Bayesian Methods for Ecology

Conserving koalas: A review of the contrasting regional trends, outlooks and policy challenges

How climate impacts organisms depends not only on their physiology, but also whether they can buffer themselves against climate variability via their behaviour. One of the way species can withstand hot temperatures is by seeking out cool microclimates, but only if their habitat provides such refugia. Here, we describe a novel thermoregulatory strategy in an arboreal mammal, the koala Phascolarctos cinereus. During hot weather, koalas enhanced conductive heat loss by seeking out and resting against tree trunks that were substantially cooler than ambient air temperature. Using a biophysical model of heat exchange, we show that this behaviour greatly reduces the amount of heat that must be lost via evaporative cooling, potentially increasing koala survival during extreme heat events. While it has long been known that internal temperatures of trees differ from ambient air temperatures, the relevance of this for arboreal and semi-arboreal mammals has not previously been explored. Our results highlight the important role of tree trunks as aboveground 'heat sinks', providing cool local microenvironments not only for koalas, but also for all tree-dwelling species.

https://royalsocietypublishing.org/doi/pdf/10.1098/rsbl.2014.0235

Tree-hugging koalas demonstrate a novel thermoregulatory mechanism for arboreal mammals.

Climate refugia are regions that animals can retreat to, persist in and potentially then expand from under changing environmental conditions. Most forecasts of climate change refugia for species are based on correlative species distribution models (SDMs) using long-term climate averages, projected to future climate scenarios. Limitations of such methods include the need to extrapolate into novel environments and uncertainty regarding the extent to which proximate variables included in the model capture processes driving distribution limits (and thus can be assumed to provide reliable predictions under new conditions). These limitations are well documented; however, their impact on the quality of climate refugia predictions is difficult to quantify. Here, we develop a detailed bioenergetics model for the koala. It indicates that range limits are driven by heat-induced water stress, with the timing of rainfall and heat waves limiting the koala in the warmer parts of its range. We compare refugia predictions from the bioenergetics model with predictions from a suite of competing correlative SDMs under a range of future climate scenarios. SDMs were fitted using combinations of long-term climate and weather extremes variables, to test how well each set of predictions captures the knowledge embedded in the bioenergetics model. Correlative models produced broadly similar predictions to the bioenergetics model across much of the species' current range - with SDMs that included weather extremes showing highest congruence. However, predictions in some regions diverged significantly when projecting to future climates due to the breakdown in correlation between climate variables. We provide unique insight into the mechanisms driving koala distribution and illustrate the importance of subtle relationships between the timing of weather events, particularly rain relative to hot-spells, in driving species-climate relationships and distributions. By unpacking the mechanisms captured by correlative SDMs, we can increase our certainty in forecasts of climate change impacts on species.

Unpacking the mechanisms captured by a correlative species distribution model to improve predictions of climate refugia

Aim: The koala is a widely distributed Australian marsupial with regional populations that are in rapid decline, are stable or have increased in size. This study examined whether it is possible to use expert elicitation to estimate abundance and trends of populations of this species. Diverse opinions exist about estimates of abundance and, consequently, the status of populations. Location: Eastern and south-eastern Australia Methods: Using a structured, four-step question format, a panel of 15 experts estimated population sizes of koalas and changes in those sizes for bioregions within four states. They provided their lowest plausible estimate, highest plausible estimate, best estimate and their degree of confidence that the true values were contained within these upper and lower estimates. We derived estimates of the mean population size of koalas and associated uncertainties for each bioregion and state. Results: On the basis of estimates of mean population sizes for each bioregion and state, we estimated that the total number of koalas for Australia is 329,000 (range 144,000-605,000) with an estimated average decline of 24% over the past three generations and the next three generations. Estimated percentage of loss in Queensland, New South Wales, Victoria and South Australia was 53%, 26%, 14% and 3%, respectively. Main conclusions: It was not necessary to achieve high levels of certainty or consensus among experts before making informed estimates. A quantitative, scientific method for deriving estimates of koala populations and trends was possible, in the absence of empirical data on abundances.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/ddi.12400

