Intermediate level of environmental changes promotes higher community richness since 15,500 years

Background/Question/Methods

Biodiversity assessment in a changing world is a key issue that is fairly neglected in paleoecological studies, although its assessment could highlight the long-term effect of environmental changes on biodiversity patterns. The present study paves the way for new developments bridging biodiversity dynamics and ecosystem processes at millennial scale, thus enabling to test ecological theories under a unique larger range of environmental conditions. Here we focused on the North American boreal forest-taiga biome (NABT) since the last 15,500 years.    We developed a framework for biodiversity assessment using fossil pollen records stored in databases. Long term biodiversity was decomposed into three indices: past ecosystem richness (α-diversity), temporal Rate of Change between pollen assemblages (ROC), and spatial turnover between fossil pollen taxa (spatial β-diversity, β_S). The ROC depicts population rearrangements that could result from immigration, climatic fluctuations or disturbances and, thus, environmental changes. 

Results/Conclusions

Richness (α) was mostly stable during the last 15,500 years that argue in favor of saturated and niche determined richness within the NABT. The ROC decreased from the Lateglacial to the Holocene, towards stable and low values attained at c. 4500-500 cal BP, which depicts equilibrium in community dynamics. Considering the entire NABT, we found that both α and ROC are limited below a certain level of spatial heterogeneity. During the last 15,500 years, intermediates levels in ROC were associated to higher species richness, whereas situations with higher or lower ROC (greater and lower environmental changes, respectively) are associated to depauperated richness in the biome. This biodiversity framework assessment is the necessary first step to further acquire more ambitious knowledge on long term ecosystem dynamics and, to address the question of past biodiversity effect on ecosystem properties, such as productivity, carbon storage, biotic interaction or, resilience to disturbances.