Finding exotic needles in a haystack: Early detection of colonizing invaders when rare

Background/Question/Methods

The establishment and spread of non-indigenous species (NIS) is a growing global concern. To combat this problem, we need to reduce vectors of spread, but also detect new NIS before they become established. Early detection of colonizing NIS is essential for eradication efforts. In this study, we estimate the sampling effort required to detect a new NIS when it is still rare. To address this question, we sampled Hamilton Harbour, Lake Ontario, for the fishhook waterflea (Cercopagis pengoi). C. pengoi is an established NIS in this system, but its unique life history and biphasic morphology make it the perfect and ethical model species. C. pengoi is parthenogenetic and overwinters as eggs in lake sediment. Individuals that emerge in spring are morphologically distinct from subsequent generations of asexually-produced individuals and thus readily identifiable as the initial generation. This biphasic seasonal morphology allowed us to target sampling to the initial generation animals only, which, when first hatching from eggs, would be rare, analogous to a new invader. One hundred stratified samples (vertical plankton hauls), spaced 100m apart, were taken on six dates throughout spring and summer 2007 across the emergence period. C. pengoi presence/absence data were used to construct sample-based rarefraction curves for each sampling date.
Results/Conclusions

Capture rates ranged from 0 to 92%, with a mean of 0 to 2.17 C. pengoi per sample. Rarefraction curves demonstrated a clear positive association between the probability of detecting C. pengoi and the number of samples taken. Yet, there were significant differences between the number of samples required to detect C. pengoi early and late in the season than when at its peak density. C. pengoi was found in 2 of 100 samples on the first date of detection and in 3 of 100 samples on the final sampling date. Rarefration curves based on these probabilities indicate a minimum of 30 samples in a single habitat to detect NIS at rare densities. Furthermore, the typical standard of 3-5 samples per lake would be sufficient to detect this NIS only when at peak density. These results suggest that a much higher sampling intensity is required to detect colonizing NIS than standard performed protocols. Because newly colonizing NIS can only be detected with intense sampling effort, sampling protocols must be augmented above those typically used for routine monitoring.