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Chapter 52: Population ecology. birth, death, immigration, and emigration. Populations grow due to birth and immigration , which occurs when individuals enter a population by moving from another population.
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Chapter 52: Population ecology • 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.
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 construct a life table for this species
Survivorship • Survivorship is 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.
Survivorship • In a type I curve, survivorship throughout life is high, and most individuals approach the maximum life span of the species (e.g. humans ) • 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.
Fecundity • Fecundity is 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.
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.
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.
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.
Exponential Growth • Exponential growth cannot continue indefinitely. (Environments have finite resources) • 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.
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.
Predicting pop size: 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 (density dependent).
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 decrease (N increases) when competition for density-dependent factors begins to occur. • Growth rate reaches 0 at the carrying capacity (N vs. t is flat).
What Limits Growth Rates and Population Sizes? • Population sizes change as a result of density-independent and density-dependent factors. • Density-independent factors are usually abiotic; they change birthrates and death rates irrespective of population size. • Density-dependent factors are usually biotic; they change in intensity as a function of population size.
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. • migration (rescue effect) • 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.
Age Structure in Human Populations • The age structure of human populations in different countries, which varies dramatically, can be represented by age pyramids—graphs with horizontal bars representing the numbers of males and females of each age group. • The age structure of a population tends to be uniform in developed countries and bottom-heavy in developing countries. • Analyzing an age pyramid can give biologists important information about a population’s history, and also help them predict a population’s future.
Analyzing Change in Human Population Growth Rate • The rate of human population growth has increased over the past 250 years, leading to a very steeply rising curve over the past few decades. • It is almost impossible to overemphasize just how dramatically the human population has grown recently.
Ch 53 Introduction, community ecology • A biological community consists of interacting species, usually living within a defined area. • Biologists want to know how communities work, and how to manage them in a way that will preserve species and create an environment that people want to live in.
Species Interactions • There are four general types of interactions among species in a community: • Competition occurs when individuals use the same resources—resulting in lower fitness for both (/). • Consumption occurs when one organism eats or absorbs nutrients from another, increasing the consumer’s fitness but decreasing the victim’s fitness (+/). • Mutualism occurs when two species interact in a way that confers fitness benefits to both (+/+). • Commensalism occurs when one species benefits but the other species is unaffected (+/0).
Three Themes • As you analyze each type of species interaction, watch for three key themes: • Species interactions may affect the distribution and abundance of a particular species. • Species act as agents of natural selection when they interact. In biology, a coevolutionary arms race occurs between predators and prey, between parasites and hosts, and between other types of interacting species. • The outcome of interactions among species is dynamic
Competition • Competition is a –/– interaction that lowers the fitness of the individuals involved. When competitors use resources, those resources are not available to help individuals survive better and produce more offspring. • Intraspecific competition occurs between members of the same species. • Because intraspecific competition for resources intensifies as a population’s density increases, it is a major cause of density-dependent growth. • Interspecific competition occurs when members of different species use the same limiting resources.
Using the Niche Concept to Analyze Competition • Early work on interspecific competition focused on the concept of the niche—the range of resources that the species is able to use or the range of conditions it can tolerate. • Interspecific competition occurs when the niches of two species overlap.
When One Species Is a Better Competitor • The competitive exclusion principle, formulated by G. F. Gause, states that it is not possible for species within the same niche to coexist. • The hypothesis was inspired by a series of experiments Gause did with similar species of the unicellular pond-dweller Paramecium. • Grown in separate cultures, both species exhibited logistic growth. • When the two species grew in the same culture together, only one species exhibited logistic growth; the other species was driven to extinction.
When One Species Is a Better Competitor • Asymmetric competition occurs when one species suffers a much greater fitness decline than the other. • In symmetric competition, each species experiences a roughly equal decrease in fitness. • If asymmetric competition occurs and the two species have completely overlapping niches, the stronger competitor is likely to drive the weaker competitor to extinction.
When One Species Is a Better Competitor • Gause’s experiments illuminated an important distinction: • A species’ fundamental niche is the resources it uses or conditions it tolerates in the absence of competitors. • A species’ realized niche is the resources it uses or conditions it tolerates when competition occurs. • If asymmetric competition occurs and the niches of the two species do not overlap completely, the weaker competitor will move from its fundamental niche to a realized niche, ceding some resources to the stronger competitor.
Mechanisms of Coexistence: Niche Differentiation • Because competition is a –/– interaction, there is strong natural selection on both species to avoid it. • The predicted eventual outcome is an evolutionary change in traits that reduces the amount of niche overlap and the amount of competition. • This change in resource use is called niche differentiation or resource partitioning. • The change in species’ traits is called character displacement.
Competition and Conservation • One of the goals of conservation biology is to keep biological communities intact. • One of the major threats to communities is invasive species. • Recent experiments have shown that communities that contain a large number of different species are more resistant to invasion than communities with a smaller number of species.
Consumption • Consumption is a +/– interaction that occurs when one organism eats another. • There are three major types of consumption: • Herbivory is the consumption of plant tissues by herbivores. • Parasitism is the consumption of small amounts of tissues from another organism, or host, by a parasite. • Predation is the killing and consumption of most or all of another individual (the prey) by a predator.
Constitutive Defenses • Some of the best-studied constitutive defenses involve mimicry—the close resemblance of one species to another. • There are two forms of mimicry: • Müllerian mimicry is the resemblance of two harmful prey species. • Batesian mimicry is the resemblance of an innocuous prey species to a dangerous prey species.
How Do Keystone Species Structure Communities? • Even though species are not predictable assemblages, the structure of a community can change dramatically if a single species of predator or herbivore is removed from or added to a community. • A keystone species is a species that has a much greater impact on the surrounding species than its abundance would suggest. • For example, the sea star Pisaster is a keystone species in some intertidal areas. When Pisaster was removed from experimental areas, the number of species present and the complexity of the habitat changed radically.
Succession • Succession is the recovery, the development of communities, that follows a severe disturbance. • Primary succession occurs when a disturbance removes the soil and its organisms, as well as organisms that live above the surface. • Secondary succession occurs when a disturbance removes some or all of the organisms from an area but leaves the soil intact.
Succession • Early successional communities are dominated by species that are short lived and small in stature, and that disperse their seeds over long distances. • Late successional communities are dominated by species that tend to be long lived, large, and good competitors for resources such as light and nutrients. • The specific sequence of species that appears over time is called the successional pathway.
Species Richness in Ecological Communities • Species richness is the number of species present in a given community. • Species diversity is a weighted measure that incorporates a species’ relative abundance, as well as its presence or absence.
The Role of Island Size and Isolation • MacArthur and Wilson formulated the model called the theory of island biogeography. • Their theory makes two predictions—species richness should be higher on: • Larger islands than smaller islands. • Nearshore islands versus remote islands.
Ch 54: Introduction to ecosystem ecology • An ecosystem consists of the multiple communities of organisms that live in an area along with abiotic components such as the soil, climate, water, and atmosphere. • The biotic and abiotic components of an ecosystem are linked by flows of energy and nutrients.
Trophic Cascades and Top-Down Control • When a consumerlimits a prey population, biologists say that top-down control is occurring. • A trophic cascade occurs when changes in top-down control cause conspicuous effects two or three links away in a food web. • For example, the reintroduction of wolves in Yellowstone National Park has led to far-reaching changes in the food web.
