Biological equilibrium in ecosystems 1. A theory of biological equilibrium

In this paper the process of establishing equilibrium in ecosystems is explored, and compared to the process of transformation. This process changes the state of organisms from absent to present, and the reverse, and if functioning randomly on very large numbers of individuals, results in biological equilibrium which can be expressed by the ratio (e?x): 1, withx=1 at Natural Biological Equilibrium (NBE). A model of ecosystems shows that four basic factors are operative in ecosystems: the constraint, or habitat-type; the regulator, or carrying capacity of the substrate; the input, or organisms; and the operator, death. The Theory of Biological Equilibrium (TBE) proposed, postulates that Natural Biological Equilibrium is reached when the individuals in an ecosystem are in dynamic balance with the carrying capacity of the substrate of the ecosystem. The TBE provides the theoretical basis for the Canonical Hypothesis, which postulates that the parameters in eco- systems are fixed, and can be computed from the number of species in the system. The TBE is in agreement with many well-known ecological phenomena, provides the basis for several hypotheses, and forces the rejection of some traditional hypotheses, viz., the hypothesis of competition as a factor in ecosystems.     

Drivers of extinction risk in African mammals: the interplay of distribution state,human pressure,conservation response and species biology

Although conservation intervention has reversed the decline of some species, our success is outweighed by a much larger number of species moving towards extinction. Extinction risk modelling can identify correlates of risk and species not yet recognized to be threatened. Here, we use machine learning models to identify correlates of extinction risk in African terrestrial mammals using a set of variables belonging to four classes: species distribution state, human pressures, conservation response and species biology. We derived information on distribution state and human pressure from satellite-borne imagery. Variables in all four classes were identified as important predictors of extinction risk, and interactions were observed among variables in different classes (e.g. level of protection, human threats, species distribution ranges). Species biology had a key role in mediating the effect of external variables. The model was 90% accurate in classifying extinction risk status of species, but in a few cases the observed and modelled extinction risk mismatched. Species in this condition might suffer from an incorrect classification of extinction risk (hence require reassessment). An increased availability of satellite imagery combined with improved resolution and classification accuracy of the resulting maps will play a progressively greater role in conservation monitoring.     

Evolutionary regime shifts in simulated ecosystems

Catastrophic regime shifts in ecosystems occur when the system is tipped into a new attractor state under some external forcing. Here we consider whether evolutionary adaptations within ecosystems can trigger similar transitions. We use an individual‐based, evolutionary model of interconnected ecosystems to analyze nonlinear changes in global state resulting from local adaptations. Transitions between periods of stability occur when new traits arise that allow exploitation of under‐utilized resources. Subsequent rapid growth of the population carrying the new trait causes abrupt environmental change that drives incumbent species extinct. We call these transitions ‘evolutionary regime shifts’. These internally generated perturbations can result in ecosystem collapse, followed by recovery to an alternate stable state, or occasionally system‐wide extinction. While these disruptions may have a negative impact on ecosystem productivity in individual simulation runs, mean results over many simulations show a trend for increasing ecosystem productivity and stability over time. Feedback between life and the abiotic environment in the model creates a ‘long‐tailed’ distribution of extinction sizes without any external trigger for large extinction events.     

Prospects for monitoring freshwater ecosystems towards the 2010 targets

Human activities have severely affected the condition of freshwater ecosystems worldwide. Physical alteration, habitat loss, water withdrawal, pollution, overexploitation and the introduction of non-native species all contribute to the decline in freshwater species. Today, freshwater species are, in general, at higher risk of extinction than those in forests, grasslands and coastal ecosystems. For North America alone, the projected extinction rate for freshwater fauna is five times greater than that for terrestrial fauna--a rate comparable to the species loss in tropical rainforest. Because many of these extinctions go unseen, the level of assessment and knowledge of the status and trends of freshwater species are still very poor, with species going extinct before they are even taxonomically classified. Increasing human population growth and achieving the sustainable development targets set forth in 2002 will place even higher demands on the already stressed freshwater ecosystems, unless an integrated approach to managing water for people and ecosystems is implemented by a broad constituency. To inform and implement policies that support an integrated approach to water management, as well as to measure progress in halting the rapid decline in freshwater species, basin-level indicators describing the condition and threats to freshwater ecosystems and species are required. This paper discusses the extent and quality of data available on the number and size of populations of freshwater species, as well as the change in the extent and condition of natural freshwater habitats. The paper presents indicators that can be applied at multiple scales, highlighting the usefulness of using remote sensing and geographical information systems technologies to fill some of the existing information gaps. Finally, the paper includes an analysis of major data gaps and information needs with respect to freshwater species to measure progress towards the 2010 biodiversity targets.     

