Recently, it was proposed that stable isotope patterns can be used to quantify the width of the ecological niche of animals. However, the potential effects of habitat use on isotopic patterns of consumers have not been fully explored and consequently isotopic patterns may yield deceptive estimates of niche width. Here, we simulated four different scenarios of a consumer foraging across an isotopically heterogeneous landscape to test the combined effects of habitat and diet selection on the widths of the isotopic niche. We then modeled the actions of a naïve researcher who randomly sampled consumers from the simulated populations, and used these results to assess the overlap and partitioning of the isotopic and the ecological niches when habitat-derived differences among isotope signatures are not considered. Our results suggest that populations of dietary specialists exhibited broader isotopic niches than populations composed of dietary generalists, and habitat generalists exhibited narrower isotopic niche widths compared with populations of individuals that foraged in specific habitats. The conversion of isotopic niches to ecological niches without knowledge of foraging behavior and habitat-derived isotopic differences transformed an informative δ-space into ‘a blurry p-space’. Therefore, knowledge of habitat-derived differences in stable isotope values and understanding of habitat use and individual foraging behavior are critical for the correct quantification of the ecological niche.