Fig. 2. Summary of the factors affecting the size and direction of road effects on animal abundance, with 10 possible cases. Each case is defined by all the conditions leading to it through the arrows above. “neg,” “pos,” and “neut” refer to negative, positive, and neutral effects of roads on abundance (respectively). “m,” “h,” and “p” refer to the mechanisms creating road effects on the populations: mortality, habitat loss or increase, and predation release (respectively). Mortality and habitat loss are negative effects, and habitat increase and predation release are positive effects of roads on animal abundance. Examples of species in each case are: some turtles and snakes (A), vultures (B), large mammals, some mid-sized mammals, and some large birds (C), some small birds (D and E), small mammals (F, G, H, I), and amphibians (J).