20
Biotic Manipulations
of Aquatic Ecosystems
Daniel E. Schindler, Brian R. Herwig, and Stephen R. Carpenter
Introduction
Large-scale biotic manipulations have been performed in aquatic habitats to evaluate how ecosystem structure alters processes and dynamics. These
experiments have involved either direct manipulation of species or the habitat structure that mediates
species interactions. Manipulations at smaller experimental scales can not encompass the natural biology of wide-ranging species or the system-level
heterogeneity that has critical effects on the interactions between community and ecosystem dynamics (e.g., Schindler et al. 1997). Manipulations of
aquatic ecosystems are often used to explore the
responses of possible management actions or, in
fact, are large management experiments (e.g.,
Kitchell 1992; Hansson et al. 1998; Olson et al.
1998). In this chapter we review briefly some of the
techniques used in controlled experimental settings
and in natural resource management that have involved biotic manipulations of aquatic ecosystems.
We also discuss some considerations for design of
ecosystem experiments, and refer to important directions for future ecosystem research. We have
limited our review to experiments in freshwater
systems, and cite only selected examples of biotic
manipulations. For more detailed descriptions of
some of the methods we discuss, readers should
consult Nielsen and Johnson (1983), Downing and
Rigler (1984), and Cooke et al. (1993).
Manipulations of Species
Manipulations of species in ecosystems involve either supplementation or removal of existing populations, or introductions of novel species. Both
308
types of manipulations usually aim to restructure
food webs to determine the consequences for ecosystem processes. Although some manipulations
have involved introduction of exotic species, we
urge extreme caution in such experiments and encourage careful consideration of potential ecological damage that may result (Li and Moyle 1981).
Species Removals
Manipulations that remove a single species are difficult to perform. Most removals impact a broad
spectrum of ecologically similar species. Elimination of fishes is usually accomplished by physical
removal, application of chemical piscicides, or a
combination of both (Meronek et al. 1996). Methods for physical removal include use of seines, gill
nets, trawls, trap nets, electrofishing, angling, and
water drawdowns. In shallow systems, the efficiency of electrofishing and seining can be improved by using blocking nets to minimize gear
avoidance (Moss et al. 1996). These methods are
generally labor intensive and are only moderately
successful because of difficulties in removing all
individuals from a population. Physical removal is
most effective if spawning aggregations allow mass
removal of the target species. Physical removal
methods are preferred over chemical treatment if
the fishes are to be transferred to another system,
or temporary perturbation of the entire community,
including nontarget species, is not desired.
Physical removal experiments should include a
monitoring program that allows estimation of the
magnitude of the manipUlation. The easiest way to
accomplish this is by monitoring the change in a
Biotic Manipulations
of Aquatic Ecosystems
Daniel E. Schindler, Brian R. Herwig, and Stephen R. Carpenter
Introduction
Large-scale biotic manipulations have been performed in aquatic habitats to evaluate how ecosystem structure alters processes and dynamics. These
experiments have involved either direct manipulation of species or the habitat structure that mediates
species interactions. Manipulations at smaller experimental scales can not encompass the natural biology of wide-ranging species or the system-level
heterogeneity that has critical effects on the interactions between community and ecosystem dynamics (e.g., Schindler et al. 1997). Manipulations of
aquatic ecosystems are often used to explore the
responses of possible management actions or, in
fact, are large management experiments (e.g.,
Kitchell 1992; Hansson et al. 1998; Olson et al.
1998). In this chapter we review briefly some of the
techniques used in controlled experimental settings
and in natural resource management that have involved biotic manipulations of aquatic ecosystems.
We also discuss some considerations for design of
ecosystem experiments, and refer to important directions for future ecosystem research. We have
limited our review to experiments in freshwater
systems, and cite only selected examples of biotic
manipulations. For more detailed descriptions of
some of the methods we discuss, readers should
consult Nielsen and Johnson (1983), Downing and
Rigler (1984), and Cooke et al. (1993).
Manipulations of Species
Manipulations of species in ecosystems involve either supplementation or removal of existing populations, or introductions of novel species. Both
308
types of manipulations usually aim to restructure
food webs to determine the consequences for ecosystem processes. Although some manipulations
have involved introduction of exotic species, we
urge extreme caution in such experiments and encourage careful consideration of potential ecological damage that may result (Li and Moyle 1981).
Species Removals
Manipulations that remove a single species are difficult to perform. Most removals impact a broad
spectrum of ecologically similar species. Elimination of fishes is usually accomplished by physical
removal, application of chemical piscicides, or a
combination of both (Meronek et al. 1996). Methods for physical removal include use of seines, gill
nets, trawls, trap nets, electrofishing, angling, and
water drawdowns. In shallow systems, the efficiency of electrofishing and seining can be improved by using blocking nets to minimize gear
avoidance (Moss et al. 1996). These methods are
generally labor intensive and are only moderately
successful because of difficulties in removing all
individuals from a population. Physical removal is
most effective if spawning aggregations allow mass
removal of the target species. Physical removal
methods are preferred over chemical treatment if
the fishes are to be transferred to another system,
or temporary perturbation of the entire community,
including nontarget species, is not desired.
Physical removal experiments should include a
monitoring program that allows estimation of the
magnitude of the manipUlation. The easiest way to
accomplish this is by monitoring the change in a