Fruit and seed traits of native and invasive plant species in Hawai‘i: implications for seed dispersal by non-native birds

Abstract

For alien invasive plant species dependent on frugivores for seed dispersal, traits that influence consumption can be important determinants of invasion and spread. However, trait comparisons between native and invasive species have documented mixed results. In Hawai‘i, one of the most invaded systems in the world, nearly all frugivory, and thus seed dispersal, is dependent on non-native birds. Moreover, the majority of dispersal events also involve invasive plants, suggesting the potential for an “invasional meltdown”. Here we compare fruit and seed traits between native and invasive plants and evaluate how those traits influence consumption by non-native avian frugivores. Although we found that most traits examined were similar between native and invasive fruiting species, invasive species tended to have higher fruit protein content, longer fruiting duration, were less likely to have orange fruits, and have less variable seed length. Longer fruiting duration and smaller seed size were important predictors for avian frugivore consumption. In combination, these results suggest that traits that increase probability of encounter (fruiting duration) and ability to be consumed (seed size) are more important for dispersal by frugivores than traits associated with fruit preferences and, thus, are driving the spread of invasive species and limiting dispersal of some native species. Further, we document an apparent seed size threshold for avian consumption (~ 7.0 mm), supporting previous work suggesting that large seeded species, particularly native plant species adapted for extinct large-bodied frugivores, are likely dispersal limited based on gape size limitation of the current non-native frugivore community.