Reproductive ecology of Pyracantha angustifolia in afromontane grasslands of the eastern Free State
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Date
2020-04
Authors
Adams, Lehlohonolo Donald
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Publisher
University of the Free State
Abstract
Invasion by alien plants is one of biodiversity’s major threats. Several plant traits are associated with invasiveness, such as fast growth rate, having long flowering and fruiting periods, higher fecundity and higher specific leaf area and efficient seed dispersal. Most of these traits are reproductive traits such as effective seed production, dispersal and pollination. It is important to study such traits in order to understand the process of invasion. These invaders are capable of modifying ecosystem functioning and disrupt ecosystem services, changing the plant community diversity. There are also economic losses such as loss in crop production. In South Africa, invasive fleshy-fruited shrubs in the Maloti-Drakensberg Mountains use native frugivores to disperse their seed. They also transform the habitat and threaten the agricultural productivity of the region. One of these invasive shrubs is the notorious Pyracantha angustifolia. This species was introduced to South Africa for ornamental purposes and used as hedgerows along highways. It has escaped cultivation and invaded grasslands. Scientifically sound information is needed in order to advise on invasive alien plant management strategies. The aim of this study was to determine the reproductive ecology and associated mechanisms of spread of Pyracantha angustifolia (Rosaceae) in the eastern Free State. Objectives of the study included determining: (1) the seed biology of the plant, (2) the effect of ingestion by frugivorous birds on germination rate and success, and (3) the role of floral visitors in pollination and reproductive success. Seed biology was determined through seed production, dispersal and soil seed bank studies. Fruit estimates indicated that the shrub produces enormous amounts of fruit (>5 million per square metre) available to frugivores for dispersal. Observations and seed traps confirmed frugivorous birds, livestock and wildlife to be seed dispersers. Soil cores revealed the soil seed bank to be more dense under (46400 seed per m2 ± 8934 SE) and near (10000 seed per m2 ± 3611 SE) source plants. Seed away from the parent plants were largely not viable. Seed traps indicated that seeds do also reach far distances under perching structures. Aviary trials involving feeding fruits to frugivorous birds and germination trials with defecated seeds were performed to investigate the effect of ingestion on germination. Ingestion by the four frugivorous birds, Dark-capped Bulbuls (Pycnonotus tricolor), Purple-crested Turacos (Gallirex porphyreolophus), Speckled Mousebirds (Colius striatus), and Red-winged Starlings (Onychognathus morio), under study did not affect germination rate and success; instead, the birds facilitate the spread and germination of seeds by removing the fruit pulp. Cape White-eyes (Zosterops virens) refused to feed on fruits. Overall, native South African birds were shown to play a pivotal role in the spread and germination success of this invasive alien plant. Larger birds are important in dispersing Pyracantha angustifolia seeds further distances and smaller birds deposit seed at a faster rate. Floral visitor observations, pollinator exclusion of flowers and pollen load counts on insect floral visitors addressed the effect of insect floral visitors on pollination and reproductive success. The study showed that hymenopterans, specifically Apis mellifera, were the most important pollinating agents. Pyracantha angustifolia pollen dominated pollen loads of all insect floral visitors in the study site. Results revealed that this invader might be sharing pollinators with important crops. Pyracantha angustifolia produced healthy seeds without the aid of pollen vectors but the fruit yield was increased in the presence of a pollen vector. This plant used generalist pollinators to ensure increased fruit yield and thus it is likely to attract pollen vectors in newly invaded ecosystems. It uses native bird species as the main seed dispersers; they enhance germination by removing the pulp. Despite high fruit and seed production rates, this species’ spread may be hampered by potential seed predation by rodents and low longevity of seeds in the soil seed bank.
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Keywords
Dissertation (M.Sc. (Plant Sciences))--University of the Free State, 2020, Fleshy-fruited plants, Germination, Invasive alien plants, Pollination, Seed bank, Seed production, Seed dispersal