36 Ecology and Applied Environmental Science
• Predators that consume only a part of numerous preys without killing
them (mostly herbivores)
• Predators that kill only one prey during their life span (mainly some
insects that are parasites of other insects)
• Predators that consume a part of only one prey in their life span
(most parasites)
Trophic relations do not always coincide with the notion of predation as
a quantitative interaction between two populations. In nature, trophic relations between two populations can result in the decrease of one population
and the increase of the other, but sometimes they may also be beneficial for
both populations or have a neutral effect on one and positive on the other etc.
In nature, observed interactions present a great diversity due to the existing differences between species, and also due to numerous environmental factors. Generally, the predator-prey interaction does not result in the
elimination of one or both populations. A typical example is the permanent
oscillation of both populations with a phase difference (Figure 3.7). Prey
decrease may be followed by predator decrease (due to lack of food), which,
though, leads to an increase in prey numbers (due to decrease of predation),
which in turn may cause an increase in predator numbers (due to excessive
food), which brings about a decrease in prey numbers etc. As a rule, such a
system exhibits a notable elasticity, but if a great oscillation width is present,
one of the populations might shrink and come under the threat of extinction due to a random event. If this population represents the predator, then
the prey will have the opportunity to increase unobstructed, whereas if it
represents the prey, the predator population will be eliminated due to the
absence of food. However, in natural ecosystems, relations are multilateral,
not bilateral; thus the prey population might be reduced by other factors or
an alternative food source might be available to the predator.
0
t
N
Prey
Predator
Figure 3.7 Predator-prey interaction. (From Hadjibiros 2007, modified. Ecology.
Ecosystems and Environmental Protection, 3rd edition. Symmetria, Athens.
With permission.)
• Predators that consume only a part of numerous preys without killing
them (mostly herbivores)
• Predators that kill only one prey during their life span (mainly some
insects that are parasites of other insects)
• Predators that consume a part of only one prey in their life span
(most parasites)
Trophic relations do not always coincide with the notion of predation as
a quantitative interaction between two populations. In nature, trophic relations between two populations can result in the decrease of one population
and the increase of the other, but sometimes they may also be beneficial for
both populations or have a neutral effect on one and positive on the other etc.
In nature, observed interactions present a great diversity due to the existing differences between species, and also due to numerous environmental factors. Generally, the predator-prey interaction does not result in the
elimination of one or both populations. A typical example is the permanent
oscillation of both populations with a phase difference (Figure 3.7). Prey
decrease may be followed by predator decrease (due to lack of food), which,
though, leads to an increase in prey numbers (due to decrease of predation),
which in turn may cause an increase in predator numbers (due to excessive
food), which brings about a decrease in prey numbers etc. As a rule, such a
system exhibits a notable elasticity, but if a great oscillation width is present,
one of the populations might shrink and come under the threat of extinction due to a random event. If this population represents the predator, then
the prey will have the opportunity to increase unobstructed, whereas if it
represents the prey, the predator population will be eliminated due to the
absence of food. However, in natural ecosystems, relations are multilateral,
not bilateral; thus the prey population might be reduced by other factors or
an alternative food source might be available to the predator.
0
t
N
Prey
Predator
Figure 3.7 Predator-prey interaction. (From Hadjibiros 2007, modified. Ecology.
Ecosystems and Environmental Protection, 3rd edition. Symmetria, Athens.
With permission.)