Consumption and performance responses of the amphipod Echinogammarus berilloni change during laboratory incubation

Abstract

Microcosms try to mimic the behaviour of natural ecosystems; however, differences in experimental conditions reduce comparability among studies. A standard incubation duration may increase comparability between experiments without losing realism in the response. Some response variables can respond fast, acting as early warning signals of other, more important oncoming changes. In this experiment, we test the effects of resource quality and incubation duration on the performance of the amphipod Echinogammarus berilloni. Individuals were fed on five leaf species of contrasting quality and we measured their consumption rate, death rate, RNA:body mass (RNA:BM), growth rate, mass body condition and lipid body condition over time. We predicted that (i) resource quality would affect the response variables, (ii) consumption rate and RNA:BM ratio would act as early warning signals of oncoming changes in growth or death rates, and (iii) the inter-individual variation would gradually decrease with time. Resource quality was positively related to consumption rates, although it did not correlate to nutrient concentration and toughness of the materials. Amphipod body mass condition changed with diet, animals feeding on oak and beech showing the lowest values. Death rate, growth rate, RNA:BM, and lipid mass condition did not change with food resources. Consumption, growth rate and mass body condition changed with time. Moreover, consumption, mass body condition and RNA:BM significantly interacted with incubation duration. Variability among individuals in consumption and growth rate decreased with time, as predicted. Our results pointed that special care should be taken when comparing microcosm experiments with different incubation duration.