Otter occupancy in the Cape Peninsula: Estimating the probability of river habitat use by Cape clawless otters, Aonyx capensis, across a gradient of human influence

Abstract

1. The distribution of Cape clawless otters, Aonyx capensis, in South Africa and their habitat requirements in freshwater and marine systems has been well established. There is, however, a lack of information on how otters are adapting to urban development and the transformation of critical freshwater habitat.2. Within the Western Cape, the Cape Peninsula exhibits substantial variation in levels of human impact over a small geographic range, offering an excellent opportunity to explore the hypothesis that otters are adversely affected by habitat transformation.3. A single season occupancy model was used based on otter sign to determine the probability of otter occupancy across a gradient of habitat transformation at both landscape and local scales.4. The probability of otters occupying river habitat in the Cape Peninsula was low (P = 0.29) but increased with proximity to marine protected areas (MPAs) that included estuaries and wetland habitat. Otter presence was not influenced by proximity to urban areas at the landscape scale, but declined in canalized sections of river that were heavily degraded by human activity. 5. Despite being heavily transformed, lowland aquatic ecosystems may still provide critical resources in the form of fresh water and breeding sites, and together with food within the marine habitat may be sustaining the peninsula's otter population.6. In order for otters to persist on the peninsula, conservation authorities must maintain wetland, estuarine and river habitat in close proximity to MPAs. Further research is needed to determine the long-term impacts on otters persisting in degraded ecosystems. 7. Annual single season occupancy surveys provide a rapid, cost-effective method for monitoring changes in otter occupancy which should be incorporated into current monitoring efforts to provide much needed long-term monitoring of a top predator in freshwater ecosystems.

Habitat selection and diet of the Neotropical otter (Lontra longicaudis) on the Osa Peninsula, Costa Rica, and range-wide monitoring recommendations

“…Bayesian variable selection methods are difficult when variables are strongly collinear (such as in our data [41]). We therefore selected which variables to include in the Bayesian analysis using an AICc approach, creating N-mixture models in program unmarked, including river as a fixed effect (to somewhat account for segment non-independence) in the state part of each model, along with each habitat covariate singly (similar methods as [15]). We also tested whether to include number of scat and track substrates as a detection covariate by creating the above model set with and without substrate variables included (totaling 22 models).…”

“…We believe the robust methodology which is becoming commonly employed for other otter species (e.g. [4,15,16]) is currently the most feasible method for addressing these key priorities. The IUCN's Conservation Actions for this Near Threatened species includes the standardization of field survey techniques to map the species' range, the identification of key habitats, and the protection of areas where large populations remain [1].…”

“…Many otter studies have modified repeated temporal visits to a site for repeated spatial replications of a site. This assumes that a site (a river "segment") has similar otter use along its entire length; it is therefore safe to assume that dividing the site into sub-sections of equal length ("sub-segments") should mean each subsection is also occupied, and can serve as a replicate for the larger site [2,4,15,16]. In this way, river "sub-segments" are spatial replicates for the larger river "segment".…”

“…This method has been used across 3 species of otters and 4 recent publications [4,6,15,16]. Inferences that include detection probability are in theory more accurate [16], and able to be compared between studies and across extremely diverse environments.…”

“…Perhaps as a result of this debate, many recent studies have used occupancy, i.e., the simple presence or absence of signs, to study habitat quality (e.g. [15,16]). Johnson (1980) [21] hypothesized that animals select resources at different hierarchical scales, including home ranges across the landscape (henceforth "landscape-scale"), within a home-range (henceforth "localscale"), and at finer scales (on the order of tens of meters; henceforth "micro-scale").…”