Populations and communities Chapter 35

What Is Ecology? Ecology is the study of how organisms interact with each other and with their environment. Ecology also includes the study of the distribution and abundance of organisms; ecology can be studied at progressively more encompassing levels of organization.

What Is Ecology? Levels of ecological organization: 1. Populations - individuals of the same organism that live together are members of a population. 2. Species - consists of all the populations of a particular organism. 3. Communities - populations of different species that live together in the same place constitute a community.

What Is Ecology? 4. Ecosystems - a community and the nonliving factors with which it interacts is called an ecosystem. 5. Biomes - major terrestrial assemblages of plants, animals, and microorganisms that occur over wide geographic areas and have distinctive physical characteristics are called biomes. 6. Biosphere- all the world’s biomes, along with its marine and freshwater assemblages, together constitute an interactive system called the biosphere.

What Is Ecology? The nature of the physical environment determines to a large extent which organisms live in a certain climate or region. Key elements of the environment include: Temperature Water Sunlight Soil

What Is Ecology? Many organisms are able to adapt to environmental changes by making morphological, physiological, or behavioral adaptations. For example: The gray wolf grows a thicker coat of fur in the winter. The green iguana lizard escapes to the shade in the heat of the day.

Population Range Organisms live as members of populations, groups of individuals that occur together at one place or time. Five aspects of populations are particularly important: Population range Population distribution Population size Population density Population growth

Species that occur in only one place Most species have relatively limited geographical ranges. Organisms must be adapted for the environment in which they occur. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Devil's hole pupfish Socorro isopod Iriomote cat Iiwi Hawaiian bird New Guinea tree kangaroo Catalina Island mahogany tree Northern white rhinoceros Species that occur in only one place

Population Range Population ranges are not static; rather, they change through time. These changes occur for two reasons: In some cases, the environment changes For example, the range for trees that survive better in colder temperatures shifts farther up a mountain when temperature increases in an area. Present Grassland, chaparral, and desert scrub Alpine tundra Spruce-fir forests Mixed conifer forest Woodlands Elevation (km) 3 km 2 km 1 km 0 km Grassland, chaparral, and desert scrub 15,000 years ago

Range expansion of the cattle egret In addition, populations can expand their ranges when they are able to move from inhospitable habitats to suitable, previously unoccupied areas For example, cattle egret expansion 1966 1964 1970 1965 1960 Immigration from Africa 1961 1951 1943 1958 1937 Equator 1956 1970

Population Distribution A key characteristic affecting a species’ range is the way in which individuals of its populations are distributed; they may be: Randomly spaced Individuals do not interact strongly with one another. Uniformly spaced Often results from competition for resources. Clumped Clumped spacing results from uneven distribution of resources in the individuals’ immediate environment.

Population Growth A population is a group of individuals of a species that live together and influence each other’s survival.

Population Growth Populations have several properties: population size is the number of individuals in the population. population density is the population size that occurs in a given area.

Population Growth Another characteristic about any population is its capacity to grow. Population growth can be modeled in different ways that identify what factors in nature limit growth.

Population Growth Biotic potential, symbolized by r, is the rate at which a population of a given species will increase when no limits are placed on its rate of growth. The simplest model of population growth assumes a population growing without limits at its maximal rate.

Population Growth The exponential growth model is defined by the following formula: growth rate = G = riN N is the population size G is the change in its numbers over time ri is the intrinsic rate of natural increase for that population

Population Growth The actual rate of population increase, r, is defined as: r = (b – d) + (i – e) b is the birthrate, d is the death rate. e is the amount of emigration out of the area and i is the amount of immigration into the area.

Population Growth The innate capacity for growth of any population is exponential. even when the rate of increase remains constant, the actual increase in the number of individuals accelerates rapidly as the population grows. in practice, such patterns prevail for only short periods, usually when an organism reaches a new habitat with abundant resources.

Population Growth No matter how rapidly populations grow, they eventually reach a limit imposed by shortages of important environmental factors. A population ultimately stabilizes at a certain size, called the carrying capacity. the carrying capacity is symbolized by K and is defined as the maximum number of individuals that an area can support.

