Scale dependent effects of native prey diversity,prey biomass and natural disturbance on the invasion success of an exotic predator

There is growing support for the general notion that the drivers of invasion success often shift from biotic to abiotic factors with increasing spatial scale. Most of this research, however, has been conducted on a single trophic level; i.e. it has primarily looked at how the diversity of native competitors may influence invasion success. Less attention has been paid to understanding how native prey diversity may influence the invasion success of exotic predators and whether such biotic factors are scale-dependent. We used a hierarchical spatial survey of 17 stream communities to test whether native prey diversity, along with native prey biomass, algal resource abundance and annual stream discharge, influenced the abundance of an exotic crayfish predator, and whether the importance of these factors were scale-dependent. We used a hierarchical generalized linear model to evaluate the influence of these community and stream characteristics on exotic crayfish abundance at both the transect scale (1 m²) and the stream scale (400 m²). Our results indicated that at the stream scale, high stream discharge significantly limited invader abundance. However, at the smaller transect scale, native prey biomass was a significant driver of invasion success and positively correlated with invader abundance. We suggest that our results add to the emerging pattern that abiotic processes are stronger determinants of invasion success at large spatial scales, whereas biotic processes become more important with decreasing spatial scale. However, for predator invasions, prey biomass, not prey diversity may be a more important for driver of invasion success at small spatial scales.     

Spatial heterogeneity of soil respiration and related properties at the plant scale

Geostatistical techniques were used to quantify the scale and degree of soil heterogeneity in 2 m² plots around 9-year-old poplar trees and within a wheat field. Samples were taken during two years, on an unaligned grid, for analysis of soil respiration, C and N content, available P, gravimetric moisture, pH, nitrification potential, and root biomass. Kriged maps of soil respiration, moisture, and C content showed strong spatial structure associated with poplar trees but not with wheat rows. All soil properties showed higher autocorrelation in June than in April. Isopleth patchiness for all variates was less in June. This was associated with lower respiration rates due to lower litter decomposition. From the degree and scale of heterogeneity seen in this study, we conclude that the main causes of soil heterogeneity at this scale (2 m²) are likely to be found at micro scales controlled in part by plant root and plant residue patterns. These must be understood in the evaluation of ecosystem processes. This revised version was published online in June 2006 with corrections to the Cover Date.     

Spatial population structure in the banner-tailed kangaroo rat,Dipodomys spectabilis

I attempted to characterize spatial units of local dynamics and dispersal in banner-tailed kangaroo rats (Dipodomys spectabilis), to determine if spatial structure influenced population dynamics in the way predicted by current metapopulation models. D. spectabilis exhibited a hierarchical spatial structure. Local populations that appeared as discrete entities on a scale of kilometers were subdivided into clusters of mounds on a scale of meters. This structure, however, cannot be characyerized in terms of the discrete habitat patches envisioned by the metapopulation models. Occupied areas were statistically distinguishable from the surrounding matrix, but this difference was only quantitative. There were no discrete boundaries between occupied areas and the matrix. Habitat within occupied areas was heterogeneous, and occupied areas in different locations were statistically distinguishable from each other. High heterogeneity within occupied areas, and high contrast among them, make it difficult to define what is a suitable habitat patch for D. spectabilis. On a smaller spatial scale, there was significant aggregation of resident mounds within occupied areas. These aggregations, however, do not correspond to discrete habitat patches. Rather, they appear to result from an interaction between fine-scale habitat heterogeneity and limited dispersal due to natal philopatry and low adult vagility. These complications make it difficult to identify habitat patches independent of the species' distribution. For species like D. spectabilis that are patchily distributed but do not occupy discrete habitat patches, a patch occupancy approach does not seem appropriate for describing spatial structure. Hierarchical spatial structure underscores the need for a framework that incorporates multiple scales of spatial structure, rather than one that pre-imposes a single spatial scale as being important for population dynamics. A framework that (i) considers patchiness as a combination of both habitat heterogeneity, and life-history and behavioral characteristics, and (ii) incorporates hierarchical spatial structure, appears to be the most suitable for conceptualizing spatial dynamics of behaviorally complex vertebrates such as D. spectabilis.     

