Giant otters (Pteronura brasiliensis) and humans in the lower Yasuní Basin, Ecuador : spacio-temporal activity patterns and their relevance for conservation

Graduation date: 2007Giant otters (Pteronura brasiliensis) and humans in the Lower Yasuní Basin (Ecuador) have similar food and space requirements: they consume comparable arrays of fish species, and they use similar aquatic and terrestrial habitats. Resource partitioning could facilitate coexistence by allowing each species exclusive access to some resources.\ud My research examines the correlations between the use of spatial resources by giant otters and humans and the hydrological pulse of their ecosystem. My objectives were (1) to estimate the extent of terrestrial and aquatic habitat available for giant otters and humans;(2) to recognize the main patterns of space occupancy and its overlap between them; (3)to explain these patterns in relation to natural gradients in the watershed; (4) to describe the giant otter’s diet; and (5) to depict the foraging activities of giant otters and humans\ud relative to the distribution of prey. Between October 2004 and March 2005, I surveyed the Jatuncocha and Tambococha Creeks (Yasuní National Park) for direct and indirect signs of giant otters and humans, and I collected giant otter scat. Habitat availability was estimated by a geo-referenced map model that integrated virtual and field data, portraying four seasonal scenarios.\ud Space use overlaps between giant otters and humans are a function of water level and the\ud distance between terrestrial and aquatic habitats. The floodplain comprises 43% of the\ud total study area during the flooding season, and is reduced to 25% of the total area at\ud minimum water levels. Polygons obtained containing suitable habitat for giant otters are in average 41% larger than those obtained for humans; these contain areas that are inaccessible to humans (isolated pools and swamps, tributaries, and the upper portions of the main channel). Differences in space availability for and use by giant otters and humans depend on water level fluctuations. Area occupancy by both species is proportional to the estimated availability; but more so for giant otters, as humans increase area occupancy\ud during the driest periods. Giant otter activity was primarily concentrated around the largest tributaries of each creek, and secondarily wherever the distance between suitable terrestrial and aquatic habitat was smaller. Human activity signs were distributed along the hydrographical gradient, i.e. in the lower portion of the Jatuncocha System (lagoon) or in the middle portion of the Tambococha Creek (large tributary).\ud To identify correlations between habitat use by giant otters and humans and prey\ud availability, I analyzed scat samples and collected fish along several creek stretches. Giant otters consumed at least 47 of the 73 species available for them. Identifiable hard parts in the scat samples consisted primarily of Hoplias malabaricus (11.3%), Hypselecara termporalis (5.3%), Acestrorrhynchus sp. and Prochilodus nigricans (4.9% each). Foraging activity of giant otters was positively correlated with fish abundance and relative diversity,\ud with some exceptions. Human activity patterns were related to fish abundance and\ud diversity in Tambococha, but showed less correlation to fish distribution in Jatuncocha.\ud Overlap in resource availability and habitat use by otters and humans is highly variable. In the floodplain, extent and depth are critical variables that determine the distribution of resources in time and space, as well as the amount of resource partitioning possible.\ud Competitive interactions during stressful conditions could be reduced by ensuring the\ud availability of exclusive resources for giant otters during the driest periods, allocating suitable habitats within a short distance from one another along the longitudinal gradient of each watershed. Locally adapted and biologically sound regulations within this and\ud comparable areas of the YNP could promote the persistence of giant otters without\ud compromising the well-being of the local human inhabitants