An experimental approach to assess the role of nest predation in the population dynamics of the sociable weaver (Philetairus socius)
Master Thesis
2016
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University of Cape Town
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Predation is a widespread population process that has been shown to affect the distribution, abundance and dynamics of populations in ecosystems. This is the first study that used an experimental approach to assess the effect of nest predation on the population dynamics of the sociable weaver (Philetairus socius), a keystone species in the semi-arid savannas of the Kalahari and Namib regions. Snakes were excluded from five colonies for five breeding seasons and two colonies for three breeding seasons, with another eight colonies acting as the controls. Reproductive output, colony size, dispersal events and several environmental variables were measured between 2010 and 2015. This was done to determine (1) what effect nest predator exclusion had on reproductive output; (2) how this related to colony and population size trends by using a matrix-projection metapopulation model; (3) how protected colonies influence movement patterns; and (4) if nest predation had a compensatory or positive effect by reducing the intraspecific competition of a colony. The fourth aim was investigated by tracking the foraging paths of eight colonies of varying sizes, with foraging distance acting as a proxy for intraspecific competition. Colonies that were protected from snake predation produced, on average, more than double the number of fledglings per female per breeding season that were produced in unprotected colonies. However, the magnitude of this effect decreased with increasing colony size of protected colonies, most likely due to the negative effects that large colony sizes have on reproductive output. Increasing aridity was found to have a negative effect on reproductive output and warmer winter minimum temperatures were found to have a positive effect. My results suggested that protecting a subset of colonies in the metapopulation may be sufficient in preventing population declines under climate change conditions. The protected colonies played an important role in structuring and connecting the movement network of the metapopulation, whilst colony size explained the migration rates of colonies. However, predation was not found to have a compensatory effect in reducing the intraspecific competition (measured as foraging distance) of a colony. Instead, foraging distance was probably determined by the ability to thermoregulate under hot and humid conditions. To fully understand the effects of nest predation on sociable weaver population dynamics, future studies need to investigate the response of snake predators to sociable weaver behaviour and environmental conditions.
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Lloyd, K. 2016. An experimental approach to assess the role of nest predation in the population dynamics of the sociable weaver (Philetairus socius). University of Cape Town.