Effects of plant diversity on productivity strengthen over time due to trait-dependent shifts in species overyielding.

Plant diversity effects on community productivity often increase over time. Whether the strengthening of diversity effects is caused by temporal shifts in species-level overyielding (i.e., higher species-level productivity in diverse communities compared with monocultures) remains unclear. Here, using data from 65 grassland and forest biodiversity experiments, we show that the temporal strength of diversity effects at the community scale is underpinned by temporal changes in the species that yield. These temporal trends of species-level overyielding are shaped by plant ecological strategies, which can be quantitatively delimited by functional traits. In grasslands, the temporal strengthening of biodiversity effects on community productivity was associated with increasing biomass overyielding of resource-conservative species increasing over time, and with overyielding of species characterized by fast resource acquisition either decreasing or increasing. In forests, temporal trends in species overyielding differ when considering above- versus belowground resource acquisition strategies. Overyielding in stem growth decreased for species with high light capture capacity but increased for those with high soil resource acquisition capacity. Our results imply that a diversity of species with different, and potentially complementary, ecological strategies is beneficial for maintaining community productivity over time in both grassland and forest ecosystems.

Site conditions determine a key native plant's contribution to invasion resistance in grasslands.

Many plant invasion studies in grasslands suggest that resident plants that share functional traits with invaders can reduce invasion by competing for limiting resources. However, since invasion studies often occur in highly controlled plots or microcosms, it is unclear how heterogeneous site conditions alter competitive interactions under realistic scenarios. To explore how landscape heterogeneity affects biotic resistance provided by competitive resident plants, we conducted a field-based experiment across four sites in California grasslands. Plots contained naturally occurring populations of native Hemizonia congesta, but differed in other characteristics, including litter cover, annual grass cover, soil moisture, and species richness. We invaded plots with the functionally similar nonnative Centaurea solstitalis (yellow starthistle) and, at one site, supplemented one-half of the established plots with water to test the effects of increasing a limiting resource. As in simplified plots and microcosms, increasing H. congesta abundance reduced starthistle biomass by competing for limited soil moisture, but only in plots with high starthistle germination. We conclude that higher abundances of native H. congesta can reduce starthistle invasion in heterogeneous grasslands, but competition is also affected by both abiotic (soil moisture) and biotic (starthistle germination number) conditions that vary across sites.

Assessment of the Conservation Measures Partnership's effort to improve conservation outcomes through adaptive management.

Conservation practice has demonstrated an increasing desire for accountability of actions, particularly with respect to effectiveness, efficiency, and impact to clearly identified objectives. This has been accompanied by increased attention to achieving adaptive management. In 2002, practitioners representing several prominent conservation nongovernmental organizations (NGOs) launched a community of practice called the Conservation Measures Partnership (CMP). The partnership CMP has worked to establish standards of conservation practice to improve accountability of conservation actions through adaptive management. The focal organizing framework for CMP has been the Open Standards for the Practice of Conservation (OS). We evaluated, through an online survey and personal interviews, the first decade of CMP and the OS. The CMP has garnered a positive reputation among agencies, NGOs, and funders and has succeeded in developing a large user base of the OS. However, CMP has not fully achieved its goal of making the OS standard operating procedure for the largest NGOs (e.g., The Nature Conservancy, World Wildlife Fund), despite it being widely used within these organizations. This lack of institutionalization is attributable to multiple causes, including an increase in the number of partially overlapping decision-support frameworks and challenges achieving full-cycle adaptive management. Users strongly believed the OS fosters better conservation practice and highly valued the OS for improving their practice. A primary objective of the OS is to assist practitioners to achieve full-cycle adaptive management to better integrate learning into improving the effectiveness and efficiency of actions. However, most practitioners had not yet achieved cycle completion for their projects. To improve the effectiveness of CMP, OS, and conservation practice in general, we recommend collaborative efforts among the proponents of multiple decision-support frameworks to foster strong institutional adoption of a common set of adaptive-management standards for conservation accountability.

Abundance declines of a native forb have nonlinear impacts on grassland invasion resistance.

The effects of declining plant biodiversity on ecosystem processes are well studied, with most investigations examining the role of species richness declines rather than declines of species abundance. Using grassland mesocosms, we examined how the abundance of a native, resident species, Hemizonia congesta (hayfield tarweed), affected exotic Centaurea solstitialis (yellow starthistle) invasion. We found that progressive H. congesta abundance declines had threshold effects on invasion resistance, with initial declines resulting in minor increases in invasion and subsequent declines leading to accelerating increases in invader performance. Reduced invasion resistance was explained by increased resource availability as H. congesta declined. We also found evidence that resident abundance might indirectly affect invasion by mediating invader impact on resident competitors; C. solstitialis disproportionately reduced H. congesta biomass in low-abundance rather than high-abundance populations. H. congesta's direct and indirect effects on invasion resistance illustrate that an individual species' declining abundance can have accelerating, deleterious effects on ecosystem functions of conservation value.

Sustaining multiple ecosystem functions in grassland communities requires higher biodiversity.

Society places value on the multiple functions of ecosystems from soil fertility to erosion control to wildlife-carrying capacity, and these functions are potentially threatened by ongoing biodiversity losses. Recent empirically based models using individual species' traits suggest that higher species richness is required to provide multiple ecosystem functions. However, no study to date has analyzed the observed functionality of communities of interacting species over multiple temporal scales to assess the relationship between biodiversity and multifunctionality. We use data from the longest-running biodiversity-functioning field experiment to date to test how species diversity affects the ability of grassland ecosystems to provide threshold levels of up to eight ecosystem functions simultaneously. Across years and every combination of ecosystem functions, minimum-required species richness consistently increases with the number of functions considered. Moreover, tradeoffs between functions and variability among years prevent any one community type from providing high levels of multiple functions, regardless of its diversity. Sustained multifunctionality, therefore, likely requires both higher species richness than single ecosystem functionality and a diversity of species assemblages across the landscape.

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.