9780521861724htl 1..2

When changing spatial scales (Fig.8.1), the range ofN-Mslopes obtained was

wider over smaller sampling areas (<20 m^2 ) reflecting the small-scale patchi-

ness of individuals and species in the stream communities. The slope values

decreased exponentially with increasing area and an asymptote is reached

at sampling areas>200 m^2 in the three streams (Fig.8.1;Table8.1). The

fitted exponential decay function predicts well the scaling exponents of the

N-M bOLS(Table8.1;Fig.8.1). Thus, theN-Mscaling appears to be roughly

constant across communities of different stream orders. However, a suffi-

ciently large area has to be sampled in order to detect an invariant pattern of

scaling exponents.

Density and biomass scaling

The data based on all 867 samples from the three stream communities illustrate

that density rapidly reaches an asymptote with increasing species richness,

following a power function (Fig.8.2). In contrast, total biomass or standing

stock (g DW m^2 ) remains roughly constant across sites of different number of

species (Fig.8.2). However, higher variability of biomass occurring in samples

with a low number of species in the first-order LL (Fig.8.1) may reflect early

050

–1.0

–0.8

–0.6

–0.4

–0.2

0.0

0.2

0.4

–1.0

bOLS = –0.702 ± 0.071

–2–1

01

23

45

67

log Density (ind m

–2)

–5 –4 –3 –2 –1 0 1 2 3 4 5log Body-mass (μg DM)

–2–1

01

23

45

6

log Density (ind m

–2)

–5 –4 –3 –2 –1 0 1 2 3 4 5log Body-mass (μg DM)

–0.8

–0.6

SB

bOLS = –0.496 ± 0.028

–2–1

01

23

45

6

log Density (ind m

–2)

–5 –4 –3 –2 –1 0 1 2 3 4 5log Body-mass (μg DM)

bOLS = –0.733 ± 0.088

LL

MY

–0.4

–0.2

0.0

0.2

0.4

–0.8

–0.6

–0.4

–0.2

0.0

0.2

0.4

100 150

Area, A (m^2 )

Slopes of log density – log body-mass relationships

for different areas,

bA – OLS

200 250 300 350

Figure 8.1Ordinary least-square slopes

(bAOLS) of the linear regressions

between log density and log body-mass

for increasing sampling areas of the

benthic invertebrate communities of

three streams (Seebach (SB), Llwch (LL)

and Mynach (MY)). Curves are

exponential decay functions with details

given in Table8.1. The insets show the

linear regressions of log mean

population density on log mean body-

mass obtained for the total sampled area

of the three streams.bOLSvalues are the

slopes for these ordinary least-square

model I regressions with1 SE. Insets

for the SB and MY are modified after

Schmidet al., 2000.

146 P.E. SCHMID AND J. M. SCHMID-ARAYA