A unified model of island biogeography sheds light on the zone of radiation

Islands acquire species through immigration and speciation. Models of island biogeography should capture both processes; however quantitative island biogeography theory has either neglected speciation or treated it unrealistically. We introduce a model where the dominance of immigration on small and near islands gives way to an increasing role for speciation as island area and isolation increase. We examine the contribution of immigration and speciation to the avifauna of 35 archipelagoes and find, consistent with our model, that the zone of radiation comprises two regions: endemic species diverged from mainland sister-species at intermediate isolation and from insular sister-species at higher levels of isolation. Our model also predicts species-area curves in accord with existing research and makes new predictions about species ages and abundances. We argue that a paucity of data and theory on species abundances on isolated islands highlights the need for island biogeography to be reconnected with mainstream ecology.     

Species richness–decomposition relationships depend on species dominance

Human disturbances both decrease the number of species in ecosystems and change their relative abundances. Here we present field evidence demonstrating that shifts in species abundances can have effects on ecosystem functioning that are as great as those from shifts in species richness. We investigated spatial and temporal variability of leaf decomposition rates and community metrics of leaf‐eating invertebrates (shredders) in streams. The shredder community composition dramatically influenced the diversity–function relationship; decomposition was much higher for a given species richness at sites with high species dominance than at sites where dominance was low. Decomposition rates also markedly depended on the identity of the dominant species. Further, dominance effects on decomposition varied seasonally and the number of species required for maintaining decomposition increased with increasing evenness. These findings reveal important but less obvious aspects of the biodiversity–ecosystem functioning relationship.     

Self-organized instability in complex ecosystems

Why are some ecosystems so rich, yet contain so many rare species? High species diversity, together with rarity, is a general trend in neotropical forests and coral reefs. However, the origin of such diversity and the consequences of food web complexity in both species abundances and temporal fluctuations are not well understood. Several regularities are observed in complex, multispecies ecosystems that suggest that these ecologies might be organized close to points of instability. We explore, in greater depth, a recent stochastic model of population dynamics that is shown to reproduce: (i) the scaling law linking species number and connectivity; (ii) the observed distributions of species abundance reported from field studies (showing long tails and thus a predominance of rare species); (iii) the complex fluctuations displayed by natural communities (including chaotic dynamics); and (iv) the species-area relations displayed by rainforest plots. It is conjectured that the conflict between the natural tendency towards higher diversity due to immigration, and the ecosystem level constraints derived from an increasing number of links, leaves the system poised at a critical boundary separating stable from unstable communities, where large fluctuations are expected to occur. We suggest that the patterns displayed by species-rich communities, including rarity, would result from such a spontaneous tendency towards instability.     

Inferring the parameters of the neutral theory of biodiversity using phylogenetic information and implications for tropical forests

We develop a statistical method to infer the parameters of Hubbell's neutral model of biodiversity using data on local species abundances and their phylogenetic relatedness. This method uses the approximate Bayesian computation (ABC) approach, where the data are summarized into a small number of informative summary statistics. We used three statistics: the number of species in the sample, Shannon H index of evenness and Shao and Sokal's B ₁ index of phylogenetic tree imbalance. Our approach was found to outperform previous methods, illustrating the potential of ABC methods in ecology. Applying it to four large tropical forest tree data sets, the best-fit immigration rates m were found to be two orders of magnitude smaller and regional diversities θ larger than previously reported for the same data. This implies that neutral-compatible regional pools of tropical trees should extend over continental scales, and that m measures, in this context, mostly the frequency of long-distance dispersal events.     

Native faunal communities depend on habitat from non-native plants in novel but not in natural ecosystems

Invasive non-native plants are a major driver of native biodiversity loss, yet native biodiversity can sometimes benefit from non-native species. Depending on habitat context, even the same non-native species can have positive and negative effects on biodiversity. Blackberry (Rubus fruticosus aggregate) is a useful model organism to better understand a non-native plant with conflicting impacts on biodiversity. We used a replicated capture-mark-recapture study across 11 consecutive seasons to examine the response of small mammal diversity and abundance to vegetation structure and density associated with non-native blackberry (R. anglocandicans) in native, hybrid and blackberry-dominated novel ecosystems in Australia. Across the three habitat types, increasing blackberry dominance had a positive influence on mammal diversity, while the strength and direction of this influence varied for abundance. At a microhabitat scale within hybrid and native habitat there were no significant differences in diversity, or the abundance of most species, between microhabitats where blackberry was absent versus dominant. In contrast, in novel ecosystems diversity and abundances were very low without blackberry, yet high (comparable to native ecosystems) within blackberry as it provided functionally-analogous vegetation structure and density to the lost native understory. Our results indicate the ecological functions of non-native plant species vary depending on habitat and need to be considered for management. Comparative studies such as ours that apply a standardized approach across a broad range of conditions at the landscape and habitat scale are crucial for guiding land managers on control options for non-native species (remove, reduce or retain and contain) that are context-sensitive and scale-dependent.     

