Joanne L. Isaac

Towards an integrated framework for assessing the vulnerability of species to climate change.

Climate change is a major threat to global biodiversity. A novel integrated framework to assess vulnerability and prioritize research and management action aims to improve our ability to respond to this emerging crisis.

Rarity of a top predator triggers continent-wide collapse of mammal prey: dingoes and marsupials in Australia

Top predators in terrestrial ecosystems may limit populations of smaller predators that could otherwise become over abundant and cause declines and extinctions of some prey. It is therefore possible that top predators indirectly protect many species of prey from excessive predation. This effect has been demonstrated in some small-scale studies, but it is not known how general or important it is in maintaining prey biodiversity. During the last 150 years, Australia has suffered the worlds highest rate of mammal decline and extinction, and most evidence points to introduced mid-sized predators (the red fox and the feral cat) as the cause. Here, we test the idea that the decline of Australias largest native predator, the dingo, played a role in these extinctions. Dingoes were persecuted from the beginning of European settlement in Australia and have been eliminated or made rare over large parts of the continent. We show a strong positive relationship between the survival of marsupials and the geographical overlap with high-density dingo populations. Our results suggest that the rarity of dingoes was a critical factor which allowed smaller predators to overwhelm marsupial prey, triggering extinction over much of the continent. This is evidence of a crucial role of top predators in maintaining prey biodiversity at large scales in terrestrial ecosystems and suggests that many remaining Australian mammals would benefit from the positive management of dingoes.

Ecological specialization and population size in a biodiversity hotspot: how rare species avoid extinction

Species with narrow environmental niches typically have small geographic ranges. Small range size is, in turn, often associated with low local abundance. Together, these factors should mean that ecological specialists have very small total populations, putting them at high risk of extinction. But some specialized and geographically restricted species are ancient, and some ecological communities have high proportions of rare and specialized endemics. We studied niche characteristics and patterns of distribution and abundance of terrestrial vertebrates in the rainforests of the Australian Wet Tropics (AWT) to identify mechanisms by which rare species might resist extinction. We show that species with narrow environmental niches and small geographic ranges tend to have high and uniform local abundances. The compensation of geographic rarity by local abundance is exact, such that total population size in the rainforest vertebrates of the AWT is independent of environmental specialization. This effect would tend to help equalize extinction risk for specialists and generalists. Phylogenetic analysis suggests that environmental specialists have been gradually accumulating in this fauna, indicating that small range size/environmental specialization can be a successful trait as long as it is compensated for by demographic commonness. These results provide an explanation of how range-restricted specialists can persist for long periods, so that they now form a major component of high-diversity assemblages such as the AWT.

Distributions, life‐history specialization, and phylogeny of the rain forest vertebrates in the Australian Wet Tropics

The purpose of this data set was to compile distributional, general life-history characteristics and phylogenies for Australian tropical rain forest vertebrates to inform a wide range of comparative studies on the determinants of biodiversity patterns and to assess the impacts of global climate change. We provide three distinct data sets: (1) a table of species-specific distributional and life-history traits for 242 vertebrate species found in the rain forests of the Australian Wet Tropics; (2) species distribution maps (GIS raster files) for 202 of the species displaying both the realized and potential distributions; and (3) phylogenies for these species. These species represent 93 birds, 31 amphibians, 31 mammals (including one monotreme), and 47 reptiles. Where information exists, the distributional and life-history data compiled here present information on: indices of environmental specialization (ENFA), habitat specialization, average body mass and size, sexual dimorphism, reproductive characteristics such as age at first reproduction, clutch/litter size, number of reproductive bouts per year and breeding seasonality, longevity, time of day when most active, and dispersal ability; distributional characteristics such as range size (potential and realized for both total and core ranges) and observed ranges in temperature, precipitation, and elevation; and niche attributes such as environmental marginality and specialization. The distribution maps provided represent a combination of presence-only ecological niche modeling (using MaxEnt) to estimate the potential distribution of a species followed by biogeographic clipping by expert opinion based on extensive field data and a subregional classification relevant to the topography and biogeographic history of the region to produce best-possible estimates of the realized distribution. Our assemblage contains many species with a shared evolutionary history, and thus many analyses of these data will need to account for phylogeny. Although a comprehensive phylogeny with branch length information does not exist for this diverse group of species, we present a best-estimate composite phylogeny constructed primarily from recently published molecular phylogenies of included groups.

Terminal reproductive effort in a marsupial

Life-history theory predicts that as organisms approach the end of their life, they should increase their reproductive effort (RE). However, studies on mammals often find that measures of RE do not vary with maternal age. This might be because offspring have some control over energy transfer which may constrain adaptive variation in RE by mothers, particularly in eutherian mammals where placental function is primarily controlled by offspring. However, in marsupials, energy transfer is primarily by lactation and under maternal control, leaving marsupial mothers free to vary RE. Here, we provide the first analysis, to our knowledge, of age-specific RE in a marsupial, the common brushtail possum. RE, measured as the proportion of maternal mass lost during lactation, was strongly correlated with offspring mass as a yearling. Older females had higher RE, gave birth earlier in the season and were more likely to produce two offspring in a year. Females with high RE in one year were lighter at the beginning of the next breeding season. These results provide the clearest support yet for terminal RE in a mammal.

