Species richness destabilizes ecosystem functioning in experimental aquatic microcosms

The relationship between species diversity and ecosystem stability has long interested ecologists, yet no consensus has been reached and the underlying mechanisms remain unclear. We used five unicellular algal species, cultured in all possible combinations, to assemble microcosms containing 1 to 5 algal species, on which a cold perturbation was imposed. Our aim was to find whether and how species richness begets ecosystem resistance and resilience. In the experiment, the species-rich communities produced more biomass than the species-poor ones, either in pre-, under- or post-perturbation conditions. The positive diversity–biomass relationship was weakened by the perturbation, and fully restored one week after the perturbation. The diverse communities showed greater absolute biomass reduction during the perturbation than did species-poor systems. Resistance of community, measured by the relative change in biomass from pre- to under-perturbation, decreased with species richness. All the species showed significant reduction in biomass when stressed, without any density compensation among species in diverse communities; and the ratio of biomass change in each species was independent of diversity. The overyielding effect, measured as relative yield total, remained constant from pre- to under-perturbation; and the selection and complementarity effects played equal roles for the biodiversity effect on biomass production, and their relative importance was not altered by the perturbation. These results suggest that similar responses of different species to environmental perturbations may limit the insurance effect of biodiversity, and lead to an inverse diversity–resistance relationship.     

Species richness in a model with resource gradient

In order to study the dependence of the species richness on heterogeneity of the habitat, we introduce an extended model of annual plants which combines the features of the island model and of gradient heterogeneity resources. First, we consider a native population of plants living on a square lattice of linear size L. After equilibration of this native population, seeds of several different species j = 2, ... , k of annual plants invade the system; they compete among themselves and the native ones. The system is exposed to a one-dimensional water gradient, and each species is characterised by a tolerance to a surplus of water, τ(j). We study the influences of the properties of the gradient of the resource (GR) on the species richness (SR) present in the system. We have shown that the relation between GR and SR is not straightforward and that several cases could be distinguished: For a large class of control parameters, SR increases linearly with GR. However, when the values of the control parameters are such as to create wide inhabitable regions, the relation between SR and GR ceases to have a monotonic character. We have also demonstrated that the average species richness as a function of the resource availability has a hump shape. Our results can be simply explained within our model and are in agreement with several previous field and theoretical works.     

Testing effects of consumer richness, evenness and body size on ecosystem functioning

1.?Numerous studies have revealed (usually positive) relationships between biodiversity and ecosystem functioning (B-EF), but the underpinning drivers are rarely addressed explicitly, hindering the development of a more predictive understanding. 2.?We developed a suite of statistical models (where we combined existing models with novel ones) to test for richness and evenness effects on detrital processing in freshwater microcosms. Instead of using consumer species as biodiversity units, we used two size classes within three species (six types). This allowed us to test for diversity effects and also to focus on the role of body size and biomass. 3.?Our statistical models tested for (i) whether performance in polyculture was more than the sum of its parts (non-additive effects), (ii) the effects of specific type combinations (assemblage identity effects) and (iii) whether types behaved differently when their absolute or relative abundances were altered (e.g. because type abundance in polyculture was lower compared with monoculture). The latter point meant we did not need additional density treatments. 4.?Process rates were independent of richness and evenness and all types performed in an additive fashion. The performance of a type was mainly driven by the consumers' metabolic requirements (connected to body size). On an assemblage level, biomass explained a large proportion of detrital processing rates. 5.?We conclude that B-EF studies would benefit from widening their statistical approaches. Further, they need to consider biomass of species assemblages and whether biomass is comprised of small or large individuals, because even if all species are present in the same biomass, small species (or individuals) will perform better.     

