The effects of colony dynamics and climate on a declining population of sociable weavers, Phlietairus Socius

This study was conducted in order to investigate the potential reasons for population decline in colonial birds in semi-arid environments by looking at the role that colony dynamics and climatic variables may play in this. This study was carried out on a declining population of Sociable Weavers, Philetairus socius, monitored in 15 years from 1993-2009 on Benfontein Game Farm, Kimberley, South Africa. Twenty-four weaver colonies were divided into four groups based on the size of their colonies as well as the population trend of the colony: 1) Large declining colonies (n=8); 2) Small declining colonies (n=4); 3) Large stable colonies (n=6); and 4) Small stable colonies (n=6). I used CMR models to evaluate patterns in survival of adult birds (>1 year old). Age did not have a strong effect on survival, the effect of fieldwork on the population is minimal and emigration was not considered. Adults in large colonies had greater survival rate than birds in small colonies, and birds in stable colonies had greater survival than those in colonies with a declining trend. Adding climatic variables failed to improve the model fit, with analysis of deviance (ANODEV) suggesting that climatic variables explained only 8% of the variance in survival data. Minimum temperature was the climatic variable that has the greatest effect (negative, linked to the costs of thermoregulation), with rainfall having the second largest effect (positive, linked to increased food availability) on survival. The number of fledglings per female in each colony was available for 1999, 2000, 2002 and 2005. Poisson regression models indicated that maximum temperature (negative) and rainfall (positive) best explained variance in reproductive success. Weavers from larger colonies and from stable colonies had fledged more chicks than birds from small and declining colonies, but reproduction was affected more by climatic conditions than by colony dynamics. Sociable weavers from larger colonies probably have higher survival rates due to the extra vigilance in anti-predator responses and a better thermoregulatory ability. Nest predation also is important, but was not included explicitly in this study, although it was incorporated in the reproduction data. Apart from these factors, the differences in survival also is likely to be due to Allee effects in smaller colonies, such as reduced predator defense, lower thermoregulatory ability, poorer colony maintainence and lower reproductive rates in which the low growth rate cannot compensate for the loss of individuals. These findings may shed light on similar species under similar conditions. 1 Un ive rsi ty Of C ap e T ow n Chapter 1: Introduction Conservation biology is concerned with the protection of species and their habitats. There are many species that are in dire need of conservation (www.iucn.org), and these species are placed in conservation categories based on various aspects of their world population (size, spatial extent and pattern, number of sub-populations, etc.) and the trends that they show (including perceived threats; www.iucnredlist.org). The field of conservation is advancing on two fronts, supported by two sets of ideas (Caughley, 1994; Asquith, 2001): the small population paradigm, concerned with the inherently higher extinction risk small populations face (Caughley, 1992; Keller & Waller, 2002), and the declining population paradigm, which deals with the processes whereby populations are driven towards extinction by external agents (Caughley, 1994; Green, 1995; Norris, 2004). Population size and population growth rate are the two main factors influencing a species’ conservation status (www.iucnredlist.org). Birdlife International has estimated that approximately 12% of the 9946 bird species on earth are threatened with extinction (www.birdlife.org, 2009), and it is estimated that 21% of these birds are extinction prone (Sekercioglu et al. 2004). Although this sounds alarming, the situation is much more severe for other vertebrate taxa, while for invertebrate taxa, the lack of comprehensive data prevents a proper evaluation of endangerment (Norris, 2004). Diagnosing the factors that cause a population to decline is a principle component of nearly all projects intending to save threatened species from extinction (Green, 1995; Norris, 2004). Population dynamics In order to diagnose declines and their causes, it is important to consider the population ecology of a species. The study of a species’ population dynamics is concerned with the fluctuations of population sizes and the demographics of the population, as well as the biotic and abiotic factors driving these fluctuations. Populations increase through births and immigration and decrease through deaths and emigration. Their dynamics are 2 Un ive rsi ty Of C ap e T ow n determined by the relationship between the size of the population and the per capita growth rate (Sibly et al., 