Tag Archives: trophic cascade

The case for a dingo reintroduction in Australia remains strong: a reply to Morgan et al., 2016

Authors: Thomas M Newsome, Aaron C Greenville, Mike Letnic, Euan G Ritchie and Christopher R Dickman

Published in: Food Webs (early view)

dingofootprint

We challenge the arguments of Morgan et al. in regard to the efficacy of dingo reintroductions Image credit: Daryll Bellingham via Flickr

In their paper “Trophic cascades and dingoes in Australia: does the Yellowstone wolf-elk- willow model apply?” Morgan et al. (2016) argue that the case for dingo reintroduction in Australia, based on trophic cascade theory, is “weak”. They conclude that, “because of climate instability, the strong top-down trophic responses reported from the Yellowstone National Park case study are unlikely to apply in arid and semi-arid south-eastern Australia and are speculative at best”.

We agree that dingoes (Canis dingo) are likely to exert different effects on ecological communities in Australia as compared to grey wolves (Canis lupus) in North America. A comparison of body sizes and dietary preferences between these canid species alludes to their functional ecological differences. Differences in the biological communities and climate between Yellowstone National Park and Australia also prevent direct comparisons of trophic cascade-processes between the two regions. These facts should not, however, preclude examination of the efficacy and consequences of dingo reintroductions in Australia.

We contend that Morgan et al. (2016):

  1. misunderstand the circumstances that make trophic cascades important to consider in Australia,
  2. do not acknowledge key reasons why dingo reintroduction has been proposed,
  3. haven’t recognised the different pathways by which dingoes could influence ecosystems via trophic cascades, and
  4. do not fully acknowledge literature and theory relevant to understanding the interplay of bottom-up and top-down processes in Australia.

Our reply is intended to assist managers and decision makers when deciding whether or not to reintroduce dingoes into Australian ecosystems.

Newsmen TM, Greenville AC, Letnic M, Ritchie EG, Dickman CR (2017) The case for a dingo reintroduction in Australia remains strong: A reply to Morgan et al., 2016, Food Webs, PDF DOI

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

Stop jumping the gun: A call for evidence-based invasive predator management

Authors: Tim S Doherty and Euan G Ritchie

Published in: Conservation Letters (early access)

Abstract

Invasive mammalian predators are major drivers of species extinctions globally.

To protect native prey, lethal control is often used with the aim of reducing or exterminating invasive predator populations. The efficacy of this practice however is often not considered despite multiple practical and ecological factors that can limit success.

Here, we summarise contemporary knowledge regarding the use and challenges of lethal control and alternative approaches for reducing invasive predator impacts.

As the prevailing management approach, we outline four key issues that can compromise the effectiveness of lethal control: release of herbivore and mesopredator populations; disruption of predator social systems; compensatory predator immigration; and ethical concerns.

We then discuss the relative merits and limitations of four alternative approaches that may enhance conservation practitioner’s ability to effectively manage invasive predators: top-predator conservation or reintroduction; maintaining habitat complexity; exclusion fencing; and behavioural and evolutionary ecology.

Considerable uncertainty remains regarding the effectiveness of management approaches in different environmental contexts.

We propose that the deficiencies and uncertainties outlined here can be addressed through a combination of adaptive management, expert elicitation, and cost-benefit analyses.

Improved management of invasive predators requires greater consideration and assessment of the full range of management approaches available.

Doherty TS, Ritchie EG (2016) Stop jumping the gun: A call for evidence-based invasive predator management. Conservation Letters PDF DOI

The Conversation: Ocean predators can help reset our planet’s thermostat

By Peter Macreadie (University of Technology Sydney), Euan Ritchie (Deakin University) Graeme Hays (Deakin University), Rod Connolly (Griffith University) and Trisha B Atwood (Utah State University).

If you knew that there was zero percent chance of being eaten by a shark, would you swim more often? Rhetorical questions aside, the fear of being eaten has a profound influence on other animals too, and on the way they use marine environments.

Turtles, for example, fear being eaten by sharks and this restricts the movement and behaviour of entire populations. But when the fear of being eaten dissipates, we see that turtles eat more, breed more, and go wherever they please.

It might sound like turtle paradise, but in an article published today in Nature Climate Change we show that loss of ocean predators can have serious, cascading effects on oceanic carbon storage and, by extension, climate change.

Sea Turtle

When sea turtles aren’t being frightened by sharks, they consume more seagrass, and the Carbon stocks stored in it. Image credit Framk_am_Main via Flickr

Cascading effects

For a long time we’ve known that changes to the structure of food webs – particularly due to loss of top predators – can alter ecosystem function. This happens most notably in situations where loss of predators at the top of the food chain releases organisms lower in the food chain from top-down regulatory control. For instance, the loss of a predator may allow numbers of its prey to increase, which may eat more of their prey, and so on. This is known as “trophic downgrading”.

With the loss of some 90% of the ocean’s top predators, trophic downgrading has become all too common. This upsets ecosystems, but in our article we also report its effects on the capacity of the oceans to trap and store carbon.

This can occur in multiple ecosystems, with the most striking examples in the coastal zone. This is where the majority of the ocean’s carbon is stored, within seagrass, saltmarsh and mangrove ecosystems – commonly known as “blue carbon” ecosystems.

Blue carbon ecosystems capture and store carbon 40 times faster than tropical rainforests (such as the Amazon) and can store the carbon for thousands of years. This makes them one of the most effective carbon sinks on the planet. Despite occupying less that 1% of the sea floor, it is estimated that coastal blue carbon ecosystems sequester more than half the ocean’s carbon.

The carbon that blue carbon ecosystems store is bound within the bodies of plants and within the ground. When predators such as sharks and other large fish are removed from blue carbon ecosystems, resulting increases in plant-eating organisms can destroy the capacity of blue carbon habitats to sequester carbon.

For example, in seagrass meadows of Bermuda and Indonesia, less predation on herbivores has resulted in spectacular losses of vegetation, with removal of 90–100% of the above-ground vegetation.

Stop killing predators

Such losses of vegetation can also destabilise carbon that has been buried and accumulated over millions of years. For example, a 1.5-square-kilometre die-off of saltmarsh in Cape Cod, Massachusetts, caused by recreational overharvesting of predatory fish and crabs, freed around 248,000 tonnes of below-ground carbon.

If only 1% of the global area of blue carbon ecosystems were affected by trophic cascades as in the latter example, this could result in around 460 million tonnes of CO2 being released annually, which is equivalent to the annual CO2 emissions of around 97 million cars, or just a bit less than Australia’s current annual greenhouse gas emissions.

So what can be done? Stronger conservation efforts and modification of fishing regulations can help restore marine predator populations, and thereby help maintain the important indirect role that predators play in climate change mitigation.

It’s about restoring balance so that we have, for example, healthy and natural numbers of both sea turtles and sharks. Policy and management need to reflect this important realisation as a matter of urgency.

More than 100 million sharks may be killed in fisheries each year, but if we can grant these predators great protection they may just help to save us in return.The Conversation

This article was originally published on The Conversation. Read the original article, including reader comments.

The Conversation

 

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