Influence of habitat heterogeneity on the distribution of larval Pacific lamprey (Lampetra tridentata) at two spatial scales

1. Spatial patterns in channel morphology and substratum composition at small (1–10 metres) and large scales (1–10 kilometres) were analysed to determine the influence of habitat heterogeneity on the distribution and abundance of larval lamprey. 2. We used a nested sampling design and multiple logistic regression to evaluate spatial heterogeneity in the abundance of larval Pacific lamprey, Lampetra tridentata, and habitat in 30 sites (each composed of twelve 1‐m² quadrat samples) distributed throughout a 55‐km section of the Middle Fork John Day River, OR, U.SA. Statistical models predicting the relative abundance of larvae both among sites (large scale) and among samples (small scale) were ranked using Akaike's Information Criterion (AIC) to identify the ‘best approximating’ models from a set of a priori candidate models determined from the literature on larval lamprey habitat associations. 3. Stream habitat variables predicted patterns in larval abundance but played different roles at different spatial scales. The abundance of larvae at large scales was positively associated with water depth and open riparian canopy, whereas patchiness in larval occurrence at small scales was associated with low water velocity, channel‐unit morphology (pool habitats), and the availability of habitat suitable for burrowing. 4. Habitat variables explained variation in larval abundance at large and small scales, but locational factors, such as longitudinal position (river km) and sample location within the channel unit, explained additional variation in the logistic regression model. The results emphasise the need for spatially explicit analysis, both in examining fish habitat relationships and in developing conservation plans for declining fish populations.     

Scales of horizontal patchiness in chlorophyll a, chemical and physical properties of landfast sea ice in the Gulf of Finland (Baltic Sea)

Horizontal variation of first-year landfast sea ice properties was studied in the Gulf of Finland, the Baltic Sea. Several scales of variation were considered; a number of arrays with core spacings of 0.2, 2 and 20 m were sampled at different stages of the ice season for small-scale patchiness. Spacing between these arrays was from hundreds of meters to kilometers to study mesoscale variability, and once an onshore–offshore 40-km transect was sampled to study regional scale variability. Measured variables included salinity, stable oxygen isotopes (¹⁸O), chlorophyll a (chl-a), nutrients and dissolved organic carbon. On a large scale, a combination of variations in the under-ice water salinity (ice porosity), nutrient supply and the stage of ice development control the build-up of ice algal biomass. At scales of hundreds of meters to kilometers, there was significant variability in several parameters (salinity, chl-a, snow depth and ice thickness). Analyses of the data from the arrays did not show evidence of significant patchiness at scales <20 m for algal biomass. The results imply that the sampling effort in Baltic Sea ice studies should be concentrated on scales of hundreds of meters to kilometers. Using the variations observed in the study area, the estimate for depth-integrated algal biomass in landfast sea ice in the Gulf of Finland (March 2003) is 5.5±4.4 mg chl-a m^–2.     

Horizontal variation of biomass and C:N:P ratios of benthic algae in lakes

The horizontal variation of chlorophyll a (chl a) and C:N:P (carbon:nitrogen:phosphorus) ratio was estimated for benthic algal communities attached to living substrates (mussels and macrophytes) and to rocks and stones in three lakes of different trophy. Samples were taken in a nested hierarchical design with replicates separated by several cm, dm, 10 m, and km. The observed horizontal variation of chl a, C:N, and C:P ratios did not differ for horizontal scales, substrates, or lakes. Although the investigated lakes were quite unlike regarding nutrient status, light regime, or morphology, the patchiness was similar in all lakes. Moreover, patchiness was also similar on stones, macrophytes and mussels, although those substrates differed in longevity and surface structure. Similar patchiness regardless of scale, substrate, or sampled lake, implies the possibility of using an optimal sampling design calculated for one lake and substrate also in other lakes and on other substrates.     

