Unravelling the complex structure of forest soil food webs: higher omnivory and more trophic levels

Abstract

Food web topologies depict the community structure as distributions of feeding interactions across populations. Although the soil ecosystem provides important functions for aboveground ecosystems, data on complex soil food webs is notoriously scarce, most likely due to the difficulty of sampling and characterizing the system. To fill this gap we assembled the complex food webs of 48 forest soil communities. The food webs comprise 89 to 168 taxa and 729 to 3344 feeding interactions. The feeding links were established by combining several molecular methods (stable isotope, fatty acid and molecular gut content analyses) with feeding trials and literature data. First, we addressed whether soil food webs (n = 48) differ significantly from those of other ecosystem types (aquatic and terrestrial aboveground, n = 77) by comparing 22 food web parameters. We found that our soil food webs are characterized by many omnivorous and cannibalistic species, more trophic chains and intraguild‐predation motifs than other food webs and high average and maximum trophic levels. Despite this, we also found that soil food webs have a similar connectance as other ecosystems, but interestingly a higher link density and clustering coefficient. These differences in network structure to other ecosystem types may be a result of ecosystem specific constraints on hunting and feeding characteristics of the species that emerge as network parameters at the food‐web level. In a second analysis of land‐use effects, we found significant but only small differences of soil food web structure between different beech and coniferous forest types, which may be explained by generally strong selection effects of the soil that are independent of human land use. Overall, our study has unravelled some systematic structures of soil food‐webs, which extends our mechanistic understanding how environmental characteristics of the soil ecosystem determine patterns at the community level.

Biodiversity and ecosystem functioning in dynamic landscapes

“…Superficially, this seems straightforward, because the general conclusion that biodiversity in a trophic group increases the efficiency of resource capture of this group seems to hold across trophic levels [1,3,9,12]. In real-world ecosystems, however, the clear separation between consumer and prey biodiversity effects is blurred by the complex structure of food webs [33]. While the group of autotrophs with their mineral resources are well defined, consumers of higher trophic levels often distribute their feeding interactions across resources of multiple trophic levels [24,33].…”

“…In real-world ecosystems, however, the clear separation between consumer and prey biodiversity effects is blurred by the complex structure of food webs [33]. While the group of autotrophs with their mineral resources are well defined, consumers of higher trophic levels often distribute their feeding interactions across resources of multiple trophic levels [24,33]. Hence, the dynamics of resource and consumer populations comprise feedback mechanisms via intraguild predation links that alter the functional consequences of biodiversity change [9,34,35].…”

The use of DNA barcodes in food web construction—terrestrial and aquatic ecologists unite!

“…A specific methodological challenge emerges for generalist insectivores where individual gut contents may contain many different prey items (with solutions offered by, e.g., Vesterinen et al 2013Vesterinen et al , 2016Pinol et al 2014;Kruger et al 2014;Paula et al 2015). When it comes to uncovering who feeds on whom, DNA-based techniques offer a clear-cut advantage: the potential for studying feeding relations among species too small (such as soil fauna: Digel et al 2014), too secretive (e.g., bats: Clare et al 2009Clare et al , 2014aClare et al , 2014bClare et al , 2014cVesterinen et al 2013Vesterinen et al , 2016Emrich et al 2014;lemmings: Soininen et al 2009), or too wide-roaming ) for their feeding habits to be followed by direct means. Targeting links among host-parasitoid species hard to distinguish in a region hard to access-the tropical rainforest of Papua New Guinea- Hrček et al (2011) were able to detect 93 previously unknown trophic links between 37 host species from Lepidoptera and 46 parasitoid species from Hymenoptera and Diptera.…”

Prohibition rules for three-node substructures in ordered food webs with cannibalistic species

“…Nodes in food webs often represent average individuals of a population (Digel et al 2014). This means that a node summarises the feeding relations of individuals with different body masses, e.g.…”

“…To determine whether the substructures that are prohibited by the two niche model variants occur in empirical food webs, we evaluated 63 food webs from different habitat types that we obtained from a large foodweb database (Riede et al 2010;Digel et al 2014). Only food webs for which body masses of all species were available were used.…”