Population ecology

• Population
• definition
• major characteristics
• dynamics
• life histories

Population definition

• Population
• definition
• group of individuals of a species living in same area at same time
• using common resources
• regulated by same natural phenomena
• flexible
• allows discourse in similar terms about any population

Population characteristics

• Populations
• major characteristics
• size
• density
• dispersion
• age distribution
• Population size
• definition
• number of individuals
• important feature of any population
• affects ability of population to survive
• small populations tend to become extinct
• endangered by random events
• inbreeding

• Population density
• definition
• number of individuals in a certain area or volume
• # trees per km² of forest
• # earthworms per m³ of soil
• important to survival of population
• individuals spaced widely apart may rarely encounter one another
• limits reproductive capacities

• how is population density measured?
• impossible or impractical to count all individuals in a population
• use sampling techniques

• sampling technique
• method to estimate population density
• direct count of organisms or indicators in small area or volume
• used to project actual density over entire area or volume
• examples

• Population dispersion
• way in which individuals of a population are spaced within their area or volume
• often depends on resource availability
• spatial pattern
• three main patterns of dispersion
• clumped
• uniform
• random

• clumped
• individuals clump into groups or clusters
• often in response to uneven distribution of resources
• most common pattern in nature

• uniform
• individuals are uniformly or evenly spaced
• often results from interactions between individuals
• relatively common in nature

• random
• individuals spaced in a pattern-less, unpredictable way
• don’t interact strongly with
• one another
• non-uniform aspects of their environment
• not common in nature

• Population age distribution
• proportions of individuals of each age
• often based on
• non-reproductive ages
• reproductive ages
• post-reproductive ages

• Population dynamics
• variables governing changes in population size
• factors that affect population size
• population growth
• types of
• limits to
• populations are dynamic
• size increases or decreases in response to
• environmental stress
• changes in environmental conditions

• Variables governing change in population size
• governed by 4 variables
• births
• deaths
• immigration
• emigration

• populations
• gain individuals by
• birth
• immigration
• lose individuals by
• death
• emigration

• population change=(births+immigrations - (deaths+emigration)

• Factors that affect size of population
• population size may increase, remain stable, or decrease
• depending on interactions between
• biotic potential
• growth factors
• environmental resistance
• decrease factors

• biotic potential
• "growth factors"
• capacity of a population for growth
• varies
• between populations
• within population over time
• factors that favor increase in size
• abiotic
• favorable light
• favorable temperature
• favorable chemical environment (optimal level of critical nutrients)
• biotic (such as)
• high reproductive rate
• generalist
• adequate food
• adequate defenses from predators
• resistance to diseases

• environmental resistance
• "decrease factors"
• all the factors acting jointly to limit growth of a population

• factors that lead to decrease in size
• abiotic
• too much, too little light
• temperature too high, too low
• unfavorable chemical environment (critical nutrients too high, too low)
• biotic (such as)
• low reproductive rate
• specialist
• inadequate food
• inadequate defenses from predators
• inability to resist diseases

• biotic potential & environmental resistance
• together determine
• carrying capacity (K)
• number of individuals of a given species that can be sustained indefinitely in a given area or volume

Types of population growth

• Two types of population growth
• exponential
• accelerating increase in population size
• occurs when growth is unregulated
• logistic
• population growth that is slowed by population-limiting factors
• tends to level off at a carrying capacity

• Exponential growth
• exhibited by a population that has few, if any, resource limitations
• starts out slowly, speeds up as population increases
• rate of expansion that occurs under ideal conditions
• entire population multiplies by a constant factor during constant time intervals
• described by equation G = rN
• G = growth rate of the population
• N = population size
• r = intrinsic rate of increase
• graph produces typical J-shaped curve
• r = intrinsic rate of increase
• rate at which a population would grow if it had unlimited resources
• remains constant for any population expanding without limits
• based on organism’s inherent capacity to reproduce
• varies by organism
• can be roughly estimated as
• birth rate minus death rate
• r = b - d
• long periods of exponential growth are not common
• bacteria example
• no population can grow indefinitely
• eventually some factor(s) limit population growth
• rapidly growing population reaches size limit imposed by shortage of limiting factors
• there are always limits to population growth in nature

• Logistic growth
• growth, slowed by limiting factors
• involves
• exponential growth when pop. is small
• steady ¯ in growth with time as pop.
• encounters environmental resistance
• approaches carrying capacity
• equation must account for limiting factors
• exponential equation is modified by a term that represents overall effect of limiting factors
• (K - N)/K where K = carrying capacity
• effects of the modifying term
• (K - N)/K
• when population is small,
• (K - N)/K has little effect
• growth rate is reduced very little
• early logistic curve is very similar to J-shaped exponential curve
• for example, if N=10 and K=1000
• (1000-10)/1000 = 0.99
• as population gets larger,
• (K - N)/K has greater effect
• growth rate is affected more (gets smaller)
• later logistic curve becomes S-shaped
• population levels off at "carrying capacity"
• limiting factors causes birth rate and death rate to be equal
• for example, N= 800 and K=1000
• (1000-800)/1000 = 0.20

• after leveling off at carrying capacity (K)
• population typically fluctuates slightly above or below K

• Exponential and logistic growth models
• both are mathematical ideals
• no natural populations fit either model perfectly

Limits to population growth

• Population growth
• limited by two general types of factors
• density-dependent factors
• limits to growth related to population density
• density-independent factors
• limits to growth not related to population density

• density-dependent factors
• affect a greater percentage of individuals in a population as density increases
• individuals compete with increasing intensity for limited resources
• such as
• food
• shelter
• light

