5. Ecology

Ecology is the study of the interactions among the organisms on a site, and of the interactions between the organisms and the physical environment. The levels of organization involved are the population, the community, and the ecosystem (and perhaps the entire Biosphere, or all parts of the entire Earth where Life can be found). The ultimate intent is to develop the necessary theories to account for the organizational dynamics of the Biosphere. An important point here is that organization at ecological levels is dynamic, or is subject to change over time. We are currently in a time period of Climate Change, potentially a major Climate shift. The controversy among Scientists is about the causes of this Climate Change, not whether or not Climate Change is real [The magnitude of the Climate Change is a matter of speculation, not of Science]. Given the challenges for Human Civilization to persist during the current Climate Change, there is some urgency to understand how (or even if) Life as we know it can survive this Climate Change. It is worth noting that Life itself can, and has, survived several major Climate shifts (between global tropical conditions and global Ice Ages), but with mass extinctions followed by complete replacement of the dominant Plants and Animals on Earth.
Understanding Nature becomes complicated because there are so many species involved, even on rather small tracts of land: on enough undeveloped land (such as a weedlot) to hold a typical suburban school (including the school grounds as well as the building) there could be 100 or so plant species, 100’s of Arthropod species, and 10’s of Chordate species. If we hope to have our students begin to understand how Nature functions, we must start by examining artificial, simplified models of natural systems of living creatures. Thus we begin with the population.

5.1 Populations

In Ecological terms, a Population is “a group of individuals of the same species occupying a single site and interacting with each other.”
We have already seen two aspects of the population in Chapter 3.2 Classical Genetics & Evolution, where we examined Population Growth Potential and Population Genetics and the implications of these for the concept of Evolution. In Genetics and Evolution, the most interesting interaction between the members of the population is reproduction, as a means for transmitting genetic information from one generation to the next. In Ecology, the more interesting interaction is competition. We already know from Genetics and Evolution that competition must occur because we can observe that populations change little from year to year in spite of the reproduction potential to grow (in number of individuals) indefinitely.
    There are five population parameters which we shall examine in this chapter: Population density, Dispersion, Migratory patterns, Age structure, and Distribution of populations.

Population density

The number of individuals in a population is called “population density.” Density seems a reasonable term here because the population is defined as occupying a given site, implying that it has a definable area. The details of the behavior of population density were initially discovered in a series of laboratory experiments in which the population could be isolated from other populations. The series of experiments began with a single species, then added a second species. The analysis of the results led to the development of tentative mathematical descriptions of population growth. However, the math involved was Calculus which was designed by Isaac Newton specifically to explain Physics (which he considered to behave as a rigorous mathematical system, with simple cause-effect relationships). The assumptions of Calculus do not seem to apply well to Ecology which appears to have multiple possible states (or effects), each with a probability of occurring given a single cause.

Single species populations

When we compare the Reproductive potential (non-overlapping generations, litter size = 6, from Chapter 3.2) to simulated data (environment [available food supply] will support only 26 mice) for “actual,” observed mouse population in a house, we see that the population does not grow as fast as its potential, due to competition. The population even drops as the competition becomes too severe. It will recover over 3 generations back up to 9 pairs, then drop again. Thus one cause of fluctuations in the population size is a somewhat cyclic rise and fall due to competition between members of the population. Since this is typical of living systems (but less so for non-living, chemical and physical, systems), the fluctuating population size is called a “dynamic equilibrium.”

House mice (Mus musculus)
		potential		population
	date	n	pairs	n	pairs
P-1	9/22/10	2	1	2	1
F-1	12/1/10	6	3	5	2
F-2	2/9/11	18	9	9	4
F-3	4/20/11	54	27	15	7
F-4	6/29/11	162	81	17	8
F-5	9/7/11	486	243	16	8
F-6	11/16/11	1458	729	18	9
F-7	1/25/12	4374	2187	15	7

Two species populations

Based on the definition of a population, it is not possible to have a population of two species, so by ‘two species populations’ we clearly mean to say “two populations.” A ‘species population’ uses ‘species’ (improperly) as a adjective to distinguish the population from a statistical population, which is a defined group of data points (the definition describes the statistical universe from which the sample data can be drawn). The distinction between a species population and a statistical population is needed because at least a majority of ecologists currently use statistical methods to design experiments and to analyze the resulting data.

