If evolution happened anew, what would the present-day plant world look like? That is, would the randomized processes that govern evolutionary change tell a different story? And particularly for plants which are sessile organisms, is the starting point of ‘who gets there first’ the most important of all?
Priority effects – the order and timing of species arrival into local communities – can affect ecological community structure and functioning, with profound effects for species persistence and ecological interactions (Chase et al., 2000; van de Voorde et al., 2011). As such, the arrival of different species at different times can dramatically alter the evolutionary tapestry of any given system on ecological time frames, but also in evolutionary time. In particular, the diversification of early arriving species can pre-empt available niche space to prevent the establishment, dominance or diversification of species that arrive later on down the road.
While priority effects have been explored in plant community ecology (Körner et al., 2008; Fukami, Mordecai & Ostling, 2016), the incorporation of evolutionary priority effects has lagged behind, in spite of the potential importance for our understanding of plant interactions with herbivores and pathogens (Chase et al., 2000). There are examples of evolutionary priority effects in lizards, bacteria and alpine angiosperms (Fukami et al., 2007; Leopold et al. 2015), but key questions remain. Do these effects diminish over time? Are priority effects less evident in disturbed ecosystems, given that disturbance and related extinctions can open niche space for resident and immigrant taxa? Do priority effects vary across gradients of abiotic conditions, given that the degree of competitive interactions and thus evolutionary priority effects are expected to be higher in benign, environmentally resource-rich environments? We don’t often think of the importance of these evolutionary priority effects when examining plant community structure and diversity, but given the Editor’s Choice for issue 104:6, perhaps we should be taking a closer look.
The Editor’s Choice article for this issue of Journal of Ecology, “Precipitation alters the strength of evolutionary priority effects in forest community assembly of pteridophytes and angiosperms,” by Brandt and colleagues focuses on community assembly of pteridophytes and angiosperms in the ancient temperate forests of New Zealand. These forests have existed throughout the Cenozoic (>60 million years), and thus have experienced partial marine inundation and major climate shifts driving species extinctions, and are widely distributed across a well-studied gradient of precipitation from the drier east to the wetter west.
Brandt et al. collected forest community data from 340 plots, in which they recorded the percentage area cover of each plant species present. They then combined this ecological data with a unique and modern focus on the analysis on 38 generic or subgeneric clades of pteridophytes and angiosperms for which time-calibrated molecular phylogenies have been published.
The headline result is that older clades of both ferns and angiosperms tended to have greater relative abundance and older angiosperm clades also had greater relative richness in these New Zealand ecosystems. Thus, being an early arriver appears to result in evolutionary priority effects, even in an environment with a long evolutionary history that has experienced substantial disturbance and extinction.
Brandt et al. also showed that the precipitation gradient changed the strength of the clade age effect on community dominance. Clade age had a stronger effect on the relative abundance of pteridophytes– moisture-loving ferns – with increasing precipitation and thus a reduced role of competition in community assembly in more benign environments. In contrast, the effect of clade age on relative abundance of angiosperms decreased with increasing rainfall. Thus variation in environmental conditions is likely to mediate the strength of evolutionary priority effects on plant community assembly.
This study provides new evidence for the generation of general predictions about which plant communities may be strongly impacted by evolutionary priority effects, using a unique gradient of ecosystems in New Zealand. It provides a valuable stepping-stone towards revealing what the biotic world would look like if species had arrived and diversified in a different order, harkening back to the vision of Stephen Jay Gould’s replaying of the tape of life.
Associate Editor, Journal of Ecology
Senior Editor, Journal of Ecology