Ecological Impacts of Exotic Species on Native Seed Dispersal Systems: A Systematic Review

Abstract

Exotic species are one of the main threats to biodiversity, leading to alterations in the structure and functioning of natural ecosystems. However, they can sometimes also provide ecological services, such as seed dispersal. Therefore, we assessed the ecological impacts of exotic species on native dispersal systems and the mechanisms underlying the disruption of mutualistic plant-disperser interactions. Exotic species negatively affect dispersal mutualisms by (i) altering dispersal behavior and visitation rates of native dispersers, (ii) predating native dispersers, (iii) transmitting forest pathogens, and (iv) predating seeds. Conversely, positive impacts include the dispersal of native plants, forest regeneration, and native habitat restoration via (i) increasing the visitation rates of frugivorous birds, (ii) facilitating the colonization and establishment of native forest trees, (iii) enhancing forest species seedling survival, and (iv) facilitating seed rain and seedling recruitment of early and late successional native plants. The reviewed studies provide similar results in some cases and opposite results in others, even within the same taxa. In almost all cases, exotic species cause negative impacts, although sometimes they are necessary to ensure native species' persistence. Therefore, exotic species management requires a comprehensive understanding of their ecological roles, since the resulting effects rely on the complexity of native-exotic species interactions.

Keywords: biological invasions; dispersal disruption; frugivory; invasive species; plant–animal mutualism; seed predation; zoochory.

Figure 1

General overview of exotic taxa distribution and their ecological impacts on native seed dispersal systems, including the number of studies reviewed for each continent. Symbols indicate the effect exerted by exotic species, classified as negative (−), positive (+), mixed (−|+), and neutral (0).

Figure 2

Mechanisms explaining the disruption of plant–disperser mutualism, based on reviewed studies: (a) forest pathogens transmitted by exotic species cause a decrease in the population of native trees, with the consequent loss of plant–frugivore interaction and facilitating the invasion of exotic plants; (b) exotic species affect dispersal behaviors and visitation rates of native dispersers; (c) feral cats predate native frugivorous lizards, disrupting seed dispersal services; (d) exotic plants compete with natives for frugivore species; and (e) exotic plants negatively influence disperser diversity and alter dispersal networks. Exotic species are represented in gray, and natives in black.

Figure 3

PRISMA flowchart summarizing the sequence of information gathering and selection for the systematic review.