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BIOL 4120 Principles of Ecology Phil Ganter 320 Harned Hall 963-5782 |
Some
populations of the cactus above reproduce sexually and disperse
when birds eat the seeds. Other populations reproduce asexually
and disperse as joints (stem sections) stick to animals. |
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Lecture 9 Populations
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Overview - Link to Course Objectives
Populations are local groups of organisms, all of the same species, in which organisms interact. These interactions (like mating, competing for the same food or territory, communicating, transmitting disease, etc.) are important because they help determine the dynamics of the group (=population)
The book points out that we must look at populations from our human perspective but must see them from the perspective of the organisms composing them. An example of this comes from asking how to count the members of a population. This is a straightforward question from our perspective. Each individual is a separate organism and each organism is a separate individual. What about a plant that grows separate stems from a single root system? (These stems can be as small as grass stems or as large as oak tree trunks.) This problem is not unique to plants (consider a population of bacteria that is a mix of clones of genetically identical individuals or a reef with many coral heads, each consisting of many individuals budded from a single founder). Botanists have come up with some terms that apply to this situation:
Assessing the size of a population with modular growth is often not a simple count of ramets. For some purposes, you might need to consider genets, not ramets and counting ramets and genets together will give you misleading results. In some cases, it is impossible to separate ramets from one another. In such cases, the total weight of the species (its Biomass) is assessed.
The above picture is a spectacular example of modular growth
Our operational definition of ecology is the study of the distribution and abundance of organisms. The population is a unit of biological organization and we can gain much insight into ecology by examining distribution and abundance at the population level. Neither aspect of ecology is as simple as one might expect.
The first complication is the problem of Spatial Scale (the size of area examined). For a species of small annual plant found throughout all of North America, one can examine the entire range, the distribution within a region (say, within the watershed of a river), or the plants that occur in a single field. It is often difficult to define the edge of a population and different important processes may operate at different scales. Mating may be very local if, for instance, the pollinator of the plant does not leave the field. Dispersal of the plant may involve longer distances if birds eat the fruit and fly from field to field depositing seeds.
Distribution is the spatial location of organisms in a population. There are two areas to consider in describing distribution: the boundary and patterns within the boundary.
Geographic Range - the outer limit of the location of members of the population, the boundary beyond which no members are located. This is often what is presented as distribution and can vary from the size of a field to the size of a continent.
- The boundary may be set by Edaphic Conditions (physical factors like climate, soil type, temperature, salinity, etc.), by geography (a coast, a river, a mountain chain), or by biological factors (competitors, presence of disease, predators, etc.)
Spatial Pattern - the manner in which organisms are distributed within the boundary of the population.
First we need to introduce a term and to recall two terms from math.
- Population Density is the average number of organisms per unit area. The unit area will differ with the organism studied. It might be per square mile when dealing with brown bears and per square meter when dealing with spiders in a field. Usually, average density is reported and, if one wants to know the total number of organisms, one multiplies average density by total area. If the density of brown bears in the Great Smoky Mountains National Park is 0.3 per square mile and the park covers 400 square miles, there are 120 bears in the park in total.
- The Average (or Mean) is calculated by totaling all observations and dividing by the number of observations.
- Variance is calculated by subtracting the mean from each observation to get the difference between the two (the difference is often called a deviation), squaring each deviation (never mind here about why the deviations are squared - ask in class if you are curious), and taking the total of the deviations. This total is then divided by the number of observations and the final figure represents the average of the squared deviations.
Organisms can be distributed within an area in one of three ways:
Uniform - when organisms are distributed so that there the distance between organisms does not vary much so that all organisms are about the average distance apart. This yields a pattern that is grid-like.
In mathematical terms, uniform patterns have a variance in density that is less than the average density. Think a second about why this should be so. If uniform distributions are grid-like (think of the points of intersection in a square grid), then all of samples that measure the density (done by counting number of organisms in an area of some standard size) will contain about the same number of organisms (some error is always present) and none will differ from the average by much. Thus, the deviations between the actual distances and the average distance will all be small, the square of those deviations will be small, and the variance will be small.
