—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.