Showing papers by "Stephen E. Williams published in 2009"

Abundance and the environmental niche: Environmental suitability estimated from niche models predicts the upper limit of local abundance

Abstract: Ecologists seek to understand patterns of distribution and abundance of species. Studies of distribution often use occurrence data to build models of the environmental niche of a species. Environmental suitability (ES) derived from such models may be used to predict the potential distributions of species. The ability of such models to predict spatial patterns in abundance is unknown; we argue that there should be a positive relationship between ES and local abundance. This will be so if ES reflects how well the species’ physiological and ecological requirements are met at a site and if those factors also determine local abundance. However, the presence of other factors may indicate that potential abundance is not attained at all sites. Therefore, ES should predict the upper limit of abundance, and the observed relationship with ES should be wedge shaped. We tested the relationship of ES with local abundance for 69 rain forest vertebrates in the Australian wet tropics. Ordinary least squares and ...

Identification and dynamics of a cryptic suture zone in tropical rainforest

Abstract: Suture zones, shared regions of secondary contact between long-isolated lineages, are natural laboratories for studying divergence and speciation. For tropical rainforest, the existence of suture zones and their significance for speciation has been controversial. Using comparative phylogeographic evidence, we locate a morphologically cryptic suture zone in the Australian Wet Tropics rainforest. Fourteen out of 18 contacts involve morphologically cryptic phylogeographic lineages, with mtDNA sequence divergences ranging from 2 to 15 per cent. Contact zones are significantly clustered in a suture zone located between two major Quaternary refugia. Within this area, there is a trend for secondary contacts to occur in regions with low environmental suitability relative to both adjacent refugia and, by inference, the parental lineages. The extent and form of reproductive isolation among interacting lineages varies across species, ranging from random admixture to speciation, in one case via reinforcement. Comparative phylogeographic studies, combined with environmental analysis at a fine-scale and across varying climates, can generate new insights into suture zone formation and to diversification processes in species-rich tropical rainforests. As arenas for evolutionary experimentation, suture zones merit special attention for conservation.

Resistance and resilience: quantifying relative extinction risk in a diverse assemblage of Australian tropical rainforest vertebrates

Abstract: Aim: Assessing the relative vulnerability of species within an assemblage to extinction is crucial for conservation planning at the regional scale Here, we quantify relative vulnerability to extinction, in terms of both resistance and resilience to environmental change, in an assemblage of tropical rainforest vertebrates Location: Wet Tropics Bioregion, north Queensland, Australia Methods: We collated data on 163 vertebrates that occur in the Australian Wet Tropics, including 24 frogs, 33 reptiles, 19 mammals and 87 birds We used the 'seven forms of rarity' model to assess relative vulnerability or resistance to environmental change We then develop a new analogous eight-celled model to assess relative resilience, or potential to recover from environmental perturbation, based on reproductive output, potential for dispersal and climatic niche marginality Results: In the rarity model, our assemblage had more species very vulnerable and very resistant than expected by chance There was a more even distribution of species over the categories in the resilience model The three traits included in each model were not independent of each other; species that were widespread were also habitat generalists, while species with narrow geographical ranges tended to be locally abundant In the resilience model, species with low reproductive output had a narrow climatic niche and also a low capacity to disperse Frogs were the most vulnerable taxonomic group overall The model categories were compared to current IUCN category of listed species, and the product of the two models was best correlated with IUCN listings Main conclusions: The models presented here offer an objective way to predict the resistance of a species to environmental change, and its capacity to recover from disturbance The new resilience model has similar advantages to the rarity model, in that it uses simple information and is therefore useful for examining patterns in assemblages with many poorly known species

New approaches to understanding late Quaternary climate fluctuations and refugial dynamics in Australian wet tropical rain forests

