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

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.

Priority effects and ecological restoration

“…First, long‐term field experiments would allow ecologists to investigate how persistent priority effects are in a variety of environments (Vaughn & Young 2015; Werner et al 2016; Weidlich et al 2017). Second, the setup of multiple experiments with the same design at different locations and/or different time points would allow tests of the importance of site and year effects for priority effects (Stuble et al 2017; Werner et al 2020). Third, we argue that some coexistence mechanisms would be more accurately tested in the field than in experiments where root growth is constrained by the size of pots or mesocosms.…”

“…Interannual variation in weather can create “year effects” in the composition of plant communities. Thus, very similar restoration approaches may produce very different outcomes depending on the year restoration was initiated (see Vaughn & Young 2010; Werner et al 2020). However, few studies have experimentally tested how the year of initiation of a priority effect experiment affects the structure and functioning of plant communities, and what would be the main environmental drivers.…”

Accounting for year effects and sampling error in temporal analyses of population and biodiversity change - Response to Seibold et al. 2019 “Arthropod decline in grasslands and forests is associated with landscape-level drivers”

“…Field studies that yield multiple observations per year from sites within a region are likely to be influenced by many shared uncontrolled variables, creating a 'year effect' because observations from the same year will often be more similar (Werner, Stuble, Groves, & Young, 2020). If this year pseudoreplication is ignored, our confidence in trends estimated across years and type I errors can be greatly inflated (Knape, 2016).…”

“…If this year pseudoreplication is ignored, our confidence in trends estimated across years and type I errors can be greatly inflated (Knape, 2016). A simple remedy for year effects is to include a year intercept random term in statistical models (Knape, 2016;Werner et al, 2020). Seibold et al 2019 presented an analysis of arthropod diversity trends across 140 (30 in some analyses) forest plots and 150 grassland plots over a 10-year period (a nine-year period in some cases).…”

Soil chemical legacies trigger species-specific and context-dependent root responses in later arriving plants

“…A ssembly history is an important determinant of the structure and functioning of ecological communities (Chase, 2003;Fukami et al, 2010;Halliday et al, 2020), and plant communities are no exception (Werner et al, 2016). The sequence and timing of multiple biotic and abiotic events that happened in the past cause plant communities to be historically contingent (Temperton et al, 2016;Werner et al, 2020). This historical contingency is often caused by priority effects, in which the order and timing of species immigration influence further assembly by determining the way species affect one another in communities (Fukami, 2015).…”