Density-dependent and Density Independent Factors
Other Factors and Relationships
|Unit 2 Objectives||Part 1: Niche Development and Interspecific Competition|
Populations tend to grow to the maximum extent possible given the environmental conditions. The rate of growth possible under ideal conditions is called the biotic potential and is dependent on innate biological characteristics which determine how many progeny are produced and how often. Although the actual numbers differ significantly from one type of organism to another, (whales, humans, fish and flies produce vastly different numbers of offspring for instance), the growth curves look remarkably similar!
Biotic potential cannot be sustained. Some form of environmental resistance will eventually limit further population growth. If this resistance comes in the form of a limit placed by non-renewable resources, or a physical limitation such as space, light, temperature, etc., or a catastrophe such as fire, flood, drought, plague, etc., the result will be a sudden decline or crash of the population as seen in the J-curve.
The factors which produce the J-curve are density
independent, which means that they can affect a
population whenever they occur, at any density. Their
effects are not dependent on the population reaching a
certain density. This type of curve can be seen
experimentally when a population with a high biotic
potential such as algae, protozoans, insects, etc., are
provided with a resource base but not the elements of the
community required to recycle or renew the resources.
The population grows until it outstrips the resource limit
and then crashes. Such a response can be seen in nature
when the resource base suddenly decreases as a result of
seasonal or weather changes, or when a catastrophe
When a population is part of a community which provides for the recycling and renewal of resources and which exerts density dependent population controls, the result is an S or Sigmoid Curve in which population growth decelerates and establishes an equilibrium related to the carrying capacity of the ecosystem. Note that both growth curves start out the same but that the result is different depending on the type of environmental resistance. Point A represents the lag phase in which growth proceeds slowly due to the initially small population. As the progeny of those individuals begin themselves to reproduce the population enters the acceleration phase (B) and soon the population growth occurs exponentially (C), the exponential phase. Exponential growth cannot occur for long before environmental resistance takes over to either produce a population crash (F), or to produce the deceleration phase (D), which leads to an equilibrium (E). The equilibrium is the point at which increases to the population from births exactly balance decreases due to deaths. The equilibrium may be only short-lived giving way to repeated fluctuations with changes in birth rates and death rates. These fluctuations occur around a carrying capacity. The carrying capacity is the population density which the ecosystem can support. The carrying capacity is not a stable unchanging parameter but rather a dynamic variable depending on environmental conditions. The result will be dynamic fluctuation of the population as seen at G.
Most populations in nature will already be in this dynamic fluctuation phase. Only experimentally, or where a species is reintroduced into a suitable habitat, will the initial phases be observed. The density dependent factors which result in an equilibrium or controlled population include intraspecific competition for resources, mates, territories, etc., as well as predation, and endemic disease. It should be noted that starvation due to lack of resources is a non-density dependent factor, as is a plague or epidemic. See [Predation Module]
As a result of these and other factors the mortality of a population increases as its density increases. In addition, as seen at left, for many populations, both plant and animal, the birthrate is also density dependent. Whether the birthrate decreases at high densities or not, eventually birthrate and death rates will become equal, and for a short time at least, no net growth will occur. This is called the K point or equilibrium point. The difference between birthrate and death rate is the growth of the population or r which stands for recruitment. By comparing the curve seen above with the Sigmoid Curve you will see that in the beginning growth is comparatively low (the lag phase), but increases as birthrate increases until it reaches maximum growth (the exponential phase on the S Curve). Subsequently environmental resistance and lowered birthrate bring the growth back down until equilibrium is reached.
Subsequent fluctuations occur because many natural populations tend to replace members lost. By plotting the growth or recruitment (r) against the population density you find that there is a density at which maximum growth occurs. (see n-curve 1 below).
In good times populations will fluctuate at densities slightly above this level. For instance if the
population is at point A (see curve 2 below) and members are lost to predation etc. bringing the
density down to point B, analysis of the graph shows that the growth rate will increase, from A'
to B' to effect replacement of the lost individuals. Management of wild game populations attempts
to use this phenomenon to allow hunting to "harvest" the population on a regular basis.
Predator and prey will often have a relationship described as delayed density dependence in which the density of each population is dependent on the density of the other, with the fluctuations of the predator occurring slightly behind those of the prey. This occurs because the increase or decrease of the predator population depends on the availability of prey. When prey are available predator numbers increase. That causes decline of the prey, and then the subsequent decline of the predator, in a never ending cycle. Prey populations depend principally on the availability of their herbivorous food supply, which in turn often depends on the weather and other non-density dependent factors. Biologists usually describe the carrying capacity in these and other fluctuations as being at the trough or low point of the population curves, since this is the only level which can be reliably sustained Also, the troughs tend to be more consistent than the peak population levels.
