9780521861724htl 1..2

adult body-size distributions, and within the size range of adult pelagic animals

(Fig.11.9). In these ‘enrichment communities’ of opportunistic species the

meiofauna/macrofauna traits dichotomy (Table11.2) breaks down; all the

species have direct benthic development and are non-selective deposit feeders

with approximately the same lifespan. This may also be the reason why the

bimodal body-size distribution also breaks down.

Discussion and conclusion

The mechanisms usually invoked to explain species size distributions, as with

most other attributes of multispecies assemblages, have generally involved local

ecological processes; species interactions (resource partitioning, predation, etc.)

or environmental constraints (habitat architecture, disturbance, etc.). This chap-

ter advocates the alternative, or additional, view that it is evolutionary processes

acting over geological time that shape the structure of the regional species pool,

1 10 100 1000

Species rank

0

20

40

60

80

100

Mesh size (μm)

63

125

250

500

1000

Cumulative % dominance

Figure 11.8k-dominance curves

averaged over all replicates for each

mesh size in the same samples as

Figure11.7 (after Warwicket al.,

2006).

Number of species

0

10

20

30

0

5

benthic

pelagic

Pollution indicators

0102030

X2 geometric weight class

Figure 11.9Species body-size

distribution of the metazoan

pelagos (solid histogram)

compared with the metazoan

benthos (upper curve) at a

station in the Celtic Sea. Also

shown (dotted histogram) is

the size distribution of the

‘pollution indicator’ species

listed in Table11.3 (after

Warwicket al.,1986).

220 R.M. WARWICK