The pioneer effect advantage in plant invasions: site priming of native grasslands by invasive grasses

Evaluating the mechanisms that drive plant invasions in grassland ecosystems can provide insight into subtle, yet critical, drivers of ecosystem function. Common hypotheses for invader success are that (1) an invader’s physiology may allow better use of resources and competitively exclude resident natives; (2) the lack of invader-specific herbivores and diseases may result in increased growth and competitive advantage over natives; (3) invaders may bring pathogens or herbivores that negatively impact native plants directly; (4) the invader may be able to establish earlier or later than natives, giving it a priority effect or phenological niche separation; (5) the invader may produce more seeds, and propagule pressure may allow it to eventually dominate; and (6) once the invader occurs on a site, its plant detritus or litter can offer a legacy effect advantage. Yet, a widely neglected aspect of invasion is that invaders may prime the conditions in places other than where they are currently established, giving them a pioneer effect advantage. These pioneer effects can come from plant materials that move from invaded to uninvaded areas through three pathways—pollen, litter, seeds, and their associated microbiomes—each of which can favor the invader directly or indirectly. In turn, unsuccessful cohorts of seedlings can change the biotic community through root exudates as well as any biotic load they bring with them, along with taking up nutrients and potentially subverting the litter decay cycle. When seedlings of invaders occur at times when the native species have evolved not to germinate, site priming impacts may increase or accelerate. We propose that monitoring of uninvaded areas adjacent to invaded areas is needed to assess the emerging significance of pioneer effects and site priming in advancing plant invasions, especially in grassland systems worldwide.