—Francis Bacon in Novum Organum, 1620
Neither the biotic nor abiotic environment are fixed—they are an ever-changing milieu of shifting conditions, traits, and abundances. While environmental variation may be regular and predictable (e.g. seasonality), there are also rare, unpredictable, and large environmental pulses (e.g. hurricanes; Ackerly 2003, Nunez and Logares 2012, Anderson et al. 2017). These extreme “Black Swan” events are increasingly recognized as biologically important, but we know relatively little about how these extreme events shape ecological and evolutionary processes, especially in the wild (Pruitt et al. 2019). Understanding how extreme Black Swan events shape ecological and evolutionary systems is a particularly pressing issue as such events appear to be increasing in frequency (Fey et al. 2015), a pattern predicted by models of global change.
Extreme events impose harsh ecological and evolutionary filters, potentially triggering large demographic changes, shifts in community composition, and strong natural selection (Fey et al. 2015, Siepielski and Benkman 2007, Little et al. 2019). For instance, rare but regular fire events can reset succession by removing most trees from a forest (Shafi and Yarranton 1973), while also strongly favoring trees with fire-resistant traits (Perry and Lotan 1979). Extreme events can also create characteristic patterns across broader spatial scales relevant to the maintenance of biodiversity. When extreme events occur in some but not other areas, they create a mosaic of abiotic heterogeneity (Siepielski and Benkman 2007, Little et al. 2019). However, when extreme events occur simultaneously over large areas (e.g. a hurricane hitting a whole island), it is generally assumed that the environment, and the corresponding ecological interactions and patterns of selection, will be homogenized (Little et al. 2019). These hetero/homo-genization of abiotic conditions caused by Black Swan events can have key landscape-level consequences for ecological and evolutionary patterns. If the environment is homogenized, it should reduce β-diversity, spatial variation in natural selection, and the evolutionary trajectory of populations. Conversely, extreme events that create abiotic and biotic heterogeneity may be crucial for maintaining biological diversity across a landscape by allowing species with different requirement to use different patches, and by creating divergent patterns of natural selection and evolutionary change (Start et al. 2020). Extreme events should influence landscape level patterns of environmental conditions and corresponding eco-evolutionary change.
Most ecological and evolutionary research is focused on investigating regular and predictable events, rather than extreme ones. The dearth of research exploring the consequences of extreme events is surprising given their likely importance (Ackerly 2003, Fey et al. 2015, Pruitt et al. 2019), but may stem from some key methodological constraints (Nunez and Logares 2012). One key issue is that, while some events like hurricanes can be described as rare and extreme events post-hoc (Pruitt et al. 2019), in most cases understanding what constitutes an extreme event requires the long-term monitoring of a biological system—a 100 year storm for one population may be a common occurrence for another. While long term datasets do exist, they generally focus on one or few communities, do not distinguish among what may be distinct population (Nunez and Logares 2012), and/or do not measure intraspecific variation (but see Pruitt et al. 2019 and Siepielski and Benkman 2013), crucial data for understanding eco-evolutionary change. To understand the broader spatial effects of Black Swan events therefore requires both long-term monitoring of species’ abundances and individual traits, and the replication of surveys across multiple distinct communities and populations—few such datasets exist. Because the landscape-level effects of extreme events likely shape eco-evolutionary dynamics, we require long-term spatially replicated datasets to gain a better understanding of the forces structuring biological systems.
Here, I present a 6-year dataset spanning 15 populations that describes the interactions between a gall-making fly and its enemies. Gall size is under selection—bird attack favors small galls, parasitoid attack favors large galls, and the net direction of selection depends on the balance of both attack rates (Abrahamson 1989, Start and Gilbert 2016). Bird attack rate (and resultant selection) is influenced by several key factors, notably whether a gall has been knocked to the ground (making them less salient; Start 2018a) and the distance of the gall from forest habitat where bird predators reside (Start et al. 2018). In the 5th year of monitoring, a windstorm homogenized one of these key environmental variables—virtually all galls were knocked to the ground. I ask how the homogenization of this key environmental variable affects the local and landscape consequences of (1) gall-maker survival, (2) species interactions, (3) selection, (4) demographic consequences in the following generation, and (5) evolutionary change.