Reducing competition vs. improving resistance to disturbances in the environment

 We study a metapopulation model for competition between two species in environments affected by regular disturbances. We give precise conditions for the coexistence of both species. We show that both species are more likely to coexist under an intermediate disturbance frequency. Since the rates of interspecific competition, as well as the frequency of the disturbances in the environment are given explicitly, it is possible to analyze the role they play in determining the final equilibrium level of the species involved. Special interest is given to comparing the effect of increasing resistance of the weak species to the environmental disturbances versus reducing the competition rate. It turns out that reducing the competition rate is never the better strategy under these conditions. Received: 29 April 1999     

Periodic coexistence in the chemostat with three species competing for three essential resources

A chemostat model of three species of microorganisms competing for three essential, growth-limiting nutrients is considered. J. Husiman and F.J. Weissing [Nature 402 (1999) 407] show numerically that this model can generate periodic oscillations. The present contribution is concerned with rigorous analysis regarding the existence of periodic oscillations in this model. Our analysis is based on the following observation made by Huisman and Weissing: there is a cyclic replacement of species, if each species becomes limited by the resource for which it is the intermediate competitor. Using a permanence theory, an index theory, and a Poincaré-Bendixson theory for three-dimensional competitive systems, we analytically succeed to give sufficient conditions for the existence of periodic orbits in the limit sets in this model. The results in this paper suggest that with a wide range of parameter values, sustained periodic oscillations of species abundances for the model are possible, without involving external disturbances. Our results also suggest that competition is not necessarily destructive, i.e., in the case of existence of sustained periodic oscillations, if one of three competitors is absent, one of the other two rivals cannot survive.     

Complex vegetation responses to soil disturbances in mountain grassland

We studied vegetation responses to disturbances originated by ants and voles in subalpine grasslands in the Eastern Pyrenees. We compared the effects of these small-scale disturbances with those of a large-scale disturbance caused by ploughing. We wanted to know if these soil disturbances promoted species richness through the existence of a specific guild of plants colonizing these areas, and if this guild was the same for all soil disturbances, independently of their extent. In general, grassland vegetation seemed to recover relatively quickly from soil-displacement disturbances, and the effects could be scaled up in time and space in terms of species richness and composition. Vole mound composition was similar to that in the surrounding grassland, suggesting that mounds were rapidly colonized by the neighbouring vegetation. Vegetation composition differed between the grassland and the ant mounds. Grasses and erect dicots coped well with repeated disturbance, while rosette-forming species and sedges were very sensitive to it. Landscape processes could be important to understanding recolonization. Species from xeric grasslands were found in mesic grasslands when disturbed by ploughing and on the tops of active ant mounds. Furrows in mesic grasslands recovered well, but decades after disturbance showed long persistence of some xeric species and increased species richness compared to terraces, while xeric grasslands showed decreased richness. This suggests that, because of those disturbances, within-habitat diversity was increased, although landscape diversity was not. However, specific disturbances showed idiosyncratic effects, which could enhance the species richness globally. In ant-affected areas, the grassland itself showed the highest plant species richness, partially associated to the presence of some species with elaiosomes not, or only rarely, found in adjacent grasslands without ant mounds. Therefore, soil disturbances occurring at different spatial scales contributed to complexity in vegetation patterns in addition to abiotic factors and grazing. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Nomenclature of the species follows Tutin et al. (1964–1980) and Bolòs et al. (1993).     

Climate interacts with anthropogenic drivers to determine extirpation dynamics

Theoretical studies suggest that the dynamics of a species’ range during a period of climate change depends upon the existence and interplay of various ecological and evolutionary processes. Here we tested how anthropogenic pressures contribute to climate‐mediated extirpation patterns of 32 freshwater fish species over the last 20 yr. We contrasted two extreme cases to determine whether extirpations were governed by patterns of climate exposure, assuming full adaptation of species to local climate, or instead by the interplay between climate exposure and the distance from the centroid of species’ climatic niches, assuming a fixed niche, and asked whether anthropogenic disturbances interact with these climatic drivers. We found strong support for the fixed niche hypothesis, but showed that species‐specific local adaptation to climate may also be important in determining extirpation dynamics. We also demonstrated that anthropogenic disturbance acted in concert with climate, ultimately determining population changes. Our results add novel evidence that unravelling the direct links between range dynamics and climate requires a multifaceted treatment, and that accounting for the cumulative effects of anthropogenic pressures deserves special attention in the context of climate change.     

