Plant Functional Group Diversity as a Mechanism for Invasion Resistance

A Mixed Model for Assessing the Effect of Numerous Plant Species Interactions on Grassland Biodiversity and Ecosystem Function Relationships

In grassland ecosystems, it is well known that increasing plant species diversity can improve ecosystem functions (i.e., ecosystem responses), for example, by increasing productivity and reducing weed invasion. Diversity-Interactions models use species proportions and their interactions as predictors in a regression framework to assess biodiversity and ecosystem function relationships. However, it can be difficult to model numerous interactions if there are many species, and interactions may be temporally variable or dependent on spatial planting patterns. We developed a new Diversity-Interactions mixed model for jointly assessing many species interactions and within-plot species planting pattern over multiple years. We model pairwise interactions using a small number of fixed parameters that incorporate spatial effects and supplement this by including all pairwise interaction variables as random effects, each constrained to have the same variance within each year. The random effects are indexed by pairs of species within plots rather than a plot-level factor as is typical in mixed models, and capture remaining variation due to pairwise species interactions parsimoniously. We apply our novel methodology to three years of weed invasion data from a 16-species grassland experiment that manipulated plant species diversity and spatial planting pattern and test its statistical properties in a simulation study.Supplementary materials accompanying this paper appear online. Supplementary materials for this article are available at 10.1007/s13253-022-00505-2.

Which factor contributes most to the invasion resistance of native plant communities under the co-invasion of two invasive plant species?

Two invasive plant species (IPS) can co-invade the same plant community. As the number of IPS increases under the co-invasion of two IPS, plant taxonomic and functional diversity, community invasibility, community stability, invasion resistance, and invasion intensity and invasiveness of IPS and their interrelationships may be altered. This study aimed to quantify the contribution of plant taxonomic and functional diversity, community invasibility, community stability, and invasion intensity and invasiveness of IPS to the invasion resistance of native plant communities under the co-invasion of the two IPS Erigeron annuus (L.) Pers. and Solidago canadensis L. in eastern China. This study also defined a method to quantify the invasion resistance of native plant communities designated the invasion resistance index. The community-weighted mean trait values of native plants and plant diversity are the factors that are the most critical to determine the invasion resistance of native plant communities. Thus, the invasion resistance of native plant communities primarily depends on the three following factors: the relative abundance of natives, the growth performance of natives, and the diversity of natives. All levels of invasion significantly decrease the invasion resistance of native plant communities. The two IPS antagonistically affect the invasion resistance of native plant communities less under co-invasion compared with their independent invasion.

The role of competition and herbivory in biotic resistance against invaders: a synergistic effect

Invasive species pose a major threat to global diversity, and once they are well established their eradication typically becomes unfeasible. However, certain natural mechanisms can increase the resistance of native communities to invaders and can be used to guide effective management policies. Both competition and herbivory have been identified as potential biotic resistance mechanisms that can limit plant invasiveness, but it is still under debate to what extent they might be effective against well-established invaders. Surprisingly, whereas biotic mechanisms are known to interact strongly, most studies to date have examined single biotic mechanisms separately, which likely influences our understanding of the strength and effectiveness of biotic resistance against invaders. Here we use long-term field data, benthic assemblage sampling, and exclusion experiments to assess the effect of native assemblage complexity and herbivory on the invasion dynamics of a successful invasive species, the alga Caulerpa cylindracea. A higher complexity of the native algal assemblage limited C. cylindracea invasion, probably through competition by canopy-forming and erect algae. Additionally, high herbivory pressure by the fish Sarpa salpa reduced C. cylindracea abundance by more than four times. However, long-term data of the invasion reflects that biotic resistance strength can vary across the invasion process and it is only where high assemblage complexity is concomitant with high herbivory pressure, that the most significant limitation is observed (synergistic effect). Overall, the findings reported in this study highlight that neglecting the interactions between biotic mechanisms during invasive processes and restricting the studied time scales may lead to underestimations of the true capacity of native assemblages to develop resistance to invaders.

