Asynchrony in individual and subpopulation fecundity stabilizes reproductive output of an alpine plant population

Abstract

Population-wide outcomes such as abundance, reproductive output, or mean survival can be stabilized by non-synchronous variation in the performance of individuals or subpopulations. Such "portfolio effects" have been increasingly documented at the scale of subpopulations and are thought to play an important role in generating stability of population phenomena in the face of environmental variation. However, few studies quantify the strength and origin of portfolio effects at the finer scale of individuals. We used 16 yr of fruit production and climate data for an alpine plant to dissect the scale of portfolio effects in reproduction, as well as the contribution of individual traits including size and flowering time in driving reproductive output. Asynchrony in reproductive success substantially reduces variation in population-level reproductive output, with approximately one-fourth of this stabilizing effect arising from individual differences, mostly not those characterized by measured traits, and approximately three-fourths from asynchrony across subpopulations. These results emphasize the different scales and causes of portfolio effects. The decomposition for portfolio effects we provide can facilitate similar breakdowns of the strength and causes of these effects in other systems.

Fishing, environment, and the erosion of a population portfolio

“…Spatial asynchrony of population abundance and demographic rates is common in numerous taxa, including insects (Ehrlich et al 1975), plants (Waddle et al 2019), birds (Ringsby et al 2002), and fishes (Hilborn et al 2003). Ecologists and conservation biologists have been increasingly interested in the existence of asynchrony among subpopulations connected through dispersal because it is directly related to metapopulation viability.…”

Understanding extinction risk and resilience in an extremely small population facing climate and ecosystem change

“…2019, Waddle et al. 2019, but see Satterthwaite and Carlson 2015) but are still surprisingly strong given that SNI lacks sharply different habitats and is regarded as highly invaded and degraded. Currently, personnel on the island are engaged in several efforts to restore plant communities that are more diverse, native‐rich, and of greater height than the invasive communities that dominate much of SNI.…”

“…2019, Waddle et al. 2019). We expected that the homogeneity of habitats across the island would weaken any portfolio effects, but tested for such effects by dividing the island into four categories of latitude and four of longitude, for a total of 14 sections (two combinations did not contain any land), summing the predicted densities for each section in each year, and then estimating the synchrony index, ϕ, the ratio of observed aggregate variance to maximum aggregate variance (Loreau and de Mazancourt 2008) across all sections (Appendix ).…”

Variation in synchrony of production among species, sites, and intertidal zones in coastal marshes

“…As a result, "bad years" are likely to result in poor ecosystem performance across the entire region. However, different vegetation types may provide a little of a "portfolio effect" to regional function (Waddle et al 2019), in which low levels of synchrony reduce fluctuations in regional productivity. Overall, however, these results suggest that coastal wetlands will tend towards boom and bust years, and that regional productivity will not be rescued by some sites or species fluctuating asynchronously with the others.…”

“…Synchrony is important because it affects regional ecosystem functioning (Bjørnstad et al 2002, Beaugrand et al 2003, Defriez et al 2016): synchronization across space can lead to coordinated temporal fluctuations across large areas, and therefore can promote population outbreaks (Micheli et al 1999, Keitt 2008) or extinctions (Earn et al 2000). In contrast, asynchronous population fluctuations, either at a single site or across sites, can stabilize ecosystem function (Tilman 1996) through the portfolio effect (Waddle et al 2019), in which populations that vary out of phase with each other increase stability in community function. The most common explanation for synchrony at large spatial scales is that multiple populations are forced by the same regional climate factors, such as temperature, precipitation, and drought (Liebhold et al 2004, Koenig and Knops 2013, Defriez and Reuman 2017, Tejedor et al 2020): this is the “Moran effect” (Moran 1953, Ranta et al 1997, Hudson and Cattadori 1999).…”