Critical thresholds for eventual extinction in randomly disturbed population growth models

This paper considers several single species growth models featuring a carrying capacity, which are subject to random disturbances that lead to instantaneous population reduction at the disturbance times. This is motivated in part by growing concerns about the impacts of climate change. Our main goal is to understand whether or not the species can persist in the long run. We consider the discrete-time stochastic process obtained by sampling the system immediately after the disturbances, and find various thresholds for several modes of convergence of this discrete process, including thresholds for the absence or existence of a positively supported invariant distribution. These thresholds are given explicitly in terms of the intensity and frequency of the disturbances on the one hand, and the population’s growth characteristics on the other. We also perform a similar threshold analysis for the original continuous-time stochastic process, and obtain a formula that allows us to express the invariant distribution for this continuous-time process in terms of the invariant distribution of the discrete-time process, and vice versa. Examples illustrate that these distributions can differ, and this sends a cautionary message to practitioners who wish to parameterize these and related models using field data. Our analysis relies heavily on a particular feature shared by all the deterministic growth models considered here, namely that their solutions exhibit an exponentially weighted averaging property between a function of the initial condition, and the same function applied to the carrying capacity. This property is due to the fact that these systems can be transformed into affine systems.

     

Understanding the Spatio-Temporal Response of Coral Reef Fish Communities to Natural Disturbances: Insights from Beta-Diversity Decomposition

Understanding how communities respond to natural disturbances is fundamental to assess the mechanisms of ecosystem resistance and resilience. However, ecosystem responses to natural disturbances are rarely monitored both through space and time, while the factors promoting ecosystem stability act at various temporal and spatial scales. Hence, assessing both the spatial and temporal variations in species composition is important to comprehensively explore the effects of natural disturbances. Here, we suggest a framework to better scrutinize the mechanisms underlying community responses to disturbances through both time and space. Our analytical approach is based on beta diversity decomposition into two components, replacement and biomass difference. We illustrate this approach using a 9-year monitoring of coral reef fish communities off Moorea Island (French Polynesia), which encompassed two severe natural disturbances: a crown-of-thorns starfish outbreak and a hurricane. These disturbances triggered a fast logistic decline in coral cover, which suffered a 90% decrease on all reefs. However, we found that the coral reef fish composition remained largely stable through time and space whereas compensatory changes in biomass among species were responsible for most of the temporal fluctuations, as outlined by the overall high contribution of the replacement component to total beta diversity. This suggests that, despite the severity of the two disturbances, fish communities exhibited high resistance and the ability to reorganize their compositions to maintain the same level of total community biomass as before the disturbances. We further investigated the spatial congruence of this pattern and showed that temporal dynamics involved different species across sites; yet, herbivores controlling the proliferation of algae that compete with coral communities were consistently favored. These results suggest that compensatory changes in biomass among species and spatial heterogeneity in species responses can provide further insurance against natural disturbances in coral reef ecosystems by promoting high levels of key species (herbivores). They can also allow the ecosystem to recover more quickly.     

Plant resistance to mechanical stress: evidence of an avoidance-tolerance trade-off

External mechanical forces resulting from the pressure exerted by wind or water movement are a major stress factor for plants and may cause regular disturbances in many ecosystems. A plant's ability to resist these forces relies either on minimizing the forces encountered by the plant (avoidance strategy), or on maximizing its resistance to breakage (tolerance strategy). We investigated plant resistance strategies using aquatic vegetation as a model, and examined whether avoidance and tolerance are negatively correlated. We tested the avoidance-tolerance correlation across 28 species using a phylogenetically corrected analysis, after construction of a molecular phylogeny for the species considered. Different species demonstrated contrasting avoidance and tolerance and we demonstrated a significant negative relationship between the two strategies, which suggests an avoidance-tolerance trade-off. Negative relationships may result from costs that each strategy incurs or from constraints imposed by physical laws on plant tissues. The existence of such a trade-off has important ecological and evolutionary consequences. It would lead to constraints on the evolution and variation of both strategies, possibly limiting their evolution and may constrain many morphological, anatomical and architectural traits that underlie avoidance and tolerance.     

