POPULATIONS AND SUSTAINABILITY
(a) The factors that determine size of a population.
● To include the significance of limiting factors in determining the carrying capacity of a
given environment and the impact of these factors on final population size.
Populations grow in three stages. The rate of reproduction is low, and the growth in population
size is slow. The lag phase is the slow growth where individuals are acclimatising to their
habitat, establishing communities, and finding mates. Thereafter, in the log phase the population
grows exponentially. Resources are plentiful, reproduction can happen quickly, with the rate of
reproduction exceeding mortality. When density-dependent limiting factors become prevalent,
the growth enters a stationary phase. In this phase, the rates of reproduction and mortality
fluctuate and the growth fluctuates around the carrying capacity (K).
Limiting factors can be density-independent, abiotic factors, such as low temperatures, which
affect growth irrespective of its size. Alternatively, they may be density-dependent, where the
factor influences population more strongly as population size increases. Such factors include
availability of resources like food, water, light, oxygen, nesting sites, or shelter may decrease.
Similarly, levels of parasitism or predation may increase, as does the intensity of competition.
K-selected species are those whose population size is determined by the carrying capacity. For
these populations, limiting factors exert a more significant effect as population size gets closer
to the carrying capacity. They tend to be larger mammals, with lower reproductive rates, slow
development, late reproductive age, longer lifespan, and large body mass.
R-selected species are those whose population size increases so quickly that it exceeds the
carrying capacity before limiting factors can affect it, and the population tends to increase and
collapse. The most important influence on growth is the physical rate at which a species can
reproduce. Examples include mice, insects, and some plants, which have a high reproductive
rate, quick development, young reproductive age, short life span, and a small body mass.
(b) Interactions between populations.
● To include predator–prey relationships considering the effects on both predator and prey
populations
● interspecific and intraspecific competition.
The size of predator and prey populations are interlinked and tend to fluctuate where two
specific species are closely linked, with the number of predators lagging behind changes in the
prey, and then impacting the number of prey species.
An increase in the prey population provides more food for the predators, allowing more to
survive and reproduce. This in turn results in an increase in the predator population. (1) The
increased predator population eats more prey organisms, causing a decline in the prey
population. The death rate of the prey population is greater than its birth rate. (2) The reduced
prey population can no longer support the large predator population. Intraspecific competition
for food increases, resulting in a decrease in the size of the predator population.
However, this theoretical link rarely occurs so strongly in the wild because more predators have
a number of prey mitigating the impact of fluctuations in prey numbers, and thus also back on
the prey population. Fluctuations may also result from seasonal changes in abiotic factors.
(a) The factors that determine size of a population.
● To include the significance of limiting factors in determining the carrying capacity of a
given environment and the impact of these factors on final population size.
Populations grow in three stages. The rate of reproduction is low, and the growth in population
size is slow. The lag phase is the slow growth where individuals are acclimatising to their
habitat, establishing communities, and finding mates. Thereafter, in the log phase the population
grows exponentially. Resources are plentiful, reproduction can happen quickly, with the rate of
reproduction exceeding mortality. When density-dependent limiting factors become prevalent,
the growth enters a stationary phase. In this phase, the rates of reproduction and mortality
fluctuate and the growth fluctuates around the carrying capacity (K).
Limiting factors can be density-independent, abiotic factors, such as low temperatures, which
affect growth irrespective of its size. Alternatively, they may be density-dependent, where the
factor influences population more strongly as population size increases. Such factors include
availability of resources like food, water, light, oxygen, nesting sites, or shelter may decrease.
Similarly, levels of parasitism or predation may increase, as does the intensity of competition.
K-selected species are those whose population size is determined by the carrying capacity. For
these populations, limiting factors exert a more significant effect as population size gets closer
to the carrying capacity. They tend to be larger mammals, with lower reproductive rates, slow
development, late reproductive age, longer lifespan, and large body mass.
R-selected species are those whose population size increases so quickly that it exceeds the
carrying capacity before limiting factors can affect it, and the population tends to increase and
collapse. The most important influence on growth is the physical rate at which a species can
reproduce. Examples include mice, insects, and some plants, which have a high reproductive
rate, quick development, young reproductive age, short life span, and a small body mass.
(b) Interactions between populations.
● To include predator–prey relationships considering the effects on both predator and prey
populations
● interspecific and intraspecific competition.
The size of predator and prey populations are interlinked and tend to fluctuate where two
specific species are closely linked, with the number of predators lagging behind changes in the
prey, and then impacting the number of prey species.
An increase in the prey population provides more food for the predators, allowing more to
survive and reproduce. This in turn results in an increase in the predator population. (1) The
increased predator population eats more prey organisms, causing a decline in the prey
population. The death rate of the prey population is greater than its birth rate. (2) The reduced
prey population can no longer support the large predator population. Intraspecific competition
for food increases, resulting in a decrease in the size of the predator population.
However, this theoretical link rarely occurs so strongly in the wild because more predators have
a number of prey mitigating the impact of fluctuations in prey numbers, and thus also back on
the prey population. Fluctuations may also result from seasonal changes in abiotic factors.
