Biodiversity and environmental stability

Although levels of biological diversity may seem to be equivalent in different areas, diversity is created and maintained by a range of different ]processes: overlap of habitat on gradients; a dynamic mosaic of communities; and accumulation and evolution of taxa in extremely stable areas. These different communities will respond in very different ways to disturbance. The most fragile are those whose component taxa are genetically adapted to the stability of a predictable environment. These areas are often under pressure from local rural populations and require intensive local conservation management actions. In other areas, where diversity is adapted to dynamism, communities are more resilient to disturbance and conservation can be best effected by policy instruments.     

Phase shifts and the stability of macroalgal communities on Caribbean coral reefs

Caribbean coral reefs are widely thought to exhibit two alternate stable states with one being dominated by coral and the other by macroalgae. However, the observation of linear empirical relationships among grazing, algal cover and coral recruitment has led the existence of alternate stable states to be questioned; are reefs simply exhibiting a continuous phase shift in response to grazing or are the alternate states robust to certain changes in grazing? Here, a model of a Caribbean forereef is used to reconcile the existence of two stable community states with common empirical observations. Coral-depauperate and coral-dominated reef states are predicted to be stable on equilibrial time scales of decades to centuries and their emergence depends on the presence or absence of a bottleneck in coral recruitment, which is determined by threshold levels of grazing intensity and other process variables. Under certain physical and biological conditions, corals can be persistently depleted even while increases in grazing reduce macroalgal cover and enhance coral recruitment; only once levels of recruitment becomes sufficient to overwhelm the population bottleneck will the coral-dominated state begin to emerge. Therefore, modest increases in grazing will not necessarily allow coral populations to recover, whereas large increases, such as those associated with recovery of the urchin Diadema antillarum, are likely to exceed threshold levels of grazing intensity and set a trajectory of coral recovery. The postulated existence of alternate stable states is consistent with field observations of linear relationships between grazing, algal cover and coral recruitment when coral cover is low and algal exclusion when coral cover is high. The term ‘macroalgal dominated’ is potentially misleading because the coral-depauperate state can be associated with various levels of macroalgal cover. The term ‘coral depauperate’ is preferable to ‘macroalgal dominated’ when describing alternate states of Caribbean reefs.     

Biodiversity and the productivity and stability of ecosystems

Attempts to unveil the relationships between the taxonomic diversity, productivity and stability of ecosystems continue to generate inconclusive, contradictory and controversial conclusions. New insights from recent studies support the hypothesis that species diversity enhances productivity and stability in some ecosystems, but not in others. Appreciation is growing for the ways that particular ecosystem features, such as environmental variability and nutrient stress, can influence biotic interactions. Alternatives to the diversity-stability hypothesis have been proposed, and experimental approaches are starting to evolve to test these hypotheses and to elucidate the mechanisms underlying the functional role of species diversity.     

Consumer and resource diversity effects on marine macroalgal decomposition

Few manipulative experiments that explicitly test the relationship between biodiversity and ecosystem function have focussed on regenerative processes such as decomposition and nutrient cycling. Of the studies that have taken place, most have concentrated on the effects of leaf litter diversity rather than the effects of consumer diversity on decomposition. In the present study, we established an in‐situ mesocosm experiment on an intertidal mudflat in the Ythan Estuary, Scotland, to investigate the interactive effects of consumer diversity, resource diversity and microbial activity on algal consumption and decomposition. We assembled communities of three commonly occurring macrofaunal species (Hediste diversicolor, Hydrobia ulvae and Littorina littorea) in monoculture and all possible combinations of two and three species mixtures and supplied them with single vs two‐species mixtures of the algae Fucus spiralis and Ulva intestinalis. Further, we also investigated whether algal decomposition changes nutrient remineralisation within the sediment by determining the C:N ratio of the surficial sediment. Data were analysed using extended linear regression with generalized least squares estimation to characterise the variance structure. We found that consumer species diversity effects are best explained by compositional effects and that species richness per se may not be the single most important determinant of resource use and decomposition in this community. Algal identity and invertebrate identity effects underpin the observed response and reflect species‐specific traits associated with algal consumption and processing. The role of the microbial community is comparatively weak, but strongly linked to faunal activities and behaviour. The C:N ratio of the sediment increased with consumer species richness, indicating increased mineralisation in more diverse communities. Overall, our results suggest that although consumer species richness effects per se are weak, decomposition and subsequent incorporation of resources is nevertheless dependent on the composition of the decomposer community, which, in turn, has important implications for biogeochemical nutrient cycling in marine coastal habitats.     