Habitat subdivision causes changes in food web structure

Theory suggests that the response of communities to habitat subdivision depends on both species' characteristics and the extent to which species interact. For species with dynamics that are independent of other species, subdivision is expected to promote regional extinction as populations become small and isolated. By contrast, intermediate levels of subdivision can facilitate persistence of strongly interacting species. Consistent with this prediction, experimental subdivision lengthened persistence of some species, altering the extent of food web collapse through extinction. Extended persistence was associated with immigration rescuing a basal prey species from local extinction. As predicted by food web theory, habitat subdivision reduced population density of a top predator. Removal of this top predator from undivided microcosms increased the abundance of two other predator species, and these changes paralleled those produced by habitat subdivision. These results show that species interactions structured this community, and illustrate the need for investigations of other communities.     

Evolution in metacommunities

A metacommunity can be defined as a set of communities that are linked by migration, and extinction and recolonization. In metacommunities, evolution can occur not only by processes that occur within communities such as drift and individual selection, but also by among-community processes, such as divergent selection owing to random differences among communities in species composition, and group and community-level selection. The effect of these among-community-level processes depends on the pattern of migration among communities. Migrating units may be individuals (migrant pool model), groups of individuals (single-species propagule pool model) or multi-species associations (multi-species propagule pool model). The most interesting case is the multi-species propagule pool model. Although this pattern of migration may a priori seem rare, it becomes more plausible in small well-defined 'communities' such as symbiotic associations between two or a few species. Theoretical models and experimental studies show that community selection is potentially an effective evolutionary force. Such evolution can occur either through genetic changes within species or through changes in the species composition of the communities. Although laboratory studies show that community selection can be important, little is known about how important it is in natural populations.     

Phylogenetic correlates of extinction risk in mammals: species in older lineages are not at greater risk

Phylogenetic information is becoming a recognized basis for evaluating conservation priorities, but associations between extinction risk and properties of a phylogeny such as diversification rates and phylogenetic lineage ages remain unclear. Limited taxon-specific analyses suggest that species in older lineages are at greater risk. We calculate quantitative properties of the mammalian phylogeny and model extinction risk as an ordinal index based on International Union for Conservation of Nature Red List categories. We test for associations between lineage age, clade size, evolutionary distinctiveness and extinction risk for 3308 species of terrestrial mammals. We show no significant global or regional associations, and three significant relationships within taxonomic groups. Extinction risk increases for evolutionarily distinctive primates and decreases with lineage age when lemurs are excluded. Lagomorph species (rabbits, hares and pikas) that have more close relatives are less threatened. We examine the relationship between net diversification rates and extinction risk for 173 genera and find no pattern. We conclude that despite being under-represented in the frequency distribution of lineage ages, species in older, slower evolving and distinct lineages are not more threatened or extinction-prone. Their extinction, however, would represent a disproportionate loss of unique evolutionary history.     

Unintended consequences of conservation actions: managing disease in complex ecosystems

Infectious diseases are increasingly recognised to be a major threat to biodiversity. Disease management tools such as control of animal movements and vaccination can be used to mitigate the impact and spread of diseases in targeted species. They can reduce the risk of epidemics and in turn the risks of population decline and extinction. However, all species are embedded in communities and interactions between species can be complex, hence increasing the chance of survival of one species can have repercussions on the whole community structure. In this study, we use an example from the Serengeti ecosystem in Tanzania to explore how a vaccination campaign against Canine Distemper Virus (CDV) targeted at conserving the African lion (Panthera leo), could affect the viability of a coexisting threatened species, the cheetah (Acinonyx jubatus). Assuming that CDV plays a role in lion regulation, our results suggest that a vaccination programme, if successful, risks destabilising the simple two-species system considered, as simulations show that vaccination interventions could almost double the probability of extinction of an isolated cheetah population over the next 60 years. This work uses a simple example to illustrate how predictive modelling can be a useful tool in examining the consequence of vaccination interventions on non-target species. It also highlights the importance of carefully considering linkages between human-intervention, species viability and community structure when planning species-based conservation actions.     

