Consequences of invasion for pollen transfer and pollination revealed in a tropical island ecosystem

Abstract

Pollination is known to be sensitive to environmental change but we lack direct estimates of how quantity and quality of pollen transferred between plant species shifts along disturbance gradients. This limits our understanding of how species compositional change impacts pollen receipt per species and structure of pollen transfer networks. We constructed pollen transfer networks along a plant invasion gradient in the Hawaiian dry tropical forest ecosystem. Flowers and stigmas were collected from both native and introduced plants, pollen was identified and enumerated and floral traits were measured. We also characterized pollen loads carried by individuals of the dominant invasive pollinator, Apis mellifera. Species flowering in native-dominated sites were more tightly connected by pollen transfer than those in heavily invaded sites. Compositional turnover in the pollen loads of A. mellifera was correlated (70%) with turnover in the composition of pollen transfer networks. Floral traits predicted species roles within pollen transfer networks, but many of these differed qualitatively depending on whether plants were native or introduced. Our work indicates that pollen transfer networks change with invasion. Floral morphology and foraging behaviour of the introduced super-generalist pollinator are implicated as key in determining the roles introduced species play within native pollen transfer networks.

Patterns and effects of heterospecific pollen transfer between an invasive and two native plant species: the importance of pollen arrival time to the stigma

“…In a recently published meta‐analysis, Arceo‐Gómez and Ashman () found that invasive HP donors reduced fruit and seed production of recipients to a greater degree than native HP donors did, but to our knowledge, the effect of native HP donors on the pollination success of invasive species has not been evaluated. Knowledge on the differential responses to HP between native and invasive plants could help us understand the mechanisms that facilitate plant invasion in natural communities (Arceo‐Gómez and Ashman, ; Johnson and Ashman, ; Takemori et al., ). For instance, invasive species are able to fully integrate into native plant–pollinator networks (e.g., Lopezaraiza‐Mikel et al., ; Vilà et al., ; Maruyama et al., ; Tylianakis and Morris, ) which can be aided, at least partially, by a high level of tolerance to HP from native species, thus enhancing their success in invading new communities.…”

“…Second, invasive plants can alter pollinator visitation quality by increasing heterospecific pollen (hereafter, HP) deposition from IS onto the stigmas of native species (e.g., Lopezaraiza‐Mikel et al., ). This latter mechanism has been increasingly studied (see Bartomeus et al., ; Morales and Traveset, ; Arceo‐Gómez and Ashman, ; Bruckman and Campbell, ), and evidence has accumulated indicating that HP transfer within co‐flowering communities is common (e.g., Ashman and Arceo‐Gómez, ; Fang and Huang, ; Johnson and Ashman, ). Less studied, however, are the potential effects of HP transfer from natives to stigmas of IS (Arceo‐Gómez and Ashman, ), even though it is likely that HP transfer between invasive and native plants is not unidirectional (from invasive to native only).…”

Floral traits are associated with the quality but not quantity of heterospecific stigmatic pollen loads

“…However, patterns for the intensity or quantity of pollen transfer, i.e. the amount of CP / HP pollen received or donated, are not well explained by floral traits [7,26].…”

“…Associations between floral traits, HP reception or donation and network properties are variable and even contradictory [3,26,27]. In general, stigma size, flower symmetry and floral size increase the likelihood and intensity of HP reception [3,27,28].…”

“…The in-degree, i.e. the number of plant species that receive pollen, is positively correlated with style length [26,27]. In contrast, the number of plant species that donate pollen, i.e.…”