we will discuss the effects of advection along environmental gradients on logistic reaction-diffusion models for population growth. the local population growth rate is assumed to be spatially inhomogeneous, and the advection is taken to be a multiple of the gradient of the local population growth rate. we show that the effects of such advection depend crucially on the gemeotry of the habitats of population: if the habitat is convex, the movement in the direction of the gradient of the growth rate is beneficial to the population, while such advection could be harmful for certain non-convex habitats.