Weed evolution and community structure - Ecological Management of Agricultural Weeds - page 451

because as the number of species increases,the proportion of species that have
yet to immigrate declines, and (ii) because the species that are best dispersed
by humans tend to arrive first. The extinction rate increases as a function of
species richness for two reasons. First, the control strategies in use in a field at
a given time will, on average, extinguish more species if more are present.
Second, species that for any reason tend to form sparse populations will likely
arrive later due to low propagule density in crop seed, manure, soil picked up
by machinery,etc.(see Chapter 2),and the characteristically small populations
of these late arrivers will make them more prone to local extinction.Although
Figure 10.2a was inspired by island biogeography theory (MacArthur &
Wilson, 1967), the rising extinction curve for islands is generated by a differ-
ent mechanism, namely the smaller average population size that occurs on
islands as more species partition the available resources.No evidence supports
an increased partitioning of resources in species-rich communities of agricul-
tural weeds, and the nature of these communities indicates that such parti-
tioning is unlikely (see preceding section). Hence, the classic mechanism
generating a rising extinction curve would not apply.
Fluctuations in the species richness of a field occur as changing practices
eliminate suites of species or facilitate the invasion of others (Figure 10.2b).
On average over long time periods,strict management should keep the extinc-
tion rate higher and therefore species richness lower than would be the case if
management of the farm were lax, but species richness should still increase as
the regional pool grows (Figure 10.2a). Whether the recent decrease in weed
species richness at the field level associated with introduction of chemical
weed management (Andreasen, Stryhn & Streibig, 1996) represents a tempo-
rary fluctuation or a shift from a lax to a strict management curve remains to
be determined.
Increased species richness is likely to confer greater flexibility on a weed
community. At any given time, most weed problems are usually the result of
invasion and extinction (solid arrows).Invasion rate (species yr 1 ) is determined
largely by the species richness ofthe regional species pool.Extinction rate
(species yr 1 ) is determined largely by management,and especially by the
effectiveness ofrotation between management strategies.In many locations,
invasion exceeds extinction and species richness is not in equilibrium (dashed
arrow).(b) Species richness in a region increases asymptotically through time
following initial settlement by agriculturalists due to immigration ofspecies
from other regions and recruitment ofpreadapted native species from non-
agricultural habitats.Worldwide evolution ofnew weed species increases the
asymptote.As the regional species pool increases,the species richness of
individual fields increases due to a shift from a lower to a higher local invasion
rate.