Chapter -39

Role of predators and parasites on controlling population

Role of predators in controlling population

• A prey population increases, the predators encounter them.

• Many predators eat a variety of prey. For example coyotes might eat more field mice when the mouse population is high but switch to eating more ground squirrels as the mouse population declines, thus allowing the mouse population to recover.

Predator tries to control the population by changing it’s prey

Predation

1. Mice munching acorns.2. Wolves working together to kill an elk3. . Venus fly trap

Predators exist density-dependent effects on their prey

• Predators such as arctic fox and snowy owl, which feed heavily on lemmings, regulate the number of offspring they produce according to the number of lemmings.

• The snowy owl might produce upto 13 chicks when lemmings are abundant, but not produce at all in years when lemmings are scarce.

• In some cases, increase in predators might cause a crash of the prey population. This results in population cycles of both predators and prey.

Role of predators in maintaining population

• Some predators feed on prey made vulnerable because their populations have exceeded the carrying capacity of their environment. Such prey may be weakened by lack of food or may be exposed because they cannot find appropriate shelter.

• Example prickly pear cactus was introduced into Australia from Latin America.

Contd..

• Example prickly pear cactus was introduced into Australia from Latin America. As there was no natural predators, its population exponentially and it spread uncontrollably, destroying acres of valuable pasture and rangeland.

• In the 1920s, a cactus moth a predator of the prickly pear was imported from Argentina and released to feed on the cacti. Within a few years, the cacti were virtually eliminated.

Cactus and moth

Parasites

• Bacteria• Fungi• Intestinal worms• Ticks protists like malarial parasites• Gypsy moth feeding on plants• Parasitism is density dependent• Parasites spread more readily among hosts when their host

population density is high.• Any example/thoughts?• Parasites influence population size by weakening their hosts

and making them more susceptible to death from other causes

Parasites, predators and prey coevolve

• Parasites and predators tend to destroy the least fit of the prey, leaving the better-adapted prey to reproduce.

• This results in balance of prey population• The balance in ecosystem can be destroyed when

predators or parasites are introduced into regions in which they did not evolve, and in which local prey species have had no opportunity to evolve defenses against them through natural selection.

Competition

• Interspecific competition• Intraspecific competition• Scramble competition• Contest ompetition• Read page numbers 806 and 807 and share

your understanding with your partner• The role of competition in controlling

population

Survivorship curve

• http://www.youtube.com/watch?v=xkAnO8VjCz0

Survivorship curve

How is the Human Population changing?

• Read page numbers 808 and 812 and share your understanding within your group, and then with the class.

• Given that the U.S birth rate is currently slightly below replacement-level fertility, why is our population growing?