1

1

Population Ecology

Chapter 55

2

Outline

• Population Ecology• Population Dispersion• Metapopulations• Demography• Cost of Reproduction• Biotic Potential• Influence of Population Density• Population Growth and Life History Models• Exponential Growth

2

3

Ecology

• The distribution & abundance of an organism

• & its interactions with other organisms

• & its interactions with its environment

(biotic)

(abiotic)

Ecology

Evolution

Physiology

Gen

etic

sB

ehavior

4

Is ecology & environment the same?

Environment: immediate surroundings of an organismmacro-environment

micro-environment

Abiotic factorstemperature

pH

salinity

humidity

atmospheric pressureO2/CO2

H2O-dehydration/hydration

wind

sunlight photoperiod

3

5

Environmental Variation

• Key elements of an organism’s environment include:– temperature– water– sunlight– soil

• Many organisms actively employ mechanisms to maintain homeostasis, while others conform to their environment.

6

Responses to Environmental Change

• Individual responses – physiology– morphology– behavior

• Evolutionary responses– Shorter ears and limbs (Allen’s Rule)– Larger ears (Bergmann’s Rule)

4

7

Morphological and Behavioral Adaptation

8

Populations

• A population consists of group of individuals of a given species living together at the same place at the same time

• Population distributions– Most species: limited geographic range– Population ranges change through time.

§ environment changes§ organisms circumvent previously inhospitable

habitat

interbreeding

5

9

Altitudinal Shifts

10

Population Dispersion

• Randomly spaced– Individuals are randomly spaced within a

population.• Uniformly spaced

– Uniform spacing within a population often results from resource competition.

• Clumped spacing– Individuals clump into groups or clusters in

response to uneven distribution of resources in their immediate environment.

6

11

Population Dispersion

12

Population Dispersion

7

13

Population Dispersion

• Human effect– By altering the environment, humans have

allowed some species to expand their ranges, although the overall effect has most often been detrimental.

• Dispersal mechanisms– light seeds (wind)– hooks and hairs (animal fur)– fruits (animal digestive systems)

14

Seed Adaptations

8

15

Metapopulations

• Metapopulations are a network of distinct populations interacting with each other by exchanging individuals (genes).– degree of interaction depends on dispersal– Rate of extinction in habitats is balanced by

the rate of colonization of empty habitats.– prevent long-term extinction

• Source-sink metapopulations occur in areas where some habitats are suitable for long-term maintenance, while others are not.

16

Demography

• Demography - statistical study of populations• Factors affecting growth rates

– sex ratio§ effect also determined by mating habits

– generation time§ average interval between birth of an

individual and the birth of its offspring

9

17

Demography

• Age structure– cohort - individuals of the same age

§ fecundity - number of offspring produced in a given period of time

§ mortality - number of deaths in a given period of time

– Age structure determined by the relative number of individuals in each cohort.

18

Demography

• Life tables– constructed by following the fate of a cohort

from birth until death• Survivorship curves

– Survivorship is the percentage of an original population that survives to a given age.§ Type I - full life span§ Type II - mortality unrelated to age§ Type III - early susceptibility

10

19

Survivorship Curves

20

Cost of Reproduction

• A life history is the complete life cycle of an organism.– all involve significant trade-offs

§ Due to limited resources, increased reproduction may decrease survival and chances of future production.vcost of reproduction

ØNatural selection will favor the life history that maximizes lifetime reproductive success.

11

21

Cost of Reproduction

• Investment per offspring– Key reproductive tradeoff concerns the

amount of resources to invest in producing any single offspring.§ number of offspring versus size of each

offspringv In many species, offspring size

critically affects chances of survival.

22

Cost of Reproduction

• Reproductive events per lifetime– semelparity - organisms focus all

reproductive efforts on a single, large event– iteroparity - organisms produce offspring

several times over many seasons• Age at first reproduction

– Longer-lived animals tend to reproduce later, and provide more parental care than shorter-lived animals.

12

23

Biotic Potential

• Exponential growth model– Rate at which a population of a given

species will increase when no limits are placed on the rate of growth.§ Innate capacity for growth of any

population is exponential.vEven when rate remains constant,

actual increase in number accelerates as the population size grows.

24

Biotic Potential

• Carrying capacity– Carrying capacity (K) is the maximum

number of individuals the resources in a given area can support.

• Logistic growth– As a population approaches carrying

capacity, its growth rate slows as resources become depleted.§ sigmoidal growth curve

13

25

Population Growth

26

Influence of Population Density

• Density-dependent population controls– As populations approach their carrying

capacity, competition for resources can be severe, leading to decreased birth rate and increased risk of mortality.§ In some cases, growth rates increase

with population size.vAllee effect

14

27

Influence of Population Density

• Density-independent population controls– Growth of a population at any instant is

limited by some factor other than population size.§ external environmental conditions

• Population cycles– snowshoe hares

§ Food plants (willows) and predators (lynx) both control the population.

28

Linked Population Cycles

15

29

Population Growth and Life History Models

• K-selected species tend to produce relatively few, large offspring.– large investment in parental care

• r-selected species tend to produce many, small offspring.– small investment in parental care

30

Exponential Growth

• Throughout most of human history, human populations have been regulated by food availability, disease, and predators.– resemble K-selected species– Due to the easing of environmental

restraints, the human population has grown explosively over the last 300 years.

16

31

32

Advent of Exponential Growth

• Population pyramids– bar graph displaying number of people in

each age category§ stable - rectangular§ rapid growth - triangular§ decreasing - inverted pyramid

17

33

Population Pyramids

34

Population Pyramids

• Uncertain future– Earth’s rapidly growing human population

is possibly the greatest challenge facing facing the biosphere.

– uneven resource distribution§ population distribution§ wealth gap§ ecological footprint

• Population growth rate on the decline

18

35

Ecological Footprints

36

Summary

• Population Ecology• Population Dispersion• Metapopulations• Demography• Cost of Reproduction• Biotic Potential• Influence of Population Density• Population Growth and Life History Models• Exponential Growth