34.1 The Scope of Ecology
Ecology is the study of the interactions of organisms with each other and with their physical environment. A population is defined as all the organisms of the same species interacting with the environment in a particular area. A community consists of all the various populations at a particular locale. An ecosystem encompasses a community of populations, as well as the nonliving environment. The biosphere is the portion of the entire Earth’s surface-air, water, land- where living organisms exist.
34.2 Patterns of Population Growth
Each population has a particular pattern of growth. The population size can change according to a per capita rate of increase or growth rate. The highest possible per capita rate of increase for a population is called its biotic potential . An exponential pattern of population growth results in a
J-shaped curve with a lag phase and an exponential growth phase. Environmental resistance encompasses all the environmental conditions that prevent populations from achieving their biotic potential; it eventually causes the population growth to level off, resulting in an S-shaped pattern called logistic growth . An S -shaped pattern has a lag phase, an exponential growth phase, a logistic growth phase, and a stable equilibrium phase. The stable equilibrium phase is said to occur at the carrying capacity of the environment.
Survivorship
A cohort is a group of individuals born at the same time, which can be plotted on a survivorship curve. Three types of idealized survivorship curves are recognized: type I in which most individuals survive well past the midpoint, type II in which survivorship decreases at a constant rate throughout the life span, and type III in which most individuals die very young.
Human Population Growth
Growth in less-developed countries is still in the exponential phase while growth in more-developed countries has leveled off. The rapid growth of the human population can be appreciated by considering the doubling time , the length of time it takes for the population size to double. Currently, the doubling time for the human population is estimated to be between 50 and 60 years.
More-Developed versus Less-Developed Countries
The more-developed countries are those in which population growth is low and people enjoy a good standard of living. The less-developed countries are those in which population growth is expanding rapidly and the majority of people live in poverty.
Age Distributions
An age-structure diagram divides the population into three age groups: prereproductive, reproductive, and postreproductive.
34.3 Regulation of Population Growth
R-strategist species have populations that are small in size, mature early, and have a short life span, exhibiting an opportunistic pattern. K-strategist species tend to be fairly large, are slow to mature, and have a fairly long life span, following an equilibrium pattern. Abiotic factors, such as weather and natural diseases, are density-independent factors , meaning that the effects are the same for all sizes of populations. Biotic factors, such as competition, predation, and parasitism, are called density-dependent factors .
Competition
Competition occurs when members of different species try to utilize a resource that is in limited supply. The ecological niche is the role an organism plays in the community, including its habitat . Competition may result in resource partitioning .
Predation
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Predation prey .
occurs when one organism, called the
Predator-Prey Population Dynamics
Predators reduce the population density of prey. Mathematical formulas predict that predator and prey populations tend to cycle instead of maintaining a steady state.
Antipredator Defenses
Predators have evolved strategies to secure the maximum amount of food with minimal expenditure of energy and prey organisms have evolved strategies to escape predation. Coevolution is present when two species adapt in response to selective pressure imposed by the other.
Mimicry
Mimicry occurs when one species resembles another species that has evolved to defend against predators or resembles an object in the
Symbiosis environment.
Symbiosis refers to close interactions between members of different species. Three types of symbiotic relationships have traditionally been defined—parasitism, commensalism, and mutualism.
Parasitism
Parasitism
Commensalism
Commensalism is a symbiotic relationship between two species in which one species is benefited and the other is neither benefited nor harmed.
Mutualism
Mutualism is a symbiotic relationship in which both members of the association benefit.
is a symbiotic relationship in which the parasite derives nourishment from another organism, called the host. The parasite benefits and the host is harmed. predator , feeds on another, called the
34.4
Ecological Succession
Ecological succession is a change in a community’s composition that is directional and follows a continuous pattern of colonization by new species.
Models of Succession
The climax-pattern model of successions says that particular areas will always lead to the same type of community, called a climax community the colonists die or are damaged. The tolerance model predicts that different types of plants can colonize an area at the same time.
. How are the various stages of succession related? The facilitation model proposes that each successive community prepares the way for the next. The inhibition model says that colonists hold on to their space and inhibit the growth of other plants until
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