Grazing effects in macroalgal communities depend on timing of patch colonization

Rocky macroalgal assemblages are typically composed of patches differing in age and species composition and grazing is generally a very important modifier of such assemblages. We hypothesized that patch colonization time determines its algal community and that grazing effects depend on the colonization time and vary with depth. We created patches by placing empty substrates at two sublittoral depths over five consecutive months, manipulated grazer entry and determined the algal species composition in each patch in the next growing season. Distinct algal colonization periods resulted in different algal assemblages. Although algal communities in our study area consist mainly of opportunistic species, thus being highly dynamic, the resulting macroalgal assemblages differed in species richness, diversity, composition, and total biomass even a year after first colonization. Substrates close to the water-surface supported a higher species richness and diversity than those in the deeper littoral. The community characteristics, total density, total biomass and species richness were only slightly, if at all affected by grazing. However, individual algal species or taxa showed varying and even contrasting responses to grazing, often differently between depths and depending on colonization time. In the deeper littoral, but not close to the water surface, grazing increased the density of filamentous brown algae while reducing the green alga Cladophora glomerata. In these taxa, grazing effects were strongest in patches colonized during the early growing season. Grazing at the colonization stage had lasting consequences for the density of several individual species.     

Biodiversity, productivity and stability in real food webs

The global biodiversity crisis has motivated new theory and experiments that explore relationships between biodiversity (species richness and composition in particular), productivity and stability. Here we emphasize that these relationships are often bi-directional, such that changes in biodiversity can be both a cause and a consequence of changes in productivity and stability. We hypothesize that this bi-directionality creates feedback loops, as well as indirect effects, that influence the complex responses of communities to biodiversity losses. Important, but often neglected, mediators of this complexity are trophic interactions. Recent work shows that consumers can modify, dampen or even reverse the directionality of biodiversity-productivity-stability linkages inferred from the plant level alone. Such consumer mediation is likely to be common in many ecosystems. We suggest that merging biodiversity research and food-web theory is an exciting and pressing frontier for ecology, with implications for biodiversity conservation.     

The roles of environmental conditions and spatial factors in controlling stream macroalgal communities

In the last three decades, several studies have suggested that the structure of stream macroalgal communities is shaped by local environmental variables, but some recent papers have shown that the relevance of the environment on these communities may be overestimated. Using Partial Redundancy Analysis (pRDA), we analyzed macroalgal communities (considering all macroalgae and Phyla Chlorophyta, Cyanobacteria, and Rhodophyta individually) from 105 streams in southern Brazil to test the hypothesis that the relative contributions of the environment and space on the taxonomic composition of these communities is mainly determined by the biological traits and dispersal mechanisms typical for each group. The pRDA showed that the taxonomic composition of the entire community and green algae were explained by both space and environment, whereas for cyanobacteria, only the environment was significant, and for red algae, only space was significant. These divergences in the relative contribution among algal phyla were consistent with our initial hypothesis and can be ascribed to the differences in the ecological features of each group. Our results also support the idea that the community structure of organisms with low dispersal is influenced more significantly by spatial processes, whereas for organisms with high dispersal the local environmental variables are more influential.     

Re-structuring of marine communities exposed to environmental change: a global study on the interactive effects of species and functional richness

Species richness is the most commonly used but controversial biodiversity metric in studies on aspects of community stability such as structural composition or productivity. The apparent ambiguity of theoretical and experimental findings may in part be due to experimental shortcomings and/or heterogeneity of scales and methods in earlier studies. This has led to an urgent call for improved and more realistic experiments. In a series of experiments replicated at a global scale we translocated several hundred marine hard bottom communities to new environments simulating a rapid but moderate environmental change. Subsequently, we measured their rate of compositional change (re-structuring) which in the great majority of cases represented a compositional convergence towards local communities. Re-structuring is driven by mortality of community components (original species) and establishment of new species in the changed environmental context. The rate of this re-structuring was then related to various system properties. We show that availability of free substratum relates negatively while taxon richness relates positively to structural persistence (i.e., no or slow re-structuring). Thus, when faced with environmental change, taxon-rich communities retain their original composition longer than taxon-poor communities. The effect of taxon richness, however, interacts with another aspect of diversity, functional richness. Indeed, taxon richness relates positively to persistence in functionally depauperate communities, but not in functionally diverse communities. The interaction between taxonomic and functional diversity with regard to the behaviour of communities exposed to environmental stress may help understand some of the seemingly contrasting findings of past research.     

