Nitrogen-fixing symbiotic bacteria act as a global filter for plant establishment on islands

Island biogeography has classically focused on abiotic drivers of species distributions. However, recent work has highlighted the importance of mutualistic biotic interactions in structuring island floras. The limited occurrence of specialist pollinators and mycorrhizal fungi have been found to restrict plant colonization on oceanic islands. Another important mutualistic association occurs between nearly 15,000 plant species and nitrogen-fixing (N-fixing) bacteria. Here, we look for evidence that N-fixing bacteria limit establishment of plants that associate with them. Globally, we find that plants associating with N-fixing bacteria are disproportionately underrepresented on islands, with a 22% decline. Further, the probability of N-fixing plants occurring on islands decreases with island isolation and, where present, the proportion of N-fixing plant species decreases with distance for large, but not small islands. These findings suggest that N-fixing bacteria serve as a filter to plant establishment on islands, altering global plant biogeography, with implications for ecosystem development and introduction risks

Data from: 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

Removal Experiment

The total aboveground biomass of vascular plant species collected in 70 experimental shrub patches in 2010 and 2011. Values are total dry mass of aboveground plant material for each species harvested from a 20 x 30 cm quadrat placed at a random location within each experimental shrub patch. Different quadrat locations were sampled in 2010 and 2011

Community Cover

Absolute percent cover of vascular plants in 92 pairs of shrub covered and open habitat patches. Values are ocular estimates of cover measured in a 20 x 30 cm quadrat placed at a random location within each shrub and open habitat patch

Shrub height

Positions of non-experimental Ericameria ericoides shrubs used in community analysis. Shrub position on the stress gradient is given in m. Canopy height given in cm

Dune Species

Life-history, nativity and family information for the plant species found in the study plots

Bromus Mass

Final aboveground dry biomass of Bromus diandrus (Poaceae) plants grown either in soil from shrub patches or from outside of shrub patches and either under shrubs or outside of shrubs

Soil Carbon

Carbon content in soil samples from a subset of the 46 shrub-free and shrub covered patches

Community Abundance

Number of individual vascular plants rooted in 92 pairs of shrub covered and open habitat patches. Values are total numbers of individuals of each species counted in a 20 x 30 cm quadrat placed at a random location within each shrub and open habitat patch. Quadrat locations were the same as those used for the estimated plant cover values in "CommunityCover.csv"

Sand Size

Sand particle size and coarse organic matter content in soil samples from 46 paired shrub and shrub-free patches