Dispersal of Introduced
Species in the Coastal Oceans
Ted Grosholz
Department of Environmental
Science and Policy
University 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 Against Current
km/yr
km/yr
Carcinus maenas
Caulerpa taxifolia
Codium fragile (t.)
Elminius modestus
Ensis directus
Hemigrapsus
sanguineus
Philine auriformis
70
67
40
47
200
72
24
60
14
34
100
16
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