Experimental Approaches to Sediment Communities
339
experimental manipulative approach has never been coordinated on a global
scale and investigations have proceeded idiosyncratically. In particular, the
approach has suffered from a lack of agreed protocols for the design and
analysis of experiments - the equivalent of the IBP manuals. Although there
are now several excellent texts available (e.g. Hairston 1989; Underwood
1997), many of the early experiments were poorly designed, leading to
potential confusion in the literature (Raffaelli and Moller 2000). For instance,
many published field experiments were not in fact truly replicated (Hurlbert
1984) and many are inappropriately analysed (Underwood 1981), making any
formal meta-analysis (Raffaelli and Moller 2000) problematic.
To the authors' knowledge, the only serious attempts at a regional-scale
comparative experimental study involving manipulations of populations are
that of Bob Paine and his colleagues of the occurrence and significance of
keystone predators on rocky shores in different regions of the Pacific (Paine
1966; Paine et al. 1985), although recently there has been a Europe-wide programme on the recruitment dynamics of the barnacle Semibalanus balanoides (Jenkins et al. 1999). Shallow-water sediment ecologists have tended not
to adopt this comparative approach to field experiments, so that comparisons
along the reflective-dissipative gradient and between biogeographical regions
can only be made by drawing together a variety of different studies (see
reviews in Reise 1985; Raffaelli and Hawkins 1996). Several common processes emerge from manipulative experiments in shallow-water assemblages:
(1) keystone predators (species which mediate coexistence amongst a
superior competing species and other members of the assemblage) are not a
feature of most of the sediment shores studied; (2) in general, top-down
effects are weaker than bottom-up effects; (3) small epibenthic consumers,
such as crabs and shrimps, tend to have a larger impact than larger fish and
shorebirds, except perhaps at the local scale in the case of rays and skates, or
where shorebird densities are extremely high; (4) bioturbation, disturbance
and stabilisation can have important local-scale effects on the organisation of
infaunal communities, and in some cases these effects may scale up to the
landscape (see Chap. 7); and (5) biological interactions may have more significance for community organisation on dissipative than on reflective
shores. For key references to (1)-(5), the reader should consult Reise (1985),
Hall et al. (1994) and Raffaelli and Hawkins (1996).
The energy flow and the manipulative approaches have indicated that
similar processes might operate to organise communities in similar ways on
similar shores. Whilst both approaches deal with interactions between species, the natures of these interactions (flows of material and dynamic changes
in numbers of individuals) are quite different and can result in different perspectives as to which specific processes are important. This point is well illustrated by Paine's (1980) analysis of a simple rocky-shore food where he
showed that trophic links along which large amounts of energy flow are often
339
experimental manipulative approach has never been coordinated on a global
scale and investigations have proceeded idiosyncratically. In particular, the
approach has suffered from a lack of agreed protocols for the design and
analysis of experiments - the equivalent of the IBP manuals. Although there
are now several excellent texts available (e.g. Hairston 1989; Underwood
1997), many of the early experiments were poorly designed, leading to
potential confusion in the literature (Raffaelli and Moller 2000). For instance,
many published field experiments were not in fact truly replicated (Hurlbert
1984) and many are inappropriately analysed (Underwood 1981), making any
formal meta-analysis (Raffaelli and Moller 2000) problematic.
To the authors' knowledge, the only serious attempts at a regional-scale
comparative experimental study involving manipulations of populations are
that of Bob Paine and his colleagues of the occurrence and significance of
keystone predators on rocky shores in different regions of the Pacific (Paine
1966; Paine et al. 1985), although recently there has been a Europe-wide programme on the recruitment dynamics of the barnacle Semibalanus balanoides (Jenkins et al. 1999). Shallow-water sediment ecologists have tended not
to adopt this comparative approach to field experiments, so that comparisons
along the reflective-dissipative gradient and between biogeographical regions
can only be made by drawing together a variety of different studies (see
reviews in Reise 1985; Raffaelli and Hawkins 1996). Several common processes emerge from manipulative experiments in shallow-water assemblages:
(1) keystone predators (species which mediate coexistence amongst a
superior competing species and other members of the assemblage) are not a
feature of most of the sediment shores studied; (2) in general, top-down
effects are weaker than bottom-up effects; (3) small epibenthic consumers,
such as crabs and shrimps, tend to have a larger impact than larger fish and
shorebirds, except perhaps at the local scale in the case of rays and skates, or
where shorebird densities are extremely high; (4) bioturbation, disturbance
and stabilisation can have important local-scale effects on the organisation of
infaunal communities, and in some cases these effects may scale up to the
landscape (see Chap. 7); and (5) biological interactions may have more significance for community organisation on dissipative than on reflective
shores. For key references to (1)-(5), the reader should consult Reise (1985),
Hall et al. (1994) and Raffaelli and Hawkins (1996).
The energy flow and the manipulative approaches have indicated that
similar processes might operate to organise communities in similar ways on
similar shores. Whilst both approaches deal with interactions between species, the natures of these interactions (flows of material and dynamic changes
in numbers of individuals) are quite different and can result in different perspectives as to which specific processes are important. This point is well illustrated by Paine's (1980) analysis of a simple rocky-shore food where he
showed that trophic links along which large amounts of energy flow are often