Linear regression model for orang-utan abundance on an expedition, conditional on at least one detection setting the coefficient of log(Days) equal to 1.

Model 3a provides full model of non-zero abundance. Model 3b provides model of non-zero abundance, omitting Person. Year = year in which expedition was conducted. Person = number of people on an expedition. Log(Days) = natural logarithm of duration of expedition in days. Residual standard error in Model 3a: 1.049 on 40 degrees of freedom. Residual standard error in Model 3b: 1.067 on 41 degrees of freedom. Significance codes: ‘**’: p<0.01; ‘***’: p<0.001.