Biodiversity increases and decreases ecosystem stability.

Losses and gains in species diversity affect ecological stability1-7 and the sustainability of ecosystem functions and services8-13. Experiments and models have revealed positive, negative and no effects of diversity on individual components of stability, such as temporal variability, resistance and resilience2,3,6,11,12,14. How these stability components covary remains poorly understood15. Similarly, the effects of diversity on overall ecosystem stability16, which is conceptually akin to ecosystem multifunctionality17,18, remain unknown. Here we studied communities of aquatic ciliates to understand how temporal variability, resistance and overall ecosystem stability responded to diversity (that is, species richness) in a large experiment involving 690 micro-ecosystems sampled 19 times over 40 days, resulting in 12,939 samplings. Species richness increased temporal stability but decreased resistance to warming. Thus, two stability components covaried negatively along the diversity gradient. Previous biodiversity manipulation studies rarely reported such negative covariation despite general predictions of the negative effects of diversity on individual stability components3. Integrating our findings with the ecosystem multifunctionality concept revealed hump- and U-shaped effects of diversity on overall ecosystem stability. That is, biodiversity can increase overall ecosystem stability when biodiversity is low, and decrease it when biodiversity is high, or the opposite with a U-shaped relationship. The effects of diversity on ecosystem multifunctionality would also be hump- or U-shaped if diversity had positive effects on some functions and negative effects on others. Linking the ecosystem multifunctionality concept and ecosystem stability can transform the perceived effects of diversity on ecological stability and may help to translate this science into policy-relevant information.

Resprouting trees drive understory vegetation dynamics following logging in a temperate forest.

Removal of canopy trees by logging causes shifts in herbaceous diversity and increases invasibility of the forest understory. However, disturbed (cut) trees of many species do not die but resprout from remaining parts. Because sprouts develop vigorously immediately after disturbances, we hypothesized that sprouts of logged trees offset the changes in species richness and invasibility of the herbaceous layer by eliminating the rise in the resource availability during the time before regeneration from seeds develops. To test this, we analyzed data on herbaceous vegetation and sprout biomass collected in a broadleaved temperate forest in the Czech Republic before and for 6 years after logging. Sprouts that were produced by most of the stumps of logged trees offset large rises in species richness and cover of herbaceous plants and the resource availability that followed logging, but they affected the alien plants more significantly than the native plants. The sprouting canopy effectually eliminated most of the alien species that colonized the forest following a logging event. These findings indicate that in forests dominated by tree species with resprouting ability, sprouts drive the early post-disturbance dynamics of the herbaceous layer. By offsetting the post-disturbance vegetation shifts, resprouting supports forest resilience.

Species multidimensional effects explain idiosyncratic responses of communities to environmental change.

Environmental change can alter species' abundances within communities consistently; for example, increasing all abundances by the same percentage, or more idiosyncratically. Here, we show how comparing effects of temperature on species grown in isolation and when grown together helps our understanding of how ecological communities more generally respond to environmental change. In particular, we find that the shape of the feasibility domain (the parameter space of carrying capacities compatible with positive species' abundances) helps to explain the composition of experimental microbial communities under changing environmental conditions. First, we introduce a measure to quantify the asymmetry of a community's feasibility domain using the column vectors of the corresponding interaction matrix. These column vectors describe the effects each species has on all other species in the community (hereafter referred to as species' multidimensional effects). We show that as the asymmetry of the feasibility domain increases the relationship between species' abundance when grown together and when grown in isolation weakens. We then show that microbial communities experiencing different temperature environments exhibit patterns consistent with this theory. Specifically, communities at warmer temperatures show relatively more asymmetry; thus, the idiosyncrasy of responses is higher compared with that in communities at cooler temperatures. These results suggest that while species' interactions are typically defined at the pairwise level, multispecies dynamics can be better understood by focusing on the effects of these interactions at the community level.