Species-area Slopes Research Articles

SLOSS‐based inferences in a fragmented landscape depend on fragment area and species–area slope

AbstractAimsWhether a Single Large Or Several Small (SLOSS) habitat patches contain more species is central to the debate about how habitat fragmentation threatens species diversity. However, the geographical and biogeographical variables that affect emergent SLOSS patterns remain poorly understood. Here, we quantified SLOSS‐based diversity patterns of woody plant, bird and spider assemblages in a subtropical archipelago of land bridge islands.LocationThousand Island Lake, Zhejiang Province, China.TaxonWoody plants, birds and spiders.MethodsWe analysed species accumulation curves and species–area relationships (SARs) to quantify SLOSS‐based patterns of all, common and rare species for each taxon across different groupings of islands. Differences in the number of species between a single large island and sets of several small islands were measured in these analyses using a Saturation index (SI) and SLOSS index. Generalized additive models were used to assess the relationships between SI and SLOSS index values and the maximum area of islands included in the analysis, the slope of SARs (i.e. z‐score) and the degree of nestedness.ResultsSI values increased with the maximum area of islands for three taxa, while SLOSS index values only increased with the maximum island area for woody plants. SI values increased as z‐scores of woody plants decreased, and showed a quadratic relationship for birds and spiders. SLOSS index values decreased as z‐scores of three taxa increased. The degree of nestedness and SI values or SLOSS index values were weakly correlated with all, rare and common species in the three taxa.Main conclusionsSLOSS‐based inferences of fragmentation effects are contingent on variation in the maximum area of patches included in analyses and the slope of SARs in fragmentated landscapes. Whether conservation efforts should prioritize a single large or several small patches depends on the geographical (e.g. maximum fragment area) and biogeographical (e.g. slope of SAR) attributes of a fragmentated landscape.

Species-area and network-area relationships in host-helminth interactions.

The scaling relationship observed between species richness and the geographical area sampled (i.e. the species-area relationship (SAR)) is a widely recognized macroecological relationship. Recently, this theory has been extended to trophic interactions, suggesting that geographical area may influence the structure of species interaction networks (i.e. network-area relationships (NARs)). Here, we use a global dataset of host-helminth parasite interactions to test existing predictions from macroecological theory. Scaling between single locations to the global host-helminth network by sequentially adding networks together, we find support that geographical area influences species richness and the number of species interactions in host-helminth networks. However, species-area slopes were larger for host species relative to their helminth parasites, counter to theoretical predictions. Lastly, host-helminth network modularity-capturing the tendency of the network to form into separate subcommunities-decreased with increasing area, also counter to theoretical predictions. Reconciling this disconnect between existing theory and observed SAR and NAR will provide insight into the spatial structuring of ecological networks, and help to refine theory to highlight the effects of network type, species distributional overlap, and the specificity of trophic interactions on NARs.

Alpine plant community diversity in species–area relations at fine scale

ABSTRACT Observations of diversity in alpine vegetation appear to be scale dependent. The relations of plant species richness with surface processes and geomorphology have been studied, but patterns of beta diversity are less known. In Glacier National Park, Montana, diversity has been examined within 1 m2 plots and for 16 m2 plots across two ranges, with within-plot and across-range explanatory factors, respectively. The slopes of species–area equations for nested 4, 8, 12, and 16 m2 plots were used as an indicator of beta diversity in Glacier National Park, where smaller and larger scales have been examined. The slopes were negatively related to a field assessment of surface stability and positively to the presence of talus—two sides of the same coin. A positive relationship with bedrock outcrops may be due to a misrepresentation of area for plants. The relationship of species–area slopes to plot-level gamma diversity was negative, weak, and marginally significant, and this variable did not enter the general linear model (GLM). Beyond simple differences in diversity with differences in environment, examination of beta diversity at a scale between that of earlier studies revealed surface processes and geomorphology as drivers that were also at a scale between those previously reported.

