Abiotic and biotic drivers of African aquatic insect distribution

Deacon, Charl (2020-04)

Thesis (PhDConsEcol)--Stellenbosch University, 2020.

Thesis

ENGLISH ABSTRACT: Freshwater habitats are disproportionately rich in biodiversity, and are among the most threatened, yet poorly protected ecosystems. Aquatic insects make up much of the total freshwater fauna and contribute greatly to ecosystem functioning. At the broad-scale, aquatic insect distribution is driven by combinations of traits, as well as regional climate gradients and historical landscape context. Locally, both aquatic insect species richness and diversity are driven by various aspects related to vegetation and to physiochemical environments. Effective conservation requires thorough understanding of species distribution patterns at various spatial scales. My overall aim here is to combine broad-scale, theoretical biogeography, and local-scale empirical ecology to investigate drivers of aquatic insect distribution across Africa. Species are often binarily classified as ‘widespread generalists’ or ‘narrow-range specialists’ based on their ecological traits. Results in Chapter 2 show that ecological and biological traits are highly interactive among dragonflies, and inferring geographical range size based on ecological preference and/or biotope specialization alone should be approached with caution. Biological traits related to phenology and mobility were also strong drivers of dragonfly range size, indicating that conservation efforts should include multiple species across all habitat types. Regional climates show considerable variation across latitudinal and longitudinal gradients, and determine areas of high species richness and diversity. In Chapter 3, I show strong latitudinal and longitudinal gradients for South-African dragonfly species richness and endemism. Dragonfly assemblage-turnover boundaries coincided with significant geographical features and/or areas where contemporary climate changed from one condition to another. However, these dragonfly assemblage turnover-boundaries were gradual rather than discrete throughout South Africa. At the local scale, natural and artificial ponds contribute greatly to overall biodiversity, especially when they are of high quality and occur in networks across the landscape. I show that ponds characterized by high heterogeneity support diverse aquatic insect assemblages (Chapters 4 and 5). Chapter 4 showed artificial reservoirs, occurring alongside natural ponds in ecological networks, to expand the area of occupancy for most widespread dragonflies, aquatic beetles and true bugs. Some species with specific habitat requirements were confined to natural ponds, suggesting the significance of natural ponds for conserving the full range of insects. Dragonflies, aquatic beetles and true bugs occupy low-quality artificial reservoirs at low abundance to survive the adverse effects of drought (Chapter 5). However, many insects exclusively occupied natural ponds, emphasizing the overall importance of naturalness, and suggests that there is merit in improving artificial reservoirs. This would most likely be by having macrophytes and vegetated banks similar to those of natural ponds.Investigating aquatic insect distribution patterns is important for conservation, and here, I demonstrate the value of dragonflies as model organisms for investigating the drivers of broad-scale distribution patterns. Studying other taxa is also appropriate, as I have demonstrated at the local scale, but not always possible due to limited distribution knowledge. I recommend broad-scale investigations of other complementary taxa to determine their added value for elucidating the drivers of overall insect distribution patterns, and so address our current shortfalls to improve insect conservation.

AFRIKAANSE OPSOMMING: Varswaterhabitatte is besonders biodivers, en val onder die mees bedreigde, onder-beskermde ekosisteme. Varswaterinsekte vorm ‘n groot deel van alle varswaterdiere, en het hoë waarde vir ekosisteem-werking. Breë-skaalse waterinsekverspreiding word aangedryf deur kombinasies van eienskappe, sowel as streeksklimaat en historiese landskap-konteks. Oor plaaslike skale word beide waterinsek spesierykheid en diversiteit aangedryf deur verskeie aspekte van plantegroei, en chemiese omgewings. Effektiewe bewaring vereis goeie begrip van spesieverspreidingspatrone oor verskeie ruimtelike skale. My algehele doel is om breë-skaalse, teoretiese bio-geografie, en fyn-skaalse empiriese ekologie te kombineer, om sodoende die dryfkragte van waterinsekverspreiding oor Afrika te ondersoek. Spesies word dikwels op ‘n binêre wyse geklassifiseer as ‘wyd-verspreide generaliste’ of ‘streeksgebonde spesialiste’, gebaseer op hul ekologiese eienskappe. Bevindinge in Hoofstuk 2 toon dat ekologiese en biologiese eienskappe onder naaldekokers hoogs interaktief is. Afleidings van geografiese verspreiding, gebaseer op ekologiese voorkeur en/of biotoop spesialisme, hoort versigtig benader te word. Biologiese eienskappe verwant aan fenologie en beweeglikheid was ook beduidende dryfkragte van geografiese verspreiding onder naaldekokers, wat aandui dat bewaringspogings verskeie spesies vanaf alle habitat moet betrek. Streeksklimaat verskil aansienlik oor breedte- en lengtegradiënte, en bepaal waar areas van hoë spesierykheid en diversiteit voorkom. Ek bewys in Hoofstuk 3 dat sterk breedte- en lengtegradiënte vir Suid-Afrikaanse naaldekoker spesierykheid en inheemsheid bestaan. Naaldekoker gemeenskapsomsetgrense stem ooreen met beduidende geografiese strukture en/of areas waar kontemporêre klimaat verander tussen streke. Hierdie naaldekoker gemeenskapsomsetgrense is egter geleidelik eerder as diskreet oor Suid-Afrika. Natuurlike en kunsmatige damme dra by tot algehele biodiversiteit oor die plaaslike skaal, veral wanneer dié damme van hoë kwaliteit is, en aangetref word in netwerke wat strek oor die landskap. My bevindinge bewys dat damme wat gekenmerk word deur hoë variasie diverse waterinsek-gemeenskappe ondersteun (Hoofstukke 4 en 5). Bevindinge in Hoofstuk 4 bewys dat kunsmatige damme, tesame met natuurlike damme in ekologiese netwerke, die besettingsarea van meeste wyd-verspreide naaldekokers, waterkewers en ware watergoggas vergroot. Sommige spesies met spesifieke habitatvereistes was beperk tot natuurlike damme, wat aandui dat natuurlike damme belangrik is vir die bewaring van die volle spektrum van waterinsekte. Naaldekokers, waterkewers en ware watergoggas beset lae-gehalte kunsmatige damme in lae hoeveelhede, om die ongunstige toestande van droogte te oorleef (Hoofstuk 5). Heelwat waterinsekte word egter slegs in en rondom natuurlike damme aangetref, wat beklemtoon dat die natuurlikheid van damme belangrik is. Hierdie bevindinge dui aan dat daar meriete is om kunsmatige damme te verbeter, waarskynlik deur om plantegroei wat soortgelyk aan dié van natuurlike damme is, te stimuleer. Om ondersoek in te stel op waterinsek-verspreidingspatrone is belangrik vir natuurbewaring, en hier bewys ek dat naaldekokers waardevol is om die drywers van breë-skaalse verspreidingspatrone aan te dui. Om ander insek-groepe te ondersoek is hoogs gepas, soos hier aangedui vir plaaslike studies, alhoewel dit nie altyd moontlik is nie, as gevolg van beperkte kennis met betrekking tot hul verspreidingspatrone. Ek beveel breë-skaalse studies aan vir ander ooreenstemmende insek-groepe, om te bevestig wat hul bydraende waarde is om die dryfkragte van algehele insekverspreiding te verklaar. Sodoende kan ons huidige tekortkominge aanspreek om insekbewaring te verbeter.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/107966
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