Avifaunal responses to environmental conditions and land-use changes in South Africa : diversity, composition and body size

Greve, Michelle (2007-03)

Thesis (MSc)--University of Stellenbosch, 2007.

Thesis

ENGLISH ABSTRACT: In macroecology, body sizes in animal assemblages have traditionally been investigated from two perspectives: body size frequency distributions (BSFDs) and geographic variation in body size. Neither of these has been investigated for the South African avifauna; one objective of this study was therefore to explore these. The regional BSFD of South African birds was found to be right-skewed, as is usually found for assemblages at large scales. This suggests that mechanisms driving the shape of BSFDs elsewhere are also acting for the South African avifauna. The Southern African Bird Atlas database was used to calculate median body sizes of avian assemblages in quarter degree grid cells. Median sizes were then used to investigate geographic variation in body size across the country. Of the mechanisms previously proposed to explain geographic variation in body size, only the starvation resistance hypothesis, which states that large size confers starvation resistance during seasonally resource shortages, was supported, though weakly, as median body size decreased with increasing productivity. The ability of null models to predict the variation in body size was subsequently explored, and it was found that much of the variation in median size of assemblages could be predicted by randomly drawing species from the regional BSFD, particularly at high species richness values. This provides empirical support for a continuum between the dominance of niche-based processes (where assemblages are a product of organisms' response to their environment) at low richness and neutral processes (where organisms assemble at random) at higher richness. In addition, it emphasizes the need to consider null expectations in investigations of the geographic variation in size. The importance of the regional BSFD and species richness for body sizes of local assemblages is highlighted. Body size is one of several life history and community characteristics of animals that may be affected by anthropogenic disturbance to the environment. World-wide, landscapes are increasingly being altered by people, though few studies have investigated the effect of such disturbances on the avifauna of South Africa. The consequence of land-use changes on avian assemblages was therefore assessed in three South African regions which experience different environmental conditions and are threatened by different land-use changes. Birds were recorded in transects in undisturbed protected areas and the disturbed landscape outside the protected areas in the three regions. The effect of land-use change on avian assemblages varied between regions, and avian assemblages were most affected where disturbance was most intense. While species richness was not affected in a consistent manner across regions, species composition always changed in response to disturbance. This led to higher regional species richness as natural and disturbed areas supported different avian assemblages, and heterogeneity of assemblages between vegetation types usually became less pronounced in disturbed areas. Functional diversity was also compromised by land-use changes: the relative proportion of feeding guilds was altered, indicating that changes in food availability affect composition of assemblages. In contrast, mean body size of birds did not change in disturbed landscapes, which suggests that habitat architecture has little effect on body size. This study therefore highlights the importance of natural and protected areas for conserving species, assemblages and ecosystem processes.

AFRIKAANSE OPSOMMING: In makro-ekologie is liggaamsgrootte van diergemeenskappe tradisioneel uit twee oogpunte ondersoek: liggaamsgrootte-frekwensieverspreidings (LGFVs) en geografiese variasie in liggaamsgrootte. Nie een van hierdie twee is al vir SuidAfrikaanse voels bepaal nie, en dit is dus 'n doel van hierdie studie om dit te ondersoek. Daar is gevind dat die land-wye LGFV 'n patroon van skuins na regs gewys het, soos gewwonlik die geval is vir diergemeenskappe op groat skaal. Prosesse wat die LGFV elders bepaal, is dus waarskynlik ook belangrik vir die LGFV van Suid-Afrikaanse voels. Die Suider Afrikaanse Voel Atlas is gebruik om die mediaan van die liggaamsgrootte van voelspesies in elke 15' x 15' vierkant te bepaal. Die waardes is gebruik om die geografiese variasie in liggaamsgrootte oor die hele land te ondersoek. Daar is al verskeie meganismes voorgestel wat geografiese variasie in liggaamsgrootte kan be"invloed. Hier is ondersteuning, alhoewel swak, gevind (liggaamsgrootte neem toe soos produktiwiteit afneem) vir die verhongeringweerstands hipotese, wat se dat groat liggaamsgrootte weerstand teen verhongering bied tydens tydperke van seisoenale tekortkominge. Daar is ook ondersoek hoe nulmodelle variasie in liggaamsgrootte kan voorspel, en daar is gevind dat baie van die variasie in liggaamsgrootte voorspel kon word deur spesies willekeurig vanuit die land-wye LGFV te kies. Dit was veral die geval vir gemeenskappe met hoe spesierykheid. Die resultate verskaf empiriese steun vir 'n kontinuum tussen nisgebaseerde prosesse (waar gemeenskappe ontstaan as gevolg van die manier hoe organismes op hulle omgewing reageer) wat by lae spesierykheid domineer, en neutrale prosesse (waar gemeenskappe willekeurig saamgestel word) wat by hoer spesierykheid domineer. Dit beklemtoon ook die noodsaakliheid van nul-modelle in die ondersoek van geografiese variasie in liggaamsgrootte, sowel as die belangrikheid van spesierykheid eri die grootskaalse LGFV vir liggaamsgroottes in plaaslike gemeenskappe. Liggaamsgrootte en ander kenmerke van dieregemeenskappe kan be'fnvloed word deur menslike steuringe aan die omgewing. Landskappe word wereldwyd al hoe meer deur mense verander. Daar bestaan tans min studies wat die invloed van hierdie antropogeniese steuringe op voels in Suid-Afrika bestudeer het. Die gevolge van landskapveranderinge vir voelgemeenskappe in drie Suid-Afrikaanse streke wat van mekaar verskil op grand van die omgewing en van landskapveranderinge is dus hier ondersoek. Voels is getel in natuurlike bewaarde gebiede en in versteurde areas buite die bewaringsgebiede. Die invloed van landskapveranderinge op voelgemeenskappe was verskillend in die drie streke, en die voelgemeenskappe is meeste geaffekter waar die landskapsteuring die meeste intens was. Terwyl spesierykheid nie op 'n konsekwente manier deur landskapveranderinge be'invloed is nie, het die samestelling van die gemeenskappe altyd verander. Die gemeenskappe in natuurlike en versteurde gebiede besit dus verskillende spesiesamestellings, wat tot hoer spesierykheid in die streke lei. Heterogeniteit in voelgemeenskappe tussen verskillende soorte vegetasie was meestal ook minder in versteurde as in natuurlike gebiede. Die funksionele diversiteit van voelgemeenskappe word boonop bedreig deur landskapveranderinge: in versteurde gebiede het die relatiewe proporsies van voedinggildes verander, wat aandui dat voedselbeskikbaarheid die samestelling van gemeenskappe affekteer, maar die gemiddelde grootte van voels het nie verander nie; die argitektuur van die omgewing het dus min invloed op die voels se . liggaamsgrootte. Die studie beklemtoon dus dat ongesteurde en bewaarde gebiede belangrik is om spesies, gemeenskappe en ekosisteemprosese te bewaar.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/50690
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