Invasion intensity influences scale-dependent effects of an exotic species on native plant diversity

Invasive plant species reduce the diversity of natives by altering habitats or disturbance regimes, but it is less clear whether they do so via competitive exclusion. Here, we show that invader abundance alters scale-dependent competitive effects of invasion on native plant richness. Large-seeded exotic annual Erodium cicutarium invaded a site that manipulated rodent granivores. The invader became dominant on all plots but attained its highest abundance on plots that removed rodents. Invasion reduced plant abundance but not evenness; site-wide richness did not change over time on control plots but declined significantly on rodent removal plots. Species-area relationships within plots changed differently with invasion intensity: slopes increased and y-intercepts decreased on control plots relative to rodent removal plots. Changes in species-area slopes and y-intercepts following invasion suggest that common rather than rare species were most strongly impacted at small spatial scales on control plots, while common and rare species were both negatively impacted at all spatial scales on rodent removal plots. Small-seeded species declined in abundance following invasion more so than large-seeded species, indicative of competitive interactions mediated by seed size. These results reveal variation in scale-dependent competitive effects of invasion on native richness associated with invasion intensity.

A stochastic model for landscape patterns of biodiversity

AbstractMany factors have been proposed to affect biodiversity patterns across landscapes, including patch area, patch isolation, edge distances, and matrix quality, but existing models emphasize only one or two of these factors at a time. Here we introduce a synthetic but simple individual‐based model that generates realistic patterns of species richness and density as a function of landscape structure. In this model, we simulated the stochastic placement of home ranges in landscapes, thus combining features of existing random placement and mid‐domain effect models. As such, the model allows investigation of whether and how geometric constraints on home range placement of individuals scale up to affect species abundance and richness in landscapes. The model encompassed a gradient of possible landscapes, from a strictly homogeneous landscape to an archipelago of discrete patches. The model incorporated only variation in home range size of individuals of different species, with a simple suitability index that controlled home range spread into areas of habitat and areas of inter‐habitat matrix. Demographic details of birth, death, and migration, as well as effects of species interactions were not included. Nevertheless, this simple model generated realistic patterns of biodiversity, including species–area curves and increases in diversity and abundance with decreasing isolation and increasing distance from patch edges. Species–area slopes (z values) generated by the model fell within the range observed in empirical studies on both true islands and habitat patches. Isolation and edge effects were stronger when the matrix was unsuitable, and became progressively weaker as matrix suitability increased, again in accordance with many empirical studies. When applied to a real data set on the abundance of 20 small mammal species sampled in forest patches in the Atlantic Forest of Brazil, the model predicted increases in abundance and richness with increasing patch size, consistent with the general pattern observed with field data. The ability of this simple model to reproduce realistic qualitative patterns of biodiversity suggests that such patterns may be driven, at least in part, by geometric constraints acting on the placement of individual ranges, which ultimately affect biodiversity patterns at the landscape level.

Habitat Fragmentation and Species Loss across Three Interacting Trophic Levels: Effects of Life‐History and Food‐Web Traits

Not all species are likely to be equally affected by habitat fragmentation; thus, we evaluated the effects of size of forest remnants on trophically linked communities of plants, leaf-mining insects, and their parasitoids. We explored the possibility of differential vulnerability to habitat area reduction in relation to species-specific and food-web traits by comparing species-area regression slopes. Moreover, we searched for a synergistic effect of these traits and of trophic level. We collected mined leaves and recorded plant, leaf miner, and parasitoid species interactions in five 100-m2 transects in 19 Chaco Serrano woodland remnants in central Argentina. Species were classified into extreme categories according to body size, natural abundance, trophic breadth, and trophic level. Species-area slopes differed between groups with extreme values of natural abundance or trophic specialization. Nevertheless, synergistic effects of life-history and food-web traits were only found for trophic level and trophic breadth: area-related species loss was highest for specialist parasitoids. It has been suggested that species position within interaction webs could determine their vulnerability to extinction. Our results provide evidence that food-web parameters, such as trophic level and trophic breadth, affect species sensitivity to habitat fragmentation.

Evolutionary species–area curves as revealed by single‐island endemics: insights for the inter‐provincial species–area relationship

The species–area relationship (SAR) between different biological provinces is one of the most interesting, but least explored aspects of the well‐known species–area pattern. Following the usage that a biological province is a region whose species have for the most part evolved within it, rather than immigrating from somewhere else, we propose that islands can be considered equivalent to biological provinces for single island endemic species (SIEs). Hence, analyses of the relationships between numbers of SIEs and island area can be used as model systems to explore the form of inter‐provincial SARs. We analyzed 13 different data sets derived from 11 sources, using the power (log–log) model. Eleven of the SIE–area relationships were statistically significant, explaining high proportions of the variance in SIE numbers (R2 0.57–0.95). The z‐values (slopes) of the statistically significant SIE–area relationships ranged from 0.47 to 1.13, with a mean value of 0.80 (SD±0.24).All the island systems in which SIE represent >50% of species exhibited z‐values for the SARs of native species higher than those deemed typical of archipelagic SARs. The SIE–area slopes are quite similar to those previously calculated for inter‐provincial SARs, indicating that, when speciation becomes the dominant process adding to the species richness of assemblages, high z‐values should be anticipated, regardless of the biogeographical scale of the study system.

