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).

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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.

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Common Ringtail Possum (Pseudocheirus peregrinus)
(Source: Barbara Hardy Institute http://www.unisa.edu.au/barbarahardy/ 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)

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