Similar yet distinct distributional patterns characterize native and exotic plant species richness across northern New Zealand islands

Abstract

Aim A better understanding of plant invasions on islands can be gained from comparing patterns of exotic and native species richness. We asked four questions: (1) Is exotic species richness on islands related to native species richness? (2) If they are related, does this result from similar responses of native and exotic species to specific island characteristics? (3) Is residual variation in native‐exotic richness relationships associated with distinctive island characteristics? (4) Are relationships between species richness and island characteristics scale‐dependent, and do they differ between native and exotic species? Location Northern New Zealand. Taxon Vascular plants. Methods We conducted field surveys and augmented our field data with previously published surveys to quantify the number of native and exotic plant species on 264 islands. We then explored the relationship of species richness and several island characteristics (e.g. area and isolation) using multiple and iterative regression techniques. Results Seventy‐two percent of among‐island variation in exotic species richness was positively related to native species richness. Both native and exotic richness increased with island area and declined with isolation and exposure to ocean‐borne disturbances (a proxy for salt spray, wave action, etc). However, exotic species responded more strongly to these three variables. Exotic richness also decreased with latitude and the distance from the nearest urban area, but native species did not. Island area was a better predictor of species richness on larger islands, whereas isolation and exposure were better predictors on smaller islands. Scale‐dependent relationships between species richness and island characteristics were stronger for exotic species. Main Conclusions Insular distribution patterns of native and exotic plant species richness are governed by similar biogeographic principles. However, in New Zealand, exotic species exhibited subtle, yet distinctive, invasion patterns preferring larger, less isolated, less exposed islands that were located at higher latitudes and closer to urban areas.

Different levels of disturbance influence the distributional patterns of native but not exotic plant species on New Zealand small islands

“…Only a few, well-adapted exotic species appear to be able to withstand harsh coastal environments, indicating that biological invasions are not always favoured by disturbances (Moles et al 2012). Previous work indicates that exotic species are particularly susceptible to ocean-borne disturbances, especially on small islands (Mologni et al 2021). I suggest exotic species management not to treat these islands differently depending on levels of disturbance.…”

“…In protected locations, disturbance events are generally less intense and frequent, and conditions more lenient for vascular plants (Abbott 1977, Morrison 2014. Results to date vary as to whether exotic species are facilitated (Bellingham et al 2005, Burns 2016) or disadvantaged (Mologni et al 2021) by ocean-borne disturbances.…”

“…Species richness on islands often declines with ocean-borne disturbances (Abbott 1977, Morrison 2002, Mologni et al 2021). Waves and winds can reduce the habitable surface area 4 of an island (Neufeld et al 2017).…”

“…Waves and winds can reduce the habitable surface area 4 of an island (Neufeld et al 2017). Thus, given two small islands of similar size and height, the one in a more disturbed location will have fewer species (Abbott 1977, Neufeld et al 2017, Mologni et al 2021. Additionally, species on smaller islands are more prone to extinction, a process enhanced by perturbation (Whittaker 1995).…”

Island biogeography of plants and humans

“…In addition, current estimates of human disturbance, such as current inhabitation, are likely to be confounded with island area, since larger islands are preferentially inhabited (mean area inhabited islands = 15 km2, x̄ uninhabited = 0.12 km 2 , χ 2 = 14.64, p = 0.0001). Past disturbances, such as fires, may be underestimated and require better resolution in my study system, hence I believe better proxies for estimating human disturbance are needed to predict plant invasions more accurately (Mologni et al 2021).…”