2005). Population growth in mammals, birds, fish and insects, generally tends to be high at low population densities and low at high population densities (Sibly et al., 2005). However, small populations may falter due to Allee effects, a syndrome that causes poor performance among small populations (Courchamp et al. 1999). Population growth rate frequently is considered to be the main natural response variable to be used in the statistical analysis of the factors that may influence the population dynamics of a particular species (Godfray & Rees 2002). Although factors affecting population growth rates have been a major focus of conservation biology and population ecology for several decades, surprisingly few generalizations have been developed that would allow us to predict variation in the growth rates of natural populations (Saether & Engen, 2002). The estimation of population growth rate using population census data faces several challenges with regards to its precision. The first challenge arises from the fact that these data are often collected over relatively short time periods, usually 5-10 years, with studies rarely longer than 30 years (Humbert et al. 2009). Other challenges include uneven time intervals arising from monetary or logistical constraints, environmental noise, estimation error, and the choice of statistical method to analyse the data (Humbert et al. 2009). Limited sample sizes may also pose major problems, especially in terms of space as populations are often divided into smaller sub-populations that may have slightly different characteristics. The balance between the loss of individuals through emigration and mortality and the recruitment of new breeders through immigration and reproduction determines local population sizes (Pulliam 1988). Populations can be divided into smaller sub-populations on a smaller spatial scale and these populations may differ in their life history components and thus make up source or sink populations depending on their success or overall fitness (Pulliam, 1988; Opdam, 1991) The biotic and abiotic conditions present within a particular environment will determine if the area is favourable or unfavourable and whether or not the population will increase or decrease. With survival and 3 Un ive rsi ty Of C ap e T ow n reproduction being the factors that typically have the largest influence on population size, factors that affect these two traits need to be considered when analysing possible reasons for a decline in population size. There are many factors that affect survival and reproduction as well as the distribution of a species, ranging from limited food or habitat availability, either through competition or poor production due to environmental factors, to predation and disease (Grosbois et al. 2008). Humans may also play an important role in population fluctuations through activities such as harvesting and habitat degradation. Whether it is stochastic events or density dependent effects that cause population fluctuations, quantifying the sources of these fluctuations may provide insights into the various underlying processes of population dynamics and can play a direct role in the conservation of wildlife (Altwegg et al., 2003, Bond et al., 2009). Understanding which factors influence population dynamics may also help us determine how populations could respond under environmental change (Grosbois et al. 2008, Karell et al. 2009). Fluctuations in population size often seem to be stochastic, but this may simply result from ignorance regarding the detailed causes of dispersal, reproduction and mortality (Lande et al., 2003). The role of climate in population dynamics Although many populations are affected by climatic variation (Stenseth et al. 2002, Walther et al. 2002, Grosbois et al 2008), relatively little is known about environmental forcing of the factors affecting survival of birds in arid regions where unpredictable, variable rainfall is the dominant environmental variable (Altwegg & Anderson, 2009). The amount and variability of rainfall in these regions is an important determinant of survival in several arid-zone bird species (Saether et al. 2004, Altwegg & Anderson, 2009). Breeding success may be affected by rainfall, through its influence on food availability (Covas et al 2008, Altwegg & Anderson, 2009) and hence understanding how climatic variables affect these parameters is crucial to get a better grasp on population dynamics. Knowing the effect of climatic variables on survival and breeding success is essential to infer population responses to different climatic scenarios (Grosbois & Thompson, 2005) 4 Un ive sit y O f C ap e T wn With a better grasp on the effects that climatic variables have on populations, and a basic knowledge of the average growth rate of the species, it may be possible to model population size and thus conduct less frequent surveys (Jonzen, et al. 2010). However, population models lack real world accuracy and should thus not be too heavily relied upon for important conservation acti