Use of expert knowledge to elicit population trends for the koala (Phascolarctos cinereus)

The decisions that animals must make to achieve a balance between quantity and quality of resources become more difficult when their habitats are patchy and differ greatly in quality across space and time. Koalas are a prime subject to study this problem because they have a specialised diet of eucalypt leaves and need to balance nutrient and water intake against toxins in the leaves, all of which can change with soil type and climate. Koalas are nocturnal and spend most of the day resting and therefore choose trees for reasons other than feeding, particularly for thermoregulation. We GPS-tracked 40 koalas over 3 yr to determine their shift in tree selection between day and night, and in relation to daily maximum temperature, in a patchy rural landscape in north-western NSW, Australia. The species, degree of shelter, diameter, height and elevation of each visited tree were recorded. We used generalised linear mixed effects models to compare tree use between day and night and maximum daily temperature. Koalas used more feed-trees during the night, and more shelter-trees during the day. They also selected taller trees with more shelter in the day compared with night. As daytime temperatures rose, koalas increasingly selected taller trees at lower elevations. Our results demonstrate that koalas need taller trees, and non-feed species with shadier/denser foliage, to provide shelter from heat. This highlights the need both for the retention of taller, mature trees, such as remnant paddock trees, and the planting of both food and shelter trees to increase habitat area and connectivity across the landscape for arboreal species. Retaining and planting trees that provide optimum habitat will help arboreal folivores cope with the more frequent droughts and heatwaves expected with climate change.

Climate‐mediated habitat selection in an arboreal folivore

Koalas are prime candidates to study the impact of climate change because they are specialised folivores and lack any ready means of avoiding weather extremes. Koalas are widely but patchily distributed throughout eastern mainland Australia. Efforts to protect them from landscape-scale threats have been identified in the NSW 2008 Koala Recovery Plan, the 2010 NSW Priorities for biodiversity adaptation to climate change and the 2009-14 National Koala Conservation and Management Strategy. The statements in the formal strategies and recovery plans identify a number of problems, two of which we address in this paper. The first problem is that of extreme weather and the second is the change of leaf quality from rising levels of carbon dioxide. This paper capitalises on our field study in Gunnedah, in north-west NSW, which examined a 1990s success story where the local koala population benefited from the plantings of trees and shrubs to hold down the water table in the face of a rising salinity crisis. In late 2009, heatwaves killed an estimated 25% of the Gunnedah koala population. This foreshadows how increased climate variability will impact on koala populations. In 2008, chlamydiosis - a disease causing infertility - had been established as being present in the Gunnedah population. The likely spread of this disease throughout the Gunnedah koala population presents a further challenge to wildlife managers in the context of a changing climate. The potential indirect effects of global climate change - how increasing concentrations of atmospheric CO2 may reduce the availability of the nutrients in Eucalyptus foliage to koalas - is described and the implication drawn that elevated concentrations of atmospheric CO2 may threaten some populations of free-ranging koalas. The Liverpool Plains are among Australia's prime agricultural landscapes where the conservation of biodiversity occurs largely on private land. Consequently, we need to integrate climate change adaptation with rural land management and restoration practices. The research demonstrates the contribution from the cross-disciplinary links. It adds to our ability to monitor sustainable native fauna populations and threatened species by distinguishing among the multiple causes of population change, and it can also be viewed as a pilot demonstrating the value of longitudinal wild population disease monitoring.

