Ralph Mac Nally

Regression and model-building in conservation biology, biogeography and ecology: The distinction between – and reconciliation of – ‘predictive’ and ‘explanatory’ models

In many large-scale conservation or ecological problems where experiments are intractable or unethical, regression methods are used to attempt to gauge the impact of a set of nominally independent variables (X) upon a dependent variable (Y). Workers often want to assert that a given X has a major influence on Y, and so, by using this indirection to infer a probable causal relationship. There are two difficulties apart from the demonstrability issue itself: (1) multiple regression is plagued by collinear relationships in X; and (2) any regression is designed to produce a function that in some way minimizes the overall difference between the observed and ‘predicted’ Ys, which does not necessarily equate to determining probable influence in a multivariate setting. Problem (1) may be explored by comparing two avenues, one in which a single ‘best’ regression model is sought and the other where all possible regression models are considered contemporaneously. It is suggested that if the two approaches do not agree upon which of the independent variables are likely to be ‘significant’, then the deductions must be subject to doubt.In many large-scale conservation or ecological problems where experiments are intractable or unethical, regression methods are used to attempt to gauge the impact of a set of nominally independent variables (X) upon a dependent variable (Y). Workers often want to assert that a given X has a major influence on Y, and so, by using this indirection to infer a probable causal relationship. There are two difficulties apart from the demonstrability issue itself: (1) multiple regression is plagued by collinear relationships in X; and (2) any regression is designed to produce a function that in some way minimizes the overall difference between the observed and ‘predicted’ Ys, which does not necessarily equate to determining probable influence in a multivariate setting. Problem (1) may be explored by comparing two avenues, one in which a single ‘best’ regression model is sought and the other where all possible regression models are considered contemporaneously. It is suggested that if the two approaches do not agree upon which of the independent variables are likely to be ‘significant’, then the deductions must be subject to doubt.

Forecasting the impacts of habitat fragmentation. Evaluation of species-specific predictions of the impact of habitat fragmentation on birds in the box-ironbark forests of central Victoria, Australia.

Abstract We tested forecasts made by Mac Nally and Bennett (Mac Nally, R., Bennett, A.F., 1997. Species-specific predictions of the impact of habitat fragmentation: local extinction of birds in the box-ironbark forests of central Victoria, Australia. Biological Conservation 82, 147–155) of relative vulnerability to habitat fragmentation for 43 species of birds in the box–ironbark system of central Victoria, Australia. The predictions were based on a simple, tripartite model linking habitat specialization, density and mobility, three of the characteristics most widely mooted in the literature to influence vulnerability to habitat fragmentation. For each species, a predicted index of ‘proneness’ to local extinction was calculated by using prior, independent data. The model system consisted of existing fragments of 10, 20, 40 and 80 ha size-classes, with between 5 and 15 ‘replicates’ of each size-class. Replicated ‘reference-areas’xa0— mapped-out areas of these same size-classes set within large (>10u2008000 ha) remnant forests of the same general vegetation as the fragmentsxa0— were surveyed also to provide ‘expectations’ of the likelihood of finding each species in a given size-class. Our results indicated that the simple model had virtually no predictive power: the index-values for proneness to local extinction were no help in forecasting whether a given species was over- or under-represented in a fragment of a given size based on the birds occurrence in reference-areas of similar size. When the three components of the model were tested separately, measures of habitat specialization and mobility showed no relationship with the observed responses of birds to fragmentation, while for population density there was a significant relationship but in a direction contrary to that expected. Evaluation of assumptions underlying the test revealed two main factors that are likely to have affected the models performance: (1) differences in anthropogenic impacts between fragments and reference-areas have resulted in differences in habitat quality, and (2) species interactions within fragments differ from those in reference-areas, a result of secondary effects arising from the altered spatial pattern of the habitat (e.g. the expansion of numbers and influence of ‘despotic’ species such as the noisy miner Manorina melanocephala). These two factors appear to be common to most fragmented systems and, therefore, need to be addressed in predictive models of species vulnerability. However, because the nature of anthropogenic impacts and the types of changes to species interactions in fragments differ between ecosystems, we conclude that it will be difficult to develop a model of species vulnerability to fragmentation that has both strong predictive ability and wide generality among ecosystems.

Forecasting New Zealand mudsnail invasion range: model comparisons using native and invaded ranges.

Evaluations of the potential distribution of invasive species can increase the efficiency of their management by focusing prevention measures. Generally, ecological models are built using occurrence data from a species native range to predict the distribution in areas that the species may invade. However, historical and geographical constraints can limit a species native distribution. Genetic Algorithm for Rule-set Production (GARP), an ecological niche modeling program, was used to predict the potential distribution of the invasive, freshwater New Zealand mudsnail, Potamopyrgus antipodarum, in Australia and North America. We compared the strength of the predictions made by models built with data from the snails native range in New Zealand to models built with data from the locations invaded by the species. A time-series analysis of the Australian models demonstrated that range-of-invasion data can make better predictions about the potential distribution of invasive species than models built with native range data. Large differences among the model forecasts indicate that uncritical choice of the data set used in training the GARP models can result in misleading predictions. The models predict a large expansion in the range of P. antipodarum in both Australia and North America unless prevention measures are implemented rapidly.

