Effect of conspecific attraction on the presence of Northern Bobwhites

Researchers and managers have used the tendency of some species of birds to settle near conspecifics (i.e., conspecific attraction) to help establish or reestablish species of conservation concern in targeted habitats. However, most studies of conspecific attraction as a recruitment tool have been conducted with either migratory or colonial-breeding species, and less is known about the possible importance of conspecific attraction for resident species. In 2017 and 2018 in Louisiana, we examined the possible use of conspecific attraction as a management tool for Northern Bobwhites (Colinus virginianus), game birds that are year-round residents. We used an alternating split experimental design across years where Northern Bobwhite vocalizations were broadcast at half of our study sites during the first year of our study and the other half were controls. The following year, experimental sites became controls, and controls became experimental sites. We also assessed land cover at each site to determine if variation in habitat composition at small spatial scales influenced bobwhite presence. We detected six times more Northern Bobwhites at treatment sites (sites with playback) than control sites and found a positive association between the proportion of grass cover and bobwhite presence. These results suggest that a non-migratory, resident species may select breeding locations based, at least in part, on the presence of conspecifics, and that playback of their calls could be incorporated into management plans. Playing back conspecific calls in early spring when Northern Bobwhites are likely searching for breeding locations may facilitate colonization and allow individuals to find locations where managers are attempting to restore their populations.     

Determinants of helminth aggregation in natural host populations: individual differences or spatial heterogeneity?

We assessed the importance of spatial heterogeneity for the aggregation of helminth populations in the bank vole Clethrionomys glareolus (Arvicolinae) using a previously published method which allows to analyse parasite aggregation at two host population levels, i.e., within and between spatial samples of the host population.
In the main empirical material from Finnish Lapland (Pallasjärvi), all five helminth species were significantly aggregated within study sites, but only three rare species showed significant aggregation among sites. In all helminth species, the total aggregation was primarily (79-98%) determined by aggregation within sites, i.e., between host individuals. Despite the larger spatial scale and more heterogeneous landscape, the comparative material from South Finland (Luhanka) supported the generally high proportion of the total aggregation due to within-site heterogeneity.
Despite the clear interspecific differences in patchiness of helminth populations, the proportion of the total aggregation due to among-site heterogeneity did not vary significantly among helminth species. The results indicate a link between aggregation at two population levels; species that showed strong within-site aggregation were also characterised by pronounced spatial heterogeneity and significant among-site aggregation.     

Deep-Sea Macrobenthic Communities at Contrasting Sites off Portugal,Preliminary Results: II Spatial Dispersion

Small-scale, biogenic processes are thought to be important in creating a mosaic of microhabitats that allow co-existence of the large numbers of rare species found in deep-sea sediments. However, the large-scale effects of hydrodynamic disturbance seem likely to be important in high-energy settings on the deep-sea bed. This is investigated by examining spatial dispersion patterns in the macrobenthic populations at two deep-sea sites off Portugal. The widely differing conditions of sediment disturbance at the two sites are related to differences in macrobenthic species diversity and dominance. Closely grouped, transponder-mapped, replicate box core samples, each divided into 25 “vegematic” subcores permitted analysis of dispersion patterns at two spatial scales at each site. One site was located on the Tagus Abyssal Plain (TAP), and the other in the nearby Setubal Canyon (SC) where ripple bedforms indicate vigorous bed flow, probably with a tidal periodicity. Greater aggregation at the scale of tens of metres is indicated from the observed numbers of SC species per box core being wide underestimates of the value predicted by rarefaction of the pooled total compared to those from TAP. Variance/mean ratios for individual species of bivalves and tanaids indicated more aggregation in spatial distribution between box cores from SC than from TAP. Dispersion chi-square analysis also indicated more non-randomness at SC between box cores than for the TAP cores. However, low faunal abundances in the TAP samples render the results of analyses for this site somewhat inconclusive and therefore the trend indicating less aggregation at TAP compared to SC must be regarded as weak for the groups examined. Analyses of the distribution of tanaids, and bivalves at a 100 × 100 mm subcore scale showed no convincing significant departure from random expectation, but low abundances in these groups reduces the sensitivity of the tests applied. Between-core aggregation detected at SC may reflect habitat variation resulting from the varying depth within the area of sampling, while within-core patchiness could be influenced by the ripple bed forms on the sediment.     

