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Intra-specific competition and other density-dependent factors regulate population growth by reducing resource availability. This article explores the various ways population density impacts population growth and the ecosystem.
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II. Density-dependent-limiting factors (intra-specific) regulate population growth because increasing population reduces resource availability and resource limitations ultimately limits population growth. Intra-specific competition: the reliance of two or more individuals of the same species on the same limited resource. Competition between individuals of the same species increases as population size increases and r or (b-d) is reduced in proportion to the intensity of the competition. -Population growth declines because death rate increases, birth rate decreases or both. -Reduction in food may limit reproductive output. e.g. eggs or seeds
e.g. The number of seeds produce by plantain decrease with increase sowing density. The average clutch size of a sparrow decreases with increase in population. -Territoriality is a behavioral mechanism that may reduce intraspecific competition since each individual protects its own area. -Health and survivorship decreases as crowding results in smaller less robust individuals. -Predators when prey population is high, takes greater percentage of prey.
-The accumulation of toxic metabolic wastes. Intrinsic factors may also play a role in limiting populations -High densities may cause stress syndrome resulting in changes hormones that delay sexual maturation, inhibiting reproduction -High densities can also suppress the immune system with stress -High densities can reduce birth rates and increase death rates. e.g. The percentage of flour beetles (Tribolium confusum) surviving from egg to adult decrease at moderate to high populations densities, reducing the adults in the next generation.
It is possible for populations to be regulated by both density-dependent and density independent factors. Population may display short-term fluctuations due to density-independent factors. The population Dungeness Crab fluctuates due to density-dependent factors e.g. intra-specific competition and cannibalism and density-independent factors e.g. changes in water temperature due to ocean currents.
e.g.Bobwhite quail (southern Wisconsin) survival varies with the depth of snow cover (density-independent factor), but if the snow depth is shallow then factors like reduction in fecundity due to limited resources (density-dependent factor) regulates the population. Together these interact to keep the population stable.
Some populations show regular fluctuations in density in a cyclic way. Mammals -Small herbivores (lemmings) 3-5 year -Large herbivores (snowshoe hares)9-11 year cycle Hypotheses for fluctuations -Stress from high densities may alter hormone balance and reduce fertility or may cause physiological changes -High density may cause changes in the immune system,
Oscillation in the population of the hare preceded changes in the lynx density. This was once considered to be interrelated but the hare shows such oscillations on islands where the lynx is absent. The lynx population oscillation may be the result of hare population but not the other way around.
The emigration of a species to reduce populations can be caused by crowding which can cause physiological changes. e.g.Aphid population is mostly wingless females, when the population increases, winged females appear. Locusts when crowded will grow longer wings, increase fat content, become darker and emigrate very readily.
Generalizations-Populations can be K-selected (equilibrium populations) or r-selected (opportunistic populations) Characteristic r-selected K-selected populations populations -Maturation time short long -Lifespan short long -Population boom-and-bust logistics growth curve curve -Death rate in young high low -Investment of none some time in offspring
Characteristic r-selected K-selected populations populations -Number of offspring many few per reductive episode -# reproductions one several per lifetime -Timing of first early in late in reproduction life life -Habitat` unstable stable
Interspecific interaction can also act as a density-dependent limitation. Predation and parasitism relationships can regulate populations. Predator-kills and eats the prey Parasite-live on or in they hosts deriving nutrients and energy. If the parasite kills the host, then it is called a parasitoid. Usually the predator and parasite population fluctuates in direct proportion to changes in the density of the prey.
The population on mites (above) that is preyed upon by another species of mites is regulated by the interaction of the two. The predator population mirrors the population of the prey with a lag time. As the prey numbers increase, there is more food available for the predator. So there will be a corresponding in increase in the predator. This had the effect of reducing the number of prey available. Thus with less food available for the predator, there ensued a corresponding decrease in predator population. This is also true for the population of moose and wolves found on Isle Royale
Herbivory is also considered a type of predation. The interactions of the herbivore and the plant can be complex. e.g. Passion flower vines produce toxic chemical that help protect leaves from herbivorous insects. A counter adaptation has evolved in the butterfly, Heliconius: Its larvae can feed on the leaves because they have digestive enzymes the break down the toxic chemicals. The females of these butterflies avoid laying eggs on passionflowers leaves that already carry bright yellow egg clusters reducing intraspecific competition. The leaves of the passionflower has responded by growing yellow nectaries that resemble eggs. The butterflies avoid laying eggs on these leaves. These nectaries attract ants and other insects that prey on butterfly eggs.
There are times when the relationship can be mutualistic (mutualism). e.g. Ants live in acacia trees. The ants harvest the trees' nectar and leaves, and they also burrow into the thorns and stems to make their nests. The acacia trees benefit by fact that the ants keep the trees relatively free from plant-eating insects and attack any mammals that are eat the tree. The ants also prevent vines from growing on the trees. The acacia provides hollow thorns for the ants to live in and Beltian bodies found at the tips of the leaves. These Beltian bodies provide a source of protein for the ants.
