Consumer trophic positions respond variably to seasonally fluctuating environments

Abstract

The effects of environmental seasonality on food web structure have been notoriously understudied in empirical ecology. Here, we focus on seasonal changes in one key attribute of a food web, consumer trophic position. We ask whether fishes inhabiting tropical river–floodplain ecosystems behave as seasonal omnivores, by shifting their trophic positions in relation to the annual flood pulse, or whether they feed at the same trophic position all year, as much empirical work implicitly assumes. Using dietary data from the Tonle Sap Lake, Cambodia, and a literature review, we find evidence that some fishes, especially small piscivores, increased consumption of invertebrates and/or plant material during the wet season, as predicted. However, nitrogen stable isotope (δ15N) data for 26 Tonle Sap fishes, spanning a broader range of functional groups, uncovered high variation in seasonal trophic position responses among species (0 to ±0.52 trophic positions). Based on these findings, species respond to the flood pulse differently. Diverse behavioral responses to seasonality, underpinned by spatiotemporal variation at multiple scales, could be central for rerouting matter and energy flow in these dynamic ecosystems. Seasonally flexible foraging behaviors warrant further study given their potential influence on food web dynamics in a range of fluctuating environments.

Fish assemblage composition within the floodplain habitat mosaic of a tropical lake (Tonle Sap, Cambodia)

“…() and McMeans et al. () as well as unpublished stable isotope data for the Tonle Sap system. Lastly, site‐level species abundances were plotted across the nine sites to explore species patterns from the exterior to the interior of the floodplain during the wet season.…”

Modelling inundation patterns and sediment dynamics in the extensive floodplain along the Tonle Sap River

“…release eggs just before or during the flood season, which allows them to migrate into the floodplain and escape predation, whereas in the dry season, the isolated water bodies in the floodplain provide shelter for them (Bayley & Li, ; Junk, Soares, & Saint‐Paul, ; Welcomme, ). Furthermore, a recent finding suggested that fish species respond to the flood pulse differently as revealed by the dietary (stomach contents) and nitrogen stable isotope ( 15 N) analysis in the fish sampled from TSL (McMeans et al, ). Although the results of this study can now provide quantitative descriptions of connectivity among hydrological elements related to fish habitat assemblage, further studies should quantitatively investigate the relationship between fish production and inundation patterns in this kind of fluctuating water environment influenced by flood pulses.…”

Monitoring of tropical freshwater fish resources for sustainable use

“…This citizen‐science reporting and observer network is complemented by a series of other survey techniques, used in combination and applied at set locations (Elliott et al, ; Gotzek & Johnson, ). Targeted ecological surveys to determine population distribution (Kong et al , ; Elliott & Chheng, ), identify spawning site locations (Elliott & Chheng, ), evaluate food‐web ecology (McMeans et al, ; Pool et al, ) and for population genetic analysis (Elliott & Uy, ), using traditional standardised nets are seasonally implemented at set locations. Both qualitative and quantitative market surveys are used to determine commercial fishing pressure (Gotzek & Johnson, ).…”

“…This citizen-science reporting and observer network is complemented by a series of other survey techniques, used in combination and applied at set locations Gotzek & Johnson, 2019). Targeted ecological surveys to determine population distribution (Kong et al, 2017;, identify spawning site locations , evaluate food-web ecology (McMeans et al, 2019;Pool et al, 2017) and for population genetic analysis FiA, 2018a;Gotzek & Johnson, 2019). To contextualise fisher catch data, to obtain estimates of consumption and to provide an independent estimate of the subsistence fishery, household surveys have also been conducted by the fisheries administration (IFReDI, 2013).…”