Different species have different strategies for dealing with predation and environmental resistance. Type I (see Survivorship Curve left) represents species like predators and other large animals which are relatively immune to predation themselves. There is some initial mortality, but most of those which remain live to old age. Humans are in this group. Type II would include most prey, especially grazing mammals, which are subject to high initial predation followed by continued predation at a fairly constant rate throughout their lifespan. Type III would be those species which produce tremendous numbers of young because most will be eaten, only a few adults surviving to reproduce. Insects, many fish, etc. make up this group.
Other Factors which influence population dynamics.
Positive vs. negative factors.
The factors discussed above are negative in the sense that they are subtractive -- they reduce population growth, often by culling members from it. But other factors are positive, they add to populations or enhance population growth. And some factors are both positive and negative.
For instance, a group of relationships which are together called symbiosis include positive and negative types. Symbiosis refers to intimate relationships between two (sometimes three) different species in which one or both of the species benefits. By an intimate relationship we mean that usually the species live very closely together, often actually on, or even in, one another. It should be noted that, like virtually all other ecological concepts, the notion of symbiosis is a human one and not all relationships fall neatly into our definition. There are some "gray areas" and some relationships are controversial.
Commensalism describes relationships in which one species benefits and the other is unaffected. In mutualism both species benefit and may even be necessary to one another's survival. In some of the following examples commensalism was assumed at first until evidence that both species benefit came to light.
lichens - these are symbionts consisting of an association between algae and fungus, sometimes with cyanobacteria included. The algae provides food through photosynthesis, fungus attaches to a rock or some other inert substrate, and the cyanobacteria, when present, can fix nitrogen from the atmosphere. Lichens are instrumental in producing organic soil where only inorganic rock formerly existed.
coral and zooxanthellae algae - [Pictures of zooxanthellae] Coral animals are coelenterates, a group of invertebrates which includes anemones (see below) and which are polyps possessing tentacles around a central mouth opening. Corals build reefs [Coral Reef Ecology Home Page] by secreting a calcium carbonate exoskeleton. Reefs are important habitats for a variety of other marine life forms, but living coral can only live within about 150 meters of the surface, even in the clearest water, because sunlight is necessary for the photosynthesis of algae living inside the coral as symbionts.
|sea anemones and clown fish
Click on image for larger view.
|Anemones are also coelenterates, who harpoon small fish with stinging nematocysts which they shoot from their tentacles. These fish are then brought in to the mouth opening to be taken in to the gut and digested. But clown fish are immune to the stinging barbs, and use the anemones for protection from larger fish. The clown fish in turn bring bits of food back to feed the anemone.|
sharks and remora - These fish swim alongside and even attach to the shark in hopes of benefiting from leftovers and receiving protection.
cleaner fish and sharks or barracuda - The cleaner fish actually remove parasites and other debris from the predator's mouths, setting up "cleaning stations" which the larger fish visit as if they were going to the dentist.
Wood termites contain one-celled protozoans that pre-digest wood cellulose in the termite gut. Without the protozoa, the termites starve with a belly full of useless wood fibers. And the
protozoa need the moist environment of the termite gut, and the termite's ability to harvest and deliver wood fiber.
yucca moths and certain species of yucca plants (an excellent site with many photos) -A classic case of mutualism cited in textbooks, it includes about 40 species, most of which occur in the southwestern United States and Mexico. Although they are often associated with arid desert regions, some species are native to the southeastern United States and the Caribbean islands. What truly sets this genus apart from other flowering plants is their unique method of pollination: A specific moth that is genetically programmed for stuffing a little ball of pollen into the cup-shaped stigma of each flower. Like fig wasps and acacia ants, the relationship is mutually beneficial to both partners, and is vital for the survival of both plant and insect. In fact, yuccas cultivated in the Old World, where yucca moths are absent, will not produce seeds unless they are hand pollinated.
humans and gut flora - See [Care and feeding of your inner pet]
Behavior Patterns - these have both positive and negative effects on populations.
