The most important factors affecting germination and growth of ruderal
vegetation are the timing of drawdown and the ability to keep the mud moist
enough for germination. For example, in the Playa Lakes Region of North
America, drawdowns in April are recommended to maximize seed production of
persicariasPolygonumspp. (Haukos and Smith 1993). Lowering of water levels in
spring can be timed so as to increase food availability to migrants (Section 14.9.4),
but may conflict with optimal management for breeding birds. Late summer is
considered the best time to re-flood to provide suitable conditions for wintering
waterfowl. Disking or ploughing the substrate in autumn prior to re-flooding can
be used to increase seed production the following growing season by increasing
the abundance of prolific seed-producing annual grasses at the expense of peren-
nial species (e.g. Gray et al. 1999). Optimal frequency of drawdowns varies
between sites, but is typically once every 5–7 years. If there is a throughput of
water, then carrying out drawdowns too frequently might flush out a high pro-
portion of nutrients made soluble during the drawdown (see below), thereby
reducing productivity.
Invertebrate biomass tends to be highest in early successional (i.e. recently
flooded) wetlands (e.g. Danell and Sjöberg 1982), and can therefore be increased
by periodically drying out and re-flooding. The initial high invertebrate biomass
is attributed to high overall productivity fueled by release of soluble nutrients
from freshly inundated soil and decomposition of flooded terrestrial vegetation,
and low levels of predation by predatory invertebrates and fish. As the wetland
matures, nutrients released from decaying terrestrial plant material decline,
numbers of predatory invertebrates and fish increase, and total invertebrate bio-
mass tends to decline. Drying out will also kill any fish present. Re-colonization
by fish following re-flooding is often accompanied by high levels of recruitment
of small fish of suitable size for fish-eating birds.
Little is known about the effects of management during a drawdown on inver-
tebrate biomass or bird use following re-flooding. Gray et al. (1999) found that
soil disturbance during drawdowns reduces the biomass of large invertebrate
prey for wildfowl the following winter, probably because it reduces the quantity
of above-ground detritus for them to feed on. In rice fields, different treatments
of rice stubble (ploughing, burning, chopping, rolling, disking, or cutting and
removing) have little or no effect on bird use following re-flooding, although rice
harvesting techniques that leave tall rice stems discourage some waterbird
species, particularly small waders (Day and Colwell 1998; Elphick and Oring
1998).
Timing of re-flooding following drawdowns can influence the biomass of
invertebrate prey for birds. Re-flooding playa wetlands in the southern USA in
354 |Habitat management