Effects of scale-dependent variation in ice cover on the distribution of wintering belted kingfishers Ceryle alcyon

Understanding why species react differently to changing environments requires detailed understanding of the factors that regulate distributions within current ranges These types of investigations require spatially and temporally explicit examination of the scales at which a species responds to environmental factors To obtain this type of information, we analyzed the relationship between ice cover and abundance, distribution, and spacing of belted kingfishers Ceryle alcyon during the autumn/winters of 1993–1994 and 1994–1995 Our results indicate that the effects of ice cover on kingfisher populations are complex and scale dependent At a small spatial scale (0 5 km) the abundance of kingfishers was negatively correlated with ice cover, as we expected At a large spatial scale (15 5 km), however, there was no relationship between these variables We argue this contradiction m our results is due to differences in the patchiness of ice cover at these scales In a year with average winter temperatures (1993–1994) the spatial and temporal autocorrelation of ice cover and kingfisher distribution were similar In a warm year (1994–1995), however, we found no such similarity Neighbor distances were shorter during the cold year than during the warm year and shorter during censuses with extensive ice cover than censuses with sparse ice cover We conclude that ice cover is a key factor regulating the distribution and abundance of belted kingfishers in our study area during cold to average years, but that during warm years other factors are likely to be more important Based on these patterns we think that patterns of ice cover may be important in mediating the response of kingfishers to changes in climate     

放牧和水淹干扰对松嫩平原碱化草地空间格局影响的分形分析

对东北松嫩平原碱化草地植物群落空间格局的分形性质进行了分析,分别用边长面积指数和Ｋｏｒｃａｋ相对斑块化指数估计了斑块边界复杂性和斑块面积大小频率分布格局。结果表明：随放牧强度增加,占优势的羊草（Ｌｅｙｍｕｓｃｈｉｎｅｎｓｉｓ（Ｔｒｉｎ．）Ｔｚｒｅｌ）斑块的斑块化加剧,而斑块的边界在中度放牧时最不规则;在水淹地,占优势的羊草斑块和次优势的碱茅（Ｐｕｃｉｎｅｌｉａｓｐｐ．）斑块、獐茅（Ａｅｌｕｒｏｐｕｓｌｉｔｏｒａｌｉｓ（Ｇｏｕａｎ）Ｐａｒｌ．ｖａｒ．ｓｉｎｅｎｓｉｓＤｅｂｅａｕｘ）斑块的斑块边界复杂性和斑块化程度比未水淹地都低,说明水淹降低了群落复合体内的异质性;在水淹地,优势斑块的边长面积指数和Ｋｏｒｃａｋ指数均低于次优势种,但在重度放牧地结果正好相反,可能是由于两种样地处于不同的演替阶段;而且,斑块边界复杂性符合同一尺度规律,在现有的面积范围内没有尺度转换,斑块面积大小的频率分布格局则存在尺度转换点。     

Scale-specific correlations between habitat heterogeneity and soil fauna diversity along a landscape structure gradient

