The physiological requirements of endemic species of microhylid frogs and their importance to accurately predict their response to different scenarios of climate change in the Wet Tropics Biogeographical Region, north-eastern Australia.
AIM
The aim of this project is quantify the real physiological limits and tolerances of microhylid frogs when exposed to different climate conditions. The project will compare relative tolerances among species and between populations in some species, and measure the actual tolerance of species and relate these directly to real conditions experienced in the field. This information will be used to produce mechanistic based distribution models and predictions of climate change impacts.
BACKGROUND
Climate change is now considered the most threatening processes to biodiversity in the Wet Tropics region. Climate change has the potential to cause mass extinction events within the Wet Tropics endemic fauna, and some authors have stated that microhylid frogs are one of the most vulnerable groups in this region. However, current predictions are based on the assumption that climate is the most important factor limiting the distribution of species. This project will directly test the mechanistic basis of these models by determining the actual physiological tolerances of representative amphibian species. Information from the study will enable us to determine which species are most threatened by climate change. This will help target management resources towards those species that are most likely to be impacted by increased temperatures, seasonality and extreme climatic events in the future.
METHODS
Physiology experiments
Three different experiments are proposed to quantify different aspects of physiological responses to variation in temperature and water availability:
1) Quantify metabolic rate and water loss under different conditions of temperature and humidity
2) Quantify temperature tolerances
3) Quantify temperature preferences.
Wherever possible these procedures will be conducted in the field as close as possible to the site of capture, to minimise stress and the duration of captivity. All the measurements proposed are designed to produce non-lethal indices of physiological performance and tolerance.
We will use flow-through respirometry to quantify metabolic rate and water loss under varying temperatures and humidities. The frogs will be held in a small (100ml) analytical chamber regulated to achieve specific conditions of temperature and humidity. Controlled air (ambient oxygen and carbon dioxide and regulated humidity) will be pumped through the chamber, and metabolism and water loss will be calculated from flow rates and changes in gas concentrations. Water loss will also be quantified gravimetrically, from mass loss of the frog during the experiment.
To quantify temperature tolerances, animals will be placed on a metal tray (containing a thin film of water) floated on a water bath. The temperature of the water bath will be slowly heated or cooled relative to ambient temperature and the performance of animals tested at regular time intervals. Performance will be quantified as the time taken to respond to an external stimulus (ie. time to recover a normal position if the animal is placed on back). Each frog will be under experimentation for 1-2 hour and initially performance will be evaluated every five degrees Celsius (range proposed: 10 – 35 ºC). Between 25 and 35oC the frog will be monitored constantly and tested at any sign of a change in behaviour.
To quantify temperature preferences, individuals will be housed in a linear temperature gradient ranging from 10 - 40 oC, but with a moist substrate such as a wet sponge, and allowed to move freely and select for optimum temperature. The position of individuals along the gradient at its associated temperature will be recorded through visual observation. The duration of experimental trial will be 24 hours. The position of the frogs in the gradient will be recorded every 30 minutes, by a video camera recorder with infrared illumination to avoid disturbing the frog.
If egg masses of Cophixalus ornatus (the most common and wide spread species of microhylid frog) are found in the field, the first experiment will be replicated to measure water loss and metabolism of the eggs (using a mass of up to 10 eggs).
Data from 10 – 20 individuals of each species will be collected to complete the three experiments. The actual number of individuals will depend on abundance of the species at the specific site, seasonality, and time availability. In addition to physiological trials, morphology of all experimental individuals will be measured. A single toe clip will also be taken for ongoing genetic work and to enable retrospective species identification. Evidence of marking via toe clip will be used to ensure that no individual is recollected and subject to physiological experiments in subsequent field visits.
Physiological experiments will be carried out in the field shortly following capture of animals. This has a number of benefits. Most importantly, the approach will minimise undue stress associated with transporting animals and housing individuals for long periods in captivity. By removing these stressors we believe that our experiments will better reflect the actual physiological tolerances of species in nature, and maximise the chances that the frogs are able to immediately reoccupy their normal home territory as it is likely that microhylid frogs have very high site fidelity and may defend their territory.
Microhabitat environmental conditions
Logs, rocks and epiphytes fallen from trees have been identified as microhylid frog diurnal refuges. The environmental conditions of the microhabitats will be recorded. Temperature and humidity dataloggers will be set in refuges used by frogs covering the altitudinal and latitudinal gradient of the Wet Tropics. This microhabitat data is being collected from the same sites where the physiology experiments are being carried out.
The microhabitat environmental information will be compared with the macroenvironmental conditions in the same site. The buffering properties of the refuges will calculated and described according to altitude, latitude, and refuge type and size.
Frog activity information will be collected in order to describe the real environmental conditions the frogs are exposed to during their circadian and seasonal cycles.
Modelling new maps of the climate change impact on microhylid frogs distribution
Finally, all the information collected from the frogs and the microhabitats will be combined to generate new maps of the impact of climate change on the distribution range of the microhylid frogs. The maps will be model under different scenarios of climate change.
These new maps will be a more realistic than the ones available at the present to predict future impacts of climate change. Biodiversity managers and conservation biologists may able to use these maps to minimize the impact of climate change on leaf-litter frog populations in the future.