Is it safe to come out? Fish responses to changes in predator density

Overfishing can cause changes in fish populations over time by removing the big fish (often predators) from marine ecosystems. Reducing predator density can have consequences for ecosystem function, including movement of prey and reproduction of the predators themselves (Madin et al. 2012). Often marine parks are established to protect marine ecosystems by preventing these changes or allowing communities to recover from fishing.

Example of a food web and the responses of lower trophic levels to a reduction in the number of top-level predators  (Cury et al. 2001).

Example of a food web and the responses of lower trophic levels to a reduction in the number of top-level predators
(Cury et al. 2001).

Professor Robert Warner from the University of California, Santa Barbara and his team of researchers study fish behaviour. Professor Warner spoke last week at Macquarie University about fish behavioural responses to predators and what happens when predators are removed from ecosystems.

Removing predators can cause a trophic cascade, which is a change in the abundance of animals and plants in the lower levels of the food chain (Baum & Worm 2009). For example, the removal of sharks means predatory fish can increase in numbers which can lead to a decrease in smaller herbivorous fish population sizes and a boom in the amount of seaweed and/or algae on the reef.

As well as these direct changes to the food chain there are also indirect changes in the ecosystem, such as the behaviour of fish (Madin et al. 2010). Prey fish change their behaviour to avoid predators and reduce their risk of being eaten. This avoidance behaviour changes in response to their environment, so when there are more predators around the prey fish are more risk averse. The behaviour can change both temporally (over time) and spatially (over an area).

The extent to which prey fish will range from shelter (blue line) in fished (lower predator density) and unfished (higher predator density) areas. Photo: Belinda Fabian.

The extent to which prey fish will range from shelter (blue line) in fished (lower predator density) and unfished (higher predator density) areas. Photo: Belinda Fabian.

The researchers compared the distances prey fish ventured from shelter and the density of predators in both fished and unfished areas. The predators in fished areas are smaller and a lower density compared to the predators in unfished areas. In unfished areas they found the prey fish ranged over a shorter distance from shelter than in fished areas (Madin et al. 2012).

Another change in fish behaviour is their foraging patterns. When predators are present, fish can change the location of their foraging and/or the time when they forage. For example, a fish that normally feeds on the reef can avoid a predator by moving to the mangroves or feed at night to avoid a predator which is active during the day.

These changes in prey fish behaviour can have flow on effects for other parts of the reef. The restriction in the distance the fish are willing to range from shelter during feeding can have an impact on the distribution pattern of algae (food of the prey fish). When there is low predator density prey fish are willing to range far and wide which leads to even consumption of algae over the reef. In contrast when there are more predators the prey fish are more risk averse and only forage close to shelter (Madin et al. 2012). This means the algae is heavily cropped close to shelter and there is low cropping at further distances from shelter. This uneven distribution and overgrowth of algae can negatively impact other organisms on the reef such as coral (Coyer et al. 1993). The heavy cropping close to shelter means some of food the fish is consuming may be less than ideal and their growth and reproduction may be limited due to energy and/or nutrient deficiencies (Heithaus et al. 2008).

Algae growing over coral in Suva, Fiji. Photo: Belinda Fabian.

Algae growing over coral in Suva, Fiji.
Photo: Belinda Fabian.

Understanding the impacts of predator density in marine ecosystems is important for fisheries management and the establishment of marine sanctuaries. The sites used in these studies include currently fished, long established protected areas (no previous fishing) and new protected areas (recently fished). The researchers included these types of areas in the study as they wanted to determine the impacts of predator removal on prey behaviour and if these effects can be reversed through the cessation of fishing and a resulting increase in predator density (Madin et al. 2012).

Reef environments have a delicate balance of species, interactions and environmental variables. Professor Warner and his team have shown that a change such as overfishing of a predator species could have far-reaching impacts on the distribution and abundance of organisms on the reef. If the interactions are permanently changed then there could be negative impacts on the functioning of the reef, especially in the current context where there are many other challenges for reefs such as pollution and climate change.

To learn more:

Baum JK and Worm B (2009). Cascading top-down effects of changing oceanic predator abundances. Journal of Animal Ecology, 78, 699-714.

