Recognition is growing that many of Earth’s biodiverse grasslands (including savannas and grassy woodlands) are ancient and require long periods of time to accumulate their diversity. Nonetheless, many ecologists and environmental policymakers continue to conceptualize grasslands as an early successional stage, particularly in places where the climate and soil can permit forests to form. The old-growth grassland concept challenges this successional narrative and posits that frequent fires and megafaunal herbivory are critical to the maintenance of ancient, species-diverse herbaceous plant communities. As a test of the old-growth concept, we performed a meta-analysis (31 studies from six continents, including 92 timepoints) to compare the richness of old-growth grasslands and secondary grasslands (i.e., grasslands recovering after destruction by agriculture or afforestation). We analysed the difference in plant species richness for each pair of grasslands using the log response ratio, and then conducted a mixed-effects meta-regression to understand the rate of species-richness recovery in secondary grasslands. If the old-growth grasslands in the meta-analyses are in fact ancient, and the paired secondary grasslands are truly early-successional, we expected to find: 1) greater herbaceous plant diversity in old-growth grasslands compared to secondary grasslands and 2) slow accumulation of species richness in secondary grasslands over time.
Results/Conclusions
We found that old-growth grasslands had on average 37% more species compared to secondary grasslands (mean log response ratio = -0.46). Our time series analysis showed that secondary grasslands need at least 100 yr, and more typically thousands of years, to recover their plant species richness (projected mean of 1400 yr). These results suggest that the old-growth concept is applicable to bioclimatically diverse grasslands globally and provide further evidence of the need to abandon the narrative that most grasslands are a successional stage. Recognition of old-growth grasslands, and appreciation for the fires and herbivores on which they depend, may be especially important in the tropics where secondary grasslands appeared to be particularly depauperate in species relative to old-growth grasslands: in supplemental analyses latitude negatively correlated with the log response ratio. We hope that the distinction between old-growth and secondary grasslands will be helpful in future studies of global change that link grassland plant diversity to e.g., productivity and carbon storage. We also hope this test of the old-growth grassland concept will offer policymakers, land managers, and the public additional evidence of the importance of conserving biodiverse grasslands and savannas.