The guest speaker for the July meeting was Dr. Nick Dunlop from the Conservation Council of Western Australia and the topic was Climate change responses in West Australian seabirds and microbats. It is generally accepted that climate change is occurring but what is not clear is how plants and animals will respond. How do we study this? At first glance sea birds and microbats would seem to be a disparate group to study but they do have one thing in common – they fly and are able to change their habitat ranges. Research into WA seabirds has focused on the role of foraging ecology in contrasting responses to a changing ocean climate in two dark terns, the Bridled Tern and the Common (or Brown) Noddy, two similar species but with distinctly different responses to changes in ocean climate. A reasonable data set for both species has been compiled over the last thirty years.
Bridled Terns have been studied on Penguin Island over that period and in the most recent nesting period two birds were captured that had first been banded there twenty five years ago. The west coast of WA experiences a tropical, south flowing current, the Leeuwin Current, that has warm, low nutrient waters flowing in a southward direction. The strength of this current is affected by El Nino and La Nina events and also by the Indian Ocean Dipole. Data shows that waters off the south west of WA have warmed by 0.9C over the last 30 years. The best point of reference for the two tern species is the Houtman Abrolhos Islands, which were previously the southern extent of the tropical species. The Dark Tern Guild states that both tern species forage and live in similar habitats. However, the Bridled Tern has been found in the Shoalwater area since the 1920’s and are now found as far south as the Recherche Archipelago, co-located with Fur Seals and Little Penguins. Their wintering area is Borneo. South West Australia is now home to the largest Bridled Tern colony in the world. They feed on small fish and also on larval growth on mats of flotsam and jetsam or sea weed mats.
The Common Noddy was not found south of the Houtman Abrolos until the early 1990’s when they established a colony on Lancelin Island. They feed on larval fish and squid – a different diet to that of the Bridled Tern. They are commonly associated with foraging tuna. Both tern species are contact dippers and do not dive. Differences between the two species may be summarised as follows. Bridled Terns have a foraging range of 20 – 80km, foraging habitat is offshore on the continental shelf, they prey on more than 20 taxa and can withstand a shift in prey species during the breeding season in the late chick rearing period. The Common Noddy has a foraging range of more than 100km, foraging habitat is oceanic, on the shelf edge canyons and beyond, they prey on only 2 – 3 taxa and cannot withstand a change in prey species within the breeding season. Chicks will often die if prey disappears before they are fully fledged.
Bridled Terns do not have any competition in their favoured areas and consequently have thrived, resulting in the large numbers that we now see. Suitable foraging areas for the Common Noddy within range of nesting areas do not occur to the south and this has limited the migration southwards. For the Common Noddy the colony at Lancelin Island increased due to migration until 2003, but since then recruitment has been from breeding pairs, evidence that natal recruitment slows or stops migration. Birds that are born at a site generally breed at that site; dispersal only occurs if something changes. However, the response is not immediate; it is generally the third year after the change that migration occurs. To test the effect of climate the Southern Oscillation Index (an indication of El Nino or La Nina weather patterns) was plotted against Bridled Tern and Common Noddy breeding. The results show that Bridled Terns breed better in El Nino events, whereas the Common Noddy does best in La Nina years. For both species laying dates are getting later by approximately one month.
The Charles Darwin Reserve is the location for research into the response of flora and fauna to climate change. The reserve is an area of uncleared land that straddles the boundary between mulga shrubland and eucalypt woodland bio-regions. Indicators being studied include vegetation communities, ant habitat, two species of Dunnarts and bats. Bats are especially good indicators because they can fly and are able to re-distribute. Microbats are mammals with a high energy demand, they go into torpor during the day and hibernate in winter. Climate change can have a significant impact. At the reserve a total of 11 species have been identified together with their foraging habitats (through isotope studies of fur). The different species have distinct foraging zones. It has been noticed that tropical bats are starting to move into the northern area of the study area. The White Striped Mastiff Bat is the only migratory bat in the area, it goes north in winter and south in summer. It is an active flyer and forages over a large area. It is prone to overheating and does not do well when average nightly temperatures exceed 20C. It is currently still breeding in the area but it is expected that they will move further south to breed if temperatures increase (this has already been observed at one colony near Brisbane).
The other species being studied are the Clinal Microbat species. These exhibit a change in forearm length in relation to environmental variables such as mean temperature and humidity so this parameter is being studied. As temperature increases so does forearm length. This research is ongoing. There were many questions for Nick after his presentation, an indication of the audience’s interest in the subject. We thank Nick for taking the time to share the results of his research with members of KRMB. Colin Prickett