1. Aim
Global trade is a key driver of human-mediated dispersal of species across biogeographical barriers, resulting in biological invasions and contributing to acceleration of species spread worldwide (Ruiz et al., 2000). In aquatic and coastal systems, many of these non-native species are dispersed by cargo ships (Wonham, Carlton, Ruiz, & Smith, 2000) that may serve as vectors of large and/or consistent releases of unwanted organisms.
Figure 1. Commercial vessel traffic of one day (May, 2nd 2018). Obtained from marinetraffic.com
Ship-borne invasions occur mainly by ballast water (water used to maintain vessel stability during voyage that can contain aquatic species) (IMO, 2011) and biofouling (undesired organisms that accumulate in submerged structures of vessels) (Davidson, Brown, Sytsma, & Ruiz, 2009). The latter mechanism has been frequently overlooked, and recent evidence suggests biofouling on hull of ships may play a more important role in the unwanted transportation of benthic invaders, given that most larval stages are not able to survive voyages in ballast tanks (Moser et al., 2016, Simkanin et al., 2016).
Figure 2. Biofouling is the community of organisms that colonize features withing submerged structures of a ship. Source: Australian Department of Agriculture and Water Resources
5. Results
5.1 Understanding the marine hitchikers: more arrivals, more species?
Propagules are the set of individuals that are released in a non-native environment.Ships as vectors can work as contributes for propagule supply and introduction of organisms overseas. However, many factors in a port area can affect a likelihood of invasion. One commonly investigated is number of arrivals where the relationship with probability of invasion is direct (Kaluza et al., 2010).
Hence, I asked the questions, does more arrivals result in greater number of invasive species identifed in this area?
## Analysis of Variance Table
##
## Response: freq
## Df Sum Sq Mean Sq F value Pr(>F)
## region 1 9126 9126 2.9642 0.22727
## year 1 55460 55460 18.0139 0.05128 .
## region:year 1 90 90 0.0293 0.87981
## Residuals 2 6157 3079
## ---
## Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1
Vessel traffic is very similar between portsthrough time (p > 0.8). Given that traffic is not signifficantly different, one would expect that number of invasives would be also similar. However, California had 3 times more established invasives recruiting to panels than Alaska. It is possible that other factors related to propagule pressure, such as seasonality of arrivals and type of vessels arriving are contributing to stronger introduction pessure in San Francisco Bay.
5.2 Monthly variation of arrivals can increase propagule number
As propagules are the set of individuals that are released in a non-native environment, and the propagule pressure is defined by two major components: the number of individuals that is released in a given environment in one release event, and propagule size, number of release events in a given locations. Based in that, the propagule pressure predicts that higher propagule pressures result in higher probabilities of establishment success.
Consistent release of propagules can enable populations to overcome problems related to small population size, such as populations subjected to stochastic processes, demographic effects, low genetic diversity etc. In San Francisco, the lower seasonality on ship arrivals might work as re - introduction of propagules of already established species and therefore result in increased invasion success. So although, Alaska and California seem to be similar in total number of arrivals the proportional number of those are discrete by month in every sampled year. Hence, the probability of a successful introduction icreases, because release events are less subjected to stochasticity.
5.3 Different ride, different guests: fleet profile affects probability of invasion
Vessels do not contribute equivalently to transportation of unwanted species. Each vessel has a particular dessign to meet its cargo type or function, and previous research suggest that there is variability in biofouling amongst vessel types (Davidson et al., 2009).
The fleet diversity arriving in San Francisco is visibily larger than Alaska. Cargo ships (i.e. Tankers, Bulkers, Containers, General cargo) usually are deeper submerged on the water column and stay longer periods on births on port areas. That creates a more suitable ondition for fouling organisms to survive during voyage and release viable propaguleson the new location.
5.4 Human transportation and biogeographic filters
Humans intentionally or accidentaly mediate transportation of organisms constantly. There are a series of biotic and abiotic factors that might prevent a non-native from establishing. However, it is likely to increase invasion probability if a port receives propagules from transoceanic sources, because that increases the probability of the fouling organism in being alien.
One may already infer from results above that San Francisco is a more internationally interconeccted port than Ketchikan. Ketchikan is an important connector on the cruise ship routes on the northeastern Pacific, but it mostly serves as starting port. In the meanwhile, San Francisco is one of the greatest ports of entry for imported goods of the west coast of the United States, which results in number of international arrivals being 2 orders of magnitude higher than Alaska.
6. Main conclusions
Studied regions displayed similar number of arrivals but very different transit profile in each port. In California, lower seasonality in shipping traffic is likely to be contributing to higher propagule number and therefore increased probability of invasion. In Alaska, it is likely that environmental mismatching is also playing an important role in preventing establishment success of non-native species.
Higher propagule pressure exerted by shipping may not result in higher probability of invasion success and a more complex suite of environmental and ecological components (e.g. environmental matching, biogeographical distance, life history traits) may act collectively with propagule pressure to influence the likelihood of successful introductions in port areas.
References
Davidson, I. C., Brown, C.W., Sytsma, M. D., & Ruiz, G. M. (2009). The role of containerships as transfer mechanisms of marine biofouling species. Biofouling 25 (7): 645-55. doi:10.1080/08927010903046268
IMO - International Maritime Organization. (2011). MEPC.207(62) Add.1 Guidelines for the Control and Management of Ships’ Biofouling to Minimize the Transfer of Invasive Aquatic Species. Annex 26 (July): 1-25.
Moser, C. S., Wier, T. P., Grant, J. F., First, M. R., Tamburri, M. N., Ruiz, G. M., Miller, A. W., & Drake, L. A. (2016). Quantifying the total wetted surface area of the world fleet: a first step in determining the potential extent of ships’ biofouling. Biological Invasions 18 (1): 265-77. doi:10.1007/s10530-015-1007-z
Ruiz, G. M., Fofonoff, P. W., Carlton, J. T., Wonham, M. J., & Hines, A. H. (2000). Invasion of coastal marine communities in North America: Apparent patterns, processes, and biases. Annual Review of Ecological Systems 31: 481-531.
Simkanin, C., Fofonoff, P.W., Larson, K., Lambert, G., Dijkstra, J. A., & Ruiz, G. M. (2016) spatial and temporal dynamics of ascidian invasions in the continental United States and Alaska. Marine Biology 163 (7). doi:10.1007/s00227-016-2924-9
Wonham, M. J., Carlton, J. T., Ruiz, G. M., & Smith, L. D. (2000). Fish and Ships: Relating Dispersal Frequency to Success in Biological Invasions. Marine Biology 136 (6): 1111-21. doi:10.1007/s002270000303