Species richness facilitates ecosystem resilience in aquatic food webs

1. Many studies indicate that biodiversity in ecosystems affects stability, either by promoting temporal stability of ecosystem attributes or by enhancing ecosystem resistance and resilience to perturbation. The effects on temporal stability are reasonably well understood and documented but effects on resistance and resilience are not. 2. Here, we report results from an aquatic mesocosm experiment in which we manipulated the species richness and composition of aquatic food webs (macrophytes, macro‐herbivores and invertebrate predators), imposed a pulse disturbance (acidification), and monitored the resistance (initial response) and resilience (recovery) of ecosystem productivity and respiration. 3. We found that species‐rich macroinvertebrate communities had higher resilience of whole‐ecosystem respiration, but were not more resistant to perturbations. We also found that resilience and resistance were unaffected by species composition, despite the strong role composition is known to play in determining mean levels of function in these communities. 4. Biodiversity’s effects on resilience were probably mediated through complex pathways affecting phytoplankton and microbial communities (e.g. via changes in nutrient regeneration, grazing or compositional changes) rather than through simpler effects (e.g. insurance effects, enhanced facilitation) although these simpler mechanisms probably played minor roles in enhancing respiration resilience. 5. Current mechanisms for understanding biodiversity’s effects on ecosystem stability have been developed primarily in the context of single‐trophic level communities. These mechanisms may be overly simplistic for understanding the consequences of species richness on ecosystem stability in complex, multi‐trophic food webs where additional factors such as indirect effects and highly variable life‐history traits of species may also be important.     

Effects of trophic skewing of species richness on ecosystem functioning in a diverse marine community

Widespread overharvesting of top consumers of the world's ecosystems has "skewed" food webs, in terms of biomass and species richness, towards a generally greater domination at lower trophic levels. This skewing is exacerbated in locations where exotic species are predominantly low-trophic level consumers such as benthic macrophytes, detritivores, and filter feeders. However, in some systems where numerous exotic predators have been added, sometimes purposefully as in many freshwater systems, food webs are skewed in the opposite direction toward consumer dominance. Little is known about how such modifications to food web topology, e.g., changes in the ratio of predator to prey species richness, affect ecosystem functioning. We experimentally measured the effects of trophic skew on production in an estuarine food web by manipulating ratios of species richness across three trophic levels in experimental mesocosms. After 24 days, increasing macroalgal richness promoted both plant biomass and grazer abundance, although the positive effect on plant biomass disappeared in the presence of grazers. The strongest trophic cascade on the experimentally stocked macroalgae emerged in communities with a greater ratio of prey to predator richness (bottom-rich food webs), while stronger cascades on the accumulation of naturally colonizing algae (primarily microalgae with some early successional macroalgae that recruited and grew in the mesocosms) generally emerged in communities with greater predator to prey richness (the more top-rich food webs). These results suggest that trophic skewing of species richness and overall changes in food web topology can influence marine community structure and food web dynamics in complex ways, emphasizing the need for multitrophic approaches to understand the consequences of marine extinctions and invasions.     

Negative effects of habitat drying and prior exploitation on the detritus resource in an ephemeral aquatic habitat

Ephemeral aquatic habitats are characterized by cycles of drying and subsequent inundation, and by production of sequential non-overlapping cohorts of organisms. Both processes may alter the quantity or quality of resources, and may therefore affect survival and development of cohorts that subsequently colonize ephemeral habitats. We examined these effects of habitat drying and non-overlapping cohorts on experimental cohorts of the tree hole mosquito Aedes triseriatus, testing specifically whether the value of leaf litter as a food resource is altered by cycles of inundation and drying, or by exploitation by a prior non-overlapping cohort. We created four treatments of leaf litter: (1) no prior cohort, continuously wet; (2) no prior cohort, one␣wet/dry cycle; (3) prior cohort, continuously wet, and (4) prior cohort, one wet/dry cycle, and tested for effects on individual fitness components (survivorship, mean dry mass at, and median days to eclosion) and on population growth (estimated finite rate of increase –λ′). Both resource drying and the presence of a prior cohort negatively affected individual fitness components in tires, increasing days to eclosion, and decreasing mean dry mass at eclosion for both sexes. Resource drying also negatively affected estimated rates of increase (λ′) in tree holes. A prior cohort had no significant effects on λ′. These results indicate that intraspecific interactions among mosquito larvae may include amensalistic effects of earlier, non-overlapping cohorts, and that resource drying reduces resource quality. The latter effect indicates that enhanced production of A. triseriatus from recently filled containers is not due to resource drying per se, and may result from more complex community-level effects of habitat drying. Extreme cycles of drying and inundation seem likely to increase intraspecific resource competition among drought-adapted species like A. triseriatus. Received: 5 May 1997 / Accepted: 20 January 1998     

