Can natural disturbance-based forestry rescue a declining population of grizzly bears?

Forest managers are increasingly considering historic patterns of natural forest disturbance as a model for forest harvesting and as a coarse-filter ecosystem management tool. We evaluated the long-term (100-year) persistence of a grizzly bear population in Alberta, Canada using forest simulations and habitat modelling. Even with harvesting the same volume of timber, natural disturbance-based forestry resulted in a larger human footprint than traditional two-pass forestry with road densities reaching 1.39 km/km² or more than three times baseline conditions and suggested maximum levels of security for grizzly bears. Because bears favour young forests and edges where food resources are plentiful, a future shift to young forests and more edge habitat resulted in a 20% projected increase in habitat quality and a 10% projected increase in potential carrying capacity. Human-caused mortality risk, however, offset any projected gains in habitat and carrying capacity resulting in the loss of all secure, unprotected territories, regardless of forest harvest method, within the first 20-30 years of simulation. We suggest that natural disturbance-based forestry is an ill-suited management tool for sustaining declining populations of grizzly bears. A management model that explicitly considers road access is more likely to improve grizzly bear population persistence than changing the size of clear-cuts. In fact, large clear cuts might be counter productive for bears since a diversity of habitats within each bear’s home range is more likely to buffer against future uncertainties.     

Managing successional species: Modelling the dependence of heath fritillary populations on the spatial distribution of woodland management

Species that persist on a shifting mosaic of successional habitat offer particular challenges to conservation, to monitoring methods, and to population dynamic modelling. The conservation of the heath fritillary butterfly (Melitaea athalia) in woodland in England, for example, depends on the creation of woodland clearings by coppicing (rotational cutting). We have developed a simulation model to assist in the conservation of such populations, called MANAGE. We have parameterised the model for the M. athalia population in the Blean Woods in Kent, and used it to answer several management questions. We find that: (1) simulations predict that the observed rates of coppicing will not be enough to meet existing conservation (Biodiversity Action Plan) targets, except when the most generous modelling assumptions are made; (2) the greatest uncertainty in the model outcome arises from uncertainty in the colonisation parameters; (3) in the worst case scenario (using the most pessimistic model assumptions), a population would require 2.3% of the Blean Woods to be coppiced each year, which is around double the currently-observed rate; (4) the four management units of the Blean where coppicing is practised are not independent metapopulations– they support each other; and (5) to sustain a population in a smaller landscape would require less coppicing overall, but more as a percentage of the landscape. This modelling approach may prove useful in the development of conservation management plans for other species that inhabit successional habitats.     

Habitat loss and the habitat fragmentation threshold: an experimental evaluation of impacts on richness and total abundances using grassland invertebrates

Based on empirical analyses and computer-based modelling, it has been suggested that the impact of habitat loss is essentially independent of habitat fragmentation when >10-30% of the original habitat is left and that habitat fragmentation is influential only when less that this amount remains. This is the threshold effect of habitat fragmentation. In many systems, effects of habitat loss cannot be distinguished from those of habitat fragmentation in a way needed to critically evaluate the existence of the threshold effect. Therefore, we used an experimental model system (EMS) that was constructed to produce multiple micro-landscapes in which the habitat-loss and habitat-fragmentation impacts were potentially distinguishable. We used responses of terrestrial invertebrates to measure the impacts. We did not find an interaction between habitat-loss and habitat-fragmentation effects in the predicted fashion, although it is possible the threshold of habitat loss we used for the experiments (90%) may have still been above a critical level for the invertebrates. The only significant components were a strong ‘edge-centre’ difference in both richness and abundance, and a temporal change in both variables. Thus, in this EMS, there was little support for the threshold phenomenon or for general effects of habitat loss and fragmentation although this conclusion needs to be tempered by the limited duration of the experiment.     

Autumn-sowing of cereals reduces breeding bird numbers in a heterogeneous agricultural landscape