Life history and demographics of an island possum

Knowledge of life history and demography is essential for effective management of target species. Here, I describe life history and demographic traits of the common brushtail possum (Trichosurus vulpecula) on Magnetic Island, north Queensland. Data were collected during monthly live-trapping sessions over three years (2001–2004). Density was ~100% higher than reported in most Australian populations, at 5 possums per hectare; losses from the population were more than compensated for by births and immigration. Age-specific survival differed between the sexes: male survival declined earlier and faster than female survival. Males were significantly heavier than females as adults and sexual dimorphism appears to arise through a faster growth rate in males following weaning. Age at maturity in females varied from 1 to 3 years; 61% of females produced their first young at age 2. Males became sexually mature at age 3. Breeding was seasonal, with >50% births occurring in April–May; a smaller birth peak in spring was due to some females producing a second young after they had successfully weaned a first. The Magnetic Island possum population appears to be more similar to New Zealand populations, in terms of their life history and demographics, than to other previously studied mainland Australian populations.

The Evolution of Body Shape in Response to Habitat: Is Reproductive Output Reduced in Flat Lizards?

Body size and shape are primary determinants of reproductive output in a variety of taxa, so selection favoring specific body sizes and shapes may, in turn, have a direct affect on reproductive output, and ultimately fitness. In reptiles, species that occupy rocky habitats are often flattened, a morphological character that aids locomotion and life on rocks, but which may constrain reproductive output by reducing the amount of abdominal space available to fill with eggs or offspring. Using 20 species of tropical skink from a wide range of habitats, we quantified habitat use, body height, body volume, and reproductive output, to determine whether the evolution of a flattened body was correlated with a reduction in abdominal volume, and, in turn, with reduced reproductive output. In this group of lizards, the occupation of rocky habitats has led (1) to the evolution of a flattened body, and this shift in body shape has (2) caused a reduction in abdominal volume. Despite this reduction in abdominal volume reproductive output was unaffected, as flatter species compensate by being more “full” of eggs. Thus, we demonstrate that morphological adaptation for enhanced performance in specific habitats did not cause a reduction in instantaneous reproductive output.

How hot do nest boxes get in the tropics? A study of nest boxes for the endangered mahogany glider

As hollow-bearing trees become scarcer due to habitat loss, the use of nest boxes as a management tool for hollow-dependent species is likely to increase. However, nest-box use can be variable among species and habitats, and one possible reason may be that nest boxes offer little protection against extreme temperatures compared with natural hollows; this may be particularly important in the tropics. Here, we measured the microclimate of 16 nest boxes, installed as part of a recovery program for an endangered arboreal marsupial, the mahogany glider, during the summer in tropical north Queensland. We also measured the microclimate of 14 naturally occurring refuges (hollows in standing and fallen trees) at the same study sites. Nest boxes were significantly hotter during the day than were natural refuges (either in fallen or standing live trees) and experienced a greater range of temperatures. The most important factors explaining variation in daytime temperature in boxes was box aspect and the amount of canopy cover directly above the box: boxes that faced north, and those with greater canopy cover, were up to 7°C cooler than those that faced south or had little cover. We discuss our results in relation to the use of nest boxes in management plans for arboreal marsupials in the tropics.

Mass loss during the mating season in male common brushtail possums (Trichosurus vulpecula): implications for age-specific variation in mating strategy

Little is known of patterns of male reproductive effort and investment, and their relationship with age-related differences in male mating strategy, in mammals. In this study, the age-specific patterns of reproductive expenditure in a male marsupial, the common brushtail possum Trichosurus vulpecula were investigated. Possums were studied for two breeding seasons on Magnetic Island, north Queensland, Australia. Energy expenditure in male possums during the mating season was estimated from changes in mass taken monthly during live-trapping. Male possums lost an average of 4.2% of their mass during the main mating season from January to May, suggesting that mating and consort behaviour is energetically expensive and may constrain foraging activity. The percentage of body mass lost by individual males during the breeding season was negatively related to age; older males directed less somatic investment into reproduction than young males. Older males seemed to target their energy expenditure toward the period when oestrus synchrony was highest. The pattern of age-specific energy expenditure for reproduction in T. vulpecula differs markedly from that reported previously for male cervids, probably owing to the spatial dispersal of females leading to differences in mating strategy. The results are discussed in terms of their relevance to potential age-related differences in reproductive strategy and behaviour.

Automated feeders: new technology for food supplementation experiments with mammals

Experimental manipulations of food supply in wild populations are often confounded by a number of factors, such as induced immigration to local populations receiving food supplements and intraspecific competition for access to feed stations. Here, we describe newly developed automated feeding stations, designed to dispense weighed food supplements to specific experimental animals. The feeders incorporate a data logger that records the identification and weight of each animal and the time of the visit to the feeder. Using the common brushtail possum (Trichosurus vulpecula) as a model medium-sized mammal, we present results of an 18-month field trial. Each free-ranging experimental possum was fitted with a collar containing a transponder chip, allowing them access to a feeder. During the field trial, experimental possums were found to show a significant increase in body mass compared with control animals, which showed a slight decrease in mass. Body masses recorded by the feeders for experimental possums did not differ from mass data recorded during live-trapping sessions. The automated feeding stations represent an advance over previous methodology and the first time supplementary food has been delivered in a controlled automated fashion in a wild mammal population. The implications of the feeders to future studies of resource supplementation in mammals are discussed.