Population density refers to the population size relative to some unit of space. It is generally expressed in terms such as trees per acre, Vinegar Weeds per square meter, coyotes per square kilometer, etc.
Crude density includes all the land within the organism's range whereas Ecological density includes only that portion of land that can actually be colonized by the species. The crude density of the Checkered Fritillary in Butte County is very low but where it is found in may be very abundant. This variation in density is important to both the species in question and its predators. If the Butte County population of Checkered Fritillaries were evenly distributed throughout the county, it would be worthless as a resource for hunter-gatherers; they would expend more energy gathering than they would get. With its relatively high ecological density, the forager merely has to know its specific habitat.
Related to ecological density but on a finer scale is the internal distribution of the population. Individuals may be uniformly distributed; randomly distributed or clumped. Such patterns of distribution are usually species-specific but may derive from a variety of causes: a uniform dispersal pattern is likely to result from direct competition or territoriality; a random distribution may be the result of chance dispersal; and a clumped pattern could result from limited dispersal of propagules, behavior, or a very spotty arrangement of suitable habitat (e.g., Yellow Carpet, Blue Oaks).
While populations may fluctuate widely over short time spans, on the average they do not change significantly over a long time span. Thus, on the average each pair of titmice will produce, during their lifetime, one pair of reproducing offspring.
Reproductive strategies vary greatly. Titmice may only produce two dozen eggs over a lifetime while some marine fish will produce over two million eggs per year. A redwood tree may produce 10,000 seeds per year over a period of 1000 years.
Whatever the strategy, the biotic potential is matched by death of eggs or offspring so that each generation merely reproduces itself. Otherwise the world would be solid maple trees or they would go extinct. Stated in other terms, normally birth rate = death rate. This is evolutionarily adjusted in each species; if b>d the population grows until the food supply runs short at which d increases to balance b. Alternatively, the increased population may support a larger population of predators that help increase d.
Energy fixed by photosynthesis is largely used up by the community. For any species to increase its numbers over a long period, other species must be displaced. Short term fluctuations in weather, etc., may permit short term increases without pushing out other species but such increases must be matched by a decrease when the weather goes back to normal.
The rate of population change under these short-term, non-equilibrium conditions is a function of the species' biotic potential or difference between birth rate and death rate.
|r = b - d|
rate of increase = birth rate - death rate
Other species have low biotic potentials and have their rate of population growth locked to the environment so that population growth slows as the population approaches the carrying capacity (K) of the environment. These species have relatively stable populations and are said to be density-dependent or K - Selected.
The following curves illustrate the theoretical difference in growth between r and K selected populations. Both populations start at time 1 with 10 individuals. Both have birth rates of 2 and death rates of 1. For the first population, there is no set carrying capacity; the second has a carrying capacity of 1000. The equations (simulations of reality) graphed here are :
r - Selected Opportunists
|Very high intrinsic rate of increase.||Annual plants|
|Populations can expand rapidly to take advantage of temporarily favorable conditions.||Bacteria|
|Characteristics of K - Selected Species||Examples|
|Population responds slowly, usually with negative feedback control so that constancy is the rule.||Most birds|
Which are better, r-Selected or K-Selected? Both strategies are necessary for the biosphere. K-Selected populations help maintain ecosystem constancy. r- Selected populations are like scar tissue on the biosphere, they quickly cover disturbed areas, remove dead plants and animals, etc.
When conditions become good so that death rate drops, r - Selected species with a high biotic potential can increase populations dramatically. These species are called opportunists (or weeds). When it is a good year for rabbits, there are rabbits everywhere; when it is a good year for elephants, nobody notices. By contrast, in bad years elephants take longer to starve than rabbits.
The population of a species at a given time is a function of its history as well as present conditions with respect to b and d. All responses also have a built in lag time. In a good spring for rabbits, all rabbits are running around mating. Forty days later, when there are baby rabbits everywhere, things may be drying up and food may be scarce. The population thus exceeds the carrying capacity of the environment before any negative effect is felt. The resultant population overshoot inevitably leads to starvation and death although the associated weakening of individuals may cause death to be expressed as disease or predation rather than starvation.
