Populations and Communities

• A group of individuals of the same species, living in a shared space at a specific point in time

= Population

Factors affecting population size

births deaths

Population

Immigration Emigration

• How big is this population?

6

11

6

Counting populations I

- Counting individuals

- Counting by sampling:

Population size =

Average number of individuals per section

xArea of section

total study area

- Counting by recapture %:

Counting by recapture %

d

d

d

# of marked recaptured animals# of captured animals

# of marked animalsPopulation size

=

Population density

• Factors influencing population density:– Access to water and food– Presence of predators– Parasites, diseases

Pop. Density =# of individuals

Space occupied (area / volume)

Population distribution

= the way in which individuals are dispersed

within their habitat

clumped

random

uniform

http://www.youtube.com/watch?v=1yKxUNP_XVo&list=PL9CE54703E2B8E92F

clumpedrandom

uniform

Ecological factors

= aspect of a habitat that can affect the organisms living there

• amount of light• soil / water pH• terrain• temperature• air humidity

• birth rate• disease• amount of food• predation• competition• human activity

Related to the action of living

organismsPhysical or

chemical origin

Abiotic Biotic

Limiting factor• A factor that limits the growth of a population– Light– Temperature– Water– Food / nutrients– …

Limiting factor• Is low temperature limiting growth of vampire

bat population?

• Vampire bat colonies

Mean minimal temperature for January > 10°C

12 m200 mDistance between nest boxes

Dens

ity (p

airs

/ ha

)

pied flycatcher

Is nesting space limiting?

Providing more nesting spaces increases population density

Nesting space was limiting

Biological cycles in populations

• Population cycles are composed of alternating rise and fall of population size.

• Periods are of fixed duration and are repeated continually.

Populations

• Counting methods• Density (number/ area)• Distribution (clumped/ random / uniform)• Ecological factors – limiting factors

Polpulation cycles

commuities

organism

populationcommunity

Communities: A set of populations of different species sharing the same habitat

BiodiversityThe variety of species living in a community

Species richness:Number of different species in a community

Relative species abundance:Number of individuals of one species in relation to total community

Same species richness;

community 2: species abundance not even

Lower biodiversity

Biodiversity is high when:• number of species high• relative abundance of species similar

Interactions between individualsCompetition:Living organisms seek access to the same resource in their habitat.

PredationOne feeds on the other.

Mutualism:Interaction between two species beneficial to both.

Commensalism:One organism benefits from relationship while other remains unaffected.

Competition

interspecific

intraspecific

Predation:

predator prey

prey may or may not die

parasite hostParasite lives either on or inside the host. Host is harmed from the interaction.

mutualism

Bee gains foodFlower gains fertilization

Clown fish gains protectionAnemone gains prey (attracted by clown fish)

Commensalism

Bird gains foodHuman remains unaffected

Bird gains protectiontree remains unaffected