Christopher MacGregor

REPTILE AND ARBOREAL MARSUPIAL RESPONSE TO REPLANTED VEGETATION IN AGRICULTURAL LANDSCAPES

We report reptile and arboreal marsupial responses to vegetation planting and remnant native vegetation in agricultural landscapes in southeastern Australia. We used a hierarchical survey to select 23 landscapes that varied in the amounts of remnant native vegetation and planted native vegetation. We selected two farms within each landscape. In landscapes with plantings, we selected one farm with and one farm without plantings. We surveyed arboreal marsupials and reptiles on four sites on each farm that encompassed four vegetation types (plantings 7-20 years old, old-growth woodland, naturally occurring seedling regrowth woodland, and coppice [i.e., multistemmed] regrowth woodland). Reptiles and arboreal marsupials were less likely to occur on farms and in landscapes with comparatively large areas of plantings. Such farms and landscapes had less native vegetation, fewer paddock trees, and less woody debris within those areas of natural vegetation. The relatively large area of planting on these farms was insufficient to overcome the lack of these key structural attributes. Old-growth woodland, coppice regrowth, seedling regrowth, and planted areas had different habitat values for different reptiles and arboreal marsupials. We conclude that, although plantings may improve habitat conditions for some taxa, they may not effectively offset the negative effects of native vegetation clearing for all species, especially those reliant on old-growth woodland. Restoring suitable habitat for such species may take decades to centuries.

The combined effects of remnant vegetation and tree planting on farmland birds.

Biodiversity conservation on agricultural land is a major issue worldwide. We estimated separate and joint effects of remnant native woodland vegetation and recent tree plantings on birds on farms (approximately 500-1000 ha) in the heavily cleared wheat and sheep belt of southern Australia. Much of the variation (>70%) in bird responses was explained by 3 factors: remnant native-vegetation attributes (native grassland, scattered paddock trees, patches of remnant native woodland); presence or absence of planted native trees; and the size and shape of tree plantings. In terms of the number of species, remnant native vegetation was more important than tree planting, in a 3:1 ratio, approximately. Farms with high values for remnant native vegetation were those most likely to support declining or vulnerable species, although some individual species of conservation concern occurred on farms with large plantings. Farm management for improved bird conservation should account for the cumulative and complementary contributions of many components of remnant native-vegetation cover (e.g., scattered paddock trees and fallen timber) as well as areas of restored native vegetation.

Contrasting mammal responses to vegetation type and fire

The response of terrestrial mammals and arboreal marsupials to past burning history as well as a year prior to, and then for 4 years after, a major wildfire in 2003 at Booderee National Park, Jervis Bay Territory was quantified. The present study encompassed extensive repeated surveys at a set of 109 replicated sites stratified by vegetation type and fire history. It was found that most species exhibited significant differences in presence and abundance between major vegetation types. Detections of long-nosed bandicoot (Perameles nasuta) increased significantly in all vegetation types surveyed, in both burnt and unburnt areas. Temporal patterns in captures of three species of small mammals (bush rat (Rattus fuscipes), swamp rat (Rattus lutreolus) and brown antechinus (Antechinus stuartii)) showed a trend for lower numbers of captures on burnt sites compared with unburnt sites. Three species of arboreal marsupials, common ringtail possum (Pseudocheirus peregrinus), greater glider (Petauroides volans) and common brushtail possum (Trichosurus vulpecula), were moderately common and all showed marked differences in abundance between vegetation types. Whereas P. peregrinus and P. volans exhibited a temporal decline between 2003 and 2006, T. vulpecula exhibited a general increase from 2003 levels. However, arboreal marsupial responses did not appear to be directly fire related.