A persistence criterion for metapopulations

Simple conditions to evaluate the persistence of populations living in fragmented habitats are of primary importance in ecology. We address this need here using a spatially implicit approach that accounts for discrete individuals in a metapopulation. Demographic stochasticity is incorporated into a Markovian model in a natural way, as local extinction is characterized by the death or the dispersal of the last individual inhabiting a patch. The variables of the model are the probabilities p(i) (i=0, 1, 2...) that a patch be occupied by a finite, integer number i of individuals at a given time. We compare the stationary distributions predicted by the model with field data and discuss the role of dispersal in determining different distributions of local abundances. The analysis of the model leads to a persistence criterion which is equivalent to a condition formerly proved by Chesson (Z. Wahrscheinlichkeitstheor. 66, 97-107, 1984) namely that E(0)>1, where E(0) is the expected number of successful dispersers from a patch begun with one individual and to which immigration is excluded. We provide an analytic way of computing E(0) as a function of the main biological characteristics of the species (natality, mortality and dispersal rates, and colonizing ability). We can thus obtain persistence-extinction boundaries in the space of model parameters.     

Testing the effects of diversity on ecosystem multifunctionality using a multivariate model

Most ecosystems provide multiple services, thus the impact of biodiversity losses on ecosystem functions may be considerably underestimated by studies that only address single functions. We propose a multivariate modelling framework for quantifying the relationship between biodiversity and multiple ecosystem functions (multifunctionality). Our framework consolidates the strengths of previous approaches to analysing ecosystem multifunctionality and contributes several advances. It simultaneously assesses the drivers of multifunctionality, such as species relative abundances, richness, evenness and other manipulated treatments. It also tests the relative importance of these drivers across functions, incorporates correlations among functions and identifies conditions where all functions perform well and where trade‐offs occur among functions. We illustrate our framework using data from three ecosystem functions (sown biomass, weed suppression and nitrogen yield) in a four‐species grassland experiment. We found high variability in performance across the functions in monocultures, but as community diversity increased, performance increased and variability across functions decreased.     

Global macroecology of bird assemblages in urbanized and semi‐natural ecosystems

Aim Despite the increasing pace of urbanization, little is known about how this process affects biodiversity globally. We investigate macroecological patterns of bird assemblages in urbanized areas relative to semi‐natural ecosystems. Location World‐wide. Methods We use a database of quantitative bird surveys to compare key assemblage structure parameters for plots in urbanized and semi‐natural ecosystems controlling for spatial autocorrelation and survey methodology. We use the term ‘urbanized’ instead of ‘urban’ ecosystems as many of the plots were not located in the centre of towns but in remnant habitat patches within conurbations. Results Some macroecological relationships were conserved in urbanized landscapes. Species–area, species–abundance and species–biomass relationships did not differ significantly between urbanized and non‐urbanized environments. However, there were differences in the relationships between productivity and assemblage structure. In forests, species richness increased with productivity; in both forests and open habitats, the evenness of species abundances declined as productivity increased. Among urbanized plots, instead, both species richness and the evenness of species abundances were independent of variation in productivity. Main conclusions Remnant habitats within urbanized areas are subject to many ecological alterations, yet key macroecological patterns differ remarkably little in urbanized versus non‐urbanized plots. Our results support the need for increased conservation activities in urbanized landscapes, particularly given the additional benefits of local experiences of biodiversity for the human population. With increasing urbanization world‐wide, broad‐scale efforts are needed to understand and manage the effects of this driver of change on biodiversity.     

Dynamic phenotypic clustering in noisy ecosystems

In natural ecosystems, hundreds of species typically share the same environment and are connected by a dense network of interactions such as predation or competition for resources. Much is known about how fixed ecological niches can determine species abundances in such systems, but far less attention has been paid to patterns of abundances in randomly varying environments. Here, we study this question in a simple model of competition between many species in a patchy ecosystem with randomly fluctuating environmental conditions. Paradoxically, we find that introducing noise can actually induce ordered patterns of abundance-fluctuations, leading to a distinct periodic variation in the correlations between species as a function of the phenotypic distance between them; here, difference in growth rate. This is further accompanied by the formation of discrete, dynamic clusters of abundant species along this otherwise continuous phenotypic axis. These ordered patterns depend on the collective behavior of many species; they disappear when only individual or pairs of species are considered in isolation. We show that they arise from a balance between the tendency of shared environmental noise to synchronize species abundances and the tendency for competition among species to make them fluctuate out of step. Our results demonstrate that in highly interconnected ecosystems, noise can act as an ordering force, dynamically generating ecological patterns even in environments lacking explicit niches.     

