Drought alters the structure and functioning of complex food webs

Extreme Climatic Events Alter Aquatic Food Webs

“…Perhaps one surprising consequence of both floods and droughts is an apparent tendency for biotic interactions to intensify, at least in the initial stages, as consumers and resources are concentrated into ever smaller refugia (cf. Lancaster, 1996;Ledger et al, 2013). This should (in theory) lead to increased network connectance and reduced stability of the food web, as predators exert stronger per capita effects on prey concentrating in refugia.…”

“…The percentage of each food type for an individual was then calculated from five fields of view and expressed as a percentage of the total food particle area (Ledger et al, 2002). Quantitative food webs were compared using metrics derived from information theory (Bersier et al, 2002;Ledger et al, 2013;Tylianakis et al, 2007). For each web, we determined the quantified, weighted measures of linkage density (LD q ), interaction diversity (ID q ), interaction evenness (IE q ), generality (G q , mean number of resources per consumer) and vulnerability (V q , mean number of consumers per resource) (see Bersier et al, 2002 B978-0-12-417199-2.00006-9, 00006 AECR, 978-0-12-417199-2 to each species in the food web, and the diversity of biomass flows derived from the resource (H N , the diversity of inflows) and going to the consumers (H P ) of each taxon k was calculated as:…”

“…Secondary production (mg m À2 year À1 ) of all invertebrates was calculated using the size-frequency method (Hynes and Coleman, 1968), excepting rare taxa <1% total numbers where production was estimated by multiplying mean annual biomass by an annual P/B value of the most closely related taxon (see Ledger et al, 2011Ledger et al, , 2013. Production was estimated for each replicate control and treatment channel.…”

“…s0055 2.5. Food web construction: Quantitative webs p0490 The food web of each mesocosm community was quantified, with links expressed as flows of biomass from resources to consumers (see Ledger et al, 2013). The trophic basis of production method (Benke, 2012;Benke & Wallace, 1997) was used to quantify directly observed feeding links, with biomass flux (F ij , mg m À2 year À1 ) from resource i to consumer j estimated as follows.…”

“…Following this colonisation period, an intermittent flow regime of substratum drying and wetting (repeating cycles of mean 6 dry days [i.e. flow cessation] followed by mean 27 wet days) was applied to one mesocosm in each block, mimicking supra-seasonal drought that causes repeated patchy dewatering of river bed sediments over prolonged periods (see Ledger et al, 2013). These events are expected to become more frequent globally under most of the Intergovernmental Panel on Climate Change (IPCC) future scenarios (Beniston et al, 2007;IPCC, 2007).…”