Non-native invasions may mask evolutionary priority effects in secondary grasslands resulting from anthropogenic disturbance
Little is known about the ecological conditions promoting evolutionary priority effects, when the order of ancestral species arrival into a new habitat influences extant community structure. Evidence suggests that evolutionary priority effects operate through niche pre-emption, where occupation of niche space by early-arriving lineages that subsequently diversify precludes establishment and dominance of later arrivals. We have previously shown that earlier-arriving lineages have greater community dominance in both alpine and forest ecosystems on South Island, New Zealand, with evidence suggesting that evolutionary priority effects occur via the inferred mechanism of niche pre-emption. Here we ask whether such priority effects occur across boundaries of anthropogenic disturbance that cause a transition in habitat type, specifically from forest to grassland. We analyzed vegetation data from a complete elevational gradient above and below historic treeline to determine the potential for immigration history of native lineages to determine community structure in primary vs. secondary grasslands, respectively. We also investigated the relative role of non-native plant invasion compared to priority effects in affecting community structure.
Our data set contained 19 genera for which published stem age estimates were available for the divergence between the New Zealand clade and closest relative outside New Zealand, used as a proxy for the immigration timing of these genera. Only 12 of these focal genera occurred below historic treeline. Community dominance tended to increase with age of lineage above historic treeline (i.e. 95% confidence intervals for effect of stem age on relative generic richness at the plot-scale excluded zero in a Bayesian linear model), indicating that evolutionary priority effects occurred similar to those observed in other alpine zones in New Zealand. However, stem age was unrelated to community dominance below historic treeline, or in secondary grassland. When only native species were included in the analysis, relative generic abundance increased with stem age both above and below historic treeline. Thus, lineage age appears to confer an advantage for community dominance when new habitat becomes available through disturbance, but this priority effect may be masked by invasion of non-native species that are likely pre-adapted to anthropogenic disturbance.