Avian biodiversity across Auckland’s volcanic cone reserves

Auckland, a city with a population of approximately 1.7 million, is located directly on the Auckland Volcanic Field, a late Quaternary-era monogenetic field. There are at least 53 volcanoes across the field, many of which are of geological, cultural and ecological significance, such as for being reserves for native species; however, few assessments of the richness of avian biodiversity across the volcanoes have been made. To address this data shortfall, we conducted avian biodiversity surveys using stationary point counts within nine of Auckland's volcanic cone reserves. Thirty-eight species were detected across the sites, of which 18 were native. Our estimates of relative species abundances and detection probabilities revealed that the most common native birds within these reserves were silvereyes, tui and southern black-backed gulls, while common mynas, house sparrows, Eurasian blackbirds and eastern rosellas were the most common introduced species. In addition to tui and silvereyes, the presence of other natives critical to the functioning of native ecosystems, such as New Zealand fantails, grey warblers and New Zealand pigeon, suggest that the volcanoes possess a diverse native avifauna supported by native flora that warrant continued and intensified restoration efforts. We discuss several feasible strategies for improving faunal and floral biodiversity across the volcanic cone reserves. Continued avian biodiversity surveys are also of critical importance as they will enable us to further evaluate and prioritise restoration projects within Auckland's multitude of diverse volcanic cone reserves.     

Ground spider assemblages as indicators for habitat structure in inland sand ecosystems

Open inland sand ecosystems harbour a specialised flora and fauna and are among the most endangered habitats in Central Europe. Land-use changes and lack of habitat dynamics are acknowledged as significant drivers for habitat loss and degradation. It is imperative for nature conservation to obtain criteria such as community structure and biodiversity of model groups to assess the conservation value of threatened habitats. By investigating the correlation between ground spider assemblages and habitat structure, the study aimed to find out the indicator potential of spiders in order to promote conservation objectives and management strategies for open inland sand ecosystems. Non-metric multidimensional scaling revealed four habitat groups with distinct spider assemblages that clearly reflected the whole variety of habitat structure types within the study area. Species distribution was constrained by biotic and abiotic gradients while the ecological traits of spiders differed significantly among the groups. Generalised linear models showed that abundances of particular species were significantly correlated with environmental factors and habitat structure, making them thus suitable as focal species to assess natural habitat modifications as well as success of management efforts. Based on these findings, we derived major aims for successful habitat management of inland sand ecosystems taking into account also the needs of arthropod conservation. Management should include both small and large reserves when aiming for higher levels of disturbance, and sand dynamics to prevent increasing scrub encroachment and to create a larger number of early succession stages.     

Inferring macro‐ecological patterns from local presence/absence data

Biodiversity provides support for life, vital provisions, regulating services and has positive cultural impacts. It is therefore important to have accurate methods to measure biodiversity, in order to safeguard it when we discover it to be threatened. For practical reasons, biodiversity is usually measured at fine scales whereas diversity issues (e.g. conservation) interest regional or global scales. Moreover, biodiversity may change across spatial scales. It is therefore a key challenge to be able to translate local information on biodiversity into global patterns. Many databases give no information about the abundances of a species within an area, but only its occurrence in each of the surveyed plots. In this paper, we introduce an analytical framework (implemented in a ready‐to‐use R code) to infer species richness and abundances at large spatial scales in biodiversity‐rich ecosystems when species presence/absence information is available on various scattered samples (i.e. upscaling). This framework is based on the scale‐invariance property of the negative binomial. Our approach allows to infer and link within a unique framework important and well‐known biodiversity patterns of ecological theory, such as the species accumulation curve (SAC) and the relative species abundance (RSA) as well as a new emergent pattern, which is the relative species occupancy (RSO). Our estimates are robust and accurate, as confirmed by tests performed on both in silico‐generated and real forests. We demonstrate the accuracy of our predictions using data from two well‐studied forest stands. Moreover, we compared our results with other popular methods proposed in the literature to infer species richness from presence to absence data and we showed that our framework gives better estimates. It has thus important applications to biodiversity research and conservation practice.     

