Going a bit batty – How do bats withstand so many viruses?

Dr Michelle Baker from the CSIRO Australian Animal Health Laboratory spoke at Macquarie University last week about her work on bat immune systems. Her lab recently contributed to the high impact Hendra virus vaccine for horses. Dr Baker’s work has implications for disease control and prevention and management of virus spillovers into human communities worldwide.

Bats make up 20% of mammalian diversity, are long lived for their body size and are the only mammals with powered flight. Bats are vital to ecosystem functions such as pollination, fertilisation and insect control (Calisher et al. 2006). Despite these unique characteristics they are not intensively studied like most other mammal groups. Before Dr Baker’s research group focused on bats not much was known about their immune systems (Baker et al. 2013).

Figure 1. Possible virus transmission routes from bats to humans. (adapted from image presented by Dr Michelle Baker)

Figure 1. Possible virus transmission routes
(adapted from image presented by Dr Michelle Baker).

Bats act as viral reservoirs, meaning they carry a range of viruses (He et al. 2010; Ng & Baker 2013). Fatal human viruses that can be traced back to bats include Rabies, Hendra, Ebola, Marburg and the SARS coronavirus (Ng & Baker 2013). These viruses occasionally spill over from bats into other animals and that’s normally how humans become infected (Figure 1). But even with this load of up to 100 different viruses, bats are hardly ever sick (Baker et al. 2013).

The bat’s lack of symptoms from viral infections was puzzling, so Dr Baker’s team looked more closely at bat immune systems. When a viral infection occurs in other mammals the immune system quickly delivers a generic response (innate response) and then a specific response occurs more slowly (adaptive response; Katze et al. 2002). The researchers observed that bats don’t develop many antibodies in response to infections, so they thought the bat immune system might be knocking down the viruses before the immune system could mount an adaptive response (Baker et al. 2013).

Figure 2. The Black Flying Fox, Pteropus alecto.

Figure 2. The Black Flying Fox, Pteropus alecto.

To test this idea the researchers sequenced the genomes and studied the immune responses of two species of bat: Myotis davidii, a micro bat, and Pteropus alecto, a megabat (Figure 2). They found the collection of immune genes in bats is different to other mammals. For example, bats have fewer genes for interferon production (Papenfuss et al. 2012).

Interferon is a protein produced by the immune system in response to the detection of viral invaders. It starts a signaling cascade that creates an anti-viral state in cells (He et al. 2010; Figure 3). High interferon levels can be toxic for cells, so normally the interferon level is very low. When a viral infection is detected, the interferon level is dramatically increased which signals cells to start fighting the infection (Katze et al. 2002). There are multiple types of interferon, but the type Dr Baker spoke most about is interferon alpha (IFNA).

Figure 3. Interferon signaling cascade - causes expression of immune system genes and creates an antiviral state in cells (Katze et al. 2002).

Figure 3. Interferon signaling cascade – causes expression of immune system genes and creates an antiviral state in cells (Katze et al. 2002).

In contrast to expectations, bat cells were found to have their IFNA genes constantly switched on and there is no increase when cells are infected with viruses (Figure 4). Even with this high IFNA level the toxicity effect observed in other mammals isn’t seen in bats. This IFNA level in bats may be part of the reason they can carry so many viruses, but don’t often get sick from them. Other research groups have found that bat IFNA genes have been positively selected which means they must have been beneficial to bats as they lived with viruses in the past (Calisher et al. 2006; He et al. 2010). Recent work has hypothesised that there is a link between the evolution of flight and the ability of bats to harbor viruses without becoming sick (Zhang et al. 2013; O’Shea et al. 2014).

These findings were very new and unexpected so there were lots of questions from the audience after the seminar. It seems like everywhere Dr Baker turned there were more questions! These are the ‘top 5’ questions asked:

  1. Why aren’t the bats harmed by high levels of interferon in uninfected cells like other mammals?
  2. What triggers the spillover of viruses into other animals that cause outbreaks?
  3. Could there be a link between the interferon level and their long lifespan relative to their body size?
  4. What is the bat immune response to bacterial infection?
  5. Why doesn’t the bat immune response completely wipe out the viruses? How come the viruses can persist and then spill over into other animals?
Figure 4. Interferon alpha levels in infected and uninfected cells in bats and other mammals  (adapted from image presented by Dr Michelle Baker).

