed quantitatively. Recycling of nutrients is one important ecosystem process that is directly influenced by fish. Fish species vary widely in the rates at which they excrete nitrogen and phosphorus; thus, altering fish communities could affect nutrient recycling. Here, we use extensive field data on nutrient recycling rates and population sizes of fish species in a Neotropical river and Lake Tanganyika, Africa, to evaluate the effects of simulated extinctions on nutrient recycling. In both of these species-rich ecosystems, recycling was dominated by relatively few species, but contributions of individual species differed between nitrogen and phosphorus. Alternative extinction scenarios produced widely divergent patterns. Loss of the species targeted by fishermen led to faster declines in nutrient recycling than extinctions in order of rarity, body size, or trophic position. However, when surviving species were allowed to increase after extinctions, these compensatory responses had strong moderating effects even after losing many species. Our results underscore the complexity of predicting the consequences of extinctions from species-rich animal communities. Nevertheless, the importance of exploited species in nutrient recycling suggests that overfishing could have particularly detrimental effects on ecosystem functioning. biodiversity ͉ cichlid ͉ nutrient cycling ͉ stoichiometry ͉ species identity U nderstanding the consequences of species extinctions for ecosystem functioning is a critical challenge. There is substantial evidence that declining species richness alters ecosystem processes in experimental systems with simple spatial and trophic structure (1), but this relationship remains poorly understood in species-rich, natural ecosystems (2). The large size, high mobility, and complex trophic relationships of vertebrate species make assessing the potential consequences of their extinctions particularly challenging.Fish are the most species-rich group of vertebrates, and their diversity is threatened worldwide by overfishing, species introductions, and other factors (3-6). Although the collective influence of fish on food web structure (7,8), nutrient cycling (9, 10), and primary productivity (11) is well documented, the ecosystem-level effects of eroding fish species richness are unclear. Tropical freshwater fish are of special concern because they represent Ͼ10% of all vertebrate species (12, 13) and support Ϸ72% of global fish harvests from inland waters (14). These fisheries provide vital animal protein for hundreds of millions of people in developing countries and benefit terrestrial conservation efforts by alleviating demand for bush meat (15).Nutrient recycling offers an ideal quantitative basis for directly linking fish species and ecosys...

Light, nutrients, and food-chain length constrain planktonic energy transfer efficiency across multiple trophic levels

“…Thus, understanding how light and nutrient supplies mediate FCE may help explain some of the observed variation in the relationship between primary produc- tion and fishery yields (29). Conversely, overfishing may affect functional food-chain length (2,30,31) possibly shifting food chains between odd-and even-number systems, with additional implications for FCE. Given the manifold importance of FCE, it is imperative that future studies elucidate the particular mechanisms by which it is mediated by light, nutrients, and food-chain length, especially in systems where ecosystem services depend on this efficiency.…”

Introduced lake trout alter nitrogen cycling beyond Yellowstone Lake

“…Thus, after the introduction of lake trout in Yellowstone Lake, nutrient fluxes from cutthroat trout were greatly reduced in tributary streams. McIntyre et al (2007) simulated how the random and nonrandom loss of species can reduce nutrient cycling. Here we show a prediction from McIntyre et al (2007), whereby the near extinction of a dominant species can reduce rates of nutrient cycling in an ecosystem.…”

“…McIntyre et al (2007) simulated how the random and nonrandom loss of species can reduce nutrient cycling. Here we show a prediction from McIntyre et al (2007), whereby the near extinction of a dominant species can reduce rates of nutrient cycling in an ecosystem. Reduced animal density or extinctions from invasive animal introductions can strongly reduce excretion fluxes from native animals.…”

Disentangling the influences of mean body size and size structure on ecosystem functioning: an example of nutrient recycling by a non‐native crayfish

“…Species extinctions ordered by maximum body size had a larger effect on nutrient recycling than random extinction scenarios when population energy was conserved (McIntyre et al. 2007). Likewise, Hall et al.…”