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