The Moran effect revisited: spatial population synchrony under global warming

Abstract

The world is spatially autocorrelated. Both abiotic and biotic properties are more similar among neighboring than distant locations, and their temporal co‐fluctuations also decrease with distance. P. A. P. Moran realized the ecological importance of such ‘spatial synchrony’ when he predicted that isolated populations subject to identical log‐linear density‐dependent processes should have the same correlation in fluctuations of abundance as the correlation in environmental noise. The contribution from correlated weather to synchrony of populations has later been coined the ‘Moran effect’. Here, we investigate the potential role of the Moran effect in large‐scale ecological outcomes of global warming. Although difficult to disentangle from dispersal and species interaction effects, there is compelling evidence from across taxa and ecosystems that spatial environmental synchrony causes population synchrony. Given this, and the accelerating number of studies reporting climate change effects on local population dynamics, surprisingly little attention has been paid to the implications of global warming for spatial population synchrony. However, a handful of studies of insects, birds, plants, mammals and marine plankton indicate decadal‐scale changes in population synchrony due to trends in environmental synchrony. We combine a literature review with modeling to outline potential pathways for how global warming, through changes in the mean, variability and spatial autocorrelation of weather, can impact population synchrony over time. This is particularly likely under a ‘generalized Moran effect’, i.e. when relaxing Moran's strict assumption of identical log‐linear density‐dependence, which is highly unrealistic in the wild. Furthermore, climate change can influence spatial population synchrony indirectly, through its effects on dispersal and species interactions. Because changes in population synchrony may cascade through food‐webs, we argue that the (generalized) Moran effect is key to understanding and predicting impacts of global warming on large‐scale ecological dynamics, with implications for extinctions, conservation and management.

Synchronous effects produce cycles in deer populations and deer‐vehicle collisions

“…We also presented what is, to our knowledge, the first demonstration that synchrony can be transmitted in a Moran‐like effect from ecological to human systems (deer to DVCs). If climate change alters synchrony, as a growing number of papers indicate is likely occurring (Post and Forchhammer, 2004; Allstadt et al ., 2015; Defriez et al ., 2016; Koenig and Liebhold, 2016; Sheppard et al ., 2016; Shestakova et al ., 2016; Black et al ., 2018; Hansen et al ., 2020), our results show that cycles in populations and human systems may change as a consequence.…”

“…Such fluctuations may influence insurance and accident recovery industries, though we have not inquired whether a fluctuation of this magnitude is big enough to be of substantial concern to these industries. Our results provide another reason that additional studies of climate‐change influences on synchrony (Hansen et al ., 2020) should be undertaken: such influences may alter the cycling of populations, including populations of socioeconomic importance.…”

Phenological sensitivity to temperature mediates herbivory

“…This could be because phenologically sensitive plants and their insect herbivores are shifting timing of leaf‐out and emergence, respectively, in synchrony. In warm years, more sensitive plants may also be more synchronized with each other and with early season herbivores (Hansen et al, 2020; Pearse, Funk, et al, 2015). However, it is also possible that either generalist herbivores or a diversity of opportunistic herbivores dominate the insect herbivore community—which might be relatively common in temperate latitudes (Forister et al, 2015)—such that temporal escape from one herbivore increases exposure to another.…”

When does weather synchronize life‐history traits? Spatiotemporal patterns in juvenile body mass of two ungulates

“…If only a weak relationship exists between the weather variable and the population property (e.g. body mass), other stochastic processes may mask the synchronizing effect of the weather variable (Hansen, Grøtan, Herfindal, & Lee, 2020). Accordingly, we predicted a stronger synchronizing effect of the weather variables that best explained the temporal variation in juvenile body mass in moose and reindeer—a prediction that was generally supported by our results (Figure 6a–h).…”