Population Ecology

Population Ecology

Introduction All populations of organisms are dynamic. Many factors, such as predation, available resources, or environmental changes, influence the changes in a species’ population. Population dynamicsis the study of the long term changes in population sizes and the factors that cause a change.

History of Population Dynamics Thomas Malthus proposed the first mathematical equation to characterize human population growth over 200 years ago. This lead to the development of many equations for the various kinds of population growth, some of which we will cover in this unit.

There are four factors that affect a population’s growth: • Births (natality) (B) • Deaths (mortality) (D) • Immigration (I) • Emigration (E) These factors can be put into a simple equation to summarize the changes in a population from one moment in time (N0) to another (N1): N1 = N0 + (B – D) + (I – E)

Population Density Population Density is a measure of how many individuals of a given species are found in a given area. There are two types of density: 1) Crude density is the total number of individuals divided by the total area of the entire habitat. 2) Ecological density is the total number of individuals divided by the total useable area in the habitat. The equation for population density (D) is: D = N (population size)/S (area)

Population Dispersion Population dispersion is how individuals of a species are arranged in their environment. There are three kinds. Clumped dispersion is usually due to a species being concentrated in areas most favourable for survival (ex. Cattails in wet soils lining ponds or lakes) or social behaviour (ex. fish in schools) for protection from predators

Uniform dispersion is when the individuals of a species are spaced equally throughout a habitat. This usually occurs with territorial organisms (ex. penguins).

Random dispersion is when organisms are spread throughout a habitat in an unpredictable and patternless way

Measuring Populations Population density is found differently depending on whether the species being studied is mobile or stationary. Quadrat studies are used for non-mobile populations such as plants. Mark-Recapture studies are used for mobile populations such as animals.

Quadrat Studies A quadrat is a small sample frame (usually a square) that is placed randomly throughout a larger ecosystem in order to estimate the population density (D). All the individuals counted in the quadrats are added together in the following equation: Estimated density = total number of sampled individuals / total sample area The population size (N) can then be estimated: N = (estimated density)(total area of studied habitat)

Example Calculations A student wants to estimate the population of ragweed plants in a large field which measures 100 m x 100 m. She randomly places three 2.0 m x 2.0 m quadrats in the field. Estimate the population density and size if she finds 18, 11, and 24 plants in her three quadrats. average sample density = total number of individuals total sample area = 18 + 11 + 24 (4.0m)² + (4.0m)² + (4.0m)² = 4.4 ragweed plants/m² estimated population size = (estimated population density) (total size of study area) = (4.4 plants/m²) x (10 000m²) = 44 000 plants

Mark-Recapture Method To start, traps are laid out in the study area and any subjects that are captured are marked and returned to the environment. A short time later the traps are set again and individuals are captured. This time it is noted how many individuals were recaptures and how many were new captures. All this data can be plugged into an equation to estimate population density.

The equation is: M (total # marked on 1st day) = m (# of recaptures) N (total estimated population) n (total # captured on 2nd day)

Example Calculation Consider a fish population of unknown size where 26 individuals are randomly captured, marked, and released. Some time later, 21 individuals are captured and three of those appear to already have been marked. What is the estimated population size? total # marked individuals in population  26 = 3 # marked in 2nd sample estimated population size  N 21  size of 2nd sample N = 26 x 21 N = 182 3 Therefore, the estimated population size is 182.

Homework Page 659, # 3, 4, 5, 6

Measuring and Modelling Population Change

Fecundity is the potential for a species to produce offspring in one lifetime. This relates to the species’ ability to increase population rapidly or over a long period of time. High fecundity is when a female of a species can produce large numbers of offspring (ex. star fish lay over 1 million eggs per year). Low fecundity is when a female can produce a much more limited number of offspring in their lifetime (ex. a hippopotamus could produce maybe 20 young over an average life of 45 years).

Carrying capacity is the maximum number of organisms that can be sustained by the available resources of a habitat over a given period of time. It is always changing as the resource levels are never constant and depend on the changing abiotic elements of habitat (ex. climate). Biotic potential is the maximum rate a population could increase under ideal conditions. It is represented mathematically by r.

Survivorship Patterns Biologists recognize three general patterns of survivorship among species. Type I Curve - Low mortality rate until they are past their reproductive age - Long life expectancy - Slow to reach sexual maturity and produce low numbers of offspring - Ex. Humans and most large mammals

Type II Curve • Intermediate between types I and III • Have a uniform risk of mortality over their lifetime (i.e. coral, squirrels, most reptiles, honeybees…)

Type III Curve • Very high mortality rate when young • Those that reach sexual maturity have a greatly reduced mortality rate • Very low average life expectancy • Ex. Green Sea Turtle, plants, oysters…

Population Change Population change (%) = [(birth+immigration)–(deaths+emigration)] x 100 initial population size (n) A negative result means population is declining. A positive result means population is growing. In an open population all four factors come into play. In a closed population, normally an island, only births and deaths are a factor.