To what extent can ecosystem services motivate protecting biodiversity?

Society increasingly focuses on managing nature for the services it provides people rather than for the existence of particular species. How much biodiversity protection would result from this modified focus? Although biodiversity contributes to ecosystem services, the details of which species are critical, and whether they will go functionally extinct in the future, are fraught with uncertainty. Explicitly considering this uncertainty, we develop an analytical framework to determine how much biodiversity protection would arise solely from optimising net value from an ecosystem service. Using stochastic dynamic programming, we find that protecting a threshold number of species is optimal, and uncertainty surrounding how biodiversity produces services makes it optimal to protect more species than are presumed critical. We define conditions under which the economically optimal protection strategy is to protect all species, no species, and cases in between. We show how the optimal number of species to protect depends upon different relationships between species and services, including considering multiple services. Our analysis provides simple criteria to evaluate when managing for particular ecosystem services could warrant protecting all species, given uncertainty. Evaluating this criterion with empirical estimates from different ecosystems suggests that optimising some services will be more likely to protect most species than others.     

Drought resistance increases with species richness in restored populations and communities

It is unknown to what extent or by what mechanisms introducing biodiversity influences stability of high-stress ecosystems undergoing restoration. Opportunity to investigate patterns of biodiversity and resistance to disturbance in a high-stress environment was presented when severe drought struck a restoration experiment underway on abandoned limestone quarry floors in Ontario, Canada. Experimental communities were previously established within small quarry-floor plots by sowing native grass and forb species considered to be characteristic of rare natural limestone pavements called alvars. Despite adding an identical 18-species seed-mixture to all plots, realized communities varied extensively with respect to the numbers of species established (species richness), the total number of individuals established (community abundance), and the number of individuals belonging to each species (population abundances). We investigated the relationship between species richness and resistance of community abundance to drought, while accounting for background richness–abundance correlation, by contrasting slopes and intercepts of the richness–abundance relationship immediately before vs. 6 weeks after the drought. This relationship was significantly positive prior to drought but 72% steeper in slope following drought, while the abundance intercept exhibited a 44% drop. Plots featuring richer, more abundant communities prior to drought thus suffered considerably less damage than species-poor, low-abundance plots. Population abundance was weakly related to richness prior to drought, but strongly and positively related to richness after the drought. At the individual species level, no species experienced greater losses of abundance with increased plot richness, but six species experienced reduced abundance losses where they co-occurred with more neighbour species. Facilitation or other mechanisms capable of increasing population resistance may thus underlie community resistance in high-stress environments. Though controlled experiments are required to establish causes of relationships reported here, the forms of these relationships suggest that managers may be able to promote resistance in high-stress ecosystems by establishing species-rich communities.     

Predator selectivity alters the effect of dispersal on coexistence among apparent competitors

While the majority of studies on dispersal effects on patterns of coexistence among species in a metacommunity have focused on resource competitors, dispersal in systems with predator–prey interactions may provide very different results. Here, we use an analytical model to study the effect of dispersal rates on coexistence of two prey species sharing a predator (apparent competition), when the traits of that predator vary. Specifically, we explore the range in immigration rates where apparent competitors are able to coexist, and how that range changes with predator selectivity and efficiency. We find that if the inferior apparent competitor has a higher probability of being consumed, it will require less immigration to invade and to exclude the superior prey as the predator becomes more opportunistic. However, if the inferior apparent competitor has a lower probability of being consumed (and lower growth rates), higher immigration is required for the inferior prey to invade and exclude the superior prey as the predator becomes more opportunistic. We further find that the largest range of immigration rates where prey coexist occurs when predator selectivity is intermediate (i.e. they do not show much bias towards consuming one species or the other). Increasing predator efficiency generally reduces the immigration rates necessary for the inferior apparent competitor to invade and exclude the superior apparent competitor, but also reduces the range of immigration rates where the two apparent competitors can coexist. However, when the superior apparent competitor has a higher probability of being consumed, increased predator efficiency can increase the range of parameters where the species can coexist. Our results are consistent with some of the variation observed in the effect of dispersal on prey species richness in empirical systems with top predators.     

Emigration and immigration can be important determinants of benthic diatom assemblages in streams

SUMMARY.

