Negative density dependence of sympatric Hinge-back

A series of 59 transect surveys was conducted in selected wet forest habitats,along the coast of West Africa, to estimate the density distribution of AfricanHinge-back tortoises (Kinixys homeana and K. erosa). Line transect data were fedinto a simple model to derive a detection function. The parameters estimated by themodel produced an elaborate characterisation of tortoise distribution, which proved tobe useful in the formulation of hypotheses about tortoise densities. Line transect datawere analysed by DISTANCE, with a series of key and the series adjustment: the uniformfunction, the 1-parameter half-normal function, and the 2-parameter hazard-ratefunction were considered as key functions; the cosine series, simple polynomials, andHermite polynomials were considered as series expansions. The detection functionwas estimated separately for Kinixys homeana and K. erosa, and for transects groupedfor each study area by considering all the combinations of the above key functions andseries expansions. The Akaike Information Criterion (AIC) was computed for eachcandidate model and used for model selection. The best model of the detection function,for both the tortoise species was the uniform function with no series expansion.Model results indicated that the density of the two species was inversely related at thelocal scale, and complementary across the region; such that the density of one speciesincreases from West to East while the other one declines. Overall, the comparison ofdensity estimates between the two tortoises is consistent with a former hypothesis suggestinginter-specific competition and consequent resource partitioning. Other causesmay contribute to explain the observed patterns, including the low productivity ofrainforest habitats and long-term human perturbation.