Interspecific competition can b a density-dependent limitation. The more two species go for the same limited resource, the greater the competition. Niche-refers the to functional role and position of an organism in an ecosystem. Every aspect of an organism's existence helps define that organism's niche. A niche, theoretically, can be quantified by graphing two or more resources. For instance with a bird, the density of the branches for nesting, versus, temperature tolerance, versus food size. Where these intersect would define the hypervolume of the niche. Many more factors could be added to the graph, giving more multidimensional enclosed space for the species.
When 2 species are competing for the same limited resource their niche's are said to overlap. The more similar two niches are, the more likely that both species will compete for at least one limited resource. There is a limit on the amount of niche overlap compatible with coexistence. Competition for one most limited resource therefore usually leads to one or two of four possible outcomes 1. One species will become extinct. 2. One species may superior in some regions and the second in other regions. Sympatry will disappear. 3. One species may be superior under normal conditions but at a strong disadvantage under periodic crises, which will reduce the population size of the superior species. Competition will then begin anew as long as generations overlap and the periodic crises are frequent enough to prevent the extinction of the inferior species.
4. Given enough time and slight differences in niche, selection may act to produce character displacement, which reduces competition. e.g. Paramecium cultures of 2 different species demonstrate #1. The graphs shows the population growth of P. aurelia and P. caudatum by themselves in a culture. They both exhibit typical logistics curves, but when both are put together in a culture, P. aurelia outcompetes P. caudatum. The growth of P. caudatum actually decreases.
e.g.#2 Joseph Connell carried out studies using barnacles. He cleared a rocky area and observed repopulation patterns of 2 species. Semibalanus by itself lived in the lower regions of the tidal zone. It can not survive the periodic drying out in the upper part of the tidal zone. When Chthamalus lived by itself it could exploit both the upper and lower tidal zones however when both species were living together, whenever the Chthamalus ventured in the lower tidal zone of the Semibalanus, it was crushed by the Chthamalus. So that being the case the Chthamalus restricted itself to the upper tidal zone and the Semibalanus was found in the lower tidal zone.
This leads to the concept of a fundamental niche vs. a realized niche.A fundamental niche is the full environmental range that a species can occupy if there is no direct competition. A realized niche is a niche that is narrowed from the fundamental niche due to competition. Competitive exclusion principle- No two species can occupy exactly the same niche at the same time
Communities where competition occurs between species are characterized by resource partitioning, meaning that every species uses a different aspect of the resource. Three different species of warbler occupies the same spruce tree but they are found in different areas of the tree. The Cape May warbler feed on the tips of the highest branches; the yellow-rumped warbler feeds among the lower branches; and the bay-breasted warbler feed in the middle part of the tree. Species with narrow niches (they can only use one or a few types of food or growth sites) are termed specialist and those species adapted to a wide range of conditions are termed generalist.
These warblers are specialists and the common crow is a generalist living from the farms to urban areas.
Social organization often plays an important role in limiting the size of populations. Sociality carries the cost of increase competition and the benefit of increase food supply through cooperative hunting, cooperative defense of food territory against predators and sharing of information about food availability. Dominance pays an important role in access to resources, particularly resources critical to breeding. Male-contest sexual selection has a profound effect on gene frequency, since certain phenotype are more likely to succeed than others.
selection has a profound effect on gene frequency, since certain phenotype are more likely to succeed than others. By hunting together, these African wild dogs can capture a much larger prey (wildebeest) than an individual could take on its own.
These bees cooperate to regulate the internal temperature of the hive. They fan air out of the hive entrance, drawing cooler air in else-where. When the internal temperature is even higher, bee collect water and spread it on the comb then fan it to cool the hive evaporatively.
Symbiosis are two organisms living and interacting together. • Types • Mutualism-Both species benefit . • Commensalism-One species benefits and the other is neither harmed or benefits. • Parasitism-One species benefits and the other is harmed. • Enslavement-One species controls the reproduction and destination of another for its own benefit.
Forming communities from populations- The species of a community interacts with each other and with the physical environment. The biotic community they form can be considered a unit of life, with its own characteristic structure and functional interrelationships. Species diversity and complexity of interaction influence community stability. A simple community may respond more violently to a disturbance but often recovers quickly. The complex community may sometimes respond less dramatically but may continue to show effects over a longer period. A diverse physical environment seems to favor community stability. The number of species in a particular area varies. The rule of thumb, the closer the environment is to the equator, there is an increase in species diversity.
Type Locality # of species group Seed Costa Rica 8,000Plants 18,400 sq. mi. Coastal California 3,050 24,520 sq. mi. Baja California 1,500 24,100 sq. mi. British Isles 1,600