Density-independent factors often cause fluctuations in populations, but seldom control them:
|Examples of Density-independent factors||Some percent of the population is killed regardless of size or there is a fractional decrease in either birthrate or survival|
Density-dependent factors usually bring about population control.
|Examples of Density-dependent factors||The bigger (denser) the population, the greater the % effected.|
|Predators whose population is dependent on food supply||1% of pop of 10,000|
|crowding/stress factors||20% of 100,000|
|Food/starvation relationships||80% of 1,000,00|
Consider an herbivorous insect. Possible factors which might affect its population include:
Since all these factors together bring about the regulation of our hypothetical herbivorous insect, it is almost impossible to collect enough data to be able to point to a single factor as the controlling agent. The factor most limiting to a population may even change from day to day.
Despite all the forces acting on them, most natural populations, whether K-Selected or r - Selected, remain relatively constant. The K- Selected species are generally controlling their own populations either directly or in some symbiotic association with other species. The r - Selected species are more likely to be controlled by interactions with K - Selected neighbors. This could be by competition as where annual "weeds" are kept in check by perennial grasses or as a function of predation or parasitism. Consequently, short term graphs of r - Selected and K - Selected species would not look obviously different.
r - Selected species do occasionally show population irruptions. During an irruption, population growth follows a J curve and the organism becomes so numerous that it will deplete its food supply and may threaten the survival of other species. Population irruptions are most common in human-dominated systems, but do occur in natural systems. Understanding the causes and control of pest irruptions is an important goal of ecologists, farmers, foresters, and others.
It would be a mistake to imply that we knew the cause of all population irruptions but one generalization can be made. Population irruptions often arise following a drastic reduction of the population by some density-independent factor. Most probably the r-selected population had been kept in check by interaction with one or more K - Selected species. A density-independent factor such as a severe winter depresses populations of both r and K - Selected species. As the populations begin to recover, the high biotic potential of the r - Selected species allows it to recover more quickly than its controlling agents and an irruption ensues. Even though controlling agents such as predators and parasites, having an unlimited food supply, may develop relatively large populations, there is no way they can catch up with the r - S elected species until its population crashes for some other reason (usually running out of food).
Suppose we (intelligent humans) step in and control the irruption with pesticides (a density-independent mechanism). Typically populations of both the pest and it's controlling species are depressed and again the r - Selected population rebounds ahead of the K - Selected ones. That result is very good for the chemical companies. Use of pesticide will require the use of more pesticide. Now it is also likely that the environment contained other r - Selected species that did not irrupt as a result of the first density-independent phenomenon. Some of these are likely to irrupt following the pesticide application, especially if broad-spectrum insecticides were used. The initial use of pesticide not only increases the likelihood of need for further treatment of the first pest, it is very likely to create additional pests.
|Effect of DDT Treatment on Red Scale (a Citrus Pest) in Southern California|
|Location||Applications||Untreated Population Density||Treated Population Density|
In the Central Valley serious target-pest resurgence and upsets on cotton were experimentally induced and studied in large field experiments by university personnel. Azodrin-treated plots suffered significantly more damage than untreated plots. Bidrin-treated plots sustained a cabbage looper infestation about three times as great as controls with considerably more leaf damage. With toxephene-DDT tests, beet armyworms were increased in all of the treated plots up to 18 times higher than controls. The overall conclusions have been that the serious lepidopterous pest problems in the Central Valley involving bollworm, beet armyworm and cabbage looper are largely pesticide-induced upsets, and that the cotton industry is actually losing more money with insecticide use than it would with no treratment. Paul Debach.1974. Biological Control by Natural Enemies. Cambridge University Press, p15.Now, if that were the case and it was known as early as 1974, why have not agricultural entomologists devoted their research time to alternative pest-control methods that do not induce further irruptions? Consider the case of the vedalia beetle. In 1868 the cottony-cushion scale insect invaded California citrus. By 1886 the entire industry was near destruction. No pesticides were available, they would not be invented for nearly 100 years. Entomologists began looking for the native home of the scale insect in hopes of finding a predator to control it. They found the Vedalia beetle (a type of lady-bird beetle) in Australia. In April, 1889, 129 individuals were experimentally introduced into one grove in California. They were highly successful. By June of 1889, beetles had been distributed to 208 groves throughout the state. By the end of 1889, the cottony cushion scale was no longer a problem in California. Total cost of research and treatment: $5 million in today's dollars. The scale remained under control until the introduction of DDT in 1946. The vedalia beetle also successfully controlled cottony cushion scale in 40 other countries. But who made a profit? How can you market something which spreads from grove to grove on its own? If no one profits, who will fund the research? Chemical companies fund the bulk of agriculture school research. They are well repaid; rice farmers in California alone spend $1.5 million annually on pesticides.