51Introduction to Ecology: Population Ecology
Lecture Outline
- Populations possess characteristic features
- Population ecology includes the study of the numbers of individuals of a particular species at a given time and location, and the interactions of that population with other populations and the environment
- Density and dispersion are important features of populations
- Population size, expressed in terms of density, is most useful
a)Density is the number of individuals of a species per unit space
- Dispersion describes the spacing of individuals
a)Uniform dispersion occurs when individuals are more evenly distributed than would be expected randomly
(1)Uniform dispersion is often a result of agonistic behavior such as territoriality
b)Random dispersion results from lack of interaction between individuals or a homogeneous environment
c)Clumped dispersion is the most common type of distribution, and results from patchy distribution of resources or by asexual reproduction
(1)Clumped distribution may include social behavior in animals
- Mathematical models describe population growth
- Population numbers change due to natality and mortality
- ∆N / ∆t = b – d
a)∆N is the change in numbers in the population
b)∆t is the change in time
c)b is the natality rate
d)d is the mortality
- r = b – d
a)r is the growth rate of the population
b)If r = 0, the population is stable; if r > 1, the population is increasing in size; if r < 1, the population is declining
- These equations can be expressed as dN / dt = rN
- Dispersal affects the growth rate in some populations
- Dispersal is the movement of individuals among populations
- Immigration (i) is the migration of individuals into the population
- Emigration (e) is the migration of individuals out of the population
- r = (b – d) + (i – e)
- Each population has a characteristic intrinsic rate of increase
- The intrinsic rate of increase is the maximum rate of increase under ideal conditions for that population
- Life history characteristics determine the biotic potential for a species
- Typically, larger animals have a greater biotic potential
- No population can increase exponentially indefinitely
- Exponential growth may exist for a period of time until limiting factors become important
- Environmental resistance prevents continued exponential growth
a)As N increases, environmental resistance increases
- The carrying capacity is the largest population that can be sustained for an indefinite period of time
- Growth regulated by environmental resistance shows an S shaped growth curve
- This growth may be described by dN / dt = rN [(K – N) / K]; the logistic equation
a)When N is small, the expression [(K – N) / K] has a value of almost 1 and minimally limits growth
b)When N is large, the expression [(K – N) / K] has a value that approaches 0 and greatly limits growth
- Populations do not always approach and stabilize at K, but may overshoot K, followed by a population crash
- Factors influencing population size
- Density-dependent factors regulate population size
- Density-dependent factors are those that increase in proportional effect with increasing population size
a)Predation and disease are examples of density-dependent factors
- Competition is an important density-dependent factor
a)Intraspecific competition exists between members of the same species
(1)In scramble competition, members of the population share the resource equally
(2)In contest competition, some members of the population obtain a "lion’s share" of the resource
b)Interspecific competition exists between members of different species
- The effects of different density-dependent factors are difficult to assess in nature
a)Many populations are regulated by multiple density-dependent factors
- Density-independent factors limit population size
- Environmental factors that affect population regardless of size are density-independent factors
- Many climatic factors are density-independent
- Most density-independent factors have some relationship to population density
- Life history traits
- Species that expend their energy in a single, immense reproductive effort are semelparous; iteroparous species exhibit repeated reproductive cycles
- Based on life history strategies, organisms may be classified as r-strategists and K-strategists
- r-strategists have a high r, small size, large numbers of offspring, and may live in unpredictable habitats
- Many weeds or pest species are r-strategists
- Many r-strategists reproduce asexually
- K-strategists have a population size near K, are larger in size, have fewer offspring, and may care for their young
- Many endangered species are K-strategists
- K-strategists typically live in stable environments
- Survivorship is related to r and K selection
- Survivorship curves may be classified as type I, type II, or type III
a)Humans exhibit type I survivorship, which is typical of K-selected organisms
(1)Type I curves are characterized by high mortality late in life
b)Some birds and lizards have a type II survivorship, characterized by constant mortality over time
c)Type III survivorship is characterized by high mortality early in life, as seen in r-selected organisms
- Metapopulations
- Many species are not distributed as one large population across the landscape
- Many species exist as a series of local populations distributed in distinct habitat patches
- Such a population is called a metapopulation
- Good habitats are called sources and are areas where local reproductive success is greater than local mortality
- Sinks are areas where local reproductive success is less than local mortality
- Human populations
- Thomas Malthus recognized that the human population cannot continue in exponential growth indefinitely
- Human population growth has been marked by a large decrease in the death rate
- Human population size will increase to unknown proportions due to uncertainty of the carrying capacity for our species
- Not all countries have the same growth rate
- Demographics describe population statistics
- The growth rate is higher in developing countries, which have a lower degree of industrialization and prosperity than the developed countries
a)Moderately developed countries include most countries of South America, as well as Mexico and Turkey
b)Less developed countries have the highest growth rates, and include many African and Southeast Asian countries
- The doubling time is the time needed for a population to double in size
a)Doubling time is less than 20 years in some developing countries, and hundreds of years in some developed countries
- Replacement-level fertility is the number of children a couple must have to replace themselves
a)Replacement-level fertility is typically 2.1 in developed countries, and 2.7 children in developing countries
- The total fertility rate is the average number of children born to a woman during her lifetime
- The age structure of a country can be used to predict future population growth
- Age structure shows the percentages of the population at different ages
- A broader base to the age structure indicates a growing population; a narrower base indicates a stable or shrinking population
- Environmental degradation is related to population growth and resource consumption
- Developed nations require more resources per capita and cause more environmental problems than developing nations
- People overpopulation is a result of too many people living in a certain area; consumption overpopulation is a result of people over-utilizing resources
Research and Discussion Topics
- One of the major concerns of biologists who concentrate on preserving tropical diversity is the continued growth of the populations of these tropical countries. Look up the growth rates of several tropical countries, and describe the potential effects of this growth on deforestation. Consider the impact of increasing populations in these developing countries on deforestation, as well as the exploitation by developed countries.
- Are conservation biologists typically concerned with r- or K- selected species? If you were in charge of attempts to repopulate an endangered species, what characteristics of the life history of that organism might hamper your efforts?