Dispersal of Introduced Species in the Coastal Oceans

Ted GrosholzDepartment of Environmental

Science and PolicyUniversity of California, Davis

Reducing Likelihood of New Invasions:How Far Can They Go?

• Among goals of developing ABWEAs is minimize likelihood of new species becoming established

• Key issue is how far can they go?• Physical models of transport one critical part• Studies have used genetic distances, small

scale dispersal to infer dispersal distances• To what degree do plankton (nekton?)

disperse independently of physical processes?

Averages vs. Rare Events

• Many models of physical and biological processes focus on long-term averages

• What may be important for establishment are rare transport events

• Hurricanes, earthquakes, etc. nearly impossible to measure/predict

• Theory of rare events (e.g. rogue waves) is relatively well developed (Gaines and Denny 1993)

• Use statistical methods with long term data to determine extreme values (dispersal?)

Rare Dispersal Events• Dispersal of Mole Crab Emerita analoga• Northern most reproductive population in Oregon• MacGinities in 1930s found new recruits is S.E. Alaska

Earlier Studies of Invasion

• Historical studies of the spread of introduced species can provide insights into how far organisms move

• Measure spread or range expansion (km/yr)

• Relate these to possible spread by larval dispersal vs. spread by other means

The Case of Elminius modestus

• Invasion of the Australasian barnacle Elminius modestus in the UK provides good example of careful (painful) study of spread

• Barnacle introduce to British coast just prior to 1945 and successfully spread throughout southeast coast

• Dozens of sites along the British coast were surveyed at sites colonized and not yet colonized

Conclusions from Elminius

• Rates of spread are episodic

• Longer distance dispersal produces “beachheads” or new sites from which more local dispersal occurs

• Dispersal distances are generally modest (tens of kilometers)

Asymmetry in the Invasion Front

• One of the repeatable patterns of spreading invasions is the asymmetry of the invasion front

• Species generally spread faster in one direction than the other

• Many factors could produce this– Higher rates of advection of surface waters– Human mediated movements– Reporting biases, differential availability of

habitat

Asymmetry in the Invasion Front

• Do introduced species show similar patterns of asymmetry (is in the same direction)?

• Do introduced species show similar rates of spread (same distance in km/yr)?

• Do we have sufficient data to answer this?

Asymmetry in the Invasion Front

Taxa With Current km/yr

Against Current km/yr

Carcinus maenas 70 24 Caulerpa taxifolia 67 60 Codium fragile (t.) 40 14 Elminius modestus 47 34 Ensis directus 200 100 Hemigrapsus sanguineus

72 16

Philine auriformis 55 0

Range Expansion of European Green Crabs

• Among the fastest rates of spread on record

• However quite variable rates of spread:– Spread of marine species is very episodic,

more so than terrestrial species (Grosholz 1996, Thresher et al. 2003, Kinlan et al. 2005)

• Hard to predict rates of spread even for the same species

European Green Crabs

• Spread for green crabs quite variable– 200 km/yr in western U.S., – 8.7 km/yr in eastern U.S. (much faster over

shorter time frame)– 1.9 km/yr in South Africa– 1.7 km/yr in South Australia and Tasmania

• Spread is highly variable among even on the same continent at approximately the same latitude

European Green Crabs

• Spread for green crabs quite variable– 200 km/yr in western U.S., – 8.7 km/yr in eastern U.S. (much faster over

shorter time frame)– 1.9 km/yr in South Africa– 1.7 km/yr in South Australia and Tasmania

• Spread is highly variable among even on the same continent at approximately the same latitude

Conclusions

• Rates of dispersal (range expansion, spread) are likely a function of rare rather than average transport events

• Dispersal is significantly asymmetric and is a persistent feature of range expansion

• Dispersal is very site (coast) specific even for the same species

Recommendations

• Develop monitoring protocols to measure absence as well as presence of introduced species

• Gather and analyze rates of spread among existing records

• Develop hierachical modelling approaches across scales combining physical processes with biological mechanisms