Global patterns of invertebrate nutrient content in relation to temperature and latitude
Ecologists have long been interested in latitudinal diversity patterns because these reveal the influence of climate and geology on biodiversity. One clear link between latitudinal variation and biodiversity is the spatial variability of the chemical content of living organisms. Despite the valuable insights provided by studies focused on primary producers, to date, we know relatively little about large-scale patterns of variation in the stoichiometry of upper trophic levels, although higher trophic levels play a key role in ecosystem structure and functioning. Given the key role of nitrogen (N) and phosphorus (P) in the metabolism of organisms, and these nutrients influence over growth and development, we also looked for a relationship between invertebrate N and P content. We used a global dataset, consisting of more than 2,800 observations of invertebrate N and P content for about 1,000 species from terrestrial and aquatic habitats. We explored global invertebrate stoichiometry to ask: (i) what are the broad latitudinal patterns of invertebrate nutrient content; and (ii) what is the scaling of invertebrate nitrogen to phosphorus within and across taxonomical groups and ecosystems.
Invertebrate N increases toward the equator as mean temperature increase, and P decreases toward the tropics as mean temperature decreases. These patterns in N content are consistent for different trophic levels: predators, herbivores and detritivores; however, P content showed contrasting results, with a decrease toward the Equator for predators, and an increase at low latitudes for herbivores and detritivores. The percentage of variance explained by latitude and mean annual temperature is small (i.e., less than 1%), suggesting weak latitudinal patterns in invertebrate N and P content. The analysis of the data grouped by terrestrial or aquatic habitat showed similar patterns of variation, but N to P ratios had a strong negative correlation with latitude in both freshwater and terrestrial habitats. Neither trophic groups nor habitats had significantly correlated N and P values. Our results support the hypotheses that: (i) invertebrates tend to show strong conservatism in their body nutrient content, and (ii) the N to P ratio of invertebrates increases with higher mean temperature and decrease with latitude. This is probably due to P limitation in tropical soils, and N limitation in temperate soils.