The role of fruit traits of bird‐dispersed plants in invasiveness and weed risk assessment

Aim  Birds play a major role in the dispersal of seeds of many fleshy‐fruited invasive plants. The fruits that birds choose to consume are influenced by fruit traits. However, little is known of how the traits of invasive plant fruits contribute to invasiveness or to their use by frugivores. We aim to gain a greater understanding of these relationships to improve invasive plant management. Location  South‐east Queensland, Australia. Methods  We measure a variety of fruit morphology, pulp nutrient and phenology traits of a suite of bird‐dispersed alien plants. Frugivore richness of these aliens was derived from the literature. Using regressions and multivariate methods, we investigate relationships between fruit traits, frugivore richness and invasiveness. Results  Plant invasiveness was negatively correlated to fruit size, and all highly invasive species had quite similar fruit morphology [smaller fruits, seeds of intermediate size and few (< 10) seeds per fruit]. Lower pulp water was the only pulp nutrient trait associated with invasiveness. There were strong positive relationships between the diversity of bird frugivores and plant invasiveness, and in the diversity of bird frugivores in the study region and another part of the plants’ alien range. Main conclusions  Our results suggest that weed risk assessments (WRA) and predictions of invasive success for bird‐dispersed plants can be improved. Scoring criteria for WRA regarding fruit size would need to be system‐specific, depending on the fruit‐processing capabilities of local frugivores. Frugivore richness could be quantified in the plant’s natural range, its invasive range elsewhere, or predictions made based on functionally similar fruits.