Dynamics of Murray‐Darling floodplain forests under multiple stressors: The past, present, and future of an Australian icon

[1]xa0We review the human actions, proximal stressors and ecological responses for floodplain forests Australias largest river system—the Murray-Darling Basin. A conceptual model for the floodplain forests was built from extensive published information and some unpublished results for the system, which should provide a basis for understanding, studying and managing the ecology of floodplains that face similar environmental stresses. Since European settlement, lowlands areas of the basin have been extensively cleared for agriculture and remnant forests heavily harvested for timber. The most significant human intervention is modification of river flows, and the reduction in frequency, duration and timing of flooding, which are compounded by climate change (higher temperatures and reduced rainfall) and deteriorating groundwater conditions (depth and salinity). This has created unfavorable conditions for all life-history stages of the dominant floodplain tree (Eucalyptus camaldulensis Dehnh.). Lack of extensive flooding has led to widespread dieback across the Murray River floodplain (currently 79% by area). Management for timber resources has altered the structure of these forests from one dominated by large, widely spreading trees to mixed-aged stands of smaller pole trees. Reductions in numbers of birds and other vertebrates followed the decline in habitat quality (hollow-bearing trees, fallen timber). Restoration of these forests is dependent on substantial increases in the frequency and extent of flooding, improvements in groundwater conditions, re-establishing a diversity of forest structures, removal of grazing and consideration of these interacting stressors.

The conservation value of mesic gullies in dry forest landscapes: mammal populations in the box–ironbark ecosystem of southern Australia

Abstract Within the context of landscape variation in habitat quality, areas of unusually high productivity may serve as refuges for fauna during environmentally stressful periods and as sources of recruitment into less productive areas. In dry landscapes, water availability often defines habitat quality, so that drainages that concentrate run-off can facilitate relatively lush, and sometimes distinct, vegetation communities. The dry box–ironbark forests of south-eastern Australia may be such an ecosystem. Recent clearing of vegetation for agriculture and mining has eliminated 85% of the original vegetation in this area, with the fertile, alluvial soils being most affected. Fragmentation and degradation of remnant forest in this ecosystem has increased the potential importance of the remaining high-quality habitat to conservation. In this study, we tested the hypothesis that the abundance of mammals was higher in moist gullies than on dry hilltops, slopes and ridges in the box–ironbark forests of central Victoria. We compared 57 spatially paired gully and ridge sites. Habitat analyses indicated that gullies had a tree canopy height that was 51% taller than on ridges, 53% more trees with hollows in the upper bole and branches, and almost six times more very large trees. Ridges had 71% more small trees than gullies. Among marsupials surveyed, the common brushtail possum Trichosurus vulpecula and common ringtail possum Pseudocheirus peregrinus were significantly more abundant in gullies. Nearly all the reproduction observed in these two species occurred in gullies. The yellow-footed antechinus Antechinus flavipes was also significantly more abundant in gullies; numbers of the sugar glider Petaurus breviceps differed little. Whereas gullies occupy a very limited total area in this ecosystem, they may nonetheless be critical to some species of mammals that can survive in dry habitats but thrive only in mesic areas. Given the precariously degraded state of the box–ironbark ecosystem as a whole, protection of gullies from proposed alluvial gold mining and timber harvesting could have a disproportionate benefit to biodiversity conservation.

Allocating surveillance effort in the management of invasive species: A spatially-explicit model

Invasive organisms often exist at low densities at the beginning and end of eradication programs. As a consequence, such organisms are often difficult to find, particularly if they are dispersed long distances to unknown locations. In such circumstances, large amounts of money can be spent searching for invasive organisms without finding any. However, chance encounters between invasive organisms and private citizens can occur even when invasive organisms exist at low densities. Reports of these passive detections may be a critically important source of information for public pest management agencies. Rates of reporting may be improved using bounty payments and increasing public awareness about the presence of the invader. To explore the importance of passive surveillance in general, and its interaction with active surveillance by pest management agencies, we developed a simulation model of the spread of an invasive species. Simulations conducted under alternative scenarios for detection rates and search effort applied demonstrate that even small increases in detection or reporting rates substantially reduced eradication costs and increased the probability of eradication. In circumstances where resources are insufficient to achieve eradication, the simulation model provides useful information on the minimum expenditure required to contain the invasion.

The conservation value of mesic gullies in dry forest landscapes: avian assemblages in the box-ironbark ecosystem of southern Australia.

