NONLETHAL EFFECTS OF PREDATORS ON PREY POPULATIONS: PREDATOR-MEDIATED CHANGE IN BIVALVE GROWTH

Abstract

Predators affect dynamics of prey populations not only by decreasing their survivorship, but also by changing other vital rates such as growth and reproduction. However, such nonlethal effects have rarely been examined experimentally in the field. In North Carolina estuaries, the hard clam Mercenaria mercenaria generally shows lower growth rates in habitats where predation rates are higher. Predator avoidance behavior of the clams (feeding inhibition) might be one of the factors responsible for this correlation. To examine possible effects of predators on clam growth, I first compared variation in clam growth rate among habitats with different predation pressure, and between predator-exclusion cages and uncaged controls. I then carried out a predator-enclosure experiment using the clam and the whelk Busycon carica, a major predator on adult clams. Among-habitat variation in survivorship, predation rate, and growth rate was generally small or not detected in winter, while large variation was found for adult clams in spring, their major growing season. Growth rate of adults was highest in a mud bottom where survivorship was highest (>60%) due to low predation rates, and it was lowest in an oyster shell bottom where survivorship was lowest (<10%). For juveniles, on the other hand, growth rate did not differ significantly among habitats, nor did survivorship. Clams in the predator-exclusion cages showed nearly twice the growth rate of uncaged controls in spring but not in winter. Clam growth rates were thus lower in habitats with lower survivorship and higher predation rates, suggesting that the predators can affect growth rates. In the whelk-enclosure experiment, the clams were enclosed in cages with three different treatments: (1) with whelks capable of attacking clams, (2) with whelks isolated from the prey, and (3) without whelks (control). The shell and somatic growth rates of the clam were at least twice as high in the control than in the two experimental treatments. However, the growth rates did not differ significantly between the two experimental treatments. The clam growth rates were therefore reduced in the presence of whelks even with no direct contact between whelks and clams, suggesting that the decrease in growth rates is due to a behavioral response of the prey to indirect cues, such as water-borne chemical signals produced by whelks. These findings show that predatory whelks can affect clam population dynamics not only by decreasing survivorship directly, but also by decreasing future reproduction and survivorship indirectly by suppressing growth rates, and that the predator-avoidance behavior observed in previous studies has significant demographic consequences. The nonlethal effects of predation on clam growth rate may also be responsible for observed variation in growth rates among habitats.