competition

When we add a second population, there is inter-species competition in addition to the intra-species competition. The second species will make demands on the same resources over which the members of the first species were already competing. At the dynamic equilibrium, as one species declines, the other will grow, and the fluctuations in either species population can be expected to be rather larger than in the artificial single population model. By extension, we should not be surprised [although the environmentalists tend to be not only surprised, but alarmed as if the natural world were tetering on the brink of collapse] if the fluctuations increase even more as more species are added to the system. These systems of multiple species are called Communities, defined as “all populations (usually of species within the same Order) occupying the same site.” Plant commuities of more than 200 species are not unusual; Insect communities and Spider communities have more species than do Plant communities; Amphibian and Reptile [called herpetological] communities, Bird communities, and Mammal communities) communities generally have fewer species.

populations of predators and prey

A special case is when one population (predator) eats the other population (prey). In such cases, there is no competition for the same resources; the prey species is the resource for the predator. In such model systems under laboratory situations, an increase in the prey species population results in more food for the predators, so the predator population grows. The increase in predator population increases the number of prey eaten, causing a decline in the prey population. The decline in prey population decreases the food supply for the predators, causing a decline in the predator population. The decline in the predator population decreases the numbers of prey eaten, causing the prey population to increase. Thus the two populations exhibit strongly coupled cyclic fluctuations.
    Adding more prey species allows the predators to ‘switch’ prey as the initial prey population declines; and adding more predator species causes competition between predator species for the prey. In both the case of multiple prey species and the case of multiple predator species, the coupling of the predator and prey species fluctuations becomes weaker. It is believed that this leads to a damping of the flucuations in the wild compared to what is seen in the laboratory. To my knowledge, there is no data to support this hypothesis, and ample data to support the notion that fluctuations in population density occur in natural systems.

Other Population Parameters

Dispersion describes the spacing between individuals of the population, and the mechanisms utilized by the species to create and maintain this spacing. In Plant Ecology, we consider “evenly dispersed” to mean that the individuals are randomly distributed within the population; “clumped” refers to those species in which the probability of finding another individual near an observed individual is higher than what would occur if the individuals were evenly dispersed; and “over-dispersed” refers to individuals maintaining an area of fewer individuals near an observed individual. Clumped dispersions tend to occur when seeds travel only short distances from the parent plant, and when asexual reproduction is commonplace. The most spectacular instances of over-dispersion occur in species whose leaves release a growth-suppressing chemical as the leaves decompose [for example creasote bush in Utah and California semi-deserts release creasote to suppress seed germination reducing competition for limited water; creasote is highly flammable, and burns with high temperature flames, leading to the spectacular wildfires of southern California]. In Animal Ecology, there are behavioral patterns [such as territoriality, in which a breeding pair will defend an area around the nest to preserve the food supply for feeding the young]. “Home range” is the area in which the individuals search for food, but without defending the home range from invaders (although many Animals ‘mark’ their home range with scents to discourage invaders).

    Migratory patterns. “Migration” as used in Animal Ecology implies the intent to return, and usually applies to seasonal movements between the breeding season (Summer) grounds and the off-season (Winter) grounds, which may involve North-South migrations or high altitude-low altitude migrations. When the young adults migrate from their parents home range in search of breeding sites, they emigrate from one area, and immigrate to another as a ‘permanent’ move. A mneumonic to remember which is which: “Emigrate begins with ‘E’ as in EXIT while Immigrate begins with ‘I’ as in IN.”

    Age structure was introduced to predict future population densities. The procedure required creating a “Life Table” (l_xm_x-table) based on cohort (a group of individuals born during a given time period, usually a year. The calculations involved are difficult (and confusing), but not worth repeating here because the Life Table is rarely used today. I will show you three examples just so you can get an idea of how this works. The terms in the table are:
  x = age group, usually a calendar year,
  N_x = number alive at start of interval,
  l_x = proportion surving to end of interval,
  d_x = number dying during interval,
  q_x = probability of dying, or age-specific mortality,
  e_x = life expectancy for survivors, as the number of intervals that 50% of the individuals in the current age group can be expected to survive.

        Example 1. A full Life Table for a population of Red deer, a wild animal found in southern England.

Red Deer females Lowe, 1969
x	Nx	lx	dx	qx	ex
2	1000	1.000	61	0.0610	3.35
3	939	0.939	185	0.1970	2.53
4	754	0.754	249	0.3302	2.03
5	505	0.505	200	0.3960	1.79
6	305	0.305	119	0.3901	1.63
7	186	0.186	54	0.2903	1.35
8	132	0.132	107	0.8105	0.70
9	25	0.025	25	1.0000	0.50

        Example 2. an abbreviated Life Table for a population of Rhodendron sp, a wild plant found in the Smokey Mountains of the U.S. southeast

Rhododendron sp McGraw, 1989
x		lx	dx	qx	ex
0		1.000	0	0	5.60
1		1.000	0.016	0.016	4.60
2		0.984	0	0	3.67
3		0.984	0.075	0.077	2.67
4		0.909	0.185	0.024	1.85
5		0.724	0.346	0.477	1.19
6		0.378	0.270	0.714	0.82
7		0.108	0.095	0.882	0.62
8		0.013	0.013	1.000	0.50

        Example 3. a highly abbreviated Life Table for the 1980 Human population of the United States (U.S. Census Bureau data). The interval is one year, but only every 10^th year is shown to shorten the table. The current full table can be viewed at 2011 Statistical Abstracts, U.S. Census Bureau.