Scale-dependence of fish habitat associations in a North American river system

1. Determining the appropriate measurement scale to assess habitat variables is critical for ecologists assessing biological or ecological conditions. Depth, velocity, substrate, woody debris and other fish cover variables occur on both reach and microhabitat scales, and fish habitat associations with these variables may be scale-dependent. The aim of this work was to better understand the importance of scale for fish–habitat associations with these variables in a framework consistent with environmental filtering and to test the hypothesis that habitat variable importance is scale-dependent.
2. I used prepositioned areal electrofishing in wadeable streams of the Delaware River basin to evaluate the associations of fish with the same variables summarised on different reach and microhabitat scales. The importance of scale for fish–habitat associations was assessed using two approaches that approximate an environmental filtering framework: variance partitioning with (1) ordination and (2) generalised linear mixed models.
3. Variables on both the reach and microhabitat scales explained a significant fraction of the total variation in fish community composition (p < 0.05). Variation decomposition of reach- and microhabitat-scale effects revealed 20.2% and 2.0% of all variation were due uniquely to reach and microhabitat scales, respectively. Measures of coarseness, embeddedness, amount of riffle and areal coverage of five fish cover variables were significant explanatory variables of community composition at the reach scale only (p < 0.05). Velocity and mesohabitat (amount or presence of riffle) were the only two habitat features that were significant explanatory variables of fish community composition at both the reach and microhabitat scales (p < 0.05). Individual models of species occurrence revealed similar patterns as seen with analyses of community composition.
4. For many fishes, habitat features quantified at the reach scale were more explanatory than at the microhabitat scale. Longnose dace (Rhinichthys cataractae) were more dependent upon microhabitat variables than reach-scale variables, relative to other fishes. Mean velocity at the reach scale was the most important explanatory variable for explaining fish community composition and indicated support for the concept of environmental filtering at the reach and microhabitat scales.
5. Few studies of fish occurrence have incorporated a study design and analytical framework that approximates the hierarchical nature of habitat. This study identifies important scales and predictors, demonstrates the importance of a multiscale approach, and provides support for the environmental filtering concept at the reach and microhabitat scales. These findings will allow ecologists to better account for scale-dependent habitat associations and justify the use of fish habitat associations on reach and microhabitat scales for assessing biotic integrity, restoration and conservation of fishes.

     

Spatial dependence of phenotype-environment associations for tadpoles in natural ponds

Within natural habitats, phenotypes are shaped by many environmental factors. Consequently, heterogeneity of these factors can promote phenotypic divergence. However, because environments exhibit heterogeneity at different spatial scales, phenotypic divergence should also exhibit such scale-dependence. Using hierarchical linear models, I determined how multiple environmental factors at two spatial scales affected the morphology of wood frog (Rana (Lithobates) sylvatica) tadpoles collected from natural ponds. Among ponds, predation risk intensity and tadpole density were strong predictors of tadpole morphology, while within ponds, other environmental variables such as water depth and leaf litter were more important. Spatial analyses revealed that water depth and leaf litter, but not predation risk intensity or tadpole density, exhibited heterogeneous spatial distributions within ponds, suggesting that spatial properties of environmental variables influenced the scale at which they shaped phenotypes. Furthermore, patterns of phenotypic variation with respect to predation risk intensity and tadpole density among ponds largely matched observations from previous laboratory studies. These results emphasize the importance of considering phenotype-environment associations across multiple spatial scales.     

Scale dependence in desert plant diversity

The relationship between the number of species and the area sampled is one of the oldest and best-documented patterns in community ecology. An equation of the form S = cA ^z describing more precisely the species–area relation for plant species in smaller area is proposed as a result of intensive examination of species presence. Several study and field data from a wild range of plant and animal taxa suggest that the slope, z, of a graph of the logarithm of species richness against the logarithm of area is not a constant to the grassland or woodland community. We collected replicated and randomized plant data at 6 spatial scales from 1 m² to 1 km² in the desert region of northwest China to identify the scale dependence in desert plant biodiversity. The results showed that the slope of the log–log plot varied systematically with spatial scale. The value of z was high (0.37) at small scales from 1 to 10 m² and it decreased with increased spatial scale subsequently. When spatial scales varied from 1 m² to 1 km², the value of z varied from 0.37 to 0.035, suggesting that desert plant diversity has strongly scale-dependence at the small scales (less than 100 m²). The result is different from the research of grassland and woodland communities.     

Using animal movement paths to measure response to spatial scale

Animals live in an environment that is patchy and hierarchical. I present a method of detecting the scales at which animals perceive their world. The hierarchical nature of habitat causes movement path structure to vary with spatial scale, and the patchy nature of habitat causes movement path structure to vary throughout space. These responses can be measured by a combination of path tortuousity (measured with fractal dimension) versus spatial scale, the variation in tortuousity of small path segments along the movement path, and the correlation between tortuousities of adjacent path segments. These statistics were tested using simulated animal movements. When movement paths contained no spatial heterogeneity, then fractal D and variance continuously increased with scale, and correlation was zero at all scales. When movement paths contained spatial heterogeneity, then fractal D sometimes showed a discontinuity at transitions between domains of scale, variation showed peaks at transitions, and correlations showed a statistically significant positive value at scales smaller than patch size, decreasing to below zero at scales greater than patch size. I illustrated these techniques with movement paths from deer mice and red-backed voles. These new analyses should help understand how animals perceive and react to their landscape structure at various spatial scales, and to answer questions about how habitat structure affects animal movement patterns.     

