Chapter Population Ecology Population an interbreeding group of

Chapter- Population Ecology

Population- an interbreeding group of individuals of a single species that occupy the same general area Community-the assemblage of interacting populations that inhabit the same area. Ecosystem- comprised of 1 or more communities and the abiotic environment within an area.

Major Characteristics of a Population • Population dynamics is the study of how populations change in: – Size – ( total # of individuals) – Density ( # individuals in a certain space) – Age distribution ( the proportion of individuals in each age in a population) in response to changes in environmental conditions.

The characteristics of populations are shaped by the interactions between individuals and their environment • Populations have size and geographical boundaries. – The density of a population is measured as the number of individuals per unit area. – The dispersion of a population is the pattern of spacing among individuals within the geographic boundaries. Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

MEASURING DENSITY Density – Number of individuals per unit of area. • Determination of Density • Counting Individuals • Estimates By Indirect Indicators • Mark-recapture Method N = (Number Marked) X (Catch Second Time) Number Of Marked Recaptures

• Measuring density of populations is a difficult task. – We can count individuals; we can estimate population numbers. Fig. 52. 1 Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

PATTERN OF DISPERSION UNIFORM CLUMPED RANDOM

• Patterns of dispersion. – Within a population’s geographic range, local densities may vary considerably. – Different dispersion patterns result within the range. – Overall, dispersion depends on resource distribution. Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Clumped Dispersion • Resources a species needs varies greatly place to place • Living in herds/flocks for protection • Hunting packs increases survival of gaining food • Temporary groups for mating Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Uniformed Uniform Dispersion • Species maintain consistent distance between individuals • Access to scarce sources and less competition

Random Dispersion • Organisms with random distribution is rare. Even within forest you find clumps of species, or vegetation around light sources, water , or another resource. Fig. 52. 2 c Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Demography is the study of factors that affect the growth and decline of populations • Additions occur through birth, and subtractions occur through death. – Demography studies the vital statistics that affect population size. • Life tables and survivorship curves. – A life table is an age-specific summary of the survival pattern of a population. Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Population Dynamics • Characteristics of Dynamics • Size • Density • Dispersal • Immigration • Emigration • Births • Deaths • Survivorship

Parameters that effect size or density of a population: Immigration Birth Population (N) Death Emigration Figure 1. The size of a population is determined by a balance between births, immigration, deaths and emigration

Age Structure: the proportion of individuals in each age class of a population Figure 2. Age pyramid. Notice that it is split into two halves for male and female members of the population.

• The best way to construct life table is to follow a cohort, a group of individuals of the same age throughout their lifetime. Table 52. 1 Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Survivorship Curves

–A graphic way of representing the data is a survivorship curve. • This is a plot of the number of individuals in a cohort still alive at each age. –A Type I curve shows a low death rate early in life (humans). –The Type II curve shows constant mortality (squirrels). –Type III curve shows a high death rate early in life (oysters).

Survivorship Curve

• Reproductive rates. – Demographers that study populations usually ignore males, and focus on females because only females give birth to offspring. – A reproductive table is an age-specific summary of the reproductive rates in a population. • For sexual species, the table tallies the number of female offspring produced by each age group. Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Reproductive Table 52. 2 Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Life History • The traits that affect an organism’s schedule of reproduction and survival make up its life history. Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Life histories are very diverse, but they exhibit patterns in their variability • Life histories are a result of natural selection, and often parallel environmental factors. • Some organisms, such as the agave plant, exhibit what is known as big-bang reproduction, where large numbers of offspring are produced in each reproduction, after which the individual often dies. • This is also known as semelparity Agaves

• By contrast, some organisms produce only a few eggs during repeated reproductive episodes. – This is also known as iteroparity. • What factors contribute to the evolution of semelparity and iteroparity? Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Limited resources mandate trade-offs between investments in reproduction and survival • The life-histories represent an evolutionary resolution of several conflicting demands. – Sometimes we see trade-offs between survival and reproduction when resources are limited. Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

• For example, red deer show a higher mortality rate in winters following reproductive episodes. Fig. 52. 5 Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

• Variations also occur in seed crop size in plants. – The number of offspring produced at each reproductive episode exhibits a trade-off between number and quality of offspring. dandelion Coconut palm

The exponential model of population describes an idealized population in an unlimited environment • We define a change in population size based on the following verbal equation. Change in population = size during time interval Births during time interval Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings – Deaths during time interval

• Using mathematical notation we can express this relationship as follows: – If N represents population size, and t represents time, then N is the change is population size and t represents the change in time, then: • N/ t = B-D • Where B is the number of births and D is the number of deaths Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

