Population Ecology

1. Population Ecology 52

2. Key Concepts • Life tables summarize how likely it is that individuals of each age class in a population will survive and reproduce. • The growth rate of a population can be calculated from life-table data or from the direct observation of changes in population size over time.

3. Key Concepts • Researchers observe a wide variety of patterns when they track changes in population size over time, ranging from growth rates that slow when populations are at high density, to regular cycles, to continued growth independent of population size. • Data from population ecology studies help biologists evaluate prospects for endangered species and design effective management strategies, as well as to predict changes in human populations.

4. Introduction • A population is a group of individuals from the same species that live in the same area at the same time. • Population ecology is the study of how and why the number of individuals in a population changes over time. • The mathematical and analytical tools used in population ecology help biologists predict changes in population size and design management strategies to save threatened species.

5. Demography • The number of individuals present in a population depends on four processes: birth, death, immigration, and emigration. • Populations grow due to birth and immigration, which occurs when individuals enter a population by moving from another population. • Populations decline due to deaths and emigration, which occurs when individuals leave a population to join another population. • Demography is the study of factors such as these that determine the size and structure of populations through time.

6. Demography • To make predictions about the future of a population, biologists need to know how many individuals of each age are alive, how likely individuals of different ages are likely to survive to the following year, and how many offspring are produced by females of different ages. • They also need to know how many individuals of different ages immigrate and emigrate each generation—the average time between a mother’s first offspring and her daughter’s first offspring.

7. Demography • If a population consists primarily of young individuals with a high survival rate and reproductive rate, the population size should increase over time. • On the other hand, if a population comprises chiefly old individuals with low reproductive rates and low survival rates, then it is almost certain to decline over time. • To understand a population’s dynamics, biologists turn to the data contained in a life table.

8. Life Tables • A life table summarizes the probability that an individual will survive and reproduce in any given time interval over the course of its lifetime. • The lizard Lacerta viviparais a common resident of open, grassy habitats in western Europe. Most populations give birth to live young. • Biologists were able to calculate the number of individuals that survived each year in each particular age group as well as how many offspring each female produced by monitoring a population daily for seven years.

11. Survivorship • Survivorship is a key component of a life table and is defined as the proportion of offspring produced that survive, on average, to a particular age. • To recognize general patterns in survivorship and make comparisons among populations or species, biologists create a graph called a survivorship curve. • The survivorship curve is a plot of the logarithm of the number of survivors versus age. • There are three general types of survivorship curves.

12. Survivorship • In a type I curve, survivorship throughout life is high, and most individuals approach the maximum life span of the species; humans show this type of survivorship curve. • In a type II curve, most individuals experience relatively constant survivorship over their lifetimes; songbirds have this curve. • Type III curves result from high death rates early in life, with high survivorship after maturity; many plants have type III curves.

16. Fecundity • Fecundity is also a key part of a life table; it is defined as the number of female offspring produced by each female in the population. • Age-specific fecundity is the average number of female offspring produced by a female in a given age class—a group of individuals of a specific age. • Data on survivorship and fecundity allow researchers to calculate the growth rate of a population.

17. The Role of Life History • In many species, key aspects of the life table vary dramatically among populations.

18. What Are Fitness Trade-Offs? • Fitnesstrade-offs occur because every individual has a restricted amount of time and energy at its disposal―its resources are limited. • For example, if a female devotes a great deal of energy to producing a large number of offspring, it is not possible for her to devote that same energy to her immune system, growth, nutrient stores, or other traits that increase survival. • A female can maximize fecundity, maximize survival, or strike a balance between the two.

20. Life History Is Based on Resource Allocation • An organism’s life historydescribes how an organism allocates its resources to growth, reproduction, and activities or structures related to survival. • Traits such as survivorship, age-specific fecundity, age at first reproduction, and growth rate are all aspects of an organism’s life history. • Understanding variation in life history is all about understanding fitness trade-offs.

21. Patterns across Species • Life-history traits form a continuum. • In general, organisms with high fecundity tend to grow quickly, reach sexual maturity at a young age, and produce many small eggs or seeds. • In contrast, organisms with high survivorship tend to grow slowly, and invest their energy and time in traits that reduce damage from enemies and increase their own ability to compete for resources.

23. Population Growth • The most fundamental questions that biologists ask about populations involve growth or decline in numbers of individuals. • For conservationists, analyzing and predicting changes in population size is fundamental to managing threatened species. • A population’s overall growth rate is a function of birthrates, death rates, immigration rates, and emigration rates.

24. Quantifying the Growth Rate • A population’s growth rate is the change in the number of individuals in the population (N) per unit time (t). • If no immigration or emigration is occurring: growth rate = N r • The per-capita rate of increase(r) is the difference between the birthrate and death rate per individual. r = b − d

25. Quantifying the Growth Rate • If the per-capita birthrate is greater than the per-capita death rate, then r is positive and the population is growing. If the per-capita death rate begins to exceed the per-capita birthrate, then r is negative and the population declines. • Within populations, r varies through time. Its value can be positive, negative, or 0.

