Mechanisms behind bottom-up effects: eutrophication increases fecundity by shortening the interspawning interval in stickleback

Anne Saarinen,Ulrika Candolin

doi:10.7717/peerj.9521

Abstract

Anthropogenic eutrophication is altering aquatic environments by promoting primary production. This influences the population dynamics of consumers through bottom-up effects, but the underlying mechanisms and pathways are not always clear. To evaluate and mitigate effects of eutrophication on ecological communities, more research is needed on the underlying factors. Here we show that anthropogenic eutrophication increases population fecundity in the threespine stickleback (Gasterosteus aculeatus) by increasing the number of times females reproduce—lifetime fecundity—rather than instantaneous fecundity. When we exposed females to nutrient-enriched waters with enhanced algal growth, their interspawning interval shortened but the size of their egg clutches, or the size of their eggs, did not change. The shortening of the interspawning interval was probably caused by higher food intake, as algae growth promotes the growth of preferred prey populations. Enhanced female lifetime fecundity could increase offspring production and, hence, influence population dynamics. In support of this, earlier studies show that more offspring are emerging in habitats with denser algae growth. Thus, our results stress the importance of considering lifetime fecundity, in addition to instantaneous fecundity, when investigating the impact of human-induced eutrophication on population processes. At a broader level, our results highlight the importance of following individuals over longer time spans when evaluating the pathways and processes through which environmental changes influence individual fitness and population processes.

Highlights

Human activities are altering habitats around the world at an unprecedented rate and scale
The increase in egg production rate was most likely caused by the increased prey availability and, higher food intake, as the abundance of prey was higher in denser algae growth
Our results show that eutrophication and increased algae growth enhances lifetime fecundity of threespine stickleback females, but not their instantaneous fecundity

Summary

Introduction

Human activities are altering habitats around the world at an unprecedented rate and scale. A growing human-induced problem in aquatic ecosystems is eutrophication, the enrichment of ecosystems with nutrients (Smith, 2003) It promotes primary production, which in turn has a range of secondary effects on ecosystems, such as the accumulation of decomposing organic matter that uses up oxygen, the promotion of toxic algal blooms, and the proliferation of ephemeral algae in coastal systems (Le Moal et al, 2019; Paerl et al, 2014). These changes cause the decline of some species, while others profit and increase in. To unravel the effects of eutrophication on species, the underlying mechanisms and pathways need to be identified and delineated

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Conclusion