Phylogenetic diversity is a better predictor of wetland community resistance to Alternanthera philoxeroides invasion than species richness

Highly biodiversity communities have been shown to better resist plant invasions through complementarity effects. Species richness (SR) is a widely used biodiversity metric but lacks explanatory power when there are only a few species. Communities with low SR can have a wide variety of phylogenetic diversities (PD), which might allow for a better prediction of invasibility. We assessed the effect of diversity reduction of a wetland community assemblage typical of the Beijing area on biotic resistance to invasion of the exotic weed Alternanthera philoxeroides and compared the reduction in SR and PD in predicting community invasibility. The eight studied resident species performed similarly when grown alone and when grown in eight-species communities together with the invasive A. philoxeroides. Variation partitioning showed that PD contributed more to variation in both A. philoxeroides traits and community indicators than SR. All A. philoxeroides traits and community indicators, except for evenness index, showed a linear relationship with PD. However, only stem length of A. philoxeroides differed between the one- and two-species treatments, and the diversity index of the communities differed between the one- and two-species treatments and between the one- and four-species treatments. Our results showed that in natural or semi-natural wetlands with relatively low SR, PD may be a better predictor of invasibility than SR. When designing management strategies for mitigating A. philoxeroides invasion, deliberately raising PD is expected to be more efficient than simply increasing species number.

Competition between the invasive Impatiens glandulifera and UK native species: the role of soil conditioning and pre-existing resident communities

Himalayan balsam (Impatiens glandulifera) is a highly invasive annual herb that has become extremely prevalent in riparian zones across the UK. The competitive ability of I. glandulifera, both in terms of resource exploitation and allelopathy (i.e., the release of biochemicals that may be toxic to neighbouring plants), is considered a key determinant of its success. Little is known, however, about the effects of the resident community on the establishment and growth of I. glandulifera. Here, we aim to increase our understanding of the competitive ability of this highly invasive plant by investigating the effects of soil conditioning on the performance of four co-occurring native species (Tanacetum vulgare, Urtica dioica, Chelidonium majus and Arabidopsis thaliana). In addition, we also aim to investigate the effect that the pre-existing species composition have on the performance of I. glandulifera seedlings by establishing artificial communities (monocultures and mixtures of four UK native species, including U. dioica). We found negative effects of soil conditioning by I. glandulifera in all four species, either by reducing above-ground biomass, chlorophyll content or both. Monocultures of U. dioica were the only artificial communities that reduced growth of I. glandulifera, and we did not find any support for the idea that a more diverse community may be more resistant to invasion. Our results confirm the high competitive ability of I. glandulifera and highlight how the identity of the natives in the resident community may be key to limit its success.

The exotic species Senecio inaequidens pays the price for arriving late in temperate European grassland communities

The exotic South African ragwort (Senecio inaequidens DC.) rapidly spread across Central Europe after its introduction, but we still do not know to what extent its timing of arrival in a plant community (i.e. before or after natives) and the composition of the native community being invaded affect (1) its capacity to invade a European grassland, (2) the performance of the native species, and (3) the direction and strength of priority effects. In a greenhouse experiment, we manipulated the timing of arrival of the exotic species (Senecio) and the composition of the native community to test the influence of these factors on the productivity and N content of exotic and native species. We also investigated if the plant species origin (native or exotic) and the native community composition affected the benefit of arriving early and the cost of arriving late in the community. The establishment success of Senecio strongly depended on its timing of arrival in a grassland community. Senecio benefited more from arriving early than did the natives. The presence of legumes in the community did not favour invasion by Senecio. When natives arrived later than Senecio, however, priority effects were weaker when legumes were part of the native community. Our results showed that inhibitory priority effects created by natives can lower the risk of invasion by Senecio. An early arrival of this species at a site with low native species abundance is a scenario that could favour invasion.

Increasing Sustainability of Residential Areas Using Rain Gardens to Improve Pollutant Capture, Biodiversity and Ecosystem Resilience

