23.1. Scope of Ecology (p. 384)
A. Ecology
1. The study of interactions of organisms with their environment.
2. Concept of ecology was first voiced by German zoologist Ernst Haeckel.
3. Modern ecology studies how environmental factors determine the distribution and abundance of populations.
4. Ecology and evolution are intertwined because natural selection has long-term effects.
5. A population is a group of the same species occupying a certain area. (Fig. 23.1a)
6. A community consists of all populations at one locale (e.g., a coral reef population, etc.).
7. An ecosystem contains the community organisms and abiotic factors (e.g., energy flow, chemical cycling).
8. Modern ecology is both descriptive and predictive, with application to wildlife management, agriculture, etc.
B. Density and Distribution of Populations
1. Density of organisms refers to how many live per unit of area.
2. Population distribution of organisms can vary from uniform to random to clumped. (Fig. 23.2)
3. Ecologists study the causes for "patchiness" of organisms across space and through time.
4. Environment includes both biotic (living) and abiotic (physical) factors.
5. Physical (abiotic) factors include types of precipitation and amounts, averages, and daily and seasonal variations in temperature; type of soil (sand, clay or loam) and few nutrients, moisture or temperature may serve as limiting factors.
6. Biotic factors are illustrated by red kangaroos limited to inland Australia by their food source.
23.2. Characteristics of Populations (p. 386)
A. Population Size
1. Population size is number of individuals contributing to gene pool of the population.
2. At any one point in time, populations have a certain size.
3. Future population size depends on births and deaths, immigration and emigration (often immigration and emigration are presumed equal).
4. Birthrate and death rate are used to calculate the net reproductive rate.
5. Net reproductive rate is used to calculate the growth and size of a population per unit time.
B. Patterns of Population Growth (Fig. 23.3)
1. There are two general patterns: organisms that reproduce once cease to grow as adults and expend energy in reproduction and die, and organisms that reproduce through their lifetime, which invests energy in future survival.
2. Most organisms do not exactly fit these two patterns.
a. Slime molds produce spores that swarm together into a sticky mass.
b. Many plants can reproduce both by seeds and vegetative extensions.
c. Aphids can switch between sexual and asexual reproduction according to weather.
a. Soil seed banks allow the next generation to come from many previous years' seeds.
C. Exponential Growth (Fig. 23.4)
1. The J-shaped exponential growth curve has two phases.
a. Lag phase (growth is slow because population is small).
b. Exponential growth phase (growth is accelerating).
2. A mathematical equation calculates exponential growth and size for any population that has discrete generations. (Fig. 23.4c)
3. Biotic potential (Fig. 23.6)
a. Exhibited during esponential growth, this is the maximum population growth under ideal circumstances.
b. Includes plenty of room for each member, unlimited resources (e.g., food, water), and no hinderances (e.g., predators).
4. Environmental resistance curbs exponential growth; includes all environmental factors that limit population size.
D. Logistic Growth (Fig. 23.5)
1. Organisms with repeated reproductive events experience an S-shaped or logistic growth curve.
2. Pearl (1930) estimated growth in a yeast and arrived at a graph and formula for logistic growth.
(Fig. 23.3b,c)
3. In addition to the lag phase and exponential growth, there is a deceleration phase where rate of population growth slows down and a stable equilibrium phase with little if any growth because births equal deaths.
4. Curve is called logistic because exponential portion of curve would plot as a straight line as log of N.
5. A mathematical equation calculates logistics growth (Fig. 23.5c)
6. Environmental resistance results in the deceleration phase and the stable equilibrium phase; population is at carrying capacity.
E. Carrying Capacity
1. Carrying capacity (K) is maximum size population that can be supported by environment year after year.
2. When N is small, a large portion of the carrying capacity has not been utilized, but as N approaches K,
population growth slows down because
3. Examples: overfishing drives a population into the lag phase; it is best to maintain populations in a growth phase; and reducing crop pests places them in exponential phase again.
F. Mortality Patterns
1. A life table shows how many members of a cohort (group born at one time) are surviving at different ages.
2. Survivorship is the percentage of remaining survivors of a population over time; usually shown graphically. (Fig. 23.7) [transp. 126]
a. Type I survivorship curve: most individuals live out their life span and die of old age (e.g., humans).
b. Type II survivorship curve: individuals die at a constant rate (e.g., birds, rodents, and perennial plants).
c. Type III survivorship curve: most individuals die early in life (e.g., fishes, invertebrates, and plants).