Figure 4. Interferon alpha levels in infected and
uninfected cells in bats and other mammals
(adapted from image by Dr Michelle Baker).

So much is currently unknown about how bats can carry so many viruses without being sick. Dr Baker and her team are working to find answers to these questions and more. As human populations increasingly overlap with bat habitats there is more chance of spillover events affecting human and animal populations. Dr Baker’s research could be used to understand human responses to viruses and develop anti-viral treatments in the future.

Learn more:

Baker ML, Schountz T & Wang L-F (2013). Antiviral Immune Responses of Bats: A Review. Zoonoses and Public Health, 60, 104-116. doi: 10.1111/j.1863-2378.2012.01528.x

Calisher CH, Childs JE, Field HE, Holmes KV & Schountz T (2006). Bats: Important Reservoir Hosts of Emerging Viruses. Clinical Microbiology Reviews, 19(3), 531-545. doi:  10.1128/CMR.00017-06

He G, He B, Racey PA & Cui J (2010). Positive Selection of the Bat Interferon Alpha Gene Family. Biochemical Genetics, 48(9-10), 840-846. doi: 10.1007/s10528-010-9365-9

Katze M, He Y & Gale M (2002). Viruses and interferon: A fight for supremacy. Nature Reviews Immunology, 2(9), 675-687. doi: 10.1038/nri888

Ng J & Baker ML (2013). Bats and bat-borne diseases: a perspective on Australian megabats. Australian Journal of Zoology, 61, 48-57. doi: 10.1071/ZO12126

O’Shea T, Cryan P, Cunningham A, Fooks A, Hayman D, Luis A, Peel A, Plowright R, & Wood J (2014). Bat Flight and Zoonotic Viruses. Emerging Infectious Diseases, 20(5), 741-745. doi: 10.3201/eid2005.130539

Papenfuss AT, Baker ML, Feng Z-P, Tachedjian M, Crameri G, Cowled C, Ng J, Janardhana V, Field HE, Wang L-F (2012). The immune gene repertoire of an important viral reservoir, the Australian black flying fox. BMC Genomics, 13:261, doi: 10.1186/1471-2164-13-261.

Zhang G, Cowled C, Shi Z, Huang Z, Bishop-Lilly KA, Fang X, Wynne JW, Xiong Z, Baker ML, Zhao W, Tachedjian M, Zhu Y, Zhou P, Jiang X, Ng J, Yang L, Wu L, Xiao J, Feng Y, Chen Y, Sun X, Zhang Y, Marsh GA, Crameri G, Broder CC, Frey KG, Wang L-F & Wang J (2013). Comparative Analysis of Bat Genomes Provides Insight into the Evolution of Flight and Immunity. Science, 339, 456-460. doi: 10.1126/science.1230835

Silhouette images in Figure 1 sourced from: horse, bat, pig, humans.

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.

Playing well with others? Sociality in huntsman spiders

Why do some huntsman spiders live in groups while the majority are perfectly happy living a solitary life? Dr Linda Rayor from Cornell University in New York spoke at Macquarie University recently about her work with Australian social huntsman spiders (Delena cancerides) and her excitement about finding more social huntsman species in Australia last year. Dr Rayor is passionate about social spiders and her findings could shed light on the evolution of parental care in a broad range of animals.

Sociality in arachnids is very rare – less than 1% are social beyond a short time of maternal care just after hatching. This may be due to the challenges a spider species has to overcome to become social. Aggressiveness in spiders means the majority can’t tolerate any other spiders being around them and this can lead to cannibalism (Riechert & Lockley 1984). The existence of three social huntsman species in Australia is a ‘big deal’ according to Dr Rayor as they have overcome the inbuilt aggressiveness that seems to come naturally to so many spiders.

