Realistic Species Loss Research Articles

Why are plant communities stable? Disentangling the role of dominance, asynchrony and averaging effect following realistic species loss scenario

Abstract A growing number of studies have demonstrated that biodiversity is a strong and positive predictor of ecosystem temporal stability by simultaneously affecting multiple underlying mechanisms of stability, that is dominance, asynchrony and averaging effects. However, to date, no study has disentangled the relative role of these key mechanisms of stability in biodiversity experiments. We created a species richness gradient by mimicking a loss of rare species and assessed the role of species richness on community stability and, more importantly, quantified the relative role of three stabilizing mechanisms, that is dominance (stabilization due to stable dominants compared to the rest of the species in the community), asynchrony (stabilization due to temporal asynchrony between species), and averaging effects (pure effect of diversity) on community stability across a species richness gradient. We found that extreme species loss negatively impacted community stability, but just three species were enough to stabilize biomass production to a level similar to highly diverse communities. However, the similar stability of communities resulted from differing contributions from each stabilizing mechanism, depending on the community diversity. Since less abundant species were more temporally variable, species loss stabilized the populations of the remaining species. The loss of rare and subordinate species reduced the dominance and averaging effects, but increased the asynchrony effect. Hence, the asynchrony effect played a major role in the stability of species poor communities, while the averaging effect drove most of the stability of species rich communities. Overall, dominance played only a minor role, accounting for 5%–15% of the stabilization, while asynchrony and averaging effects were dominating forces contributing to ~85%–95% of the total stabilization. Synthesis. This study highlights the importance of biodiversity and roles of dominant and rare species for long‐term community stability and, for the first time, disentangles relative roles of dominance effect, asynchrony and averaging effect on community stability in a real‐world biodiversity experiment.

Community biomass is driven by dominants and their characteristics – The insight from a field biodiversity experiment with realistic species loss scenario

Abstract Revealing the role of biodiversity in ecosystem functioning (BEF) has been a major focus of ecological research over recent decades. In general, results from artificially assembled communities point to the important role of biodiversity showing that loss of species has a negative effect on various ecosystem functions (mostly assessed by above‐ground peak biomass). However, the evidence from manipulations of natural communities is scarce, and results are often contradictory between these two approaches. In particular, the importance of species dominance for ecosystem functioning remains poorly understood. We created a gradient of plant species richness in a meadow community following a realistic species loss scenario (removal of less abundant species) to test the effect of diversity on community biomass and assess the importance of subordinate species compared with dominants in a 5‐year experiment. Contrasting with results of BEF experiments with artificial assembly, we did not find any relationship between plant species diversity and above‐ground biomass across the timeframe of the experiment. We provide evidence that dominant species' identity and traits are the main drivers of community biomass because dominant species were able to maintain biomass production after substantial species loss. Furthermore, dominants prevented community biomass from declining and biomass was indirectly influenced not by species richness but through differences in functional diversity. Our results support the mass ratio hypothesis, showing much bigger effect of dominant species on community biomass production and hints to the rather minor importance of the complementarity effect between species. We emphasize that BEF research should more focus on the role of dominant species in maintaining various ecosystem functions. Synthesis. Species diversity is a poor predictor of community above‐ground biomass production and dominant species can effectively compensate the total production after substantial loss of other species in a grassland community.

Trait-based filtering mediates the effects of realistic biodiversity losses on ecosystem functioning

Biodiversity losses are a major driver of global changes in ecosystem functioning. While most studies of the relationship between biodiversity and ecosystem functioning have examined randomized species losses, trait-based filtering associated with species-specific vulnerability to drivers of diversity loss can strongly influence how ecosystem functioning responds to declining biodiversity. Moreover, the responses of ecosystem functioning to diversity loss may be mediated by environmental variability interacting with the suite of traits remaining in depauperate communities. We do not yet understand how communities resulting from realistic diversity losses (filtered by response traits) influence ecosystem functioning (via effect traits of the remaining community), especially under variable environmental conditions. Here, we directly test how realistic and randomized plant diversity losses influence productivity and invasion resistance across multiple years in a California grassland. Compared with communities based on randomized diversity losses, communities resulting from realistic (drought-driven) species losses had higher invasion resistance under climatic conditions that matched the trait-based filtering they experienced. However, productivity declined more with realistic than with randomized species losses across all years, regardless of climatic conditions. Functional response traits aligned with effect traits for productivity but not for invasion resistance. Our findings illustrate that the effects of biodiversity losses depend not only on the identities of lost species but also on how the traits of remaining species interact with varying environmental conditions. Understanding the consequences of biodiversity change requires studies that evaluate trait-mediated effects of species losses and incorporate the increasingly variable climatic conditions that future communities are expected to experience.

