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Population ecology The branch of ecology & environmental science that deals with characterizing the makeup, growth, and impact of populations of organisms on the environment
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What is a community? A community is a group of interacting populations living in the same area
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Population Species may consist of multiple populations that are geographically isolated from one another A group of the same species living in the same place at the same time Ex. Earthworms living in a garden or maple trees that live in a particular forest
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How many individuals make up a population?
Actual size The total # of individuals in the population Direct count or estimation Population density The # of individuals to a given unit of area Ex. Dandelion population in a field many be the # of dandelion plants per square meter
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3 methods for measuring population size…
1. Direct count method (census) Just as it says… Count all individuals in population Done every 10 years in U.S. Not exact due to immigration, emigration, births & deaths
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2. QUADRAT METHOD Construct a grid of sample squares, each of known area, within the larger area where population lives Individuals counted in a fraction of those sample squares & then total population is estimated Works best for counting organisms that don’t move (trees)
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3. CAPTURE-RECAPTURE Used to estimate size of a mobile population
Count & mark a sample of individuals in an area on a given day & then return them to the habitat. Next day, go back and mark another sample of individuals Continue over the course of several days Ex. Japanese beetles in a field.
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Population density problem
Population density= The # of individuals to a given unit of area Equation: number of individuals divided by given unit of area 1,270 deer are living on an island that is 830km2 in size. What is the population density of the deer per square kilometer?
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Population density problem
1,270 deer are living on an island that is 830km2 in size. What is the population density of the deer per square kilometer? 1,270/830= deer/km2
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The ability to predict growth or decline is useful in monitoring and managing threatened or endangered species
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The Passenger Pigeon Used to be most abundant bird in North America
Early 1800s, an ornithologist watched 2 billion birds that took 5 hours to fly over him What happened? We started cutting down the forests & the birds became easy targets for hunters The last passenger pigeon died in 1914 in the Cincinnati Zoo
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Population density In general, larger organisms have lower population densities because they require more resources– and thus more area– to survive
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High vs Low Population Density
High density Easy to find mates Conflict over space, food Vulnerable to predators Increase transmission of disease Low density Hard time finding mate Less competition for space, food
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Population Distribution
1. Random Distribution individuals are not grouped in any particular pattern across an area. 2. Uniform Distribution Individuals are equally spaced across a given area 3. Clumped Distribution individuals associate in small groups or clusters that are unequally spaced across a given area Most populations live in clumps although other patterns occur based on resource distribution
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Random Distribution Most unusual
Occurs when resources are found throughout an area and other organisms do not strongly influence where members of a population settle Windblown seeds may settle and establish themselves in an area in this manner Some spider species
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Uniform Distribution Can occur due to… Territoriality
Limited resources Space or Water Plants that exhibit allelopathy
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Clumped Distribution Most common because resources are usually concentrated in specific areas Protection (zebra herd) Social species (bees/ants)
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Calculating birth and death rates
To calculate the birth or death rate of a population, just take the number of births/deaths and then divide by the total population Ex. If a population has 29 births a year and the original population size is 1,000, what is the birth rate for this population? 29/1,000= .029= 2.9%
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How do populations grow?
