EFFECTS OF A TAPEWORM PARASITE ON THE COMPETITION OF TRIBOLIUM BEETLES

The effect of parasites on host community structure and biodiversity i s an important issue of community ecology. Both verbal and mathematica l models suggest that host species with higher fitness costs associate d with parasitism should bear a competitive disadvantage in the presen ce of parasites. However, few rigorous empirical tests exist. This stu dy examined the effects of a tapeworm parasite (Hymenolepis diminuta) on competition between two flour beetle species (Tribolium confusum an d T. castaneum). The flour beetles are the intermediate host of this r at tapeworm parasite. Previous studies demonstrated that T. castaneum is usually the superior competitor in competition with T. confusum in parasite-free environments. However, because T. castaneum is more susc eptible to parasitism and shows higher fitness costs when infected tha n T. confusum, we expected that T. castaneum would bear a competitive disadvantage in the presence of the parasite. In contrast, we found th at tapeworm infection significantly increased the likelihood of T. cas taneum winning the competition, reduced the time needed for T. castane um to win, and increased T. castaneum population density compared to p arasite-free populations. Therefore, our results suggest that the tape worm infection confers some advantage to T. castaneum and are in contr ast to the expectation based on mathematical studies and verbal argume nts. Disagreements between experimental results and the hypothesized p redictions probably reflect the effect of parasite-induced changes in intraguild predation. Our results suggest that theories on parasite ef fects of competition need to consider the complex nature of host-paras ite associations, including the effect of parasites on host behaviors.