Random species loss underestimates dilution effects of host diversity on foliar fungal diseases under fertilization.

With increasing attention being paid to the consequences of global biodiversity losses, several recent studies have demonstrated that realistic species losses can have larger impacts than random species losses on community productivity and resilience. However, little is known about the effects of the order in which species are lost on biodiversity–disease relationships. Using a multiyear nitrogen addition and artificial warming experiment in natural assemblages of alpine meadow vegetation on the Qinghai‐Tibetan Plateau, we inferred the sequence of plant species losses under fertilization/warming. Then the sequence of species losses under fertilization/warming was used to simulate the species loss orders (both realistic and random) in an adjacently novel removal experiment manipulating plot‐level plant diversity. We explicitly compared the effect sizes of random versus realistic species losses simulated from fertilization/warming on plant foliar fungal diseases. We found that realistic species losses simulated from fertilization had greater effects than random losses on fungal diseases, and that species identity drove the diversity–disease relationship. Moreover, the plant species most prone to foliar fungal diseases were also the least vulnerable to extinction under fertilization, demonstrating the importance of protecting low competence species (the ability to maintain and transmit fungal infections was low) to impede the spread of infectious disease. In contrast, there was no difference between random and realistic species loss scenarios simulated from experimental warming (or the combination of warming and fertilization) on the diversity–disease relationship, indicating that the functional consequences of species losses may vary under different drivers.

Manipulation of Non-random Species Loss in Natural Phytoplankton: Qualitative and Quantitative Evaluation of Different Approaches

Ecological research in recent decades revealed that species loss has a predominantly negative effect on ecosystem functioning and stability. Most of these studies were based on random species loss scenarios, but extinctions in nature are not random. Recent experimental studies using macroscopic communities largely advanced knowledge about the effects of non-random species loss. However, in microscopic communities like the phytoplankton, implementing realistic species loss scenarios is challenging and experimental data are scarce. Creating more realistic experiments to study the role of phytoplankton diversity for ecosystem functioning is particularly important, as they provide up to 50% of global primary productivity, form the basis of all pelagic food webs, and are important for biogeochemical cycling. In this study, we experimentally tested and evaluated three methods for non-random species loss in a natural marine phytoplankton community. Dilution, filtration, and heat stress removed the targeted rare, large, and sensitive species, respectively. All these species groups are extremely vulnerable to extinction in future climate scenarios and play important roles in the communities. Dilution and filtration with a fine mesh additionally decreased initial biomass, which increased the variability of species left in the respective replicates. The methods tested in this study can be used to non-randomly manipulate phytoplankton species diversity in communities used for experiments. However, in studies where species identities are more important than species richness, the dilution and filtration methods should be modified to eliminate the effect of decreasing initial biomass.

Differential responses and mechanisms of productivity following experimental species loss scenarios.

Experiments that simulate nonrandom species loss from natural communities can offer a fundamentally different understanding of the impacts of species loss on ecosystem function and their underlying mechanisms compared to seeding experiments where species are randomly assembled from a local species pool. We examined the mechanisms underlying changes in primary productivity following experimental species loss scenarios in Mongolian grassland. The range of species loss scenarios was based on natural patterns of species abundance that reflect the species' contributions to ecosystem processes. We found a clear reduction in productivity due to species loss only when species were lost randomly. Grassland productivity was relatively robust following nonrandom species loss scenarios. Even in the context of density compensation, the decrease in dominant trait values for leaf height would explain the reduction in productivity with random species loss. In contrast, the maintenance of dominant trait values of key productivity traits such as leaf dry matter content and leaf height might contribute to the maintenance of productivity in response to nonrandom species loss. Our experiment demonstrated that the responses and mechanisms of primary productivity to species loss differ according to the scenarios of species loss in natural grassland communities. The effects of diversity on productivity might be weak in mature natural systems when species loss is nonrandom. Understanding the consequences of realistic species loss on ecosystem functioning based on field-based removal experiments will give insights into real conservation strategies in the face of global biodiversity change.

