Summary
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.
Methodology
Our study began as a pilot experiment that commenced on 11 April 2015, the purpose of which was to assess what food treatments would result in individuals with either high or low lipid stores in both D. pulicaria and S. oregonensis. Zooplankton store energy primarily as lipids, and the amount of stored lipids affects their survival and reproductive capabilities (Lee et al. 2006; Arts et al. 2012), therefore potentially influencing colonist success in a new environment. Lipids are stored by cladoceran and copepod zooplankton as visible droplets which can be assessed and scored using a lipid index (Tessier and Goulden 1982; Arts and Evans 1991; Vanderploeg et al. 1992). The lipid index typically ranges from 0 (no visible lipids) to 3 (high quantity of visible lipids). The goal of the pilot experiment was therefore to assess what food treatments produced a mean lipid score around 0 and 3 in each zooplankton species. Such extremes in zooplankton lipid stores are commonly observed over the course of the growing season in natural populations (e.g. Tessier and Goulden 1982; Vanderploeg et al. 1992), and are therefore relevant to natural fluctuations in reproduction and food availability. Both species were fed using a laboratory culture of Chlamydomonas reinhardtii.
On 11 April 2015, we collected D. pulicaria and S. oregonensis from the pelagic region of Elbow Lake (see Online Resource Table S1 for information on all lakes involved in the experiments) using vertical tows with an 80 µm, 15 cm diameter net. On the same day, they were then transported in cooled containers to temperature- and light-controlled environmental chambers at Queen’s University in Ontario, Canada. For D. pulicaria, we initially isolated 150 adult females into separate 100 mL containers of Elbow Lake water that had been filtered through a G4 glass fibre filter (1.2 µm pore size; Fisher Scientific, Waltham, Massachusetts, USA). This ensured enough offspring were produced on any given day for use in our experiments. Only third-generation female neonates (juveniles newly released from the brood pouch) were used in our experiments to minimize maternal effects. For S. oregonensis, since juvenile sex determination is unreliable, only adult male and non-gravid female individuals were used from a collective pool maintained in filtered Elbow Lake water. All lab zooplankton were fed an amount of C. reinhardtii equivalent to 20 µg C individual−1 day−1, and slowly acclimated over the course of 2 weeks in environmental chambers to a 16:8 h day-night cycle, with 20 °C daytime and 15 °C nighttime temperatures, similar to conditions they would eventually encounter when introduced to the field experiment (Arnott, unpublished data).