Tag Archives: apex predators

Trophic cascades in 3D: Network analysis reveals how apex predators structure ecosystems

Authors: Arian D Wallach, Anthony H Dekker, Miguel Lurgi, Jose M Montoya, Damien A Fordham and Euan G Ritchie

Published in: Methods in Ecology and Evolution

Abstract

Trophic cascade theory predicts that apex predators structure ecosystems by regulating mesopredator and herbivore abundance and behaviour. Studies on trophic cascades have typically focused on short linear chains of species interactions. A framework that integrates more realistic and complex interactions is needed to make broader predictions on ecosystem structuring.

Network analysis is used to study food webs and other types of species interaction networks. These often comprise large numbers of species but rarely account for multiple interaction types and strengths. Here we develop an intermediate complexity theoretical framework that allows specification of multiple interaction types and strengths for the study of trophic cascades. This ecological network is designed to suit data typically derived from field-based studies. The trophic cascade network contains fewer nodes than food webs, but provides semi-weighted directional links that enable different types of interactions to be included in a single model.

We use this trophic cascade network model to explore how an apex predator shapes ecosystem structure in an Australian arid ecosystem. We compared two networks that contrasted in the dominance of an apex predator, the dingo (Canis dingo), using published results ranking the direction and strength of key interactions. Nodes and links interacted dynamically to shape these networks. We examined how changes to an apex predator population affects ecosystem structure through their direct and indirect influences on different components of this ecological community.

Under strong apex predator influence, the network structure was denser and more complex, even, and top-down driven; and dingo predation and soil commensalism formed denser interactive modules. Under weak apex predator influence (e.g. reflecting predator control) the resulting network structure was frayed, with mesopredator predation and grazing forming modules.

Our study demonstrates that networks of intermediate complexity can provide a powerful tool for elucidating potential ecosystem-wide effects of apex predators, and predicting the consequences of management interventions such as predator control. Integrating trophic cascades, with their array of complex interactions, with the three-dimensional structure of ecological networks, has the potential to reveal ‘ecological architecture’ that neither captures on its own.

Wallach AD, Dekker AH, Lurgi M, Montoya JM, Fordham DA, Ritchie EG (2016) Trophic cascades in 3D: Network analysis reveals how apex predators structure ecosystems. Methods in Ecology and EvolutionPDF DOI

Crying wolf: limitations of predator–prey studies need not preclude their salient messages

Authors: Euan G Ritchie, Jannik Schultner, Dale G Nimmo, Joern Fischer, Jan Hanspach, Tobias Kuemmerle, Laura Kehoe and Ine Dorresteijn

Published in: Proceedings of the Royal Society B, volume 283, issue 1834 (July 2016)

A rapidly growing body of the literature reveals the important roles apex predators play in shaping the composition and functioning of ecological communities worldwide.

The principal effects of apex predators — namely herbivore and mesopredator population suppression — are often evident following their removal from environments, or their reintroduction, including rewilding initiatives. What remains less clear, however, is to what extent humans versus other apex predators affect ecosystems, how both interact across gradients of anthropogenic pressure and how such interactions can be affected by underlying bottom-up processes.

Such questions are critical to answer in the Anthropocene, where effective management of ecosystems and conservation of biodiversity requires a better understanding of how top-down and bottom-up processes vary according to anthropogenic influences…

Ritchie EG, Schultner J, Nimmo DG, Fischer J, Hanspach J, Kuemmerle T, Kehoe L, Dorresteijn I (2016) Crying wolf: limitations of predator–prey studies need not preclude their salient messages, Proceedings of the Royal Society B, 283:1834 PDF DOI

Incorporating anthropogenic effects into trophic ecology: predator–prey interactions in a human-dominated landscape

Authors: Ine Dorresteijn, Jannik Schultner, Dale G Nimmo, Joern Fischer, Jan Hanspach, Tobias Kuemmerle, Laura Kehoe and Euan G Ritchie

Published in: Proceedings of the Royal Society B, volume 282 (September 2015)

Apex predators perform important functions that regulate ecosystems world- wide. However, little is known about how ecosystem regulation by predators is influenced by human activities. In particular, how important are top-down effects of predators relative to direct and indirect human-mediated bottom-up and top-down processes?

Combining data on species’ occurrence from camera traps and hunting records, we aimed to quantify the relative effects of top-down and bottom-up processes in shaping predator and prey distributions in a human-dominated landscape in Transylvania, Romania. By global standards this system is diverse, including apex predators (brown bear and wolf), mesopredators (red fox) and large herbivores (roe and red deer). Humans and free-ranging dogs represent additional predators in the system.

Using structural equation modelling, we found that apex predators suppress lower trophic levels, especially herbivores. However, direct and indirect top- down effects of humans affected the ecosystem more strongly, influencing species at all trophic levels.

Our study highlights the need to explicitly embed humans and their influences within trophic cascade theory. This will greatly expand our understanding of species interactions in human-modified landscapes, which compose the majority of the Earth’s terrestrial surface.

Dorresteijn I, Schultner J, Nimmo DG, Fischer J, Hanspach J, Kuemmerle T, Kehoe L, Ritchie EG (2015) Incorporating anthropogenic effects into trophic ecology: predator–prey interactions in a human-dominated landscape, Proceedings of the Royal Society B, 282: 20151602 PDF DOI