February Balwyn meeting report
Guest speaker: Caroline Wilson
Caroline Wilson trained as an urban ecologist and has extensively studied bats, although she has previously studied Whiskered Terns on Mud Island and Grey-crowned Babbler habitat in north east Victoria. She is now a Project Officer studying woodland bird environments for BirdLife Victoria.
Bats fly more efficiently than birds; they fly faster and use less energy. This is because they have a wing membrane, less air resistance and joints in their wing bones. In fact, bats can out-manoeuvre owls. They prefer to nest in hollows in trees and, like birds, they feed on nectar and insects. There are over 1000 species in the world, from micro-bats to the large fruit bats. Their economic value in the agricultural industry is about $3 billion annually in the US, where farmers even put out nest boxes for the bats. Bats are important for long distance dispersal of seed and pollen, especially fruit bats along the east coast of Australia.
Microbats are the most common kind of bat in Melbourne, in particular Gould’s wattled bat and the Lesser long-eared bat. Although the sounds of most bats are beyond the frequency of our hearing, and can only be heard using bat detectors which lower the frequency of the echo-location calls, we can actually hear the call of the white striped freetailed bat. There are 16 species of micro-bats in Melbourne, and are found throughout the suburbs. However, as you get closer to the city, urbanisation reduces the number and species present. In general, the narrowing bats can fly better over urbanised spaces, can use artificial nesting places, and so survive better in built up areas. This is important in urbanised areas, where the numbers of trees with hollows have declined from about 22 per hectare to six per hectare.
Because Caroline needed a large sample size for her studies, she chose the Gould’s wattled bat, for it is the species most common and widespread. She chose three sites; the Royal botanic Gardens, a typical ornamental city park, with many exotic trees; and Valley Reserve and Blackburn Lake as examples of bush parks, with many 40-year-old native trees. Small radios, weighing less than 0.5 g, were fitted to the bats, using silicon glue, in order to to track the bats. The radios were slowly lost through grooming.
Because bats change roosts regularly, in order to reduce parasites and predation, the bats had to be tracked daily. A total of 135 roost trees were found, mostly within the parks and reserves, although at Blackburn Lake many were also found in nearby gardens, which often contained mature native trees. When a roost tree was found, characteristics (state of decay, diameter, height, species) of the tree were measured, together with those of sample trees within a 50 m radius. This was to see what was available for roosts, and what was chosen.
In parks, bats tended to choose cavities in decayed trees, especially beside walking tracks, which gave the bats more open space to escape into or hunt. Unfortunately, these trees are often removed during park maintenance. In the Royal Botanic Gardens there are not many decayed trees. Instead the bats use the eight cypress pines there, with deep crevices in the bark, and the dead fronds of palms. Such roosts are less insulated than tree hollows. In these trees, the most common roosting site was unusually low and much wider than the bat, making it more susceptible to predators and weather than usual. In the Botanic Gardens and in urban areas, where there are fewer preferred roosting sites, bats were re-using roosts more often than in the bush parks.
In order to discover what habitat features they are using, 21 bats were tracked over night, recording their positions ((GPS position, direction and signal strength) at 15 minute intervals. Bats foraged up to 1500 m from the roost site, preferring sites with a high density of trees and proximity to water. Thus they spent most of their time within the reserves, especially within the Botanic Gardens, where there is little green space and water within the nearby urban area.
How does urbanisation affect bats?
Six study sites of different degrees of urbanisation, were defined in terms of numbers of tree and amount of closed space (roads, buildings), with 10 observations within each. A light trap and bat detector was set up at each site.
Some 70,000 insects were caught in the light traps and identified down to order. In terms of biomass, there was a negative correlation with the roads (urbanisation), distance to water and urban light and a positive correlation with temperature and numbers of trees. The bat detector results showed that bat numbers increased linearly with insect biomass over the 60 observatins.
Caroline left me with many issues in my mind of these cute (I think of them as gothically beautiful) animals.
Dead trees are important for urban bats: Councils could help to conserve bats by trimming only those dead branches by walking tracks, leaving a safe part attached to the tree. The same could apply to our gardens with benefit for bats and birds.
Palm trees can be used as alternative roosting sites, as well as trees with suitable (rough or stringy) bark.
More trees mean more prey for bats (and birds).
Although nest boxes are used by bats, only six of the 16 species in Melbourne use them. There needs to be studies on designs suitable for other bat species.
Since preferred foraging locations have lots of trees near water, perhaps we need more ponds or fountains in parks, to provide more food for bats and birds.
Thank you Caroline, for a fascinating talk.
Contributor: Ron Garrett