Productivity, disturbance and ecosystem size have no influence on food chain length in seasonally connected rivers.

Danielle M Warfe,Stuart E Bunn,Neil E Pettit,Stephen K Hamilton,Bradley J Pusey,Michael M Douglas,Timothy D Jardine,Peter M Davies,Nicolas Mouquet

doi:10.1371/journal.pone.0066240

Abstract

The food web is one of the oldest and most central organising concepts in ecology and for decades, food chain length has been hypothesised to be controlled by productivity, disturbance, and/or ecosystem size; each of which may be mediated by the functional trophic role of the top predator. We characterised aquatic food webs using carbon and nitrogen stable isotopes from 66 river and floodplain sites across the wet-dry tropics of northern Australia to determine the relative importance of productivity (indicated by nutrient concentrations), disturbance (indicated by hydrological isolation) and ecosystem size, and how they may be affected by food web architecture. We show that variation in food chain length was unrelated to these classic environmental determinants, and unrelated to the trophic role of the top predator. This finding is a striking exception to the literature and is the first published example of food chain length being unaffected by any of these determinants. We suggest the distinctive seasonal hydrology of northern Australia allows the movement of fish predators, linking isolated food webs and potentially creating a regional food web that overrides local effects of productivity, disturbance and ecosystem size. This finding supports ecological theory suggesting that mobile consumers promote more stable food webs. It also illustrates how food webs, and energy transfer, may function in the absence of the human modifications to landscape hydrological connectivity that are ubiquitous in more populated regions.

Highlights

The food web is a central organizing theme in ecology, depicting the feeding relationships between species in a community [1,2] and providing a framework for understanding energy transfer and biogeochemical processes [3], biodiversity and trophic interactions [4], consumer behaviour and movement [5,6], and community stability and persistence in the face of perturbation [2,7,8]
Contrary to our expectations and the well-established patterns in the literature [25] (Table 1), we found none of the classic environmental determinants had any relationship with food chain length (FCL), nor was FCL related to the trophic role of the top predator
Food chain length in our food webs averaged 4.560.6, which does not support previous predictions of short food chains in these wet-dry tropical rivers [31], but is still within the range observed in aquatic systems elsewhere [44,64]

Summary

Introduction

The food web is a central organizing theme in ecology, depicting the feeding relationships between species in a community [1,2] and providing a framework for understanding energy transfer and biogeochemical processes [3], biodiversity and trophic interactions [4], consumer behaviour and movement [5,6], and community stability and persistence in the face of perturbation [2,7,8]. Food chain length influences structural attributes of communities such as species diversity, trophic interactions and predator abundance [10,11], as well as functional attributes such as population stability, primary and secondary production, material cycling, and contaminant bioaccumulation [1,12,13,14]. The ecosystem size hypothesis [20] predicts that larger ecosystems will have longer food chains because they support greater species richness [21], support more basal resources [22], promote coexistence of predators and prey [15,23], promote population persistence through enhanced colonisation opportunity [11,23], and/or support greater functional trophic diversity and less omnivory [20]

Methods

Results

Conclusion