Rain gardens have become a widespread stormwater practice in the United States, and their use is poised to continue expanding as they are an aesthetically pleasing way to improve the quality of stormwater runoff. The terms rain garden and bioretention, are now often used interchangeably to denote a landscape area that treats stormwater runoff. Rain gardens are an effective, attractive, and sustainable stormwater management solution for residential areas and urban green spaces. They can restore the hydrologic function of urban landscapes and capture stormwater runoff pollutants, such as phosphorus (P), a main pollutant in urban cities and residential neighborhoods. Although design considerations such as size, substrate depth, substrate type, and stormwater holding time have been rigorously tested, little research has been conducted on the living portion of rain gardens. This paper reviews two studies—one that evaluated the effects of flooding and drought tolerance on the physiological responses of native plant species recommended for use in rain gardens, and another that evaluated P removal in monoculture and polyculture rain garden plantings. In the second study, plants and substrate were evaluated for their ability to retain P, a typical water pollutant. Although plant growth across species was sometimes lower when exposed to repeated flooding, plant visual quality was generally not compromised. Although plant selection was limited to species native to the southeastern U.S., some findings may be translated regardless of region. Plant tissue P was higher than either leachate or substrate, indicating the critical role plants play in P accumulation and removal. Additionally, polyculture plantings had the lowest leachate P, suggesting a polyculture planting may be more effective in preventing excess P from entering waterways from bioretention gardens. The findings included that, although monoculture plantings are common in bioretention gardens, polyculture plantings can improve biodiversity, ecosystem resilience, and rain garden functionality.

Characterization of Adult Functional Traits of Local Populations and Cultivars of Sandberg Bluegrass and Bottlebrush Squirreltail Perennial Bunchgrasses

Plant functional traits offer an understanding of the plant's ability to cope with varying environmental impositions. The objective of this study was to evaluate the above and belowground adult morphological and chemical composition traits of local populations of Sandberg bluegrass (Poa secunda J. Presl) and Bottlebrush squirreltail (Elymus elymoides (Raf.) Swezey) collected in Nevada and their cultivated varieties. A total of six replications (one seedling each) from each population and cultivar of the two native perennial bunchgrasses were used in a randomized complete block design experiment. Each of the six seedlings from each sourced population was transplanted into individual tree pots (28 cm diameter × 61 cm height) containing 20.4 kg of air-dried Orr gravelly sandy loam soil in mid-November, 2015 and remained in the pots for the duration of the study (23 June, 2016). Traits evaluated were, plant height, leaf length, inflorescence length, shoot biomass, forage nutritive value, root morphological traits, and root carbon and nitrogen content. Traits means were considered different at P < 0.05. For Sandberg bluegrass, the cultivar 'Mountain Home' and the population from Panther Valley tended to have greater biomass than the population from Button Point but overall, the average of the two cultivars (10.8 g/plant) did not differ in shoot biomass relative to the local populations (7.6 g/plant). For squirreltail, plant height for the George St. Sonoma and Grass Valley populations (71.3 cm) was greater than the cultivars 'Toe Jam Creek' and 'Vale' (40.5 cm) but cultivars had greater biomass (12.6 g/plant) than the local populations (5.8 g/plant). Total root length and root diameter were not different among the Sanberg bluegrass and squirreltail populations. The results from traits expounded on in this study indicate the closeness of these populations for both species at their adult stage and provide insights for building a unified framework approach among the different agencies and restoration practitioners to aid in plant assemblages for restoration success in the Great Basin and beyond.

Mechanisms of Invasion Resistance of Aquatic Plant Communities

Invasive plant species are among the major threats to freshwater biodiversity. Few experimental studies have investigated whether native plant diversity can provide biotic resistance to invaders in freshwater ecosystems. At small spatial scales, invasion resistance may increase with plant species richness due to a better use of available resources, leaving less available for a potential invader (Complementarity effect) and/or the greater probability to have a highly competitive (or productive) native species in the community (Selection effect). In submerged aquatic plant communities, we tested the following hypotheses: (1) invader establishment success is greatest in the absence of a native plant community; (2) lower in plant communities with greater native species richness, due to complementary and/or selection effects; and (3) invader establishment success would be lowest in rooted plant communities, based on the limiting similarity theory as the invader is a rooted submerged species. In a greenhouse experiment, we established mesocosms planted with 0 (bare sediment), 1, 2, and 4 submerged plant species native to NW Europe and subjected these to the South African invader Lagarosiphon major (Ridl.) Moss. We used two rooted (Myriophyllum spicatum L., Potamogeton perfoliatus L.) and two non-rooted native species (Ceratophyllum demersum L., Utricularia vulgaris L.) representing two distinct functional groups considering their nutrient acquisition strategy which follows from their growth form, with, respectively, the sediment and water column as their main nutrient source. We found that the presence of native vegetation overall decreased the establishment success of an alien aquatic plant species. The strength of this observed biotic resistance increased with increasing species richness of the native community. Mainly due to a selection effect, the native biomass of mixed communities overyielded, and this further lowered the establishment success of the invader in our experiment. The strongest biotic resistance was caused by the two native plant species that were of the same functional group, i.e., functionally most similar to the invader. These results support the prediction of Elton's biotic resistance hypothesis in aquatic ecosystems and indicate that both species richness and functional group identity can play an important role in decreasing establishment success of alien plant species.

