Showing papers by "Thorsten Wiegand published in 2010"

Species associations in an old‐growth temperate forest in north‐eastern China

Abstract: Summary 1. Studying the spatial pattern of plants may provide significant insights into processes and mechanisms that maintain species richness. We used data from a fully mapped 25-ha temperate forest plot at Changbaishan (CBS), north-eastern China, to conduct a community-wide assessment of the type and frequency of intra- and interspecific spatial association patterns. We analysed complex scale effects in the patterning of large trees of 15 common species. First, we tested for overall spatial patterning at 6, 30 and 50 m neighbourhoods and classified the types of bivariate association patterns at these spatial scales (analysis 1). We then explored small-scale (0–20 m) association patterns conditioning on the larger-scale pattern (analysis 2) and tested for positive large-scale (50–250 m) association patterns (analysis 3). 2. Analysis 1 provided ample evidence for non-random spatial patterning, and the type and frequency of spatial association patterns changed with scale. Trees of most species pairs co-occurred less than expected by chance and positive associations were rare in local neighbourhoods. Analysis 2 revealed a separation of scales in which significant small-scale interactions faded away at distances of 10–15 m. One third of all species pairs showed significant and mostly negative bivariate small-scale association, which occurred more often than expected by chance between species sharing attributes such as family, fruit type and habitat association. This suggests the occurrence of competitive interactions. Analysis 3 showed that only 8% of all species pairs co-occurred at large scales. 3. Comparison of our results with an analogous study conducted in the species-rich tropical forest at Sinharaja, Sri Lanka, revealed several structural similarities including the dominance of segregation and partial overlap in the overall patterning (analysis 1) and the separation of scales (analysis 2). However, species pairs at CBS showed considerably more significant negative small-scale associations (31% vs. 6% at Sinharaja). 4. Synthesis. The techniques presented here allow for a detailed analysis of the complex spatial associations in species-rich forests and have the potential to reveal indicative patterns that may allow researchers to discriminate among competing hypotheses of community assemblage and dynamics. However, this will require comparative studies involving a large number of plots.

Evidence for the spatial segregation hypothesis: a test with nine-year survivorship data in a Mediterranean shrubland

Abstract: A current focus of ecology is the investigation of spatial effects on population and community dynamics; however, spatiotemporal theory remains largely untested by empirical observations or experimental studies. For example, the segregation hypothesis predicts that intraspecific aggregation should increase the importance of intraspecific competition relative to interspecific competition, thereby enhancing local coexistence in plant communities. We applied recent methods of point pattern analysis to analyze a unique long-term data set on fully mapped seedling emergence and subsequent survival in a Mediterranean gorse shrubland after experimental fires and simulated torrential rainfall events. Our overall aim was to test if the observed spatial patterns were consistent with the segregation hypothesis during the entire community dynamics from early seedling emergence to the establishment of a mature community, i.e., we explored if the observed initial segregation did indeed prevent interspecific competition from becoming dominant. We used random labeling as the null model and specific test statistics to evaluate different biological effects of the spatial interactions that determine mortality. We found that mortality was clearly not random. Comparison of the probability of mortality in dependence on the distance to conspecific and to heterospecific plants showed that mortality was controlled almost entirely by intraspecific interactions, which is consistent with the segregation hypothesis. Dead plants were aggregated and segregated from surviving plants, indicating two-sided scramble competition. Spatial interactions were density dependent and changed their sign over the course of time from positive to negative when plants grew to maturity. The simulated torrential rainfall events and subsequent erosion caused nonspecific mortality of seedlings but did not reduce the prevalence of intraspecific competition. Our results provide support for the hypothesis that the spatial distribution of plants may profoundly affect competition and can be an important determinant in the coexistence of species and biodiversity.

Spatial pattern of adult trees and the mammal-generated seed rain in the Iberian pear

