Dispersal, migration and management - Ecological Applications: Toward a Sustainable World - page 88

4.2.4 Designing
marine reserves

In contrast to the adult damselfl ies in Section 4.2.3, the main dispersal phase of
organisms that live on the seabed is an immature stage. Thus, many marine species,
including algae, invertebrates and fi sh, release spores, eggs or larvae that spend time
in the water column, moving in ocean currents, before settling and maturing on the
bed. Viable populations need a suffi cient rate of replenishment by offspring pro-
duced by local adults or by distant adults whose 'propagules' (spores, etc.) are
brought on ocean currents. When designing marine reserves, self-replenishment can
be achieved if a reserve is big enough to contain a substantial amount of local
dispersal. Alternatively, in a system of reserves, self-replenishment can occur if
neighboring reserves are at appropriate distances for transport of propagules from
one to another. In either case, management decisions require knowledge about dis-
persal distances.
Shanks et al. (2003) reviewed a considerable body of literature to obtain estimates
of the time propagules spend in the water column, and the distance they disperse,
to provide information of value when designing reserves. Duration in the fl oating
phase ranges from less than 2 minutes to 293 days, and estimated dispersal distance
ranges from less than 1 m to 4400 km. For the most part, there is a convincing rela-
tionship between dispersal distance and propagule duration (Figure 4.4), but some
species travel less far, given their time in the water column, and fall below the
regression line in the fi gure. This applies to spores, eggs and larvae that are nega-
tively buoyant or that swim toward the bottom, spending their time close to the bed
where current speeds are lower.
Figure 4.4 reveals an intriguing pattern. There appears to be a bimodal distribu-
tion of dispersal distances, most being less than 1 km or more that 20 km. The
short-distance dispersers are spores or nonfeeding larvae of a variety of plants and
animals. Long-distance species, on the other hand, have propagules that feed while
dispersing and require longer periods of larval development before they are ready
to settle. The lack of propagules competent to travel between about 1 and 20 km has
signifi cant implications for reserve design. Shanks and his team suggest that reserves
should be at least 4-6 km in diameter to contain the short-distance dispersing prop-
agules, and should be spaced from 10 to 20 km apart to capture long-distance dis-
persers released from neighboring reserves.

Fig. 4.4 Dispersal
distance (km) plotted
against propagule
duration in the water
column (hours) for a
variety of marine plants
and animals (note the
log scales used). The
points labeled A-F (for
a coral, a seaweed, a
shrimp, a fi sh, an
abalone and a giant
kelp, respectively) fall
below the regression
line, dispersing less far
than would be
predicted from their
propagule duration;
these spend their time
close to the bed where
currents are slower.
(After Shanks et al.,
2003.)

10,000

1000

100

10

1

D

0.1

C

F

E

0.01

B

0.001

A

0.000 0.01

0.1 1 10 100 1000 10,000

Propagule duration (h)