– We can simplify the equation and use r to represent the difference in per capita birth and death rates. • N/ t = r. N OR d. N/dt = r. N – If B = D then there is zero population growth (ZPG). – Under ideal conditions, a population grows rapidly. • Exponential population growth is said to be happening • Under these conditions, we may assume the maximum growth rate for the population (rmax) to give us the following exponential growth • d. N/dt = rmax. N

Fig. 52. 9 Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

The logistic model of population growth incorporates the concept of carrying capacity • Typically, unlimited resources are rare. – Population growth is therefore regulated by carrying capacity (K), which is the maximum stable population size a particular environment can support. Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Example of Exponential Growth Kruger National Park, South Africa

POPULATION GROWTH RATE LOGISTIC GROWTH RATE Assumes that the rate of population growth slows as the population size approaches carrying capacity, leveling to a constant level. S-shaped curve CARRYING CAPACITY The maximum sustainable population a particular environment can support over a long period of time.

Figure 52. 11 Population growth predicted by the logistic model

• How well does the logistic model fit the growth of real populations? – The growth of laboratory populations of some animals fits the S-shaped curves fairly well. Stable population Seasonal increase

– Some of the assumptions built into the logistic model do not apply to all populations. • It is a model which provides a basis from which we can compare real populations. Severe Environmental Impact

• The logistic population growth model and life histories. – This model predicts different growth rates for different populations, relative to carrying capacity. • Resource availability depends on the situation. • The life history traits that natural selection favors may vary with population density and environmental conditions. • In K-selection, organisms live and reproduce around K, and are sensitive to population density. • In r-selection, organisms exhibit high rates of reproduction and occur in variable environments in which population densities fluctuate well below K. Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Species Reproductive Patterns • r-Selected species, opportunists – species with a capacity for a high rate of population increase – Many small offspring – Little to no parental care or protection – Reproductive opportunists • K-selected species, competitors – reproduce later in life and have a small number of offspring with fairly long life spans – Few large offspring – High parental care

K-Selected Species • • • Poor colonizers Slow maturity Long-lived Low fecundity (reproductive rate) High investment in care for the young • Specialist • Good competitors

r-Selected Species • • • Good colonizers Reach sexual maturity rapidly Short-lived High fecundity Low investment in care for the young • Generalists • Poor competitors

Positions of r- and K-Selected Species on the S-Shaped Population Growth Curve

Transitioning between J and S curves… • Carry capacity isn’t fixed – Varies depending on climate and season – Unpredictable changes can be devastating to the species AND the habitat • Reproductive time lag – period needed for the birth rate to fall and the death rate to rise in response to resource overconsumption – May lead to overshoot – Dieback (crash)

Types of Population Change • S- Stable – population fluctuates slightly above and below its carrying capacity – Characteristic of undisturbed rain forests – Late loss curve • R- Irruptive – short-lived rapidly reproducing species – Linked to seasonal changes in weather or nutrient availability – Algal Blooms – Early loss curves

r-Curve Fluctuations

S-Curve Fluctuations

Types of Population Change • Cyclic fluctuations, boom-and-bust cycles – Top-down population regulation • Controlled by predation – Bottom-up population regulation • Controlled by scarcity of one or more resources • Irregular – changes in population size with no recurring pattern – chaos

Introduction • Why do all populations eventually stop growing? • What environmental factors stop a population from growing? • The first step to answering these questions is to examine the effects of increased population density. Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Density-Dependent Factors • • • limiting resources (e. g. , food & shelter) production of toxic wastes infectious diseases predation stress emigration

Density-Independent Factors • • severe storms and flooding sudden unpredictable severe cold spells earthquakes and volcanoes catastrophic meteorite impacts

• Density-dependent factors increase their affect on a population as population density increases. – This is a type of negative feedback. • Density-independent factors are unrelated to population density, and there is no feedback to slow population growth. Fig. 52. 13 Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Negative feedback prevents unlimited population growth • A variety of factors can cause negative feedback. – Resource limitation in crowded populations can stop population growth by reducing reproduction.

• Intraspecific competition for food can also cause density-dependent behavior of populations. – Territoriality. – Predation.

– Waste accumulation is another component that can regulate population size. • In wine, as yeast populations increase, they make more alcohol during fermentation. • However, yeast can only withstand an alcohol percentage of approximately 13% before they begin to die. – Disease can also regulate population growth, because it spreads more rapidly in dense populations. Copyright © 2002 Pearson Education, Inc. , publishing as Benjamin Cummings

Population dynamics reflect a complex interaction of biotic and abiotic influences • Carrying capacity can vary. • Year-to-year data can be helpful in analyzing population growth.