By Dr Chris Wacker, UNE Discovery

During Australian wet weather events, the news is full of images of saturated, soggy and sodden koalas and exhausted kangaroos swimming through the rising flood waters. While the diversity of Australian mammals is substantial, with approximately 380 species, most of our native mammals are small and rarely seen. Because of their tiny size, it can be difficult to remember that they, too, are affected when the skies open and gullies turn into white water rapids.

Gulliver, the koala, was found during the floods in New South Wales in February 2022. He was released back into the wild after 6 months in care. Photo credit: Friends of the Koala.

While many environments in Australia depend on frequent flooding and drought cycles, the frequency, duration, timing, and severity of these cycles have changed significantly over the past 20 years. Because of this, not much is known about how floods affect our wildlife.

The Namoi River in flood at Gunnedah in 2021. Photo credit: www.abc.net.au (Zak Insch).

Why should we care about small mammal communities?

While our larger wildlife usually receives all the airtime, our small mammals are just as important, and the diversity of small mammals in an area can even tell us a lot about the overall health of an ecosystem. Small mammals also provide food for birds of prey and reptiles, and they can keep an area healthy through ecosystem engineering, seed caching, and pruning grasses and bushes.

The native broad-toothed rat creates runways through the thick alpine bushes that are then also used by small native marsupials. Photo credit: Chris Wacker.

The invasive house mouse has been extremely successful in Australia. Photo credit: The Australian Museum, Dick Whitford.

What affects an animal’s ability to survive floods?

Mammal surveys taken before and after short-term flooding events often show that there have been no changes in their population. This means that the population of small mammals can recover after the flood receded either because animals have survived the floods or because, even if animals have died, new individuals have moved into the area.

Whether or not small mammals survive floods or drown depends on many factors, including the biological characteristics of the species, such as its diet and home range, the normal conditions of the environment, such as the soil type, elevation, type and height of the predominant vegetation, and the characteristics of the flood itself, such as the height of the water and how long it lasts.

There has been much research comparing the ability of native and invasive species to survive during and after a flood. Even though invasive species such as house mice, the brown rat, and European hare tend to be more robust and numerous than many of our native species, there doesn’t seem to be much of a difference in the overall ability of introduced animals to survive a large flooding event.

A kangaroo swimming at Coffs Harbour. Most animals can swim to some extent. Photo credit: Brendo Lewis.

What are the options for small mammals during floods? What do we know?

Swimming
Most animals can swim. While they may not be able to swim across Bass Strait, they can at least do an awkward ‘dog paddle’ until exhaustion sets in.

Going upwards
It is possible that for short-term flooding, many little mammals can climb into the trees and wait for the flood waters to recede. It is also possible that they can move away from a river before it floods; however, with small legs and bodies, they are unable to outrun flood waters which rise and spread rapidly.

Torpor
Torpor is a nifty mechanism where animals can lower their metabolic rate so they don’t need as much food. We know that animals use torpor during rain, storms, cyclones, blizzards etc. but what about floods? If they do survive the flooding, torpor may help them to hide somewhere dry, if they can find it, and survive until the waters recede.

What are the challenges for small mammals during and after floods?

If animals can survive the flood, there are other challenges they must overcome when the waters recede:

• An increase in pathogens and exposure to heavy metals after floods
• Silt and topsoil can wash into rivers and kill plants and fish that may be food for small mammals
• The particular plant or insect species that some small mammals eat may be lacking. However, there may also be an increase in one or two insect species, such as mosquitos.

Conclusions:

While not much is known about how floods affect small mammals, it is clear that floods are a significant and disruptive event for most animals. Floods not only mean that many animals may die but also that the whole population structure and the animal community can change. They may be little, but over time changes in small mammal populations can have a huge impact.

References:

Andersen, D.C., Wilson, K.R., Miller, M.S., and Falck, M. (2000) Movement patterns of riparian small mammals during predictable floodplain inundation. Journal of Mammalogy, Volume 81, Issue 4, Pages 1087–1099.

Boulton, A.J. and Lloyd, L.N. (1992) Flooding frequency and invertebrate emergence from dry floodplain sediments of the river Murray, Australia. Regulated Rivers Research and Management, Volume 7, pages 137-151.

Golet, G.H., Hunt, J.W., and Koenig, D. (2013) Decline and recovery of small mammals after flooding: implications for pest management and floodplain community dynamics. River Research and Applications, Volume 29, Issue 2, pages 183–194.

Greenville, A.C., Wardle, G.M., and Dickman, C.R. (2012) Extreme climatic events drive mammal irruptions: regression analysis of 100-year trends in desert rainfall and temperature. Ecology and Evolution. Volume 2, issue 11, pages 2645-2658.

Lada, H., MacNally, R., and Taylor, A.C. (2008) Responses of a carnivorous marsupial (Antechinus flavipes) to local habitat factors in two forest types, Journal of Mammalogy, Volume 89, Issue 2.

Nowack, J., Rojas, A.D., Körtner, G., and Geiser, F. (2015) Snoozing through the storm: torpor use during a natural disaster. Scientific Reports. Volume 15, Issue 5.

Thibault, K. and Brown, J. (2008) Impact of an extreme climatic event on community assembly. Proceedings of the National Academy of Sciences of the United States of America. Volume 105, pages 3410-3415.

Van Dyck, S, and Strahan, R. (Editors). (2008). The mammals of Australia. New Holland Pub Pty Limited.