Habitat heterogeneity contributes to the maintenance of diversity, but the extent that landscape-scale rather than local-scale heterogeneity influences the diversity of soil invertebrates—species with small range sizes—is less clear. Using a Scottish habitat heterogeneity gradient we correlated Collembola and lumbricid worm species richness and abundance with different elements (forest cover, habitat richness and patchiness) and qualities (plant species richness, soil variables) of habitat heterogeneity, at landscape (1 km²) and local (up to 200 m²) scales. Soil fauna assemblages showed considerable turnover in species composition along this habitat heterogeneity gradient. Soil fauna species richness and turnover was greatest in landscapes that were a mosaic of habitats. Soil fauna diversity was hump-shaped along a gradient of forest cover, peaking where there was a mixture of forest and open habitats in the landscape. Landscape-scale habitat richness was positively correlated with lumbricid diversity, while Collembola and lumbricid abundances were negatively and positively related to landscape spatial patchiness. Furthermore, soil fauna diversity was positively correlated with plant diversity, which in turn peaked in the sites that were a mosaic of forest and open habitat patches. There was less evidence that local-scale habitat variables (habitat richness, tree cover, plant species richness, litter cover, soil pH, depth of organic horizon) affected soil fauna diversity: Collembola diversity was independent of all these measures, while lumbricid diversity positively and negatively correlated with vascular plant species richness and tree canopy density. Landscape-scale habitat heterogeneity affects soil diversity regardless of taxon, while the influence of habitat heterogeneity at local scales is dependent on taxon identity, and hence ecological traits, e.g. body size. Landscape-scale habitat heterogeneity by providing different niches and refuges, together with passive dispersal and population patch dynamics, positively contributes to soil faunal diversity. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

The influence of spatial heterogeneity on regeneration by seed in a limestone grassland

Abstract. The spatial distribution of seedlings in fertilized and non-fertilized grassland patches (1 m x 2 m) was examined in alvar vegetation on the island of Öland, Sweden. We established whether variation in seedling distribution in grassland patches reflects the distribution of conspecific adults. We also established the significance of microsite turf attributes for prediction of seedling distribution at a small scale (10 cm x 10 cm), and the significance of conspecific adults abundance, assemblage of adult species and cover of lichens, mosses, litter and bare ground. Our results indicate that fertilization reduces the importance of regeneration by seed of perennial species and enhances seedling recruitment of winter annuals and biennials. At the patch scale, enrichment does not affect species richness. Community patchiness contributes to the maintenance of diversity by providing a variety of environments with differential regeneration of the component species. Cover of lichens, mosses, litter and bare ground, accounts, in part, for the spatial variation in seedling numbers of certain species. The strong association of seedlings of some species with conspecific adults indicates that limitations in dispersal and/or the occurrence of a more suitable environment for emergence and/or establishment close to conspecific adult plants are likely to constrain the spatial distribution of new individuals in many cases. For other species, the lack of association of seedling densities with the cover of their conspecific adults, suggests that long-distance dispersal or a long-lived seed bank can play a role in counteracting the effect of short-distance dispersal.     

Characterizing fish–habitat associations in streams as the first step in ecological restoration

Abstract Habitat availability is often regarded as the primary factor that limits population and community recovery in degraded ecosystems, and physical habitat is thus often targeted in restoration. The identification of which habitat(s) to attempt to restore is a critical step in the restoration process, but one for which there is often a paucity of useful information. Here we examine the distribution of fish in three lowland streams in Victoria, Australia, that have been degraded by severe sedimentation. We aim to identify habitats that are associated with high abundances of native fish, and that thus might be appropriate to target in habitat restoration. Associations between native fish abundances and physical habitat characteristics were examined at three spatial scales (among streams, among sites and within sites) to determine the types of habitat to which fish respond, and the scales over which these responses occur. Of the four species of native fish found in the streams, three (Galaxias olidus Günther, Gadopsis marmoratus (Richardson) and Nanoperca australis Günther) showed significant habitat associations at small spatial scales (i.e. within sites). In particular, these species were generally found in deeper water, and in close proximity to cover (typically either coarse or fine woody debris or vegetation). Differences in habitat availability among sites and streams were less influential, except in the case of G. marmoratus, which was completely absent from both the ephemeral streams. Although our results suggest that these species collectively respond to habitat at several spatial scales, fish distributions were allied to the presence of habitat structure at the scale of metres, the smallest spatial scale examined. We hypothesize that fish abundances are currently limited by the low availability of habitat at these small spatial scales. It may therefore be possible to increase fish abundances in these creeks by augmenting the amount of available habitat via stream restoration.     