Abstract: Aim: We created spatially explicit models of palaeovegetation stability for the rain forests of the Australia Wet Tropics. We accounted for the climatic fluctuations of the late Quaternary, improving upon previous palaeovegetation modelling for the region in terms of data, approach and coverage of predictions. Location: Australian Wet Tropics. Methods: We generated climate-based distribution models for broad rain forest vegetation types using contemporary and reconstructed ‘pre-clearing’ vegetation data. Models were projected onto previously published palaeoclimate scenarios dating to c. 18 kyr bp. Vegetation stability was estimated as the average likelihood that a location was suitable for rain forest through all climate scenarios. Uncertainty associated with model projections onto novel environmental conditions was also tracked. Results: Upland rain forest was found to be the most stable of the wet forest vegetation types examined. We provide evidence that the lowland rain forests were largely extirpated from the region during the last glacial maximum, with only small, marginally suitable fragments persisting in two areas. Models generated using contemporary vegetation data underestimated the area of environmental space suitable for rain forest in historical time periods. Model uncertainty resulting from projection onto novel environmental conditions was low, but generally increased with the number of years before present being modelled. Main conclusions: Climate fluctuations of the late Quaternary probably resulted in dramatic change in the extent of rain forest in the region. Pockets of highstability upland rain forest were identified, but extreme bottlenecks of area were predicted for lowland rain forest. These factors are expected to have had a dramatic impact on the historical dynamics of population connectivity and patterns of extinction and recolonization of dependent fauna. Finally, we found that models trained on contemporary vegetation data can be problematic for reconstructing vegetation patterns under novel environmental conditions. Climatic tolerances and the historical extent of vegetation may be underestimated when artificial vegetation boundaries imposed by land clearing are not taken into account.

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

Abstract: 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.

Variable responses of skinks to a common history of rainforest fluctuation: concordance between phylogeography and palaeo-distribution models

Abstract: There is a growing appreciation of impacts of late-Quaternary climate fluctuations on spatial patterns of species and genetic diversity. A major challenge is to understand how and why species respond individualistically to a common history of climate-induced habitat fluctuation. Here, we combine modelling of palaeo-distributions and mitochondrial-DNA phylogeographies to compare spatial patterns of population persistence and isolation across three species of rainforest skinks (Saproscincus spp.) with varying climatic preferences. Using Akaike Information Criterion model-averaged projections, all three species are predicted to have maintained one or more small populations in the northern Wet Tropics, multiple or larger populations in the central region, and few if any in the south. For the high-elevation species, Saproscincus czechurai, the warm–wet climate of the mid Holocene was most restrictive, whereas for the generalist S. basiliscus and lower-elevation S. tetradactyla, the cool–dry last glacial maximum was most restrictive. As expected, S. czechurai was the most genetically structured species, although relative to modelled distributions, S. basiliscus had surprisingly deep phylogeographical structure among southern rainforest isolates, implying long-term isolation and persistence. For both S. basiliscus and S. tetradactyla, there was high genetic diversity and complex phylogeographical patterns in the central Wet Tropics, reflecting persistence of large, structured populations. A previously identified vicariant barrier separating northern and central regions is supported, and results from these species also emphasize a historical persistence of populations south of another biogeographical break, the Tully Gorge. Overall, the results support the contention that in a topographically heterogeneous landscape, species with broader climatic niches may maintain higher and more structured genetic diversity due to persistence through varying climates.

How do species respond to climate change along an elevation gradient? A case study of the grey‐headed robin (Heteromyias albispecularis)

