Overexploitation, Interference Competition, and Excess Density Compensation in Insular Faunas

When overexploitation and/or interference are predominant features of competition systems, the total number of individuals in a guild of competitors may be lower than that possible if interference or resource harvesting were less intense. Some islands exhibit summed guild densities far above that found in comparable situations on the mainland or on species-rich islands. Such "excess density compensation" may be interpreted as either a lack of interference or a lack of overexploitation on these islands. These two alternatives may be distinguished by measuring insular and mainland resource levels. If these islands have higher resource levels than the mainland, overexploitation on the mainland is implicated. If insular resource levels are lower, excessive interference on the mainland may be the cause. As yet no single study has presented definitive evidence to decide between these hypotheses. However, observations on birds on islands off New Guinea suggest the former hypothesis may be operating, while the latter may be more important in explaining density patterns of lizards on islands in the Gulf of California.