Random - neither clumped or uniform. In this situation, there is no relationship between the locations of individuals.
In mathematical terms, random is defined as the pattern that results in the variance in distance between organisms is equal to the average distance between organisms.
Clumped - ramets or genets occur as clumps (areas of higher density than random expectation) surrounded by areas where fewer than expected organisms occur, like islands in a lake.
In mathematical terms, clumped distributions have a variation in density that is larger than the average distance or density. The variance is large because some of the densities will be small when the sample comes from a location between clumps and other sample densities will be large if they contain one or more clumps. The mean density will be somewhere in between the large and small values. When calculating the deviations, observations will be large or small and the average in between, giving larger deviations than, for instance, a uniform distribution. The squares of the deviations will be large and the resulting variance will be large.
Knowledge of spatial pattern can be useful to an ecologist interested in the interactions among the members of a population under study. Different spatial patterns arise from different causes.
Uniform distributions often indicate that interactions among the population members is setting a minimum distance between them. The interaction might be competition for light (one tree doesn't want to be shaded by another), each organism is defending a territory,
Clumped distributions are common and can result from a number of causes. If there is Heterogeneity (also called Patchiness) in the quality of the area within the boundary, then patches of high quality would support a higher density of organisms and result in a clump on the high-quality patch. Modular growth of plants can cause clumps of individuals to develop around a single genet or ramet. Animal social groups are clumps.
Abundance of organisms in a population can be considered from two perspectives. Total abundance is simply the size of the population, whether you measure it as organisms, ramets, genets, or even biomass. However, as we have seen above, spatial pattern in important to distribution. It can also affect the meaning of population abundance.
- Consider a population of desert fig trees. There are only 10 trees in 200 square miles. This is not a large population and the density is so low that there should be very little interaction among the trees. However, the population distribution is clumped. There is one stream that runs through the 200 square miles and all of the trees occur along its banks. Now, water taken by one tree may reduce the water available to trees farther downstream.
- Many ecologists have worked on the difference between density as an average and the real density experienced by members of the population. In clumped situations, the usual means of calculating density is often misleading.
- The book separates the two ways of looking at abundance into Crude Density and Ecological Density. Many ecologists call the average density simply the density and refer to the second aspect as the Crowding experienced by the population. Clumped populations experience more crowding. Means of calculating crowding have been devised but we will not cover them here.
One of the basic things one should know about a population is its size. It is rarely possible to count or weigh all members of a population, and so one needs to take Samples in order to predict population size. Whenever you use samples to make predictions about the entire population, you enter the realm where statistics rule.
The method of assessing population size depends on the organism being studied. The most important distinction is between those that are motile and those that are sessile (sedentary)
Sessile organisms are often sampled by using Quadrat Sampling - plots of appropriate size are set out and the number of organisms are counted in each of them. Statistical techniques are used to estimate the total population size from a sample of plots.
An example of a statistical approach to abundance of motile organisms is the use of the Lincoln-Peterson Index to estimate population size. It is based on the idea that the proportion of marked individuals in a sample should be the same as the proportion of marked individuals in the entire population.
- You would make this assumption in a heart beat.
- Suppose I told you that the huge jar of jelly beans contained 30% lemon and 70% cherry flavored beans. They are thoroughly mixed and I pour out 200 beans. How many should be lemon.
- Assume the sample's proportion of marked beans is the same as the population's proportion of marked beans. The sample is the 200 beans, the marked beans are the lemon beans, and the beans in the jar are the population. Then 30% of the sample should be lemon, or 60 beans out of 200 should be lemon.
- Lincoln-Peterson reverses the logic.
- If you mark 50 individuals, let them return to the population and mix thoroughly, and take a sample that contains 300 individuals and 2 of the 300 are marked, you can predict the populations size.
- The proportion of marked individuals in the sample (2/300) should be the same as the proportion of marked individuals in the population.
- You marked 50 individuals, which is the numerator, and the population size is the denominator (50/PopSize). So, 2/300 must be the same ratio as 50/PopSize. Fifty is 25 times greater than 2 and so PopSize must be 25 times greater than 300. The population size is 7,500.