Distribution patterns of tree species in an evergreen broadleaved forest in eastern China

Ecological assembly rules in evergreen broad-leaved forest are far from clear understanding. Spatial dispersion of individuals in a species is central in ecological theory. We analyzed the spatial patterns as well as associations between adult and juvenile of each tree species in a 5-ha subtropical evergreen broad-leaved forest plot in eastern China. Out of the 74 species occurring with more than 10 individuals, 88.4% of these species are aggregated. Most of them are aggregated from small to large scales. Spatial distributions of some species corre-spond with topography. Many bad dispersed species in the Baishanzu exhibit a highly aggregated distribution at small scales. These suggest that environmental heterogeneity and/or dispersal limitation may be the most important mechanisms that control the distribution patterns of these species. Our observations of the aggregations of abundant species basically support the hypothesis that dispersal limitation decreases as the number of reproductive trees increases. The rest species are randomly distributed, with less than 10 individuals. For most common species, spatial aggregation is weaker in larger diameter classes, and the distance between adults is larger than that between juveniles and adults, suggesting that density-dependence works on loosing aggregation and excluding conspecific juveniles away from adult trees. However, the density-dependent effect is not strong enough to eliminate all seedlings near adult trees and to result in a regular distribution of trees; thus the density dependence is usually masked by the refuge effect.     

A Test of the Utility of Exotic Tree Plantations for Understory Birds and Food Resources in the Colombian Andes1

The spatial distribution and abundance of animals are in part determined by the distribution and abundance of their resource base. In the Central Andes of Colombia, monospecific tree plantations have been used to recover vegetative cover in watershed protection programs, but these plantations differ from natural forests in structure, composition, and resources available to wildlife. To evaluate these plantations as habitat for wildlife, we compared the diversity and abundance of understory birds, flowers, and fruits between juxtaposed 40‐yr‐old patches of exotic Chinese ash (Fraxinus chinensis) plantations and naturally regenerated forest of the same age. We observed more fruiting species in the regenerated forest, but there were no differences in total flower and fruit availability between habitat types. We also observed differences between habitats in the size of some understory shrubs, but these differences did not lead to differences between habitats in the diversity and abundance of birds. Capture rates of nectarivores correlated with resource abundance at sampling plot scale, but no such correlation was found for frugivores. Understory frugivorous birds are apparently functioning at a larger spatial scale than the patchiness created by the two habitats. Nectarivorous birds are responding to small‐scale patchiness in resource availability, but not to the different habitats. These results indicate that at small scales, the ash plantation understory provides adequate habitat for birds.     

Physical variability,diversity gradients and the ecology of temperate and tropical reefs

Traditionally, compared with the tropics, temperate systems are believed to: (i) have environments which are less favourable (i.e. harsher, more variable and less predictable); and (ii) support communities which are less diverse. Explanations for differences between temperate and tropical communities, including differences in diversity, generally rely on the former notion. The evidence for these ideas is, at best, equivocal. Organisms are subjected to physical stresses and disturbances on both temperate and tropical reefs. Communities on temperate reefs are not invariably less diverse than those in the tropics, at least at small spatial scales. Finally, there is as yet little evidence of genuine differences between the ecology of temperate and tropical communities. There is, however, much small-scale, spatial patchiness in the structure of reef communities and their physical environment. This patchiness in structure may result from patchiness in biological factors (e.g. recruitment) or in the physical environment. This small-scale variation in environmental factors may prove to be a more important determinant of community structure than the large-scale, latitudinal trends some ecologists have been obsessed with.     

Behavioural mechanisms that undermine species envelope models: the causes of patchiness in the distribution of great bustards Otis tarda in Spain

