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S. Dittmann and J.A. Vargas
these paradigms are applicable to the studied tropical tidal flats and also look
for site-specific variation. The following hypotheses are tested and discussed:
I. Species diversity is higher in tropical than in temperate tidal flats, while
abundances are lower. Soft sediments are usually home to a great variety
of invertebrate species. It has been postulated by Sanders (1968) that
diversity increases in the tropics, but Thorson (1957) did not record an
increase in infaunal species numbers towards the tropics. While tidal flats
were not included in the examples of Sanders and Thorson, several studies have reported very high species numbers from tropical tidal flats
(e.g. Macnae and Kalk 1962; Day 1974; Chap. 12). Furthermore, the entire
species stock has not always been fully assessed (Reise 1991; Dittmann
1995). Contrary to this latitudinal trend in species numbers, Reise (1991)
found lower abundances in tropical tidal flats than in temperate tidal
flats.
II. Intertidal benthic fauna shows a zoned distribution following an
environmental gradient between the tide marks. Sediment composition
and food availability are among known abiotic factors that determine
animal distributions in soft sediments around the world (Petersen 1991;
Snelgrove and Butman 1994). In most temperate tidal flats, the infauna is
distributed along environmental gradients and can be separated into
distinct faunal groups (see e.g. Beukema 1976; Whitlatch 1977; Dankers
and Beukema 1983). Dittmann (2000) reported a similar distribution of
benthic fauna in a tropical tidal flat which followed this general zonation
pattern.
III. The majority of macrobenthic organisms in tidal flats are depositfeeders. Deposit-feeders occur preferentially in sediments with higher
mud contents and dominate in many temperate tidal flats (Sanders et al.
1962; Whitlatch 1977). Abundances of suspension-feeders increase
towards the lower intertidal (Beukema 1976) and are further determined
by tidal range and exchange rates of tidal basins. Although little is known
on the biology of most tropical benthic organisms, a rough classification
was made on their feeding modes, based on observations and literature
records on related species from other parts of the world (e. g. Fauchald
and Jumars 1979). As many benthic organisms demonstrate a high versatility in their feeding behavior (see Chap. 4), a classification based on
trophic modes always remains tentative.
IV. Small-scale distributions are structured and regulated by interactions
between species. Reise (l985) provided a conceptual hierarchy of ecological processes affecting population sizes of tidal flat organisms. From
comparing temperate and tropical tidal flats it appeared that unrelated
species can have similar ecological functions at different latitudes (Reise
1991; Vargas 1996). Here, we set out to test whether this assumption is also
S. Dittmann and J.A. Vargas
these paradigms are applicable to the studied tropical tidal flats and also look
for site-specific variation. The following hypotheses are tested and discussed:
I. Species diversity is higher in tropical than in temperate tidal flats, while
abundances are lower. Soft sediments are usually home to a great variety
of invertebrate species. It has been postulated by Sanders (1968) that
diversity increases in the tropics, but Thorson (1957) did not record an
increase in infaunal species numbers towards the tropics. While tidal flats
were not included in the examples of Sanders and Thorson, several studies have reported very high species numbers from tropical tidal flats
(e.g. Macnae and Kalk 1962; Day 1974; Chap. 12). Furthermore, the entire
species stock has not always been fully assessed (Reise 1991; Dittmann
1995). Contrary to this latitudinal trend in species numbers, Reise (1991)
found lower abundances in tropical tidal flats than in temperate tidal
flats.
II. Intertidal benthic fauna shows a zoned distribution following an
environmental gradient between the tide marks. Sediment composition
and food availability are among known abiotic factors that determine
animal distributions in soft sediments around the world (Petersen 1991;
Snelgrove and Butman 1994). In most temperate tidal flats, the infauna is
distributed along environmental gradients and can be separated into
distinct faunal groups (see e.g. Beukema 1976; Whitlatch 1977; Dankers
and Beukema 1983). Dittmann (2000) reported a similar distribution of
benthic fauna in a tropical tidal flat which followed this general zonation
pattern.
III. The majority of macrobenthic organisms in tidal flats are depositfeeders. Deposit-feeders occur preferentially in sediments with higher
mud contents and dominate in many temperate tidal flats (Sanders et al.
1962; Whitlatch 1977). Abundances of suspension-feeders increase
towards the lower intertidal (Beukema 1976) and are further determined
by tidal range and exchange rates of tidal basins. Although little is known
on the biology of most tropical benthic organisms, a rough classification
was made on their feeding modes, based on observations and literature
records on related species from other parts of the world (e. g. Fauchald
and Jumars 1979). As many benthic organisms demonstrate a high versatility in their feeding behavior (see Chap. 4), a classification based on
trophic modes always remains tentative.
IV. Small-scale distributions are structured and regulated by interactions
between species. Reise (l985) provided a conceptual hierarchy of ecological processes affecting population sizes of tidal flat organisms. From
comparing temperate and tropical tidal flats it appeared that unrelated
species can have similar ecological functions at different latitudes (Reise
1991; Vargas 1996). Here, we set out to test whether this assumption is also