4. The grass Poa annua is intermediate; most survive till 6-9 months and then chances of surviving diminish.
G. Age Distribution (Fig. 23.8) [transp. 127]
1. There are three major age groups in a population: prereproductive, reproductive and postreproductive.
2. An age structure diagram is a representation of the number of individuals in each age group in a population.
3. A pyramid-shape indicates the population has high birthrates; population is undergoing exponential growth.
4. A bell-shape indicates that prereproductive and reproductive age groups are more nearly equal, with the postreproductive group being smallest due to mortality; this is characteristic of stable populations.
5. An urn-shaped diagram indicates the postreproductive group is largest and the prereproductive group is smallest, a result of the birthrate falling below the death rate; this is characteristic of declining populations.
A. The J-shaped and S-shaped growth curve models do not always predict real populations.
1. In the winter moth (pages 394-395), many eggs did not survive winter and exponential growth did not occur.
2. Growth curve of reindeer herd introduced to St. Paul Island, Alaska, overshoots carrying capacity. (Fig. 23.9)
B. Populations do not increase in size year after year because environmental resistance, including both density-independent and density-dependent factors, regulates the number of organisms.
1. Some populations were considered to be regulated primarily by density-independent factors.
a. The number of organisms present does not affect the influence of the factor.
b. The damage to a population from an accidental fire does not change with the number of organisms present.
c. Density-independent factors show no correlation with the size of the population.
2. Populations regulated by density-dependent factors are affected by the number of organisms present.
a. Predation, parasitism, competition are considered density-dependent; the more organisms crowd together, the more damaging are food shortages, parasites, and predators.
b. Density-dependent factors have some effect relative to the size of the population.
3. Ecologists state that most important regulators seem to be weather, food, other animals, pathogens, habitat.
4. While the above factors are all external, it is also likely that internal factors influence population size.
5. Recruitment, immigration, and emigration are other means by which complex organisms limit densities.
6. New theories on chaos help us understand severe fluctuations over time.
23.4. Life History Patterns (p. 396)
23.5. Human Population Growth (p. 399)
A. The Human Population Is Growing (Fig. 23.14) [transp. 128]
1. The human population is now in an exponential part of a J-shaped growth curve.
2. World population increases equivalent of one medium-sized city (200,000) per day and 88 million per year.
3. Growth rate is the difference between birthrate and death rate per 1,000 persons.
4. Doubling time is the length of time for population size to double, now 47 years.
5. Zero population growth is when birthrate equals deathrate and population size remains steady.
B. More-Developed versus Less-Developed Countries (Fig. 23.16) [transp. 129]
1. More developed countries underwent demographic transition 1950-1975; their growth rate is now 0.6%.
a. More developed countries (MDCs) were first industrialized (e.g., Europe, North America, Japan, etc.).
b. Demographic transition is decline in death rate followed by declining birthrate; results in slower growth.
2. Less developed countries (LDCs) are now undergoing demographic transition.
a. Less developed countries (LDCs) are fully industrialized (e.g., countries in Africa, Asia, Latin America).
b. LDC growth rate peaked at 2.5% between 1960-1965; it is declining slowly to about 1.8% at year 2000.
C. Comparing Age Distributions (Fig. 23.15) [transp. 130]
1. Replacement reproduction will cause population growth to continue due to the age structure of the population.
2. Mere replacement does not produce zero population growth because more women enter reproductive years than leave them.
3. The MDCs have a low growth rate because of a stabilized age structure.
4. The LDCs have a higher growth rate because of a youthful age structure.
D. A Sustainable World
1. The decision is not between preservation of ecosystems or human survival.
2. Both the growing populations of the LDCs and the high consumption of the MDCs stress the environment.
3. An average American family, in terms of consumption and waste production, is equal to thirty people in India.
4. Borrowed carrying capacity is used to describe how cities borrow resources from the rural areas.
5. Overpopulation and overconsumption contribute to pollution and extinction of species.
6. Sustainable practices include logging by draft horses, etc.