Published: 2 July 2012

Trees and non-flying mammals: a hollow understanding

Ross Goldingay

For many of Australia’s mammals, tree hollows are critical habitat. Ross Goldingay argues that to make informed decisions about the conservation of species that use tree hollows, we need more research on the role that hollows play, and on the shelter needs of the animals that use them.

A common brushtail possum uses a large trunk hollow (entrance diameter >10 cm) in a scribbly gum (<i>Eucalyptus haemastoma</i>).

A common brushtail possum uses a large trunk hollow (entrance diameter >10 cm) in a scribbly gum (Eucalyptus haemastoma).

Credit: Photo by Ross Goldingay.

How much do we really know about the habitat needs of Australia’s tree-dwelling mammals?

We know that about 40 species of non-flying mammals use hollows or cavities within trees for shelter, and that many of these species depend on such sites for their survival. Among these are two of Australia’s most endangered mammals: Leadbeater’s possum in Victoria, and the mahogany glider in north Queensland. However, our knowledge of the kinds of hollows required – and how their on-going availability may be managed – is poor.

In Australia, hollows are produced as a consequence of fungal decay, insect attack and fire. Trees typically need to be at least 100 years old before these agents start to hollow out the heart wood. Harvesting trees for timber or firewood, clearing for agriculture, tree dieback and increased fire frequency all lead to a gradual loss of very old trees from our forests and woodlands, therefore reducing hollow availability.

I recently reviewed available information on the characteristics of tree hollows used by Australian non-flying mammals, published in the Australian Journal of Zoology. In addition to possums and gliders, other non-flying mammals that use tree hollows include dasyurid marsupials, such as the brush-tailed phascogale and the agile antechinus, and rodents, such as the brush-tailed rabbit rat.

Tree hollow characteristics have been described for less than half of the species that use them. In fact, only six species (15 per cent of tree-dwelling mammal species) have been studied in sufficient detail to assist their management and conservation.

Some hollow-using species are not completely reliant on tree hollows and will use other shelter types. However, without detailed studies, it is difficult to reach a conclusion about the role tree hollows play, and whether shelter needs are adequate in different locations.

All species studied use standing dead trees (stags). A key reason why stags are favoured is their higher probability of containing hollows than live trees. One concern that arises where mammals rely heavily on stags is that these trees may only remain standing for several decades. If the surrounding live trees have not reached at least 100 years of age when the stags collapse, then a shortage of shelter sites will result. This will force the local population of hollow-using wildlife into decline or collapse. This scenario becomes more probable when wildfires are factored in, because they make stags much more prone to destruction or collapse.

Standing dead tree (stag) used as a den by a squirrel glider in Brisbane. The arrow shows the location of the hollow entrance shown in the inset.

Credit: Photos by B. Taylor.

Some tree species also produce hollows more readily than other species, forming them at a much smaller size and younger age. This explains why these tree species are preferred by some mammals. In some cases, however, preferences for particular tree species may simply be because they have better-quality hollows.

For example, in north Queensland, flooded gum (Eucalyptus grandis) accounted for 91 per cent of the dens of the yellow-bellied glider, but just 11 per cent of the trees in the forest. Hollow availability was not quantified, but it appeared that flooded gum was favoured due to the size of its hollows (author’s personal observation).

Scenarios of hollow-bearing tree loss, such as the loss of stags due to wildfire, begin to highlight the challenge to land managers. There might be sufficient numbers of hollows now, but will this be the case in 20 or 50 years’ time? This question can only be answered by field studies that document the rate of tree fall over a period of years. These data and other information can then be used in computer simulations to predict future outcomes. Very few studies in Australia document rates of fall of hollow-bearing trees. Because these rates will vary across environmental gradients, local studies are required.

One curious finding arising from my review was the variation among studies in the number of hollow-bearing trees used for shelter by individuals of some species. The number used was mostly in excess of five trees, but this varied with study length; the longer the study, the greater the opportunity for additional trees to be used. The mountain brushtail possum was recorded as having used 23 trees, the highest number for one individual in any study.

