Factors that regulate Natural populations

Chapter 11.3McGraw-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