1. Population with no immigration or emigration occurs dN/dt(r)= (b-d)/N N= population size b= births d= deaths dN/dt (r)= growth rate of the population
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populations grow with immigration & emigration
dN/dt (r)= (b + i) –(d + e)/N N= population size b= births d= deaths i= immigration E= emigration dN/dt (r)= growth rate of the population
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Population doubling time
To calculate the time it takes for a population to double- we use the Rule of 70 70 r How long would it take a population with a growth rate of 2% to double in size? 70 = years 2
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Exponential Growth J curve “Malthusian Growth”
Exponential Growth Curve of a Hypothetical, Ideal Population This usually occurs with small populations & when a lot of resources are available Population Size (N) J curve Time (t) “Malthusian Growth”
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05_T03.JPG Table 05-T03 Title: Exponential growth in a savings account with 5% annual compound interest. Caption: Notes: Keywords: populations, growth rates, exponential
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However, populations do not grow indefinitely…
Every population is constrained by limiting factors Physical, chemical & biological characteristics of the environment Populations grow until they reach the Carrying capacity (k) Carrying capacity= the maximum population size of a species that a given environment can sustain This can vary, depending on changes in the environment
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Exceeding Carrying Capacity: Move, Switch Habits, or Decline in Size
Members of populations which exceed their resources will die unless they adapt or move to an area with more resources. Figure 8-6
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Logistical Growth Curve of a
Hypothetical, Real Population Carrying Capacity (K) Population Size (N) S- curve Time (t)
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Exponential and Logistic Population Growth: J-Curves and S-Curves
Populations grow rapidly with ample resources, but as resources become limited, its growth rate slows and levels off. Figure 8-4
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Exponential and Logistic Population Growth: J-Curves and S-Curves
As a population levels off, it often fluctuates slightly above and below the carrying capacity. Figure 8-4
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Biotic Potential and Environmental Resistance affect population size
Ability to Have offspring Limiting factors The more offspring you produce the higher your biotic potential
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Environmental resistance: Limiting Factors
Density independent factors Factors NOT influenced by the population density They will affect the population the same, regardless of its size Density dependent factors Factors influenced by population density They increase as population increases
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Density Independent Factors
Temperature changes in the environment Weather Habitat destruction Availability of water Sunlight Salinity Soil chemistry Suitable Breeding Sites Dissolved Oxygen Fire, flood, earthquake, etc.
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Density dependent factors
Competition for food resources Predation Disease Territoriality Changes in reproductive capacity (some slow down when populations become overcrowded like mice/rats) Behavioral changes (some species resort to cannibalism and killing their young when they become overcrowded)
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biotic potential: organisms fall into one of the following two groups
r-strategists/r-selected species k- strategists/k-selected species Mortality due to density independent factors J shaped growth curve Insects, fish, bacteria, algae, rodents, weeds Mortality due to density dependent factors S shaped growth curve Sharks, birds, some reptiles, most mammals
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Little or no parental care and protection of offspring
r-Selected Species Cockroach Dandelion Many small offspring Little or no parental care and protection of offspring Early reproductive age Most offspring die before reaching reproductive age Small adults Adapted to unstable climate and environmental conditions High population growth rate (r) Population size fluctuates wildly above and below carrying capacity (K) Generalist niche Low ability to compete Early successional species Figure 8.10 Natural capital: generalized characteristics of r-selected (opportunist) species and K-selected (competitor) species. Many species have characteristics between these two extremes. Fig. 8-10a, p. 168
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Fewer, larger offspring High parental care and protection of offspring
K-Selected Species Elephant Saguaro Fewer, larger offspring High parental care and protection of offspring Later reproductive age Most offspring survive to reproductive age Larger adults Adapted to stable climate and environmental conditions Lower population growth rate (r) Population size fairly stable and usually close to carrying capacity (K) Specialist niche High ability to compete Late successional species Figure 8.10 Natural capital: generalized characteristics of r-selected (opportunist) species and K-selected (competitor) species. Many species have characteristics between these two extremes. Fig. 8-10b, p. 168
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Types of Population Change Curves in Nature
Population sizes may stay the same, increase, decrease, vary in regular cycles, or change erratically. Stable: exhibits dynamic equilibrium. Irruptive: when populations explode and crash. Cyclic: populations fluctuate up and down Irregular: erratic changes.
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Types of Population Change Curves in Nature
Population sizes often vary in regular cycles when the predator and prey populations are controlled by the scarcity of resources.
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Survivorship curves Proportion of individuals in a population that survive to a particular age
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Survivorship curves Long life span
Type I: Long life span Young survive, death rates high among elderly Top consumers & k-strategists Humans Type II: Constant death rate at all ages Birds, rodents, flowering plants Type III: High death rates among young; if live, likely to live entire life span Fish, insects, amphibians, sea turtles, oysters, redwood trees, etc. (r-strategists)
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