Relative importance of colonist quantity, quality, and arrival frequency to the extinction of two zooplankton species.

Colonist quantity, quality, and arrival frequency can all individually drive the dynamics and extinction of new populations. However, we do not understand which has the strongest influence, nor the circumstances under which their relative importance may change. We conducted a field mesocosm experiment that manipulated colonist quantity, quality, and arrival frequency in two zooplankton species (Daphnia pulicaria and Skistodiaptomus oregonensis). Individuals of each species were cultured under either high or low food concentrations to produce, respectively, 'good' and 'poor' quality colonists. Each species was then introduced at either small (2 individuals) or large introduction quantities (8 individuals) divided over single or multiple introduction events. We found that the extinction of Daphnia pulicaria was not particularly affected by any of our treatments. Introductions of just two individuals performed as well as larger or more frequent introductions, regardless of quality. Conversely, Skistodiaptomus oregonensis extinction was strongly driven by arrival frequency. Populations that arrived in a single event exhibited high rates of extinction (75-83%), with this probability declining dramatically when colonists were introduced over multiple events (33% extinction). Our results show that other less studied aspects of the colonist pool, such as arrival frequency, could be as important to population persistence as the initial quantity of arriving colonists. Additionally, there are potentially numerous species that are well suited to succeeding with a small number of founders, and whose success is therefore not necessarily dependent upon colonist quantity, quality, or arrival frequency.