9.5 The Biotic Ingredients of the Wadden Sea
251
favourable (less physiological stress. more predictable environmental conditions,
less exposed to disturbances). Therefore. subtidal populations are likely to buffer
negative influences on intertidal populations (cf. Chesson & Huntley 1988; Gunther 1990). However. there are only a few studies of subtidal populations in the
Wadden Sea (but see the ELA W AT study of Carcinus maenas, Chap. 5.6).
Finally, even at smaller spatial scales of a few metres or so (Chap. 6), resilience
relies on "migration". In this case, in addition to mobile larvae, the mobility of
juveniles and adults comes into play.
Functional diversity - As a consequence of the high variability of the abiotic
boundary conditions in the Wadden Sea within a year and from year to year, the
quality and quantity of food supply for macrobenthic species change all the time.
For this reason, many macrobenthic species are generalist feeders, i.e. they can
utilize a more or less wide spectrum of food resources. This may even mean that
they can change their feeding mode. This generalist feeding behaviour is, like the
above-mentioned phenotypic plasticity. another mechanism at the level of the
individual which at the population level promotes resistance to environmental
t1uctuations.
Thus, the relatively low number of macrobenthic species, i.e. low diversity, may
be misleading if stability properties are to be assessed, because low resilience is
often inferred from low diversity. However, Costanza et al. (1993) point out that
the wide food spectra of macrobenthic species living in tidal f1ats or estuaries
mean that there is a high functional diversity. Instead of many different species,
which would allow for alternative paths in a food web, the different "feeding functions" of just a few species enable food webs with certain stability properties. Only
if there are alternative paths in a food web will the temporary loss of certain single
paths have no fatal consequences for the entire web (MacArthur 1955).
Extreme scenarios
An extreme scenario involving macrobenthic species would be a Wadden Sea with
a short circuit of nutrient cycles, i.e. the existence of just algal primary production
and bacterial remineralization. There would be no macrobenthos at all. The consequences of this scenario for all the species which depend on macrobenthic species
as the major food resource would, of course, be disastrous: birds and young fish
would have no more food.
However, this scenario is not very likely for two reasons. Firstly, two preconditions for the existence and the high abundance of the macrobenthos possess
marked stability properties themselves: the existence of an oxic sediment layer and
high primary production. Secondly, the macrobenthic species of the Wadden Sea
have so many very effective stability mechanisms that their total disappearance is
very unlikely under natural conditions. Anthropogenic influences have, however,
caused some changes in the species composition of the macrobenthos (Reise 1982,
1991, 1994).
251
favourable (less physiological stress. more predictable environmental conditions,
less exposed to disturbances). Therefore. subtidal populations are likely to buffer
negative influences on intertidal populations (cf. Chesson & Huntley 1988; Gunther 1990). However. there are only a few studies of subtidal populations in the
Wadden Sea (but see the ELA W AT study of Carcinus maenas, Chap. 5.6).
Finally, even at smaller spatial scales of a few metres or so (Chap. 6), resilience
relies on "migration". In this case, in addition to mobile larvae, the mobility of
juveniles and adults comes into play.
Functional diversity - As a consequence of the high variability of the abiotic
boundary conditions in the Wadden Sea within a year and from year to year, the
quality and quantity of food supply for macrobenthic species change all the time.
For this reason, many macrobenthic species are generalist feeders, i.e. they can
utilize a more or less wide spectrum of food resources. This may even mean that
they can change their feeding mode. This generalist feeding behaviour is, like the
above-mentioned phenotypic plasticity. another mechanism at the level of the
individual which at the population level promotes resistance to environmental
t1uctuations.
Thus, the relatively low number of macrobenthic species, i.e. low diversity, may
be misleading if stability properties are to be assessed, because low resilience is
often inferred from low diversity. However, Costanza et al. (1993) point out that
the wide food spectra of macrobenthic species living in tidal f1ats or estuaries
mean that there is a high functional diversity. Instead of many different species,
which would allow for alternative paths in a food web, the different "feeding functions" of just a few species enable food webs with certain stability properties. Only
if there are alternative paths in a food web will the temporary loss of certain single
paths have no fatal consequences for the entire web (MacArthur 1955).
Extreme scenarios
An extreme scenario involving macrobenthic species would be a Wadden Sea with
a short circuit of nutrient cycles, i.e. the existence of just algal primary production
and bacterial remineralization. There would be no macrobenthos at all. The consequences of this scenario for all the species which depend on macrobenthic species
as the major food resource would, of course, be disastrous: birds and young fish
would have no more food.
However, this scenario is not very likely for two reasons. Firstly, two preconditions for the existence and the high abundance of the macrobenthos possess
marked stability properties themselves: the existence of an oxic sediment layer and
high primary production. Secondly, the macrobenthic species of the Wadden Sea
have so many very effective stability mechanisms that their total disappearance is
very unlikely under natural conditions. Anthropogenic influences have, however,
caused some changes in the species composition of the macrobenthos (Reise 1982,
1991, 1994).