Factors that affect owl ecology in an agricultural matrix in east Gauteng

Abstract

Anthropogenic impacts are extensive and affect species at individual and population levels as well as communities. To understand these anthropogenic impacts we have to understand various factors, such as agriculture, and the expansion of roads and how these affect populations, species and communities. The Boesmanspruit Highveld Grassland ecosystem, between Springs and Devon, Gauteng, is a critically endangered ecosystem, dominated by agriculture in the area, urbanisation and extensive road networks. The agricultural crops include maize, soya beans, and cultivated fields for animal feed. The region also contains the vulnerable African grass-owl (Tyto capensis), and three other owl species namely, the western barn owl (Tyto alba), the spotted eagle-owl (Bubo africanus) and the marsh owl (Asio capensis). The extensive road network, and particularly the N17 highway in this area have resulted in continued owl roadkills in the area. During 2002-2003 an average of 9.2 dead owls per kilometre per annum was reported. Various factors contribute to these high mortality numbers, one being that maize trucks carrying grain along the N17 may spill grain onto the road, which lures rodents to feed on the waste and in turn attracts owls to the road where they can easily hunt, putting the owls in danger of being struck by vehicles at night. The aim of this research was to assess the species of owls in the area and their abundance, estimate their occupancy and unpack the ecological and anthropogenic factors that may be driving owl occupancy in the area. Road surveys were done for 5 nights, in October 2018, January 2019 and April 2019 allowing for a 15 night occupancy estimate. Only three of the reported four owl species were found, namely the western barn owl, the spotted eagle-owl and the marsh owl. The data collected during these surveys were used in an occupancy model to determine the occupancy of owls in the area, providing an estimate of 0.817 (SE = 0.102). The abundance was determined to be approximately 2 owls per 4 km2 (SE = 1.21). Covariates were also used to assess the factors that may be affecting the high occupancy of owls in the area. The covariate that had the greatest effect on the occupancy of owls in the area was water sources. With water sources incorporated into the models, occupancy increased to 1 (SE = 3.427 x 10-16) from the initial 0.817. This increased the abundance estimates to approximately 6 owls per 4 km2 (SE = 3.091). Comparing occupancy across the different months that were sampled, it was found that owl occupancy was highest in October 2018 at 0.996 (SE = 0.155), and lowest in April 2019 at 0.526 (SE = 0.173). Food resources are a key driver for owl occupancy. Rodent communities and density was assessed in the area. The rodents caught were mostly Mastomys species (92%) and the remaining rodents were identified as Rhabdomys dilectus (8%). Up to 55 rodents were caught in a hectare, with the highest number of rodents caught in the April 2019 sampling and the lowest number of rodents caught in the October 2018 sampling. The abundance of rodents increased as the height of the crops increased and fields where soya beans were being grown had the largest number of rodents. Comparing the species caught in the live traps of the rodent survey with an unpublished small pellet analysis done in the area, the barn owls regurgitated pellets contained four species of small mammals (Otomys spp., Gerbilliscus spp., Mastomys spp. and a shrew species Myosorex spp.), while only two rodent species were trapped in the fields. The asynchronous owl-rodent abundance reported in the study indicate that the three owl species are likely more driven by habitat availability and nesting opportunities, rather than purely by food availability. It is therefore important that the owl breeding seasons, and in particular grass-owl breeding seasons are taken into consideration when managing agriculture in the area. It is also important that when making decisions on where to build new infrastructure, areas where owls may be more likely to nest should be avoided. It should also be noted that where rodents are being controlled by ecologically based methods, such as by owls, it is likely that these methods alone are not effective enough to control the levels of rodents in the area. Therefore, it is important that other ecologically responsible methods of rodent control are used in the area.