Biology Reference
In-Depth Information
Many plants also possess adaptations for dispersal by animals by a vari-
ety of techniques, such as burs that entangle in animal fur or spiny joints
that embed in animal skin. In central Mexico, a surprising number of
cropland weeds possess burrs, hooks, or awns that appear to adapt them
for dispersal on the clothing of farmers (Vibrams 1999). Other plants pro-
duce fruit that contains seeds that pass intact through the digestive tract
of animals. In most cases, animal-assisted dispersal functions only for short
distances. Although waterfowl and other plant-feeding birds may fly long
distances on migration, the probability of transport of viable seed in their
digestive tracts to distant, yet suitable, habitats is small (Charalambidou
and Santamaría 2002; Clausen et al. 2002). Most seeds eaten by waterfowl
prior to migratory flights, for example, are probably discharged within
300 km of the feeding area.
Many animals possess active means of dispersal by locomotion on land
or through air or water.Aquatic animals that live attached to the substrate
as adults produce larvae that can swim or float to new locations. Insects
with flightless aquatic or terrestrial larvae disperse as winged adults. For
many terrestrial and aquatic animals with moderate locomotion capabili-
ties, dispersal tends to follow closely the distribution of suitable, closely
spaced habitat areas.Vertebrates and many invertebrates that are capable of
long-distance movements often rely on complex behavioral and sensory
abilities to recognize appropriate new habitats.
Dispersal and establishment by parasites and diseases require a suitable
host plant or animal and, in some cases, a different alternate host or
required vector species. Parasites or diseases with direct life cycles, in
which the infective agent or its reproductive stage is transferred to a new
host without using an alternate host or vector, are more likely to invade
new regions (Dobson and May 1986). Parasites of freshwater fish, for
example, show this pattern, with 39 of 48 parasites that are successful
invaders being capable of direct transmission.
Natural dispersal mechanisms, however, seem to play a very small role
in the dispersal of alien species across major geographical barriers such as
major mountain ranges, deserts, and oceans. In particular, dispersal of
plants across ocean barriers by wind or water currents is not known to
have led to the recent introduction of invasive plants anywhere in the
world (Mack and Lonsdale 2001).Within continental areas, however, nat-
ural dispersal mechanisms have enabled many arriving alien species to
spread rapidly through areas of favorable habitat.
Dispersal of vertebrate animals introduced to new continents has often
been extremely rapid. In North America, birds such as the house sparrow