Population Growth The growth curve of a population that is approaching its carrying capacity can be approximated by the logistic growth equation: G = rN [(K – N)/K] As N approaches K, the rate of population growth (G) begins to slow, until it reaches zero at N = K.

Two models of population growth Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1,250 Carrying capacity 1,000 Exponential growth model 750 Population size (N) Logistic growth model 500 250 5 10 15 Number of generations (t)

Most natural populations exhibit logistic growth Population Growth The sigmoid growth curve is characteristic of most biological populations. The processes of competition and emigration tend to increase as a population approaches its carrying capacity. 10 8 6 Breeding male fur seals (thousands) 4 2 1915 1925 1935 1945 Time (years) Most natural populations exhibit logistic growth

The Influence of Population density Many factors act to regulate the growth of populations in nature: density-independent effects these effects regulate population growth regardless of population size. for example, weather effects or geological events (i.e., volcanoes). density-dependent effects the effect that these factors have on population growth depends on population size. these effects grow stronger as the population size increases.

Density-dependent effects 5.0 4.0 3.0 Number of surviving young per female 2.0 1.0 10 20 30 40 50 60 70 80 Number of breeding females

Life History Adaptations Life history describes the complete life cycle of an organism. r-selected adaptations Favor rapid growth in a habitat with unlimited resources or in unpredictable environments - take advantage of resources when they are available. K-selected adaptations Favor reproduction near the carrying capacity of the environment. help survival in an environment in which individuals are competing for limited resources.

Population Demography Demography is the statistical study of populations. measures characteristics of populations and helps predict how population sizes will change in the future. populations grow if births outnumber deaths and shrink if deaths outnumber births. birth and death rates are dependent on age and sex.

Population Demography A cohort is a group of individuals of the same age. Within a population, every cohort has the following characteristics: fecundity, or birthrate, which is defined as the number of offspring produced in a standard time. mortality, or deathrate, which is the number of individuals that die in that period. The relative number of individuals in each cohort defines a population’s age structure.

Population Demography Sex ratio is the proportion of males and females in a population. the number of births is usually directly related to the number of females. Age distribution is the proportion of individuals in different age categories. when a population lives in a constant environment for a few generations, its age distribution tends to stabilize.

Population Demography A survivorship curve is one way to express the age distribution characteristics of a population. Survivorship is defined as the percentage of an original population that survives to a given age. There are three types of survivorship curves: type I has the highest mortality for the oldest individuals type II has relatively the same mortality risk for all ages type III has the highest mortality for the youngest individuals

Survivorship curves 1,000 Human (type I) Hydra (type II) 100 Survival per thousand Oyster (type III) 10 1 25 50 75 100 Percent of maximum life span

Communities Community refers to the species that occur at any given locality. Interactions among community members govern many ecological and evolutionary processes. for example, predation, competition, and mutualism affect the population biology of a particular species, as well as the way in which energy and nutrients cycle through the ecosystem.

The niche and competition The niche an organism occupies is the sum total of all the ways it utilizes the resources of its environment. Sometimes species are not able to occupy their entire niche because of the presence or absence of other species. Competition describes the interaction when two organisms attempt to use the same resource when there is not enough of the resource to satisfy both. interspecific competition occurs between individuals of different species. intraspecific competition occurs between individuals of the same species.

The niche and competition Competition among two species of barnacles limits niche use Fundamental niche is the entire niche that an organism may theoretically occupy. Realized niche is the actual niche that the organism is able to occupy because of competition. Chthamalus Semibalanus Fundamental niches Realized niches

The niche and competition G. F. Gause demonstrated the principle of competitive exclusion. If two species are competing for a resource, the species that uses the resource more efficiently will eventually eliminate the other locally. In other words, no two species with the same niche can coexist.

Competitive exclusion among three species of Paramecium 50 100 150 (measured by volume) Population density 4 8 Days P.bursaria P.caudatum P.aurelia 12 16 20 24 200 (a) 4 8 (measured by volume) Population density 50 75 25 P.caudatum P.aurelia P.bursaria Days (c) (b) 200 100 16 20 24 12

The niche and competition Species in communities act to avoid competition whenever possible. When niches overlap, two outcomes are possible: Competitive exclusion (i.e., winner takes all). Resource partitioning, which divides up resources to create two niches. Thus, persistent competition between two species is rare in natural communities Either one species drives the other to extinction or natural selection reduces the competition between the them.