Koalas and climate change: a case study on the Liverpool Plains, north-west New South Wales

Cryptococcosis, caused by environmental fungi in the Cryptococcus neoformans and Cryptococcus gattii species complexes, affects a variety of hosts, including koalas (Phascolarctos cinereus). Cryptococcal antigenemia and nasal colonization are well characterized in captive koalas, but free-ranging populations have not been studied systematically. Free-ranging koalas (181) from the Liverpool Plains region of New South Wales, Australia, were tested for cryptococcal antigenemia (lateral flow immunoassay) and nasal colonization (bird seed agar culture). Results were related to environmental and individual koala characteristics. Eucalypt trees (14) were also randomly tested for the presence of Cryptococcus spp. by bird seed agar culture. In sum, 5.5% (10/181) and 6.6% (12/181) of koalas were positive for antigenemia and nasal colonization, respectively, on at least one occasion. And 64.3% (9/14) of eucalypts were culture-positive for Cryptococcus spp. URA5 restriction fragment length polymorphism analysis identified most isolates as C. gattii VGI, while C. neoformans VNI was only found in one koala and one tree. Colonized koalas were significantly more likely to test positive for antigenemia. No associations between antigenemia or colonization, and external environmental characteristics (the relative abundance of Eucalyptus camaldulensis and season), or individual koala characteristics (body condition, sex, and age), could be established, suggesting that antigenemia and colonization are random outcomes of host-pathogen-environment interactions. The relationship between positive antigenemia status and a relatively high abundance of E. camaldulensis requires further investigation. This study characterizes cryptococcosis in a free-ranging koala population, expands the ecological niche of the C. gattii/C. neoformans species complexes and highlights free-ranging koalas as important sentinels for this disease.

Prevalence of cryptococcal antigenemia and nasal colonization in a free-ranging koala population

Context Precise and accurate estimates of animal numbers are often essential for population and epidemiological models, as well as for guidance for population management and conservation. This is particularly true for threatened species in landscapes facing multiple threats. Estimates can be derived by different methods, but the question remains as to whether these estimates are comparable. Aims We compared three methods to estimate population numbers, namely, distance sampling, mark–recapture analysis, and home-range overlap analysis, for a population of the iconic threatened species, the koala (Phascolarctos cinereus). This population occupies a heavily fragmented forest and woodland habitat on the Liverpool Plains, north-western New South Wales, Australia, on a mosaic of agricultural and mining lands. Key results All three methods produced similar estimates, with overlapping confidence intervals. Distance sampling required less expertise and time and had less impact on animals, but also had less precision; however, future estimates using the method could be improved by increasing both the number and expertise of the observers. Conclusions When less intrusive methods are preferred, or fewer specialised practitioners are available, we recommend distance sampling to obtain reliable estimates of koala numbers. Although its precision is lower with a low number of sightings, it does produce estimates of numbers similar to those from the other methods. However, combining multiple methods can be useful when other material (genetic, health and demographic) is also needed, or when decisions based on estimates are for high-profile threatened species requiring greater confidence. We recommend that all estimates of population numbers, and their precision or variation, be recorded and reported so that future studies can use them as prior information, increasing the precision of future surveys through Bayesian analyses.

Comparison of three methods of estimating the population size of an arboreal mammal in a fragmented rural landscape

Koalas (Phascolarctos cinereus) are difficult to detect due to their cryptic nature, occurrence at low densities and broad distribution. We compare the relative effectiveness of two common, direct survey techniques used to detect koalas: spotlighting and diurnal searches. Seventy-six 2-ha sites were surveyed using both spotlighting and diurnal searching. Each site was surveyed for 0.5 person-hours, such that search area and effort were equal. In this study, spotlighting was found to be 3.25 times more effective at detecting koalas than day searches. Therefore, where access and terrain allows, spotlighting surveys offer a significant advantage over diurnal searches in the detection of koalas.

George Madani

Papers

Climate‐mediated habitat selection in an arboreal folivore

Koalas and climate change: a case study on the Liverpool Plains, north-west New South Wales

Prevalence of cryptococcal antigenemia and nasal colonization in a free-ranging koala population

Comparison of three methods of estimating the population size of an arboreal mammal in a fragmented rural landscape

Are koalas detected more effectively by systematic spotlighting or diurnal searches