The reproductive ecology of Aloe reitzii var. reitzii (Asphodelaceae): a South African grassland endemic
Date
2021
Authors
Minnaar, Jessica J
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Abstract
According to the pollination syndrome hypothesis, floral traits associated with attracting particular pollinators are used to infer likely pollinators. Most Aloe species are characterized by a bird-pollination (ornithophilous) syndrome, as seen by their red/orange floral displays and copious nectar production. Indeed, many aloe species are primarily bird-pollinated, but small non-flying mammals and insects, particularly bees, are also effective pollinators. Additionally, aloes have been shown to host a variety of phytophages and their associated parasatoids, but little is known about the effect these parasites have on the plants’ reproductive output. Aloe reitzii var. reitzii is a summer-flowering aloe, endemic to the grasslands in the eastern regions of South Africa; it is abundantly visited by birds and bees. However, the roles of these two pollinator groups in the pollination of A. reitziivar. reitzii have not been assessed. This study aimed to investigate the breeding system of A. reitzii var. reitzii and the relative roles of birds and insects in its reproductive biology – both in pollination and fruit and seed predation. During the 2020 flowering season, observations of floral visitors were conducted to identify the main floral visitors to A. reitzii var. reitzii, and their visitation rates. The role of these floral visitors in the pollination of A. reitzii var. reitzii was determined by swabbing birds and insects for pollen, and their relative contributions to the reproductive output of A. reitzii var. reitzii was determined using pollinator exclusion experiments. Since generalist and specialist nectarivorous birds are common visitors to A. reitzii var. reitzii, their roles in its pollination were assessed by i) measuring nectar properties, ii) comparing main sites of pollen deposition, and iii) comparing their bill morphology to floral characteristics of A. reitzii var. reitzii. Contrary to most Aloe species, Aloe reitzii var. reitzii is self-compatible, a condition previously reported in only one other South African Aloe species. However, A. reitzii var. reitzii is not capable of autonomous self-pollination, requiring pollinators to transfer pollen between flowers within a raceme and between racemes on a plant. Bees are shown to facilitate self-pollination, suggesting a mixed-mating system in A .reitzii var. reitzii. Aloe reitzii var. reitzii is predominantly bird-pollinated, which is consistent with its floral traits and nectar properties. Pollinator exclusion experiments show birds are the predominant pollinator in A. reitzii var. reitzii, and bees only play a minor role. The long floral tubes and generalist nectar properties of A. reitzii var. reitzii suggest floral traits are an intermediate between a generalist and specialist pollination syndrome, enabling effective pollination by both generalist and specialist nectarivores, Cape Weavers (Ploceus capensis) and Malachite Sunbirds (Nectarinia famosa), respectively. Although pollen loads did not differ between these two species, due to their consistent abundance and frequent visitation at its flowers over multiple years, A. reitzii var. reitzii is primarily pollinated by Cape Weavers. Aloe reitzii var. reitzii is host to a variety of phytophages and parasitoids, with the chalcid wasp Euryoma aloineae as the predominant pre-dispersal seed predator in A. reitzii var. reitzii. Fruit and seed predation by phytophagous and parasitoid insects greatly reduces the reproductive output of A. reitzii var. reitzii, which may considerably impact the persistence of its populations, especially its fragmented populations
Description
A dissertation submitted to the Faculty of Science, University of the Witwatersrand in fulfilment of the requirements for the degree of Master of Science, 2021