Species richness and trophic diversity increase decomposition in a co-evolved food web

Ecological communities show great variation in species richness, composition and food web structure across similar and diverse ecosystems. Knowledge of how this biodiversity relates to ecosystem functioning is important for understanding the maintenance of diversity and the potential effects of species losses and gains on ecosystems. While research often focuses on how variation in species richness influences ecosystem processes, assessing species richness in a food web context can provide further insight into the relationship between diversity and ecosystem functioning and elucidate potential mechanisms underpinning this relationship. Here, we assessed how species richness and trophic diversity affect decomposition rates in a complete aquatic food web: the five trophic level web that occurs within water-filled leaves of the northern pitcher plant, Sarracenia purpurea. We identified a trophic cascade in which top-predators--larvae of the pitcher-plant mosquito--indirectly increased bacterial decomposition by preying on bactivorous protozoa. Our data also revealed a facultative relationship in which larvae of the pitcher-plant midge increased bacterial decomposition by shredding detritus. These important interactions occur only in food webs with high trophic diversity, which in turn only occur in food webs with high species richness. We show that species richness and trophic diversity underlie strong linkages between food web structure and dynamics that influence ecosystem functioning. The importance of trophic diversity and species interactions in determining how biodiversity relates to ecosystem functioning suggests that simply focusing on species richness does not give a complete picture as to how ecosystems may change with the loss or gain of species.     

Legacy effects of canopy disturbance on ecosystem functioning in macroalgal assemblages

Macroalgal assemblages are some of the most productive systems on earth and they contribute significantly to nearshore ecosystems. Globally, macroalgal assemblages are increasingly threatened by anthropogenic activities such as sedimentation, eutrophication and climate change. Despite this, very little research has considered the potential effects of canopy loss on primary productivity, although the literature is rich with evidence showing the ecological effects of canopy disturbance. In this study we used experimental removal plots of habitat-dominating algae (Order Fucales) that had been initiated several years previously to construct a chronosequence of disturbed macroalgal communities and to test if there were legacy effects of canopy loss on primary productivity. We used in situ photo-respirometry to test the primary productivity of algal assemblages in control and removal plots at two intertidal elevations. In the mid tidal zone assemblage, the removal plots at two sites had average primary productivity values of only 40% and 60% that of control areas after 90 months. Differences in productivity were associated with lower biomass and density of the fucoid algal canopy and lower taxa richness in the removal plots after 90 months. Low-shore plots, established three years earlier, showed that the loss of the large, dominant fucoid resulted in at least 50% less primary productivity of the algal assemblage than controls, which lasted for 90 months; other smaller fucoid species had recruited but they were far less productive. The long term reduction in primary productivity following a single episode of canopy loss of a dominant species in two tidal zones suggests that these assemblages are not very resilient to large perturbations. Decreased production output may have severe and long-lasting consequences on the surrounding communities and has the potential to alter nutrient cycling in the wider nearshore environment.     

Placing biodiversity and ecosystem functioning in context: environmental perturbations and the effects of species richness in a stream field experiment