Resource partitioning among lizard species

The niche and competition Resource partitioning can often be seen in similar species that occupy the same geographical area. Such species are sympatric. When a pair of species occupy the same habitat (i.e., when they are sympatric), they tend to exhibit greater differences in morphology and behavior than the same two species do when living in different habitats (i.e., when they are allopatric). The evident differences are called character displacement and are favored by natural selection to facilitate habitat partitioning and reduce competition.

Coevolution and Symbiosis Coevolution is the adaptation of two or more species to each other. Examples of coevolution include: plants and animal pollinators predator-prey interactions symbiotic relationships

Coevolution and Symbiosis In a symbiotic relationship, two or more kinds of organisms live together in often elaborate and more or less permanent relationships. There are three major kinds of symbiotic relationships: mutualism parasitism commensalism

Coevolution and Symbiosis Mutualism is a symbiotic relationship in which both species benefit. For example, ants tend to aphids, feeding on the honeydew that aphids excrete continuously, moving the aphids around, and protecting them from potential predators.

Coevolution and Symbiosis Parasitism is a symbiotic relationship in which one species benefits while the other is harmed. This interaction is really a form of predator-prey relationship but a parasite usually does not kill its host. The parasite is much smaller than the host and remains closely associated with it.

Coevolution and Symbiosis There are many forms of parasitism in nature: External parasites - also known as ectoparasites, these parasites feed on the exterior surface of a host. parasitoids are insects that lay eggs on living hosts. Internal parasites - also known as endoparasites, these parasites feed internally on their hosts. Brood parasitism is a form of parasitism in which the parasite does not consume the body of its host. brood parasites are birds, such as cowbirds and cuckoos, that lay their eggs in the nest of other species for the host to raise.

Coevolution and Symbiosis Commensalism is a symbiotic relationship that benefits one species but neither hurts nor helps the other. Note: There is no clear-cut boundary between commensalism and mutualism

Predator-prey interactions Predation is the consuming of one organism by another. Under laboratory conditions, predators may exhaust their prey species and then starve. In nature, predators often have large effects on prey populations.

Predator-prey interactions Number of pelts (in thousands) (a) Year 1935 1925 1915 1905 1895 1885 1875 1865 1855 1845 120 160 80 40 Snowshoe hare Lynx Population cycles may be, in some situations, stimulated by predators. A classic example is the “10-year cycle” of the snowshoe hare, Lepus americanus, that appears to be under the influence of food plants and predators.

Predator-prey interactions Predator-prey interactions are an essential factor in the maintenance of communities that are rich and diverse in species. Predators prevent or greatly reduce competitive exclusion by reducing the number of individuals of competing species. Examples of key predators include sea stars, wolves, and mountain lions.

Mimicry Batesian mimicry is when a palatable species resembles a poisonous one. Müllerian mimicry is when several unrelated, but protected, species come to resemble one another. For example, the colors black, yellow, and red tend to be common color patterns that warn predators relying on vision.

Ecological succession Succession is the orderly replacement of one community with another. Primary succession occurs on bare, lifeless substrates, such as those left behind when a glacier retreats or when a volcanic island emerges. Pioneering community is the first to become established. Secondary succession occurs after an already established community has been disturbed.

Ecological succession Succession happens because species alter the habitat and the resources available in it, often in ways that favor other species. Three dynamic concepts are of critical importance: Tolerance - early successional stages are characterized by weedy r-selected species that tolerate harsh conditions but do not compete well. Facilitation - the weedy species introduce local changes in the habitat that favor nonweedy species. Inhibition - sometimes the changes in habitat caused by one species may inhibit the growth of the species that caused them.

Plant succession produces progressive changes in the soil Year 100 Nitrogen concentration (g/m2 of surface) a b c d 300 250 200 150 100 50 Year 1 Pioneer mosses Year 200 Invading alders Alder thickets Spruce forest Nitrogen in mineral soil in forest floor