• density-independent factors
• population-limiting affects that are independent of population density
• include abiotic factors
• weather
• physical disruption of habitat

• Population fluctuations
• occur in nature, over time
• four general types exist
• stable
• irruptive
• irregular
• cyclic
• most are poorly or incompletely understood

• stable
• population size fluctuates around carrying capacity
• slightly above
• slightly below
• typical of species in undisturbed tropical rainforests
• little variation in average temperature or rainfall

• irruptive
• population is normally fairly stable
• occasionally explodes (irrupts) to peak
• then crashes to
• stable lower level
• very low level
• due to factor (ie temp) that temporarily increases carrying capacity
• examples: raccoon, house mouse

• irregular
• irregular, chaotic behavior in population size
• no apparent recurring pattern
• may be due to
• chaos in system
• poorly understood interactions

• cyclic
• fluctuations in size that occur over a regular time period
• most are poorly understood
• include predator-prey cycles

• predator-prey cycles
• seen in some groups of species that interact as predator and prey
• characterized by
• sharp increases in numbers followed by
• seemingly periodic crashes
• classic example
• snowshoe hare, Canadian lynx
• explained by two hypotheses
• top-down control
• bottom-up control
• top-down control hypothesis
• lynx prey on hare
• reduces hare population
• fewer hares support fewer lynxes
• causes periodic reduction in lynx population
• lag-time, offset from hare reduction

• reduced numbers of predators (lynx) allows population of prey (hare) to recover and increase
• increased numbers of prey (hare) support increased numbers of predators and lynx population increases
• cycle continues
• doubt has been cast on this explanation
• snowshoe hares have been found to exhibit similar 10-year "boom-or-bust" cycles on islands where lynx are absent
• leading to 2nd hypothesis
• bottom-up control
• bottom-up control hypothesis
• rather than cycle being driven by predator at top
• might be driven by food source of prey (hare) at bottom
• reduction in quantity or quality of food source (plants) of hare leads to crash of hare population
• fewer hare support fewer predators and lynx population crashes
• reduction in hare population gives plant population time to recover
• increased plant population supports more hares and hare population increases
• increased hare population supports more lynx and lynx population increases
• cycle continues, driven by plant availability

• genuine examples of both top-down and bottom-up control exist in nature

• Survivorship and life history strategies
• survivorship
• life tables
• survivorship curves
• life history strategies
• opportunisitc life history
• equilibrial life history

• Survivorship
• percentage of an original population that survives to a given age
• requires compilation of data (life table)
• for each defined age interval
• number living at start of interval
• number dying during interval
• from which can be calculated
• mortality (death rate)
• chance of surviving age interval

• Survivorship curves
• way to express age distribution characteristics of a population
• graph of life table data
• varies with species
• uses percentage scale instead of actual life span on horizontal axis
• allows comparison of species with different life spans on same graph
• three primary types of survivorship curves
• type I survivorship curve
• type II survivorship curve
• type III survivorship curve

• type I survivorship curve
• exhibited by population in which mortality rates rise steeply in post-reproductive years
• also known as "late loss" curve
• most individuals die in older age intervals
• species with this type curve
• produce few offspring & give them intense care to insure their survival
• examples
• humans, whales, elephants

• type II survivorship curve
• exhibited by population in which individuals are equally likely to die at any age
• also known as "constant loss" curve
• mortality is constant over life span
• intermediate to types I and III
• examples
• jellyfish
• hydra
• some rodents

• type III survivorship curve
• exhibited by population in which individuals produce vast numbers of offspring
• also known as "early loss" curve
• only a small number of offspring survive to reproductive age
• survivors become established, reproductive, with low mortality rate
• examples
• oysters, some plants

Life History Strategies

• Life history of an organism
• series of events from birth through reproduction to death
• life history strategies influence growth rate of a population, including
• age of first reproduction
• number of offspring
• amount of parental care given to offspring
• energy cost of reproduction
• shaped by evolution
• operating through natural selection
• every population has a life history strategy adapted to its environment
• two main life history strategies
• opportunistic (r-selected)
• equilibrial (K-selected)

• Opportunistic (r-selected) life history
• put most of their energy into reproduction
• rather than long term survival of individuals
• are poor competitors
• considered opportunists
• take advantage of favorable conditions, changes in environment
• when favorable conditions are gone population may crash
• population go through irregular or unstable cycles
• characteristics
• organisms
• small-bodied
• reproduce when young
• produce many offspring
• provide little to no parental care of offspring
• most offspring die before reaching reproductive age
• populations
• tends to grow exponentially
• thus the name r-selected
• due to high intrinsic rate of growth
• live in unpredictable environments
• controlled by density-independent factors
• exhibit type III survivorship curve
• examples
• bacteria
• algae
• most annual plants
• dandelions
• most insects
• cockroaches
• rodents
• oysters

• equilibrial (K-selected) life history
• put fairly little energy into reproduction
• put most energy into long term survival
• for purpose of being able to put lots of energy into nurturing and protecting offspring
• are good competitors
• are not considered opportunistic
• thrive best in ecosystems with fairly constant environmental conditions
• populations remain close to carrying capacity (K) over long periods of time
• characteristics
• organisms
• larger-bodied
• reproduce later in life
• produce fewer offspring
• provide high parental care
• most offspring survive to reproductive age
• populations
• size tends to be stable
• thus the name K-selected
• populations tends to stay near carrying capacity (K)
• live in predictable environments
• controlled by density-dependent factors
• exhibit type I survivorship curve
• examples
• humans
• large trees
• polar bears
• elephants

• Intermediate life history
• many organisms have life histories that fall between opportunistic and equilibrial
• exhibit type II survivorship curve
• examples
• many birds
• squirrels
• hydra