elements that prevent a population from attaining its biotic potential. Limiting factors are categorized into density-dependent and density-independent factors, as follows:
• Density-dependent factors are those agents whose limiting effect becomes more intense as the population density
increases. Examples include parasites and disease (transmission rates increase with population density),
competition for resources (food, space, sunlight for photosynthesis), and the toxic effect of waste products.
Also, predation is frequently density-dependent. In some animals, reproductive behavior may be abandoned
when populations attain high densities. In such cases, stress may be a density-dependent limiting factor.
• Density-independent factors occur independently of the density of the population. Natural disasters (fires,
earthquakes, volcanic eruptions) and extremes of climate (storms, frosts) are common examples.
fluctuations in population size in response to varying effects of limiting factors. For example, since many limiting factors are density-dependent, they will have a greater effect when the population size is large as compared to when the population is small. In addition, a newly introduced population may grow exponentially beyond the carrying capacity of the habitat before limiting factors inhibit growth (Figure 15-5). When limiting factors do bring the population under control, the population size may decline to levels lower than the carrying capacity (or it may even crash to extinction). Once reduced below carrying capacity, however, limiting factors may ease, and population growth may renew. In some cases, a new carrying capacity, lower than the original, may be established (perhaps because the habitat was damaged by the excessively large population). The population may continue to fluctuate about the carrying
capacity as limiting factors exert negative feedback on population growth when population size is large. When population size is small, limiting factors exert little negative feedback, and population growth renews.
The niche that an organism occupies in the absence of competing species is its fundamental
niche. When competitors are present, however, one or both species may be able to coexist by occupying their realized
niches, that part of their existence where niche overlap is absent, that is, where they do not compete for
the same resources.
• Under experimental conditions, one species of barnacle can live on rocks that are exposed to the full range of tides. The full range, from the lowest to the highest tide levels, is its fundamental niche. In the natural environment,
however, a second species of barnacle outcompetes the first species, but only at the lower tide levels where desiccation is minimal. The first species, then, survives only in its realized niche, the higher tide levels.
relationship in which both species benefit (+,+).
• Certain acacia trees provide food and housing for ants. In exchange, the resident ants kill any insects or fungi
found on the tree. In addition, the ants crop any neighboring vegetation that makes contact with the tree, thereby
providing growing space and sunlight for the acacia.
• Lichens, symbiotic associations of fungi and algae, are often cited as examples of mutualism. The algae supply
sugars produced from photosynthesis, and the fungi provide minerals, water, a place to attach, and protection
from herbivores and ultraviolet radiation. In some cases, however, fungal hyphae invade and kill some of their
symbiotic algae cells. For this and other reasons, some researchers consider the lichen symbiosis closer to
change in the composition of species over time
traditional view of succession describes how one community with certain species is gradually and predictably replaced by another community consisting of different species.
As succession progresses, species diversity (the number of species in a community) and total biomass (the total mass of all living organisms) increase. Eventually, a final successional stage of constant species composition, called the climax community, is attained. The climax community persists relatively unchanged until destroyed by some catastrophic event, such as a fire.
not as predictable as once thought; some species are established at random, influenced by season, by climatic conditions, or by which species happen to arrive first; climax community is not always attained because
fires or other disturbances occur so frequently
occurs in some regions over thousands of years due to climate change;
occurs over shorter period b/c species that make up communities alter the habitat by their presence;
In both cases, the physical
and biological conditions which made the habitat initially attractive to the resident species may no longer exist, and the habitat may be more favorable to new species.
occurs on substrates that never previously supported living things. For example, primary
succession occurs on volcanic islands, on lava flows, and on rock left behind by retreating glaciers. Two examples
• Succession on rock or lava usually begins with the establishment of lichens. Hyphae of the fungal component
of the lichen attach to rocks, the fungal mycelia hold moisture that would otherwise drain away, and the lichen
secretes acids which help erode rock into soil. As soil accumulates, bacteria, protists, mosses, and fungi appear,
followed by insects and other arthropods. Since the new soil is typically nutrient deficient, various nitrogenfixing
bacteria appear early. Grasses, herbs, weeds, and other r-selected species are established next. Depending
upon local climatic conditions, r-selected species are eventually replaced by K-selected species such as perennial
shrubs and trees.
