Functional traits explain non-native plant species richness and occupancy on northern New Zealand islands

Abstract

Plant functional traits can greatly influence invasion success on islands. However, interrelationships between traits and invasion success are rarely integrated with the island biogeography theory. Here, we explored relations between functional traits and plant distributions to assess which traits are associated with invasion success (i.e. high island occupancy), test whether non-native richness and seed mass of species with distinct growth forms and dispersal modes vary differently with island characteristics (e.g. area, isolation, exposure to ocean-borne disturbances, distance from the nearest urban area, and whether islands were managed for conservation), and whether results differ from native species. We assembled a database of 264 northern New Zealand offshore islands, amalgamating species lists from field surveys and previously published data, and comprising 822 native and 855 non-native species. Non-native graminoids occurred on islands more frequently than forbs and woody species, and long-distance dispersal modes (wind, animal, unspecialized) more than species with short-distance dispersal modes. Most differences among trait categories of non-native species were associated with human-related variables (i.e. distance from the nearest urban area, and whether islands were conservation areas). Non-native plant species with high island occupancy were less commonly associated with human-related variables than non-native species with low island occupancy. Instead, they were more similar in their distributional patterns to native species within the same trait category, suggesting comparable processes regulate both sets of species. Our results illustrate that integrating trait-based approaches in the island biogeography framework can be a useful tool in understanding and predicting plant invasions.

Time of naturalization is key to explaining non-native plant invasions on islands

“…However, once time of first naturalization was accounted for, and in contrast to an earlier study (Mologni et al, 2022), there were no differences across categories of growth forms, dispersal modes and biogeographic origin.…”

“…Northern Aotearoa New Zealand is the most populous region of the country (Statistics NZ Tatauranga Aotearoa, 2018), and 41 islands are still inhabited within the study system. A more detailed description of the study system is provided by Mologni et al, 2022.…”

“…Five categories of growth forms were identified: graminoids, forbs, woody species, lycophytes and lianas and climbers. Five categories of dispersal modes were defined based on morphological adaptations to long-distance dispersal: wind-, water-, animal-dispersed, unspecialized and short-distance (Arjona et al, 2018;Burns, 2019;Mologni et al, 2022;Negoita et al, 2016). Nine categories of biogeographic origin were delineated: Cosmopolitan, Eurasia (excl.…”

“…Time lags might be shorter for species that disperse more efficiently, either because of shorter life cycles (e.g. herbs) or because of morphological adaptations allowing for long-distance dispersal (Arjona et al, 2018;Mologni et al, 2022;Negoita et al, 2016but see Duncan, 2021. Similarly, species that originate from biogeographic areas with similar climates (i.e.…”

A test of island plant syndromes using resource‐use traits

“…Island plant syndrome predictions are typically investigated by comparing the traits of native island species against nonnative continental plants growing in situ (Mologni et al, 2022) or by comparing a single species growing on a continent to the same species on an island (Burns, 2016). For example, a number of previous studies have tested whether natives and invasives on islands differed in their stature, dispersal mode, and growth rates (Heberling & Mason, 2018; Mathakutha et al, 2019; Sperry et al, 2021) because these traits are thought to be associated with competitive ability and persistence.…”