When should we save the most endangered species?

At the heart of our efforts to protect threatened species, there is a controversial debate about whether to give priority to cost-effective actions or whether focusing solely on the most endangered species will ultimately lead to preservation of the greatest number of species. By framing this debate within a decision-analytic framework, we show that allocating resources solely to the most endangered species will typically not minimise the number of extinctions in the long-term, as this does not account for the risk of less endangered species going extinct in the future. It is only favoured when our planning timeframe is short or we have a long-term view and we are optimistic about future conditions. Conservation funding tends to be short-term in nature, which biases allocations to more endangered species. Our work highlights the need to consider resource allocation for biodiversity over the long-term; 'preventive conservation', rather than just short-term fire-fighting.     

How does spatio-temporal disturbance influence species diversity in a hierarchical competitive system? Prospective order of species coexistence and extinction

To address how habitat destruction and hierarchical competition among species affect the spatio-temporal dynamics of a multi-species community, we present a compartment model in which multiple species undergo dispersal and competitive interactions in a patchy habitat arranged in a two-dimensional lattice. We assume that disturbances are periodically imposed on some parts of the lattice in a block, followed by a period free of disturbance. For convenience, species are ranked in order of competitive ability. We further assume that the intrinsic growth rate of species i, _i , and the dispersal ability, D _i , increase in decreasing order of rank. Our model can analytically determine the exact number of surviving species when disturbance is absent. In the presence of disturbance, we numerically examine how spatio-temporal changes in environmental heterogeneity affect species coexistence and extinction, for the case in which the value of _i /D _i monotonically increases or decreases with rank. The results demonstrate that (1) when the interspecific competition is smaller than the intraspecific competition, we can provide predictions on the prospective order of species to be driven extinct and the order of potential species to revive with increasing extents of disturbance; (2) when the interspecific competition is stronger than intraspecific competition, a small difference in the disturbance level can lead to drastic changes in the species composition, their densities and the order of species extinction. In addition, comparison with other similar models reveals that differences in species interaction in local population dynamics critically affect the disturbance-mediated species diversity.     

When does ecosystem engineering cause invasion and species replacement?

Introduced exotic species can dominate communities and replace native species that should be better adapted to their local environment, a paradox that is usually explained by the absence of natural enemies and by habitat alteration resulting from anthropogenic disturbance. Additionally, introduced species can enhance their invasion success and impact on native species by modifying selection pressures in their new environment through ecosystem engineering. We analyse a simple dynamic model of indirect competition for habitat between a non-engineering resident species and an engineering exotic species. The conditions for invasion and competitive exclusion of the resident by the exotic species and the range of dynamic outcomes suggested by the model are determined by the form of density dependence. We give simple criteria for the success of the invading species on dimensionless quantities involving rates of ecosystem engineering and of habitat degradation. The model's predictions offer an additional explanation for a range of invasion dynamics reported in the literature, including lag times between introduction and establishment. One intriguing result is that a series of failed invasions may successively reduce environmental resistance to subsequent invasion, through a cumulative effect of habitat transformation. More work is needed to determine the frequency and conditions in which engineering is required for successful establishment, and whether highly-successful (or high-impact) invaders are more likely to possess ecosystem engineering traits.     

How does temporal variation in habitat connectivity influence metapopulation dynamics?

Metapopulation persistence depends on connectivity between habitat patches. While emphasis has been placed on the spatial dynamics of connectivity, much less has been placed on its short‐term temporal dynamics. In many terrestrial and aquatic ecosystems, however, transient (short‐term) changes in connectivity occur as habitat patches are connected and disconnected due, for example, to climatic or hydrological variability. We evaluated the implications of transient connectivity using a network‐based metapopulation model and a series of scenarios representing temporal changes in connectivity. The transient loss of connectivity can influence metapopulation persistence, and more strongly autocorrelated temporal dynamics affect metapopulation persistence more severely. Given that many ecosystems experience short‐term and temporary loss of habitat connectivity, it is important that these dynamics are adequately represented in metapopulation models; failing to do so may yield overly optimistic‐estimates of metapopulation persistence in fragmented landscapes.     