TEMPORAL CHANGES IN VERTEBRATES DURING LANDSCAPE TRANSFORMATION : A LARGE-SCALE NATURAL EXPERIMENT

Plantation development is a significant form of landscape change worldwide. We report findings from a large-scale longitudinal natural experiment that quantified changes in Australian vertebrates as a former grazing landscape was transformed to one dominated by a radiata pine (Pinus radiata) plantation. The study included four main “treatments”: woodland remnants surrounded by emerging radiata pine (52 sites, termed “woodland treatments”), stands of radiata pine (10 sites, “pine controls”), woodland remnants where the surrounding landscape remained unchanged (56 sites, “woodland controls”), and paddocks with scattered woodland trees that surrounded the 56 woodland remnants (10 sites, “paddock controls”). In our study region, woodland is distinguished from forest by differences in tree height, tree spacing, bole length, and canopy development. Between 1998 and 2006, occupancy rates of “woodland treatments” by most mammals and reptiles increased linearly. Similar trends occurred in the “woodland controls,” suggesting that species had increased landscape-wide, rather than displaying year × treatment interaction effects. We cross-classified birds according to the statistical significance and nature of time trajectories. Groups included those that: (1) declined in woodland treatments in comparison with woodland controls, (2) decreased within woodland treatments but increased in woodland controls, (3) declined across the entire study area, (4) increased within woodland treatments in comparison with woodland controls, (5) increased within woodland treatments but declined in woodland controls, and (6) increased across the entire study area. Attributes of woodland treatments significantly associated with temporal changes in bird occupancy included: (1) age of surrounding pine stands; (2) number of boundaries with surrounding pines; (3) size of the woodland patches; (4) dominant vegetation type of woodland patches; and (5) temporal changes in vegetation structure in the woodland treatments. Bird species associated with open country and woodland environments were disadvantaged by landscape transformation, whereas those that benefited were forest taxa and/or habitat generalists capable of inhabiting pine stands and adjacent woodland patches. Beyond this generalization, an unanticipated finding was a lack of association between life history attributes and landscape transformation. We suggest that several key processes are likely drivers of change at multiple spatial scales. Recognition of such processes is important for conservation in landscapes transformed by plantation expansion.

TESTING HYPOTHESES ASSOCIATED WITH BIRD RESPONSES TO WILDFIRE

Disturbance is a key ecological process influencing the distribution and abundance of many elements of the earths biota. Predicting the response of biota to disturbance is therefore important, but it nevertheless remains difficult to make accurate forecasts of response. We tested predictions from disturbance-related theories and concepts in 10 vegetation types at Booderee National Park (southeastern Australia) using a retrospective study of bird responses to fire history (over 35 years) on 110 sites and a prospective study following a single wildfire event in 2003 at 59 of these sites. Our data did not support predictions from the intermediate-disturbance hypothesis; observed bird species richness at a site was significantly (F(1,99) = 6.30, P = 0.014) negatively related to the number of fires since 1972 and was 8.7% lower (95% CI, 1.8-15.1%) for each additional fire. In contrast to fire history effects, we found that after the 2003 fire, the vast majority of individual species and the bird assemblage per se in most vegetation types recovered within two years. Thus, recovery after a single fire did not reflect long-term effects of multiple fires on overall bird species richness at a site. We postulated that the recovery of bird species richness and bird assemblage composition after the 2003 fire would be fastest in structurally simple vegetation types and slowest in structurally complex vegetation, but observed the opposite. Although observed bird species richness in vertically heterogeneous forest and woodland had returned to prefire levels by 2006, bird species richness in structurally simple vegetation types (e.g., sedgeland) had not. Postfire vegetation regeneration, together with a paucity of early-successional specialists, would explain the speed of recovery of the bird assemblage and why it changed relatively little during our investigation.

Do observer differences in bird detection affect inferences from large-scale ecological studies?

