Outside the envelope: rare events disrupt the relationshipbetween climate factors and species interactions

Abstract

The order in which species arrive during community assembly can be an important driver of community composition and function. However, the strength of these priority effects can be variable, in part because of strong site and year effects. To understand how priority effects vary in importance with abiotic conditions, we initiated identical community assembly experiments in which we varied the timing of arrival of native and exotic grass species in each of 4 yr across three grassland sites in northern California. This uniquely replicated experiment tested the power of priority to determine initial community structure in a restoration context across a natural range of conditions. There were large and significant differences in both total seeded cover and the strength of priority across sites and years of initiation, confirming the suspicion that most ecological experiments may lack spatial and temporal generality. On the other hand, much of the variation in strength of priority could be related to climate. Strikingly, however, the model fit across the three sites and the first 3 yr of the study (the first nine experiments) was radically altered when we included the fourth year, which was characterized by an unusual weather pattern with higher temporal variability in rainfall (a rainfall pattern predicted to increase with climate change). This year produced relatively low strength of priority, supporting the suggestion that highly variable climates may be associated with lower strength of priority effects. Experiments that examine community assembly over a range of naturally occurring abiotic conditions enhance our ability to predict when priority effects will be important, allowing us to explore shifting patterns of community assembly in the face of climate change and optimize restoration strategies based on environmental conditions.

Priority Treatment Leaves Grassland Restoration Vulnerable to Invasion

“…Insertion of native species into a restoration, before non-native species invade, can promote native cover while depressing non-native cover [10,28,29]. In addition, priority effects can be used to promote certain native species when seeded earlier than other native species [4,7,9], but these effects are likely often contingent on factors, including species identity [7,11,18], environmental context [10,12,30], and background levels of invasion. As such, it can be important to evaluate the specifics of a particular system to determine when and where priority can be a useful restoration tool, and when it may leave a system vulnerable to unintended consequences such as invasion.…”

“…In particular, practitioners can use priority effects to enhance desirable but difficult-to-establish species by planting them early [9]. In fact, as little as two-weeks of a head start has been shown to significantly enhance a species representation in a community in some cases [10,11]. However, there has been no exploration of how the use of priority effects in restoration practices may influence non-target species in the community.…”

Year effects: Interannual variation as a driver of community assembly dynamics

“…Including multiple types of climatic variables, such as temperature, precipitation, or cumulative variables like climatic water deficit in multivariate analyses can be useful in elucidating the drivers of year effects (Young et al 2019). These variables can be considered on an annual level or broken down by month (as with temperature in Stuble et al 2017 b ). Another approach is to use ecological and system‐specific knowledge to determine which variables to consider in multivariate models (Stuble et al 2017 b ).…”

Toward a more temporally explicit framework for community ecology

“…In general, extreme events during seasonal windows of opportunity (e.g., Yang & Cenzer, ) may be more likely to have strong effects on growth or reproduction, as windows of opportunity may also represent important weak‐link stages that are potentially vulnerable to perturbation. On an interannual scale, many ecological experiments show strong year‐to‐year variability in results (Stuble et al, ; Stuble, Fick, & Young, ; Vaughn & Young, ); this environmental variability can both be caused by event‐driven dynamics (e.g., Stuble et al, ) and affect the outcomes of event‐driven dynamics (Jentsch et al, ). By comparison, the potential for persistent effects resulting from episodic events has been more clearly documented.…”