Ecosystem Components, Dynamics, Limiting Factors (NOTES)

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					Ecosystem Components, Dynamics, Limiting Factors
We have been discussing natural selection and selective pressure — the idea that
environmental conditions can put stress/pressure on certain phenotypes that are less
fit. Now, we will look at what some of those environmental factors are, and how they
influence/interact with organisms. For example, plants need specific amounts of
certain minerals in the soil to grow and reproduce.

Recall that Shelford’s Law of Tolerance states that if an organism has too much or
too little of a substance/condition, it will not be able to grow well, if at all. Organisms
can be affected by too little or too much of water ( including humidity), temperature
of the surrounding medium (and therefore the amount of heat available to be
absorbed), food (nutrients), light (including intensity, quality/color, and duration),
salinity, partial pressure of various gases, and/or current/pressure (a moving vs
stagnant medium). An organism’s or species’ tolerance to conditions may vary
seasonally, geographically, or depending on stage in the organism's life cycle. A
number of insect species overwinter as cold-tolerant eggs or pupae at temperatures
that would kill adults who are not cold-resistant/tolerant, and in many cases the insect
will only go on to the next stage in its life cycle if it has received adequate exposure to
cold temperatures.

Seeds of some species of pine need exposure to fire to germinate and grow and many
plant seeds around here need exposure to cold temperatures or to a certain amount of
light to germinate and grow. Also, organisms’ ranges of tolerance vary. Some
organism can tolerate a wide range of conditions, while others will tolerate only a
narrow range of specific conditions. Thus, it could be said, for example, that an
organism is stenothermal or eurythermal with reference to range of temperature
tolerance, and stenohaline or euryhaline with reference to range of salt-concentration
There are several other
important, related terms of
which to be aware.
The medium in which an
organism lives is the material
(liquid water or gaseous air) in
direct contact with the
organism and with which
materials are exchanged. An
organism’s response to
current/flow of the medium is
calledrheotaxis if the organism
moves in response to the
current orrheotropism if the
organism leans or grows in
response to the current. The substrate or substratum is the surface in/on which an
organism lives. This serves as a place to rest, attach, for protection, and/or for
nourishment. An organism’s response to the substrate is called thigmotaxis if the
organism moves in response to touch or thigmotropism or stereotropism when an
organism grows or leans in response to touch or contact with something solid. Some
animals, such as rats and roaches, like to feel closed in and live in tight spaces, thus
they are said to be positively thigmotaxic, while a cat uses its long whiskers to avoid
tight places and is said to be negatively thigmotaxic. The tendrils on vines would
bepositively thigmotropic.

Hopefully, you recall that an ecosystem includes all the organisms within the system
plus the physical environment as well as the various interactions, cycles, and energy
exchanges that tie the whole thing together. Remember that the term “ecosystem” was
first used by A. G. Tansley in a discussion of how the organisms and inorganic
components/factors fit together to make up a system. An ecosystem can include both
major and minor communities.

 A major community is a self-sustaining      A minor community is dependent on
 community, for example: a whole forest.     another community for its energy input, for
                                             example: a rotting log or a dead crow
(Note: When the second photo was taken, there were five Question Marks and one
Red Admiral on this dead crow. The Red Admiral is the butterfly on the left. At least
three or four of the Question Marks are visible here.)

The abiotic input in an ecosystem includes:

      heat and light (collectively referred to as radiant energy), especially from the
       Sun, which influence temperature cycles, humidity cycles, and overall energy
       input, therefore the productive capabilities of the system
      inorganic materials/matter, including nutrients, O2, H2O, CO2, etc., some of
       which are used for growth and repair, others for energy flow, and still others,
       especially pesticides introduced by humans, are harmful or toxic

The abiotic components of an ecosystem include things like:

      soil
      various particles/chemicals in H2O
      dead organic material in detritus or leaf litter

The biotic components of an ecosystem include:

      producers or autotrophs, which usually are plants
      consumers or heterotrophs, which can include:
          o primary consumers or herbivores which eat plant material
          o secondary and tertiary consumers (or higher) or carnivores which eat
            other animals
          o decomposers, which are important to nutrient cycles and energy flow
            because they help to break down the bodies of dead organisms and return
            the nutrients to the soil
The indicator species in an ecosystem are significant not because of their numbers,
but because they indicate existing conditions. For example, blueberries indicate/will
only grow in acidic soil, thistle and ironweed indicate overgrazing, broomsedge
indicates acidic soil, and hydrangea indicates landslides/erosion.