Vulnerability of moorland plant communities to environmental change: consequences of realistic species loss on functional diversity

Summary Understanding the consequences of realistic species loss on the functioning and persistence of vulnerable ecosystems is key to devising conservation strategies when environmental changes are immediate threats. Yet, few studies have provided direct evidence for conservation prioritization and decision‐making. We incorporated the quantification of functional diversity based on the Rao index of diversity (FDrao) into this applied context and examined the consequences of realistic species loss on functional diversity in moorland plant communities widely interspersed within a subalpine zone in northern Japan. The realistic order of species loss was derived from the nested subset pattern in the moorland communities, which was corroborated by selective species tolerance and selective extinction. We analysed the relationships between the FDrao half‐life, as an index of each moorland's vulnerability to species loss, and a range of environmental variables describing the moorlands. We then mapped this index across the entire landscape. At most sites, ordered species loss caused a relatively small decline in FDrao until a certain number of species was lost and an accelerating decline thereafter, suggesting relatively low initial vulnerability to species loss. At the other sites, however, ordered species loss caused an approximately proportional decrease in FDrao, suggesting relatively high vulnerability to species loss. The model indicated that sites with higher elevation, higher carrying capacities, or increasing isolation have a shorter FDrao half‐life. The mapping of this index allowed us to identify the geographical distribution of sites of high conservation priority. Synthesis and applications. We performed fine‐scale assessments of the vulnerability of moorland plant communities to species loss, which is likely to occur under future environmental conditions, by simulating the consequences of realistic species loss for functional diversity. The methods used here can provide urgently needed information to support the prioritization and decision‐making involved in conserving ecosystems in the face of global biodiversity loss.

Realistic diversity loss and variation in soil depth independently affect community‐level plant nitrogen use

Numerous experiments have demonstrated that diverse plant communities use nitrogen (N) more completely and efficiently, with implications for how species conservation efforts might influence N cycling and retention in terrestrial ecosystems. However, most such experiments have randomly manipulated species richness and minimized environmental heterogeneity, two design aspects that may reduce applicability to real ecosystems. Here we present results from an experiment directly comparing how realistic and randomized plant species losses affect plant N use across a gradient of soil depth in a native-dominated serpentine grassland in California. We found that the strength of the species richness effect on plant N use did not increase with soil depth in either the realistic or randomized species loss scenarios, indicating that the increased vertical heterogeneity conferred by deeper soils did not lead to greater complementarity among species in this ecosystem. Realistic species losses significantly reduced plant N uptake and altered N-use efficiency, while randomized species losses had no effect on plant N use. Increasing soil depth positively affected plant N uptake in both loss order scenarios but had a weaker effect on plant N use than did realistic species losses. Our results illustrate that realistic species losses can have functional consequences that differ distinctly from randomized losses, and that species diversity effects can be independent of and outweigh those of environmental heterogeneity on ecosystem functioning. Our findings also support the value of conservation efforts aimed at maintaining biodiversity to help buffer ecosystems against increasing anthropogenic N loading.

Realistic plant species losses reduce invasion resistance in a California serpentine grassland

Summary1. The majority of experiments examining effects of species diversity on ecosystem functioning have randomly manipulated species richness. More recent studies demonstrate that realistic species losses have dramatically different effects on ecosystem functioning than those of randomized losses, but these results are based primarily on microcosm experiments or modelling efforts.2. We conducted a field‐based experiment directly comparing the consequences of realistic and randomized plant species losses on invasion resistance and productivity in a native‐dominated serpentine grassland in California, USA. The realistic species loss order was based on nested subset analysis of long‐term presence/absence data from our research site and reflects differing species sensitivities to prolonged drought.3. Biomass of exotic invasive plant species was inversely related to native species richness in the realistic loss order. In contrast, invader biomass was consistently low across species richness levels in the randomized species loss order, with no effect of native species richness on invader biomass among randomized assemblages. Although total above‐ground plant biomass increased with soil depth (a proxy for resource availability) in both realistic and randomized assemblages, soil depth influenced invader biomass only in the randomized assemblages.4. Synthesis. Our results illustrate that the functional consequences of realistic species losses can differ distinctly from those of randomized species losses and that incorporation of realistic species loss scenarios can increase the relevance of experiments linking biodiversity and ecosystem functioning to conservation in the face of anthropogenic global change.