Abstract Use of multiple observers in large-scale bird surveys is often unavoidable. But how significant are observer differences in bird detection? Do observer effects significantly influence inferences about environmental factors on birds? We conducted a field experiment to quantify differences between observers in the detection of birds at Booderee National Park, south-eastern Australia. We also re-analysed a large dataset from an observational study where multiple observers had participated in bird surveys. We identified highly significant observer differences for estimates of bird species richness and the probability of detection of three exemplar taxa. We demonstrated that observer effects would not substantially alter inferences we made about relationships between bird species and vegetation type or burning history. We believe that four features of our survey design and protocol limited the magnitude of observer effects on environmental inferences: (1) high levels of replication of classes of field sites—critical for relative comparisons of site (vegetation) types; (2) pre-survey screening to ensure that only experienced ornithologists participated in surveys; (3) repeat sampling of field sites by a different observer on a different day to reduce the impacts of observer heterogeneity and ‘day’ effects; and (4) precluding surveys during poor weather or long after dawn, also to limit ‘day’ effects.

Experimental evidence of the effects of a changed matrix on conserving biodiversity within patches of native forest in an industrial plantation landscape

We implemented a replicated before-after-control-impact (BACI) experiment to quantify vertebrate response in native forest patches to a major change in the surrounding exotic Radiata Pine (Pinus radiata) plantation. We contrasted vertebrate occupancy of patches of native eucalypt forest where the surrounding stands of exotic Radiata Pine (Pinus radiata) were clearfelled (termed “treatment patches”) with matched “control patches” where surrounding pine stands remained unlogged. Different species of arboreal marsupials varied in their response to our experimental treatments. The Common Ringtail Possum was unaffected by cutting of the surrounding pine stands, whereas all sightings of the Mountain Brushtail Possum were in control patches. For birds, species richness was significantly reduced by 4–9 species in treatment patches. Birds with cup and dome nests were those negatively affected by the cutting of the surrounding pine stands. They may be susceptible to altered microclimatic conditions or increasing levels of nest predation when the surrounding pine matrix is clearfelled. Our study emphasized how the biota inhabiting retained patches of native forest within plantation landscapes can be changed when stands of surrounding Radiata Pine are clearfelled. In the case of birds, more species will be maintained within eucalypt patches if logging is scheduled so that not all the surrounding pine plantation is clearfelled at once.

Unintended consequences of invasive predator control in an Australian forest: overabundant wallabies and vegetation change.

Over-abundance of native herbivores is a problem in many forests worldwide. The abundance of native macropod wallabies is extremely high at Booderee National Park (BNP) in south-eastern Australia. This has occurred because of the reduction of exotic predators through an intensive baiting program, coupled with the absence of other predators. The high density of wallabies at BNP may be inhibiting the recruitment of many plant species following fire-induced recruitment events. We experimentally examined the post-fire response of a range of plant species to browsing by wallabies in a forest heavily infested with the invasive species, bitou bush Chrysanthemoides monilifera. We recorded the abundance and size of a range of plant species in 18 unfenced (browsed) and 16 fenced (unbrowsed) plots. We found the abundance and size of bitou bush was suppressed in browsed plots compared to unbrowsed plots. Regenerating seedlings of the canopy or middle storey tree species Eucalyptus pilularis, Acacia implexa, Allocasuarina littoralis, Breynia oblongifolia and Banksia integrifolia were either smaller or fewer in number in grazed plots than treatment plots as were the vines Kennedia rubicunda, Glycine tabacina and Glycine clandestina. In contrast, the understorey fern, Pteridium esculentum increased in abundance in the browsed plots relative to unbrowsed plots probably because of reduced competition with more palatable angiosperms. Twelve months after plots were installed the community structure of the browsed and unbrowsed plots was significantly different (P = 0.023, Global R = 0.091). The relative abundance of C. monilifera and P. esculentum contributed most to the differences. We discuss the possible development of a low diversity bracken fern parkland in Booderee National Park through a trophic cascade, similar to that caused by overabundant deer in the northern hemisphere. We also discuss its implications for broad scale fox control in southern Australian forests.