The effects of macroalgal cover on the spatial distribution of macrobenthic invertebrates: the effect of macroalgal morphology

The growth of green macro-algae in response to nutrient inputs is a common phenomenon in marine estuaries and sheltered bays. While the ecological effects of the growth of the most commonly occurring macroalgal taxa (Enteromorpha, Chaetomorpha, Ulva, Cladophora) have been well studied, the effects of a morphologically very different species, Vaucheria subsimplex, have not been investigated. This study investigated the ecological effects of the establishment of V. subsimplex on a relatively exposed intertidal sandflat, Drum Sands, Firth of Forth, Scotland. Because of the spatially heterogeneous development of the weed, the short term (4 weeks) and long term (20 weeks) effects of the weed could be studied using a survey approach in which the weed-affected and weed-free plots were interspersed. After 4 weeks, V. subsimplex significantly increased the mean number of individuals and diversity of the macrofauna, eight of the ten most abundant species showed significant increases in abundance compared to weed-free areas. After 20 weeks, mean number of species and individuals were significantly higher under weed patches, while species diversity was reduced due to the numerical dominance of Pygospio elegans (Claparède). The weed, therefore, had an enriching effect on the macrofaunal communities on Drum Sands. The increased numbers of P. elegans, the numerical dominant infaunal species on Drum Sands, resulted mainly from enhanced larval recruitment to weed-affected areas. The effects of V. subsimplex on sediment characteristics were similar to those reported for other macroalgal taxa, i.e., increased water, organic and silt/clay contents, medium particle size and sorting coefficients, and reduced redox potentials. The results from this study are compared to those for other, morphologically different macroalgal species, with particular reference to an Enteromorpha-implanted experiment on the same sandflat. Since the general effect of such macroalgal taxa on macrofaunal communities is a detrimental one, the present study supports the contention that macroalgal morphology is an important feature in algal–faunal interactions.     

Biodiversity under rocks: the role of microhabitats in structuring invertebrate communities in Brazilian outcrops

There is little knowledge on the factors that can structure communities of tropical arthropods that live under rocks (lapidicolous communities). Using 28 rock shelters in a small granitic outcrop, we examined the relations among area, inclination, organic matter content, substrate moisture and distance from the border of the outcrop and richness and diversity of the communities. In total, we collected 1216 individuals from 106 morphospecies of Araneida, Acarina, Polyxenida, Spirostreptida, Dermaptera, Neuroptera, Orthoptera, Dictyoptera, Psocoptera, Coleoptera, Hymenoptera, Diptera, Embioptera, Homoptera, Isoptera, Heteroptera, Lepidoptera, Collembola, Nematoda and Oligochaeta. The richness and diversity of the lapidicolous communities were significantly correlated with area, inclination and distance from the edge of the outcrop in the dry season. The richness and diversity of lapidicolous communities were significantly correlated with organic matter, substrate moisture and area in the rainy season. Space availability seemed to be an important structuring factor for lapidicolous communities in the present study, the most significant variables (substrate moisture and organic matter) were related to shelter area.     

Fertilization effects on species density and primary productivity in herbaceous plant communities

Fertilization experiments in plant communities are often interpreted in the context of a hump-shaped relationship between species richness and productivity. We analyze results of fertilization experiments from seven terrestrial plant communities representing a productivity gradient (arctic and alpine tundra, two old-field habitats, desert, short- and tall-grass prairie) to determine if the response of species richness to experimentally increased productivity is consistent with the hump-shaped curve. In this analysis, we compared ratios of the mean response in nitrogen-fertilized plots to the mean in control plots for aboveground net primary productivity (ANPP) and species density ( D ; number of species per plot of fixed unit area). In general, ANPP increased and plant species density decreased following nitrogen addition, although considerable variation characterized the magnitude of response. We also analyzed a subset of the data limited to the longest running studies at each site (≥4 yr), and found that adding 9 to 13 g N m⁻² yr⁻¹ (the consistent amount used at all sites) increased ANPP in all communities by approximately 50% over control levels and reduced species density by approximately 30%. The magnitude of response of ANPP and species density to fertilization was independent of initial community productivity. There was as much variation in the magnitude of response among communities within sites as among sites, suggesting community-specific mechanisms of response. Based on these results, we argue that even long-term fertilization experiments are not good predictors of the relationship between species richness and productivity because they are relatively small-scale perturbations whereas the pattern of species richness over natural productivity gradients is influenced by long-term ecological and evolutionary processes.     

