Genetic connectivity, population dynamics and habitat selection of the southern ground hornbill (Bucorvus leadbeateri) in the Limpopo province

Abstract

Southern ground hornbills (Bucorvus leadbeateri) (SGH) are co-operative breeders that occur in groups of 2-9 individuals. Long life spans, large territory sizes (100km²), and low reproductive rates render these birds vulnerable to threats such as loss of habitat, persecution for their habit of breaking windows through territorial aggression, poisoning and loss of suitable nesting sites. As a result, SGH are listed as vulnerable in the red data book of South Africa as well as globally. The main objective of this study was to contribute to our overall understanding of the ecology and biology of the SGH for conservation planning. Data collection was completed in the nonprotected, semi-arid landscape of the Limpopo Valley from June 2008 - September 2009. The seasonal habitat use by a group of SGH, seasonal abundance (numbers) and biomass (volume) of invertebrates using pitfall and sweep net methods was investigated. Furthermore, a total of eight groups and 23 birds were captured in the Limpopo Valley and different statistical analysis were performed to investigate levels of inbreeding, relatedness, sex-biased dispersal and the effects the recent re-colonisation has had on the genetic structure of SGH in the Limpopo Valley. Finally the genetic variation of the species in the rest of Africa was determined using samples from Kenya, Tanzania and three populations in South Africa namely the Limpopo Valley, Kruger National Park (KNP) and Kwa Zulu-Natal (KZN). Genetic analysis revealed SGH have retained comparatively high levels of genetic diversity, even though there are indications of genetic bottlenecks in the Limpopo, KNP and Kenyan populations. The SGH populations studied were grouped into two clusters corresponding to the geographic origin of samples. The birds from Tanzania and Kenya clustered together while the KNP and KZN birds clustered together with the Limpopo population grouping more or less equally between the Kenyan/Tanzanian and South African populations. A large percentage of genetic variation was found within populations while among population variation was low, indicating there is little molecular evidence for the presence of SGH subspecies. The overall home range of one group was approximately 20 000 ha while seasonal home ranges varied between 5000 ha in winter to 13 500 ha in summer. The response of organisms to environmental variables in this extremely seasonal habitat was further revealed by the positive correlations found between the number of invertebrates with mean monthly maximum and minimum temperatures, and the volume of invertebrates with mean monthly rainfall. No significant differences were found between numbers and volume of invertebrates per order, between sites, which was expected in this homogenous vegetation type dominated by mopani shrub and trees (Colophospermum mopane). The re-colonisation of the Limpopo Valley was shown to have occurred by a number of unrelated individuals. This was demonstrable by very low levels of inbreeding and average relatedness of the population, as well as the favourable levels of heterozygosity across age and sex categories. Within group relatedness was high with juveniles related to at least one parent from their natal group. Insights were also gained into the breeding behaviour of SGH, providing evidence for the first time that SGH are not as monogamous as previously thought, with two instances of extra pair copulations recorded between four groups. This study shows that a holistic approach combining genetic techniques, radio telemetry studies and ecological principles has great potential to further investigate SGH, thereby contributing to the preservation of this enigmatic species of the savannah biome.