Sociality in spiders has evolved independently at least 18 times (Yip & Rayor 2013b), so there must be something in it. Social huntsman spiders are found in south-west and south-eastern Australia and live in groups of 20 to 200 individuals, with a dominant female and her offspring of all different ages. In contrast to social insects, such as bees and termites, living together doesn’t increase the reproductive output of social spiders so there must be a different driving force behind their sociality (Whitehouse & Lubin 2005).

Delena Cancerides collected at Mount Ainslie, ACT in March 2014 Source: Canberra Times / Photo: Jay Cronan

Delena cancerides collected at Mount Ainslie, ACT – March 2014
Source: Canberra Times
Photo: Jay Cronan

Research from Dr Rayor’s group suggests a lack of available habitat was the driving force for the evolution of sociality in these spiders. Their preferred habitat of tight spaces under peeling acacia bark is normally 80-100% occupied, so there’s not much room to spread out. The spiders are forced together due to a lack of available housing.

Social huntsman spiders aren’t attached to their family members; they only live together because there’s no other option. This is demonstrated when a colony has to move because their home is destroyed (usually the bark falling off the tree). The family doesn’t move as a group – the spiders go in search of new homes by themselves with no regard for their siblings and the small ones generally get eaten by predators.

These social spiders live (mostly) peacefully in family groups, but if there are big spiders trying to immigrate into their colony they will become aggressive and deny entry to the invaders (Beavis et al. 2007). This is consistent with the limited habitat concept as the spiders are protecting their home (a valuable resource). Young spiders don’t leave the family home until they are big enough to compete with others for the sparse housing options.

To test this idea the researchers looked at the relationship between the availability of suitable habitat and the occupants of the bark spaces. They found that as suitable habitat becomes rarer the number of spiders in each colony increases and there are more large spiders in the colonies (Yip 2012). In addition, the frequency of takeovers of bark spaces also increases when available habitat decreases.

Huntsman family

Delena cancerides siblings of varying ages sharing food / Photo: Linda Rayor

Instead of using a web to trap prey, huntsman spiders roam around at night and hunt their prey (hence the name ‘huntsman’). This food is brought back to the colony for consumption. Food is shared about 5% of the time – mainly between mothers and children and sometime older siblings even share with their younger brothers and sisters (Yip & Rayor 2013a). Even this small amount of sharing is very different to solitary spiders who share food less than 1% of the time.

Prey sharing means all the spiders in the group have some food often, so there is less variability in the amount of food they consume. This is very different to spiders living alone that can have erratic food availability. D. cancerides has a lower metabolic rate than solitary spiders which means they can survive on lower amounts of food (Zimmerman 2007). The researchers aren’t sure whether sociality arose because of the lower metabolic rate in these spiders or whether sociality allows them to share prey and they have developed a lower metabolic rate as a result – it’s a bit of a ‘which came first: the chicken or the egg?’ discussion in the research group at the moment.

It would be interesting to study the metabolic rate of some closely related social and solitary huntsman species to see if all social huntsman species have a low metabolic rate or just D. cancerides. This could possibly shed some light on the evolution of sociality in this group.

To learn more:

Click here to see Linda Rayor talking about social huntsman prey sharing dynamics and click here to learn about her field collection of spiders in Canberra this month.

Beavis AS, Rowell DM and Evans T (2007). Cannibalism and kin recognition in Delena cancerides (Araneae: Sparassidae), a social huntsman spider. Journal of Zoology, 271:2, 233-237.

Riechert SE and Lockley T (1984). Spiders as Biological Control Agents. Annual Review of Entomology, 29, 299-320.

Whitehouse MEA & Lubin Y (2005). The functions of societies and the evolution of group living: spider societies as a test case. Biological Reviews. 80, 347-361.

Yip E (2012). ‘Costs and benefits of group living in an unusual social spider, Delena cancerides’. PhD thesis, Cornell University, New York.

Yip EC & Rayor LS (2013a). The influence of siblings on body condition in a social spider: is prey sharing cooperation or competition? Animal Behaviour. 85, 1161-1168.