The switch from spring-sown to autumn-sown cereals and the loss of habitat heterogeneity are often suggested to be key drivers of breeding bird decline on arable farmland. Yet, both factors are interlinked and it remains uncertain whether autumn-sown cereals reduce breeding bird numbers also in the structurally complex arable farmland of northern Europe. We tested whether autumn-sowing of cereals at both local and landscape scales affected the breeding bird community in a heterogeneous agricultural landscape of south-central Sweden. Rotation between sowing types was used as a semi-experiment based on 34 spring- vs. 41 autumn-sown cereal plots centred on infield non-crop islands of similar structure, size and surroundings. Species richness and territory abundance of ground-foraging species were significantly lower in autumn- than in spring-sown cereal plots both in the crop fields and the infield non-crop islands during the breeding season. No such effect was observed among foliage gleaning birds. Species richness in spring-sown cereal plots was less the more autumn-sown crops in the surrounding landscape within a 500 m radius. Average skylark densities did not differ between autumn- and spring-sown cereal plots because habitat preferences changed; densities declined in autumn-sown cereals during the growing season whereas they increased on spring-sown fields which had shorter swards throughout the breeding season. Our results indicate that negative effects of autumn-sown crops on breeding bird numbers spill over into both neighbouring non-crop and crop habitats. We conclude that agri-environmental schemes should place more emphasis on facilitating the value of the cropped area of fields as a foraging and nesting habitat. The retention of various non-crop habitats alone may not provide sufficient food close to nest sites for farmland birds that rely on crop fields for foraging.     

Influence of stand size and quality of tree hollows on saproxylic beetles in Sweden

The habitat requirements of 11 beetle species associated with tree hollows were studied by correlating their occupancy with characteristics of the oaks and the number of hollow oaks in each stand. The species richness was higher in trunks with a large girth and in tree hollows with entrances situated high up on the trunks and not directed upwards. Several species occurred with higher frequencies in larger stands, which suggests that these species are sensitive to habitat fragmentation. In some cases at least, the present species richness may reflect higher densities of hollow trees in the past. It is therefore suggested that, in order to preserve the rare fauna in old oaks, the stand size must be increased at many sites. In this study two species,Elater ferrugineus and Tenebrio opacus, were absent from almost all stands with <10 hollow oaks, whereas the other species occurred at least to some extent also in smaller stands. Presence/absence data of E. ferrugineus, T. opacus and Osmoderma eremita may be used to evaluate to what extent the fauna in old trees is affected by habitat fragmentation today as well as in the past.     

Declining birds in Australian agricultural landscapes may benefit from aspects of the European agri-environment model

Temperate Australia’s wheat/sheep zone and much of Western Europe have both experienced dramatic declines in native bird populations associated with agricultural landscapes. We compare recent conservation strategies on private land in the context of each region’s historical agricultural development and the ecology of its bird fauna. Specifically, we consider which aspects of the conservation instruments and practices employed in European agricultural landscapes might be used to augment and inform approaches to private-land biodiversity conservation in Australia. Australian biodiversity conservation activities have focussed predominantly on remnant native vegetation and rarely target the agricultural matrix (i.e. land that is primarily used for agricultural production). However, declining species include those that not only primarily inhabit woodland, but also species for which components of the agricultural matrix are important, or even their main, habitat. In contrast, in Europe a range of conservation activities undertaken through agri-environment schemes focus explicitly on the management of the agricultural matrix. Whilst the different approaches to conservation on private land in Australia and Europe reflect the two continents’ different ecologies, land-use histories and political economies of agriculture, there are a number of parallels between bird population declines in the two regions, and an opportunity may exist to incorporate some of the successful aspects of the European agri-environment approach into emerging stewardship schemes in Australia. We suggest that the long-term nature of European agri-environment agreements, the principle of landholder payments more commensurate with reduced production opportunity and management actions specifically targeted at the agricultural matrix, are features of the European scheme that could benefit both woodland- and matrix-inhabiting bird species in Australian agricultural landscapes.     

Habitat selection by the common wombat (Vombatus ursinus) in disturbed environments: Implications for the conservation of a ‘common’ species

The construction of habitat models is a repeatable technique for describing and mapping species distributions, the utility of which lies in enabling management to predict where a species is likely to occur within a landscape. Typically, habitat models have been used to establish habitat requirements for threatened species; however they have equal applicability for modelling local populations of common species. Often, few data exist on local populations of common species, and issues of abundance and habitat selection at varying scales are rarely addressed. We provide a habitat suitability model for the common wombat (Vombatus ursinus) in southern New South Wales. This species is currently perceived as abundant throughout its extensive range across temperate regions of eastern Australia, yet little factual survey data exist and populations appear under threat. We use wombat burrows to reflect habitat selection and as our basis for ecological modelling. We found that environmental variables representing proximity to cover, measures of vegetation and proximity to watercourses are important predictors of burrow presence. Extrapolation of habitat models identified an abundance of habitat suitable for burrows. However, burrows in many suitable areas were abandoned. Our estimate of the population size was similar to the total annual mortality associated with road-kill. Theoretically, given the availability of suitable habitat, common wombat populations in the region should be thriving. It seems likely that this area once supported a much higher number of wombats; however limiting factors such as road mortality and disease have reduced the populations. The persistence of wombats in the study region must be supported by migration from other populations. Our findings challenge the perception that wombats are currently common and not in need of monitoring, suggesting that perceptions of abundance are often clouded by socio-political motives rather than informed by biological and ecological factors.     