A strong species–area relationship for eukaryotic soil microbes: island size matters for ectomycorrhizal fungi

While the effects of habitat size and isolation have been successfully studied for macro-organisms, there is currently debate about their relative importance in explaining patterns of microbial species richness. In this study, we examine the species richness of a dominant group of eukaryotic soil microbes, ectomycorrhizal fungi, on 'tree islands' of constant age and host composition that range in size from < 10 to > 10,000 m(2). Our results show that ectomycorrhizal species richness is significantly reduced on smaller and more isolated tree islands, and the species-area slope that we observe (0.20-0.23) is similar to average slopes reported for macro-organisms. Additionally, species' occurrence patterns across tree islands and investment trends in fungal fruit bodies suggest that a trade-off between competition and dispersal could play an important role in structuring ectomycorrhizal assemblages.

Distribution Patterns of Great Basin Conifers: Implications of Extinction and Immigration

Factors influencing the distribution of scattered montane conifers on mountaintops in the Great Basin of North America were investigated. The sources of data were collections and observations on more than 300 mountain ranges in the region. All mountains in the region with at least one montane conifer species and all adjacent source areas were included in the data set. In all, 164 montane island sites and 40 mainland sites were used in the analyses. Physical data for each site were compiled and regression analyses were conducted to test the predictions of three island biogeography models: immigration, extinction, and equilibrium. These models were treated as alternatives to the Random Placement Hypothesis. The Random Placement Hypothesis was refuted. However, none of the island biogeography models explained the observed patterns because mainland species-area slopes were within the expected range of island slopes. The pattern that emerges is one in which both the mainlands and islands represent the remnants of a preexisting regional conifer flora. The vertical relief of a site's montane zone is a better estimate of habitat diversity than area, and explains 65% of all variation in species richness on mountain ranges in the region. Although not all predictions of the island biogeography models were supported, it appears that both immigration and extinction of montane and subalpine conifers have occurred in the region during the Late Quaternary. Extinction was more important than immigration in shaping modern conifer distributions because area, not distance to sources, has a stronger influence on species richness.

Are ecotone properties scale-dependent? A test from a Nothofagus treeline in southern New Zealand

Species richness, the location of exotic species and heterogeneity (investigated via dissimilarity and via species-area relations) were investigated in relation to spatial scale, in an ecotone between Nothofagus forest and sub-alpine shrubland. The rate of change in ordination score as well as tree diameter and the dripline were used to locate the position of the ecotone. Patterns of species richness were largely scale-independent, with species richness lowest in the forest community, intermediate in the ecotone, highest a short distance into the shrubland, and lower again in the shrubland further from the ecotone. High richness just into the shrubland is attributed to the existence of a fine-scale spatial mosaic pattern of vegetation, though the spatial mass effect may have a role. Exotic species were absent in the forest, but occurred sparsely in the ecotone and in the shrubland, possibly with decreasing frequency away from the ecotone. Community pattern, expressed by species-based dissimilarity and species-area curves, also differed. The ecotone community was the most heterogeneous (indicated by higher dissimilarity values and a steeper species-area slope with higher Arrhenius z-values), with the forest the least heterogeneous and the shrubland intermediate. We conclude that z-values are inter-woven with both habitat and spatial scale, and that this argues against a universal relationship between species and area.