What is the role of disturbance in catalyzing spatial shifts in forest composition and tree species biomass under climate change?

Mounting evidence suggests that climate change will cause shifts of tree species range and abundance (biomass). Abundance changes under climate change are likely to occur prior to a detectable range shift. Disturbances are expected to directly affect tree species abundance and composition, and could profoundly influence tree species spatial distribution within a geographical region. However, how multiple disturbance regimes will interact with changing climate to alter the spatial distribution of species abundance remains unclear. We simulated such forest demographic processes using a forest landscape succession and disturbance model (LANDIS-II) parameterized with forest inventory data in the northeastern United States. Our study incorporated climate change under a high-emission future and disturbance regimes varying with gradients of intensities and spatial extents. The results suggest that disturbances catalyze changes in tree species abundance and composition under a changing climate, but the effects of disturbances differ by intensity and extent. Moderate disturbances and large extent disturbances have limited effects, while high-intensity disturbances accelerate changes by removing cohorts of mid- and late-successional species, creating opportunities for early-successional species. High-intensity disturbances result in the northern movement of early-successional species and the southern movement of late-successional species abundances. Our study is among the first to systematically investigate how disturbance extent and intensity interact to determine the spatial distribution of changes in species abundance and forest composition.     

Invaders' control on post-disturbance succession in coastal mangroves

Aims In recent years, coastal mangroves have been frequently affected by large disturbances (cyclones, hurricanes, flooding and tsunamis) and post-disturbance vegetation is often dominated by small stature mangrove, mangrove-associate and non-mangrove species potentially affecting ecosystem functioning. Knowledge on the processes of mangrove vegetation development and recovery (succession) following normal and large disturbances will benefit practitioners in designing robust ecosystem management/restoration plans. Here we propose a conceptual model of disturbance-mediated succession in mangroves.Methods Based on field observations and species' life history traits, we develop conceptual models of mangrove succession under normal disturbance regime and recently experienced increased frequency of large disturbances. We evaluate our conceptual models by conducting a scenario testing experiment.Important findings We suggest two predominant processes affecting mangrove succession after disturbance: propagule limitation due to damage of seed producing mature trees and dispersal barrier resulting from biological invasion associated with large disturbance. We argue that large disturbances affect mature trees more than the small-stature non-tree (shrubs, herbs and climbers) species creating a larger propagule shortage for mangrove tree species than non-tree species. Secondly, large disturbances facilitate invasion of free-floating aquatics, which may interfere with the flow-facilitated propagule dispersal and seedling establishment of mangrove species. In a scenario testing experiment, we have shown that similar levels of disturbance impact vegetation development and recovery differently depending on the presence or absence of invasive species. We conclude that since biological invasion is one of the major drivers of post-disturbance mangrove succession, the dimension of biological invasion should be included in prediction, management and restoration of mangrove forests.     

Functional traits and climate drive interspecific differences in disturbance-induced tree mortality

With climate change, natural disturbances such as storm or fire are reshuffled, inducing pervasive shifts in forest dynamics. To predict how it will impact forest structure and composition, it is crucial to understand how tree species differ in their sensitivity to disturbances. In this study, we investigated how functional traits and species mean climate affect their sensitivity to disturbances while controlling for tree size and stand structure. With data on 130,594 trees located on 7617 plots that were disturbed by storm, fire, snow, biotic or other disturbances from the French, Spanish, and Finnish National Forest Inventory, we modeled annual mortality probability for 40 European tree species as a function of tree size, dominance status, disturbance type, and intensity. We tested the correlation of our estimated species probability of disturbance mortality with their traits and their mean climate niches. We found that different trait combinations controlled species sensitivity to disturbances. Storm-sensitive species had a high height-dbh ratio, low wood density and high maximum growth, while fire-sensitive species had low bark thickness and high P50. Species from warmer and drier climates, where fires are more frequent, were more resistant to fire. The ranking in disturbance sensitivity between species was overall consistent across disturbance types. Productive conifer species were the most disturbance sensitive, while Mediterranean oaks were the least disturbance sensitive. Our study identified key relations between species functional traits and disturbance sensitivity, that allows more reliable predictions of how changing climate and disturbance regimes will impact future forest structure and species composition at large spatial scales.     

