Biodiversity of Freshwaters - Freshwater Ecology: Concepts and Environmental Applications - page 192

A consistent ecological pattern relating spatial scale and diversity is the
positive relationship between the size of an island of habitat and the num-
ber of species that are found in that habitat (Preston, 1962). If S is the
number of species, then

cA z

S

or, in a form easier for calculations,

log S log c z (log A )

where c is a constant measuring the number of species per one unit area,
and z is the slope of the line on a plot of the log of the area versus the log
number of species. The relationship between size of habitat and area holds
well for many types of organisms. This includes diatoms colonizing glass
slides (Fig. 10.7), invertebrates on stones in streams (Fig. 10.8A), and fish
in lakes (Fig. 10.8B). This relationship can be used to estimate the effects
of habitat reduction on species diversity (Example 10.2).
Spatial considerations also determine diversity of a given habitat. The
distance from a source of colonists paired with the area of the habitat can
be used to estimate a relative expected diversity (MacArthur and Wilson,
1967). Although the specifics of these models are somewhat complex to
treat here, they include two important points related to biodiversity and
success of invaders. First, lowering the connectivity of habitats (i.e., sources
of natural colonists) can disrupt the natural diversity. Second, human trans-
port of organisms can vastly reduce the effective isolation of some habitats.
Ramifications of such transport of organisms will be discussed later.
Succession, the sequence of species inhabiting newly formed habitat, is
a dominant and long-standing ecological principle. The central idea related

Summer

50

Autumn

40

30

20

10

0

9 mm 2 36 mm 2 625 mm 2 144 mm 2 625 mm 2

Area of glass slide

FIGURE 10.7 Number of diatom species on glass plates of various surface areas after 1
week in Ridley Creek (data from Patrick, 1967).