Dispersal, migration and management - Ecological Applications: Toward a Sustainable World - page 92

species that play a critical role in the functioning of wetland forest ecosystems have
restricted powers of recolonization. Just as important, the nature of these systems
depends on fl oods that periodically inundate the land. These were engineered away
to permit agricultural production. Now, human settlement in the fl oodplains limits
the opportunity to re-engineer the appropriate fl ood dynamics.
Middleton (2003) set out to examine the natural restoration potential of farmland
that had originally been baldy cypress swamp ( Ta xodium distichum ) along the Cache
River in Illinois, USA. She compared the seed-banks of 51 ex-swamps that had been
farmed for up to 50 years with those from nine intact swamps, collecting soil
samples for germination under laboratory conditions. Herbaceous species proved to
be common in the seed-banks of both farmed (207 species) and intact (173 species)
swamps. On the other hand, neither farmed nor intact sites were well represented
by seeds of woody species, including those of the baldy cypress itself.
Baldy cypress seeds are short lived and are naturally dispersed to seed-banks by
water during a fl ood (after winter rain or snowmelt), germinating once the fl ood-
waters recede. Middleton argues that the re-establishment and maintenance of the
dominant tree species will be best served by hydrological engineering (e.g. removing
levees and restoring the linkage between the river and its fl oodplains). This will re-
establish the supply of tree seeds to seed-banks. Although it is good news that many
swamp herbs remain abundant in the seed-bank, not all will prosper unless the
hydrological regime is restored to regain the necessary physicochemical conditions.
Moreover, some herbaceous species rely on vegetative organs for dispersal (e.g.
Heteranthera dubia and Lemna minor ) and are absent from seed-banks: these will
require active reintroduction. Middleton looks forward to the possibility of restora-
tion of a natural fl ood regime in newly acquired conservation land in the Cache
River fl oodplain.

4.4 Predicting the
arrival and spread of
invaders

Human travel and commerce have opened up novel 'dispersal' routes for thousands
of invaders. Just like the seeds and mites that hitch a ride on animal dispersers,
countless plants, animals and microorganisms travel the world's trade and tourist
routes on people and their goods.
The best way to deal with invaders is to prevent their arrival in the fi rst place and
this involves identifying these novel 'dispersal' pathways. If we know the major
invader routes, such as hitchhiking in mail or cargo, on aircraft and in ships, we
can plan to manage the associated risks (Wittenberg & Cock, 2001). Biosecurity
precautions are now routine in many parts of the world, including the screening
and cleaning of cargo at the point of embarkation and on arrival, and inspection of
camping gear and boots of incoming passengers. As noted before (Section 3.3.3),
however, it is not just the likelihood of invader arrival that must be attended to, but
also the probability of spread from the point of entry. The Great Lakes of North
America provide an excellent case study of both biosecurity to prevent invasion and
the prediction of subsequent spread.

4.4.1 The Great Lakes
- a great place for
invaders

The Great Lakes of North America have been invaded by a motley collection of more
than 145 alien species, including fi sh, mussels, amphipods, cladocerans and snails.
Many of these arrived in ships' ballast water taken aboard at the other end of an
important trade route in the Black and Caspian Seas (Ricciardi & MacIsaac, 2000).
An ocean freighter before taking on cargo in the Great Lakes might discharge