9780521861724htl 1..2

community (ANCOVA:F1,28¼2.49,P¼0.126). A higher value ofzfor the SAR of

the LL is attributed to a rapid increase in species richness with increasing area,

with a ten-fold increase in species number over a spatial scale encompassing an

order of magnitude (Fig.8.8). In contrast, over the same spatial scale species

richness increased four-fold in the SB but only two-fold in the MY. Moreover, a

wider range of the scale-dependent coefficientcfor the SB and LL invertebrate

communities implies higher sample-to-sample variations in species richness at

small spatial scales compared to the MY.

Empirical observations suggested that the parameterzof the SAR is about 1/4

for many ecosystems (Rosenzweig, 1995 ). Thez-values of the SAR differed from a

1/4 assumption for the LL (t¼13.45;df¼13,P<0.001) and MY (t¼9.46,df¼13,

P<0.001) but were close to 1/4 for the SB community (t¼0.49,df¼15,

P¼0.628). The results indicate thatz-values, and therefore SARs, do not display

a common scale-invariant behaviour across stream systems.

Table 8.4Regression parameters of the power-law relationship between the mean number of

species and sampled areas of sizel¼k^2 in the streams Seebach (SB), Llwch (LL) and

Mynach (MY).cgives the intercept, z the slope,r^2 is the variance explained by the correlation

of area with mean number of species per area,Fis the value of the variance ratio test,dfare

the degrees of freedom,P-values are given for theF-test. Bootstrap confidence limits (95%)

are given in parentheses forcandzvalues.

Stream c(95% CL) z(95% CL) r^2 FdfP

SB 134.50 0.233 0.903 139.46 1,15 <0.001

(107.90, 205.12) (0.140, 0.282)

LL 34.36 0.448 0.968 395.06 1,13 <0.001

(28.64, 56.89) (0.338, 0.498)

MY 92.05 0.200 0.992 1694.93 1,13 <0.001

(87.30, 96.16) (0.190, 0.214)

Area, λ (m–2)

Mean number of species, < 1 10 100 1000

S^ > (

λ)

1

10

100

1000

SB

LL

MY

Figure 8.8Species-area relationship (SAR)

of all benthic invertebrate species for

three geographically separate stream

ecosystem, the SB, LL and MY. The plot

shows the linear regression of log mean

number of species on log area, inl ¼k^2

intervals (see details in text). The

regression equations are given in

Table8.4.

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