Multiple mechanisms confer stability to isolated populations of a rare endemic plant

Abstract

The persistence of small populations remains a puzzle for ecology and conservation. Especially interesting is how naturally small, isolated populations are able to persist in the face of multiple environmental forces that create fluctuating conditions and should, theory predicts, lead to high probabilities of extirpation. We used a combination of long‐term census data and a five‐year demographic study of a naturally rare, endemic plant, Yermo xanthocephalus, to evaluate the importance of several possible mechanisms for small population persistence: negative density dependence, vital rate buffering, demographic compensation, asynchrony in dynamics among sub‐populations, and source–sink dynamics. These non‐exclusive explanations for population persistence all have been shown to operate in some systems, but have rarely if ever been simultaneously examined for the same population or species. We hypothesized that asynchrony in dynamics and demographic compensation would be more powerful than the other three mechanisms. We found partial support for our hypothesis: density dependence, asynchrony among population segments, and source–sink patterns appear to be the most important mechanisms maintaining population viability in this species. Importantly, these processes all appear to operate strongly at very fine spatial scales for Yermo, allowing the only two, extremely small, populations to persist. We also found considerable differences in the results of our census and demographic analyses. In general, we estimated substantially greater chances of population survival from the census data than from the shorter‐term demographic studies. In part, this difference is due to drier than average climate conditions during the years of the demographic work. These results emphasize that while demographic information is necessary to understand various components of population dynamics, longer term studies, even if much less detailed, can be more powerful in uncovering some mechanisms that may be critical in stabilizing population numbers, especially in variable environments.

Quantifying the relative importance of variation in predation and the environment for species coexistence

“…Measurements of demographic rates and food web interactions are usually made separately – often entire studies in their own rights (e.g. Dibner et al , 2019; Gripenberg et al , 2019), and thus all the necessary information is difficult to obtain for many systems. As such, we encourage future work to examine both demographic rates along with trophic links and their corresponding strengths.…”

Quantifying the relative importance of competition, predation, and environmental variation for species coexistence

“…Measurements of demographic rates and food web interactions are usually made separately—often entire studies in their own rights (e.g. Dibner et al (2019); Gripenberg et al (2019)), and thus all the necessary information is difficult to obtain for many systems. As such, we encourage future work to examine both demographic rates along with trophic links and their corresponding strengths.…”

From global to local scale: where is the best for conservation purpose?

“…Indeed, previous studies demonstrate as populations of the same species, despite the proximity to the areal of distribution, could show strong local adaptations and a low correlation with the overall population performance (e.g. García 2008;Abeli et al 2014;Dibner et al 2019). This adaptation to local conditions at range edges (e.g.…”

“…A recovery plan for a threatened plant is impossible without a good knowledge of population demography (Schemske et al 1994;Menges and Dolan 1998;Bell et al 2003;Cogoni et al 2019). Therefore, demographic analysis, combined with extensive ecological and genetic studies, constitute an essential conservation tool allowing to identify the long-term population trends, the contribution of particular life history stages to the population growth rate, and causes and consequences of differences in life stages among populations and years (Caswell 2001;Morris and Doak 2002;Dibner et al 2019;Cogoni et al 2019). In addition, management actions for species recovery require understanding differences among populations in different parts of the species range, especially if the range includes distinct ecosystems.…”

“…Although demographic analysis have been successfully applied in plant ecology and evolutionary biology (e.g. Rees and Rose 2002;Williams 2009;Coulson 2012;Dibner et al 2019), their use in conservation biology remains still lacking (e.g. Zuidema et al 2010;Nicolè et al 2011;Ferrer-Cervantes et al 2012), especially in the Mediterranean Basin (e.g.…”