Medium and large mammal community assemblages across city of Cape Town nature reserves

Abstract

Urbanisation is associated with the loss and fragmentation of natural land, the disruption of ecosystem functioning and services, and the loss of biodiversity. Small remnants of natural land within cities not only serve as recreational green spaces that contribute to human wellbeing, but also as refugia for a variety of indigenous flora and fauna. While large mammal species, in particular those that pose a threat to humans and are rarely tolerated in urban reserves, small and medium mammals may persist and even thrive in human modified landscapes. Understanding which species survive best in urban protected areas and how reserve attributes such as size, shape and connectedness influence mammal assemblages and species richness is important for the conservation of urban ecosystems globally. Cape Town is situated in the Cape Floristic Region (CFR) - a renowned biodiversity hotspot, with high rates of endemism. Cape Town is however one of the fastest growing cities in South Africa and both agricultural and housing demands are increasing pressure on remaining patches of natural land. Currently most of this land is conserved within 17 nature reserves that together comprise roughly 9% of the total surface area of the City of Cape Town (CCT) municipal area. Existing mammal species lists suggest that 22 mammal species still survive in these reserves but no formal, standardised surveys of the existing reserves have been conducted with a method that allows for comparisons between reserves and within reserves over time. The primary aim of this study was therefore to develop a standardised monitoring protocol for medium and large mammal species within the CCT reserves (range 30 - 8 400 ha). The secondary goal was to understand how reserve size, area to perimeter ratio, connectivity, vegetation heterogeneity and presence of permanent freshwater aquatic habitat might influence mammal community composition. A standardised camera trap protocol was developed for the 12 CCT reserves larger than 30 ha and conducted from June 2017 to Feb 2019 with cameras positioned within every square kilometre of a reserve, with a minimum of five cameras per reserve irrespective of reserve size. Additional cameras were placed in unique habitat types not included or underrepresented in the standardised grid and a minimum of 1000 camera days of data were collected for each reserve. A total of 13 360 independent trigger events by medium and large mammals revealed 19 native species (11 carnivores, 7 herbivores, 1 omnivore), which was 86% of the 22 species listed in the databases (based on records of 2012 to 2017), and 49% of the 39 species believed to have been present historically. Species richness varied from 1 – 12 species (mean ± SD = 7±3.6) and Cape porcupine (Hystrix africaeaustralis), Cape grysbok (Raphicerus melanotis) and small grey mongoose (Galerella pulverulenta) were present in most reserves. The minimum survey effort required to effectively sample the reserves varied from 210 to more than 1840 camera days and was affected by both reserve size and levels of connectivity. The use of camera traps with a placement protocol as used in this study together with the minimum camera day effort estimates presented for each reserve should allow for regular monitoring and provide comparable results. Species richness was best explained by reserve area-perimeter ratio with richness lower in reserves with large perimeters relative to their total area. Large, better connected reserves also had higher species richness and included wide ranging large carnivores such as leopard (Panthera pardus), while species with specialist habitat requirements such as otter (Aonyx capensis) were notably absent from reserves without the appropriate habitats. This study suggests that reductions in the size of existing CCT reserves and/or an increase in hard edges that reduce the core area may lower species richness and potentially drive more medium and large mammals to local extinctions. Extending existing reserves through the addition of core natural habitat and improved connectivity to tracts of natural land are both management interventions likely to maintain and improve the ability of urban reserves to sustain diverse, ecologically functional mammal assemblages.