Yip EC & Rayor LS (2013b). Maternal care and subsocial behaviour in spiders. Biological Reviews, doi: 10.1111/brv: 12060

Zimmerman A (2007). ‘Assessing the Costs of Group Living: Comparing Metabolic Physiology and Growth in Social and Solitary Spiders’. PhD thesis, Cornell University, New York.

Weaker Mussels in Warm Water?

You’d be forgiven for thinking that mussels attach to rocks and other substrates using a muscular foot. After all, that’s what their name implies. But mussels actually hang on using byssal threads – small fibres constructed by the mussel that are very strong while also being highly flexible.

MusselFoot300x267Researchers at the University of Washington are looking at the impacts of environmental conditions on the strength of byssal threads. They found the strength and flexibility of the threads varies with temperature and ocean pH, which could have far reaching consequences in the not too distant future.

Mussel foot (right) and byssal thread (left). Photo: Laura Coutts

The researchers compared the strength of byssal threads at 10oC and 25oC. In warmer water the mussels produced fewer threads and those that were produced were weaker than the corresponding threads created in cooler conditions. These changes were seen even as a result of short-term variations in temperature.

The warming of the ocean due to climate change could impact mussel populations by reducing their attachment strength. They may not be able to hold on as tightly to the substrate and could be washed away by waves and currents. Existing sites may no longer be habitable by mussels and there could be increased mortality if feeding is impacted by the inability to remain attached to the substrate.

Byssus threads 300x271When these temperature impacts are combined with other environmental stressors, such as ocean acidification and a change in the frequency and intensity of storms, mussels could be detrimentally affected. Mussels have a larval stage at the beginning of their life cycle, so the colonisation of cooler and calmer environments is theoretically possible.

Mussels attaching to substrate using byssal threads. Photo: Emily Carrington

Mussel migration due to changes in ocean temperature has the potential to dramatically impact intertidal ecosystem composition and dynamics. Changes in water temperature and mussel attachment strength will also have ramifications for the aquaculture industry as mussel attachment to ropes is important for productive mussel farming.

The mussel species in these experiments was Mytilus trossulus which lives mainly in the intertidal zone of the northern Pacific Ocean. Mussels are also found in warmer environments around the world, but these findings seem to imply that they may not be able to hang on to the substrate in turbulent conditions as well as their counterparts in cooler environments.

Maybe mussels in warmer environments may be more successful in habitats with calmer conditions? It would be interesting to extend these experiments to warmer conditions and possibly freshwater mussels to see if the same limitations apply to their byssal threads.

To find out more:

Newcomb LA, Carrington E, George MN & O’Donnell MJ (2014). Short−term exposure to elevated temperature and low pH alters mussel attachment strength. Abstract of presentation to The Society of Integrative & Comparative Biology, Austin, Texas, 3-7 January.

O’Donnell MJ, George MN & Carrington E (2013). Mussel Byssus Attachment Weakened by Ocean Acidification. Nature Climate Change. doi: 10.1038/nclimate1846.

Hear Professor Emily Carrington discussing this research and Professor Phillip Messersmith talking about the applications of mussel attachment for medical research on the ‘The Science Show’ Radio National podcast here.

Travelling geckos – coping with climate change

Researchers from Macquarie University have been studying geckos (Gehyra variegata) in arid areas of Australia to determine the impacts of climate change and the possible responses of gecko populations.

The pace of the change in climate expected over the next 70 years is greater than any other change in climate in human history. Even with effective climate policy and major changes in greenhouse gas emissions the world over there is still a significant possibility of exceeding a 2 degree temperature increase that will have major negative impact on ecosystems.

Increases in global average temperature, changes in rainfall patterns and more extreme events (such as drought, fire, flood and cyclones) are the major factors that all organisms on earth potentially have to face. Each of these factors will have differing levels of impacts so the actual changes in climate and ecosystems will differ between areas.

