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Concepts of Biocontrol
In this section, general concepts of population biology and ecology are presented
in an effort to make clear how the use of Biocontrol agents is not only feasible
but practical.
Overview of Biocontrol
Biological control, in the classical sense, is the purposeful introduction by man
of parasites, predators, and/or pathogenic microorganisms to reduce or suppress
populations of plant or animal pests.
This concept of classical biological control is not new, having been practiced in
many forms around the world since the earliest days of recorded history. Biological
control has been and is currently used as a viable management strategy for insect
pests, unwanted plants, and the control of nuisance reptiles and mammals.
The most successful cases of using biological control occur when a problem species
has been introduced into a new region of the world without a complex or assemblage
of organisms that feed directly upon it, attack its seeds or progeny (i.e., predators
or parasites), or cause severe or debilitating diseases (i.e., pathogenic microorganisms).
Such is the case with the major problem aquatic plants; they were introduced without
their associated complex of predators or herbivores, parasites, and disease-causing
organisms.
The reason biological control is practical is that pest populations are maintained
at lower densities mainly by the combined action of predators, herbivores, parasites,
disease-causing organisms and a variety of abiotic factors such as climate, localized
environmental conditions, etc., with only minimal intervention by man.
To clearly comprehend the potential that the interactions between various species
have on the regulation of population size, and hence biological control, it is important
to understand some basic concepts of population biology and ecology, including population
growth, population regulatory factors, and the potential competition between and
within different species.
Such information is important for a clear and concise understanding of Biocontrol
as a viable alternative to more traditional aquatic plant management strategies.
Population Increase in Absence of Regulatory Factors
Living organisms have a great potential for large increases in population numbers.
As operational personnel you have witnessed countless times the invasive potential
of plants such as waterhyacinth, waterlettuce, hydrilla, alligatorweed, and Brazilian
elodea. Water that contained only a handful of potentially nuisance aquatic plants
one day can seemingly be covered overnight. A familiar scenario is the accidental
introduction of waterhyacinth in a small pond in spring, leading to rapid surface
coverage of the pond by summer's end.
Houseflies offer an excellent example of the potential living organisms have for
increases in population size. If we assume that house flies have an average generation
time from egg to adult of about 10 days and each female can lay about 120 eggs,
the resulting offspring in just three months from this single female would number
>325 trillion individuals. If we were to line up all of the resulting individuals
lengthwise, the resulting line of flies (assuming an average housefly length of
7 mm) would encircle the equator >57,000 times.
This type of unchecked population increase is known as exponential growth and is
represented by this type of graph or curve.
Note how population growth is slow initially followed by sharp increases until population
growth rises almost vertically.
But houseflies and other organisms do not reach such tremendously high and damaging
levels. If that was possible there would be drastic impact to the world as we know
it. For example, we would all go to work in the morning walking through 10-foot-deep
layers of houseflies and other organisms. There are many important factors that
work together to maintain or regulate populations at more manageable and realistic
sizes.
Abiotic and Biotic Factors
Biologists recognize two broad categories of factors that regulate population size.
These categories are known as abiotic and biotic.
Abiotic factors include such items as weather, climate, shelter availability, and
geographic barriers.
Biotic factors include developmental times, mortality of the Biocontrol agents according
to age, and the mortality of the target population. Also, biotic regulatory factors
include the interactions between members of the same species (intra-specific competition)
as well as interactions with different species (inter-specific competition). Inter-specific
competition can include the regulatory effects of herbivores or predators; interactions
with other animal or plant species that utilize the same or similar resources such
as food, shelter, light; or the effects of parasites and other disease-causing organisms.
Both abiotic and biotic factors can have profound effects on population size.
Exponential vs. Logistic Population Growth
Graphical unchecked or non-regulated growth is commonly represented by the exponential
growth curve as illustrated in a previous help topic (see Population Increase in
Absence of Regulatory Factors).
This is in contrast to the logistic curve where population size is regulated by
a complex of abiotic and biotic factors.
The logistic curve, sometimes referred to as an 'S-shaped' curve, initially follows
a similar pattern as the exponential growth curve; i.e., population growth is slow
initially, then enters into a point where growth is rising rapidly.
Eventually the population stops increasing and reaches its maximum level or "carrying
capacity". The maximum population size that can be reached is based on the availability
of light, in the case of plants, or food, shelter, etc. Most populations never approach
the "carrying capacity" but instead remain at lower levels because of the regulating
effects of both abiotic and biotic factors.
Note that populations do not typically remain at a steady state continually but
instead tend to fluctuate or oscillate around some characteristic density.
Factors Governing Populations at Max and Min
Each of the major factors that regulate populations act differently with regards
to how it exerts control over a population.
For example, biotic factors interacting within a population (i.e., intra-specific
competition) work together to maintain populations below the "carrying capacity".
When populations become too large, the individuals of the same species begin to
compete for the same resources such as food, shelter, egg-laying sites, etc. This
interaction between members of the same species tends to be the most important factor
maintaining population levels below the "carrying capacity".
Conversely, harsh abiotic factors such as hurricanes, unusual freezing conditions,
high temperatures, shifts in climate, and change in sheltering conditions can all
act to reduce populations to very low or minimal levels, levels where the population
may be eradicated from a particular area.
Factors Governing Populations Below Carrying Capacity
Biological control agents (parasites, pathogens, and predators) as well as competition
between species with similar environmental requirements (i.e., interspecific competition)
act together to regulate populations below the carrying capacity.
While other factors, most notably abiotic factors, may influence these fluctuations,
biological factors seem to be the most important.
The importance of biotic factors is their influence on the fluctuations of population
size above or below the characteristic population size.
A good example is the introduction of an insect Biocontrol agent on an exotic plant
in the absence of a complex of different species which act in concert to keep the
plant population in equilibrium. Before the introduction, the pest population levels
typically fluctuate relatively widely around the characteristic density. After the
insects are introduced, the population fluctuations tend to become smaller with
subsequent decreases in the oscillations to a point where the net effect is the
reduction of the characteristic density.
This is exactly what is observed with many introduced biological control agents.
For example, the following graph represents the population of waterhyacinth in Louisiana
over the last ca. 20 years. Samples were taken yearly, once in the early spring
and again in the fall, in an effort to quantify the yearly growth of the plants.
Note the large fluctuations in plant growth in the years immediately following the
releases of insect Biocontrol agents. However, as the population levels of the insect
Biocontrol agents increase, as evidenced by the swarming or wholesale dispersal
of the insect Biocontrol agents, the fluctuations have become drastically reduced,
with a net effect of reducing the overall characteristic density of waterhyacinth
in Louisiana.
Biological control agents in conjunction with pathogens, competition with other
plant and animal species, abiotic factors, as well as parasites all work to gradually
reduce population levels within the region of some theoretical maximum and minimum.
The biotic factors, in essence, tend to change the population equilibrium to some
new or lower level.
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