The world without birds : an experimental test of the ecological significance of pollinating birds for plant communities

Botha, Pieter Willem (2017-03)

Thesis (MSc)--Stellenbosch University, 2017.

Thesis

ENGLISH ABSTRACT: Since pollinators are declining in many parts of the world, it is vital to understand the consequences of pollinator loss. How will plant communities be affected? If pollinators are keystone mutualists, then their loss may cause cascading extinctions with far-reaching consequences for communities. Alternatively, loss of a pollinator may have little effect, since many pollination systems are generalised and the typical structure of plant-pollinator interaction networks suggest flexibility and functional redundancy. Yet relatively few studies have assessed the effects of pollinator loss in the field. This thesis considers the consequences of bird pollinator loss for plant communities in the megadiverse Cape Floristic Region of South Africa, where bird pollinators are particularly important. I constructed 20 x 20 m cages to exclude birds, but not insects, from six plant communities in the Jonkershoek Nature Reserve near Stellenbosch. Communities without birds were compared to neighbouring communities with birds. The scale of the experiment made it possible to assess the demographic and community-level consequences of pollinator loss. Many Cape Proteaceae are pollinated by birds and reliant on seeds to regenerate after periodic fires. One such a species is the showy Leucospermum lineare (Proteaceae), the focus of Chapter 2. Leucospermum lineare was pollinated by Cape Sugarbirds. In the absence of birds, nectar volume and concentration increased, which seemed to amplify visitation by nectar thieving ants. Seed set was maintained without birds through autonomous selfing. However, seeds from bird-excluded plants had lower seed viability in laboratory trials. In the field, post-fire seedling recruitment was reduced significantly in plots where birds had been excluded, most likely as a result of inbreeding. Leucospermum lineare populations may thus decline without birds. A number of other bird-pollinated plant species in the study communities also saw declines in fecundity when birds were excluded (Chapter 3). Protea neriifolia (Proteaceae), a dominant plant in many of the communities, was mostly pollinated by Cape Sugarbirds and its seed set declined by half in the absence of birds. Erica plukenetii (Ericaceae) was pollinated by Orange-breasted Sunbirds and its fruit set was reduced substantially without birds. In Mimetes cucullatus (Proteaceae) seed set collapsed in the absence of birds. In contrast, Protea repens, also a dominant species, and Protea nitida maintained their seed set, likely thanks to insect pollination. Seedling recruitment of Protea species in the presence of bird pollinators was well above adult replacement levels after a fire swept the six year old vegetation; Protea seedling recruitment in the absence of birds could not be assessed. Overall my results suggest that extinction of bird pollinators may reduce the biomass and diversity of plant communities in the Cape Floristic Region. Plant fecundity can also be reduced by animals that eat flowers. I quantified the effects of florivory by rodents and baboons on Protea neriifolia. Seed cone production was reduced substantially. Striped mice, Rhabdomys pumilio, climbed up to 1.6 m high into P. neriifolia plants to feed on inflorescences. Rodents also severed inflorescences by gnawing through subtending stems.

AFRIKAANSE OPSOMMING: In die lig van die wêreldwye afname in die volopheid van bestuiwers is dit noodsaaklik om die gevolge van dié afname te verstaan. Hoe gaan plantgemeenskappe geaffekteer word? Enersyds, indien bestuiwers hoeksteen-mutualiste is, kan die verlies van bestuiwers tot trapsgewyse uitsterwings lei, met verreikende gevolge vir gemeenskappe. Andersyds kan die verlies van bestuiwers weinig impak maak, aangesien baie bestuiwingsisteme gegeneraliseerd is en die tipiese struktuur van plant-bestuiwer-interaksienetwerke dui op buigsaamheid en oortolligheid van funksie. Relatief min studies het egter tot dusver die effekte van bestuiwerverlies in die veld ondersoek. Hierdie tesis handel oor die gevolge van die verlies van voëlbestuiwers vir plantgemeenskappe in die hiperdiverse Kaapse Floristiese Streek, waar voëlbestuiwers buitengewoon belangrik is. Hokke van 20 x 20 m is in die Jonkershoek Natuurreservaat buite Stellenbosch opgerig om voëls (maar nie insekte nie) uit te sluit. Gemeenskappe sonder voëls is vergelyk met gemeenskappe met voëls. Die skaal van die eksperiment het dit moontlik gemaak om die gemeenskapsvlak- en demografiese gevolge van bestuiwerverlies te ondersoek. Baie Kaapse Proteaceae word deur voëls bestuif en is afhanklik van sade om na periodieke brande te regenereer. Die pronkerige Leucospermum lineare (Proteaceae) is ’n voorbeeld van só ’n spesie en is die fokus van Hoofstuk 2. Leucospermum lineare is deur Kaapse suikervoëls bestuif. In die afwesigheid van voëls het nektarvolume en -konsentrasie toegeneem, wat oënskynlik besoeke van nektar-stelende miere laat styg het. Danksy selfbestuiwing het saadproduksie nie afgeneem sonder voëls nie. Die kiemkrag van sade afkomstig van plante waar voëls uitgesluit is, was egter laer in laboratoriumproewe. In die veld was saailingvestiging beduidend laer in plotte waar voëls uitgesluit is, bes moontlik weens inteling. Gevolglik kan Leucospermum lineare-populasies kwyn sonder voël-bestuiwers. Die fekunditeit van ’n aantal ander voëlbestuifde plantspesies in die studiegemeenskappe het ook afgeneem toe voëls uitgesluit is (Hoofstuk 3). Protea neriifolia (Proteaceae), ’n dominante spesie in baie van die gemeenskappe, is hoofsaaklik deur Kaapse suikervoëls bestuif en saadproduksie het met die helfte verminder sonder voëls. Erica plukenetii (Ericaceae) is deur Oranjeborssuikerbekkies bestuif en vrugproduksie het beduidend verminder in die afwesigheid van voëls. Mimetes cucullatus (Proteaceae) het sonder voëls byna geen sade geproduseer nie. Daarenteen kon Protea repens, ook ’n dominante spesie, en Protea nitida wel sade produseer sonder voëls, heel moontlik danksy insekbestuiwing. In die aanwesigheid van voëlbestuiwers was die saailingvestiging van Protea-spesies, nadat die sesjaar oue veld afgebrand het, aansienlik hoër as die vervangingsvlak vir volwassenes. Protea-saailingvestiging in plotte sonder voëls kon nie gemeet word nie. In geheel gesien dui hierdie resultate aan dat die uitsterwing van voëlbestuiwers die biomassa en diversiteit van plantgemeenskappe in die Kaapse Floristiese Streek kan verminder. Plante se fekunditeit kan ook verminder word deur diere wat blomme eet. Die effekte van florivorie deur knaagdiere en bobbejane op Protea neriifolia is ondersoek. Die produksie van saadkoppe het beduidend verminder. Streepmuise, Rhabdomys pumilio, het tot 1,6 m hoog in P. neriifolia-plante geklim om op bloeiwyses te voed. Knaagdiere het ook bloeiwyses afgesny deur deur stingels te knaag.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/101186
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