Predicting Ecosystem Wide Impacts of Wallaby Management Using a Fuzzy Cognitive Map

At Booderee National Park, south-eastern Australia, the intensive control of the introduced red fox (Vulpes vulpes) resulted in a major increase in the abundance of a browsing macropod, the swamp wallaby (Wallabia bicolor). This has led to a major decrease in the abundance and biomass of a range of palatable plant species. Fox control has also started a trophic cascade that has resulted in a decline in the abundance of the greater glider (Petauroides volans) a folivorous arboreal marsupial, mediated either through increased predation by owls or increased competition with common brushtail possums (Trichosurus vulpecula). We identified five potential scenarios for managing the effects of over-abundant swamp wallabies on the ecosystem as a whole. These were (1) the present scenario of continued intensive fox control and four possible scenarios to redress the problem: (2) ceasing fox control; (3) intensive fox control and intensive wallaby control; (4) introducing dingoes and ceasing fox control; and (5) introducing dingoes and maintaining fox control. We used an ecosystem modelling approach based on a fuzzy cognitive map (FCM) to predict relative estimates of abundance for each scenario for a wide range of taxa in the Booderee National Park ecosystem likely to be affected by each scenario. We addressed uncertainty in our knowledge of the interactions between species by creating alternative models of the system by removing one or more of the uncertain links between species and varying the strength of the remaining interactions in the FCM and aggregated predictions from 100,000 models to estimate the effect of uncertainty on the predictions from our FCM model. In comparison with the current scenario of intensive fox control, scenario 3 had the greatest likelihood of improving the status of palatable plants. Scenarios 2 and 4 reduced the abundance of a range of medium-sized mammals but improved the status of greater gliders, whereas the predicted effects of scenario 5 were uncertain. The FCM modelling approach developed here provided a valuable tool for managers to learn about the potential ecosystem wide effects of management actions while incorporating the likely effects of uncertain knowledge on system outcomes.

The use of hollows and dreys by the common ringtail possum (Pseudocheirus peregrinus) in different vegetation types

Tree hollows are a key habitat component for a large number of Australian vertebrates and understanding how these resources are used is critical for developing successful management and conservation strategies for particular species or sets of species. Some hollow-using vertebrates are capable of using other kinds of nest sites. The common ringtail possum (Pseudocheirus peregrinus) is one of these species and it is known to use tree hollows and also to construct nests (dreys) made from sticks and leaves. Nest site selection by P. peregrinus may be a function of hollow availability. This proposition and several related questions were tested in a radio-tracking study that examined patterns of tree hollow and drey use by P. peregrinus in a range of vegetation types in Booderee National Park in Jervis Bay Territory. In addition, this study explored whether hollow and drey use was influenced by a wildfire that occurred in the study region in late 2003. It was found that use of hollows or dreys appeared to be a function of the availability of these resources. Most individuals were either primarily hollow users or primarily drey users. These patterns conformed to differences we recorded in hollow abundance between vegetation types: drey use was most pronounced in shrubland where hollows were rare and hollow use was most prevalent in forest where hollows were abundant. We found no evidence to suggest that hollow or drey use was influenced by fire. There was a trend pattern in our data suggesting that home-range size of P. peregrinus was larger in burnt versus unburnt sites but this effect was not statistically significant. Evidence was found that P. peregrinus selected particular kinds of trees as nest sites. Hollows in dead trees were more often used than those in living trees. Smaller dreys were most likely to be used. Several kinds of nest-selection effects that were contrary to the findings of studies of other arboreal marsupials were identified. These included more frequent use of smaller diameter trees with fewer cavities. The reasons for these unexpected results remain unclear. Findings such as those quantifying gender differences in the frequency of drey use, as well as marked between-vegetation-type differences in nest-type selection, indicate that nest use and nesting behaviour in P. peregrinus may be more complex than previously recognised. These findings also indicate that a deep understanding of the nesting biology of a species may require careful studies of both sexes, across a range of environments, and where disturbances have and have not occurred.