Ecological application of biotic resistance to control the invasion of an invasive plant, Ageratina altissima

Biotic resistance is the ability of species in a community to limit the invasion of other species. However, biotic resistance is not widely used to control invasive plants. Experimental, functional, and modeling approaches were combined to investigate the processes of invasion by Ageratina altissima (white snakeroot), a model invasive species in South Korea. We hypothesized that (1) functional group identity would be a good predictor of biotic resistance to A. altissima, whereas a species identity effect would be redundant within a functional group, and (2) mixtures of species would be more resistant to invasion than monocultures. We classified 37 species of native plants into three functional groups based on seven functional traits. The classification of functional groups was based primarily on differences in life longevity and woodiness. A competition experiment was conducted based on an additive competition design with A. altissima and monocultures or mixtures of resident plants. As an indicator of biotic resistance, we calculated a relative competition index (RCI_avg) based on the average performance of A. altissima in a competition treatment compared with that of the control where only seeds of A. altissima were sown. To further explain the effect of diversity, we tested several diversity–interaction models. In monoculture treatments, RCI_avg of resident plants was significantly different among functional groups but not within each functional group. Fast‐growing annuals (FG1) had the highest RCI_avg, suggesting priority effects (niche pre‐emption). RCI_avg of resident plants was significantly greater in a mixture than in a monoculture. According to the diversity–interaction models, species interaction patterns in mixtures were best described by interactions between functional groups, which implied niche partitioning. Functional group identity and diversity of resident plant communities were good indicators of biotic resistance to invasion by introduced A. altissima, with the underlying mechanisms likely niche pre‐emption and niche partitioning. This method has most potential in assisted restoration contexts, where there is a desire to reintroduce natives or boost their population size due to some previous level of degradation.

The Importance of Being First: Exploring Priority and Diversity Effects in a Grassland Field Experiment

Diversity of species and order of arrival can have strong effects on ecosystem functioning and community composition, but these two have rarely been explicitly combined in experimental setups. We measured the effects of both species diversity and order of arrival on ecosystem function and community composition in a grassland field experiment, thus combining biodiversity and assembly approaches. We studied the effect of order of arrival of three plant functional groups (PFGs: grasses, legumes, and non-leguminous forbs) and of sowing low and high diversity seed mixtures (9 or 21 species) on species composition and aboveground biomass. The experiment was set up in two different soil types. Differences in PFG order of arrival affected the biomass, the number of species and community composition. As expected, we found higher aboveground biomass when sowing legumes before the other PFGs, but this effect was not continuous over time. We did not find a positive effect of sown diversity on aboveground biomass (even if it influenced species richness as expected). No interaction were found between the two studied factors. We found that sowing legumes first may be a good method for increasing productivity whilst maintaining diversity of central European grasslands, although the potential for long-lasting effects needs further study. In addition, the mechanisms behind the non-continuous priority effects we found need to be further researched, taking weather and plant-soil feedbacks into account.

A promising future for integrative biodiversity research: an increased role of scale-dependency and functional biology

Studies into the complex interaction between an organism and changes to its biotic and abiotic environment are fundamental to understanding what regulates biodiversity. These investigations occur at many phylogenetic, temporal and spatial scales and within a variety of biological and geological disciplines but often in relative isolation. This issue focuses on what can be achieved when ecological mechanisms are integrated into analyses of deep-time biodiversity patterns through the union of fossil and extant data and methods. We expand upon this perspective to argue that, given its direct relevance to the current biodiversity crisis, greater integration is needed across biodiversity research. We focus on the need to understand scaling effects, how lower-level ecological and evolutionary processes scale up and vice versa, and the importance of incorporating functional biology. Placing function at the core of biodiversity research is fundamental, as it establishes how an organism interacts with its abiotic and biotic environment and it is functional diversity that ultimately determines important ecosystem processes. To achieve full integration, concerted and ongoing efforts are needed to build a united and interactive community of biodiversity researchers, with education and interdisciplinary training at its heart.

