Misalignments between where species are found vs. where they should be found reveals shape of persistence-area relationships and their drivers

Background/Question/Methods

Species-area relationships describe how, as spatial scale increases, biodiversity increases and saturates. Despite being a canonical "law" in ecology, species-area relationships nevertheless fail to provide a framework for identifying the underlying processes that drive it, and the coexistence outcomes of the species it describes. In particular, it remains unclear how species persistence (positive growth rates) versus transience (negative growth rates) scales across space and maps onto the classic species-area relationship. To examine this, we transplanted four annual plant species across a serpentine grassland in CA, spanning scales from microsites to regions (1 - 8050 m2). We returned the next year to compare natural occurrence patterns to species persistence measures via low density growth rates across abiotic conditions with and without competition. By parametrizing and comparing species-area relationships to persistence-area relationships, we identified how drivers of persistence and transience vary across spatial scales. These relationships correspond to (1) persistence in abiotic conditions (effects of abiotic filters), (2) persistence with biotic interactions (effects of biotic interactions), and (3) occurrence in natural conditions (effects of dispersal).

Results/Conclusions

We found that this grassland system is characterized by transient dynamics, where species occurrence and persistence patterns often differ. Specifically, we found a mismatch between persistence with biotic interactions (intercept = 3.48 ± 0.025 CI; slope = 0.70 ± 0.035 CI) and natural occurrence (intercept = 3.72 ± 0.033 CI; slope = 0.39 ± 0.048 CI), from small to large scales. We infer that dispersal and competition act in tandem to create transience. Critically, the transient outcomes that are driven by dispersal and competition differ with spatial scale: at small scales, species are transient due to dispersal limitation (species do not naturally occur where they can persist), whereas at mid to large scales species are transient due to source-sink dynamics (species occur where they cannot persist). The influence of competition increased with spatial scale, such that in the absence of biotic interactions, persistence was common at large scales, whereas transience dominated with biotic interactions. Our results suggest that transient dynamics are an under-considered phenomenon and could be confounding many of our inferences in ecological research, especially as transience is expected to increase in frequency with climate change, as species fail to track moving ranges or adapt.