Demographic and functional determinants of large-scale population dynamics and ecological niches of 26 serotinous Proteaceae

Treurnicht, Martina (2018-03)

Thesis (PhDAgric)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: Understanding how organisms respond to the environment at large spatial scales is central to ecology, biodiversity research and conservation. Environmental variation affects the fitness (or performance), population dynamics and geographical distributions of species via morphological traits. Quantifying how demographic rates and functional traits vary across environmental gradients may thus yield insights into the underlying determinants of ecological performance and geographical distribution. However, studies of demographic and trait variation widely rely on observations from a few species, at small spatial scales and seldom include multiple abiotic and biotic drivers. A basic understanding of the drivers of large-scale demographic variation and how functional traits relate to population dynamics and species’ niches remains limited. Using the Hutchinsonian niche concept (i.e. the set of environmental conditions in which populations can grow), I investigated how environmental conditions and functional traits affect the demography, population dynamics and ecological niches of 26 serotinous Proteaceae species with fire-dependent life cycles from the Cape Floristic Region (South Africa). My objectives were to: (i) identify the environmental drivers of large-scale demographic variation, (ii) investigate whether plant functional traits explain demographic performance and Hutchinsonian niches, and (iii) study geographical variation in population sensitivity to wildflower harvesting. I addressed these objectives using data on key demographic rates and plant functional traits sampled across species’ entire geographical ranges. Environmental drivers (climate, fire disturbance, soil nutrient status and population density) explained variation in key demographic rates of reproduction and survival across species’ geographical distributions. The relative importance of these drivers varied throughout the life cycle of the study species: fecundity was mostly driven by fire interval whereas recruitment depended more on climate. A trade-off between survival and reproduction was also found where species with fire-protected buds (resprouters) had substantially higher fire survival compared to species without fire-protected buds (nonsprouters). Overall, intraspecific variation in fecundity and recruitment was greater than that of fire survival. I also investigated whether variation in functional traits (leaf, plant-architectural and seed traits) explains the Hutchinsonian niches of species. Results showed that interspecific trait variation explained considerable variation in global maximum population growth rates (rmax), as well as niche optima and widths along different environmental gradients. Intraspecific trait variation had positive effects on niche widths. Overall, relatively few individual traits stood out as predictors of species’ demographic niches. Finally, I integrated range-wide demographic data and dynamic population models to assess spatial variation in sensitivity to harvesting across species’ geographical distributions. I detected considerable variation in sensitivity to harvesting across species and populations. Range-wide intraspecific variation in sensitivity to harvesting showed distinct geographical and environmental relationships. Notably, sensitivity to harvesting was highest at the environmental limits of species’ ranges. Combined, these range-wide demographic and functional approaches on species niches provide fundamental and applied perspectives in ecology and conservation biogeography. These are necessary steps to understand how range dynamics emerge from variation in demography and functional traits, and how species may be affected by ongoing global change.

AFRIKAANSE OPSOMMING: ’n Begrip van hoe organismes op ’n groot ruimtelike skaal op die omgewing reageer, is van belang vir ekologie, biodiversiteitsnavorsing en bewaring. Omgewingsvariasie beïnvloed die geskiktheid (of prestasie), populasiedinamiek en geografiese verspreiding van spesies deur middel van morfologiese kenmerke. ’n Kwantifisering van hoe demografiese koerse en funksionele kenmerke oor omgewingsgradiënte wissel, kan dus waardevolle insig bied in die onderliggende bepalers van ekologiese prestasie en geografiese verspreiding. Bestaande studies van demografiese en kenmerkvariasie maak egter meestal staat op waarnemings van slegs ’n paar spesies op ’n klein ruimtelike skaal, en neem selde ’n verskeidenheid abiotiese en biotiese dryfvere in ag. ’n Basiese begrip van die dryfvere van grootskaalse demografiese variasie, en die verband tussen funksionele kenmerke, populasiedinamiek en spesie-nisse, is nog beperk. Met behulp van Hutchinson se niskonsep (d.w.s. die stel omgewingsfaktore waarin populasies kan groei) het ek ondersoek watter invloed omgewingsomstandighede en funksionele kenmerke het op die demografie, populasiedinamiek en ekologiese nisse van 26 laatbloeiende Proteaceaespesies met brandafhanklike lewensiklusse in die Kaapse planteryk (Suid-Afrika). My oogmerke was (i) om te bepaal watter omgewingsdryfvere grootskaalse demografiese variasie veroorsaak, (ii) om te ondersoek watter funksionele plantkenmerke demografiese prestasie en Hutchinson se nisse verklaar, en (iii) om geografiese variasie in populasiesensitiwiteit vir veldblomoesting te bestudeer. Om hierdie oogmerke te verwesenlik, het ek gebruik gemaak van data oor die vernaamste demografiese koerse en funksionele plantkenmerke wat ingesamel is oor geografiese verspreidingsgebiede van spesies. Omgewingsdryfvere (klimaat, brandontwrigting, grondvoedingstatus en populasiedigtheid) het ’n verklaring gebied vir variasie in die vernaamste demografiese voortplantings- en oorlewingskoerse oor geografiese spesieverspreidings heen. Die relatiewe belang van hierdie dryfvere wissel deur die lewensiklus van die studiespesies: Vrugbaarheid word meestal deur brandgereeldheid bepaal, terwyl aanwas meer op klimaat berus. ’n Kompromis tussen oorlewing en voortplanting blyk ook uit die beduidend hoër brandoorlewingskoerse onder spesies met brandbeskermde knoppe (heruitlopers) vergeleke met spesies daarsónder (nieuitlopers). Intraspesifieke variasie in vrugbaarheid en aanwas was oor die algemeen hoër as variasie in brandoorlewing. Daarbenewens het ek ondersoek of variasie in funksionele kenmerke (blaar-, plantargitektuuren saadkenmerke) Hutchinson se spesie-nisse verklaar. Resultate toon dat interspesifieke kenmerkvariasie beduidende variasie in globale maksimum populasiegroeitempo’s (rmaks) sowel as nis-optima en -breedtes oor verskillende omgewingsgradiënte verklaar. Intraspesifieke kenmerkvariasie het’n positiewe uitwerking op nisbreedtes. Oor die algemeen staan betreklik min individuele kenmerke uit as voorspellers van demografiese spesie-nisse. Laastens het ek grootskaalse demografiese data en dinamiese populasiemodelle geïntegreer om ruimtelike variasie in oestingsensitiwiteit oor die geografiese verspreidings van spesies heen te beoordeel. Ek het bevind dat daar beduidende variasie in oestingsensitiwiteit is tussen spesies en populasies. Intraspesifieke variasie in oestingsensitiwiteit oor spesies se verspreidingsgebiede heen bring duidelike geografiese en omgewingsverwantskappe aan die lig. Oestingsensitiwiteit is veral die hoogste op die omgewingsperke van spesies se verspreidingsgebiede. Tesame bied hierdie grootskaalse demografiese en funksionele benaderings tot spesie-nisse fundamentele en toegepaste perspektiewe vir ekologie en bewaringsbiogeografie. Dit voorsien nodige inligting om te verstaan hoe verspreidingsgebieddinamiek uit variasie in demografie en funksionele kenmerke ontstaan, en hoe voortdurende globale verandering spesies kan raak.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/103909
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