Life expectancy Human, USA 1980
	male	female
x	ex	ex
0	70.83	75.83
10	61.66	66.53
20	52.37	57.04
30	43.24	47.65
40	34.05	38.36
50	25.36	29.53
60	17.51	21.25
70	10.96	13.67
80	6.18	7.48
90	3.18	3.45
100	0.50	0.50

If you were a 30 year old male in 1980, you could have expected to live another 43 years and about 3 months or to the age of 73. If you were a 30 year old female in 1980, you could have expected to live another 47 years and about 6 months or to the age of 77. More accurately, of all 30 year old males alive in 1980, 50% would have been expected to die before the age of 73 yrs 3 months, and the other 50% would have been expected to live longer than an additional 43 yrs 3 months; and of all 30 year old females alive in 1980, 50% would have been expected to die before the age of 77 yrs 6 months, and the other 50% would have been expected to live longer than an additional 47 yrs 6 months. For what it is worth, out of 10,000 30yr old men buying $20,000 simple life insurance policies in 1980, the insurance company would expect to pay out $20,000 to 5,000 policy holders or $100,000,000 by 2023, so would need to collect $232.56 per year (or $19.38 per month) from each of the 10,000 policy holders to break even [this is the legal definition of simple life insurance]. The insurance company would profit on the investment of the net collected premiums until the last surviving policy holder died. Each surviving policy holder would pay the $232.56 per year until death. Of course, the insurance company gets to keep all collected premiums on each policy which lapsed due to non-payment of the annual premiums.

Distribution of populations

Most populations, and species, have limited distribution. An extreme of limited distribution is Kirkland's warbler (Dendroica kirklandii, Songbird Family) with a breeding range including only sandy jack pine barrens of northern lower Michigan, or about 15,500 km² (6,000 sq miles) or 1,600,000 hectares (3,800,000 acres). This illustrates the two aspects of distribution: geographic limits [northern lower Michigan] (often stated as “county records” or a list of all counties where specimens have been collected or observed by reliable observers) and habitat (“a general description of the environmental conditions where the population can be expected to be present”) [sandy Jack pine barrens]. For any of the readers who are interested, Kirkland's warbler winters in the Bahamas and nearby islands (Turks, Caicos and Hispaniola).
    In 1840, Leibig published the Law of the Minimum [Sprengel actually publish the same hypothesis in 1828] which hypothesizes that, for Plants, one of the factors in the physical-chemical environment will limit the growth of a plant (and if the amount of the limiting factor is increased, another factor will become limiting). For Animals, the limiting factor will usually be the food supply. The Sprengel-Leibig Law is still used in agriculture to design fertilizer regimes to increase yields.

The principle of limiting factors was generalized by Shelford in 1911 as the Law of (Ecological) Tolerance. As now understood (after research on the physiological response of plants to the levels of environmental factors), Ecological tolerance is a bell-shaped curve (probably the statistical Normal distribution familiar to teachers as the grading curve) showing the relationship of plant response to environmental factors. The highest point on the curve is the optimum condition at which the plant has the best growth, reproduction, etc. Shelford divided the rest of the curve into ‘zones:’ an optimal zone [the “C” grades] on either side of the optimum point where growth is good; sub-optimal zones on either side of the optimal zone (the “B” and “D” grades]; beyond the sub-optimal zones are the marginal zones [“A” and “F” grades] where survival is barely possible; and the lethal zones where the plant cannot survive. If we knew the physiological response of the species to the environmental factors involved, we should be able to predict how well the population would do under any level of the environmental factor. When the environment forms a gradient, such as depth to the water table (and therefore availability of water, or risk of flooding) from a stream, across the floodplain and up into the surrounding hills, this model predicts that the population density will change along the gradient. Other species populations on the site should, according to modern interpretation of the theory of Evolution, have different optimum points. This is ‘plot material’ for the next two sections (Chapter 5.2 Communities, and Chapter 5.3 Ecosystems).

Works Cited

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