Species-rich ecosystems are vulnerable to cascading extinctions in an increasingly variable world

Global warming leads to increased intensity and frequency of weather extremes. Such increased environmental variability might in turn result in increased variation in the demographic rates of interacting species with potentially important consequences for the dynamics of food webs. Using a theoretical approach, we here explore the response of food webs to a highly variable environment. We investigate how species richness and correlation in the responses of species to environmental fluctuations affect the risk of extinction cascades. We find that the risk of extinction cascades increases with increasing species richness, especially when correlation among species is low. Initial extinctions of primary producer species unleash bottom-up extinction cascades, especially in webs with specialist consumers. In this sense, species-rich ecosystems are less robust to increasing levels of environmental variability than species-poor ones. Our study thus suggests that highly species-rich ecosystems such as coral reefs and tropical rainforests might be particularly vulnerable to increased climate variability.     

Sociality, density-dependence and microclimates determine the persistence of populations suffering from a novel fungal disease, white-nose syndrome

Disease has caused striking declines in wildlife and threatens numerous species with extinction. Theory suggests that the ecology and density-dependence of transmission dynamics can determine the probability of disease-caused extinction, but few empirical studies have simultaneously examined multiple factors influencing disease impact. We show, in hibernating bats infected with Geomyces destructans, that impacts of disease on solitary species were lower in smaller populations, whereas in socially gregarious species declines were equally severe in populations spanning four orders of magnitude. However, as these gregarious species declined, we observed decreases in social group size that reduced the likelihood of extinction. In addition, disease impacts in these species increased with humidity and temperature such that the coldest and driest roosts provided initial refuge from disease. These results expand our theoretical framework and provide an empirical basis for determining which host species are likely to be driven extinct while management action is still possible.     

Potential extinction debt due to habitat loss and fragmentation in subalpine moorland ecosystems

Habitat loss and fragmentation would often induce delayed extinction, referred to as extinction debt. Understanding potential extinction debts would allow us to reduce future extinction risk by restoring habitats or implementing conservation actions. Although growing empirical evidence has predicted extinction debts in various ecosystems exposed to direct human disturbances, potential extinction debts in natural ecosystems with minimal direct human disturbance are little studied. Ongoing climate change may cause habitat loss and fragmentation, particularly in natural ecosystems vulnerable to environmental change, potentially leading to future local extinctions. Recent climate change would lead to extended growing season caused by earlier snowmelt in spring, resulting in expansion of shrubby species and thereby habitat loss and fragmentation of mountainous moorlands. We examined the potential extinction debts of species diversity and functional diversity (FD; trait variation or multivariate trait differences within a community) in subalpine moorland ecosystems subjected to few direct human disturbances. Plant species richness for all species and for moorland specialists were primarily explained by the past kernel density of focal moorlands (a proxy for spatial clustering of moorlands around them) but not the past area of the focal moorlands, suggesting potential extinction debt in subalpine moorland ecosystems. The higher kernel density of the focal moorland in the past indicates that it was originally surrounded by more neighborhood moorlands and/or had been locally highly fragmented. Patterns in current plant species richness have been shaped by the historical spatial configuration of moorlands, which have disappeared over time. In contrast, we found no significant relationships between the FD and historical and current landscape variables depicting each moorland. The prevalence of trait convergence might result in a less sensitive response of FD to habitat loss and fragmentation compared to that of species richness. Our finding has an important implication that climate change induced by human activities may threaten biodiversity in natural ecosystems through habitat loss and fragmentation.

     

Application to the terrestrial vertebrates of Italy of a system proposed by IUCN for a new classification of national Red List categories

The criteria for classification set down in the new system of the IUCN Red List categories were applied to national distribution areas. Application of the new criteria to national distribution areas calls for an estimate of the rate of immigration. Classification was applied to the Italian autochthonous terrestrial vertebrate species that reproduce in Italy (Amphibia, Reptilia, Aves and Mammalia). Species with both marine and land habitats were included, but Cetaceans were excluded. Eighty-eight species of the autochthonous species that reproduce in Italy were classified in the threatened categories at the national level. One of the main purposes of the present classification of the risk of extinction in national distribution areas is to provide the information needed to define global conservation priorities of species present on national territory. The species nationally and/or globally endangered or at lower risk were grouped into four priority classes, on the basis of their reproductive phenology on national territory and the size of temporarily present contingents. Of the 551 species present regularly or seasonally on Italian territory, 149 (27.0%) are in the initial indicative list of conservation priorities.     