Dominance, Biomass and Extinction Resistance Determine the Consequences of Biodiversity Loss for Multiple Coastal Ecosystem Processes

Key ecosystem processes such as carbon and nutrient cycling could be deteriorating as a result of biodiversity loss. However, currently we lack the ability to predict the consequences of realistic species loss on ecosystem processes. The aim of this study was to test whether species contributions to community biomass can be used as surrogate measures of their contribution to ecosystem processes. These were gross community productivity in a salt marsh plant assemblage and an intertidal macroalgae assemblage; community clearance of microalgae in sessile suspension feeding invertebrate assemblage; and nutrient uptake in an intertidal macroalgae assemblage. We conducted a series of biodiversity manipulations that represented realistic species extinction sequences in each of the three contrasting assemblages. Species were removed in a subtractive fashion so that biomass was allowed to vary with each species removal, and key ecosystem processes were measured at each stage of community disassembly. The functional contribution of species was directly proportional to their contribution to community biomass in a 1∶1 ratio, a relationship that was consistent across three contrasting marine ecosystems and three ecosystem processes. This suggests that the biomass contributed by a species to an assemblage can be used to approximately predict the proportional decline in an ecosystem process when that species is lost. Such predictions represent “worst case scenarios” because, over time, extinction resilient species can offset the loss of biomass associated with the extinction of competitors. We also modelled a “best case scenario” that accounts for compensatory responses by the extant species with the highest per capita contribution to ecosystem processes. These worst and best case scenarios could be used to predict the minimum and maximum species required to sustain threshold values of ecosystem processes in the future.

Realistic Variation in Species Composition Affects Grassland Production, Resource Use and Invasion Resistance

We investigated the effects of realistic variation in plant species and functional group composition, with species occurring at realistic abundances, on ecosystem processes in exotic-dominated California grassland communities. Progressive species removals from microcosm communities, designed to mimic nested variation in diversity observed in the field, reduced grassland production, resistance to intentional invasions, and resistance to natural colonization by new species. Three lines of evidence point to the particular importance of intensified competition within a single functional group—late-active forbs—in explaining the observed effects of realistic species loss order on community resistance. First, reduced success of naturally colonizing species in more diverse assemblages was dominated by declining colonization by late-active forbs. Second, increasing late-active forb biomass appeared to reduce the biomass of intentionally introduced yellow starthistle (Centaurea solstitialis, a late-season forb) both within and across diversity levels. Finally, starthistle addition reduced biomass of resident late-season forbs but not of any other functional group. Increasing diversity increased light levels and soil moisture availability in spring and summer, providing a proximate mechanism linking our realistic species loss order to decreased community resistance. Starthistle addition reduced light and soil moisture availability but not N across richness levels, mirroring the apparent effects of the additional late-active forb species present in higher diversity treatments. Species losses that entail the early loss of whole or key functional groups could, through mechanisms like those we explore, have greater ecosystem consequences than those suggested by randomized-loss experiments.

Realistic Species Losses Disproportionately Reduce Grassland Resistance to Biological Invaders

Consequences of progressive biodiversity declines depend on the functional roles of individual species and the order in which species are lost. Most studies of the biodiversity-ecosystem functioning relation tackle only the first of these factors. We used observed variation in grassland diversity to design an experimental test of how realistic species losses affect invasion resistance. Because entire plant functional groups disappeared faster than expected by chance, resistance declined dramatically with progressive species losses. Realistic biodiversity losses, even of rare species, can thus affect ecosystem processes far more than indicated by randomized-loss experiments.