Biodiversity characterisation and hydrodynamic consequences of marine fouling communities on marine renewable energy infrastructure in the Orkney Islands Archipelago,Scotland, UK

As part of ongoing commitments to produce electricity from renewable energy sources in Scotland, Orkney waters have been targeted for potential large-scale deployment of wave and tidal energy converting devices. Orkney has a well-developed infrastructure supporting the marine energy industry; recently enhanced by the construction of additional piers. A major concern to marine industries is biofouling on submerged structures, including energy converters and measurement instrumentation. In this study, the marine energy infrastructure and instrumentation were surveyed to characterise the biofouling. Fouling communities varied between deployment habitats; key species were identified allowing recommendations for scheduling device maintenance and preventing spread of invasive organisms. A method to measure the impact of biofouling on hydrodynamic response is described and applied to data from a wave-monitoring buoy deployed at a test site in Orkney. The results are discussed in relation to the accuracy of the measurement resources for power generation. Further applications are suggested for future testing in other scenarios, including tidal energy.     

Functional diversity and climate change: effects on the invasibility of macroalgal assemblages

Climate-driven and biodiversity effects on the structure and functioning of ecosystems are increasingly studied as multiple stressors, which subsequently may influence species invasions. We used a mesocosm experiment to test how increases in temperature and CO₂ partial pressure (pCO₂) interact with functional diversity of resident macroalgal assemblages and affect the invasion success of the non-indigenous macroalga Sargassum muticum. Early settlement of S. muticum germlings was assessed in the laboratory under common environmental conditions across three monocultures and a polyculture of functional groups of native macroalgae, which had previously grown for 3 weeks under crossed treatments of temperature and pCO₂. Functional diversity was a key driver shaping early settlement of the invader, with significant identity and richness effects: higher settlement occurred in low-diversity and low-stature assemblages, even after accounting for treatment biomass. Overall, early survivorship of settled germlings responded to an interaction of temperature and pCO₂ treatments, with survivorship enhanced in one treatment (high pCO₂ at ambient Temperature) after 3 days, and reduced in another (ambient pCO₂ at high Temperature) after 10 days, although size was enhanced in this same treatment. After 6 months in the field, legacy effects of laboratory treatments remained, with S. muticum reaching higher cover in most assemblages previously subjected to ambient pCO₂, but ephemeral green algae appearing disproportionately after elevated-pCO₂ treatment. These results caution that invasion outcomes may change at multiple points in the life cycle under higher-CO₂, higher-temperature conditions, in addition to supporting a role for intact, functionally diverse assemblages in limiting invader colonization.     

The impact of environmental variability and species composition on the stability of experimental microbial populations and communities

Understanding how environmental fluctuations affect the stability of populations and communities is complex, for example, because direct effects of environmental variability on populations may be modified and propagated across communities by species interactions. One way to explore and further understand these complexities is via a factorial manipulation of community composition and environmental conditions. Using laboratory based aquatic microcosms we manipulated environmental fluctuation by creating two environments; one with variable light and one with constant light. Within these environments, community composition was manipulated by constructing communities from all possible combinations of three species that vary in their reliance on light for growth (an autotroph: a diatom completely reliant on light, a heterotroph: a Paramecium species not reliant on light, and a mixotroph: a Paramecium species somewhat reliant on light). Community composition was predicted to affect populations and communities by introducing and altering competitive interactions between species and affecting the degree of niche differentiation between species. We found that population stability was predominantly influenced by an interaction between community composition and environmental variability, whereby the effect of environmental variability synergistically combined with effects of community composition to reduce population stability. Covariance of populations was determined by an interaction between community composition and environmental variability, though this did not result from the effect of niche differentiation between species. Species interactions drove correlations between population biomass and the environment which otherwise did not exist. Our results demonstrate the complex and interrelated effects of abiotic and biotic factors on population and community stability, and suggest the need to consider aspects of community composition when predicting the impact of environmental fluctuations.     