Habitat type richness associations with environmental variables: a case study in the Greek Natura 2000 aquatic ecosystems

We investigated the potential associations of habitat type richness patterns with a series of environmental variables in 61 protected aquatic ecosystems of the Greek Natura 2000 network. Habitat type classification followed the Natura 2000 classification scheme. Habitat type richness was measured as the number of different habitat types in an area. To overcome a potential area effect in quantifying habitat type richness, we applied the “moving window” technique. The environmental variables were selected to account for some of the major threats to biodiversity, such as fragmentation, habitat loss and climate change. We run GLMs to associate habitat type richness with different combinations of climatic, spatial and topographic variables. Habitat type richness seemed to significantly associate with climatic variables, more than spatial or topographic ones. In particular, for the climatic ones, the importance of precipitation surpassed that of temperature and especially the precipitation of the wettest and driest month had a limiting contribution to richness unlike average climate estimators. Moreover, the landscape’s latitude and longitude and fragmentation were significantly associated to richness. Our findings are in accordance to those observed in recent literature at lower (i.e. species) levels of ecological organization, fact showing that large-scale phenomena (such as climate change) can also be observed at the habitat type level, at least in our case. Thus, following the context of the Habitats Directive (92/43/EEC), that habitat types and not solely species of community interest should be protected and restored, this study serves as a first step towards investigating habitat type richness patterns.     

Assessing extinction risk in the absence of species-level data: quantitative criteria for terrestrial ecosystems

The conservation of individual plant and animal species has been advanced greatly by the World Conservation Union’s (IUCN) development of objective, repeatable, and transparent criteria for assessing extinction risk, which explicitly separate the process of risk assessment from priority-setting. Here we present an analogous procedure for assessing the extinction risk of terrestrial ecosystems, which may complement traditional species-specific risk assessments, or may provide an alternative when only landscape-level data are available. We developed four quantitative risk criteria, derived primarily from remotely sensed spatial data, information on one of which must be available to permit classification. Using a naming system analogous to the present IUCN species-specific system, our four criteria were: (A) reduction of land cover and continuing threat, (B) rapid rate of land cover change, (C) increased fragmentation, and (D) highly restricted geographical distribution. We applied these criteria to five ecosystems covering a range of spatial and temporal scales, regions of the world, and ecosystem types, and found that Indonesian Borneo’s lowland tropical forests and the Brazilian Atlantic rainforest were Critically Endangered, while South Africa’s grasslands and Brazil’s Mato Grosso were Vulnerable. Furthermore, at a finer grain of analysis, one region of Venezuela’s coastal dry forests (Margarita Island) qualified as Vulnerable, while another (the Guasare River watershed) was Critically Endangered. In northern Venezuela, deciduous forests were classified as Endangered, semi-deciduous forests Vulnerable, and evergreen forests of Least Concern. We conclude that adoption of such a standardized system will facilitate globally comparable, repeatable geographic analyses that clearly separate risk assessment (a fundamentally scientific process), from the definition of conservation priorities, which should take into account additional factors, such as ecological distinctiveness, costs, logistics, likelihood of success, and societal preferences. Jon Paul Rodríguez and Jennifer K. Balch are contributed equally to this work     

Stratigraphic distribution and ecology of European Jurassic bivalves

Documentation of bivalve generic and species diversity and times of first and last appearance through successive Jurassic stages in Europe, together with data on turnover and changes in taxonomic and ecological composition of the faunas, indicate an approximation to the establishment of an equilibrium fauna by early Middle Jurassic times. Subsequently faunal change was slight compared with the early Jurassic. A diversity increase through the Lower into the Middle Jurassic correlates with an increase in the area of epicontinental seas, while a major species extinction in the early Toarcian is bound up with the onset of widespread stagnation associated with a rise of sea level. An increase of the generic extinction rate at the end of the period correlates with a regional marine regression. The mean species longevity is estimated at 15×10⁶ years. The ecological factors thought to control bivalve distribution are reviewed and four ecological associations distinguished: the reefal, lagoonal and nearshore and basinal marine.     

Palaeontologic and biogeochemical characterization of the Cyrtograptus lundgreni event in the black shales of eastern Mid-Sardinia, Italy

