122
J.H. Lawton
Fig. 2a-c. The general pat. • MAx
tern illustrated in Fig. 1b can
•
be generated by: a the sam•
pIing hypothesis; or b niche
complementarity or positive
species interactions. In a, the
dots are hypothetical data
a
(from left to right): individual
species in monoculture, various two-species combinations etc. up to a single
polyculture containing all
I:I.l
species. Over yielding is imI:I.l
r;ri:I
possible - the maximum
U
MAx productivity of mixtures
0
(MAX) never exceeds that of
c::z::
the best monoculture. In b
~
overyielding
over yielding is possible. c In
~
fluctuating environments
r;ri:I
with extreme perturbations,
Eb
the insurance hypothesis preI:I.l
>qui red to maintain ecosystem
I:I.l
0
processes at a given level, the
U
harsher, and/or the larger the
r;ri:I
1
number (1-3) of kinds of per2
turbations
3
SPECIES RICHNESS
Similar patterns could clearly also be generated by loss of functional groups of
plant species, that is sets of species with similar growth-forms and ecologies. Functional groups can be broadly defined (e.g. legumes, grasses, and herbs), or more
finely divided (e.g. early and late annuals, perennial grasses) (Hooper and Vitousek
1998).
2.1.2 More Formal Theory
Providing the species pool is reasonably large (thereby avoiding strong stochastic
effects), the most likely theoretical relationship between loss of species and ecosys-