American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Trophic position of consumers and size structure of food webs across aquatic and terrestrial ecosystems”

Posted on

Anton Potapov, Ulrich Brose, Stefan Scheu, and Alexei Tiunov (Dec 2019)

Read the Article (Just Accepted)

Do large organisms occupy higher trophic levels? Size matters in water, but on land top predators can be of any size

A soil mite (Laelapidae, about 1&nbsp;mm long) successfully caught a springtail. He is “white shark” in his small world.<br />(Photo credit: Andy Murray, <a href="https://www.chaosofdelight.org/">https://www.chaosofdelight.org/</a>)
A soil mite (Laelapidae, about 1 mm long) successfully caught a springtail. He is “white shark” in his small world.
(Photo credit: Andy Murray, https://www.chaosofdelight.org/)

Thinking about a “superpredator” we would imagine a white shark, or a tiger. As humans, we intuitively expect large predators to occupy top trophic position in food webs. This is true for marine food webs, that have been long recognized as size-structured – they are based on unicellular phytoplankton, that is being eaten by zooplankton, which is further being eaten by fish and mammals. Thus, organic matter is transferred all the way from unicellular producers to large predators, such as white sharks. On the other hand, terrestrial ecosystems are dominated by multicellular vascular plants, which can be eaten by both small animals, such as leafhoppers, and large animals, such as megaherbivores. In this case, organic matter is unlikely to be transferred from small herbivores to large predators; in other words, tigers do not hunt leafhoppers, or animals that feed on leafhoppers. This straightforward idea appears from theory, but has never been tested empirically across invertebrate and vertebrate consumers and across ecosystems.

A group of researchers from Germany (University of Göttingen and German Centre for Integrative Biodiversity Research) and Russia (Institute of Ecology and Evolution) leaded by Anton Potapov compiled published data on stable isotope composition and sizes of consumers, from microscopic fauna to whales, from marine, freshwater and terrestrial ecosystems and both grazing and detrital food webs. The results show that size matters in water, but not on land – trophic position in terrestrial consumers (including those belowground) does not depend on body size. Namely, a small mite living in soil may be positioned higher in a food web than a large mammalian predator. As predators are systematically larger than their prey, results of the study suggest that small and large consumers on land (and partly in fresh water) belong to different food webs, or “size compartments”. If we want to predict responses of natural interaction networks to perturbations, we need to know that these responses will be intrinsically different in size-compartmentalized terrestrial and size-structured marine food webs.


Abstract

Do large organisms occupy higher trophic levels? Predators are often larger than their prey in food chains, but empirical evidence for positive body mass – trophic level scaling for entire food webs mostly comes from marine communities based on unicellular producers. Using published data on stable isotope compositions of 1093 consumer species, we explored how trophic level scales with body size, food-web type (green vs. brown) and phylogenetic group across biomes. In contrast to widespread assumptions, the relationship between body size and trophic level of consumers, from protists to large vertebrates, was not significant per se, but varied among ecosystem types and animal groups. The correlation between body size and trophic level was strong in marine, weak in freshwater and absent in terrestrial consumers, which was observed also at the scale of local food webs. Vertebrates occupied higher trophic positions than invertebrates and green trophic chains were longer than brown ones in aquatic (primarily marine) but not in terrestrial food webs. Variations in body size of top predators suggest that terrestrial and many freshwater food webs are size-compartmentalized, implying different trophic dynamics and responses to perturbations than in size-structured marine food webs.