Site Preparation Drives Long-Term Plant Community Dynamics in Restored Tallgrass Prairie: A Case Study in Southeastern South Dakota

Most tallgrass prairies have been destroyed or altered, making restoration an important component to their conservation. Our goal was to evaluate progress 12-years post-restoration at Spirit Mound Historic Prairie and determine whether the outcomes varied based on different land use and restoration histories across the site. We examined changes in plant diversity, richness, evenness, non-native species relative abundance, and community composition from 2004 to 2013. Areas with different restoration treatments and land-use histories showed divergent results. Seventy percent of the site, previously annual row crop, was reconstructed using herbicide application followed by native seeding (hereafter reconstruction). Areas that were previously grazed, 15 % of the site, were restored with only partial seeding and no herbicide treatment (hereafter rehabilitation). Species richness and diversity increased over 40 % in the reconstruction since 2004 and remained over 1.9 times higher in the reconstructed areas than rehabilitated areas. Diversity did not change in the rehabilitation, but richness increased 47 % since 2004. Evenness decreased 11-26 % over time in both areas. Non-native species relative abundance did not change from 2004 to 2013, and remained five times higher in the rehabilitation than the reconstruction. Native C4 grass and forb abundance increased over time in the reconstruction, whereas non-native C3 grasses remained dominant in the rehabilitation. These results showed that restoration outcomes were radically different 12-years post-restoration among areas with different prior land uses that were subjected to different restoration practices. Long-term assessments are important to accurately determine restoration progress and inform management decisions.

Effects of soil nitrogen availability and native grass diversity on exotic forb dominance

Exotic plants are often most successful in high resource environments. By drawing down available resources, species-rich communities may be able to reduce exotic success when resource supply is elevated. We tested the prediction that exotic success would be greatest in species-poor communities when nitrogen availability is high. We also tested two underlying assumptions of this prediction: species-rich communities draw down soil nitrogen availability more than species-poor communities following fertilization and exotic success increases when soil nitrogen availability is high. In a restored grassland where native grass diversity was manipulated (one, three, or five-species) seven years earlier to form a gradient in species richness, we manipulated nitrogen availability directly via fertilization, and indirectly via burning. We then examined the success of the exotic forb Galium verum L. Contrary to our prediction, diversity and nutrient treatments did not jointly influence exotic success. Instead, one-time fertilization increased exotic biomass in the first year of the study. This likely occurred because the effect of nutrient treatments on nitrogen availability was independent of diversity treatment. Thus, we found no evidence that species-rich communities are better able to reduce exotic biomass when nitrogen is added than are species-poor communities. This suggests that in some systems, the effects of increasing species richness can be overwhelmed by the effects of nutrient addition that promote exotic success.

Transforming ecosystems: When, where, and how to restore contaminated sites

Chemical contamination has impaired ecosystems, reducing biodiversity and the provisioning of functions and services. This has spurred a movement to restore contaminated ecosystems and develop and implement national and international regulations that require it. Nevertheless, ecological restoration remains a young and rapidly growing discipline and its intersection with toxicology is even more nascent and underdeveloped. Consequently, we provide guidance to scientists and practitioners on when, where, and how to restore contaminated ecosystems. Although restoration has many benefits, it also can be expensive, and in many cases systems can recover without human intervention. Hence, the first question we address is: "When should we restore contaminated ecosystems?" Second, we provide suggestions on what to restore-biodiversity, functions, services, all 3, or something else--and where to restore given expected changes to habitats driven by global climate change. Finally, we provide guidance on how to restore contaminated ecosystems. To do this, we analyze critical aspects of the literature dealing with the ecology of restoring contaminated ecosystems. Additionally, we review approaches for translating the science of restoration to on-the-ground actions, which includes discussions of market incentives and the finances of restoration, stakeholder outreach and governance models for ecosystem restoration, and working with contractors to implement restoration plans. By explicitly considering the mechanisms and strategies that maximize the success of the restoration of contaminated sites, we hope that our synthesis serves to increase and improve collaborations between restoration ecologists and ecotoxicologists and set a roadmap for the restoration of contaminated ecosystems.

