How environmental drivers of spatial synchrony interact

Abstract

Spatial synchrony, the tendency for populations across space to show correlated fluctuations, is a fundamental feature of population dynamics, linked to central topics of ecology such as population cycling, extinction risk, and ecosystem stability. A common mechanism of spatial synchrony is the Moran effect, whereby spatially synchronized environmental signals drive population dynamics and hence induce population synchrony. After reviewing recent progress in understanding Moran effects, we here elaborate a general theory of how Moran effects of different environmental drivers acting on the same populations can interact, either synergistically or destructively, to produce either substantially more or markedly less population synchrony than would otherwise occur. We provide intuition for how this newly recognized mechanism works through theoretical case studies and application of our theory to California populations of giant kelp. We argue that Moran interactions should be common. Our theory and analysis explain an important new aspect of a fundamental feature of spatiotemporal population dynamics.

Metapopulations, the inflationary effect, and consequences for public health

“…As a result, biodiversity may enhance and buffer ecosystem processes by virtue of periods of inflationary growth and spatial exchanges among local ecosystems, even when such effects do not occur in a closed homogeneous system. Factors that synchronize population dynamics across ecosystems (Reuman et al, in press) or constrain dispersal will erode the multispecies inflationary dynamics that contribute to metacommunity stability.…”

Dispersal synchronizes giant kelp forests

“…Moreover, it is not known whether the synchronizing effects of nutrients and waves on giant kelp (Moran effects) remain strong after controlling for the potentially synchronizing effects of dispersal (Castorani et al, 2022). Lastly, although patterns of giant kelp synchrony have been described for the California mainland (Castorani et al, 2022;Reuman et al, 2023;Walter et al, 2022), it is unknown whether patterns of synchrony and their drivers are similar for populations surrounding the California Channel Islands. These islands support 64% of southern California's kelp forests (measured as canopy biomass, 1984Bell, 2023) and are subject to patterns of nitrate supply, wave exposure, ocean currents, and geographic isolation that differ from those of the mainland (Bell et al, 2015;Castorani et al, 2017).…”

“…Coastal upwelling delivers nitrate-rich water that fuels rapid growth (Fram et al, 2008), and periods of low nitrate have been associated with declines in giant kelp biomass, reduced recruitment, and delayed population recovery (Bell et al, 2015;Cavanaugh et al, 2011Cavanaugh et al, , 2019Edwards, 2019). Therefore, spatial variation in nitrate concentrations and wave heights partly determine geographic patterns of giant kelp synchrony (Castorani et al, 2022;Reuman et al, 2023;Walter et al, 2022Walter et al, , 2024. Cavanaugh et al (2013) speculated that dispersal may be an additional factor synchronizing giant kelp populations based on an observed rapid decay of synchrony with increasing distance that roughly coincided with the typical distances of giant kelp spore dispersal (up to several kilometers).…”

“…In addition to these metrics of dispersal, we investigated two environmental variables as potential drivers of giant kelp synchrony via Moran effects: disturbance in the form of destructive waves and resources in the form of seawater nutrient availability. These factors were recently shown to induce synchrony in giant kelp populations across the California mainland (Castorani et al, 2022;Cavanaugh et al, 2013;Reuman et al, 2023;Walter et al, 2024), but such studies did not quantify effects at the California Channel Islands, nor did they control for the potential synchronizing effects of dispersal.…”

“…Here, we examine this hypothesis. We focused our investigation on giant kelp Macrocystis pyrifera, a broadly distributed marine foundation species that has served as an effective system for studies of synchrony in natural populations (Castorani et al, 2022;Cavanaugh et al, 2013;Reuman et al, 2023;Walter et al, 2022Walter et al, , 2024. Giant kelp is patchily distributed on rocky reefs in shallow coastal seas and demographically linked by ocean currents that disperse kelp spores (Castorani et al, 2015(Castorani et al, , 2017Reed et al, 2006).…”