Propagule pressure has consistently been identified as a primary factor in invader success, and reducing it can be one of the most effective methods for preventing the establishment of non-native species. However, when policy is implemented to reduce propagule pressure it almost exclusively focuses on the size of individual introduction events (‘propagule size’), with little confirmation that controlling this single aspect of propagule pressure is the most effective strategy. The number of introduction events (‘propagule number’) can play as much, or more, of a role in invader success, yet only a small portion of propagule pressure research has studied the relative importance of size and number. We investigated the relative roles of propagule size and number in the establishment of a sexually reproducing species using a field mesocosm experiment that introduced Hemimysis anomala (a non-native mysid) across a range of propagule sizes and numbers. We found that single, large introductions had higher abundances and probabilities of survival than smaller, more frequent additions. This experiment illustrated that, for sexual reproducers, focusing on lowering propagule size can be the most effective method for reducing non-native establishment.
We established the mesocosm experiment between June 12–13 using one-hundred and eight 200 L cylindrical tanks (84 cm diameter × 53 cm high). Each tank was filled with 180 L of lake water from nearby Lake Opinicon (see Online Resource Table S1 for information on all lakes involved in the experiment) filtered through an 80 µm mesh to remove large zooplankton while allowing most phytoplankton and some small zooplankton to pass through. Shards of a 14 cm diameter and an 8 cm diameter unglazed ceramic pot were added to each mesocosm to provide daytime cover as Hemimysis is sensitive to daytime light levels and prefers rocky rather than soft substrates (Ricciardi et al. 2012). Tanks were also covered with 1 mm mesh to provide additional shade and to protect from insects and debris. We provided a natural resident community and food source by collecting zooplankton from the pelagic and littoral regions of four nearby lakes uninvaded by Hemimysis. We added zooplankton to each mesocosm at an ambient density on June 20. All mesocosms were then allowed to acclimate for 1 week prior to commencement of the Hemimysis additions.
Hemimysis were collected after sunset on June 25, July 2, July 9, and July 16 from the St. Lawrence River at Montreal, QC, CA (45°29′58.9″N, 73°33′01.7″W) using vertical plankton tows with a 250 µm net. All individuals were then transported that same night in dark containers to a laboratory near the field site. Over the following 2 days, all Hemimysis remained in dark containers and were fed Cyclop-eeze® daily. The second night after capture, all individuals were identified and added to the mesocosms. Only adult males and non-gravid females (based on Pothoven et al. 2007) were used in the experiment, with approximately equal numbers of large (about 9–11 mm) and small (about 6–8 mm) individuals used for each treatment. Tanks were haphazardly assigned to treatments, and Hemimysis were transported from the lab to the field site in 1 L of 80 µm filtered lake water and added to the mesocosms. Initial introductions for all propagule number treatments were made on June 27, with additional introductions for propagules divided over two additions made on July 4, and for those divided over four additions on July 4, 11, and 18.
Hemimysis abundance was monitored weekly beginning on July 10 to allow for 2 weeks between initial introduction and initial sampling to minimize stress. For each tank, ceramic cover was carefully removed, Hemimysis were then captured and counted using aquarium nets (10 cm × 15 cm), stored in mesocosm water, then returned following replacement of their cover. All sampling began 1 h after sunset and concluded prior to sunrise (~9 p.m.–5:30 a.m.) and was performed using red-light headlamps. Sampling generally required two nights, with any Hemimysis additions occurring on the second night. If we were unable to detect Hemimysis for three consecutive weeks after the last addition, we considered the population extirpated and sampling was not continued.