Abstract
Environmental conditions that vary from year to year can be strong drivers of ecological dynamics, including the composition of newly assembled communities. However, ecologists often chalk such dynamics up to "noise" in ecological experiments. Our lack of attention to such "year effects" hampers our understanding of contingencies in ecological assembly mechanisms and limits the generalizability of research findings. Here, we provide examples from published research demonstrating the importance of year effects during community assembly across study systems. We further quantify these year effects with two case studies-a grassland restoration experiment and a study of postfire conifer recruitment-finding that the effects of initiation year on community composition can dictate community as much, if not more, than the effects of experimental treatments or site. The evidence strongly suggests that year effects are pervasive and profound, and that year effects early in community assembly can drive strong and enduring divergence in community structure and function. Explicit attention to year effects in ecological research serves to illuminate basic ecological principles, allowing for better understanding of contingencies in ecology. These dynamics also have strong implications for applied ecological research, offering new insights into ecological restoration as well as future climate change.
Highlights
Community assembly is a central process in both wild and managed ecosystems
We review existing studies of the effects of interannual variation on community assembly and argue that year effects are both pervasive and important, yet currently undervalued in ecology
To quantify the relative importance of year effects on community-level metrics, we explored two empirical data sets assessing interannual variation in the outcomes of community assembly: (1) an experimental California grassland restoration study that manipulated order of species arrival during assembly across four initiation years and (2) an exploration of postfire pine establishment across an experimental precipitation manipulation across two initiation years
Summary
Community assembly is a central process in both wild and managed ecosystems. It drives the nature of regeneration—after natural disturbances (e.g., floods, fires, tree falls), yearly in annual-dominated systems (Pitt and Heady 1978), following human land use (e.g., agriculture, Cramer et al 2008; timber harvest, Belote et al 2012), and during ecological restoration (Young et al 2001, 2005, Brudvig 2011). We review existing studies of the effects of interannual variation on community assembly and argue that year effects are both pervasive and important, yet currently undervalued in ecology. In their 3-yr grassland restoration experiment, Bakker et al (2003) observed strong interannual variation in the establishment and survival of native species, depending on the weather.
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