Search

• Extinction risk of a species population with negative growth rate analyzed. • Extinction risk partitioned according to key deterministic and stochastic processes. • Deterministic processes drive extinction risk when growth rate is large. • Stochastic processes often drive extinction risk when growth rate is small. • Demographic and environmental variance often have sub-additive extinction effects. Species populations are subjected to deterministic and stochastic processes, both of which contribute to their risk of extinction. However, current understanding of the relative contributions of these processes to species extinction risk is far from complete. Here, we address this knowledge gap by analyzing a suite of models representing species populations with negative intrinsic growth rates, to partition extinction risk according to deterministic processes and two broad classes of stochastic processes – demographic and environmental variance. Demographic variance refers to random variations in population abundance arising from random sampling of events given a particular set of intrinsic demographic rates, whereas environmental variance refers to random abundance variations arising from random changes in intrinsic demographic rates over time. When the intrinsic growth rate was not close to zero, we found that deterministic growth was the main driver of mean time to extinction, even when population size was small. This contradicts the intuition that demographic variance is always an important determinant of extinction risk for small populations. In contrast, when the intrinsic growth rate was close to zero, stochastic processes exerted substantial negative effects on the mean time to extinction. Demographic variance had a greater effect than environmental variance at low abundances, with the reverse occurring at higher abundances. In addition, we found that the combined effects of demographic and environmental variance were often substantially lower than the sum of their effects in isolation from each other. This sub-additivity indicates redundancy in the way the two stochastic processes increase extinction risk, and probably arises because both processes ultimately increase extinction risk by boosting variation in abundance over time. [ABSTRACT FROM AUTHOR]

Species populations are subjected to fluctuations in their surrounding environment, and the strength of these fluctuations has been hypothesized to be a major determinant of the extinction risk of these populations. Therefore, a key question is: How does temporal environmental variance affect the extinction risk of species populations? Previous theory based on the dynamics of single populations typically predicts an increased risk of extinction from the effects of environmental variance. However, previous studies have focused mainly on the case where environmental effects are temporally uncorrelated (white environmental noise), whereas such effects are typically correlated (colored environmental noise) in nature. Thus, further work on the case of colored environmental noise is required, but this has been hindered by the analytical intractability of corresponding stochastic models. In our study, we address this limitation by developing a new discrete-time Markov chain model of a species population fluctuating under colored environmental noise, with the simplification that the effects of demographic variance are manifested indirectly as an extinction threshold. This simplifying assumption allows us to derive analytical solutions, which show that the expected extinction time of model species declines with the strength of environmental variance under a variety of different scenarios, reflecting greater extinction risk. Our study thus clarifies the situations under which environmental variance tends to increase extinction risk, and provides a novel analytically tractable framework for modeling temporal environmental variance. We also discuss the possible implications of our results for species richness in ecological communities. [ABSTRACT FROM AUTHOR]

• Observed neighbourhood species richness (SR), phylogenetic diversity (PD) and beta diversity (BD) all decreased from early-, mid- to late-successional species. • Early-successional trees had higher NSR but a lower PD than homogeneous Poisson null model (HomoPNM). • Mid-successional trees showed no significant departure in SR and PD from HomoPNM. • Late-successional trees had lower SR and lower PD than HomoPNM. • Both SR and PD of all three species groups were well fitted by the heterogeneous Poisson model. A long-standing paradigm in ecology is that species diversity in a community accrues over succession. Central to this process are the mechanisms that regulate neighbourhood diversity around species of different functional groups such as early vs late-successional species. However, there is poor understanding of the accumulation of this neighbourhood diversity and its contribution to community diversity. We calculated neighbourhood species and phylogenetic diversities for adult trees of 10 species belonging to three groups: early-, mid-, and late-successional species in a 25-ha forest stem-mapping plot in subtropical China. We compared these diversities with homogeneous and heterogeneous Poisson null models to assess the roles of individual species and habitat heterogeneity in community assembly. We also calculated the neighbourhood species composition to quantify beta diversity turnover across different species groups. We observed that species richness, phylogenetic diversity and beta diversity all decreased from early- to mid- and late-successional species. Compared with homogeneous Poisson null model, early-successional trees had higher neighbourhood species but a lower phylogenetic diversity. Mid-successional trees showed no significant departure in richness and phylogenetic diversity from the null model. In contrast, late-successional trees had lower neighbourhood species richness and also a lower phylogenetic diversity than the null model. Both neighbourhood richness and phylogenetic diversity of all three species groups were well fitted by the heterogeneous Poisson model, indicating that environmental filtering predominated neighbourhood diversity. Our results underscore the importance of the early successional species in the accumulation of neighbourhood diversity and their contributions to diversity of plant communities. We suggest that secondary forests, which are mostly composed of early successional species, warrant a due attention in biodiversity conservation. [ABSTRACT FROM AUTHOR]

The article presents a study regarding the use of info-gap theory to choose between two alternative land uses such as the establishment of an exotic timber plantation and native vegetation conservation. The study made use the linear equations by ordinary least-squares regression to define the reward functions for the two alternative land-use decisions. The result of the study shows the sensitivity of the info-gap analyses to the choice of uncertainty model.

The article focuses on a study related to incorporating landscape dynamics into population viability analysis (PVA). Landscape stochasticity contributes much to the variance of model outputs in some PVAs on stochastic dynamic landscapes. A dynamic landscape metapopulation (DLMP) model of the Brown Creeper in the boreal forests of northern Ontario was used in the study. It was found that in case of this model, most of the variance in the expected minimum population size (EMP) is associated with population stochasticity. The RAMAS Landscape computer software was also used during the study.