Contents - LAC Biosafety

128 Population dynamics: what makes an insect a pest?
interaction with other biological populations and their physical environments,
only one or a few of these interactions is likely to dominate the dynamics at any
particular time and place. Therefore we need not know all the details of the web
of interrelationships to understand and predict the dynamics of a particular
population. In other words, some of the feedback loops act as limiting factors.
This is a simplifying principle, although the limiting factors can change in
response to changing population density and environmental conditions.
Simulations show that this can lead to unpredictable population dynamics
including population explosion and collapse (Berryman, 1999).
Populations governed by these five principles, that is, geometrical growth,
cooperative interaction between individuals, competitive interaction between
individuals, circular causality between the population and its environment and
limiting factors, can display a wide array of dynamic behaviour patterns.
7.5 Types of forest insect outbreaks
What do we observe in real life situations? Chapter 10 will show that
pest incidence can take many different forms, from low density infestations to
very heavy outbreaks which may be regular or sporadic. Is there any consistent
pattern? Unfortunately we have only qualitative knowledge of pest incidence in
tropical forests. Most information on the dynamics of forest insect populations
has come from studies in temperate forests. Berryman (1988) compiled detailed
information on the dynamics of 27 well-known forest insect pests across the
world, mostly from Europe, North America and Australia, but including one,
the teak defoliator, from India. Largely based on data from temperate forest
insects, Berryman (1986, 1987, 1999) has made an attempt to develop a classification
system for forest insect outbreaks. A classification system tries to
organize the observed patterns of population fluctuations into groups or classes
according to their common characteristics. In turn, it helps us to organize the
observed phenomena and probe into the cause–effect relationship. The rationale
is that if we know how a pest outbreak originates, we are better able to prevent
or control it.
Theoretically, population fluctuations may be caused either by endogenous
factors (density-induced feedback loops) or by exogenous factors (weather, host
condition etc.). However, different causes can lead to the same type of population
growth behaviour (Berryman, 1999). Fig. 7.4 shows the commonly observed
types of insect population growth. Two basic types have been recognized,
gradient and eruptive.
Gradient population growth occurs when the environment is favourable for a
particular insect. In this case, changes in the favourableness of the exogenous