Despite the general success of species envelope models, capturing the fine-scale detail of patchiness in the distributions of some species is problematic. For great bustards in Spain, apparently suitable habitat patches remain unoccupied and cannot be distinguished from occupied patches in current distribution models. We consider philopatry and conspecific attraction as main behavioural mechanisms which could account for this patchiness, and then look for evidence of their influence on the distribution of great bustards across the whole of Spain. We compared the characteristics of habitat patches classed as suitable by a distribution model according to whether they were actually used or not. Occupied patches were larger than unoccupied patches and over-used in proportion to their size, suggesting aggregation and a metapopulation structure. Arguing that conspecific attraction may serve to transfer information about site history and environmental predictability (at least over a short time period), we compared the coefficients of variation in time-series of vegetation and climatic factors at occupied and unoccupied sites. Great bustards chose sites which were more environmentally stable at critical periods in the breeding cycle, "public information" that can only be gained from others rather than through sampling. There is thus evidence that both metapopulation dynamics and conspecific attraction influence the large scale distribution of great bustards in Spain. We discuss how alternative predictor variables and multi-stage analyses may help us to incorporate behavioural mechanisms into distribution models, but acknowledge that there are limits to the value of species envelope models for animal species making decisions.     

Distribution patterns of tree species in an evergreen broadleaved forest in eastern China

Ecological assembly rules in evergreen broad-leaved forest are far from clear understanding. Spatial dispersion of individuals in a species is central in ecological theory. We analyzed the spatial patterns as well as associations between adult and juvenile of each tree species in a 5-ha subtropical evergreen broad-leaved forest plot in eastern China. Out of the 74 species occurring with more than 10 individuals, 88.4% of these species are aggregated. Most of them are aggregated from small to large scales. Spatial distributions of some species correspond with topography. Many bad dispersed species in the Baishanzu exhibit a highly aggregated distribution at small scales. These suggest that environmental heterogeneity and/or dispersal limitation may be the most important mechanisms that control the distribution patterns of these species. Our observations of the aggregations of abundant species basically support the hypothesis that dispersal limitation decreases as the number of reproductive trees increases. The rest species are randomly distributed, with less than 10 individuals. For most common species, spatial aggregation is weaker in larger diameter classes, and the distance between adults is larger than that between juveniles and adults, suggesting that density-dependence works on loosing aggregation and excluding conspecific juveniles away from adult trees. However, the density-dependent effect is not strong enough to eliminate all seedlings near adult trees and to result in a regular distribution of trees; thus the density dependence is usually masked by the refuge effect.     

Assembly rules of ground-foraging ant assemblages are contingent on disturbance, habitat and spatial scale

Aim A major endeavour of community ecology is documenting non‐random patterns in the composition and body size of coexisting species, and inferring the processes, or assembly rules, that may have given rise to the observed patterns. Such assembly rules include species sorting resulting from interspecific competition, aggregation at patchily distributed resources, and co‐evolutionary dynamics. However, for any given taxon, relatively little is known about how these patterns and processes change through time and vary with habitat type, disturbance history, and spatial scale. Here, we tested for non‐random patterns of species co‐occurrence and body size in assemblages of ground‐foraging ants and asked whether those patterns varied with habitat type, disturbance history, and spatial scale. Location Burned and unburned forests and fens in the Siskiyou Mountains of southern Oregon and northern California, USA. Methods We describe ground‐foraging ant assemblages sampled over two years in two discrete habitat types, namely Darlingtonia fens and upland forests. Half of these sites had been subject to a large‐scale, discrete disturbance – a major fire – in the year prior to our first sample. We used null model analyses to compare observed species co‐occurrence patterns and body‐size distributions in these assemblages with randomly generated assemblages unstructured by competition both within (i.e. at a local spatial scale) and among (i.e. at a regional scale) sites. Results At local spatial scales, species co‐occurrence patterns and body‐size ratios did not differ from randomness. At regional scales, co‐occurrence patterns were random or aggregated, and there was evidence for constant body‐size ratios of forest ants. Although these patterns varied between habitats and years, they did not differ between burned and unburned sites. Main conclusions Our results suggest that the operation of assembly rules depends on spatial scale and habitat type, but that it was not affected by disturbance history from fire.     