Remember, 7500 is a prediction made from a sample, not an actual count of the population size. The methods of prediction can get much more complicated to improve the accuracy (Lincoln-Peterson is not very accurate) and can allow a more dynamic look at population size as organisms die, migrate, or age. We will see some of these methods in the laboratory sessions.
Population Characteristics related to Abundance
We have seen that abundance is a more complicated phenomenon when examined in depth. There are other characteristics of populations to consider that are related to abundance.
Organisms are born (or germinate) and age until death. Age structure is the distribution of members of the population according to their age. We usually calculate it as a proportion of the total population in each age class. We use classes because age is a continuous variable but we usually divide age into classes of equal length. Sometimes the classes are not of equal length. For insects, the age classes are often life cycle stages (egg, larva, pupa, adult).
Different age structures can lead to very different predictions about the fate of a population. A population composed of individuals too old to reproduce is a population fated for extinction unless younger individuals are added through migration.
One means of depicting age structure is to make a horizontal histogram so that the ages stack up from youngest to oldest. From this, one can quickly assess some of the history of the population and some of its future. A wide pyramid is characteristic of a growing population (lots of young). A tower is characteristic of a stationary or declining population (few young). Events that affected one or two age classes can be seen (after WWII, age distributions in some European countries were missing most military age men and this could be seen even as this age class aged into retirement)
The ratio of males to females in a population has great consequence.
Only females produce new offspring and so the number of females sets the limit to the growth rate of the population. However, the genetic makeup of each generation comes half from the females and half from the males. Compare the potential growth in a population with 50 females and 50 males compared to a population with 90 females and 10 males. Assuming that the 10 males can fertilize all of the eggs produced by the 90 females, the second population can produce many more offspring. However, half of the genes in each population come from the males, whether it is from the 50 in the first population or the 10 in the second.
Optimizing the sex ratio is a complicated business and has lead to an entire body of mathematical theory, Sex Ratio Theory. We will deal with some aspects of it in subsequent lectures.
Dispersal - the movement of organisms - is another of those things that becomes more complicated under close examination.
Dispersal is a time of stress and danger for organisms.
All organisms have a dispersal phase as part of their life history.
These two statements might seem to be incompatible. Why move if one is endangering oneself in doing so? The fact that all organisms do must mean that the consequences of not moving are even more dire.
If one assumes that all things change, then any area occupied by an organism will, at some point, undergo fluctuation is some important characteristic. It may be a flood due to a local storm, the arrival of a disease with an immigrant, the arrival of a predator, or the exhaustion of non-renewable resources. If all of your offspring are in the same patch with you when things go bad, your lineage is extinct. Dispersal may increase the mortality of you offspring but it also avoids the ultimate loss.
Dispersal phases can be any portion of the life history. In animals, adults may disperse (birds) or juveniles may disperse (larvae of animals attached to rocks, like mussels and barnacles). In plants, seeds often have special adaptations for dispersal (wings to aid wind dispersal, fruits to encourage animal dispersal). Even microbes must disperse. Some microbes produce special cells that can better withstand the rigors of dispersal (Spores and Cysts) and some form relationships with animals that disperse the microbes.
Populations are usually not stand-alone entities. Most populations exchange migrants with other populations. This interconnectedness of populations means that one can view a set of interconnected populations as a Metapopulation. When we are referring to a metapopulation, those populations that compose the metapopulation are called Subpopulations.
This can be confusing if you think that the term subpopulation refers to a division of a population. It doesn't. It refers to a division of a metapopulation. Each subpopulation is what we have been calling a population.
Modular Growth, Clone, Genet, Ramet, Biomass, Spatial Scale, Distribution, Geographic Range, Edaphic Conditions, Spatial Pattern, Population Density, Average, Mean, Variance, Uniform, Random, Clumped, Heterogeneity, Patchiness, Crude Density, Ecological Density, Crowding, Samples, Motile, Sessile, Quadrat Sampling, Lincoln-Peterson Index, Age structure, Sex Ratio, Dispersal, Migration, Emigration, Immigration, Metapopulation, Subpopulations
Last updated February 3, 2007