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Half a century ago, Janzen and Connell hypothesized that the high tree species diversity in tropical forests is maintained by specialized natural enemies. Along with other mechanisms, these can cause conspecific negative density dependence (CNDD) and thus maintain species diversity. Numerous studies have measured proxies of CNDD worldwide, but doubt about its relative importance remains. We find ample evidence for CNDD in local populations, but methodological limitations make it difficult to assess if CNDD scales up to control community diversity and thereby local and global biodiversity patterns. A combination of more robust statistical methods, new study designs, and eco-evolutionary models are needed to provide a more definite evaluation of the importance of CNDD for geographic variation in plant species diversity. Stabilizing conspecific negative density dependence (CNDD) is a classical explanation for diversity maintenance in species-rich ecosystems, particularly in tropical forests. Fifty years after Janzen and Connell popularized this idea, a plethora of empirical evidence suggests that CNDD can be found at all latitudes and for many tree species. On closer inspection, however, it is still unclear if locally measured density effects are indeed stabilizing community dynamics and, moreover, if they have a causal effect on large-scale diversity and abundance patterns, such as the latitudinal diversity gradient. More robust and comparable CNDD estimates are needed, coupled with a theoretical research program that aims at understanding the role of CNDD for coexistence in stochastic multispecies communities as well as macroecological and macroevolutionary diversity patterns. [ABSTRACT FROM AUTHOR]

• 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]

Abstract: The wedge-tailed eagle is Australia’s largest bird of prey and one of the largest eagles in the world. Aquila audax fleayi is an endemic Tasmanian subspecies isolated for 10,000years from the nominate subspecies on the Australian mainland. The Tasmanian wedge-tailed eagle is classified nationally and at a State level as endangered due to its small number of breeding pairs, low breeding success and high rate of mortality from unnatural causes. The subspecies experiences mortality throughout its range from shooting, poisoning, trapping, road accidents, electrocutions and collisions with wind turbines, aircraft, fences and overhead wires, which we term ‘un-natural mortality’. A portion of the subspecies’ range is managed for timber production, which can lead to disturbance of nest sites and the loss of nest trees. We use a model of the eagle population from the Bass District in northeast Tasmania to explore the relative importance of different sources of mortality and nesting habitat loss, and the potential for mitigating impacts associated with unnatural mortality, disturbance, nesting habitat loss and human access to forests. We create a habitat map including suitable nest sites and link it to a dynamic landscape population model based on life history traits and disturbance responses. Using the program RAMAS-Landscape, we model alternative forest management scenarios, ranging from no timber harvesting and a natural wildfire regime, to scenarios prescribing native forest harvesting and regeneration and different levels of conversion of native forest to plantation under the same natural wildfire regime. The results indicate that the Tasmanian wedge-tailed eagle is sensitive to unnatural mortality, plantation establishment and native forest harvesting. The predicted decline over the next 160years (∼65%) will most likely be driven largely by loss of current and potential future nest sites associated with harvesting activities, exacerbated by unnatural mortality in the wider landscape. Interventions that minimise unnatural mortality, reduce nest disturbance, and retain breeding habitat and nest sites may improve the prospects for the subspecies in the Bass District. If nest disturbance and unnatural mortality continue at the rates modelled here, the species appears to face a high risk of declining substantially in the region. [Copyright &y& Elsevier]

• Tea Bag Index is originally designed to characterise microbial-driven decomposition. • Application of Tea Bag Index in termite-rich habitats greatly reduced sample size. • Using termite-exclusion barrier preserved full sample size in termite-rich habitats. • We also propose extending the Tea Bag Index to incorporate termite-driven mass loss. • Extended Tea Bag Index validated – contribute to pan-tropical decomposition data. The Tea Bag Index (TBI) is a standardised and cheap method to quantify microbial-driven decomposition by measuring the mass loss of tea within tea bags. Termites are known to damage the bags to access the content, rendering the method less suitable for termite-rich ecosystems. Extension of the TBI to accommodate and incorporate the influence of termites would broaden its applicability to include termite-rich ecosystems, such as tropical forests. We extended the original TBI by applying physical and chemical termite-exclusion methods. Tea mass loss and the proportion of tea bags detected by termites in the original TBI were also recorded to infer the role of termites in litter decomposition. TBI estimates derived from the original and extended TBI were compared, benchmarked against global estimates, and validated with time-series mass loss data. Using the original TBI, we found that termites damaged up to 80 % of tea bags and consumed the recalcitrant fraction of tea in several of them, leaving only 20 % of tea bags from which TBI estimates could be retrieved. The physical termite-exclusion treatment completely eliminated termite-infringement, thus preserving the full sample size for estimating TBI parameters. The chemical termite-exclusion treatment also successfully excluded termites, but potentially inhibited microbial decomposition and made TBI estimates unreliable. In the absence of termite-infringement, both the TBI estimates and time-series analysis revealed a low decomposition rate compared to other measurements in tropical and temperate regions. We propose an extended TBI, in which the physical termite-exclusion treatment is used to preserve the retrieval rate of TBI parameters and reliably measure microbial-driven decomposition, while the original TBI is used to incorporate the contribution of termites in driving litter mass loss. By characterising both termite- and microbial-driven decomposition, the extended TBI will provide a comprehensive understanding of decomposition and its drivers in termite-rich ecosystems, and permit global comparisons. [ABSTRACT FROM AUTHOR]

Insects as a group are suffering rapid declines in many parts of the world but are also poorly studied relative to vertebrate taxa. Comprehensive assessments of insect declines must account for both detected and undetected species. We studied extirpations among butterflies, a particularly well-known insect group, in the highly developed and biologically well-surveyed island city-state of Singapore. Building on existing butterfly species lists, we collated museum and naturalist records over the last two centuries and used statistical models to estimate the total extirpation rate since the first major collections in 1854. In addition, we compiled a set of traits for each butterfly species and explored how they relate to species discovery and extirpation. With a database of 413 native species, 132 (32%) of which are recorded as extirpated in Singapore, we used a statistical model to infer that, in addition, 104 unknown species (95% CI 60–162) were likely extirpated before they were ever discovered, suggesting a total extirpation rate of 46% (41–51%). In the trait analyses, we found that butterfly species that were discovered later were weakly associated with rarer larval host plants and smaller wingspans, while species that persisted for longer were weakly associated with higher larval host plant abundance and lower forest-dependence. This exercise is one of the first to offer a holistic estimate of extirpations for a group of insects by accounting for undetected extirpations. It suggests that extirpations among insects, specifically in the tropics, may be higher than naïve estimates based only on known records. [ABSTRACT FROM AUTHOR]