Does bird community structure vary with landscape patchiness? A Chihuahuan Desert perspective

During the springs of 1995–1997, we studied birds and landscapes at 70 sites in the Chihuahuan Desert to assess relations between bird community structure and landscape patchiness. Within each of two spatial extents (1‐km and 2‐km‐radius areas centered on each site), we measured the number of patches of individual land‐cover types and the total number of patches of all land‐cover types. Mean bird richness, and the mean abundance and probability of occurrence of most bird species were significantly correlated with one or more of these variables. Contrary to evidence from other systems, positive association with landscape patchiness did not increase with the degree to which species were habitat generalists, was not negatively related to body size, and did not differ between neotropical migrants and nonmigrants. For the communities’ primary constituent species as a group, the strength of positive and negative associations with patchiness did not differ between landscape extents. Within the 1‐km but not the 2‐km extent, habitat specialists were more positively and negatively associated with patchiness than were habitat generalists. In general, however, neither habitat breadth, body size, nor migratory status seemed to be responsible for associations with landscape patchiness. Mean richness, and the mean abundance and probability of occurrence of most species were significantly correlated with patchiness within one or both extents, and patchiness of all of the most extensive land‐cover types was influential. The simplest explanation for most of the bird‐patchiness relations we found is that the associations reflected species‐specific habitat needs. Through effects on avian richness, abundance, and occurrence, landscape patchiness affected bird community structure. A more complete understanding of the effects of landscape patchiness on bird community structure is likely to emerge when ecologists study the patchiness of major land‐cover types at various spatial extents.     

Native and naturalized plant diversity are positively correlated in scrub communities of California and Chile

Abstract. An emerging body of literature suggests that the richness of native and naturalized plant species are often positively correlated. It is unclear, however, whether this relationship is robust across spatial scales, and how a disturbance regime may affect it. Here, I examine the relationships of both richness and abundance between native and naturalized species of plants in two mediterranean scrub communities: coastal sage scrub (CSS) in California and xeric-sloped matorral (XSM) in Chile. In each vegetation type I surveyed multiple sites, where I identified vascular plant species and estimated their relative cover. Herbaceous species richness was higher in XSM, while cover of woody species was higher in CSS, where woody species have a strong impact upon herbaceous species. As there were few naturalized species with a woody growth form, the analyses performed here relate primarily to herbaceous species. Relationships between the herbaceous cover of native and naturalized species were not significant in CSS, but were nearly significant in XSM. The herbaceous species richness of native and naturalized plants were not significantly correlated on sites that had burned less than one year prior to sampling in CSS, and too few sites were available to examine this relationship in XSM. In post 1-year burn sites, however, herbaceous richness of native and naturalized species were positively correlated in both CSS and XSM. This relationship occurred at all spatial scales, from 400 m² to 1 m² plots. The consistency of this relationship in this study, together with its reported occurrence in the literature, suggests that this relationship may be general. Finally, the residuals from the correlations between native and naturalized species richness and cover, when plotted against site age (i.e. time since the last fire), show that richness and cover of naturalized species are strongly favoured on recently burned sites in XSM; this suggests that herbaceous species native to Chile are relatively poorly adapted to fire.     

The effect of overstorey proteas on plant species richness in South African mountain fynbos

Two South African mountain fynbos sites, similar in drainage, elevation, slope angle, slope aspect and soil type but with differing fire histories, were studied to measure how the effect of high densities of overstorey proteas in one fire cycle affects the α-diversity levels of the plant community in the following fire-cycle, how their repeated absence due to several short fire-cycles affects their species richness and finally, at what spatial scale such patterns are most appropriately measured. High prefire canopy cover percentages and densities of overstorey proteas increase the postfire α-diversity of understorey species. In addition, the increase in species richness observed occurred for all higher plant life history types present. At sites where one or more short fire cycles resulted in the repeated absence of overstorey proteas, the number of plant species present in the understorey was lower than at a site where overstorey proteas persisted. These results are dependent on the spatial scale at which the α-diversity of understorey species is measured. At small quadrat sizes (< 5 m²), overstorey proteas decrease the number of understorey species present, while at larger quadrat sizes (100 m²) higher species richness is observed. The contradiction in conclusions when α-diversity is measured at different spatial scales can be attributed to the patchiness of fynbos communities. Overstorey proteas play an important role in maintaining the patchiness component of fynbos communities by diminishing the effect of understorey resprouting species, making available regeneration niches for the maintenance of plant species richness. Where small quadrats are used, the effect of patchiness on the dynamics of the mountain fynbos community is lost. Thus, it is the fire history prior to the last fire and how it affects overstorey proteas that is important in the determination of α-diversity levels in mountain fynbos plant communities.     

