Feeding selectivity in relation to territory size in a herbivorous reef fish.

Territory size in the herbivorous reef fish, Parma victoriae, (Pisces: Pomacentridae) is primarily determined by local density patterns and is independent of changes in food levels (Norman and Jones 1984). However, while individual readily expand territories in response to having their neighbours removed, they appear not to increase total food consumption. Individuals with large territories may benefit from improved food quality. Here we test the hypothesis that expanded territories increase access to preferred food types.A preliminary comparison of the relative proportions of algal species consumed and those present in the field indicated that feeding was highly selective. Strong feeding preferences were indicated by a field 'cafeteria' experiment, in which algal species were presented to individuals in equal amounts. Selectivity indices calculated from diet and availability measures were positively correlated with the feeding preference rankings.Individuals examined from two territory size categories (<10 m2 and > 10 m2) did not exhibit significant differences in feeding prefernces. However, they differed greatly in terms of the makeup of their diets. Higher ranking algae such as Champia zostricola and Rhodoglossum sp. were consumed in greater proportions on large territories. Algal abundances within territories also differed for the two groups. Some of the less abundant, high ranking food species were found in greater quantities per unit area on large territories. The differences in diet were not solely explained by changes in availability. The appropriate comparison showed that whereas individuals on large territories fed in a highly selective manner, those on small territories consumed algae in proportion to their availability. A neighbour removal experiment supported the hypothesis that territory expansion results in an increase in the consumption of preferred algal species.