Answer: Mesquite shrubs
Explanation: In majority of the food webs, plants are the primary source of energy flow as they are the producers. Here also, mesquite shrubs is a kind of plant species which is consumed by cactus finch, which is a species of bird. Coyotes lie on the top of this food web, as they are the last ones to carry forward the energy flow.
The food web would look like this-
Mesquite shrubs ----> Beaded lizards -----> Cactus finch ------> Coyotes.