Diversity-Function Relationships Changed in a Long-Term Restoration Experiment

The central tenet of biodiversity-ecosystem function (BEF) theory, that species richness increases function, could motivate restoration practitioners to incorporate a greater number of species into their projects. But it is not yet clear how well BEF theory predicts outcomes of restoration, because it has been developed through tests involving short-run and tightly controlled (e.g., weeded) experiments. Thus, we resampled our 1997 BEF experiment in a restored salt marsh to test for long-term effects of species richness (plantings with 1, 3, and 6 species per 2 x 2 m plot), with multiple ecosystem functions as response variables. Over 11 years, 1- and 6-species assemblages converged on intermediate richness (mean = 3.9 species/ 0.25-m2 plot), and composition changed nonrandomly throughout the site. While three species became rare, the two most productive species became co-dominant. The two dominants controlled and increased shoot biomass, which appeared to decrease species richness. Diversity-function relationships became less positive over 11 years and differed significantly with (a) the species-richness metric (planted vs. measured), and (b) the indicator of function (shoot biomass, height, and canopy layering). The loss of positive relationships between species richness and function in our restored site began soon after we stopped weeding and continued with increasing dominance by productive species. Where species-rich plantings are unlikely to ensure long-term restoration of functions, as in our salt marsh, we recommend dual efforts to establish (1) dominant species that provide high levels of target functions, and (2) subordinate species, which might provide additional functions under current or future conditions

Biotic Resistance Via Granivory: Establishment by Invasive, Naturalized, and Native Asters Reflects Generalist Preference

Escape from specialist natural enemies is frequently invoked to explain exotic plant invasions, but little attention has been paid to how generalist consumers in the recipient range may influence invasion. We examined how seed preferences of the widespread generalist granivore Peromyscus maniculatus related to recruitment of the strongly invasive exotic Centaurea stoebe and several weakly invasive exotics and natives by conducting laboratory feeding trials and seed addition experiments in the field. Laboratory feeding trials showed that P. maniculatus avoided consuming seeds of C. stoebe relative to the 12 other species tested, even when seeds of alternative species were 53–94% smaller than those of C. stoebe. Seed addition experiments conducted in and out of rodent exclosures revealed that weakly invasive exotics experienced relatively greater release from seed predation than C. stoebe, although this was not the case for natives. Seed mass explained 81% of the variation in recruitment associated with rodent exclusion for natives and weak invaders, with larger-seeded species benefiting most from protection from granivores. However, recruitment of C. stoebe was unaffected by rodent exclusion, even though the regression model predicted seeds of correspondingly large mass should experience substantial predation. These combined laboratory and field results suggest that generalist granivores can be an important biological filter in plant communities and that species-specific seed attributes that determine seed predation may help to explain variation in native plant recruitment and the success of exotic species invasions

Climate Change and San Francisco Bay-Delta Tidal Wetlands

Climate change will affect tidal wetlands with higher rates of sea-level rise and higher concentrations of salt in brackish and freshwater tidal systems, in addition to causing increases in atmospheric CO2 concentration, warmer temperatures, and shifts in precipitation. In the San Francisco Bay–Delta, the areas most likely to be affected—brackish and freshwater tidal wetlands—are also the sites with the majority of endemic plant species and the greater biodiversity and productivity. Effects on the San Francisco Bay– Delta estuary are complex and difficult to predict, but a few things are clear. Biodiversity of the tidal wetland system in the San Francisco Bay–Delta region will decline, with subsequent effects on ecosystem functioning and services. Altered plant production, physiological tolerances, and shifts in rates of mortality will modify wetland plant communities in ways not yet predictable. Lower ecosystem productivity from salinity increases will affect both primary and detrital-based food webs. Such changes will cascade via the food webs into invertebrate, bird, and pelagic systems. Tidal wetlands are especially sensitive to processes that climate change will alter. Several of these altered processes are exacerbated by water diversions from the Delta