A succession of biotic and geochemical changes that occurred during the Cyrtograptus lundgreni Event (Late Wenlock) have been recorded from the 'pelagic' black-shales in the Goni section, eastern mid-Sardinia, Italy. The studied interval encompasses the Cyrtograptus rigidus to Pristiograptus dubius-Gothograptus nassa zones. The fossil association includes graptolites, chitinozoans and microplankton i.e. probable linings of agglutinated foraminifera and radiolaria capsular membranes. Analysis of the chitinozoan distribution revealed a succession of several chitinozoan associations with low species diversity and dominated by opportunistic species. Three chitinozoan faunal turnovers and three extinction events have been recorded. Two of them coincide with graptolite extinctions whereas one probably is of local significance. Disappearance of the chitinozoan and microplankton associations occurred during four consecutive graptolite zones. Geochemical data (trace elements analysis) showed significantly higher (up to c. 100%) values for Co and Cd in the sedimentary organic matter (SOM) than in the whole rock samples. Possible relationships between peaks of metal enrichment, the major faunal changes among chitinozoans, extinction events among chitinozoans and graptolites and, to a certain extent, oceanic events may be inferred. The first extinction datum is older that those occurring in Gotland, Sweden and Thüringen, Germany and is so far considered to be of local significance. The second extinction datum of Sardinia can be matched with Datum 1 of Gotland and Thüringen. A close correlation between the third extinction datum of Sardinia and Datum 2 of Thüringen and Gotland reinforces the importance of these events at global scale.     

Impact of the Late Triassic mass extinction on functional diversity and composition of marine ecosystems

Mass extinctions have profoundly influenced the history of life, not only through the death of species but also through changes in ecosystem function and structure. Importantly, these events allow us the opportunity to study ecological dynamics under levels of environmental stress for which there are no recent analogues. Here, we examine the impact and selectivity of the Late Triassic mass extinction event on the functional diversity and functional composition of the global marine ecosystem, and test whether post‐extinction communities in the Early Jurassic represent a regime shift away from pre‐extinction communities in the Late Triassic. Our analyses show that, despite severe taxonomic losses, there is no unequivocal loss of global functional diversity associated with the extinction. Even though no functional groups were lost, the extinction event was, however, highly selective against some modes of life, in particular sessile suspension feeders. Although taxa with heavily calcified skeletons suffered higher extinction than other taxa, lightly calcified taxa also appear to have been selected against. The extinction appears to have invigorated the already ongoing faunal turnover associated with the Mesozoic Marine Revolution. The ecological effects of the Late Triassic mass extinction were preferentially felt in the tropical latitudes, especially amongst reefs, and it took until the Middle Jurassic for reef ecosystems to fully recover to pre‐extinction levels.     

Botanical diversity in British mires

Although mires are sometimes regarded as a single, specific and even esoteric habitat-type, in their full range of development-calcareous fens to acidic bogs, lowland swamps to upland flushes, woodlands, sedgelands and mosslands-they occupy a wide range of environmental conditions and sustain a rich botanical resource. Here an outline is given of the types of mires that are widespread in Britain and their botanical resource. It is discussed with reference to some of the environmental variables and management conditions that have been shown to influence the species composition and diversity of their vegetation. Some implications that this has for the classification of mires are outlined.     

Trapped in the web of water: Groundwater‐fed springs are island‐like ecosystems for the meiofauna

We investigated whether the equilibrium theory of island biogeography (ETIB) can be applied to the meiofauna of groundwater‐fed springs. We tested whether copepod species richness was related with spring area, discharge, and elevation. Additionally, five hypotheses are tested based on species distribution patterns, dispersal ability, and life‐history characteristics of several guilds (stygobiotic, nonstygobiotic, cold stenotherm, and noncold stenotherm species). Thirty springs in the central Apennines (Italy) were considered. A multimodel selection procedure was applied to select best‐fit models using both ordinary least‐squares regressions and autoregressive models. Mantel tests were used to investigate the impact of spatial autocorrelation in determining interspring similarity (ßsor), pure turnover (ßsim), intersite nestedness (ßnest = ßsor ? ßsim), and matrix nestedness (measured using NODF and other metrics). Explicit consideration of spatial correlations reduced the importance of predictors of overall species richness, noncold stenotherm species (both negatively affected by elevation), cold stenotherm species, and nonstygobiotic species, but increased the importance of area for the stygobiotic species. We detected nested patterns in all cases, except for the stygobites. Interspring distances were positively correlated with ßsor and ßnest (but not with ßsim) for the entire data set and for nonstygobiotic, cold stenotherm, and noncold stenotherm species. In the case of stygobites, interspring geographical distances were marginally correlated with ßsor and no correlation was found for ßsim and ßnest. We found support for ETIB predictions about species richness, which was positively influenced by area and negatively by elevation (which expresses the size of source of immigrants). Low turnover and high nestedness are consistent with an equilibrium scenario mainly regulated by immigration and extinction. Stygobites, which include many distributional and evolutionary relicts, have a low capability to disperse through the aquifers and tend to be mainly confined to the springs where they drifted out and were trapped by springbed sediments.