Environmental Management: Principles and practice

Holdridge Life Zone Model
The Holdridge Life Zone Model is a widely used eco-climatic classification system,
based on the relationship of current vegetation biomes to three general climatic
parameters: annual temperature, annual precipitation and estimated potential
evapotranspiration. It is an approach often used in land use classification. The model
predicts eco-climatic areas but does not directly model actual vegetation or land
cover distribution (Holdridge, 1964).
Ecological stability
Ecosystems adjust to perturbation through regulatory mechanisms. When the
relationship between input and output to the system is inverse (e.g. increased sunlight
causes more cloud, which reduces the impact of that sunlight on the surface), it is
termed a negative feedback. The opposite is a positive feedback, whereby an effect
is magnified (e.g. global warming might release methane hydrates trapped in the
ocean causing increased warming). There is a risk that a positive feedback could
result in an uncontrollable runaway reaction affecting a critical biogeochemical or
biogeophysical cycle, so one of the tasks of environmental management should be to
warn of such threats.
The environmental manager needs to know whether the environment and
ecological processes are stable (Smith, 1996). It is widely held that, given long enough,
a steady state will be reached by an ecosystem because a web of relationships will
allow it to adjust to serious localized or moderate widespread disturbances. Such an
ecosystem should remain in steady state unless a critical parameter alters sufficiently.
If change then occurs, it is termed ‘ecological succession’ or ‘biotic development’.
Over a very long period of time organisms may evolve to an evolutionary maturity;
over a shorter period a successional maturity may be reached before such evolution
can occur (Johnson and Steere, 1974:8)
The concept of ecological succession, pioneered by Clements (1916), is
complex and still debated. According to the concept, organisms occupying an
environment may modify it, sometimes assisting others—a birch wood may act as a
nursery for a pine forest, which ultimately replaces the birch—thus birch is a
successional stage en route to a pine stage. These transitional stages leading to a
mature climax community are known as seres. Each vegetational stage or sere will
have a characteristic assemblage of macrofauna and micro-organisms. Two types of
succession are recognized: (1) primary succession and (2) secondary succession.
The former is the sequential development of biotic communities from a bare lifeless

---

SCIENCE
141
area (the site of a fire, volcanic ash, newly deglaciated land, etc). The latter is the
sequential development of biotic communities from an area where the environment
has been altered but has not had all life destroyed (cut forest, abandoned farmland,
land that has suffered a flood or been lightly burnt, etc). Where succession is taking
place from a bare area, the first stage is known as the pioneer stage, although in
practice the expression may be applied to growth taking place in areas that do have
some life—such as regrowth after logging (natural forests may be assumed to maintain
maturity, rather than becoming senile and degenerating, through ‘patch and gap’
dynamics—clearings caused by storms, etc., that allow regeneration). Pioneer
communities have a high proportion of plants and animals that are hardy, have catholic
niche demands, and disperse well (weeds with wind-carried seeds, insects which can
fly, etc). Mature, climax communities are supposed to have more species diversity,
recycle dead matter better, and be more stable.
Many communities do not reach maturity before being disturbed by natural
forces or humans. It is often argued that an ecosystem with greater species diversity
is more stable than one with less. In practice many variables are involved in
determining ecosystem stability, and in a given situation the path of succession can
be unpredictable (Figure 7.4).
Until quite recently, the world population was non-urban; now, after rapid
urbanization since the 1800s, over 50 per cent of people live in cities, and the
FIGURE 7.4 Abrupt boundary between cleared lowland tropical rainforest and young oil-
palm plantation, Peninsular Malaysia. A contrast between rich diversity of plant species in
the forest, and the oil-palm/ground-cover species (planted to try to reduce erosion and weed
growth) of the plantation

---

CHAPTER SEVEN
142
percentage is increasing, Many of the largest, fastest growing cities are in poor
countries and pose severe environmental problems. Even in developed countries urban
growth is a challenge for environmental management. In recent years there has been
a shift in interest from just coping with city problems to seeking strategies for
‘sustainable cities’—however, there is a long way to go before there are practical
solutions in most, if not all, countries. Engineering and institutional developments
alone will not provide solutions for urban transport, water supply, sanitation, control
of crime, improving social cohesion, etc. For effective environmental management
there must be better understanding of urban and peri-urban environments, societies
and economies and how they interact with rural surroundings.