Will variation among genetic individuals influence species responses to global climate change?

Increased anthropogenic CO₂ emissions in the last two centuries have lead to rising sea surface temperature and falling ocean pH, and it is predicted that current global trends will worsen over the next few decades. There is limited understanding of how genetic variation among individuals will influence the responses of populations and species to these changes. A microcosm system was set up to study the effects of predicted temperature and CO₂ levels on the bryozoan Celleporella hyalina. In this marine species, colonies grow by the addition of male, female and feeding modular individuals (zooids) and can be physically subdivided to produce a clone of genetically identical colonies. We studied colony growth rate (the addition of zooids), reproductive investment (the ratio of sexual to feeding zooids) and sex ratio (male to female zooids) in four genetically distinct clonal lines. There was a significant effect of clone on growth rate, reproductive investment and sex ratio, with clones showing contrasting responses to the various temperature and pH combinations. Overall, decreasing pH and increasing temperature caused reduction of growth, and eventual cessation of growth was often observed at the highest temperature, especially during the latter half of the 15‐day trials. Reproductive investment increased with increasing temperature and decreasing pH, varying more widely with temperature at the lowest pH. The increased production of males, a general stress response of the bryozoan, was seen upon exposure to reduced pH, but was not expressed at the highest temperature tested, presumably due to the frequent cessation of growth. Further to the significant effect of pH on the measured whole‐colony parameters, observation by scanning electron microscopy revealed surface pitting of the calcified exoskeleton in colonies that were exposed to increased acidity. Studying ecologically relevant processes of growth and reproduction, we demonstrate the existence of relevant levels of variation among genetic individuals which may enable future adaptation via non‐mutational natural selection to falling pH and rising temperature.     

When is a parasite species a species?

Regrettably, 140 years after the publication of Darwin's Origin of Species, we face the grotesque situation that we still do not know what is a species whose origin Darwin wanted to explain. A generally applicable species definition is not available. Is there a basic unit of biodiversity above the level of individuals? Do we try to define something that does not exist in reality? The strong potential for the evolution of genetic variability in parasites together with the importance of species diagnosis for applied fields of parasite research make biodiversity research a key role in parasitology. Frequent occurrence of sympatric speciation, clonal reproduction, selfing, sib mating or parthenogenesis imply exceptional conditions for the evolution of gene pool diversities in parasites.     

Does energy availability influence classical patterns of spatial variation in exotic species richness?

Aim At macroecological scales, exotic species richness is frequently positively correlated with human population density. Such patterns are typically thought to arise because high human densities are associated with increased introduction effort and/or habitat modification and disturbance. Exotic and native species richness are also frequently positively correlated, although the causal mechanisms remain unclear. Energy availability frequently explains much of the variation in species richness and we test whether such species–energy relationships may influence the relationships of exotic species richness with human population density and native species richness. Location Great Britain. Methods We first investigate how spatial variation in the distributions of the 10 exotic bird species is related to energy availability. We then model exotic species richness using native avian species richness, human population density and energy availability as predictors. Species richness is modelled using two sets of models: one assumes independent errors and the other takes spatial correlation into account. Results The probability of each exotic species occurring, in a 10‐km quadrat, increases with energy availability. Exotic species richness is positively correlated with energy availability, human population density and native species richness in univariate tests. When taking energy availability into account, exotic species richness is negligibly influenced by human population density, but remains positively associated with native species richness. Main conclusions We provide one of the few demonstrations that energy availability exerts a strong positive influence on exotic species richness. Within our data, the positive relationship between exotic species richness and human population density probably arises because both variables increase with energy availability, and may be independent of the influence of human density on the probability of establishment. Positive correlations between exotic and native species richness remain when controlling for the influence of energy on species richness. The relevance of such a finding to the debate on the relationship between diversity and invasibility is discussed.     

Big plants—do they limit species coexistence?

