9780521861724htl 1..2

communities reside on the constant yield line and total biomass was not a

function of the size distribution of individuals. However, the slopes in analysis

two and three were significantly less than unity, indicating that communities

containing larger individuals contained greater total biomass.

Species richness tended to increase the intercept of the relationship between

average mass and total density in both analysis one and three (Figs.13.4and

13.6; Tables13.4and13.6). This indicates that more speciose communities

contain greater total biomass than less rich communities and has been observed

directly in these communities before (McGrady-Steed & Morin, 2000 ; Petchey

et al., 2002). This could result from a variety of mechanisms, including comple-

mentarity among species and more probabilistic processes (Loreauet al., 2001).

2 species

6 species

10 species

14 species

0

2

4

6

–8 –7 –6 –5 –4 –3 –2

Log 10 (Cell mass (mg))

Log

(Density per ml) 10

(a)

Low temperature

High temperature

0

2

4

6

–8 –7 –6 –5 –4 –3 –2

Log 10 (Cell mass (mg))

Log

10

(Density per ml)

(c)

Autotroph

Bacterivore

Omnivore

0

2

4

6

–8 –7 –6 –5 –4 –3 –2

Log 10 (Cell mass (mg))

Log

(Density per ml) 10

(b)

Figure 13.2Size-abundance relationship for

analysis two, which tests for influence of

(a) species richness, (b) species trophic position,

and (c) environmental temperature on the

slope of the relationship.

CONSEQUENCES OF BODY SIZE IN MODEL MICROBIAL ECOSYSTEMS 253