It appears that the number of trees used by individuals is, in part, influenced by local availability. Nevertheless, the average number needed could be important: if land managers need to manage individuals of a species, it would be useful to know how many tree hollows should be provided. Available data are insufficient to answer this at present.

A burnt stump used as a den by a radio-collared male eastern pygmy-possum in Royal National Park, Sydney. The inset shows the view looking into the stump from above.

Credit: Photos by R. Goldingay.

Why do individual animals need more than one tree hollow? A set of hypotheses have been advanced to explain this:

to minimise predation risk
to minimise foraging costs – moving hollows may reduce distances required to travel to sources of food
to provide home-range defence – shifting of den sites may more readily advertise that an area is occupied, pre-empting the incursion of trespassers
to optimise thermal buffering – den sites vary in their thermal properties, which may require changing dens on a seasonal basis to ensure exposure to an optimal microclimate in the nest
to reduce the parasite load – fleas, ticks and lice, for example, build up in shelter sites.

These hypotheses are not mutually exclusive, with several potentially applying to a given species.

Increasingly, artificial hollows (nest or roost boxes) are being installed to compensate for the loss of hollow-bearing trees. However, there is much debate about how effective nest boxes are as replacements for natural hollows. Little is known, for example, about how frequently nest boxes are used by different species. There is also debate about whether the cost of providing and maintaining nest boxes on a large scale and over a long period is justified, given the lack of knowledge about outcomes.

It is clear that nest boxes should not be used to justify the removal of hollow-bearing trees or unsustainable forestry practices. However, nest and roost boxes do play an important role in alleviating hollow loss caused by the expansion of major roadways and mine sites in eastern Australia, and by urbanisation.

This brings us back to the need to improve the state of scientific knowledge of species that depend on tree hollows for their survival. Here, nest boxes offer a way forward in the search for knowledge. Nest box characteristics, such as size, placement and number, can be easily manipulated to gain an understanding of the key attributes of tree hollows favoured by hollow-using mammals. Such knowledge may then be used to manage natural hollows and conserve the species that depend on them.

Dr Ross Goldingay is Associate Professor in the School of Environment, Science & Engineering at Southern Cross University, and is currently the editor of Australian Mammalogy, the journal of the Australian Mammal Society.

More information

Goldingay RL (2011) Characteristics of tree hollows used by Australian arboreal and scansorial mammals. Australian Journal of Zoology 59, 277–294.

Goldingay RL (2009) Characteristics of tree hollows used by Australian birds and bats. Wildlife Research 36, 394–409.

Goldingay RL and Stevens JL (2009) Use of artificial tree hollows by Australian birds and bats. Wildlife Research 36, 81–97.

Published: 25 November 2014

Things warm up as the East Australian Current heads south

Jaci Brown

Occasional erratic bursts southward of the East Australian Current (EAC) are thought to have moderated the weather of south-east Australia this autumn and winter and they continue to introduce tropical and sub-tropical marine species to Tasmanian waters.

Tasmania’s east coast: tropical and sub-tropical marine species normally found off NSW are finding their way further south, thanks to changes in the East Australian Current.

Credit: spelio/flickr under CC BY-NC-SA 2.0

Ocean monitoring by Australia’s Integrated Marine Observing System is providing scientists with significant new insights into the changing structure of the EAC. Over the past 50 years sporadic warm bursts have become more common as the EAC moves further south. With global warming, the warm burst we’ve seen this year may also become the norm.

Had our little friend Nemo the clownfish been riding the EAC this year he might have found himself holidaying in Tasmania rather than admiring the Sydney Opera House. He wouldn’t have been on the trip alone, though. Sea nettles (Chrysaora spp.) have headed from their usual home in Sydney to be found for the first time ever in Tasmania and the Gippsland Lakes.

<i>Chrysaora woodbridge</i>, or sea nettle, was found in surprising numbers in Tasmania this year.

Chrysaora woodbridge, or sea nettle, was found in surprising numbers in Tasmania this year.