Abstract Drainage lines and shallow gullies generally have different microclimates and hence flora to surrounding upland ridges and slopes (abbreviated throughout as ‘ridges). Gullies often have mesic, or at least less xeric, conditions compared with surrounding ridges. Gullies are frequently subjected to greater human impacts, such as clearance for agriculture, logging, grazing-damage and mining for alluvial deposits. We tested the hypothesis that mesic gullies in the generally dry (400–700 mm precipitation p. a.) box–ironbark forests of central Victoria, Australia, harbour a richer and different avifauna to surrounding ridges. Ten pairs of adjacent gullies and ridge sites were surveyed eight times over 1 year. Species richness was one-third greater in gullies than in ridges, while mean total densities of birds were almost twice as great. Assemblage composition also differed significantly, which reflected: (1) significantly different densities of those species common to both gullies and ridges (several more abundant in gullies, some more abundant in ridges); and (2) differences in composition wherein some species occurred only in one or the other habitat, even though pairs of gullies/ridges were only 400–1900 m apart. These results indicate that avian assemblages within gullies are distinct from those in ridges and slopes, and that integrated management is required to conserve the entire avifauna of these dry forests and woodlands. The depletion of gully habitats through human disturbance makes prescriptive management of gullies the highest priority at present.

Landscape-scale conservation of an endangered migrant : the Swift Parrot (Lathamus discolor) in its winter range

Abstract The endangered Swift Parrot ( Lathamus discolor ) is threatened by disturbance processes in both its breeding and its over-wintering ranges. Here we report on the spatial distribution of Swift Parrots over two successive winters in the box-ironbark system of central Victoria, Australia. The parrots showed little site-fidelity overall, and varied significantly among years in their regional distributions. Moreover, in some years it appears that relatively small remnant patches become significant elements in the landscape for the over-wintering parrots. Logistic-regression analyses suggest that the occurrence of Swift Parrots may be linked to the intensity of flowering by the Golden Wattle ( Acacia pycnantha ) and also is associated with the summed density of aggressive, nectarivorous honeyeaters of the box-ironbark system. About 17% (a statistically significant amount) of the variance in the presence of Swift Parrots was explicable by these two variables. On the other hand, densities of Swift Parrots (where present) appeared to be related to densities of other nectarivorous species and of Noisy Miners ( Manorina melanocephala ). Although statistically significant, just 5% of the variance in densities of Swift Parrots was explained by these variables. There were no relationships between measures of eucalypt flowering and the occurrence or density of Swift Parrots, nor with densities of potential aerial predators. These statistical patterns were difficult to explain because Swift Parrots seem to depend upon eucalypt nectar, with few feeding observations on racemes of Golden Wattle. Moreover, the positive correlation with the honeyeaters is difficult to reconcile with the lack of correspondence with eucalypt flowering. From a conservation standpoint, inter-annual variation in use of the major areas of the box–ironbark system by Swift Parrots is pronounced so that a very broad perspective needs to be maintained for their management in over-wintering regions. Moreover, remnants as small as 10 ha are utilized in some years by Swift Parrots so that preservation of remnant patches must be encouraged.

Model-based adaptive spatial sampling for occurrence map construction

In many environmental management problems, the construction of occurrence maps of species of interest is a prerequisite to their effective management. However, the construction of occurrence maps is a challenging problem because observations are often costly to obtain (thus incomplete) and noisy (thus imperfect). It is therefore critical to develop tools for designing efficient spatial sampling strategies and for addressing data uncertainty. Adaptive sampling strategies are known to be more efficient than non-adaptive strategies. Here, we develop a model-based adaptive spatial sampling method for the construction of occurrence maps. We apply the method to estimate the occurrence of one of the world’s worst invasive species, the red imported fire ant, in and around the city of Brisbane, Australia. Our contribution is threefold: (i)xa0axa0model of uncertainty about invasion maps using the classical image analysis probabilistic framework of Hidden Markov Random Fields (HMRF), (ii)xa0an original exact method for optimal spatial sampling with HMRF and approximate solution algorithms for this problem, both in the static and adaptive sampling cases, (iii)xa0an empirical evaluation of these methods on simulated problems inspired by the fire ants case study. Our analysis demonstrates that the adaptive strategy can lead to substantial improvement in occurrence mapping.

Building a Regionally Connected Reserve Network in a Changing and Uncertain World

Habitat connectivity is required at large spatial scales to facilitate movement of biota in response to climatic changes and to maintain viable populations of wide-ranging species. Nevertheless, it may require decades to acquire habitat linkages at such scales, and areas that could provide linkages are often developed before they can be reserved. Reserve scheduling methods usually consider only current threats, but threats change over time as development spreads and reaches presently secure areas. We investigated the importance of considering future threats when implementing projects to maintain habitat connectivity at a regional scale. To do so, we compared forward-looking scheduling strategies with strategies that consider only current threats. The strategies were applied to a Costa Rican case study, where many reserves face imminent isolation and other reserves will probably become isolated in the more distant future. We evaluated strategies in terms of two landscape-scale connectivity metrics, a pure connectivity metric and a metric of connected habitat diversity. Those strategies that considered only current threats were unreliable because they often failed to complete planned habitat linkage projects. The most reliable and effective strategies considered the future spread of development and its impact on the likelihood of completing planned habitat linkage projects. Our analyses highlight the critical need to consider future threats when building connected reserve networks over time.