Do fluctuating environmental conditions have a positive or negative effect on biodiversity? This question is of profound ecological interest, and of growing practical relevance as climate variability around the world continues to increase. The answer to the question depends on the balance of two opposing forces. On the one hand, a fluctuating environment has negative effects on biodiversity by increasing stochasticity, which can lead to more extinctions by chance. On the other hand, a fluctuating environment can have positive effects on biodiversity by creating “temporal niches”. The net effect of these two opposing forces in natural communities was an outstanding knowledge gap.
A new Ecology Letters paper by Tak, Ryan and 46 collaborators from the CTFS-ForestGEO network addressed this key knowledge gap by quantifying the net effect of fluctuation-dependent mechanisms on tree species richness in 21 large forest plots, across a large latitudinal gradient. For each plot, we used tree census data over at least two censuses to quantify temporal population variability of tree species populations at the plot, which is an indicator of the strength of fluctuation-dependent mechanisms. We then fitted a mechanistic model to the observed temporal population variability at each plot, to determine whether the variability is having a net negative or positive effect on tree species richness.
We found that in our 21 forest plots, temporal population variability increased strongly with latitude, by a factor of about 3 to 4 over the latitudinal range of our data set. However, our model estimated that in these plots temporal population variability had mixed net effects on species richness: positive in some cases and negative in others. Thus, our results imply that temporal population variability makes no clear contribution to the strong latitudinal gradient in local tree species richness. This provides a nuanced perspective on the effects of temporal population variability on tree species richness.
Fung, T., R. A. Chisholm, K. Anderson-Teixeira, N. Bourg, W. Y. Brockelman, S. Bunyavejchewin, C.-H. Chang-Yang, R. Chitra-Tarak, G. Chuyong, R. Condit, H. S. Dattaraja, S. J. Davies, C. E. N. Ewango, G. Fewless, C. Fletcher, C. V. S. Gunatilleke, I. A. U. N. Gunatilleke, Z. Hao, J. A. Hogan, R. Howe, C.-F. Hsieh, D. Kenfack, Y. Lin, K. Ma, J.-R. Makana, S. McMahon, W. J. McShea, X. Mi, A, Nathalang, P. S. Ong, G. Parker, E.-P. Rau, J. Shue, S.-H. Su, R. Sukumar, I.-F. Sun, H. S. Suresh, S. Tan, D. Thomas, J. Thompson, R. Valencia, M. I. Vallejo, X. Wang, Y. Wang, P. Wijekoon, A. Wolf, S. Yap, J. Zimmermann (2019). Temporal population variability in local forest communities has mixed effects on tree species richness across a latitudinal gradient. Ecology Letters. [link: https://onlinelibrary.wiley.com/doi/full/10.1111/ele.13412]
Temporal population variability of tree species populations against absolute latitude for the 21 CTFS-ForestGEO plots that we examined.