26. Quantifying the Growth Rate • When birthrates per individual are as high as possible and death rates per individual are as low as possible, r reaches a maximum value called the intrinsic rate of increase, rmax. • When this occurs, the population's growth rate is expressed as: N/t = rmaxN

27. Quantifying the Growth Rate • Each species has a characteristic rmax that does not change. But at any specific time, the per-capita rate of increase of each population of that species is likely to be much lower than rmax. • A population’s r is also likely to be different from r values of other populations of the same species and to change over time.

28. Exponential Growth • Exponential population growth occurs when r does not change over time. It does not depend on the number of individuals in the population—it is density independent. • In nature, exponential growth is observed in two circumstances: • A few individuals found a new population in a new habitat. • A population has been devastated by a storm or some other type of catastrophe and then begins to recover, starting with a few surviving individuals.

30. Exponential Growth • Exponential growth cannot continue indefinitely. • When population density—the number of individuals per unit area—gets very high, the population’s per-capita birthrate should decrease and the per-capita death rate increase, causing r to decline. • This type of growth is density dependent.

31. Logistic Growth • Carrying capacity, K, is the maximum number of individuals in a population that can be supported in a particular habitat over a sustained period of time. K can change depending on conditions. • The carrying capacity of a habitat depends on a large number of factors: food, space, water, soil quality, and resting or nesting sites. Carrying capacity can change from year to year, depending on conditions.

32. A Logistic Growth Equation • If a population of size N is below the carrying capacity K, the population should continue to grow. Specifically, a population’s growth rate should be proportional to (K – N)/K: N/t = rmaxN((K – N)/K) • This expression is called the logistic growth equation.

33. A Logistic Growth Equation • The logistic growth equation describes logistic population growth—a change in growth rate that occurs as a function of population size. • Logistic growth is density dependent. As a population approaches a habitat’s carrying capacity, its growth rate should slow.

34. Graphing Logistic Growth • In a hypothetical population, density-dependent growth has three distinct stages: • Initially, growth is exponential (r is constant). • Growth rate begins to decline (N increases) when competition for density-dependent factors begins to occur. • Growth rate reaches 0 at the carrying capacity (N vs. t is flat).

38. What Limits Growth Rates and Population Sizes? • Population sizes change as a result of density-independent and density-dependent factors. • Density-independent factors, such as variation in weather patterns, are usually abiotic; they change birthrates and death rates irrespective of population size. • Density-dependent factors, such as increased predation when a deer population increases, are usually biotic; they change in intensity as a function of population size. • Density-dependent changes in survivorship and fecundity cause logistic population growth.

39. A Closer Look at Density Dependence • An experimental study of a coral-reef fish called the bridled goby showed a strong density-dependent relationship in survivorship. • Likewise, a long-term study of song sparrows on Mandarte Island, British Columbia, showed a strong density-dependent relationship in fecundity. • Density-dependent changes in survivorship and fecundity cause logistic population growth.

43. Carrying Capacity Is Not Fixed • K varies among species and populations. • K varies because for any particular species, some habitats are better than other habitats due to differences in food availability, space, and other density-dependent factors. Stated another way, K varies in space. • It also varies with time, as conditions in some years are better than in others. • In addition, the same habitat may have a very different carrying capacity for different species.

44. Modeling Population Growth Web Activity: Modeling Population Growth

45. Population Dynamics • Population dynamics are the changes in populations through time. • Research on population dynamics has uncovered a wide array of patterns in natural populations, in addition to exponential and logistic growth.

46. How Do Metapopulations Change through Time? • If individuals from a species occupy many small patches of habitat so that they form many independent populations, they represent metapopulations―a population of populations. • Because humans are reducing large, contiguous areas of forest and grasslands to isolated patches or reserves, more and more species are being forced into a metapopulation structure. • Glanville fritillaries—an endangered species of butterfly native to the Åland islands off the coast of Finland—exist naturally as metapopulations.

48. Metapopulations Should Be Dynamic • Ilkka Hanski and colleagues determined the number of Glanville fritillary breeding pairs in each patch within a metapopulation. • Over time, each population within the larger metapopulation is expected to go extinct due to any number of potential causes. • However, migration can reestablish populations in empty patches. • There is thus a balance between extinction and recolonization within a metapopulation. Subpopulations may blink on and off over time, but the overall population is maintained at a stable number of individuals.

50. Metapopulations Should Be Dynamic • Experimental studies supported the dynamic nature of metapopulations in Glanville fritillaries; the overall population size was relatively stable even though constituent populations came and went. • To summarize, the history and future of a metapopulation is driven by the birth and death of populations, just as the dynamics of a single population are driven by the birth and death of individuals.