The effect of agriculture and alien plants on natural communities of plants, insect herbivores and parasitoids

Moxley, Courtney (2016-03)

Thesis (MSc)--Stellenbosch University, 2016.

Thesis

ENGLISH ABSTRACT: Habitat transformation and invasions by non-native (alien) plants are two of the most concerning drivers of global environmental change. These factors cause biodiversity declines that disrupt species interactions, with cascading effects throughout ecosystems. On farmlands, this has implications for the provision of ecosystem services and disservices by insects, including crop damage by herbivores, some of which are crop pests, and pest control by natural enemies. In this study, I investigated how plants, insects and their interactions involved in these processes are affected by habitat transformation and alien plants in the Kruger to Cayons Biosphere Region, South Africa. I first determined whether insect pests spill-over from habitats transformed for agriculture into surrounding natural vegetation in a fragmented landscape. Patches of preserved natural vegetation alongside farmlands are believed to be the source of crop pests and farmers manage the natural vegetation as a form of pest control. Using a case study with fruit flies (Ceratitis spp.), cultivated mango (Mangifera indica, Anacardiaceae) and the marula tree (Sclerocarya birrea, Anacardiaceae) as a host species in nearby natural vegetation, I showed that pests appear to spill-over in the reverse direction, from crop fields to natural vegetation when mango is out of season. Marula fruit alongside mango farms were 25 times more likely to be infested by Ceratitis than in the distant vegetation. Ceratitis appears to spill-over into natural vegetation when marula replaces mango as the most apparent resource in the landscape. Marula may represent an important reservoir for Ceratitis to maintain its population between crop seasons, but this may depend on seasonality and the relative timing of marula-mango fruiting. Secondly, I investigated the interactive effects between habitat transformation and alien plants on the structure and composition of communities of plants, herbivores and parasitoids, and their interactions such as herbivory. Insect herbivores and parasitoids were reared from native and alien seeds collected along transects in mango fields, natural vegetation and disturbed margins, and the % alien seed abundance was determined for each transect. Mango fields had the lowest abundance and diversity of plants, herbivores and parasitoids. Across the landscape, high alien seed abundance was associated with lower herbivore and parasitoid species richness. Seed herbivory was lowest in mango fields and was influenced by interactive effects between habitat transformation and alien plants, with high and low alien seed abundance associated with high and low herbivory in mango fields and natural vegetation, respectively. In showing that habitat transformation and alien plants have both independent and interactive effects throughout this food web, this research is important for predicting future declines among plants, insects and their interactions in agricultural landscapes. Managing the negative effects of habitat transformation and alien plants requires co-operation between farmers and conservationists in an area-wide approach. Farmers should manage pests and alien plants in crop fields to limit their dispersal into surrounding natural habitats. Conservation efforts should focus on improving habitat quality in agricultural landscapes by promoting natural vegetation alongside farms, and limiting harmful activities in crop fields, such as the use of pesticides and mowing. By reducing impacts on native plants, insects and their ecological interactions, these efforts will contribute to long-term sustainability of agriculture in the future.

AFRIKAANSE OPSOMMING: Habitat verandering en indringer spesies is twee van die hoof drywers van globale omgewings veranderinge. Hierdie faktore veroorsaak dalings in biodiversiteit wat spesie-interaksies ontwrig, en die gevolge kan gesien word regdeur ekosisteme. Op plaaslande het dit implikasies vir die voorsiening van ekosisteem dienste deur insekte. Dit sluit in gewaskade deur herbivore, waarvan sommige oes peste is, en plaagbeheer deur natuurlike vyande, parasitoïed perdebye ingesluit. In hierdie studie, bespreek ek die invloed van habitat transformasie en indringer plante op die interaksies tussen plante en insekte in die Kruger to Cayons biosfeer, Suid Afrika. Ek het eers bepaal hoe grond transformasie die oordrag van peste en plae tussen plase en natuurlike areas binne gefragmenteerde landskappe beïnvloed. Daar word geglo dat die natuurlike beweiding langs bewerkte lande die bron is van peste en plae. Dus, probeer boere om die natuurlike lande langs hul bewerkte lande te beheer vir plaagbeheer. Deur gebruik te maak van vrugte vlieë (Ceratitis spp.), gekultiveerde mango (Mangifera indica, Anacardiaceae) asook die Marula boom (Sclerocarya birrea, Anacardiaceae) as gasheer spesies in nabygeleë natuurlike areas, is daar `n gevallestudie opgestel. Dit het bewys dat die teenoorgestelde waar is; die peste en plae se oorloopgevolge vind plaas vanaf die bewerkte landerye na die natuurlike omgewing. Marula vrugte langs mango boorde het ‘n 25 keer groter kans om deur Ceratitis besmet te word as die wat in afgleë gebiede gevestig is. Dit dui daarop dat dit onwaarskynlik is dat natuurlike areas die bron is van vrugtevlieë vir mango velde, en lê klem op die negatiewe gevolge wat landbou het op die gasheer-plaag interaksies tussen bewerkte lande en die natuurlike omgewing. Gevolglik, het ek die interaksie tussen habitat transformasie en indringer plante op gemeenskappe van plante, herbivore en parasitoids, en hul interaksies soos saad predasie, ondersoek. Mango boorde het ‘n kleiner verskeidenheid herbivore en parasiete as natuurlike beweidinge en versteurde habitat marges gehad. Oor die landskap, toenemende hoeveelhede van uitheemse saad het dalings veroorsaak in herbivoor en parasitoïed spesierykheid. Saad predasie deur herbivore was die laagste in mango boorde, beïnvloed deur interaktiewe effekte tussen habitat transformasie en indringerplante, soos dat hoë uitheemse saad oorvloed is wat verband hou met hoë saad predasie in mango velde en lae saad predasie in natuurlike areas. Deur te bewys dat habitat transformasie en indringer spesies beide onafhanklike en interaktiewe verhoudings het in die voedsel-web, kan die plant- en insek-bevolkings dalings in die toekoms voorspel word vir die landbou bedryf. Beheer van die negatiewe effekte van habitat transformasie en indringer spesies verg samewerking tussen boere en natuurbewaarders. Boere moet peste en indringer plant spesies op hulle bewerkte lande beheer om die verspreiding na omliggende natuurlike areas te verminder. Natuurbewaarders moet fokus op die bevordering van die natuurlike landskappe deur om die natuurlike plantlewe langs bewerkte landerye te bevorder, en skadelike aktiwiteite in bewerkte landerye, soos die gebruik van plaagdoders en gras sny, te beperk. Deur die impak op die inheemse plante, insekte en hulle ekologiese interaksies te verminder, sal hierdie pogings bydra tot die volhoubaarheid van die landbou in die toekoms.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/98462
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