Life-history theory and management - Ecological Applications: Toward a Sustainable World - page 61

Several schemes have been developed that link particular groups of traits to particular kinds of
environments (see Chapter 4 in Begon et al., 2006, for a detailed treatment). Two of these schemes
will fi gure here - the r / K and CSR concepts.

The r / K concept
The potential of a species to multiply rapidly - producing large numbers of progeny early in life - is
advantageous in environments that are short-lived (created, for example, by a disturbance such as
the falling of a forest tree, or the storm-battering of a rocky reef), allowing the organisms to quickly
colonize and exploit the new habitat. Such species have been called r -species because they spend
most of their lives in a near-exponential phase of population growth where their intrinsic rate of
increase ( r ) is being fully expressed. The habitats where they are favored have been called
r -selecting.
At the other end of the scale are organisms with life histories that enable them to survive where
there is often intense competition for limited resources. In this case the individuals leaving most
descendants are those that capture a larger share of resources. They are called K -species because
they spend most of their lives bumping up against the environment's carrying capacity ( K ). Habitats
where they are favored have been called K -selecting.
The r / K concept (MacArthur & Wilson, 1967; Pianka, 1970) can be useful in the interpretation
of contrasting patterns in nature. For example, many forest trees are excellent examples of K -
species. They compete for light in the canopy and the survivors are those that put their resources
into growth so they can overtop their neighbors. They usually delay reproduction until their
branches have secured a place in the forest canopy, and hold on to their position and live for a very
long time. Overall, they make a relatively low allocation to reproduction but many produce large,
well-provisioned seeds. By contrast, in the more disturbed circumstances of r -selecting habitats,
plants tend to conform to a contrasting group of r characteristics: a greater reproductive allocation,
but smaller seeds, smaller size, earlier reproduction and a shorter life.

The CSR concept
Grime (1974, Grime et al., 1988) produced a different, but not unrelated, classifi cation of habitats
and plant life histories. Habitats are seen as varying in two signifi cant ways - in their level of dis-
turbance (brought about by grazing, disease, trampling or adverse weather) and in the extent to
which they experience 'stress' (shortages of light, water or nutrients that limit photosynthesis).
Grime argued that a stress-tolerant strategy (S) is appropriate when 'stress' is severe but
disturbance is uncommon. Conversely, a so-called ruderal strategy (R) is appropriate when dis-
turbance levels are high but conditions are benign and resources abundant (R-species are essen-
tially good colonizers). Finally, a competitive strategy (C) is appropriate when disturbance is rare,
resources are abundant and crowded populations develop. Grime then classifi ed a large number of
plants on the basis of ecological characteristics he thought suited them for one or other of these
strategies (C, S or R), or some intermediate combination (CR, CS, SR or even CSR). Grime suggests
that strong competitors have a high relative growth rate, the ability to spread by vegetative means
and a tall stature. Stress tolerators, on the other hand, are small in stature with a low relative
growth rate. Finally, ruderal species are generally annuals or short-lived herbaceous perennials
with a capacity for rapid seedling establishment and growth, and a tendency for a high proportion
of photosynthate to be directed into seeds. Intermediate species possess combinations of these
traits.

3.2 Species traits as
predictors for
effective restoration

Native biodiversity is often compromised as a result of human activity, whether by
mining, agriculture, forestry or urban development. The desire to restore these
places to something approaching their pristine state may lead, through a political
process, to legislation or economic incentives that foster recovery. In other cases,
economic circumstances change and previous activities are no longer sustainable -
mines close, agricultural land is put out of production, forestry is no longer viable.
Whatever the background, effective restoration needs to be based on knowledge of