Scale‐dependence of environmental effects on species richness in oak savannas

Abstract. In order to investigate how scale (grain size) affects the relationships between species richness and environmental drivers (such as stress and disturbance), we collected 12 nested quadrats (from 0.25 m² to 1023 m²) from seven remnant oak savannas located in the floodplain of the Chippewa River in western Wisconsin, USA. Large and small‐scale richness were not significantly correlated, suggesting that small‐scale richness is not strongly controlled by sampling effects of the local species pool. Linear and curvilinear regressions between species richness and disturbance, canopy cover, biomass, and soil organic matter were dependent on sampling scale (grain size). Disturbance by fire was strongly related to richness at small scales, while tree canopy cover was strongly related to richness at larger scales. While there was some evidence suggesting the transition from disturbance to canopy effects occurs between 10 and 100 m², the transition was not particularly abrupt. The results cast doubt on the general importance of local species pools in affecting small‐scale richness as well as our ability to make generalizations that do not explicity include scale.     

Spatial patterns and scaling behaviors of Steller sea lion (Eumetopias jubatus) distributions and their environment

Fractal geometry and other multi-scale analyses have become popular tools for investigating spatial patterns of animal distributions in heterogeneous environments. In theory, changes in patterns of animal distributions with changes in scale reflect transitions between the controlling influences of one environmental factor or process over another. In an effort to find linkages between Steller sea lions (Eumetopias jubatus) and their environment, the objective of this study was to determine if the spatial distribution of Steller sea lions at sea displayed similar scaling properties to the variation of two environmental features, including bathymetry and sea surface temperature (SST). Additionally, distributions of Steller sea lion point patterns were examined with respect to measurements of bathymetric complexity. From February 2000 to May 2004, satellite transmitters were deployed on 10 groups of juvenile Steller sea lions (n=52) at eight different locations within the Aleutian Islands and Gulf of Alaska. Indices of fractal dimension were calculated for each group of sea lions using a unit square box-counting method, whereas indices of bathymetry and SST patchiness were derived by conducting a variance ratio analysis over the same scales. Distributions of Steller sea lions at sea displayed self-similar fractal patterns, suggesting that individuals were distributed in a continuous hierarchical set of clumps within clumps across scales, and foraging behavior was likely influenced by a scale invariant mechanism. Patterns of bathymetric variability also were self-similar, whereas patterns of SST variability were scale dependent and failed to retain self-similar spatial structure at larger scales. These results indicate that the distributions of Steller sea lions at sea were more influenced by bathymetry than SST at the scales examined, but scale-dependent patterns in the distribution of Steller sea lions at sea or linkages with SST may have been apparent if analyses were conducted at finer spatial scales.     

Patterns of resource tracking by avian frugivores at multiple spatial scales: two case studies on discordance among scales

Scaling is relevant for the analysis of plant‐frugivore interaction, since the ecological and evolutionary outcomes of seed dispersal depend on the spatial and temporal scale at which frugivory patterns emerge. We analyse the relationship between fruit abundance and frugivore activity at local and landscape spatial scales in two different systems composed, respectively, by the bird‐dispersed woody plants Juniperus communis and Bursera fagaroides, and their frugivore assemblages. We use a hierarchical approach of nested patchiness of fruit‐resource, where patches are defined by individual plants within site, at the local scale, and by sites within region, at the landscape scale. The structure of patches is also described in terms of contrast (differences in fruit availability among patches) and aggregation (spatial distribution of patches). For J. communis, frugivore activity was positively related to fruit availability at the landscape scale, this pattern seldom emerging at the local scale; conversely, B. fagaroides showed a general trend of positive local pattern that disappeared at the landscape scale. These particular trends might be partially explained by differences in contrast and aggregation. The strong contrast among plants within site together with a high aggregation among sites would promote the B. fagaroides pattern to be only local, whereas in J. communis, low aggregation among sites within region would favour a sharp landscape‐scale pattern. Both systems showed discordant patterns of fruit‐resource tracking among consecutive spatial scales, but the sense of discordance differed among systems. These results, and the available multi‐scale frugivory data, suggest that discordance among successive scales allows to link directly frugivory patterns to resource‐tracking mechanisms acting at particular scales, resulting, thus, more informative than concordance observational data, in which landscape patterns might result from accumulated effect of local mechanisms. In this context, we propose new methodological approaches for a better understanding of the hierarchical behavioural mechanisms underpinning the multi‐scale resource tracking by frugivores.     