• 1 Interspecific differences in diatom abundances in stream drift (plankton), immigration, and natural benthic assemblages were compared to assess the importance of emigration and immigration in benthic diatom community dynamics. Water samples were collected throughout a 24-h period to measure diel changes in diatom drift abundances and to estimate benthic diatom emigration rates. Immigration was assessed with 24-h colonization of bare tiles.
• 2 Dissimilarity in species composition of drift, immigration, and natural substrate assemblages indicated differential emigration and immigration among diatom species.
• 3 A mathematical model indicated that reproduction by diatoms in the plankton could not account for diel drift peaks and that diel variation in drift was an informative measure of benthic diatom emigration.
• 4 Emigration and immigration of some species constituted substantial proportions of diatom abundances on natural substrata. We conclude that emigration into the drift and immigration onto substrata can be important processes that regulate benthic diatom species composition and standing crop in streams.

     

VARIATION IN BENTHIC DIATOM (BACILLARIOPHYCEAE) IMMIGRATION WITH HABITAT CHARACTERISTICS AND CELL MORPHOLOGY1

We studied the hypothesis that diatom immigration abilities are related to their fitness for colonizing stream substrates. Diatom abundances on artificial substrates exposed for 24 h (the measure of immigration rate) and abundances of stream plankton were determined in six habitats. Diatom immigration varied among habitats from 50–2500 cells·cm^?2·d^?1. Immigration rates decreased 10-fold with increases in current from 10 to 30 cm·s^?1 but changed little during a 40-d summer period. Immigration abilities of diatom taxa were characterized as ratios of either their abundances or relative abundances in immigration assemblages versus in the plankton. Immigration abilities varied over 100-fold among different species. Immigration of some species could be characterized as slower than others in different streams; however, variation in immigration abilities of other species among streams indicated that environment also affected immigration. Diurnal variation in abundance and species composition of the immigration pool (stream plankton) can be important in assessing immigration abilities. Immigration ability may affect benthic diatom fitness. Monoraphid diatoms had a lower probability of immigrating from the plankton than araphid diatoms.     

城市生物多样性分布格局研究进展

城市生物多样性分布格局由自然生态环境和城市化过程所决定;其动态和机理与自然生态系统迥然不同.城市生物多样性为城市生态系统提供了诸多生态系统功能和服务,对改善城市环境、维持城市可持续发展有着重要的意义和作用.城市化过程深刻改变了城市的生物多样性分布格局,导致了诸如本地物种多样性降低、外来物种多样性增加、物种同质化等一系列问题.近年来,城市生物多样性受到学界高度关注,大量研究结果既回答了一些关键性问题,又提出了诸多新的论题和挑战.分析了当前城市生物多样性分布格局研究的若干热点问题,总结了影响城市生物多样性格局的主要因素,探讨了城市生物多样性格局研究方法的关键问题,指出了未来城市生物多样性研究的发展方向,特别强调了城市生物多样性的生态系统功能研究在未来城市生物多样性研究中的重要地位.     

A novel genealogical approach to neutral biodiversity theory

Current neutral theory in community ecology views local biodiversity as a result of the interplay between speciation, extinction and immigration. Simulations and a mean‐field approximation have been used to study this neutral theory. As simulations have limitations of convergence and the mean‐field approximation ignores dependencies between species’ abundances when applied to species‐abundance data, there is still no final conclusion whether the neutral theory or the traditional lognormal model describes community structure best. We present a novel analytical framework, based on the genealogy of individuals in the local community, to overcome the problems of previous approaches, and show, using Bayesian statistics, that the lognormal model provides a slightly better fit to the species‐abundance distribution of a much‐discussed tropical tree community. A key feature of our approach is that it shows the tight link between genetic and species diversity, which creates important perspectives to future integration of evolutionary and community ecological theory.     

Biodiversity in model ecosystems, II: species assembly and food web structure

This is the second of two papers dedicated to the relationship between population models of competition and biodiversity. Here, we consider species assembly models where the population dynamics is kept far from fixed points through the continuous introduction of new species, and generalize to such models the coexistence condition derived for systems at the fixed point. The ecological overlap between species and shared preys, that we define here, provides a quantitative measure of the effective interspecies competition and of the trophic network topology. We obtain distributions of the overlap from simulations of a new model based both on immigration and speciation, and show that they are in good agreement with those measured for three large natural food webs. As discussed in the first paper, rapid environmental fluctuations, interacting with the condition for coexistence of competing species, limit the maximal biodiversity that a trophic level can host. This horizontal limitation to biodiversity is here combined with either dissipation of energy or growth of fluctuations, which in our model limit the length of food webs in the vertical direction. These ingredients yield an effective model of food webs that produce a biodiversity profile with a maximum at an intermediate trophic level, in agreement with field studies.