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The importance of Proteaceae diversity to nectar-feeding birds

30 Jul 2020 - 10:00

Nectar-feeding birds and the plants that they feed from benefit each other and they are expected to be highly dependent on each other. Anthropogenic effects on one of these parties will thus likely affect the other party, or even whole communities. A study (http://dx.doi.org/10.1111/1440-1703.12148) investigated the relationship between nectar-feeding birds in the Cape Floristic Region (CFR) and one of their most wide-spread and common food sources: bird-pollinated Proteaceae species. At least 71 species from the genera Protea, Leucospermum and Mimetes depend, to varying degrees, on the Cape Sugarbird and three other sunbird species.

At the community level scale, i.e. one-hectare plots, bird abundances and species richness were most strongly linked to the amount of nectar at the site. There was also a threshold of nectar levels (found in Proteaceae communities of 4-5 years old) necessary to attract significant numbers of birds. Most bird-pollinated Proteaceae are slow growing, thus young communities, produce few flowers and attract few birds. Such communities, as well as small and isolated communities, may therefore suffer low pollination rates.

This highlights the threat that high fire frequencies pose to these communities. Many bird-pollinated Proteaceae species only reproduce through seeds and are thus highly dependent on birds for their pollination services. Fires at intervals of less than 5 years at a single site could result in local extinction of some of these species. In fact, Cape Sugarbirds only visited Proteaceae communities that are 4-years or older, strongly indicating that young communities are vulnerable.

Two citizen science projects, the South African Bird Atlas Project and the Protea Atlas Project, were combined to assess the relationship between nectarivorous birds and bird-pollinated Proteaceae species across the whole CFR. This revealed that the bird species richness and the abundance of three of the bird species depended more on the diversity of these Proteaceae than their floral abundance. This may be because of the complementary flowering times of species from the different genera, which could provide food for the birds in one area all-year-round. The fact that all regions in the CFR had low floral abundances in the summer, further indicates that birds rely on local food sources throughout the year, rather than migrating between mountain ranges. The conservation of diverse Proteaceae communities is thus critical, particularly considering the fragmentation of natural habitats through land-use change that possibly restrict birds to fragments.