Frequently, ecosystems are named for the dominant (= most numerous) species in the
community or for a geographical feature. For example, “beech-maple forest,” “oak-
hickory forest,” “young foredune,” and “open beach” are all names given to specific
types of ecosystems. Just as with any other system, an ecosystem is, we hope, in a
state of dynamic equilibrium which is always changing but balanced in terms of its
various cycles, fluctuations, etc.

Some Factors/Cycles Include:

    Light possesses three qualities/properties which must be considered when
    studying its effect on an ecosystem: intensity, quality/color, and duration
    (including daily and yearly fluctuations). Daily changes in light levels also
    change humidity and temperature levels. As light decreases, temperature
    decreases and humidity increases.
      Photoperiodism refers to an organism’s response to
      changing amounts of light. Typically, most
      organisms exhibit circadian rhythm which means
      that they exhibit an activity cycle of about 24 hours.
      This daily activity cycle involves or is triggered by
      both the organism’s internal biological
      clock and photoreceptors and by the organism’s
      response to the actual photoperiod. Most organisms
      have one of three basic kinds of cycles, depending on
      when they are most active:

             Diurnal organisms are active during the
             Nocturnal organisms are active during the
             Crepuscular organisms (such as owl
              butterflies) are active at dawn and/or dusk.

Photoperiodism is tied to navigation in some species. For example, honeybees
communicate the location of a nectar source to other bees in the colony through the
“Waggle Dance.” The angle from the top of the hive to the direction of the “waggle”
is the same as the angle from wherever the sun is to the location of the flowers. As the
sun moves across the sky, the direction of the “waggle” changes. Honeybees are even
capable of orienting correctly to the location of the sun at night when the sun is on the
other side of the earth!
       Bioluminescence is a term used to describe organisms, such as lightening bugs,
       that “glow in the dark.” In most cases, light is produced as a result of the action
       of the enzyme luciferase on the substrate luciferin. This is a very energy-
       efficient reaction, and almost no heat is produced/given off.

       An organism’s response to light is called phototaxis if the organism moves in
       response to light and phototropism if the organism grows or leans in response
       to light. Organisms may be said to be positively or negatively phototaxic or
       phototropic. For example, fruit flies are positively phototaxic and roaches are
       negatively phototaxic, houseplants sitting on a windowsill are positively

    The yearly light cycle is influenced by the earth’s journey around the sun.
    Special points to note include the spring and autumn equinoxes and the
    summer and winter solstices. These changes in incoming light and heat also
    cause cycles in airflow and ocean currents.

       Especially in temperate zones with widely-varying seasons and fluctuations in
       light, temperature, etc., organisms’ sensitivities to changing day length trigger
       various phases in their annual life cycles. For example, the terms “long-day
       plants” and “short-day plants” refer to whether flower-set (blooming) is
       triggered by increasing or decreasing day length.

 The angle of incidence of incoming light depends on latitude and is important in determining
 how much light reaches understory plants. In tropical rainforests, almost no light reaches the
 floor, but in our eastern North American oak-hickory forests, about 35% of the incident light
 reaches the floor. Here, however, the amount of light which reaches the floor varies depending
 on season, with more light able to “get through” in winter when the angle is lower than in
 summer when the angle is higher. Also, many woodland flowers bloom in spring when there
 are few leaves on the trees and more light gets to the floor.
    There are both daily and seasonal heat cycles, and organisms must be able to
    respond/cope with these changes. Also, there is quite a wide range of
    temperatures on earth, from the poles to the equator to thermal vents,
    volcanoes, hot springs, etc. The temperature of a given habitat and an
    organism’s tolerance to temperature both vary or are affected by time of day
    and season. An endothermic or homeothermic or “warm-blooded,” animal
    maintains body heat from within, making use of blood flow and countercurrent
    heat exchange to help maintain a constant temperature.

