Chapter 4 Cont'd. Population

We looked briefly at the Ecology of individuals. Then, we shifted to an overview of the Biome and Vegetation, or Landscape. Once we finally started a more detailed explanation of the Principles which govern the distribution of communities, we saw the limits imposed on population density by the environment. We now turn our attention to the mechanisms which allow populations to grow to these limits.

population = group of individuals of same species

We have already defined the population to be “all of the individuals of a species which occupy the same site and interact with each other.” One of the most important of the interactions within a population is reproduction, if for no other reason, because the individuals which make up the population are mortal… and will die. If the population is to survive longer than the life expectancy of its members, which it normally does for many generations of individuals, it is essential that the individuals must reproduce. For Animals, reproduction is always (with exceptions) sexual, but Plants have mechanisms for asexual reproduction in addition to sexual reproduction. Based on a study by a colleague of mine, even in Milkweeds (Asclepias spp) which produce large numbers of seeds, on new ‘road cuts’ [widening of highways by cutting into the hillsides along the right-of-way] in Illinois about 60% of the first growth of the milkweeds comes from previously buried root stocks and only 40% from seed. Many plants will form adventitious shoot buds at injury sites on the root stock, and many can form adventitious root buds on broken ends of fallen limbs [for example, willows (Salix spp)]. Some can do both. In a curious case, the dandelion (Teraxacum officinale) produces seeds both sexually (pollination by at least 93 different species of Insects [botanical.com]), or asexually (apomixis, “which occurs in ca. 60% of the British flora” [Richards, A. J., 2003. “Apomixis in flowering plants: an overview” Philosophical Transactions of the Royal Society of London, B Biological Science. 2003 June 29; 358(1434): 1085–1093.]).

    The population is also considered to be a fundamental unit of Genetics, along with the individual (as a mating pair). In Genetics, a trait is “an inheritable characteristic of a living organism.” Each individual has a genotype which is a listing of both alleles for each gene (or at least for the interesting traits) of the species. For an imaginary creature (“Cute & Fuzzy Bunnies©” from the Warner Brothers™ movie One Crazy Summer) which may have brown hair (B, dominant) or tan hair (b, recessive) and long hair (L, dominant) or short hair (l, recessive), the individuals would be {BB LL, BB Ll, BB ll, Bb LL, Bb Ll, Bb ll, bb LL, bb Ll, bb ll} [{members of the set} is the mathematical symbol to represent a set.] The population equivalent to the individual's genotype is the ‘gene pool.’ The gene pool is a listing of all possible alleles (sometimes there are more than 2 per gene, and only 1 for a particular gene would make the trait controlled by this gene ‘uninteresting’) for the species, plus the relative frequencies of each allele in the population. ‘Relative frequency’ is the total count of the allele across the population divided by the sum of the counts of the alleles for the gene. For a population of our Cute & Fuzzy Bunnies© with 12 pure-bred brown, short haired individuals (BB ll); 6 hybrid brown, pure-bred long haired individuals (Bb LL); and 2 tan, hybrid long haired individuals (bb Ll):

Thus, the gene pool illustrated for this imaginary “population” is: {pr(B) = 0.75, pr(b) = 0.25; pr(L) = 0.35, pr(l) = 0.65}.

population parameters

Populations exhibit a population density or the number of individuals of the species per unit area. An issue unique to Plant Ecology is how to define one individual. Many plants [such as Staghorn sumac (Rhus typhina)] grow in clumps which result from branching of the roots or rhizomes [a horizontal underground stem]. It can be argued that each stem is a separate individual (produced by asexual reproduction), or that the entire clump is a single individual (with each stem as a branch). Some Ecologists have even proposed defining the flower to be effectively the genetic individual for flowering plants. Although the interpretation of data on population density would be strongly influenced by the definition of an individual used, most plant population data is collected on the presumption that we all know what we mean by ‘an individual.’
    Populations also exhibit “dispersion,” which is a (statistical) description of how the individuals are distributed within the population. They can be randomly distributed (the statistical expectation), clumped or clustered (under dispersed), or more evenly distributed than random expectation (over dispersed). The under dispersed pattern most often results from the seeds landing, germinating, and growing close to the parent plant. The most familiar example of an overdispersed population occurs in semi-arid conditions where the parent plant releases a chemical to the soil causing inhibition of germination next to the parent [for example, Creosote bush (Larrea tridentata) which as the name implies deposits creosote to inhibit germination of competitors (of all species) for the limited water in a roughly circular area around the plant].
    For animals the mechanisms for dispersion are mostly “territorial behavior” where territory is a piece of real estate which the resident mating pair defends from invasion by other individuals, “Home range” which is the real estate within which the animals searches for food, and is generally larger than the territory.

    Each population has a characteristic reproductive potential defined as the maximum number of offspring which can be produced by an individual. This value determines how quickly an immigrant individual (or reproductive pair) can grow to the population density which can be supported within the limits set by the environment. We will return to this in a later chapter as we develop the basis for a theory of plant community ecology.

Life Table (l_xm_x-tables) based on cohort

The life span of a species or a population is the age at death of the oldest known individual of the species (or population) [for Humans, the Life Span is 122 yrs 164 days (a French woman), and 115 yrs 252 days (an American man)]. A more important measure of population age is life expectancy defined as the age by which 50% of the population can be expected to die.
The age structure of a population can be calculated to predict short-term changes in population density, for which we set up a “Life Table.” The population age structure is valid only if each ‘age group’ has the same age range (most frequently one year since the year is divided into a growing season and a non-growing season, but sometimes some other age range if there is a reason to select a different age range). We need to define the parameters used in the Life Table analysis, but will skip the mathematical procedures to calculate those parameters derived from the age structure:
        x = age group, usually years but may be any time interval
        N_x = number alive at start of interval (the age structure from sample data)
        l_x = proportion surving to end of interval (usually equal to the data value for the next oldest age group)
        d_x = number dying during interval
        q_x = probability of dying, age-specific mortality
        e_x = life expectancy for survivors
Notice, life expectancy is stated as the number of additional intervals that 50% of the surviving population at the beginning of the next interval can expect to live.
To illustrate a Life Table, we can use the red deer (Cervus elaphus)

We can also set up Life Tables for plants, for example the following summary for Rhododendrum

The best available data for Human (Homo sapiens) Life Tables comes from the U.S. Census Bureau, for example from the 1980 census

Normally the population age structure will be a pyramid with declining population density in the older age groups. Sometimes, there will be a cohort (“all of the individuals born in the same time period”) produced which is unusually large [such as the Human ‘baby boomers’], or which is unusually small, and this group will move up through the population age structure. In addition, as the unusual cohort reaches reproductive age they can be expected to produce a secondary boom (or decline). The population density should continue to change (increasing or declining) through several generations.

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revised: 25 Feb 2011