How to make a common species rare: A case against conservation complacency

A traditional focus in conservation biology has been on rare species as they are often those most at risk of decline or extinction. However, we argue in this paper that some kinds of currently common species also can be susceptible to decline. Those at particular risk are species that are specialized on widespread environmental conditions. Such specialization may make such species vulnerable to a range of drivers of environmental change, placing them at risk of significant decline or even local extinction. We illustrate this with a case study of the arboreal marsupial the Greater Glider (Petauroides volans) in south-eastern Australia. The Greater Glider was formerly common in two large-scale studies but in one it suffered rapid extinction (within a 3 year period) and in another it is declining at an annual rate of 8.8%. We therefore argue for more research to better predict those kinds of currently common species which might be at risk of future rapid decline or extinction. In addition, we suggest there will often be a need to take pro-active conservation and management action to reduce the number of potential environmental stressors on populations of common species to ensure they do not become uncommon or rare. We also argue that conserving common species will ensure the retention of their key ecological and functional roles in ecosystems. Finally, we believe there is a need to develop better monitoring programs that can detect changes in the population trajectories of common species, help identify the reasons for temporal changes in such populations, and underpin timely management interventions. Despite these good intentions, we acknowledge that in one of our own long-term investigations we: (1) failed to anticipate the extremely rapid decline (and likely local extinction) of the Greater Glider, (2) were unable to diagnose the reason/s underpinning the population collapse, and (3) nor were we able to instigate a timely intervention program of management to prevent this from occurring. The key lesson from this sobering result is that common species can sometimes be at risk of rapid decline and it is wise to avoid complacency in conservation.     

The role of timber tree species in the nutritional ecology of spider monkeys in a certified logging concession,Bolivia

Selective harvesting of timber can lead to population declines in some primate species. As frugivorous primates are important seed dispersers in tropical forests, the reduction of their populations may affect the ecological sustainability of selectively logged forests. This paper is the first to quantify the importance of timber tree species in the diet and nutritional ecology of a primate species. We studied spider monkeys (Ateles chamek) inhabiting a certified forestry concession in Bolivia where post-logging population declines of this species have been recorded. We show that spider monkeys occupying unlogged areas obtained approximately 50% of their total intake of macro-nutrients from timber tree species and exhibited a distinct preference for foraging within trees that were of harvestable size. Timber tree species dominated the spider monkeys’ diet both during peak fruiting periods and during periods of fruit scarcity. We estimate that under current timber extraction intensities spider monkeys lose significant proportions of their food sources. Our results indicate that further extraction limits could be considered for Ficus boliviana, Spondias mombin and Pouteria nemorosa. We suggest that to ensure long-term ecological sustainability of certified forestry concessions, the importance of timber tree species in the ecology of seed dispersers needs to be taken into account.     

Benchmark stem densities for forests and woodlands in south-eastern Australia under conditions of relatively little modification by humans since European settlement

Estimates of forest and woodland structure prior to major periods of modification (e.g. prior to European settlement) are routinely used to inform decisions relating to biodiversity conservation, silviculture and carbon sequestration potential in natural forests and woodlands. The techniques used to derive these estimates often demand that data be collected from specific geographic locations (e.g. locations for which historic survey records exist, where pollen accumulates, or where there is little modification by humans since European settlement) and therefore are often inherently biased. In this study we predicted numbers of trees by diameter class for several widespread forest and woodland types in south-eastern Australia under conditions of relatively little modification by humans since European settlement. To do this we fitted Generalised Additive Models (GAMs) separately to counts of stems in eight diameter classes from 495 plots using explanatory variables representing human modification, environmental variation and natural disturbances. We predicted stem densities under conditions of relatively little modification by humans since European settlement from these models by holding the significant explanatory variables representing modification by humans at minimum observed values. We compared these predictions with published estimates of pre-European stem densities and estimates that we derived for stands at theoretical equilibrium using the quotient of diminution (q). Our mean predictions were broadly comparable with estimates derived from both of these sources; however, we appeared to over-estimate numbers of stems in the smaller diameter classes for some vegetation alliances. A key outcome of this research – and rarely reflected in other techniques used to predict pre-European forest structure – was the amount of variation in stem numbers even within a single diameter class and vegetation alliance. For example, in the white box vegetation alliance, flat parts of the landscape supported 5 times the number of large trees (>80 cm DBH) found on upper slopes under conditions of relatively little modification by humans since European settlement. Our results therefore suggest that these forests and woodlands are more structurally heterogeneous than typically reflected in pre-European estimates and vegetation alliance should not be the unit for managing these stands. The methodology we present is applicable in many forests where the objective is to predict forest structure under conditions of relatively little modification by humans.     