Spatial Heterogeneity in Habitat Quality and Cross-Scale Interactions in Metapopulations

Abstract Integration of habitat heterogeneity into spatially realistic metapopulation approaches reveals the potential for key cross-scale interactions. Broad-scale environmental gradients and land-use practices can create autocorrelation of habitat quality of suitable patches at intermediate spatial scales. Patch occupancy then depends not only on habitat quality at the patch scale but also on feedbacks from surrounding neighborhoods of autocorrelated patches. Metapopulation dynamics emerge from how demographic and dispersal processes interact with relevant habitat heterogeneity. We provide an empirical example from a metapopulation of round-tailed muskrats (Neofiber alleni) in which habitat quality of suitable patches was spatially autocorrelated most strongly within 1,000 m, which was within the expected dispersal range of the species. After controlling for factors typically considered in metapopulation studies—patch size, local patch quality, patch connectivity—we use a cross-variogram analysis to demonstrate that patch occupancy by muskrats was correlated with habitat quality across scales ≤1,171 m. We also discuss general consequences of spatial heterogeneity of habitat quality for metapopulations related to potential cross-scale interactions. We focus on spatially correlated extinctions and metapopulation persistence, hierarchical scaling of source–sink dynamics, and dispersal decisions by individuals in relation to information constraints.     

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.     

湖南省阳明山山顶苔藓矮林优势种空间分布格局

山顶苔藓矮林是亚热带山地常绿阔叶林区在极端气候与环境条件下发育的一种群落变型,通过研究优势植物的分布格局与规模,可以有效地指导植被保护与管理活动。在湖南阳明山国家级自然保护区的山顶苔藓矮林区设置3个样地,应用相邻格子法对优势种云锦杜鹃(Rhododendron fortunei)进行每木调查,在不同取样尺度(面积)上采用方差/均值比率法(v/m)、负二项参数(k)、丛生指数(I)、平均拥挤度(m~*)、聚块性指数(m~*/m)、聚集指数(C_a)和格林指数(GI)等聚集强度指数对云锦杜鹃种群分布格局进行分析。结果表明:阳明山山顶苔藓矮林优势种云锦杜鹃种群在北、南、东三个方位上的空间分布格局总体上为聚集分布,随着取样尺度的增大,聚集格局强度和规模依次减弱;而山顶苔藓矮林优势种群不同年龄阶段的空间分布特征为随着种群年龄的增大,其空间分布格局呈聚集强向聚集弱、以至于向随机分布发展;阳明山山顶苔藓矮林优势种群及其分布格局形成的主要原因是云锦杜鹃种群本身的种子繁殖及传播方式和类营养繁殖等生物学特性及方位、坡向等环境因子影响所致。     

Patterns of species richness in a limestone grassland under different treatments in relation to spatial scale

Abstract. In an experiment in a limestone grassland on the Baltic island of Öland, SE Sweden, nutrient and water supply, light intensity and grazing regime were altered in 10 combinations during four years in 10 plots of 0.25 m² with subplots of 0.01 m² and 0.0004 m². Only the combined application of fertilizer and shade led to a strong decrease in average species richness (S₁) at all scales. When comparing species numbers summed up over all 10 replicates of each treatment (S_n) at the three quadrat sizes, differences in effect of these treatments were much smaller, and were so already at the finest scale. α-diversity, measured as (S_n - S₁ was quite constant over different scales for most treatments, i.e. diversity did not increase with an increase in scale. The ‘richness ratio’S_n/S₁ decreased with increasing scale, indicating an increasing degree of homogeneity at larger scales. Treatments which only included fertilizer or shade, maintained high species richness; this high richness was also maintained in combination with grazing and could then be explained by the denser packing of vegetation. Patterns of species richness were correlated at the large scale, but not at the finer scales, indicating a high degree of spatial and temporal heterogeneity at the finer scales. With increasing quadrat size species persistence increased which explains the small effect of certain treatments. Clearly, a range of scales has to be sampled in this type of vegetation to be able to measure different patterns, which may occur under different experimental treatments. The finest scale in this study can become too small, when certain treatments result in a coarse-grained vegetation pattern. The quadrat size of 10 cm x 10 cm should be included in the range of scales. It combines accuracy in sampling with efficiency in time effort, a reasonably large number of species sampled, and a strong differentiation in the effects of the various treatments.     

