Food-Web Structure in Relation to Environmental Gradients and Predator-Prey Ratios in Tank-Bromeliad Ecosystems

Abstract

Little is known of how linkage patterns between species change along environmental gradients. The small, spatially discrete food webs inhabiting tank-bromeliads provide an excellent opportunity to analyse patterns of community diversity and food-web topology (connectance, linkage density, nestedness) in relation to key environmental variables (habitat size, detrital resource, incident radiation) and predators:prey ratios. We sampled 365 bromeliads in a wide range of understorey environments in French Guiana and used gut contents of invertebrates to draw the corresponding 365 connectance webs. At the bromeliad scale, habitat size (water volume) determined the number of species that constitute food-web nodes, the proportion of predators, and food-web topology. The number of species as well as the proportion of predators within bromeliads declined from open to forested habitats, where the volume of water collected by bromeliads was generally lower because of rainfall interception by the canopy. A core group of microorganisms and generalist detritivores remained relatively constant across environments. This suggests that (i) a highly-connected core ensures food-web stability and key ecosystem functions across environments, and (ii) larger deviations in food-web structures can be expected following disturbance if detritivores share traits that determine responses to environmental changes. While linkage density and nestedness were lower in bromeliads in the forest than in open areas, experiments are needed to confirm a trend for lower food-web stability in the understorey of primary forests.

The Multidimensional Stoichiometric Niche

“…Costa Rica is dominated by primary and secondary tropical rain forests with sparse open clearings whereas Cardoso Island corresponds to a "resting" ecoregion (i.e., coastal vegetation located on nutrient-poor sand deposits; Magnago et al, 2012). Tank bromeliads are flowering plants that accumulate rainwater in their leaf rosettes creating an aquatic habitat-from a few milliliters up to several liters of water per plant-for a diverse community of macroinvertebrates (e.g., Diptera, Coleoptera, Haplotaxida, Ostracoda; Dézerald et al, 2013). Macroinvertebrates were sampled from tank bromeliads in each study site either by dissection of the plant or with a large-mouthed pipette.…”

Dominant predators mediate the impact of habitat size on trophic structure in bromeliad invertebrate communities

“…Predators were defined as those organisms that feed on other macroscopic invertebrates, including engulfer predators as well as piercers. Although there is some intraguild predation in bromeliad invertebrate food webs (De´zerald et al 2013), this is usually incidental to the main prey of detritivorous invertebrates, so we felt justified in simplifying the food webs in our study and considering all predators as a single trophic level. All other taxa were classified as detritivores.…”

“…This is supported by our finding that detritivore per capita biomass increased along the habitat-size gradient at sites without odonates, where most of the predators were smaller-bodied species such as ceratopogonids or chironomids. In bromeliads, predatory chironomids and ceratopogonids only prey on a subset of the detritivore food web, unlike odonates (De´zerald et al 2013), and have insignificant effects on detritivore abundances (Starzomski et al 2010, LeCraw 2014. These size-class shifts are a typical response to changes in predation pressure, as, for example, shown for the response of macroinvertebrates to fish predation in pond ecosystems (Crowder and Cooper 1982).…”

Assessing the role of canopy cover on the colonization of phytotelmata by aquatic invertebrates: an experiment with the tank-bromeliad Aechmea lingulata

“…Moreover, these organic particles can be carried by the wind; therefore, FPOM input rates are not necessarily related to the presence of canopy cover (Ngai and Srivastava, 2006;Brouard et al, 2011). The amount of FPOM found in tanks of A. lingulata was independent of canopy cover and colonization times, as was observed by Dézerald et al (2013) in tank-bromeliads sampled in different vegetation types. As expected, larger amounts of CPOM were found in shaded bromeliads.…”

“…Abiotic (leaching) and biotic processes (microorganisms and invertebrate detritivores) (Graça, 2001) successively decompose this material. As in other aquatic ecosystems (Graça et al, 2015), the availability of organic detritus is a relevant factor to phytotelm communities (Walker et al, 1997;Brouard et al, 2012;Dézerald et al, 2013). Thus, both the quantity and quality of organic detritus can influence invertebrate abundance (Armbruster et al, 2002) and taxonomic richness (Kitching, 2000(Kitching, , 2001, as well as the biomass for functional feeding groups (Srivastava, 2006;Ptatscheck and Traunspurger, 2015).…”