Dispersal, migration and management - Ecological Applications: Toward a Sustainable World - page 96

function in a similar way, unless the organisms' powers of dispersal are so strong
that the habitats are functionally continuous. Thus, the timing and pattern of inva-
sive spread depends on both the geography of the discontinuous habitat and an
organism's dispersal potential.
Then again there are habitats that are 'continuous', including large expanses of
grassland (prairies, steppes, pampas, agricultural land) or forest (rainforest, boreal
forest, plantations). In t hese cases the spread of invaders can be envisaged as a
progressive wave of diffusion across the landscape, at a rate that depends on the
species' intrinsic rate of population growth and its 'coeffi cient of diffusion'. A high
coeffi cient of diffusion may be associated with the ability to hitch a ride with
humans or other animals or on the wind, or the capacity to walk or fl y quickly
through the landscape.
The red fi re ant ( Solenopsis invicta ) has spread rapidly through much of the south-
ern USA with dramatic economic consequences (Section 1.2.5). The species, which
originated in Argentina, occurs in two distinct social forms. The single-queened
(monogyne) form and the multiple-queened (polygyne) form differ in patterns of
reproduction and modes of dispersal. Queens from monogyne colonies take part in
mating fl ights and create new colonies, whereas queens from polygyne colonies are
adopted into already existing nests after mating. As a result, the monogyne popula-
tions spread a thousand times faster than their polygyne counterparts (Holway &
Suarez, 1999). The ability of managers to predict potentially problematic invaders
and devise strategies to counter their spread depends on a thorough understanding
of dispersal behavior.

4.4.4 How to manage
invasions under
globalization

We now have some power to predict the species that pose the greatest risk as invad-
ers, and know the major invasion routes; national biosecurity strategies are based
on this information. However, as Perrings et al. (2005) point out, protecting national
borders is diffi cult because those whose actions result in invasions usually bear no
legal responsibility and do not have to pay the costs associated with invaders. In
economic terms, invasion costs are an 'externality' of global trade - an unintended
side effect whose cost is not refl ected in the market price of the goods that pose the
risk. Invasions are a form of biological pollution; the same externality problem
occurs if the costs of chemical pollution are borne by society in general rather than
by those who pollute. It is now more generally accepted that polluters should pay
for the damage they cause - the 'polluter pays principle'. This is an example where
what was an externality (cost of pollution) becomes internalized (pollution costs
refl ected in the market price of the industrial products). Can the same thing be done
for invasion costs?
You saw in Section 3.3.3 that the probability of a successful invasion by a parrot
species was positively correlated with the development of the international parrot
market. There are many other examples among both animals and plants. What is
needed, according to Perrings' team, is a measure that confronts exporters with the
costs of their actions - the introduction of invasion risk-related tariffs. Import tariffs
can be expected to reduce export activities that are particularly risky, which is
desirable, but such tariffs could disproportionately hurt the economies of poor
countries. Thus, the team suggests that tariffs should be coupled with international
fi nancial support for low-income countries that adopt biosecurity-enhancing
measures and, thus, confer a worldwide benefi t.