Factors that regulate Natural populations

Chapter 11.3

McGraw-Hill Ryerson (2011)

Factors Affecting Population Change A. Density-dependent factors limit population growth and intensify as the population increases in size (i.e. Competition for resources, disease...)

Density-dependent factors

1. Intraspecific competition is when individuals of the same species compete for resources. If this is high then the population will have a low growth rate.

Density-dependent factors

2. Predation is the consumption of prey by a predator. If there is more prey available they will be chosen more by predators.

Density-dependent factors

3. Allee effect: Warder Allee found that some density-dependent factors reduce population growth when the population is at a low density rather than high density. – Ex. harder for individuals to find a mate and successfully reproduce thus lowering the growth rate of species.

Allee-effect continued

• Small populations also may go through inbreeding depression which reduces the populations' genetic variability and may prevent successful population growth.

Allee-effect continued

• • The minimum viable population size is the smallest number of individuals that ensures the population will persist for a certain period of time. Allows biologists to determine whether a species is endangered.

Factors Affecting Population Change B. Density-independent factors limit population growth no matter what the population size  i.e. Natural disaster, human intervention...

Density-independent factors

• The resource in the ecosystem that is in the shortest supply is known as the limiting factor since it is preventing massive population growth.

Population Change Affects the Entire Ecosystem • A change in one population of species can affect the entire hierarchy of living things in that ecosystem.

 Ex. The disappearance of beaver from this ecosystem causes a decrease in the wolf population which would cause an increase in its other prey

INTERACTIONS WITHIN COMMUNITIES • Community: Populations of different species living in the same area.

• INTERACTIONS WITHIN COMMUNITIES Ecological niche: The role an organism fills within a community (what it does, eats, its pattern of living)

• INTERACTIONS WITHIN COMMUNITIES Interspecific competition: competition for resources among members of two or more different species.

Hyenas battle with a lioness and win the day. Photo by Brittany Gunther, 2008

Types of Niches:

• Fundamental niche: the role the organism would fill under ideal enviromental conditions (if there was no interspecific competition).

• Realized niche: the portion of the fundamental niche the organism actually fills (due to competition).

A. Symbiosis

• • Interactions in which members of two (or more) species maintain a close association. There are 3 main types:

1. Parasitism

• • The interaction is beneficial to one species and harmful (but not fatal) to the other species. Parasites can be both micro- and macroscopic as well as ecto- and endoparasites.

• Social parasites mimic the behaviour of another species in order to complete their lifecycle (i.e. cowbirds).

2. Mutualism

• • The interaction is beneficial to both species. Ex. Bees and flowers Obligatory mutualism is when neither species can survive without the other (gut bacteria in herbivores, oxpecker birds).

3. Commensalism

• • The interaction is beneficial to one species while the other is unaffected. Remoras and sharks are a possible example.

B. Types of Interspecific Competition: 1. Interference

Competition: two species are actually fighting over the resources (birds over birdhouses, lion vs. hyena).

B. Types of Interspecific Competition: 2. Exploitative

Competition: two species are using a common resource and one species is more efficient at obtaining it ex. arctic foxes and snowy owls eating arctic hares, canopy trees in rainforest.

Gause's Principle/ Principle of Competitive Exclusion:

• • if the resources are limited, no two species can remain in competition for exactly the same niche indefinitely. One species will always out compete the other.

Resource Partitioning

• The avoidance of, or reduction in, competition for similar resources by individuals of different species that do not occupy the same niche

Resource Partitioning

• i.e. Plant root systems, lizards/insects/birds in different parts of the tree.

C. Predation

• Predator-prey relationships are an important interaction in a community.

C. Predation

• When the prey population increases, the predator population will increase shortly thereafter. Predator prey populations follow a cyclical pattern.

C. Predation

• • Predator-prey interactions have caused prey to evolve diverse defence mechanisms in order not to be eaten. Predators are evolving to bypass these defences (Evolutionary Arms Race).

D. Defense Mechanisms

• Plants use morphological defenses such as thorns, hooks, needles, spines and chemical defenses such as toxins, hormones, and other chemicals to deter herbivores from eating them.

D. Defense Mechanisms

• Animals use passive defences such as hiding / being poisonous or active defences such as fleeing/ fighting/ producing venom.

D. Defense Mechanisms

• Some animals use mimicry, which is one species appearing very similar to another species.

D. Defense Mechanisms

• Batesian mimicry: a harmless species mimics a harmful one (ie. An edible butterfly mimics a toxic species so it won't get eaten)

D. Defense Mechanisms

• Mullerian mimicry: dangerous species all appear similar which causes predators to learn quickly to avoid them.

Homework

• Pg 519 #1, 3, 5, 6, 7, 9, 10, 11, 14, 15