Aims According to conventional theory, larger plant species are likely to inflict more intense competition on other (smaller) species. We tested a deducible prediction from this: that a larger species should generally be expected to impose greater limits on the number of species that can coexist with it.Methods Species richness was sampled under plant canopies for a selection of woody species ('host' species) that display a wide range of adult sizes (from small shrubs to large trees), growing within natural vegetation of the Interior Douglas-fir zone of southern British Columbia, Canada. These data were compared with species richness levels sampled within randomly placed plots within the host species habitat.Important findings A prominent host species size effect on species richness was detected but only narrowly at the small end of the species size range. Across most (90%) of the increasing size range of host species, the number of species residing under the host canopy showed no significant decrease relative to the number expected by random assembly, based on species richness within randomly defined equivalent areas within the habitat of the host species. This apparent 'null effect', we suggest, is explained not because these larger species have no effect on community assembly. We postulate that larger species are indeed likely to be more effective in causing competitive exclusion of some smaller species (as expected from conventional theory), but that any potential limitation effect of this on resident species richness is offset for two reasons: (i) larger species also generate niche spaces that they cannot exploit under their own canopies and so have minimal impact (as competitors) on smaller species that can occupy these niches and (ii) certain other small species—despite small size—have effective competitive abilities under the severe competition that occurs within host neighbourhoods of larger species. These and other recent studies call for re-evaluation of traditional views on the role of plant size in affecting competitive ability and community assembly.     

When does coevolution promote diversification?

Coevolutionary interactions between species are thought to be an important cause of evolutionary diversification. Despite this general belief, little theoretical basis exists for distinguishing between the types of interactions that promote diversification and those types that have no effect or that even restrict it. Using analytical models and simulations of phenotypic evolution across a metapopulation, we show that coevolutionary interactions promote diversification when they impose a cost of phenotype matching, as is the case for competition or host-parasite antagonism. In contrast, classical coevolutionary arms races have no tendency to promote or inhibit diversification, and mutualistic interactions actually restrict diversification. Together with the results of recent phylogenetic and ecological studies, these results suggest that the causes of diversification in many coevolutionary systems may require reassessment.     

When does grazing benefit plants?

Many recent studies have attempted to support the hypothesis that grazing can have positive effects on plant growth and fitness. However, a recent critical survey has shown that many of these studies suffer from poor experimental design, and consequently that the hypothesis may only be tenable under very particular circumstances.     

When does invasive species removal lead to ecological recovery? Implications for management success

The primary goal of invasive species management is to eliminate or reduce populations of invasive species. Although management efforts are often motivated by broader goals such as to reduce the negative impacts of invasive species on ecosystems and society, there has been little assessment of the consistency between population-based (e.g., removing invaders) and broader goals (e.g., recovery of ecological systems) for invasive species management. To address this, we conducted a comprehensive review of studies (N = 151) that removed invasive species and assessed ecological recovery over time. We found positive or mixed outcomes in most cases, but 31% of the time ecological recovery did not occur or there were negative ecological outcomes, such as increases in non-target invasive species. Ecological recovery was more likely in areas with relatively little anthropogenic disturbance and few other invaders, and for the recovery of animal populations and communities compared to plant communities and ecosystem processes. Elements of management protocols, such as whether invaders were eradicated (completely removed) versus aggressively suppressed (≥90% removed), did not affect the likelihood of ecological recovery. Our findings highlight the importance of considering broader goals and unintended outcomes when designing and implementing invasive species management programs.     

How does variation in total and relative abundance contribute to gradients of species diversity?

Patterns of biodiversity provide insights into the processes that shape biological communities around the world. Variation in species diversity along biogeographical or ecological gradients, such as latitude or precipitation, can be attributed to variation in different components of biodiversity: changes in the total abundance (i.e., more‐individual effects) and changes in the regional species abundance distribution (SAD). Rarefaction curves can provide a tool to partition these sources of variation on diversity, but first must be converted to a common unit of measurement. Here, we partition species diversity gradients into components of the SAD and abundance using the effective number of species (ENS) transformation of the individual‐based rarefaction curve. Because the ENS curve is unconstrained by sample size, it can act as a standardized unit of measurement when comparing effect sizes among different components of biodiversity change. We illustrate the utility of the approach using two data sets spanning latitudinal diversity gradients in trees and marine reef fish and find contrasting results. Whereas the diversity gradient of fish was mostly associated with variation in abundance (86%), the tree diversity gradient was mostly associated with variation in the SAD (59%). These results suggest that local fish diversity may be limited by energy through the more‐individuals effect, while species pool effects are the larger determinant of tree diversity. We suggest that the framework of the ENS‐curve has the potential to quantify the underlying factors influencing most aspects of diversity change.     

