Dominant Assembly Process Research Articles

The functional assembly of experimental grasslands in relation to fertility and resource heterogeneity

SummaryNiche differences among species are considered key drivers of coexistence (limiting similarity), and increased heterogeneity of resources should promote species diversity through niche partitioning (niche theory). Two predictions follow – co‐occurring species should be more different than expected by chance, especially in productive communities due to increased competition, and heterogeneity should increase both species and functional diversity. However, evidence for these predictions in plant communities is limited.We examined niche overlap in functional traits among co‐occurring grassland species in relation to fertility and resource heterogeneity to determine whether species are more different than expected and reveal dominant assembly processes. We conducted a mesocosm experiment with five treatments, with three levels of soil nutrient availability (low, medium, high) and scale of resource heterogeneity (at medium fertility, large‐scale heterogeneous – patch size 156 cm2, small‐scale heterogeneous – patch size 39 cm2).We measured community weighted means and niche overlap for three functional traits related to resource acquisition and competitive ability (specific leaf area, leaf size and plant height), and compared our observed values to those expected when abundance was randomly allocated to species in the experimental treatments.For leaf size and plant height, we found more niche overlap with increased fertility, and a positive relationship between shoot biomass and niche overlap, whereas the opposite pattern was found for specific leaf area. For all traits, the heterogeneity treatments generally had similar niche overlap values as the high‐fertility treatment and differed significantly from the homogeneous treatment of the same overall fertility. Patch type was important in most cases; significant differences were detected in functional composition and niche overlap between the low‐ and high‐fertility patches in the heterogeneity treatments.Increased niche overlap with increased fertility and heterogeneity can be due to both habitat filters restricting the range of trait values and competition – when resources are either homogenously high or high‐fertility patches are available. The small‐scale distribution of soil resources impacts on community assembly processes and should be incorporated into future studies examining functional diversity, especially when the aim is to determine the influence of productivity gradients.

Experimentally induced habitat filtering in marine bacterial communities

We tested the habitat filtering hypothesis by measuring the phylogenetic structure in marine bacterial communities before and after experimentally induced stress. The habitat filtering hypothesis predicts that phylogenetic clustering (mean relatedness) should increase as the environment becomes suitable for only a subset of the original community. We show that community composition and phylogenetic structure were considerably changed with changes in salinity and dissolved organic carbon. Community composition showed no consistent patterns, while the phylogenetic relatedness between species consistently increased with treatment. We have no information about species interactions in our system, but the phylogenetic signal is strong enough to suggest that habitat filtering is the dominant assembly process. Our results support the hypothesis that habitat characteristics and environmental stress can 'filter' a community so that only closely related species can persist. This non-random phylogenetic signal also implies a relationship between ecologically relevant characteristics and species relatedness.

Positive long-term effect of mulching on species and functional trait diversity in a nutrient-poor mountain meadow in Central Europe

Oligotrophic mountain meadows are threatened biodiversity hotspots throughout Europe. The traditional summer hay-making followed by autumn grazing is no longer economic and question is whether alternative managements can maintain both plant species and functional diversity typical of these habitats. In the Bohemian Forest Mts., we applied three treatments (mowing once a year – i.e., cutting and removing the biomass, mulching once a year – cutting and leaving the crushed biomass to decompose in situ, fallowing – no cutting) in order to assess temporal changes in meadow vegetation, plant trait composition and biomass production in a 13-year experiment. We recorded for each species twenty-five traits as to be most informative of plant strategies related to growth, resource acquisition and carbon-water economy. We compared different components of trait composition (community averages that mostly reflect traits of dominant species vs. the Rao index of functional diversity that reflects trait dissimilarity among species) and their impact on biomass production. We show that mulching promotes species and functional diversity by facilitating heliophilous forbs and legumes with more acquisitive strategies in resource use and release, e.g., higher foliar N and P content. This occurs at the expense of tall grasses (with resource-retentive strategies, e.g., high leaf dry matter content) which dominate the mown and fallow plots. The divergence in most quantitative traits indicates that niche complementarity is the dominant assembly process in mulched plots, which can prevent competitive exclusion and enable species coexistence. The divergent development was detected only after 5–6 years. This slow floristic and functional response is caused by acidity of soil and severe mountain climate that preclude rapid responses of vegetation to land-use changes. We conclude that mulching represents a good compromise maintaining both plant species and functional diversity as well as a relatively high biomass production. Mowing without grazing leads to gradual nutrient loss and thus reduces the productivity and diversity in these oligotrophic ecosystems. Fallowing causes gradual loss in diversity by increased grass competition and litter accumulation.