Species–area relationships arise from interaction of habitat heterogeneity and species pool

Species–area relationships (SARs) represent a ubiquitous and useful empirical regularity characterizing biodiversity. The rate of species accumulation, captured by the value of the exponent, z, varies substantially and for many reasons. We hypothesized that one of the major contributors to this variation is heterogeneity and its change with scale. To test this hypothesis, we used an array of natural microcosms for which we had invertebrate species composition and physical properties of habitat. Using GIS and cluster analysis, we organized the species data into four sets: communities grouped by spatial proximity in the field, randomly, by similarity of their physical habitat and by dissimilarity of their physical habitat. These groupings produced varying levels of heterogeneity at different scales. We fitted species–area and species–volume relationships to the four groups of communities, and obtained z-values for each group or a portion of the group if the slope of the relationship varied. As predicted, we recovered a number of properties reported by others. More interestingly, we found that small- and large-scale habitat heterogeneity produced scale-dependent z-values while the random grouping of pool habitats produced z-values more robust across scales but also susceptible to initial values of habitat richness. Habitat area affected rate at which species accumulated much less than the mean degree of inter-habitat differences: increasing area that is heterogeneous at broader scales produces higher z-values than increasing an area that shows heterogeneity at small scale only. Our results, while from a microcosm system, rely on logic transferable to larger scale data sets.     

Searching clusters of community composition along multiple spatial scales: a case study on aquatic invertebrate communities in bamboo stumps in West Timor

We examined spatial heterogeneity at multiple scales in composition of the aquatic invertebrate communities in bamboo stumps in a mountainous area of West Timor. We partitioned the study area (ca. 15,000 m²) at five levels of patchiness: two sites, four sub-sites, eight super-clumps, 14 clumps, and 84 stumps. Similarity of community composition between stumps varied more than expected from independent occurrence of each taxon in comparisons within any levels of patches. Negative association was frequently detected among taxa. These results indicate heterogeneity in community composition at a stump level. At higher levels, similarity among stumps within each site was greater than expected from null models which assumed no spatial heterogeneity, and similarities among super-clumps, sub-sites and between sites in a whole area were lower than expected from the null models. The observed patterns in similarities among subsets of the community and distribution of each taxon were mostly consistent with the models which assumed site-level heterogeneity. Therefore, we conclude that the community in this area was spatially heterogeneous at stump and site levels. The relationship between mean intra- and inter-specific crowding suggested that the site level habitat heterogeneity might reduce the chance of encounters between two predators, the larvae of the Toxorhynchites mosquito and the Brachyceran fly.     

Guild-specific responses of avian species richness to LiDAR-derived habitat heterogeneity

Ecological niche theory implies that more heterogeneous habitats have the potential to support greater biodiversity. Positive heterogeneity-diversity relationships have been found for most studies investigating animal taxa, although negative relationships also occur and the scale dependence of heterogeneity-diversity relationships is little known. We investigated multi-scale, heterogeneity-diversity relationships for bird communities in a semi-arid riparian landscape, using airborne LiDAR data to derive key measures of structural habitat complexity. Habitat heterogeneity-diversity relationships were generally positive, although the overall strength of relationships varied across avian life history guilds (R² range: 0.03–0.41). Best predicted were the species richness indices of cavity nesters, habitat generalists, woodland specialists, and foliage foragers. Heterogeneity-diversity relationships were also strongly scale-dependent, with strongest associations at the 200-m scale (4 ha) and weakest associations at the 50-m scale (0.25 ha). Our results underscore the value of LiDAR data for fine-grained quantification of habitat structure, as well as the need for biodiversity studies to incorporate variation among life-history guilds and to simultaneously consider multiple guild functional types (e.g. nesting, foraging, habitat). Results suggest that certain life-history guilds (foliage foragers, cavity nesters, woodland specialists) are more susceptible than others (ground foragers, ground nesters, low nesters) to experiencing declines in local species richness if functional elements of habitat heterogeneity are lost. Positive heterogeneity-diversity relationships imply that riparian conservation efforts need to not only provide high-quality riparian habitat locally, but also to provide habitat heterogeneity across multiple scales.