Can short-billed nectar thieving sunbirds replace long-billed sunbird pollinators in transformed landscapes?

A generalized bird pollination system in Schotia brachypetala (Fabaceae)

Bird pollination systems are diverse, ranging from narrow-tubed flowers pollinated by specialist nectarivores such as hummingbirds and sunbirds, to relatively open flowers pollinated by opportunistic (i.e. generalist) nectarivores. The role of opportunistic avian nectarivores as pollinators has historically been under-appreciated. A key aspect to understanding the importance of opportunistic birds as pollinators is to investigate how efficiently they transfer pollen among flowers. Here, we document the pollination and breeding systems of Schotia brachypetala, a southern African tree known as the 'weeping boer-bean' on account of its prolific production of dilute hexose-dominated nectar. The cup-shaped flowers of this tree attract a large number of bird species, including both opportunistic and specialist nectarivores. We identified floral visitors using observations and camera traps and quantified the floral traits responsible for animal attraction. We documented the breeding system, used selective pollinator exclusion to test the contribution of birds to fecundity, and performed supplemental pollination to test for pollen limitation. Single-visit pollen deposition trials were undertaken to determine the efficacy of bird pollinators. Controlled hand-pollination experiments showed that S. brachypetala is genetically self-incompatible and therefore dependent on pollinators for seed production. Supplemental hand-pollination experiments showed that natural fecundity is limited by either the amount and/or the quality of pollen on stigmas. Flowers from which birds but not insects were experimentally excluded set fewer seeds than open control flowers. Opportunistic birds deposited more pollen per visit than did specialist sunbirds. We conclude that S. brachypetala has a generalized bird pollination system that mainly involves opportunistic nectarivores.

An invasive alien Proteaceae lures some, but not all nectar-feeding bird pollinators away from native Proteaceae in South African fynbos

Invasive alien plants often influence pollinator visitation to native plants when sharing pollinator guilds. It is of conservation concern when the invasive alien plant is characterized by floral resources that attract pollinators, thereby reducing the reproductive success of native species. This is well studied for insects, but whether the same is true for bird pollinators is largely unknown. We address this by considering the impact of an invasive alien plant (Banksia speciosa) on visitation rates of nectar-feeding bird pollinators to native Protea compacta in the Cape Floristic Region of South Africa. We determined bird pollinator visitation rate to B. speciosa and P. compacta over 21 h of observation at three sites. We also quantified how visitation rate influenced reproductive success of both study species through different breeding experiments. Sugarbird visitation to P. compacta was significantly lower in the presence of B. speciosa, while there was no effect for sunbirds as they mostly avoided B. speciosa. Protea compacta had higher nectar volumes and sucrose per flower than B. speciosa. Sucrose per hectare was higher in all P. compacta plots compared to B. speciosa. Neither study species is pollen limited and they are self-compatible to some extent. But pollinator visitation enhanced seed production in both species. We show here that the invasive alien B. speciosa flowers attract sugarbirds - but not sunbirds - away from native P. compacta. The long-term effect of reduced pollinator visitation may reduce the fitness of P. compacta, but the long-term demographic impact is unknown and would require further study.

A Bird's-Eye View of Pollination: Biotic Interactions as Drivers of Adaptation and Community Change

Nectarivorous birds and bird-pollinated plants are linked by a network of interactions. Here I ask how these interactions influence evolution and community composition. I find near complete evidence for the effect of birds on plant evolution. Experiments show the process in action—birds select among floral phenotypes in a population—and comparative studies find the resulting pattern—bird-pollinated species have long-tubed, red flowers with large nectar volumes. Speciation is accomplished in one “magical” step when adaptation for bird pollination brings about divergent morphology and reproductive isolation. In contrast, evidence that plants drive bird evolution is fragmentary. Studies of selection on population-level variation are lacking, but the resulting pattern is clear—nectarivorous birds have evolved a remarkable number of times and often have long bills and brush-tipped or tubular tongues. At the level of the ecological guild, birds select among plant species via an effect on seed set and thus determine plant community composition. Plants simultaneously influence the relative fitness of bird species and thus determine the composition of the bird guild. Interaction partners may give one guild member a constant fitness advantage, resulting in competitive exclusion and community change, or may act as limiting resources that depress the fitness of frequent species, thus stabilizing community composition and allowing the coexistence of diversity within bird and plant guilds.

Nectar robbers influence the trait-fitness relationship of Primula secundiflora

The trait-fitness relationship influences the strength and direction of floral evolution. To fully understand and predict the evolutionary trajectories of floral traits, it is critical to disentangle the direct and indirect effects of floral traits on plant fitness in natural populations. We experimentally quantified phenotypic selection on floral traits through female fitness and estimated the casual effects of nectar robbing with different nectar robbing intensities on trait-fitness relationships in both the L- (long-style and short-anther phenotype) and S-morph (short-style and long-anther phenotype) flowers among Primula secundiflora populations. A larger number of flowers and wider corolla tubes had both direct and indirect positive effects on female fitness in the P. secundiflora populations. The indirect effects of these two traits on female fitness were mediated by nectar robbers. The indirect effect of the number of flowers on female fitness increased with increasing nectar robbing intensity. In most populations, the direct and/or indirect effects of floral traits on female fitness were stronger in the S-morph flowers than in the L-morph flowers. In addition, nectar robbers had a direct positive effect on female fitness, but this effect varied between the L- and S-morph flowers. These results show the potential role of nectar robbers in influencing the trait-fitness relationships in this primrose species.