Migration - [See Migrations page] Many species migrate to find new food sources during the winter lean season. Birds and whales are well known for their long migration routes, while wildebeest and other African ungulates are known for the tremendous size of their migrations. Migration also serves to give the land a chance to recover from intense feeding pressure, and to provide an increased choice of mates for reproduction. At the same time not all individuals survive the migration and so it tends to cull out the sick and weak. [See Migration of Monarch Butterflies]
Territory Behavior - territories vary considerably from one species to another. Some birds may establish small territories on a beach or in a tree for nesting but share territories for feeding purposes. Others may defend a territory measured in acres or square miles against interlopers who might poach food or raid nests. Territories of the big cats are usually somewhere between 20 and 100 square miles or so depending on the habitat. And territories may be related to family groups or societies.
Societies and hierarchies - Ants, termites, bees, lions, wolves, baboons, from one end of the animal spectrum to the other, societies and hierarchies keep things running smoothly. Efficiency of reproduction and feeding, defense of the nest or pack, all are common threads. Wolf territories allow each unit, or pack, to establish a feeding area and reproductive unit regulated by prey density, competition, and disease. Many societies such as those of baboons and wolves permit reproduction by only dominant individuals. Yet in good years groups can expand their numbers and take advantage of increased resource levels.
Mating and courtship behavior - Courtship behavior allows individuals to signal that they are looking for a mate and provides factors by which a mate may be chosen. It can involve display of breeding plumage, ritualistic dances or movements, release of pheromones, calling and signaling (the light emitted by fireflies serves to attract a mate), etc. It is indispensable in permitting reproduction to occur, yet some are not selected. For instance female songbirds select mates with the most intense coloration. Color intensity in male songbirds has been shown to be related to their level of parasites. So the females are selecting the healthiest mates! See [Mating Selection in butterflies] and [How Females Choose their Mates]
Colors, patterns, and other physical characteristics. In a sense most every physical characteristic of every plant and animal has, or at least had, an adaptive value. Some have lost their importance as the environment has changed but they haven't been selected out. Others are critical to the survival of the species. Camouflage colors help many species avoid detection, either by their predators or their prey. Others want to be seen and so they display warning coloration. Usually these species are poisonous or toxic and want to warn potential predators. The predators have a natural aversion to their bright colors, or learn to have such an aversion after an encounter or two. Mimics take advantage of the warning of other similarly colored species even though they may or may not be poisonous themselves. Patterns such as those seen on zebras can help confuse predators, who find the mass of black and white stripes in a herd of zebras prevent them from sighting on individual prey.
Certain butterflies (models) have evolved a chemical defense system against predation. They have also evolved a distinctive coloration to warn off possible predators. Experiments confirm that birds, after they have once attacked such a butterfly, will be much less likely to try eating it again -- a warning flag. In some instances other species mimic this coloration. In other words, by resembling the unpalatable species they may avoid predation by those animals who have lea rned the warning signal. This is called batesian mimicry after the 19th century English naturalist Henry W. Bates. Wallerian mimicry involves both mimics and models which are toxic or dangerous.
|A classic example of batesian mimics is the Monarch and the Viceroy butterflies.||
|Monarchs feed extensively on milkweed, which has a toxin that the monarch sequesters. Birds eating monarchs get sick from the toxin. This causes them to avoid future encounters with both the model and its mimic.|
Click on images for larger view.
|Another example of mimics is the numerous mimics of coral snakes. Coral snakes of various species are found across North America and the world. In Virtually each case there are mimics, such as this example for the Eastern coral snake found in Florida.||
Eastern coral snake
Coral snake banding pattern
Tropical king or "false coral" snake mimic
"Red next to black, friend of Jack, Red next to yellow kills a fellow"
Sometimes populations come into conflict with humans. We may import them into a new habitat, either intentionally or accidentally, without the factors which kept them under control where they lived before. Or we may eliminate their predators and competitors and provide them with unlimited food. Such species become pests. Since the advent of chemical pesticides soon after World War II (many of these pesticides were developed as an derivative of nerve gas research and are much the same), it has been common to get rid of these pests with toxic chemicals. But the detrimental effects of these chemicals are now well known: persistence in the environment, biological magnification in food chains and decline of non-target species, cancer and other illnesses in humans, and resistance in the pest populations. [See Biological Control Virtual Information Center]
Some examples -
Predators: use of ladybugs, lacewings, mantises, wasps, flower flies, etc. which prey on or parasitize many insects and their larvae.
Diseases: use of bacterial or virus diseases, for example use of B.T. (bacillus thuringiensis) to control caterpillars such as cabbage worms, tomato worms, etc.
Use of pheromones such as sex attractants and juvenile hormones to disrupt the life cycles of pests.
Use of the sterile male technique for control of screw worm (cattle pest in SE U.S.) and med fly.