Asymptotic Behavior of a Chemostat Model with Constant Recycle Sludge Concentration

In this work, we study a several species aerobic chemostat model with constant recycle sludge concentration in continuous culture. We reduce the number of parameters by considering a dimensionless model. First, the existence of a global positive uniform attractor for the model with different removal rates is proved using the theory of dissipative dynamical systems. Hence, we investigate the asymptotic behavior of the model under small perturbations using methods of singular perturbation theory and we prove that, in the case of two species in competition, the unique equilibrium which is positive is globally asymptotically stable. Finally, we establish the link between the open problem of the chemostat with different removal rates and monotone functional responses, and our model when two species compete on the same nutrient. We give some numerical simulations to illustrate the results.     

Recurrent habitat disturbance and species diversity in a multiple-competitive species system

To address how species interactions, dispersal and environmental disturbances interplay to affect the spatial distribution and diversity of species, we present a compartment model in which multiple species undergo competitive interaction of Lotka-Volterra type in a patchy environment arranged in a square lattice. Dispersal of species occurs between adjacent patches. Disturbances are periodically imposed on a central part of the environment in a belt-like block or an island-like block of various sizes where each species is killed for a certain time interval and then allowed to recover for the rest of a disturbance cycle. We deal with a case in which the local population dynamics within each patch is analytically determinable and has multiple locally stable equilibrium states in the absence of environmental disturbance. We further assume a trade-off between the reproductive rate of species and its dispersal ability. With these settings, we numerically examine how the spatio-temporal distributions of species are affected by changes in the pattern, size and duration of disturbances. The results demonstrate that: (1) in the undisturbed area, environmental disturbances could generate spatially segregated distributions of species; (2) in the disturbed area, species with higher dispersal abilities quickly invade and preferentially recover their population during the post-disturbance period, being temporarily relieved of competition from other species. These mechanisms collectively lead to increased species diversity in the whole habitat, functioning best when both the size and duration of disturbances are intermediate. In particular, the belt-like disturbance is more effective than the island-like disturbance in sustaining spatial heterogeneity for a wider range of duration of disturbance.     

Influence of experimental soil disturbances on the diversity of plants in agricultural grasslands

Aims and Methods Disturbance is supposed to play an important role for biodiversity and ecosystem stability as described by the intermediate disturbance hypothesis (IDH), which predicts highest species richness at intermediate levels of disturbances. In this study, we tested the effects of artificial soil disturbances on diversity of annual and perennial vascular plants and bryophytes in a field experiment in 86 agricultural grasslands differing in land use in two regions of Germany. On each grassland, we implemented four treatments: three treatments differing in application time of soil disturbances and one control. One year after experimental disturbance, we recorded vegetation and measured biomass productivity and bare ground. We analysed the disturbance response taking effects of region and land-use-accompanied disturbance regimes into account.Important findings Region and land-use type strongly determined plant species richness. Experimental disturbances had small positive effects on the species richness of annuals, but none on perennials or bryophytes. Bare ground was positively related to species richness of bryophytes. However, exceeding the creation of 12% bare ground further disturbance had a detrimental effect on bryophyte species richness, which corresponds to the IDH. As biomass productivity was unaffected by disturbance our results indicate that the disturbance effect on species richness of annuals was not due to decreased overall productivity, but rather due to short-term lowered inter- and intraspecific competition at the newly created microsites. Generally, our results highlight the importance of soil disturbances for species richness of annual plants and bryophytes in agricultural grasslands. However, most grasslands were disturbed naturally or by land-use practices and our additional experimental soil disturbances only had a small short-term effect. Overall, total plant diversity in grasslands seemed to be more limited by the availability of propagules rather than by suitable microsites for germination. Thus, nature conservation efforts to increase grassland diversity should focus on overcoming propagule limitation, for instance by additional sowing of seeds, while the creation of additional open patches by disturbance might only be appropriate where natural disturbances are scarce.     

