The Relationship between Selectivity and Food Abundance in a Juvenile Lizard

Abstract

Several theoretical models of foraging behavior predict relationships between the relative abundance of a food item and a forager's selectivity for that item. Prey switching models predict that selectivity for items will be positively related to their relative abundance. Optimal foraging models based on energy optimization predict a non—negative correlation between selectivity and the abundance of valuable food items. Optimal foraging models based on nutrient constraints predict that selectivity for items should be negatively related to their relative abundance for all nutritionally important food items. The relationship between selectivity and relative abundance was investigated in the field in a juvenile lizard, Anolis aeneus. In each of 11 periods spanning the wet and dry season, sweep samples and stomach contents yielded data on selectivity, the relative and absolute abundance of different prey types in the environment, and the total abundance of food in the environment. The results tend to confirm the optimal foraging models based on nutrient constraints. All of the six important prey types eaten showed a negative correlation between selectivity and relative abundance; for four taxa the relationship was significant. In the case of four taxa, juveniles tended to eat the same proportion of the prey type regardless of its relative abundance in the habitat. In two other cases juveniles tended to eat proportionally less of a prey type as the relative abundance of that type increased in the environment. The absolute abundance of prey types also had a negative correlation with selectivity (significant for the two most frequently eaten prey types). Total food abundance was the most important determinant of selectivity in only one prey type; lizards ate more ants when food abundance was low. It is suggested that energy optimization may be relatively low priority for small ectothermic insectivores such as Anolis aeneus juveniles. Low energy demands, a relative abundance of suitable prey sizes in the environment, and low search costs may allow such animals to feed more like herbivores than like insectivorous birds or mammals.