Animals have an advantage over plants when it comes to adapting to and coping with climate change as they can move to new areas. To achieve a 1 degree temperature change an animal needs to move 100m upwards in altitude or 125km south (in the southern hemisphere or north in the northern hemisphere). This means that to combat a 2 degree temperature rise a shift of 250km would be required if a mountainous habitat was not suitable or available (and Australia is a very flat country – 99% percent of the continent is <1000m above sea level).

Paul Duckett and other scientists from Macquarie University used models to identify suitable habitats for the geckos and what proportion would make it to these new habitats. A startling conclusion they came to was that although there are places for the geckos to move to which would mitigate the effects of climate change the problem would be in them actually getting there.

Gehyra variegata – Sturt National Park NSW Australia (Wikipedia)

The modeling showed that over 40% of the gecko populations would not reach suitable areas before climate change has negative impacts on the populations, such as small population sizes and the associated genetic consequences. There are also suitable areas to colonise that won’t be used as the geckos won’t be able to migrate that far within the time span of the change in climate.

As the data used in these models is based on past conditions it is possible that the rates of gecko dispersal may differ from the model under actual climate change conditions. For example, the geckos in the past may have dispersed under specific rainfall and aridity conditions, but these may not be the same conditions under which the geckos will disperse in times of climate change as the Australian continent is expected to experience increasing aridity. In addition the predicted future distribution of these geckos is expected to overlap with areas utilised by humans, so fragmented environments may have additional impacts on the persistance of gecko populations.

And even if the geckos do make it to their new and suitable habitats far to the south of their current locations what is the chance that their food source also made the journey successfully?

Read more:

Duckett PE, Wilson PD, Stow AJ (2013). Keeping up with the neighbours: using a genetic measurement of dispersal and species distribution modelling to assess the impact of climate change on an Australian arid zone gecko (Gehyra variegata). Diversity and Distributions, DOI: 10.1111/ddi.12071

Seeing-eye sponges? Phototaxis and eye development in sponge larvae

This week I attended a seminar by Professor Bernard Degnan from the University of Queensland on the early evolution of animal complexity. He discussed many genetic aspects of the evolution of multicellularity, the similarities between sponges and more complex organisms and the differences between sponges and their unicellular ancestors. The presentation was captivating (even if some of the genetic bits did go over my head) and I loved the passion he had for the topic.

But the thing that stuck with me is his argument that sponges have organs which they use to see. This absolutely goes against the grain of everything I’ve learnt about sponges in the last 20 years. No more believing that sponges are ‘simple’ (and on a side note: no more believing that just because it’s in a textbook means it’s infallible – this part of the scientific method has been drilled in to me intellectually, but this is my first experience of having my biological world rocked in reality).

Only a tiny part of the seminar related to sponge (Amphimedon queenslandica) response to light. Opsins (photo-sensitive proteins involved in light responses of all animals with nervous systems) are not found in sponges so how do sponges respond to light? During A. queenslandica larval development (cleavage) a cryptochrome gene is expressed before any cells become pigmented. Pigmented cells then form throughout the blastula and they migrate to form a pigment ring at the posterior pole of the larvae. This pigment ring is complemented by long cilia which move in response to blue-light stimulus (440nm) possibly using cryptochrome proteins. These long cilia dictate the swimming direction of the larvae and act as a rudder.


Pigment and cilia ring at A. queenslandica larval posterior pole

From 0-24 hours old the larvae are negatively phototactic (avoid light) and from 48 hours old they lose their ability to respond to light. This negative phototaxis early in the larvae’s life could be a response to an environmental cue (light) that indicates the presence (or absence) of an appropriate location for the larvae to settle and/or a timing cue for metamorphosis.

So the pigment ring structures in a sponge fit the minimum definition of an eye – pigment adjacent to photoreceptors. The evolution the sponge eye (albeit a neuron-less, opsin-less eye that loses function as the larvae develop) is a fascinating example of convergent evolution.

Further reading:

Rivera AS, Ozturk N, Fahey B, Plachetzki DC, Degnan BM, Sancar A and Oakley T (2012) Blue-light-receptive cryptochrome is expressed in a sponge eye lacking neurons and opsin. The Journal of Experimental Biology, 215, 1278-1286.

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 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)