Greater biodiversity is often associated with increased ecosystem process rates, and is expected to enhance the stability of ecosystem functioning under abiotic stress. However, these relationships might themselves be altered by environmental factors, complicating prediction of the effects of species loss in ecosystems subjected to abiotic stress. In boreal streams, we investigated effects of biodiversity and two abiotic perturbations on three related indices of ecosystem functioning: leaf decomposition, detritivore leaf processing efficiency (LPE) and detritivore growth. Replicate field enclosures containing leaves and detritivore assemblages were exposed to liming and nutrient enrichment, raising pH and nutrient levels. Both treatments constitute perturbations for our naturally acidic and nutrient-poor streams. We also varied detritivore species richness and density. The effects of the abiotic and diversity manipulations were similar in magnitude, but whereas leaf decomposition increased by 18% and 8% following liming and nutrient enrichment, respectively, increased detritivore richness reduced leaf decomposition (6%), detritivore LPE (19%) and detritivore growth (12%). The detritivore richness effect on growth was associated with negative trait-independent complementarity, indicating interspecific interference competition. These interactions were apparently alleviated in both enriched and limed enclosures, as trait-independent complementarity became less negative. LPE increased with detritivore density in the monocultures, indicating benefits of intra-specific aggregation that outweighed the costs of intra-specific competition, and dilution of these benefits probably contributed to lowered leaf decomposition in the species mixtures. Finally, the effects of liming were reduced in most species mixtures relative to the monocultures. These results demonstrate how environmental changes might regulate the consequences of species loss for functioning in anthropogenically perturbed ecosystems, and highlight potential influences of biodiversity on functional stability. Additionally, the negative effects of richness and positive effects of density in our field study were opposite to previous laboratory observations, further illustrating the importance of environmental context for biodiversity–ecosystem functioning relationships.     

Biodiversity effects on ecosystem functioning: insights from aquatic systems

Unprecedented rates of species extinctions have prompted extensive research into the consequences of biodiversity losses on ecosystem functioning. While aquatic species are most threatened, research with freshwater and marine model systems has lagged behind progress made in terrestrial environments. This editorial to a special feature summarizes the main outcomes of a conference aimed at setting the stage for exploring the potential of aquatic systems to assess the role of biodiversity in ecosystem functioning. This series of papers proposes fresh approaches to the study of biodiversity effects on ecosystem functioning, outlines a new way of analyzing experimental data, presents a model that considers scale as an important factor determining outcomes, explores the effects of multiple stressors on species richness and ecosystem processes, and develops a food-web perspective that relates ecosystem properties to biodiversity. An insightful synthesis of lessons learned from aquatic systems is premature at present, but the papers clearly demonstrate the role that marine and freshwater systems can play in resolving open questions. The implications go well beyond the biodiversity in, and functioning of, ecosystems shaped by free-flowing or standing water.     

Shrubs as ecosystem engineers across an environmental gradient: effects on species richness and exotic plant invasion

Ecosystem-engineering plants modify the physical environment and can increase species diversity and exotic species invasion. At the individual level, the effects of ecosystem engineers on other plants often become more positive in stressful environments. In this study, we investigated whether the community-level effects of ecosystem engineers also become stronger in more stressful environments. Using comparative and experimental approaches, we assessed the ability of a native shrub (Ericameria ericoides) to act as an ecosystem engineer across a stress gradient in a coastal dune in northern California, USA. We found increased coarse organic matter and lower wind speeds within shrub patches. Growth of a dominant invasive grass (Bromus diandrus) was facilitated both by aboveground shrub biomass and by growing in soil taken from shrub patches. Experimental removal of shrubs negatively affected species most associated with shrubs and positively affected species most often found outside of shrubs. Counter to the stress-gradient hypothesis, the effects of shrubs on the physical environment and individual plant growth did not increase across the established stress gradient at this site. At the community level, shrub patches increased beta diversity, and contained greater rarified richness and exotic plant cover than shrub-free patches. Shrub effects on rarified richness increased with environmental stress, but effects on exotic cover and beta diversity did not. Our study provides evidence for the community-level effects of shrubs as ecosystem engineers in this system, but shows that these effects do not necessarily become stronger in more stressful environments.     

