LB 30-293 - Species persistence and species occurrence are decoupled across an environmental stress-gradient.

Thursday, August 18, 2022

5:00 PM – 6:30 PM EDT

Located in: ESA Exhibit Hall

Presenting Author(s)

EC

Emilie Craig

University of Wyoming, United States

Coauthor(s)

MS

Megan C. Szojka

University of Wyoming, United States
RG

Rachel Germain

University of British Columbia
Vancouver, British Columbia, Canada
LS

Lauren G. Shoemaker

University of Wyoming
Laramie, Wyoming, United States

Background/Question/Methods:

Biological diversity is well known to depend on environmental productivity gradients. However, while measures of ecological diversity predominantly focus on species occurrence, they often fail to account for what percentage of the species actually persist, i.e., those which exhibit positive growth rates. Species persistence may change across environmental gradients via three processes: species interactions, abiotic filtering, and dispersal into habitats where they can persist. Here, we present an empirical investigation into (i) how the type of species interaction changes across a stress-gradient, (ii) how persistence changes across this gradient in comparison to occurrence, and (iii) how dispersal fuels differences between persistence and occurrence. We measured invasion growth rates (i.e., persistence) of four annual plant species in serpentine grasslands of McLaughlin Nature Reserve, California USA, and compared this to each species' natural distribution (i.e., occurrence). We tested the stress-gradient hypothesis by comparing persistence with species interactions to persistence with competitors removed. We calculated productivity by using greenness as a proxy for above ground biomass. We quantified dispersal by comparing potential occupancy (i.e., persistence) to realized occupancy: where a population’s persistence without occurrence indicates dispersal limitation, whereas occurrence without persistence indicates a sink population.

Results/Conclusions:

We found that species differed drastically in their patterns of occurrence versus persistence along a productivity gradient, depending on their biotic interactions, their performance, and their ability to disperse. We found support for the stress-gradient hypothesis, where higher environmental stress leads to increased facilitation and lower environmental stress leads to increased competition in persisting species. Second, we found significant mismatches between species’ occurrence versus persistence across an environmental gradient, where occurrence was higher than persistence in productive, or less stressful conditions, and lower than persistence in harsh, or more stressful conditions. This outcome could lead to an overestimation or underestimation of species’ persistence depending on underlying abiotic conditions, which in turn leads us to overlook possible threats of species extirpation. Interestingly, dispersal more commonly increased the proportion of sink populations in less stressful environments, whereas the proportion of dispersal limited populations decreased in less stressful environments. These results highlight the importance of including persistence as a measure of biological diversity, as dispersal dynamics and biotic interactions may be fueling large mismatches between diversity versus persistence across productivity gradients.