Soil disturbance as a grassland restoration measure-effects on plant species composition and plant functional traits

Soil disturbance is recognized as an important driver of biodiversity in dry grasslands, and can therefore be implemented as a restoration measure. However, because community re-assembly following disturbance includes stochastic processes, a focus only on species richness or establishment success of particular species will not inform on how plant communities respond ecologically to disturbance. We therefore evaluated vegetation development following disturbance by quantifying species richness, species composition and functional trait composition. Degraded calcareous sandy grassland was subjected to experimental disturbance treatments (ploughing or rotavation), and the vegetation was surveyed during four subsequent years of succession. Treated plots were compared with control plots representing untreated grassland, as well as nearby plots characterized by plant communities representing the restoration target.

Species richness and functional diversity both increased in response to soil disturbance, and rotavation, but not ploughing, had a persistent positive effect on the occurrence of specialist species of calcareous sandy grassland. However, no type of soil disturbance caused the plant species composition to develop towards the target vegetation. The disturbance had an immediate and large impact on the vegetation, but the vegetation developed rapidly back towards the control sites. Plant functional composition analysis indicated that the treatments created habitats different both from control sites and target sites. Community-weighted mean Ellenberg indicator values suggested that the observed plant community response was at least partially due to an increase in nitrogen and water availability following disturbance. This study shows that a mild type of disturbance, such as rotavation, may be most successful in promoting specialist species in calcareous sandy grassland, but that further treatments are needed to reduce nutrient availability. We conclude that a functional trait based analysis provides additional information of the vegetation response and the abiotic conditions created, complementing the information from the species composition.

Rangeland dynamics: investigating vegetation composition and structure of urban and exurban prairie dog habitat

Rapid human population growth and habitat modification in the western United States has led to the formation of urban and exurban rangelands. Many of these rangelands are also home to populations of black-tailed prairie dogs (Cynomys ludovicianus). Our study aimed to compare the vegetation composition of an urban and exurban rangeland, and explore the role that prairie dogs play in these systems. The percent absolute canopy cover of graminoids (grasses and grass-likes), forbs, shrubs, litter, and bare ground were estimated at sampling areas located on and off prairie dog colonies at an urban and an exurban site. Herbaceous forage quality and quantity were determined on plant material collected from exclosure cages located on the colony during the entire growing season, while a relative estimate of prairie dog density was calculated using maximum counts. The exurban site had more litter and plant cover and less bare ground than the urban site. Graminoids were the dominant vegetation at the exurban plots. In contrast, mostly introduced forbs were found on the urban prairie dog colony. However, the forage quality and quantity tests demonstrated no difference between the two colonies. The relative prairie dog density was greater at the urban colony, which has the potential to drive greater vegetation utilization and reduced cover. Exurban rangeland showed lower levels of impact and retained all of the plant functional groups both on- and off-colony. These results suggest that activities of prairie dogs might further exacerbate the impacts of humans in fragmented urban rangeland habitats. Greater understanding of the drivers of these impacts and the spatial scales at which they occur are likely to prove valuable in the management and conservation of rangelands in and around urban areas.

Interactions between abiotic constraint, propagule pressure, and biotic resistance regulate plant invasion

With multiple species introductions and rapid global changes, there is a need for comprehensive invasion models that can predict community responses. Evidence suggests that abiotic constraint, propagule pressure, and biotic resistance of resident species each determine plant invasion success, yet their interactions are rarely tested. To understand these interactions, we conducted community assembly experiments simulating situations in which seeds of the invasive grass species Phragmites australis (Poaceae) land on bare soil along with seeds of resident wetland plant species. We used structural equation models to measure both direct abiotic constraint (here moist vs. flooded conditions) on invasion success and indirect constraint on the abundance and, therefore, biotic resistance of resident plant species. We also evaluated how propagule supply of P. australis interacts with the biotic resistance of resident species during invasion. We observed that flooding always directly reduced invasion success but had a synergistic or antagonistic effect on biotic resistance depending on the resident species involved. Biotic resistance of the most diverse resident species mixture remained strong even when abiotic conditions changed. Biotic resistance was also extremely effective under low propagule pressure of the invader. Moreover, the presence of a dense resident plant cover appeared to lower the threshold at which invasion success became stable even when propagule supply increased. Our study not only provides an analytical framework to quantify the effect of multiple interactions relevant to community assembly and species invasion, but it also proposes guidelines for innovative invasion management strategies based on a sound understanding of ecological processes.