Agricultural resource use in an andean coastal ecosystem

Five concurrent systems of agricultural resource management in the Viru Valley in Peru's arid northern coastal plain are discussed as adjustments to microenvironmental variations in soil humidity. Widespread dependence on canal irrigation in an environment characterized by uncertainty in the availability of river water affects the agrarian population in several ways. The upper socioeconomic class has adapted to uncertainty by implementing a deviation-counteracting mechanism (tubular wells) that provides water on demand, giving them flexibility in choice of agricultural activities. Another class of farmers is unable to introduce this mechanism, however, and consequently must depend on a repertoire of inflexible decisions to cope with uncertainty. Each group exploits different opportunity costs to increase economic gain. One pattern provides for expansion, whereas the other at best establishes stability and maintenance. Noncanal techniques permit expansion of cultivation in conditions where canal irrigation is not feasible, thereby improving the overall level of effectiveness of resource use. Alternative techniques do not involve regulatory mechanisms nor do they require complex, interlocking social, economic, and political components. Their presence and persistence in the agricultural system provide variation that may ultimately be amplified as the need to intensify resource exploitation increases in the future.     

Intraspecific traits change biodiversity effects on ecosystem functioning under metal stress

Studies investigating the impacts of biodiversity loss on ecosystem processes have often reached different conclusions, probably because insufficient attention has been paid to some aspects including (1) which biodiversity measure (e.g., species number, species identity or trait) better explains ecosystem functioning, (2) the mechanisms underpinning biodiversity effects, and (3) how can environmental context modulates biodiversity effects. Here, we investigated how species number (one to three species) and traits of aquatic fungal decomposers (by replacement of a functional type from an unpolluted site by another from a metal-polluted site) affect fungal production (biomass acumulation) and plant litter decomposition in the presence and absence of metal stress. To examine the putative mechanisms that explain biodiversity effects, we determined the contribution of each fungal species to the total biomass produced in multicultures by real-time PCR. In the absence of metal, positive diversity effects were observed for fungal production and leaf decomposition as a result of species complementarity. Metal stress decreased diversity effects on leaf decomposition in assemblages containing the functional type from the unpolluted site, probably due to competitive interactions between fungi. However, dominance effect maintained positive diversity effects under metal stress in assemblages containing the functional type from the metal-polluted site. These findings emphasize the importance of intraspecific diversity in modulating diversity effects under metal stress, providing evidence that trait-based diversity measures should be incorporated when examining biodiversity effects.     

Productivity and species richness in an arid ecosystem: a long-term perspective

To examine the different effects of rain pulse size on uptake of summer rains by three dominant desert plants in field conditions of desertified grasslands on the Ordos Plateau of northwestern China, we studied relationships between precipitation event size and rainwater uptake using stable isotopes of hydrogen in plant and soil water. Four natural precipitation events that represented precipitation sizes of 5.3, 8.3, 13.3, and 65.3 mm in the summer were chosen for the experiment. The perennial grass Stipa bungeana, the shrub Artemisia ordosia, and the herb Cynanchum komarovii – the dominant species in the communities – were compared for their use of summer rains with different pulse sizes based on the changes in the hydrogen isotope ratios (δD) of their stem water 7 days following each natural rain event. We found that S. bungeana and C. komarovii took advantage of shallow water sources derived from small (< 10 mm) rain events, A. ordosia took advantage of deeper soil water recharged by large (> 65 mm) rain events, and C. komarovii relied primarily on rain events of intermediate (10–20 mm) size. These different responses to rain pulses among species suggested that more frequent small rain events will promote the dominance of S. bungeana and C. komarovii, medium-sized events will facilitate development of C. komarovii, and large events will advance A. ordosia in this community. The rainwater utilization patterns of the three species would allow the coexistence of S. bungeana and A. ordosia or the coexistence of A. ordosia and C. komorovii in various successional serals following the disturbances. With an increase in variability of summer rain pulse size as predicted by climate change models, we expect that the structure of this community will undergo significant change in the future. Altered precipitation regimes, especially in combination with anthropogenic-related disturbances such as over-grazing, are likely to accelerate rates of degradation in northwestern China.     