How does feedback from phage infections influence the evolution of phase variation in Campylobacter?

Campylobacter jejuni (C. jejuni) causes gastroenteritis following the consumption of contaminated poultry meat, resulting in a large health and economic burden worldwide. Phage therapy is a promising technique for eradicating C. jejuni from poultry flocks and chicken carcasses. However, C. jejuni can resist infections by some phages through stochastic, phase-variable ON/OFF switching of the phage receptors mediated by simple sequence repeats (SSR). While selection strength and exposure time influence the evolution of SSR-mediated phase variation (PV), phages offer a more complex evolutionary environment as phage replication depends on having a permissive host organism. Here, we build and explore several continuous culture bacteria-phage computational models, each analysing different phase-variable scenarios calibrated to the experimental SSR rates of C. jejuni loci and replication parameters for the F336 phage. We simulate the evolution of PV rates via the adaptive dynamics framework for varying levels of selective pressures that act on the phage-resistant state. Our results indicate that growth reducing counter-selection on a single PV locus results in the stable maintenance of the phage, while compensatory selection between bacterial states affects the evolutionary stable mutation rates (i.e. very high and very low mutation rates are evolutionarily disadvantageous), whereas, in the absence of either selective pressure the evolution of PV rates results in mutation rates below the basal values. Contrastingly, a biologically-relevant model with two phase-variable loci resulted in phage extinction and locking of the bacteria into a phage-resistant state suggesting that another counter-selective pressure is required, instance, the use of a distinct phage whose receptor is an F336-phage-resistant state. We conclude that a delicate balance between counter-selection and phage-attack can result in both the evolution of phase-variable phage receptors and persistence of PV-receptor-specific phage.     

At what scales does aggregated dispersal lead to coexistence?

Aggregation during dispersal can allow persistence of weak competitors, by creating conditions where stronger competitors are more likely to interact with conspecifics than with heterospecifics. However, aggregation mechanisms operate over a wide range of spatial scales, and species experience space in very different ways. The net effect of dispersal aggregation on coexistence will depend on how these scales interact. We show that it is possible to approximate the effects of aggregated dispersal on coexistence by considering three empirically measurable parameters: the spatial scale of interaction (how strongly competition drops off with distance), the spatial scale of aggregation (how large propagule packets are), and the temporal scale of aggregation (how frequently packets arrive). We use a novel metacommunity moment closure based on this approximation and stochastic simulations to show that aggregated dispersal allows for coexistence only when the stronger competitor is both aggregated and interacts at the same or a smaller spatial scale than the weaker competitor. When species interact and are aggregated at the same scales, coexistence outcomes are only weakly sensitive to the absolute scales of interaction and aggregation, as long as the scale of interaction is smaller than the scale of aggregation. However, coexistence is sensitive to the time‐scale of aggregation: increasing the frequency of packet arrival substantially reduces the region of fitness inequalities where both species persist. Finally, coexistence is less likely and global extinction of both competitors is more frequent when aggregation is fixed (with a constant number of propagules per packet), compared to density‐dependent (number of propagules increases with adult density).     

A new hypothesis for species coexistence: male–male repulsion promotes coexistence of competing species

We propose a new hypothesis for species coexistence by considering behavioral interactions between individuals. The hypothesis states that repulsive behavior between conspecific males (male–male repulsion) creates space for competing species, which promotes their coexistence. This hypothesis can explain the coexistence of two competing species even when their ecological niches completely overlap in spatially homogeneous environments. In addition, the mechanisms underlying such behavior might play a role in enabling the coexistence of two species immediately after speciation, with little or no niche differentiation, as in the case of cichlid fish communities, for example. Although there is limited evidence supporting this hypothesis, it can nevertheless explain the occurrence of species coexistence and biodiversity, which cannot be explained by previous theories.