Integrated trophic position decreases in more diverse communities of stream food webs

Naoto F Ishikawa,Naohiko Ohkouchi,Tomoya Iwata,Jun-Ichi Okano,Noboru Okuda,Aya R Murakami,Michio Kondoh,Hiroyuki Togashi,Ichiro Tayasu,Yoshito Chikaraishi,Yoichiro Sakai

doi:10.1038/s41598-017-02155-8

Abstract

The relationship between biodiversity and ecosystem functioning is an important theme in environmental sciences. We propose a new index for configuration of the biomass pyramid in an ecosystem, named integrated trophic position (iTP). The iTP is defined as a sum of trophic positions (i.e. the average number of steps involved in biomass transfer) of all the animals in a food web integrated by their individual biomass. The observed iTP for stream macroinvertebrates ranged from 2.39 to 2.79 and was negatively correlated with the species density and the Shannon–Wiener diversity index of the local community. The results indicate a lower efficiency of biomass transfer in more diverse communities, which may be explained by the variance in edibility hypothesis and/or the trophic omnivory hypothesis. We found a negative effect of biodiversity on ecosystem functioning.

Highlights

The transfer of biomass via prey-predator interactions in food webs is a fundamental ecosystem process[1]
Few studies have holistically examined the mean trophic position (TP) of all animals in food webs, with the exception of a study by Williams & Martinez (2004), which reconstructed a trophic network to estimate the average TP for a whole community by using a large dataset based on gut contents[15]
We propose a new index of food web properties, the integrated trophic position, which we defined as the summed TPs of all animals in a food web integrated by their individual biomass

Summary

Introduction

The transfer of biomass via prey-predator interactions in food webs is a fundamental ecosystem process[1]. A higher efficiency of trophic transfer implies that more agricultural, livestock, and marine animal products are available per unit basal production, and such ecosystem functioning and service are critical for humanity[2] As both the theoretical and empirical evidence suggest that primary productivity is sensitive to primary producers’ biodiversity[3,4,5], the amount of biomass transferred across food webs is likely to be influenced by the diversity of preys and predators[6, 7]. Because the MTI did not consider non-fisheries species[14], the index was biased toward commercially important and large predatory fish It appears that the MTI was unable to represent the biomass flow in a whole community. The CSIA-AA method only needs the δ15N value for a focal animal and no source data, so it is independent of the δ15N fluctuations in basal resources, thereby leading to much smaller errors in the TP estimates compared to bulk δ15N analysis[22]

Methods

Results

Conclusion