Population ecology is the study of populations in relation to the environment. It includes environmental influences on population density and distribution, age structure, and variations in population size.
Population size Population density Dispersion patterns Demographics Survivorship curves Population growth
“In population genetics and population ecology, population size (usually denoted N) is the number of individual organisms in a population”. Factors that Govern Population Size1. Crude Birth Rate (CBR)2. Crude Death Rate (CDR)3. Immigration4. Emigration
Natality The birthrate, which is the ratio of total live births to total population in a particular area over a specified period of timeMortality The death rate, which is also the ratio of the total number of deaths to the total population.Immigration The number of organisms moving into area occupied by the population is called immigration.Emigration The number of organisms moving out of the area occupied by the population is called emigration.
Factors that increase population size1. Natality is recruitment to a population through reproduction.2. Immigration from external populations e.g. Bird migration. Factor reducing population size1. Mortality which is the death rate from any source e.g. predation.2. Emigration, where individuals leave the population for another habitat.
Population Change = (births + immigration) – (deaths + emigration) Parameters that effect size or density of a population
“Population density is a measurement of the number of people in an area. It is an average number”. It is usually shown as the number of people per square kilometer.Density = Population/ Area
1. How to quantify nature – ecologist role2. Estimates are allows for comparisons between different populations in terms of space and time measure.
Mobility Based on movements of these organisms 2 attributes SizeSmall animals/plants are usually more abundant than large animals/plants
Density is measured by two methods.1. Total count method2. Sampling method
Direct counting of populations. Possible for few animals. Breeding colonies can be photographed then later counted.Examples Human pop census Trees in a given area
Depends on the type of organism and its natural abundance and distribution. Two broad categories:1. Plot-based (quadrant) methods2. Capture-based methods
Widely used in plant studies Total population = Average number per quadrat × Total area/Area of quadrat
Used for very mobile or elusive species First used in the 1890s by C. G. Peterson to estimate fish abundance.
The dispersion of a population is the pattern of spacing among individuals within the geographic boundaries. random clumped 3 types uniform
Clumped is a pattern when individuals are aggregated in patches. Most frequent pattern of distribution in a populationReasons: Some area of habitat are more suitable than others Heterogeneous environment with resources concentrated in patches Tendency of offspring to remain with parents Mating or social behavior of the individuals
Evenly spaced distributions, in which members of the population maintain a minimum distance from one another. In plants due to competition for water, sunlight, or available nutrientsExampleCreosote bushes in the Mojave desert In animals due to strong territorialityExampleThe desert lizard Uta sp
It is a spacing pattern based on total unpredictability. Least common pattern of distributionReasons Members of a species do not frequently interact with one another Not heavily influenced by the microenvironments within their habitat
Density Independent Density Dependant Floods Competition for Hurricanes Resources Unseasonable Predation Weather Parasitism Fire Infectious Disease Clear Cutting Pesticide Spraying
Demography is the study of the vital statistics of a population and how they change over time Twostatistics that are of particular import are a populations age structure and a populations sex ratio.
Itis the ratio of males to females in a population. Primary sex ratio Secondary sex ratio Tertiary sex ratio The human sex ratio is of particular interest to anthropologists and demographers.
The age structure describes the number of individuals in each age class as a ratio of one class to another. Age classes can be specific categories, such as people in the same age range.
Age structure diagram Graphical illustration that shows the distribution of various age groups & sex ratio in a population. Three age categories:1. Prereproductive (ages 0-14)2. Reproductive (ages 15-44)3. Postreproductive (ages 45 and up)
A life table (mortality table ) is a table which shows, for each age, what the probability is that a person of that age will die before his or her next birthday. From this starting point, a number of inferences can be derived.1. The probability of surviving any particular year of age2. Remaining life expectancy for people at different ages Separately for men and for women because of their substantially different mortality rates.
Graph showing the number or proportion of individuals surviving at each age for a given species or group (e.g. males/females). Constructed for a given cohort (a group of individuals of roughly the same age) based on a life table. Three types1. Type I2. Type II3. Type III
Type I survivorship curves are characterized by high survival in early and middle life, followed a rapid decline in survivorship in later life.Example: Humans Type II curves are an intermediate between Type I and III, where roughly constant mortality rate is experienced regardless of age.Example: Some birds In Type III curves, the greatest mortality is experienced early on in life, with relatively low rates of death for those surviving this bottleneck.Example: Octopus
Refers to how the number of individuals in a population increases (or decreases) with time. Controlled by the rate at which new individuals are added to the population -- the birth rate, and the rate at which individuals leave the population -- the death rate.
Exponential population growth dN = rmaxN dt 2 types of pop growth Logistic populationPopulation Growth growthMathematicallyDefined dN = rmaxN (K-N) dt K
If a population has a constant birth rate through time and is never limited by food or disease, it has what is known as exponential growth. With exponential growth the birth rate alone controls how fast (or slow) the population grows.
Continuouspopulation growth in an unlimited environment can be modeled exponentially. dN / dt = rmax N As population size (N) increases, rate of population increase (dN/dt) gets larger.
For an exponentially growing population, size at any time can be calculated as: Nt = Noert Nt = number individuals at time t. N0 = initial number of individuals. e = base of natural logarithms. r (rmax ) = per capita rate of increase. t = number of time intervals.
As resources are depleted, population growth rate slows and eventually stops: logistic population growth. Sigmoid (S-shaped) curve Carryingcapacity (K) is the number of individuals of a population the environment can support. Finite amount of resources can only support a finite number of individuals.
dN/dt = rmaxN(1-N/K) rmax= Maximum per capita rate of increase under ideal conditions. When N nears K, the right side of the equation nears zero. As population size increases, logistic growth rate becomes a small fraction of growth rate. Highest when N=K/2
Carrying capacity Environmental limits result in logistic growth No limitsNew or changed environment Logistic growth curve
Environment limits population growth by altering birth and death rates. Density-dependent factors Disease, Parasites, Resource Competition Populations do not show continuous geometric increase When density increases other organisms reduces the fertility and longevity of the individuals in the population This reduces the rate of increase of the pop until eventually the pop ceases to grow The growth curve is defined as the sigmoid curve, S – shaped K = carrying capacity (upper asymptote or maximum value) – the maximum number of individuals that environment can support Density-independent factors Natural disasters Climate
K and r selection (MacArthur and Wilson 1967)r-selected species•r refers to the per capita rate ofincrease•Selection favoring rapid growth•Should be favored in new or disturbedenvironments•Less competitionK-selected species•K refers to carrying capacity•More prominent in species that aretypically at their carrying capacity•Favors more efficient use of resources•Live with competition
The history of human population growth Figure 35.8A