Abstract
Whether community membership is of limited or unlimited nature is a longstanding issue in ecology. Assembly studies have provided insight into the contributions of competition, diversity, and history on the development of community structure. In these studies however, the colonizers are drawn from the same species pool in which all members have had an evolutionary history. Thus interacting species have potentially evolved life history strategies in response to one another and have altered the resistance of native assemblages to species insertion.
The human-mediated introduction of species provides an opportunity to ask questions pertaining to the resistance or susceptibility of communities to invasion in the absence of co-evolved traits. Whether a co-adapted, potentially co-evolved species pool can resist the invasion of a species with which none of the community members have had evolutionary "experience" has rarely been experimentally examined.
The marine encrusting communities of Coos Bay estuary, Oregon, have been and continue to be inocculated by non-indigenous species from a range of donor regions. The settlement phenologies of native and introduced species in both the lower and upper bay exhibit significant differences in the duration, timing, and density of settlement.
Questions pertaining to the resistance of native communities to invasion by nonindigenous species were examined in the face of catastrophic disturbance and established adult assemblages. In the presence of introduced species, disturbed patches (bare settlement panels) quickly attained a species equilibrium and diversity. In contrast native communities continued to gain species after 17mo.
Experimental manipulation of established native and invaded assemblages (14mo) such that native assemblages were placed in direct contact with invaded assemblages in 25%, 50% and 100% treatments allowed adult-adult interactions to be controlled. These manipulated assemblages were then reciprocally transplanted between the two sites. Early mortality of introduced species and lack of larval input at the native site resulted in reduced invader success. At the invaded site invasion success was correlated with two factors, initial native space occupancy and invaded assemblage treatment density.