Coyer JA, Ambrose RF, Engle JM and Carroll JC (1993). Interactions between corals and algae on a temperate zone rocky reef: mediation by sea urchins. Journal of Experimental Marine Biology and Ecology, 167 (1): 21-37.

Cury P, Shannon L and Shin Y-J (2001). ‘The Functioning of Marine Ecosystems’, Reykjavik Conference on Responsible Fisheries in the Marine Ecosystem, Reykjavik, Iceland, 1-4 October.

Heithaus MR, Frid A, Wirsing AJ and Worm B (2008). Predicting ecological consequences of marine top predator declines. Trends in Ecology and Evolution, 23 (4), 202-210.

Madin EMP, Gaines SD and Warner RR (2010) Field evidence for pervasive indirect effects of fishing on prey foraging behaviour. Ecology, 91 (12), 3563-3571.

Madin EMP, Gaines SD, Madin JS, Link A-K, Lubchenco PJ, Selden RL and Warner RR (2012). Do Behavioral Foraging Responses of Prey to Predators Function Similarly in Restored and Pristine Foodwebs? PLoS ONE, doi: 10.1371/journal.pone.0032390.

Mind the edge: impact of habitat edges on community competition dynamics

Scientists from the Australian National University and La Trobe University have found that habitat fragmentation can have a major impact on competition and composition of marsupial folivore communities.

Human-induced landscape change is a major driver of habitat loss and species extinction. One of the major changes humans cause in landscapes is the fragmentation of habitats. This leads to the creation of pockets of habitable areas surrounded by inhospitable zones with the borders between these areas termed edges. Edges also occur naturally when changing from one landscape type to another and are very important for determining the distribution and abundance of species and populations.

The fragmentation of habitats can change the distribution of resources and this can affect species unevenly. As resources are redistributed species can gain or lose competitive advantage, which can lead to a change in the competitive dynamics of communities. Unfortunately there have been very few studies in this area, so conservation managers usually can’t take into account these impacts on communities when making decisions.

In the study recently published in Conservation Biology, the researchers focused on a community comprised of four arboreal marsupial folivores in southern New South Wales. The results showed that there is a considerable difference in the response of these marsupials to edges between natural eucalypt forest remnants and pine plantations (inhospitable zone for these animals).


Google Maps aerial view of the fragmented landscape in the study area near Tumut, NSW, Australia

Field surveys showed that there are sufficient food resources for all four marsupials all the way up to the edges of the eucalypt forest remnants. In addition, at the edges of these eucalypt forest remnants there is increased understorey and exotic plant species coverage. Although the species that feed exclusively on eucalypt leaves (such as the Greater Glider) are not disadvantaged at these edges, the generalist species (such as the Common Brushtail Possum) have more food resources available so they have a competitive advantage.

There are also advantages for marsupials that specialise on plants that are unattractive to other species due to their tannins and nitrogen content. Common Ringtail Possums which feed on these undesirable plants were found in significantly higher numbers in the eucalypt forest remnants compared with the uninterrupted forest.

Common Ringtail Possum (Pseudocheirus peregrinus)
(Source: Barbara Hardy Institute Photographer: John Hodgson)

Out of the four marsupials studied, the Greater Glider was the most edge avoidant species. The researchers hypothesised that this behaviour would lead to this species being more extinction prone than the animals with an affinity for edges. The numbers of Greater Gliders were much higher in the continuous eucalypt forest compared with the forest remnants, so this lends support to the idea that the Greater Glider has a higher risk of extinction due to its smaller populations in the forest remnants.

The findings from this study show that the impact of edges on communities can be profound and need to be taken into account when making conservation and management decisions. Forest managers should aim to minimise the amount of edges compared to area enclosed in fragmented habitats in order to reduce the competitive advantage of generalist species that may exclude specialists.

Read more:

Youngentob, K.N., Yoon, H-J., Coggan, N. and Lindenmayer, D.B. (2012) Edge effects influence competition dynamics: A case study of four sympatric arboreal marsupials. Biological Conservation 155, 68-76

(Post written as part of assessment for BIOL349 Biodiversity & Conservation at Macquarie University 2012)