Prey diversity effects on ecosystem functioning depend on consumer identity and prey composition

For migratory species, acquisition and allocation of energy after arrival on the breeding grounds largely determine reproductive decisions. Few studies have investigated underlying physiological mechanisms driving variation in breeding phenology so far. We linked physiological state to individual timing of breeding in pre-laying arctic-nesting female peregrine falcons (Falco peregrinus tundrius). We captured females from two populations 2–20 days before egg-laying to assess plasma concentration of β-hydroxybutyric acid (BUTY) and triglyceride (TRIG), two metabolites known to reflect short-term changes in fasting and fattening rate, respectively. We also assessed baseline corticosterone (CORTb), a hormone that mediates energy allocation, and the scaled mass index (SMI) as an indicator of somatic body reserves. Plasma BUTY was slightly higher during the pre-recruiting period compared to the period of rapid follicular growth, indicating a reduction in catabolism of lipid reserves before investment in follicle development. Conversely, TRIG levels increased in pre-recruiting females, and best-predicted individual variation in pre-laying interval and lay date. A marked increase in CORTb occurred concomitantly with the onset of rapid follicle growth. SMI was highly variable possibly reflecting variation in food availability or individuals at different stages. Results suggest that (1) lower rates of pre-laying fattening and/or lower mobilization rate of lipoproteins to ovarian follicles delayed laying, and (2) an elevation in pre-laying CORTb may result from, or be required to compensate for, the energetic costs of egg production. Results of this study illustrate how variation in the allocation of energy before laying can influence individual fitness-related reproductive decisions.     

Climate change impacts on ecosystem functioning: evidence from an Empetrum heathland

? The extent to which plants exert an influence over ecosystem processes, such as nitrogen cycling and fire regimes, is still largely unknown. It is also unclear how such processes may be dependent on the prevailing environmental conditions. ? Here, we applied mechanistic models of plant-environment interactions to palaeoecological time series data to determine the most likely functional relationships of Empetrum (crowberry) and Betula (birch) with millennial-scale changes in climate, fire activity, nitrogen cycling and herbivore density in an Irish heathland. ? Herbivory and fire activity preferentially removed Betula from the landscape. Empetrum had a positive feedback on fire activity, but the effect of Betula was slightly negative. Nitrogen cycling was not strongly controlled by plant population dynamics. Betula had a greater temperature-dependent population growth rate than Empetrum; thus climate warming promoted Betula expansion into the heathland and this led to reduced fire activity and greater herbivory, which further reinforced Betula dominance. ? Differences in population growth response to warming were responsible for an observed shift to an alternative community state with contrasting forms of ecosystem functioning. Self-reinforcing feedback mechanisms--which often protect plant communities from invasion--may therefore be sensitive to climate warming, particularly in arctic regions that are dominated by cold-adapted plant populations.     

A general multi-trait-based framework for studying the effects of biodiversity on ecosystem functioning

Environmental change is as multifaceted as are the species and communities that respond to these changes. Current theoretical approaches to modeling ecosystem response to environmental change often deal only with single environmental drivers or single species traits, simple ecological interactions, and/or steady states, leading to concern about how accurately these approaches will capture future responses to environmental change in real biological systems. To begin addressing this issue, we generalize a previous trait-based framework to incorporate aspects of frequency dependence, functional complementarity, and the dynamics of systems composed of species that are defined by multiple traits that are tied to multiple environmental drivers. The framework is particularly well suited for analyzing the role of temporal environmental fluctuations in maintaining trait variability and the resultant effects on community response to environmental change. Using this framework, we construct simple models to investigate two ecological problems. First, we show how complementary resource use can significantly enhance the nutrient uptake of plant communities through two different mechanisms related to increased productivity (over-yielding) and larger trait variability. Over-yielding is a hallmark of complementarity and increases the total biomass of the community and, thus, the total rate at which nutrients are consumed. Trait variability also increases due to the lower levels of competition associated with complementarity, thus speeding up the rate at which more efficient species emerge as conditions change. Second, we study systems in which multiple environmental drivers act on species defined by multiple, correlated traits. We show that correlations in these systems can increase trait variability within the community and again lead to faster responses to environmental change. The methodological advances provided here will apply to almost any function that relates species traits and environmental drivers to growth, and should prove useful for studying the effects of climate change on the dynamics of biota.