Abstract
With five large and several small ports, Osaka Bay is heavily industrialized rendering the introduction of marine species that are not native to Japan inevitable. It presents, therefore, an ideal model locality to investigate the presence and persistence of introduced marine and estuarine species. We used the results obtained from surveys by citizen scientists undertaken between 2010 and 2013 to investigate the temporal and spatial patterns in numbers of introduced species. The results show remarkable temporal consistency in numerical diversity with between 11 and 15 introduced species being recorded every year. There were, however, strong spatial differences with tendencies for greater numbers of introduced species in the inner Bay and for lower numbers in the outer Bay. We analyzed the factors responsible for these patterns statistically using a generalized linear model (GLM). The optimal model implicated salinity and the number of native species. Because the salinity of Osaka Bay lies within the tolerance range of all the introduced species, it was discounted as a factor limiting introductions into the Bay. This implies that, interestingly, the actual number of native species is the most important limiting factor. This is then a real-world validation of the ecosystem resistance hypothesis.
