# Mixed Evidence for Species Diversity Affecting Ecological Forecasts in Constant Versus Declining Light

## TL;DR

This study shows that species diversity can improve the predictability of ecological systems under changing environmental conditions, contrary to previous assumptions.

## Contribution

The paper experimentally tests the impact of species richness and environmental change on ecological forecastability using long-term microcosm data.

## Key findings

- Species richness reduced forecast accuracy in constant light but increased it in declining light.
- Aggregate properties like biomass and oxygen were unaffected by species richness or light changes.
- Results held true even when forecasts were back-transformed to the original scale.

## Abstract

Accurate forecasts of ecological dynamics are critical for ecosystem management and conservation, yet the drivers of forecastability are poorly understood. Environmental change and diversity are considered major challenges to ecological forecasting. This assumption, however, has never been tested experimentally because forecasts have high data requirements. In a long‐term microcosm experiment, we manipulated the species richness of 30 experimental protist communities and exposed them to constant or gradually decreasing light levels. We collected finely resolved time series (123 sampling dates over 41 weeks) of species abundances, community biomass, and oxygen concentrations. We then employed data‐driven forecasting methods to forecast these variables. We found that species richness and light had a weak interactive effect on forecasts of species abundances: richness tended to reduce forecast accuracy in constant light but tended to increase forecast accuracy in declining light. These effects could partially be explained by differences among time series in variability and autocorrelation. Forecasts of aggregate properties (community biomass, oxygen), however, were unaffected by richness and light and were not more accurate than those of species abundances. Our forecasts were based on time series that were detrended and standardized. Since real‐world forecasting applications require predictions at the original scale of the forecasted variable, it is important to note that the results were qualitatively identical when back‐transforming the forecasts to the original scale. Taken together, we found no strong evidence that higher diversity results in lower forecastability. Rather, our results imply that promoting diversity could make populations more predictable when environmental conditions change. From a conservation and management perspective, our findings suggest preliminary support that diversity conservation might have beneficial effects on decision‐taking by increasing the forecastability of species abundances in changing environments.

Accurate forecasts of ecological dynamics are critical for ecosystem management and conservation, yet the drivers of forecastability are poorly understood. We conducted a long‐term microcosm experiment to test how species richness and environmental change (light decline) affect ecological forecasts. We found that the accuracy of species abundance forecasts tended to decline with increasing richness when light was constant but tended to increase with increasing richness when light declined. Forecasts of aggregate properties (community biomass, oxygen), however, were unaffected by richness and light. Our results imply that promoting diversity could make populations more predictable when environmental conditions change.