Types of Population Growth Geometric growth is a pattern where organisms reproduce at fixed intervals at a constant rate. Exponential growth is a pattern where organisms reproduce continuously at a constant rate. Logistic growth is a pattern where growth levels off as the size of the population reaches the carrying capacity of their environment.

Geometric Growth Deaths occur at a relatively constant rate over time but births are restricted to a specific breeding period. These populations increase rapidly during breeding season and decline slowly the rest of the year. Appears continuous In reality…

Their growth rate is a constant (λ) and can be determined using the following equation: λ = N (t + 1) N(t) • λ is the fixed growth rate (from one year to the next) • N is the population size at year (t+1) or (t) To find the population size at any given year, the formula is: N(t) = N(0)λt • N(0) is the initial population size

Sample Problem The initial Puffin population on Gull Island, Newfoundland is 88 000. Over the course of the year they have 33 000 births and 20 000 deaths. a) What is their growth rate? ANSWER a) N (0) = 88 000 λ = N(t + 1) = 101 000 = 1.15 N (1) =101 000 N(t) 88 000 Therefore the growth rate is 1.15. b) What will the population size be in 10 years at this current growth rate? ANSWER: From a) growth rate, or λ = 1.15 N(10) = N(0)λ10 * Remember BEDMAS! = 88 000 (1.15)10 = 356 009 Therefore the population size will be 356 009 in 10 years.

Exponential Growth Many species, such as bacteria, are not limited to a breeding season. These species can reproduce at a continuous rate throughout the year. Since they grow continuously, biologists are able to determine the instantaneous growth rate, or intrinsic (per capita) growth rate, r. (r = b (births per capita) – d (deaths per capita)) Population growth rate is given by: dN = rN dt To find the time it takes a population that is reproducing exponentially to double, we use the equation: td = 0.69 r r is growth rate per capita and N is population size Instantaneous growth rate

Example Calculations A population of 2500 yeast cells in a culture tube is growing exponentially. If the intrinsic growth rate is 0.030 per hour, calculate: a) the initial instantaneous growth rate of the yeast population. b) the time it will take for the population to double in size. c) the population size after four doubling periods. a) r = 0.030 per hour and N = 2500 dN = rN = 0.030 x 2500 = 75 per hour dt When the population size is 2500 the instantaneous growth rate is 75 per hour. b) r = 0.030 td = 0.69 = 0.69 = 23 hours r 0.030 The yeast population will double in size every 23 hours.

c) After 4 doubling periods, the population of the yeast culture is 40 000 cells.

Logistic Growth The previous two models assume an unlimited resource supply, which is never the case in the real world. However, when a population is just starting out, resources are plentiful and the population grows rapidly. As the population grows, resources are being used up and the population nears the ecosystem's carrying capacity (K). The growth rate drops and a stable equilibrium exists between births and deaths. The population size is now at the carrying capacity (K). This is known as a sigmoidal curve. A: Population small, increasing slowly B: Population goes through largest increase C: Dynamic equilibrium (at carrying capacity), b=d, no net population increase

Logistic Growth, continued. Population size at given time Logistic growth represents the effect of carrying capacity on the growth of a population. It is the most common growth pattern in nature. dN = rmaxN (K – N) dt K Notice if the population size is close to the carrying capacity, there is virtually no growth (K-N = 0), thus the equation takes into account declining resources as the population increases. Population growth at a given time Carrying capacity Max intrinsic growth rate

Sample Problem A population of humans on a deserted island is growing continuously. The carrying capacity of that island is 1000 individuals and the maximum growth rate is 0.50. a) Determine the population growth rates over 5 years if the initial population size is 200. b) Describe the relationship between population size and growth rate. Answer a) b) When the population is small the rate of growth is slow. The rate of growth increases as the population gets larger and then, as it approaches carrying capacity, the growth rate declines and levels off.

Factors Affecting Population Change There are many things that can alter a population size. Density-independent factors limit population growth no matter what the population size (ex. natural disaster, human intervention, etc). Density-dependent factors limit population growth and intensify as the population increases in size (ex. competition for resources, disease, etc).

Density-dependent Factors Intraspecific competition is when individuals of the same species compete for resources. If this is high then the population will have a low growth rate. Predation is a density-dependent factor. If there is more prey available they will be chosen more by predators. Illness/disease spreads faster when a population has a higher density.

Allee Effect Warder Allee found that some density-dependent factors reduce population growth when the population is at a low density rather than high density. This is known as the Allee effect. For example, at times when a population has such a low density it is harder for individuals to find a mate and successfully reproduce thus lowering the growth rate of the species.

Small populations also may go through inbreeding depression which reduces the populations’ genetic variability and may prevent successful population growth. The minimum viable population size is the smallest number of individuals that ensures the population will persist for a certain period of time. It is different for different species. Allows biologists to determine whether a species is endangered.

Density-independent Factors The resource in the ecosystem that is in the shortest supply is known as the limiting factor since it is preventing massive population growth. Often times these are based on human influences on the ecosystem (ex. pollution, urban sprawl, etc.) but it could also be related to changes in climate (ex. a dry season that growth of plants for food) or natural disasters.

Population Ecology