Predicting invasion patterns in coastal ecosystems: relationship between vector strength and vector tempo

Abstract

Oyster transports are among the leading anthropogenic vectors of coastwise introduction of non- indigenous species. Using the oyster industry of the Netherlands as a model system, we investigated the rela- tionship between vector strength (number of invasions) and vector tempo (magnitude and frequency of transport) in analyzing and predicting invasion patterns. We re- viewed literature on oyster-associated species introduc- tions, analyzed the scale of commercial oyster imports, and collected and identified epiflora from Pacific oyster shells. A total of 35 protist, algal, and invertebrate species have been introduced to the Netherlands with oysters, and we found 41 species of macroalgae on transported oysters. However, the number of introductions and quan- tity of oysters imported are not necessarily positively cor- related, particularly in the past 20 yr, when oyster imports decreased but the rate of introductions increased. The discrepancy between vector tempo and strength can be explained by unreported imports and vector characteris- tics: a single oyster may harbor a large number of species which are introduced with their substrate, thus facilitat- ing establishment. Further, the recently developed exten- sive Pacific oyster reefs in Dutch waters provide a suit- able substrate, enabling establishment even after low propagule pressure introduction events. Assumptions that are made about crucial parameters need to be recon- sidered: reported propagule pressure is not the same as actual propagule pressure; per-episode diversity of po- tential inoculants is not at a fixed level without episodic unpredictable spikes, and the recipient environment is not static. With increasing interest in predicting invasion patterns, caution must be taken in assuming that reduced propagule pressure will lead to reduced invasions.