Abstract: Climate is predicted to change rapidly in the current century, which may lead to shifts of species’ ranges, reduced populations and extinctions. Predicting the responses of species abundance to climate change can provide valuable information to quantify climate change impacts and inform their management and conservation, but most studies have been limited to changes in habitat area due to a lack of abundance data. Here, we use generalized linear model and Bayesian information criteria to develop a predictive model based on the abundance of the grey-headed robin (GHR) and the data of climatic environmental variables. The model is validated by leave-one-out cross-validation and equivalence tests. The responses of GHR abundance, population size and habitat area by elevation are predicted under the current climate and 15 climate change scenarios. The model predicts that when temperature increases, abundance of GHR displays a positive response at high elevation, but a negative response at low elevation. High precipitation at the higher elevations is a limiting factor to GHR and any reduction in precipitation at high elevation creates a more suitable environment, leading to an increase in abundance of GHR, whereas changes in precipitation have little impact at low elevation. The loss of habitat is much more than would otherwise be assumed in response to climate change. Temperature increase is the predominant factor leading to habitat loss, whereas changes in precipitation play a secondary role. When climate changes, the species not only loses part of its habitat but also suffers a loss in its population size in the remaining habitat. Population size declines more than the habitat area under all considered climate change scenarios, which implies that the species might become extinct long before the complete loss of its habitat. This study suggests that some species might experience much more severe impacts from climate change than predicted from models of habitat area alone. Management policies based on predictions of habitat area decline using occurrence data need to be re-evaluated and alternative measures need to be developed to conserve species in the face of rapid climate change.

On the isolated population of Lewin’s Honeyeater (Meliphaga lewinii amphochlora) from the McIlwraith Range uplands, Cape York Peninsula, Australia: estimates of population size and distribution

Abstract: Subspecies amphochlora of the Lewin’s Honeyeater (Meliphaga lewinii) is confined to a small isolated population in the McIlwraith Range, Cape York Peninsula, north-eastern Australia The population is poorly known but is thought to be restricted to elevations above 500 m We aimed to establish reliable estimates of population size and geographical range to enable a better evaluation of the conservation status of the subspecies We also sought to quantify elevational patterns of density within the range and test the hypothesis that climate is the dominant factor governing range limits We estimate the area of occupancy to be 183 km2, pending the investigation of three small fragments of predicted suitable habitat in mountain ranges to the north of the McIlwraith Range Our count data indicate that the population probably consists of fewer than 5000 individuals (best estimate = 4666 individuals, 95% confidence intervals (CI) = 2868–7591) Highest densities were observed in a geographically limited subset of the range above 650 m Independent validation of our species-climate distribution model suggests that the restricted spatial distribution of the subspecies is attributable to climate associations and implies that warm temperatures in particular act to restrict the lower elevational limit of the subspecies We discuss the implications of these findings for the future survival of M l amphochlora in the context of contemporary climate warming

Volume measurements for quicker determination of forest litter standing crop

Abstract: Litter standing crop (LSC) is the quantity of plant detritus on the floor in forested environments. Knowledge of LSC is important in understanding many ecological phenomena. These include studies of litterfall, decomposition/litter turnover rates and nutrient cycling (Anderson et al. 1983, Dent et al. 2006), general plant performance (Benitez-Malvido & Kossmann-Ferraz 1999), other ecosystem processes such as the effects of fire (Odiwe & Muoghalu 2003) and fauna (Frith & Frith 1990, Giaretta et al. 1999, Levings & Windsor 1985). The determination of accurate annual average LSC data, may require monitoring over long periods due to seasonality and sometimes sporadic nature of litterfall and decomposition rates (Clark et al. 2001). Furthermore, the effects of topography and water movement create the need for both representative site selection and sufficient spatial coverage.

SHORT COMMUNICATION Volume measurements for quicker determination of forest litter standing crop

Abstract: but adequacy of sampling is usually unknown. Theapproach of taking samples from the site to determinedry weights has drawbacks in regard to the time andeffort required for accurate analyses, particularly whennumerous sites are being studied. Removing litter fromthe site also creates disturbance with possible carry-on effects to other processes that may be of interest.Significanteffortcanbenecessaryfordryingandweighingfollowing the fieldwork. The method of subsampling todetermine moisture content can also be problematic, asmany samples may be needed to accurately cover therange of moisture within the litter layer temporally andspatially. Most importantly, logistical demands inherentin these commonly used methods limit the capacity ofresearchers to sample spatial and temporal variability inthe litter layer, which can be particularly high at mostscales of interest and change seasonally (Proctor