Populations Characteristics and Issues

Population Characteristics A population is a group of individuals of the same species inhabiting the same area at the same time.

Population Density and Spatial Distribution Population density is the number of individuals per unit area. Population density is the number of individuals per unit area. –High population density injures all individuals within the population because they compete severely for necessary resources. High density housing Low density housing

Population Density and Spatial Distribution  Dispersal: Movement of individuals from densely populated locations to new areas.  Emigration: Movement from an area.  Immigration: Movement into an area. Urban Sprawl

A Population Growth Curve Biotic potential is the inherent reproductive capacity of a species (biological ability to produce offspring). Biotic potential is the inherent reproductive capacity of a species (biological ability to produce offspring). Generally, biotic potential is much above replacement level. Generally, biotic potential is much above replacement level. –There is a natural tendency for increase.

Factors That Limit Population Size Factors that prevent unlimited population growth are known as limiting factors. Factors that prevent unlimited population growth are known as limiting factors. Density-dependent limiting factors are those that become more effective as the density of the population increases. Density-dependent limiting factors are those that become more effective as the density of the population increases. –Examples include: food and space – –density-dependent factors operate as negative feedback mechanisms leading to stability or regulation of the population Density-independent limiting factors are population-controlling influences that are not related to the density of the population. Density-independent limiting factors are population-controlling influences that are not related to the density of the population. –Examples include: fire, flood, drought.

Categories of Limiting Factors For most populations, limiting factors recognized as components of environmental resistance can be placed into four main categories: For most populations, limiting factors recognized as components of environmental resistance can be placed into four main categories: –Raw material availability –Energy availability –Accumulation of waste products –Interactions among organisms

Carrying Capacity Carrying capacity is the maximum sustainable population for an area. It is not an inflexible number; it can be influenced by environmental differences such as: –S–S–S–Successional changes (changes in the habitat) –C–C–C–Climate variations –D–D–D–Disease epidemics –F–F–F–Forest fires, floods, or natural disasters –N–N–N–Nutrient levels in aquatic ecosystems

A Population Growth Curve Population growth follows a pattern consisting of a lag phase, an exponential growth phase, a deceleration phase, and a stable equilibrium phase. Population growth follows a pattern consisting of a lag phase, an exponential growth phase, a deceleration phase, and a stable equilibrium phase. –Lag Phase: This is the first portion of the curve; slow population growth.

A Population Growth Curve –Exponential Growth Phase (Log Phase): More organisms are reproducing, causing accelerated growth; this continues as long as birth rate exceeds death rate. –Deceleration Phase: The population growth rate slows as the death rate and birthrate come to equal one another. –Stable Equilibrium Phase: The death rate and birth rate become equal and the population stops growing.

A Population Growth Curve This type of growth curve is called an S curve or logistic growth. This type of growth curve is called an S curve or logistic growth. A typical population growth curve

Carrying Capacity A typical population oscillates slightly from the carrying capacity of the environment. A typical population oscillates slightly from the carrying capacity of the environment. This is due to a slight overpopulation followed by resource scarcity due to the overpopulation and then a die off. As resources become more available, population increases which again puts a strain on the resources. This is due to a slight overpopulation followed by resource scarcity due to the overpopulation and then a die off. As resources become more available, population increases which again puts a strain on the resources.

Reproductive Strategies and Population Fluctuations Species can be divided into two broad categories based on their reproductive strategies: Species can be divided into two broad categories based on their reproductive strategies: –K-strategists –r-strategists

Reproductive Strategies and Population Fluctuations K-strategists: K-strategists: –Organisms that typically reach a stable population as the population reaches the carrying capacity. r-strategists: r-strategists: –Typically, these are small organisms that have a short life, produce many offspring, exploit unstable environments, and do not reach a carrying capacity.

Reproductive Strategies and Population Fluctuations K-strategist characteristics include: K-strategist characteristics include: –Usually occupy relatively stable environments. –Large organisms –Long-lived –Produce few offspring –Provide substantial parental care –Reproductive strategy is to invest a great deal of energy in producing a few offspring that have a good chance of living to reproduce. K-strategists are controlled by density-dependent limiting factors. Examples: ???

Reproductive Strategies and Population Fluctuations r-strategist characteristics include: r-strategist characteristics include: –Small, short-lived organisms –Produce many offspring –Little if any parental care –Exploit unstable environments. – –Regulated by density-independent factors, of which weather is the most important factor – –Fertility, size of breeding territory, predation or disease are examples of density-dependent limiting factors for r-strategists. –Usually do not reach carrying capacity (boom-bust cycles). –Reproductive strategy is to produce large numbers of offspring to overcome high mortality. –Examples: ???

Reproductive Strategies and Population Fluctuations r-strategists are controlled by density- independent limiting factors. r-strategists are controlled by density- independent limiting factors. This graph depicts a r- strategist population. There is a lag phase followed by exponential growth. Instead of entering equilibrium, there is a crash followed by another lag phase and so on. This graph depicts a r- strategist population. There is a lag phase followed by exponential growth. Instead of entering equilibrium, there is a crash followed by another lag phase and so on.

Population Characteristics K and r strategists can also be explained in survivorship curves. K and r strategists can also be explained in survivorship curves. A survivorship curve shows the proportion of individuals likely to survive to each age. A survivorship curve shows the proportion of individuals likely to survive to each age. Three types of survivorship curves can be recognized: Three types of survivorship curves can be recognized: –Species with high mortality in young. –Species with mortality equitable among age classes. –Species with mortality high only in old age.

Survivorship Curve

Population Data Collection There are 2 ways to collect data on population. There are 2 ways to collect data on population. –Which method is used depends on the type of population being determined.

Population Data Collection Mark-release-capture method Mark-release-capture method –Used with mobile organisms  The particular organism of interest is captured (as many as can be caught in a particular area. and marked with an obvious identifier (eg. paint).  The organisms are released.  The organisms are recaptured and a count of how many have the paint mark are tallied.  A calculation is made and a population estimate is formulated.

Population Data Collection The calculation used is called the Lincoln Index. P = N 1 x N 2 R Where P = total population size N 1 = the size of the first sample N 2 = the size of the second sample R = The number of marked individuals recaptured in the second sample

Population Data Collection The second type of data collection is the quadrat method. The second type of data collection is the quadrat method. –Used for immobile organisms, such as plants. –Observer delineates a predetermined area. –Observer counts all organisms occurring within that area. –Usually several areas within a habitat will be marked off.