Birds in eucalypt and pine forests: landscape alteration and its implications for research models of faunal habitat use

Most studies of faunal habitat fragmentation are based on a human perspective of the landscape in which landscape elements are classified as habitat and non-habitat. Moreover, many landscape models that define “habitat patches” assume that the same set of patches will be suitable for all taxa or a broad range of taxa. McIntyre and Hobbs [Conservation Biology 13 (1999) 1282] recently proposed a model in which landscapes can be classified according to the amount of habitat remaining and in which the remaining habitat can correspond to a continuum or gradient of modification. The perception of a landscape as being intact, variegated, fragmented or relictual [sensu Conservation Biology 13 (1999)] will depend on the capacity of individual species to utilise modified habitat. We suggest that although the continuum concept of habitat use is reasonably well established for plants, faunal studies have often ignored the notion of a gradient of habitat use and have classified landscape areas simplistically and inappropriately as either habitat or non-habitat. Data on birds in southeastern Australia are used to illustrate how the binary view of habitat can be incorrect. Birds were sampled in landscapes that ranged from intact to relictual as defined from an anthropocentric perspective. Our data: (1) illustrated a wide range of bird responses to habitat modification including many that might have been overlooked using a simple binomial approach to habitat classification, and (2) highlighted the fact that the way in which humans perceive landscapes may not correspond to how some elements of the biota perceive the same landscape. Viewing landscapes as a species-specific gradient of states of remaining habitat and condition has important implications for undertaking studies of human impacts on biodiversity and also integrating conservation considerations in production environments. It also challenges the effectiveness of “quick fixes” such as species-based biodiversity surrogates schemes and the uncritical use of generic landscape indices to save “habitat” because the assumption that all species will conform to the same landscape pattern will not hold.     

Managing landscapes for the little bustard Tetrax tetrax: lessons from the study of winter habitat selection

More than half of the world's population of the little bastard Tetrax tetrax lives in the Iberian Peninsula, where it is mostly dependent for survival on extensive agricultural areas. The species has declined dramatically, chiefly due to changes in agricultural practices, and is now globally ‘Near Threatened’. Knowledge of its habitat requirements is crucial to reverse the trend of habitat deterioration. Winter habitat preferences were studied in a region dominated by extensive cereal farming in Southern Portugal, comparing the characteristics of sites used by 54 flocks with those of randomly selected sites within the study area. The birds preferred recent fallows and grassy vegetation of mid-size (about 11-20 cm) and mid-density (about 11-50% cover). They tended to concentrate on the tops of hills and to avoid disturbed areas near roads and houses. Overall, the observed preferences suggest that predator avoidance is a significant factor in habitat selection. To improve habitat suitability for little bustard, managers of key wintering areas should minimize permanent sources of human disturbance, encourage rotations with frequent fallows, and favour moderate levels of grazing to manipulate vegetation height and cover. Particular attention should be given to the upper parts of the hills.     

Incorporating uncertainty into analyses of red-crested pochard habitat selection

Studies describing habitat use in animal species need to take into account detectability of individuals in order to reach more robust conclusions. However, the importance of detectability in habitat selection analyses of rare and cryptic species has received little investigation, although robust methods for estimating detectability have been made recently available. Understanding habitat requirements should be an important management tool for the conservation of the red-crested pochard (Netta rufina), a rare duck species in France and Europe. Three different lake samples (82 lakes in total) were conjointly used in 2000 and 2001 to assess the species habitat requirements, using its presence, density and hatching dates as response variables. Risk of false absence was estimated using the double-observer approach at less than 0.001. A robust-design approach produced estimates of individual brood detectability (0.545, SE = 0.053). Observed red-crested pochard densities were adjusted to time dependent detectabilities, and modelled as a function of habitat variables. Habitat fragmentation and low permanent water levels negatively affected brood densities. Interestingly, these variables were not retained when modelling the unadjusted densities. This analysis showed that investigating temporal variation in brood detectability was a crucial prerequisite in the study of this rare species’ habitat requirements. More generally, it strongly suggests that integrating detection probability and its variations in habitat use analyses of cryptic species of conservation concern may be an essential methodological step to reach more valid conclusions on habitat management.