      Much like a heat pump for your house or your
      refrigerator coils, an animal’s circulatory system
      is involved in countercurrent
      heating/cooling of its body. Arteries and veins
      lying near each other in the extremities, but
      flowing in opposite directions can absorb heat
      from each other as needed. When the animal’s
      core temperature is too high, the arteries carry
      heat to the extremities to be dissipated. As the
      blood returns via the veins, any excess heat still
      in the blood is transferred to the arterial blood
      and sent to the extremities, again. When the core
      temperature is too low, as the blood flows out in the arteries to nourish the
      extremities, its heat is transferred to the venous blood and sent back into the
      body to keep it warm.

      Some endothermic animals are able to lower their body temperature at certain
      times to conserve energy resources. Hibernation is a long-term (overwinter)
      decrease in body functions, while estivation is a short-term (overnight)
      decrease in body functions. Hummingbirds estivate every night to conserve

      Skunk cabbage is an endothermic plant! Because it blooms in early spring, it
      generates heat from within to maintain a warm temperature in its spadix and

      An exothermic, ectothermic or poikilothermic or “cold-blooded” animal
      maintains body heat from outside sources. The term “cold-blooded” really is
      not accurate because these organisms do maintain an internal temperature that
      is different from that of their external environment. A lizard in the desert will
sun itself on a rock to warm up in the morning, and will seek a cool, shady
place to spend the afternoon.

An interesting special case is that of honeybees. Individual honeybees, like
other insects, are exothermic, but a hive collectively is endothermic. In winter,
the bees shiver to generate heat and warm the hive, and in summer, they bring
in water and fan it with their wings to evaporate the water and cool the hive.
The temperature in the area of the hive where the immature bees are being
raised is kept at a fairly constant temperature of about 93° F.

For an exothermic organism, the rates of
the various chemical reactions and
physiological processes in its body will
vary with temperature. For each of these
processes/reactions, the change in its rate
is defined in terms of a 10° C change in
temperature, and this value is called Q10.

For example, if a cricket respires 20
molecules of CO2/min @ 25° C and 40
molecules/min @ 35° C, then Q10 =
(40/20)(10/(35-25)) = 2, so for every 10° C
increase in temperature, the rate would
double. Thus, the cricket would respire 10
molecules/min @ 15° C and 80
molecules/min @ 45° C (if the cricket
could withstand that temperature).

Acclimation is when an individual organism “gets used to” its environment. In
humans, a 50° F day in spring feelswarmer than a 50° day in autumn because
we are acclimated to either the cold winter weather or the hot summer weather.
Whether or not animals are able to acclimate to a change in temperature
depends on the rate of the temperature change, the rate at which the animal can
acclimate, and other behavioral patterns such as migration, etc.
      Degree-days = the number of days (or hours, etc.) above a given minimum
      temperature × the number of degrees above that minimum temperature (= 6°
      C?). Thus, 600 degree-days could be accumulated via a long, cool season or a
      short, warm season. Often, plants need a minimum number of degree-days to
      accumulate enough warmth for growth and development. Many farmers plant
      their corn based on the number of degree-days that have accumulated, knowing
      that the soil will then be warm enough for the corn to germinate. Conversely,
      some seeds must be chilled (and must accumulate a given amount of cold) to
      break dormancy. Botanists generally refer to this as vernalization while
      horticulturists generally refer to the same process as “stratification.” Many of
      our local insects also need cold weather to trigger proper development. For
      example Cecropia moth pupae will never emerge as adults unless exposed to a
      sufficient amount of cold weather.

   Water is a key ingredient in all life. Cells are 70 to 95% water. About 75% of
   the Earth’s surface is covered with water. Water is the only common substance
   existing naturally in all three forms: solid, liquid, gas. Water has many unique
   properties due, in great part, to its hydrogen bonding. Water is important to
   living organisms as a solvent, so even land-dwelling organisms need it.
   Hopefully, you recall last year’s discussion of hypertonic, hypotonic,
   and isotonic solutions.
                                                               rain ocean, lake,
                                                               river, and ground
                                                               H2O plants
                                                               evaporation from all of
                                                               the above rain

                                                               The amount of rainfall
                                                               varies with the overall
                                                               local climate, season,
                                                               etc., and this, in turn,
                                                               causes variations in the
                                                               amount of water in the
                                                               soil, therefore available
                                                               to the local plants and
                                                               animals. The organisms,
                                                               then, must be able to
                                                               adjust to these variations
                                                               in available water.
      Absolute humidity refers to the actual amount of humidity in a given volume
      of air. Relative humidity is the percentage of the theoretical possible humidity
      the air could hold at that temperature, the percent of total saturation. Hopefully,
      you recall from your chemistry classes that the partial pressure of water or
      other gases in the air = % of mixture ×barometric pressure.