• Succession on sand dunes begins with the appearance of grasses adapted to taking root in shifting sands. These
grasses stabilize the sand after about six years. The subsequent stages of this succession can be seen on the
dunes of Lake Michigan. The stabilized sand allows the rooting of shrubs, followed by the establishment of
cottonwoods. Pines and black oaks follow over the next fifty to one hundred years. Finally, the beech-maple
climax community becomes established. The entire process may require a thousand years.
begins in habitats where communities were entirely or partially destroyed by some kind of
damaging event. For example, secondary succession begins in habitats damaged by fire, floods, insect devastations,
overgrazing, and forest clear-cutting and in disturbed areas such as abandoned agricultural fields, vacant lots, roadsides,
and construction sites. Because these habitats previously supported life, secondary succession, unlike primary
succession, begins on substrates that already bear soil. In addition, the soil contains a native seed bank. Two examples
of secondary succession follow:
• Succession on abandoned cropland (called old-field succession) typically begins with the germination of
r-selected species from seeds already in the soil (such as grasses and weeds). The trees that ultimately follow
are region specific. In some regions of the eastern United States, pines take root next, followed by various
hardwoods such as oak, hickory, and dogwood.
• Succession in lakes and ponds begins with a body of water, progresses to a marsh-like state, then a meadow,
and finally to a climax community of native vegetation. Sand and silt (carried in by a river) and decomposed
vegetation contribute to the filling of the lake. Submerged vegetation is established first, followed by emergent
vegetation whose leaves may cover the water surface. Grasses, sedges, rushes, and cattails take root at the
perimeter of the lake. Eventually, the lake fills with sediment and vegetation and is subsequently replaced by a
meadow of grasses and herbs. In many mountain regions, the meadow is replaced by shrubs and native trees,
eventually becoming a part of the surrounding coniferous forest.
expanded, more complete version of a food chain. It would show all of the major plants in the ecosystem, the various animals that eat the plants (such as insects, rodents, zebras, giraffes, antelopes), and the animals that eat the animals (lions, hyenas, jackals, vultures). Detritivores may also be included in the food web.
Arrows connect all organisms that are eaten to the animals that eat them, that is, in the direction of energy flow
The ozone layer forms in the upper atmosphere when UV radiation reacts with oxygen (O2)
to form ozone (O3). The ozone absorbs UV radiation and thus prevents it from reaching the surface of the earth
where it would damage the DNA of plants and animals. Various air pollutants, such as chlorofluorocarbons
(CFCs), enter the upper atmosphere and break down ozone molecules. CFCs have been used as refrigerants, as
propellants in aerosol sprays, and in the manufacture of plastic foams. When ozone breaks down, the ozone layer
thins, allowing UV radiation to penetrate and reach the surface of the earth. Areas of major ozone thinning, called
ozone holes, appear regularly over Antarctica, the Arctic, and northern Eurasia
Air pollution, water pollution, and land pollution contaminate the materials essential to life. Many
pollutants do not readily degrade and remain in the environment for decades. Some toxins, such as the pesticide DDT, concentrate in plants and animals. As one organism eats another, the toxin becomes more and more concentrated, a process called biological magnification. Other pollution occurs in subtle ways. A lake, for example, can
be polluted with runoff fertilizer or sewage. Abundant nutrients, especially phosphates, stimulate algal blooms,
or massive growths of algae and other phytoplankton. The phytoplankton reduce oxygen supplies at night when
they respire. In addition, when the algae eventually die, their bodies are consumed by detrivorous bacteria, whose
growth further depletes the oxygen. The result is massive oxygen starvation for many animals, including fish and
invertebrates. In the end, the lake fills with carcasses of dead animals and plants. The process of nutrient enrichment
in lakes and the subsequent increase in biomass is called eutrophication. When the process occurs naturally,
growth rates are slow and balanced. But with the influence of humans, the accelerated process often leads to the death of fish and the growth of anaerobic bacteria that produce foul-smelling gases