Credit: copyright Lisa-ann Gershwin

Waters in the EAC travel southward along the east coast of Australia, with most of it splitting from the coast near Sydney and heading for New Zealand. A small part of the current, known as the EAC Extension, works its way southward past Victoria and Tasmania.

A typical signature in this region are the large eddies, around 200 kilometres across and hundreds of metres deep. Some of the warm water is trapped here along with marine life.

The EAC starts at the Great Barrier Reef and travels south to Sydney before turning eastward to New Zealand. Some of the water can still push southward via a series of strong eddies.

Credit: Eric Oliver

This year a larger proportion of the EAC was sent southward instead of breaking away to the east. Winter ocean temperatures off Bass Strait were around 19°C, an increase of 4°C. This impacted local fishing, beach conditions and the weather.

In the video (above) the animation on the left shows the actual sea surface temperature and speed of the ocean currents. The animation on the right shows the difference in the temperature from average conditions.

Through autumn and winter, you can see two interesting changes occur. A strong warm current heads down the coast from Sydney to the coast of Victoria. At the same time, warm water peels off from the EAC and swirls around in large eddies as it meanders toward Tasmania.

An unusual catch down south

One advantage of warm eddies is the refuge they provide for tuna. They congregate in the centre of the eddy where the waters are warm and dine at the nutrient-rich edges.

Local fishers in north-east Tasmania report a remarkable year that allowed them to fish longer than usual, providing game fishers with more opportunities to catch tuna.

Last summer’s (2013–2014) warmth provided an abundance of skipjack and striped marlin, while winter brought a run of bluefin tuna.

Redmap is a website where locals can report sightings of marine species that are unusual for a given area.

Last summer a manta ray, a tropical cartilaginous fish (in a group including rays and skates), was sighted off the north-eastern coast of Tasmania. Previously the southern-most sighting of a manta ray was just south of Sydney.

<i>Manta birostris</i> spotted off north-east Tasmania on Australia Day 2014.

Manta birostris spotted off north-east Tasmania on Australia Day 2014.

Credit: Redmap/Leo Miller

It’s not just new species visiting Tassie either. Local jellyfish such as the Lion’s Mane (Cyanea) – more commonly known as ‘snotty’ – are usually quite elusive, but turned up in unprecedented numbers last summer in Tasmania.

But there’s a catch

This movement south of the EAC may have an impact on other systems, including our health. We rely on fish such as those from the Tasman Sea as a source of omega-3 fatty acids for our brain health. But the concentration of omega-3 fatty acids in the fish is likely to decrease with global warming.

Algae are the original source of fatty acids. As our waters warm, we will see more of the algae from the tropics take up residence in the south-east.

But the algae from the tropics are much smaller, which means more steps in the food chain from the algae to the fish we eat. The more steps in the food chain, the more the omega-3 fatty acids in the fish are replaced by fatty acids that are less favourable to brain health.

The warmer coastal waters also contributed to the balmy autumn and winter in south-eastern Australia this year. Afternoon sea breezes cool coastal temperatures by drawing cool oceanic air onto the coast.

Sydney’s heat wave in May this year had 19 consecutive days of 22°C or more – this is partly due to the sea breezes failing to bring in the usual cooling air.

What’s causing the EAC to move south?

Over the past 50 years the EAC Extension has stretched about 350 km further south. This extension doesn’t happen smoothly but in erratic bursts.

The southward extent of the EAC is controlled by the collective behaviour of the winds between Australia and South America. Over that same 50-year period these winds changed their pattern due to a strengthening of a climate system known as the Southern Annular Mode.

The changes to this mode have been attributed to a combination of ozone depletion and increasing atmospheric CO₂.

One of the most robust and consistent responses of the climate system to increasing CO₂ is a further strengthening of the Southern Annular Mode.

So the result will likely be a further enhancement of the EAC extension southward and even warmer waters in the Tasman Sea.

Dr Jaci Brown is a senior research scientist with the Centre for Australian Weather and Climate Research (CAWCR), a partnership between CSIRO and the Bureau of Meteorology. Her research focuses on the El Nino Southern Oscillation (ENSO) and climate change. This article was originally published on The Conversation. Read the original article.

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