Chapter 40
Population Ecology & The Distribution of Organisms
Earth’s Climate Varies
● The scientific study of ecology examines questions like what factors limit the geographic distribution of the
harlequin toad of Costa Rica or what conditions favor the red kangaroo of Australia.
● Climate, the long term prevailing weather conditions in a given area, is based on 4 main factors:
○ Temperature, precipitation, sunlight, and wind.
● Global climate patterns are determined by solar energy input and the Earth’s movement.
● Regional climate patterns can be influenced also by distance to large water bodies and altitude.
● Microclimate is a fine localized pattern encountered
within regional climates affected mostly by abiotic factors - nonliving factors such as sunlight or nutrients.
● Global climate change is having a profound affect on the biosphere.
Climate and Terrestrial Biomes
● Biomes are major life zones characterized by climate and vegetation.
● Terrestrial biomes show strong longitudinal patterns.
● Climatographs are plots of mean temperature and precipitation that show the distribution of world biomes.
● Terrestrial biomes are generally characterized by climatic features and predominant vegetation, which also influences the distribution of life.
● The area where two biomes overlap and show a graded change is called an ecotone.
● Vertical layering of vegetation is an important feature of terrestrial biomes.
● Disturbances, whether natural or man-induced can have significant effects on biomes.
Aquatic Biomes
● Aquatic biomes are characterized primarily by their physical environment and are less influenced by latitude.
● Marine biomes make up 75% of the Earth’s surface and freshwater biomes are closely linked to their surrounding terrestrial biomes.
● Most aquatic biomes are physically and chemically stratified or layered.
● Light intensity is critical for photosynthetic organisms in the photic zone, whereas the aphotic zone is void of these organisms.
● Further layering is the pelagic zone or open water and the benthic zone or bottom.
● Thermal energy differences in the water column create layers of different density water, which is greatest in the
thermocline.
● In seasonal latitudes water bodies undergo turnover which mixes the different layers of water.
Interactions Limit Distribution
● Species geographic distribution is a consequence of ecological and evolutionary interactions over time.
● To better understand species distribution, biologists focus on both biotic and abiotic factors that
influence species abundance and distribution.
● Dispersal contributes greatly to global species distribution, ie short or long range dispersal.
● Habitat selection behavior can influence distribution in otherwise suitable areas.
● Biotic or living factors such as predation or competition can affect species dispersal.
● Abiotic factors or nonliving factors such as
temperature, water and oxygen, salinity, sunlight availability, and soil composition als affect
distribution.
Population Ecology
Biological Processes Influence Populations
• Population ecology is the study of populations in relationship to their environment.
• A population is the number of individuals of the same species living in the same
place at the same time.
• How can we quantify populations?
– Density is number/area ie snails/m2 – Dispersion is how distributed across
an area ie, clumped, uniform, or random.
– Mark-recapture method helps to quantify pop size.
Births Deaths
Immigration Emigration
Births and immigration add individuals to
a population.
Deaths and emigration remove individuals from a population.
Factors that Affect Population Size
Studying Populations
Demographics
• The study of vital statistics of a population is demography.
• A cohort is a group of individuals of the same age from birth to death.
• One method of representing
demographic is survivorship curves and life tables.
• Survivorship curves can be
classified into three general types.
I II
III
The Different Growth Models
• What variables affect population size?
– b is birth rate, m is death rate, r is rate of increase, N is pop size
• We can sum up population growth with – dN/dt = rN
• The exponential growth model is growth without limits under ideal conditions
• The logistic growth model shows pop growth slow and stabilize around K or carrying capacity.
Life History Traits Influenced by Natural Selection
● Life history is are the traits that affect an organism’s schedule of reproduction and survival
● A life history includes 3 traits:
○ When reproduction begins
○ How often it reproduces
○ How many offspring are produced
● Semelparity is a one reproductive event strategy
● Iteroparity is repeated reproduction.
● What critical factors contribute to these reproductive strategies?
○ Survival rate of offspring
○ Likelihood to survive and reproduce
● K-selection for traits sensitive to high population densities
● r-selection for traits that maximize reproduction in low densities
Many Factors Affect Population Growth
• Density-dependent factors influence population size
because of some characteristic of the population, ie
competition, predation, pollutants, disease.
• Density-independent factors influence individuals in
population equally, ie drought, extreme cold, floods.
Population Dynamics
• Fluctuations in populations may be influenced by several factors.
• Some populations show booms and busts in size over time.
• The classic example is the
snowshoe hare & lynx populations.
Does one control the other?
• A metapopulation is a population of local organisms linked by
immigration and emigration.
The Human Population
• Demographic transition is the switch from high b and d rates towards low d and d rates which accompany development.
• Age structure is the relative number of individuals of each age in the population.
• Age structure diagrams predict growth trends and can suggest social conditions.
• The ecological footprint concept
summarizes the land and water needed to support a person.