Hybridization, species collapse, and species reemergence after disturbance to premating mechanisms of reproductive isolation

There are now a number of well-studied cases in which hybridization between closely related sympatric species has increased, sometimes resulting in the replacement of species pairs by hybrid swarms. Many of these cases have been linked to anthropogenic environmental change, but the mechanisms leading from environmental change to species collapse, and the long-term effects of hybridization on species pairs, remain poorly understood. We used an individual-based stochastic simulation model to explore the conditions under which disturbances that weaken premating barriers to reproduction patterns between sympatric species might lead to increased hybridization and to species collapse. Disturbances often resulted in bouts of hybridization, but in many cases strong reproductive isolation spontaneously reemerged. This was sometimes true even after hybrid swarms had replaced parental species. The reemergence of species pairs was most likely when disturbances were of short duration. Counterintuitively, incipient species pairs were more likely to reemerge after strong but temporary disturbances than after weaker disturbances of the same duration. Even temporary bouts of hybridization often led to substantial homogenization of species pairs. This suggests that ecosystem managers may be able to refill ecological niches, but in general will not be able to resurrect lost species after species collapse.     

Environmental influences on local amphibian diversity: the role of floods on river basins

We tested two hypotheses about boundary units and seven about environmental control of species diversity in order to explain geographical trends in the richness of amphibian species in the Mediterranean watershed of the southern coast of the Iberian Peninsula. The number of amphibian species tends to decrease from west to east. The longitudinal trend in the richness of amphibian species actually occurs on passing from one basin to another, but there is not any longitudinal trend within the basins. Multivariate analyses confirmed that the disturbances of episodic river-basin floodings were the principal factor which controls the richness of amphibian species. They explained 94.8% of the observed variations in the richness of amphibian species in this area, according to the intermediate disturbance hypothesis.We propose hydrographic basins as suitable geographical units for further biogeographical analysis and for considering the role of disturbances produced by floods in the environmental control of species diversity.     

Temporal variability of nestedness and idiosyncratic species in stream insect assemblages

Aims The nested subset pattern has been widely studied in the last 20 years, and recent syntheses have challenged the prevalence of this pattern in nature. We examined the degree of nestedness, its temporal variability and its environmental correlates in stream insects of a boreal drainage system. We also examined differences between nested and idiosyncratic species in site occupancy, niche position and niche breadth. Location Koutajoki drainage basin in northern Finland. Methods We used (i) nestedness analyses with three null models for testing the significance of nestedness; (ii) Spearman rank correlation to examine the correlates of nestedness; (iii) outlying mean index analysis to analyse the niche characteristics of species; (iv) and t‐test to examine differences in niche breadth, niche position and site occupancy of idiosyncratic and other nested species. Results Stream insect assemblages were significantly nested in each of the three study years. The maximally packed matrices were significantly nested according to the nestedness calculator based on null models I (species frequencies and site richness equiprobable) and II (species frequencies fixed and site richness equiprobable), but non‐significant based on a conservative null model III (species frequencies and site richness fixed to those of the observed matrix). The most important correlate of nestedness was stream size, whereas isolation, productivity (total phosphorus) and habitat heterogeneity exhibited non‐significant relationship with nestedness. Idiosyncratic species occurred, on average, at more sites than nested species, mirroring the restricted distributions of several nested species that were inclined towards species‐rich sites. Idiosyncratic and nested species also differed in niche position and niche breadth, with idiosyncratic species having, on average, less marginal niche positions and wider niches than nested species. Main conclusions Stream size correlated with nestedness, possibly because small streams were inhabited only by species able to persist under, or colonize shortly after, disturbances, while most species could occur at larger sites where disturbances are less severe. From the conservation perspective, our findings suggest that stream size really matters, given that sites with high species richness and many rare species are more likely to occur in larger streams. However, also the requirements of idiosyncratic species should be accommodated in conservation planning.     

Young-of-the-year fish assemblages as indicators of anthropogenic disturbances in large tributaries of the Seine River Basin (France)

A variety of indices targeting a number of different biological assemblages have been developed to assess aquatic ecosystem condition, identify the drivers of alteration and provide information on possible restoration measures. Fish-based indices, commonly focused on adult assemblages, are capable of indicating declines in condition, but often provide limited information on the ultimate causes for the observed changes. Using young-of-the-year (YOY) fish assemblages has been suggested as a means of improving the sensitivity of fish-based indicators. To verify this assumption, we first model, using boosted regression trees, the occurrence of 16 YOY fish species as a function of twenty two environmental factors in 227 reference (i.e., least disturbed) habitats located in the river Seine (France) and its two main tributaries (i.e., Oise and Marne rivers). We then validate the species models using an independent dataset of 74 reference habitats. Finally, using three independent data sets reflecting different categories of disturbances (i.e., physico-chemical disturbances, physical disturbances induced by navigation and a mix of both disturbances), we measure the deviation between expected and observed species occurrences to evaluate our species models ability in detecting these disturbances. The models are ecologically meaningful and overall perform well in discriminating between reference and disturbed habitats, showing a stable response for all unimpaired habitats and highlighting the main habitat disturbances tested.     

