Showing papers by "Andy Hector published in 2023"

Clarifying the effect of biodiversity on productivity in natural ecosystems with longitudinal data and methods for causal inference

Abstract: Causal effects of biodiversity on ecosystem functions can be estimated using experimental or observational designs - designs that pose a tradeoff between drawing credible causal inferences from correlations and drawing generalizable inferences. Here, we develop a design that reduces this tradeoff and revisits the question of how plant species diversity affects productivity. Our design leverages longitudinal data from 43 grasslands in 11 countries and approaches borrowed from fields outside of ecology to draw causal inferences from observational data. Contrary to many prior studies, we estimate that increases in plot-level species richness caused productivity to decline: a 10% increase in richness decreased productivity by 2.4%, 95% CI [-4.1, -0.74]. This contradiction stems from two sources. First, prior observational studies incompletely control for confounding factors. Second, most experiments plant fewer rare and non-native species than exist in nature. Although increases in native, dominant species increased productivity, increases in rare and non-native species decreased productivity, making the average effect negative in our study. By reducing the tradeoff between experimental and observational designs, our study demonstrates how observational studies can complement prior ecological experiments and inform future ones.

Experimentally induced drought and growing season stage modulate community-level functional traits in a temperate grassland

Abstract: Extreme precipitation events are expected to become more intense and frequent with climate change. This climatic shift may impact the structure and dynamics of natural communities and the key ecosystem services they provide. Changes in species abundance under these extreme conditions are thought to be driven by functional traits, morpho-physiological characteristics of an organism that impact its fitness. Future environmental conditions may, therefore, favour different functional traits to those in present-day communities. Here, we measure functional traits on 586 vascular plants in a temperate grassland where precipitation has been experimentally manipulated for six years. We calculated community-weighted means of five functional traits (plant height, leaf dry matter content, leaf thickness, specific leaf area, and leaf phosphorus concentration) and compared community-weighted means between three levels of precipitation: drought (−50%), irrigated (+50%), and control. Additionally, we contrasted treatments at two different timings along the growing season: mid-season and late-season. We expected altered community-weighted means for traits associated with a conservative use of water that will result from increased summer stress-induced intraspecific variability in the mid-season and from community composition changes in the late-season, after the field is cut, a common management action across most European grasslands. In the drought treatment, we found significantly lower community-weighted mean plant height and leaf dry matter content. However, we only observed these differences after the mid-season cut. We also observed an increase in leaf phosphorus concentration in the drought treatment before the mid-season cut. A combination of changes in community composition and intraspecific variation contributed to these differences, with community composition being more important after the cut. Species with higher height, leaf dry matter content, and lower leaf thickness showed a more pronounced abundance decline at the drought plots. We observed no changes in functional traits community-weighted means in the irrigated treatment compared to those in control and drought treatments. Synthesis. Our results suggest how the functional trait composition of grassland communities may shift under climate change-induced drought, stressing the interacting effects with growing season stages.

Evenness mediates the global relationship between forest productivity and richness

Abstract: 1. Biodiversity is an important component of natural ecosystems, with higher species richness often correlating with an increase in ecosystem productivity. Yet, this relationship varies substantially across environments, typically becoming less pronounced at high levels of species richness. However, species richness alone cannot reflect all important properties of a community, including community evenness, which may mediate the relationship between biodiversity and productivity. If the evenness of a community correlates negatively with richness across forests globally, then a greater number of species may not always increase overall diversity and productivity of the system. Theoretical work and local empirical studies have shown that the effect of evenness on ecosystem functioning may be especially strong at high richness levels, yet the consistency of this remains untested at a global scale.

Experimental drought reduces the productivity and stability of a recovering calcareous grassland

Abstract: Grasslands comprise 40% of terrestrial ecosystems and are globally important for food production, carbon storage, and other ecosystem services. However, grasslands in many areas are becoming increasingly exposed to extreme wet and dry periods resulting from global temperature increases. Therefore, understanding how grasslands will respond to climate change is a pressing issue for managing changes to biodiversity and ecosystem service provision. Here, we use experimental manipulations of precipitation (50% increase and 50% decrease of growing-season precipitation) to investigate the resistance of the diversity and productivity of a calcareous grassland community recovering from historical agricultural conversion. We found that decreasing growing season precipitation led to reductions of mean productivity (25 % decrease in peak above-ground biomass) and its temporal stability (54 % increase in biomass variance across years). However, the grassland community composition was resistant to the precipitation manipulations, with no clear difference in community compositional turnover, dissimilarity, or biodiversity indices. Furthermore, the precipitation manipulations had no effect on the path of ongoing (30 year) recovery of grassland plant diversity from the period of previous agricultural conversion. While the diversity of this calcareous grassland was resistant to precipitation extremes (at least in the short term), sustained reductions in growing-season precipitation reduced productivity and its temporal stability demonstrating that different properties of grasslands can vary in their responses to changes in precipitation.