Landscape context affects honeyeater communities and their foraging behaviour in Australia: implications for plant pollination

We investigated the species richness and composition of bird communities in mallee woodland remnants in a highly fragmented landscape, focusing specifically on honeyeaters and their foraging behaviour. We observed birds around flowering Eremophila glabra ssp. glabra plants in three replicated contexts: (1) the interior of large remnants, (2) linear remnants within ~3 km of a large remnant, and (3) linear remnants 5–7 km from a large remnant. We found species richness differed among elements, with an increase in the number of species that tolerate disturbed, open habitat and a decrease in the number of woodland-dependent species in linear elements. Honeyeater assemblages were similar in species richness and abundance among the elements, but differed in composition due to a higher number of large-sized honeyeater species in distant elements. Honeyeater movement patterns into a site and within a site were similar among the elements. Floral visitation varied among honeyeater species and was positively correlated with their abundance in the far element. Our results demonstrate that bird species respond differently to the spatial context of remnants in a fragmented landscape; however, the degree of isolation of linear remnants was not important. Linear remnants appear to be frequently used by honeyeaters, but the changes in community composition among the elements may influence the quality of pollination, which could have implications for plant reproduction.     

Inhibited development of trichostrongylid worms in grazing cattle

Inhibition of development of gastro-intestinal trichostrongylid worms was studied using successive groups of tracer calves and groups of continuously grazed calves over one year in the Tully area of North Queensland lowland wet tropics. The results, assessed by means of worms from these calves recovered at necropsy 3 weeks after their removal from pastures, showed inhibition in the development of Haemonchus placei and Cooperia punctata at the early fourth stage at the approach to and during the relatively dry period in the area. Inhibition was however minor and inhibited larvae formed but only a small percentage of worm burdens in both categories of calves, indicating that they were not in any way of major epidemiological importance. It was suggested that the minor nature of inhibition was due to the mild climatic conditions which could not produce appropriate conditioning treatment, or caused only mild selection pressure for inhibition in the area.     

Farmland bird responses to intersecting replanted areas

Despite increasing revegetation of cleared landscapes around the world, there is limited research on the implications of different types of plantings for birdlife. We examined the “intersection effect”, whereby species richness is higher at the intersection of “corridors” or vegetation strips for birds inhabiting replanted areas. We also examined individual species responses. Replicated sites at the intersections of plantings were compared with “internal controls” (located in the same plantings ∼100 m from intersections), “external controls”(sites in isolated linear plantings), and block plantings. We surveyed the 39 sites in our experimental design repeatedly – on different days by different observers and in different seasons. We found no significant difference in species richness between intersections and block plantings, but intersections had higher species richness than isolated linear strips and the internal controls. Similar results were found for bird assemblage scores derived by correspondence analysis. We found evidence of extra-variation at the farm-level for species richness and derived assemblage scores, suggesting a farm-scale response. This suggests the importance of other (often unmeasured) factors at the farm level (e.g. baiting for feral animals). Our results suggest that replanting programs aimed at maximizing bird species richness may benefit from consideration of planting geometry. In particular, linking strip plantings to create intersections and/or establishing block plantings appear to be superior to isolated strips for aggregate species richness.     

Bird populations in native forest patches in south-eastern Australia: the roles of patch width,matrix type (age) and matrix use

We surveyed birds in patches of native eucalypt forest and in surrounding exotic matrix (Radiata pine forests) in south-eastern Australia. Our objectives were: (1) to examine the influence of the width of native forest patches and the age of surrounding pine forests on bird occurrence in patches of native forest; and (2) to verify the relationship between the use of the surrounding pine matrix and bird species response to variation in width of patches of native forests. A total of 32 study sites (boundaries between eucalypt and pine forests) were surveyed. Birds were counted by the area search method within 0.5-ha quadrats. Data were analysed using generalised linear models. Wide patches of eucalypt forest supported higher species richness and greater numbers of birds, such as foliage searchers and nectarivores, than narrow patches. Matrix age also influenced the occurrence of some species in native patches. The abundance of species in wide and narrow patches of native forest was related to their use of the matrix. This was true for native forests surrounded by old but not by young pine forests. We suggest that management in wood production landscapes take into account both characteristics of native patches and the surrounding matrix. Negative impacts of fragmentation in managed landscapes might be reduced by promoting matrix types that are suitable for bird species.     

Vegetation structure moderates the effect of fire on bird assemblages in a heterogeneous landscape

Ecological theory predicting the impact of fire on ecological communities is typically focused on post-disturbance recovery processes or on disturbance-diversity dynamics. Yet the established relationship between vegetation structure and animal diversity could provide a foundation to predict the short-term effects of fire on biodiversity, but has rarely been explored. We tested the hypothesis that fire effects on bird assemblages would be moderated by increasing vegetation structure. We examined bird assemblages in burnt and unburnt sites at 1 and 6 years after a wildfire, and compared richness and composition responses among and within three structurally distinct vegetation types in the same landscape: heath, woodland and forest. We found that short-term changes in bird assemblage composition were largest in simple heath vegetation and smallest in complex forest vegetation. The short-term change in species richness was larger in forest than in heath. We also found that among-site assemblage variability was greater shortly after fire in heath and woodland vegetation compared with forest vegetation. Our results indicate that complexity in vegetation structure, particularly overstorey cover, can act as an important moderator of fire effects on bird assemblages. Mechanisms for this response include a greater loss of structure in vegetation characterised by a single low stratum, and a proportionally greater change in bird species composition despite a smaller absolute change in species richness. We discuss our results in the context of a new conceptual model that predicts contrasting richness and composition responses of bird assemblages following disturbance along a gradient of increasing vegetation structure. This model brings a different perspective to current theories of disturbance, and has implications for understanding and managing the effects of fire on biodiversity in heterogeneous landscapes.