Modelling chorotypes of invasive vertebrates in mainland Spain

We investigated the existence of chorotypes – assemblages of species with similar geographical ranges – of invasive species in a host territory, and their potential use to advocate similar control or management strategies for species in the same chorotype. We analysed the distribution of 13 exotic terrestrial vertebrate species (six birds, six mammals, and one reptile) with well‐known distributions in mainland Spain. We used the presence/absence data on a grid of 10 km × 10 km UTM cells from the Atlases of terrestrial vertebrates of Spain. These data were aggregated to a grid of 50 km × 50 km UTM cells, because it entailed no loss of meaningful information and allowed dealing with a much lower number of cells. Using cluster analysis and a probabilistic assessment of the classification, we identified seven significant chorotypes: four multispecific and three monospecific. The compound chorotypes grouped together species that tended to share certain characteristics about their introduction, release cause, establishment, and spread. We modelled the chorotypes using a favourability function based on a generalized linear model and 31 variables related to spatial situation, topography, lithology, climatic stability, energy availability, water availability, disturbances, productivity, and human activity. Climatic factors affected the favourability for every chorotype, whereas human variables had a high influence in the distribution of three chorotypes involving eight species. On the basis of these variables, we identified favourable areas for all the chorotypes in mainland Spain. The favourability for a chorotype in an area may be a useful criterion for evaluating the local conservation concern due to the whole set of species. Favourable but unoccupied areas can be used to infer possible colonization areas for each chorotype. We recommend using chorotypes to optimize broad‐scale surveillance of invasive species.     

Does an ant-dispersed plant,Viola reichenbachiana,suffer from reduced seed dispersal under inundation disturbances?

Many plant species use ants as seed dispersers. This dispersal mode is considered to be susceptible to disturbances, but the effect of natural, small-scale disturbances is still unknown. We investigated how small-scale disturbances due to inundation affect seed dispersal in Viola reichenbachiana, a dominant myrmecochorous herb in riparian forests. Inundation disturbances were high in depressions and low on hillocks of the forest floor. We found that V. reichenbachiana was similarly abundant at highly and less disturbed sites, contrary to other, non ant-dispersed species. We also found that the motivation of ants to disperse seeds was higher at highly disturbed sites. Nevertheless, the number of seeds dispersed was similar at highly disturbed and weakly disturbed sites and seedlings of V. reichenbachiana were equally abundant. We conclude that inundation disturbances do not interfere with mutualistic seed dispersal by ants in V. reichenbachiana, and suggest that this may possibly contribute to the persistence of V. reichenbachiana under inundation.     

Influence of small-scale disturbances by kangaroo rats on Chihuahuan Desert ants

Banner-tailed kangaroo rats (Dipodomys spectabilis) are prominent ecosystem engineers that build large mounds that influence the spatial structuring of fungi, plants, and some ground-dwelling animals. Ants are diverse and functionally important components of arid ecosystems; some species are also ecosystem engineers. We investigated the effects of patch disturbances created by D. spectabilis mounds on ant assemblages in a Chihuahuan Desert grassland in southern New Mexico by using pitfall traps in a paired design (mound vs. matrix). Although the disturbances did not alter species richness or harbor unique ant communities relative to the matrix, they did alter species composition; the abundances of 6 of 26 species were affected. The disturbances might also act to disrupt spatial patterning of ants caused by other environmental gradients. In contrast to previous investigations of larger-scale disturbances, we detected no effects of the disturbances on ants at the functional-group level. Whether ant communities respond to disturbance at a functional-group or within-functional-group level may depend on the size and intensity of the disturbance. Useful functional-group schemes also may be scale-dependent, however, or species may respond idiosyncratically. Interactions between disturbance-generating mammals and ants may produce a nested spatial structure of patches. Received: 11 October 1999 / Accepted: 11 March 2000