Priority effects vary with species identity and origin in an experiment varying the timing of seed arrival

Abstract

Exotic species are sometimes phenologically distinct from native species in the invaded community, allowing them to be active when there may be reduced competition for resources. In southern California, annual species are particularly problematic invaders, and prior work has shown that these species germinate earlier in the growing season, giving them a competitive advantage over later-germinating native species. Th is result begs the question, if being active earlier is advantageous, why have not native species adapted earlier cues for germination? We hypothesized native species would benefi t less from earlier germination than exotic species (potentially due to slower growth following germination), thus negating potential selection for early germination. Here we manipulated planting time for common native and exotic species, growing them in all possible species pairs, to evaluate how competitive outcomes were altered by the time of arrival and the origin of competing species. In contrast to our hypotheses, the exotic species often had lower biomass when planted fi rst, potentially due to disturbance when the second species was planted. In contrast, three out of our four native species benefi ted from earlier planting (a priority eff ect). Unlike the potential benefi t of arriving early, we found no evidence that being planted one week later resulted in a competitive disadvantage, when compared to being planted simultaneously with a competitor. Further, we found that the magnitude and even direction of priority eff ects varied depending on the identity of the interacting species. Together these results suggest that a lack of directional selection may prevent adaptation towards earlier germination times of native species. Although this experiment was conducted with a limited suite of species, the results show that the role of seasonal priority eff ects varies among species, and that native species could benefi t from seasonal priority eff ects in restoration eff orts even when in competition with fast-growing exotic annual species.

Using priority effects to manipulate competitive relationships in restoration

“…dependent on site), are sometimes asymmetrical (i.e. benefit some guilds more than others; see also Blaisdell ; Cleland et al ; Vannette & Fukami ; Werner et al in press), and are of variable persistence.…”

Priority effects: natives, but not exotics, pay to arrive late

“…Though there is evidence that exotic species may benefit from early arrival to a greater extent than do natives (Dickson, Hopwood & Wilsey ), some studies have found no difference in the strength of priority benefits incurred by native and exotic plant species (Cleland, Esch & McKinney ). Such differences in results among studies likely stem from important differences among species and study systems, such as potentially different effects of priority on annual and perennial‐dominated systems (the Dickson, Hopwood & Wilsey () study took place in a perennial‐dominated system, while the Cleland, Esch & McKinney () study was in an annual‐dominated grassland in which only two of the eight study species were perennials). Our study provides a potential avenue by which priority effects may promote the success of exotic species, even in the absence of a strong benefit associated with early arrival.…”

“…The timing of arrival also has the potential to influence our results. In this study, our early arrivers were added 2.5 weeks before late‐arriving species, which is well within the typical range of 1–3 weeks often used in studies of priority (Dickson, Hopwood & Wilsey ; Kardol, Souza & Classen ; Cleland, Esch & McKinney ). However, we might imagine that increasing the time between the addition of early‐ and late‐arriving species may serve to strengthen priority effects, though 2.5 weeks seems to be enough to elicit effects (Kardol, Souza & Classen ).…”

The role of seasonal timing and phenological shifts for species coexistence

“…Theoretically, a variety of relationships of the competition–phenology function are possible, but biological systems generally fall in two broad categories (Wilbur & Alford ; Cleland et al ; Clay et al ): (1) Early arriver advantage, where negative effects of interspecific competition on a given species declines the earlier it arrives before others, and (2) late arriver advantage, where negative effects of interspecific competition on a species decrease when it arrives later than its competitors. The effect of shifts in phenologies depends on whether interacting species fall in opposite or the same arrival advantage category.…”