Contrasted effects of diversity and immigration on ecological insurance in marine bacterioplankton communities

The ecological insurance hypothesis predicts a positive effect of species richness on ecosystem functioning in a variable environment. This effect stems from temporal and spatial complementarity among species within metacommunities coupled with optimal levels of dispersal. Despite its importance in the context of global change by human activities, empirical evidence for ecological insurance remains scarce and controversial. Here we use natural aquatic bacterial communities to explore some of the predictions of the spatial and temporal aspects of the ecological insurance hypothesis. Addressing ecological insurance with bacterioplankton is of strong relevance given their central role in fundamental ecosystem processes. Our experimental set up consisted of water and bacterioplankton communities from two contrasting coastal lagoons. In order to mimic environmental fluctuations, the bacterioplankton community from one lagoon was successively transferred between tanks containing water from each of the two lagoons. We manipulated initial bacterial diversity for experimental communities and immigration during the experiment. We found that the abundance and production of bacterioplankton communities was higher and more stable (lower temporal variance) for treatments with high initial bacterial diversity. Immigration was only marginally beneficial to bacterial communities, probably because microbial communities operate at different time scales compared to the frequency of perturbation selected in this study, and of their intrinsic high physiologic plasticity. Such local "physiological insurance" may have a strong significance for the maintenance of bacterial abundance and production in the face of environmental perturbations.     

The effects of long-term fertilization on the temporal stability of alpine meadow communities

Recent theoretical and experimental work suggests that species diversity enhances the temporal stability of communities. However, empirical support largely comes from experimental communities. The relationship between diversity and stability in natural communities, and the ones facing environmental changes in particular, has received less attention. We created a gradient of fertility in a natural alpine meadow community to test the effects of diversity and fertilization on the temporal variability of community cover and cover of component species and to determine the importance of asynchrony, portfolio effects, cover and dominance for diversity-stability relationships. Although fertilization strongly reduced species richness, the temporal stability in community cover increased with fertilization. Most species showed a decline of temporal stability in mean population cover with fertilization, but two grass species, which dominated fertilized communities after 10 years, showed an increase of stability. Detailed analysis revealed that the increased dominance of these two highly stable grass species was associated with increased community stability at high levels of fertilization. In contrast, we found little support for other mechanisms that have been proposed to contribute to community stability, such as changes in asynchrony and portfolio effects. We conclude that the presence of highly productive species that have stabilizing properties dominate fertilized assemblages and enhance ecosystem stability.     

海洋微型底栖生物的多样性与地理分布

海洋微型底栖生物是指生活于海洋沉积物中及表面的所有单细胞原核和真核微型生物,包括原核微生物、真核微藻及原生动物等光合自养和异养的类群.与水体相比,海洋底栖生境孕育了形态和功能多样性更高的微型生物,在陆架浅海单位体积沉积物中其丰度较之水体中同类生物高一至几个数量级,而深海则孕育着特殊进化环境下新奇多样且数量庞大的微型底栖生物,是维持海洋生物多样性、海洋生态系统结构和功能不可或缺的部分.迄今对于微型生物是全球性还是限定性分布一直存在争议,对其解答受到分类研究欠缺及采样不足的制约.分子生物学从正反两方面提供了理论依据,但无法得出一个普遍接受的观点.海洋微型底栖生物的多样性研究侧重于物种多样性及群落结构与分子多样性,较为显著的进展体现在原核微生物的分子多样性及底栖真核微藻的物种多样性研究,对于海洋底栖原生动物的多样性研究则相较滞后.本文综述了国内外对海洋微型底栖生物各主要类群的分类学和多样性研究进展,探讨了各类群在全球的潜在物种多样性,并就我国未来加强海洋微型底栖生物多样性构成、分布与变动及驱动变化的因子以及底栖微食物网的研究提出了建议.     

Resource availability, species composition and sown density effects on productivity of experimental plant communities

Productivity of artificial grassland communities was investigated in a 6-year field experiment on the Qinghai-Tibetan Plateau. In the experiment, assemblages varying in seven species compositions and four density gradients were grown in fertilized and non-fertilized subplots. We measured biomass of sown species as an indicator of community productivity. In general, 6-years of experiments indicated that: (i) species composition had a significant influence on community productivity. During the initial phase of the experiment, sown density significantly affected community productivity, but the effects disappear with the increase of grown years. This productivity increased with biodiversity increase and fertilization, while the biodiversity effects disappeared when the influence of composition was removed. (ii) The increase of community productivity with biodiversity was resulted from joint effects of selection and complementarity. (iii) With an increase of growth time, the selection effects become weaker while complementarities become enhanced. Influence of density on both effects was significantly different in early stages, but ultimately this all became insignificant. Fertilization dramatically increased the complementarity effects in all experiment processes, but had different influences on selection effects during different experimental period.