Food web changes associated with drought and invasive species in a tropical semiarid reservoir

Abstract

Fish and invertebrates are introduced in freshwaters around the world for commercial purposes, despite widely known impacts on food webs and biological invasions. As a proxy for artificial environments, we modeled a typical reservoir in a Brazilian semiarid region using an ecosystem approach. We compared the role of native and nonnative invasive species (NIS) in the food web, between dry and wet periods, and under the influence of an extreme drought period (from 2011 to 2015), simulating the variation in fish biomasses due to decreasing consumption. Key ecosystem groups were fishes (mainly NIS), birds, and insects. Nutrient cycling was dependent on invaders, while the trophic structure was detritus based during the drought. Biomass of detritivores was almost two times higher than herbivores, and native fish species decreased abruptly in response to invaders and volume variation. The dominance of low-trophic levels (TLII) and tilapia-Oreochromis niloticus (Linnaeus, 1758) and other tilapiines-resulted from interactions among invaders, feeding behavior on benthos, and environmental seasonality, tending toward biotic homogenization (''benthification'') at the ecosystem level. An increasing relevance of detritivores with cascading effects in ecosystems subject to drought, multiple

Benthification, biotic homogenization behind the trophic downgrading in altered ecosystems

“…Beyond serving as prey, intermediate consumers such as tilapia and the native species G. brasiliensis, H. derbyi, and C. paleatus stimulate nitrogen cycling in the food web of reservoirs by the foraging on benthos (Lowe-McConnell 2000, Canonico et al 2005, Bezerra et al 2018. Top predators had massive nitrogen accumulation rates and, consequently, higher TLs than adults of other top predator native species, such as H. malabaricus and R. quelen.…”

“…Indeed, tilapia foster detrital cycling by consuming mosquitoes (larval phase, probably their preferred prey in artificial reservoirs) and a variety of other detritivores, much more than other compartments (Starling et al 2002). It was observed in less diverse regions under drought effects, in which the available biomass of invaders in intermediate TLs was dominated by tilapia (Bezerra et al 2018). Then, benthification patterns could be expanded to the cooccurrence of tilapia (or another invader with bottom-feeding habits) and other non-native predators such as Cichla monoculus Agassiz, 1831 interacting with negative effects to native species, such as Oligosarcus longirostris Menezes & G ery, 1983 (Fig.…”

“…Along with M. salmoides, adverse ecological effects due to the introduction of tilapiine fishes have been reported throughout the world (Deines et al 2016). Under drought effects, the consumption of apparently cul-de-sac species by tilapia is huge (Bezerra et al 2018). Under drought effects, the consumption of apparently cul-de-sac species by tilapia is huge (Bezerra et al 2018).…”

“…To improve the realism in our model, we included birds, known to make up part of the food web, from an inventory in the S ıtios Novos reservoir, CE, Brazil (Bezerra et al 2018). The geographic distance to the study area did not influence our simulation since we aimed to represent trophic interactions, and because all the included birds are common in Neotropical areas (Birdlife international initiative, datazone.birdlife.org/home).…”

“…Circles indicate biomasses, connect lines follow the diet matrix, and warming colors indicate interaction strengthening (adapted fromBezerra et al 2018). Less representative groups: Other birds, Other Insectivorous, Phalacrocorax brasilianus, and Rhamdia quelen were omitted.…”