Why species matter: an experimental assessment of assumptions and predictive ability of two functional-group models

Community ecologists use functional groups based on the rarely tested assumption that within-group responses to ecological processes are similar and thus members are functionally equivalent. However, recent research suggests that functional equivalency may break down with human impacts. We tested the equivalency assumption and model predictions of responses to simulated human alterations in nutrients and large herbivores for two models of coral reef algae, the Relative Dominance Model (RDM) and the Functional Group Model (FGM). Results of both mesocosm and field experiments using assembled communities were compared to model predictions, and within- and between-group variability were assessed. Both models' predictions of group response to herbivory matched experimental outcomes, but only the RDM predicted response to nutrients. However, within-group variability was dramatic, because the RDM grouped species with opposite responses to herbivory and the FGM grouped species with unique responses to nutrients. These heterogeneous responses resulted in loss of information and masked strong interactions between herbivory and nutrients that were not included in the models. As humans continue to impact major ecological processes in ecosystems globally, we postulate that functional-group models may need to be reformulated to account for shifting baselines.

Using a multi-trait approach to manipulate plant functional diversity in a biodiversity-ecosystem function experiment

A frequent pattern emerging from biodiversity-ecosystem function studies is that functional group richness enhances ecosystem functions such as primary productivity. However, the manipulation of functional group richness goes along with major disadvantages like the transformation of functional trait data into categories or the exclusion of functional differences between organisms in the same group. In a mesocosm study we manipulated plant functional diversity based on the multi-trait Functional Diversity (FD)-approach of Petchey and Gaston by using database data of seven functional traits and information on the origin of the species in terms of being native or exotic. Along a gradient ranging from low to high FD we planted 40 randomly selected eight-species mixtures under controlled conditions. We found a significant positive linear correlation of FD with aboveground productivity and a negative correlation with invasibility of the plant communities. Based on community-weighted mean calculations for each functional trait, we figured out that the traits N-fixation and species origin, i.e. being native or exotic, played the most important role for community productivity. Our results suggest that the identification of the impact of functional trait diversity and the relative contributions of relevant traits is essential for a mechanistic understanding of the role of biodiversity for ecosystem functions such as aboveground biomass production and resistance against invasion.

Grazer exclusion alters plant spatial organization at multiple scales, increasing diversity

Grazing is one of the most important factors influencing community structure and productivity in natural grasslands. Understanding why and how grazing pressure changes species diversity is essential for the preservation and restoration of biodiversity in grasslands. We use heavily grazed subalpine meadows in the Qinghai-Tibetan Plateau to test the hypothesis that grazer exclusion alters plant diversity by changing inter- and intraspecific species distributions. Using recently developed spatial analyses combined with detailed ramet mapping of entire plant communities (91 species), we show striking differences between grazed and fenced areas that emerged at scales of just one meter. Species richness was similar at very small scales (0.0625 m²), but at larger scales diversity in grazed areas fell below 75% of corresponding fenced areas. These differences were explained by differences in spatial distributions; intra- and interspecific associations changed from aggregated at small scales to overdispersed in the fenced plots, but were consistently aggregated in the grazed ones. We conclude that grazing enhanced inter- and intraspecific aggregations and maintained high diversity at small scales, but caused decreased turnover in species at larger scales, resulting in lower species richness. Our study provides strong support to the theoretical prediction that inter- and intraspecific aggregation produces local spatial patterns that scale-up to affect species diversity in a community. It also demonstrates that the impacts of grazing can manifest through this mechanism, lowering diversity by reducing spatial turnover in species. Finally, it highlights the ecological and physiological plant processes that are likely responding to grazing and thereby altering aggregation patterns, providing new insights for monitoring, and mediating the impacts of grazing.