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How do ecologists study populations?
• Geographic range: where are they distributed?
• Density and distribution: in what manner are they distributed?
• Growth rate: how is the population increasing or declining?
• Age structure: males and females? number at reproductive age?
Clumped distribution
• Food – clumped around food source
• Protection or shelter
• Small differences in habitat: shade, soil, wind, water
• Patchy resources!
Uniform distribution
• Can be caused by competition for resources
• Some organisms have specialized mechanisms…
• Creosote bushes release terpines, chemicals which inhibit growth of other plants around
• Tree planting
• Farming
Random distribution• Happens when none of
these factors exist• Least common in nature
because biotic factors usually cause other type
• Homogeneous habitats (same all over)
• Wind dispersal of seeds• Water dispersal of larvae
Population growth
• Birth rate, death rate, immigration, emigration
• Resources are often the driver: food, water abundance or shortage, overcrowding
Logistic population growth• Stages: exponential growth, growth slows
down, growth stops
• Carrying capacity: number of organisms the area can support
Limiting factors
• Any factor that slows or stops the growth of a population
• Determine the carrying capacity
• Can be biotic or abiotic: availability of food or water, parasitism or disease, competition and predation, natural disasters such as floods or droughts
Density dependent limiting factors
• Act strongly when population density reaches a certain level
• Competition, predation, parasitism and disease, hunting, overcrowding stress
• Tend to be the BIOTIC limiting factors
Density independent limiting factors
• Affect all populations equally, regardless of population size or density
• Drought, floods, unusual weather such as hurricanes, wildfires
• Tend to be ABIOTIC limiting factors
Invasive species
• Any non-native species that is introduced to an ecosystem
• Accidental in intentional introductions• Don’t have the usual limiting factors
because the species didn’t evolve there:– No natural predators– No biological controls– Often short life-history species (rapid growth,
lots of reproduction
Examples:
• Brazilian pepper trees outcompete our mangroves
• Brown hoplo “armored catfish” in St. Johns River are outcompeting native fish we eat
• Jellyfish brought in ballast water of ships
• Invertebrates attached to oil rigs moving from ocean to ocean
Demography: study of human populations
• When you refer to “demographics” it usually means: ages, genders, ethnicities, socioeconomics
• Age structure very important: how many members of the population are reproducing?
The Demographic Transition
• Stage 1: high birth rate and high death rate
• Stage 2: high birth rate, but death rate begins to fall
• Stage 3: birth rate falls to meet the death rate
Factors that influence human birth and death rates: nutrition, sanitation, medical treatment, education, higher standard of living
Top 400 urban areas in 2006:1 million +
• Humans have a clumped distribution
• Notice that Florida has 3 dots!
Fertility rate by country: number of children (0-8)
More people = less resources
• Food competition: shortages, prices
• Other resources: fuels, water, space, trees
• Impact on the environment:– Deforestation– Air pollution– Water pollution– Overcrowding
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