放牧和围封条件下羊草碱化草地中斑发布格局研究

应用分形方法对松嫩平原羊草碱化划地中斑块分布格局进行定量估计,分析了放牧干扰和围封条件下斑块分布特征及动态。结果表明：随放牧强度增加,羊草斑块的斑块化指数增大,斑块面积范围的上下限变小,分布趋于破碎化和不均匀,斑块公布格局趋于复杂,自相似规律发生转折的尺度增多。围封后,１９８９～１９９３年羊草斑块总面积增加,斑块数目减少,斑块化指数降低,１９９４年后羊草斑块分布格局基本稳定。羊草斑块分布的双对数曲     

Scale-dependent associations between native freshwater mussels and invasive Corbicula

In North America there is conflicting evidence concerning whether the invasive Asian clam, Corbicula fluminea, and native mussels (Unionidae), can successfully co-exist. One reason underlying disparate conclusions may be the different spatial scales at which data have been collected. We compared the distribution and abundance of native unionid mussels and Corbicula at two spatial scales, stream reaches and 0.25 m² patches, within one biogeographic region, the Ouachita Highlands, of the south central U.S. We found that Corbicula abundance was negatively related to native mussel abundance at small spatial scales. While Corbicula densities varied widely in patches without native mussels, and in patches where mussels occurred at low abundance, Corbicula density was never high in patches where mussels were dense. We hypothesize that the likelihood of successful Corbicula invasion decreases with increasing abundance of adult native mussels. Several mechanisms may potentially drive this pattern including lack of space for Corbicula to colonize, physical displacement by actively burrowing mussels, and locally reduced food resources in patches where native mussels are feeding. In addition, Corbicula may be unable to withstand environmental bottlenecks as readily as unionids. When patch-scale density and biomass information were pooled to represent entire stream reaches, the negative relationship between native mussels and Corbicula was no longer as apparent, and there was not a significant relationship between native mussels and Corbicula. These results point to the importance of appropriate sample scale in examining potential associations between species.     

Diversity and Abundance of Biological Soil Crust Taxa in Relation to Fine and Coarse-Scale Disturbances in a Grassy Eucalypt Woodland in Eastern Australia

Over the last 200 years the box woodlands of eastern Australia have been considerably altered by European farming practices. These changes have been accompanied by a reduction in the size and number of patches of vegetation as well as the quality of the understorey vegetation and underlying soil surface. We measured diversity and abundance of soil crust taxa in relation to habitat complexity, remnant area and width, diversity of vascular plants as well as the number, size and separation of patches of vegetation and grass butts (coarse-scale patchiness), and an index of surface stability derived from measures of seven soil surface features of small microsites (patches of bare/crusted, litter- or grass-covered soil; micro-scale) on both coarse- and fine-textured soils at 35 sites in south-eastern Australia. Fifty taxa were recorded from the 35 sites, and there were more taxa from sites with fine-textured soils (12.7) compared with coarse-textured soils (4.4). The soil crust community was dominated by a few relatively common species, with many species occurring at only a few sites. Half the number of species accounting for <1% of total abundance. Bare and crusted microsites supported more species and greater cover compared with grassy microsites. Crust diversity declined with increasing coarse-level disturbances (i.e. declines in habitat complexity, remnant area and width, and diversity of vascular plants) but the results were not consistent between soil types. No measures of fine-scale disturbance were related significantly to any of the crust diversity or abundance measures, and there was no evidence of a recent grazing effect on crust composition. The fact that few sites had many species (and visa versa) suggests to us that many sites are probably required to conserve soil crust taxa in these highly fragmented landscapes