Abstract: The degree to which plant individuals are aggregated or dispersed co-determines how a species uses resources, how it is used as a resource, and how it reproduces Quantifying such spatial patterns, however, presents several methodological issues that can be overcome by using spatial point pattern analyses (SPPA) We used SPPA to assess the distribution of P bourgaeana adult trees and their seeds (within fecal samples) dispersed by three mammals (badger, fox, and wild boar) within a 72-ha plot across a range of spatial scales Pyrus bourgaeana trees in our study plot (n =75) were clearly aggregated with a critical spatial scale of ca 25 m, and approximately nine randomly distributed tree clusters were identified As expected from their marking behaviors, the spatial patterns of fecal deposition varied widely among mammal species Whereas badger feces and dispersed seeds were clearly clustered at small spatial scales ( < 10 m), boar and fox feces were relatively scattered across the plot A toroidal shift null model testing for independence indicated that boars tended to deliver seeds to the vicinity of adult trees and thus could contribute to the maintenance and enlargement of existing tree clusters Badgers delivered feces and seeds in a highly clumped pattern but unlike boars, away from P bourgaeana neighborhoods; thus, they are more likely to create new tree clusters than boars The strong tree aggregation is likely to be the result of one or several non-exclusive processes, such as the spatial patterning of seed delivery by disposers and seedling establishment beneath mother trees In turn, the distinctive distribution of P bourgaeana in Donana appeared to interact with the foraging behavior of its mammalian seed dispersers, leading to neighbourhood-specific dispersal patterns and fruit-removal rates Our study exemplifies how a detailed description of patterns generates testable hypotheses concerning the ecology of zoochorous Pyrus bourgaeana dispersers were unique and complementary in their spatial patterning of seed delivery, which likely confers resilience to their overall service and suggests lack of redundancy and expendability of any one species

The relative effects of habitat loss and fragmentation on population genetic variation in the red-cockaded woodpecker (Picoides borealis).

Abstract: The relative influence of habitat loss, fragmentation and matrix heterogeneity on the viability of populations is a critical area of conservation research that remains unresolved. Using simulation modelling, we provide an analysis of the influence both patch size and patch isolation have on abundance, effective population size (Ne) and FST. An individual-based, spatially explicit population model based on 15 years of field work on the red-cockaded woodpecker (Picoides borealis) was applied to different landscape configurations. The variation in landscape patterns was summarized using spatial statistics based on O-ring statistics. By regressing demographic and genetics attributes that emerged across the landscape treatments against proportion of total habitat and O-ring statistics, we show that O-ring statistics provide an explicit link between population processes, habitat area, and critical thresholds of fragmentation that affect those processes. Spatial distances among land cover classes that affect biological processes translated into critical scales at which the measures of landscape structure correlated best with genetic indices. Therefore our study infers pattern from process, which contrasts with past studies of landscape genetics. We found that population genetic structure was more strongly affected by fragmentation than population size, which suggests that examining only population size may limit recognition of fragmentation effects that erode genetic variation. If effective population size is used to set recovery goals for endangered species, then habitat fragmentation effects may be sufficiently strong to prevent evaluation of recovery based on the ratio of census:effective population size alone.

Evaluating management interventions in small populations of a perennial herb Primula vulgaris using spatio-temporal analyses of point patterns

Abstract: Summary 1. In high-intensity agricultural landscapes, small landscape elements such as hedgerows, ditch banks, and rows of pollard trees may represent the last refuge of many plant and animal species, some of them being rare or even threatened with extinction. However, due to their small size and low habitat quality, long-term population survival cannot be ascertained and often active management is needed to maintain viable populations of species forced to survive in these small landscape elements. 2. Population models are needed to assess the threats to species at risk and to evaluate alternative management actions. Here, we present a methodology to evaluate management interventions using spatio-temporal analyses of point patterns. We apply this method to several populations of primrose Primula vulgaris in Flanders, where it is rare and predominantly occurs along ditch banks. 3. The effects of ditch bank clearing on the establishment success of seedlings was investigated by comparing spatial patterns of seedling recruitment, survival and mortality between populations that were grazed and populations that were severely disturbed by mechanical clearing of ditch banks followed by annual mowing. A total of 884 seedlings were mapped and monitored during 4 consecutive years (1999–2002). 4. In all populations, plants showed significant clustering, but in cleared sites only seedlings were significantly clustered around adults. Spatial patterns of mortality varied according to the management intervention. In grazed sites, mortality was almost random, whereas in cleared sites we found clear evidence for strong negative density-dependent mortality. There was no evidence that the presence of adults affected survival of recruits in any of the sites studied. 5. Synthesis and applications. This study shows that the analysis of spatial point patterns contributes to our understanding of the population dynamics of plant species occurring in different environments. The approach can be broadly applied to other plant species to elucidate the processes that determine the number of individuals that establish and persist into later life stages and will help conservation managers to refine management strategies intended to conserve or restore plant populations. In the case of P. vulgaris, increasing the availability of microsites is most likely to result in increased growth rates, as it results in increased recruitment and establishment of recruits.