Local and Regional Species Richness in North American Lacustrine Fish

1. Species-area and local-regional species richness patterns in lacustrine fish in North America were investigated by reduced major axis regression, using a database of 651 lakes. 2. Small lakes (area < 1.5 km(2)) had steeper species-area slopes than large lakes. In one area open water species declined in frequency in small lakes, suggesting that within-lake habitat diversity is partly responsible for the species-area slope. Local lake characteristics (area, mean depth and altitude) explained only about one-third of the variation in species numbers. 3. Species-area slopes were not significantly different across regions but there were considerable differences between regions in local richness for standardized lake sizes. Local richness was linearly related to regional richness when the regional pool consisted only of native species. 4. Pseudosaturation, the detection of curvilinearity when the true relation is linear, was found when introduced species were included in the regional species pool. Introduced species tend to have more localized distributions than native species, and their inclusion over-estimated regional pool sizes but had little effect on local richness estimates. Additionally, undersampling of small species, coupled with an increase in the relative importance of small species in southern regions, led to pseudosaturation by underestimating local richness in species-rich regions. 5. Turnover diversity was not correlated with regional pool size. However, comparison of three eastern regions showed that the percentage of the region occupied per species was less in the more speciose, southern regions, consistent with increased turnover diversity and pseudosaturation. 6. These results support the notion that regional-scale processes play a major role in determining the number of species in North American lacustrine fish communities.

Biogeography of island floras in the Great Lakes. I. Species richness and composition in relation to gull nesting activities

Species–area relations of vascular plants and the effect of nesting colonies of gulls on plant species composition were investigated for 77 islands in Georgian Bay and Lake Huron in the Great Lakes region of Canada. The percentage of plant species classed as alien, annual, or biennial was significantly greater on islands with gull colonies. The slope of the species–area curve was significantly steeper on islands supporting gull colonies compared with islands lacking gull colonies. The expected decline in species richness with increased island remoteness was not detected statistically using multiple regression analysis. The difference in species–area slopes does not appear to reflect a lower propagule immigration rate to islands with gull colonies, because gulls are important in the dispersal of alien plant species to these islands. Larger islands with gull colonies tended to have richer floras than islands of similar size without gull colonies. It is suggested that on these larger islands the presence of gull colonies produces a gradient of soil nutrient and disturbance regimes, thus increasing habitat heterogeneity and species richness.

Comparative biogeography of mammals on islands

Insular faunas of terrestrial mammals and bats are examined on a worldwide basis to test the adequacy of equilibrium and historical legacy models as explanations for species-area relationships. Species numbers of bats on islands conform to predictions from equilibrium theory, whereby recurrent immigrations and extinctions influence species richness. By contrast, species numbers of terrestrial mammals on islands result from a historical legacy of very low immigration rates on oceanic islands (the faunas are colonization-limited) and by the fragmentation of once contiguous continental faunas to form relictual populations, which subsequently undergo extinctions, on landbridge islands (the faunas are extinction-limited). This explanation is supported by several lines of evidence: (1) z values (slopes of species-area curves) are lower for non-volant mammals on oceanic islands than for those on landbridge islands, but are the opposite for bats; (2) z values for non-volant mammals are lower than those for bats on oceanic islands, but are higher than those for bats on landbridge islands; and (3) landbridge island faunas are attenuated mainland faunas, whereas those on oceanic islands are ecologically incomplete. No support is found for alternative hypotheses to explain low species-area slopes for terrestrial mammals on oceanic islands.

Species-Area Slopes and Coefficients: A Caution on Their Interpretation

Attempts to explain patterns exhibited by species-area slopes and coefficients sometimes overlook the influence of confounding factors. In particular, the slope is shown to change with area; small areas exhibit higher slopes than large areas. This change in slope with area can mask the effects of isolation, latitude, vagility of species, species-area model, and interpretation of intercepts. Further, these factors can interact together to cloud biological patterns. More pointed attempts to minimize confounding effects can produce statistically significant patterns for data previously interpreted to exhibit no patterns.

The Dynamics of the Species-Area Relation in Marine Fouling Systems: 1. Biological Correlates of Changes in the Species-Area Slope

After creating an archipelago of fouling panel islands, spanning three orders of magnitude in area, in Puget Sound, Washington, we monitored colonization at frequent intervals for 13/4 yr. The larger panels fill up more slowly than the smaller panels and accumulate a larger fraction of the species pool. The latter would be expected even if the species pool were constant through time, but in this temperate region, the pool changes seasonally, further contributing to the increased species number on large panels. The relative proportion of edge-to-center space on panels appears in part to determine their relative rates of filling up. Slopes of species-area curves computed for these artificial islands rapidly rise for 10 wk, then remain fairly constant. Extinction rates are negligible in this 10-wk period; thus immigration differences are nearly entirely responsible for the species-area relation. Both colonial and solitary species are present throughout the observation period: The former predominate initially, giving way to the latter.