      The rainfall and temperature of an ecosystem can be studied simultaneously by
      combining them in a climograph (or climatograph), a graph of average
      monthly rainfall (on the X-axis) vs average monthly temperature (on the Y-
      axis). Sometimes, relative humidity may be represented by the X-axis and/or
      other modifications may be made as needed to study the data. Construction of
      these graphs is discussed in more detail in a separate Web page on climate.

      Some ecosystems depend
      on annual or periodic fires
      to release nutrients, kill
      “invading” species,
      germinate seeds, etc.
      Many humans now realize
      that controlled burns can,
      thus, be used to “manage”
      certain ecosystems. This
      prairie area in Adams
      County was purposely
      burned the previous year
      to kill unwanted
      “invaders.” The native
      prairie plants, which
      evolved in an
      environment that
      experiences periodic fire,
      were not negatively
      affected and are
    Tidal cycles (high tide, low tide) are influenced by the pull of the moon, thus
    these cycles are especially important to costal/marine organisms where
    reproduction, etc. are tied to the lunar cycles and tides. Note how much lower
    the water level is in the first picture than in the second. Note the debris in the
    second photograph indicating that the water level typically gets even higher.
    The various nutrients, minerals, and gases in an ecosystem go through cycles,
    too. For example:
    CO2 sugar molecules in plants via photosynthesis other organic
    molecules in plants herbivores carnivores decomposers
     release from all of the above CO2

      Human-introduced chemicals like DDT also are passed up the food chain, as
      they are stored in the liver (when present) and fatty tissue of organisms. For
      example, suppose that some DDT from agricultural use would run off into the
      local pond. From there, it would be absorbed and incorporated into the bodies
      of the various plants that live in the pond. If each small fish would eat ten
      plants, and each big fish would eat ten small fish, then each big fish would have
      all the DDT in 100 plants. Suppose, then, that some predatory bird would eat
      ten big fish, and a Peregrine Falcon would eat ten of the smaller, predatory
      birds. That would mean the falcon’s body would contain all the DDT in 10,000
      of the original plants! This is just a hypothetical example, and Peregrine
      Falcons eat a lot more than that. Thus, before DDT was banned, the falcons
      nearly went extinct because the DDT levels in their bodies were so high that
      they interfered with calcium metabolism, causing major problems with egg
      shell production (the eggs essentially had no shells and were destroyed when
      the adults “sat” on them to incubate them). A major problem as new pesticides
      and herbicides are developed is that the developers tend to study the effects on
      only “target” species and not the whole ecosystem.

      The trophic levels in a food chain usually include producers like plants,
      primary consumers or herbivores, secondary and tertiary consumers or
      carnivores, and decomposers, each of which eats organisms in the next-lowest
      trophic level. There are several different kinds of food chains, including:
      predator chain (probably the most familiar): plant herbivore
        carnivore larger carnivore. . .
      parasite chain: various organisms sequentially parasitize one another
      saprophyte chain: decomposers, especially fungi, which feed off dead
       organic matter, including the bodies of other decomposers.

Food webs consist of the interactions among several food chains. These can be
diagramed as pyramids.

      A numbers pyramid is based only on
       numbers of organism at each level, and does
       not take into account things like size nor
       growth rate.
      A biomass pyramid is based on both
       numbers and size, but still doesn’t account
       for turnover rate (for example, grass grows).
      An energy pyramid is based on grams or
       Calories/area/time, and so does take all those
       factors into account, but is much harder to
       construct. With an energy pyramid, it is
       possible to examine the efficiency of each
       trophic level (intake vs production) or
       compare levels (production vs production).
       Herbivores make more efficient use of food
       than carnivores. The average American is
       actually eating more grain than people in
       Third World countries, but as pigs, cows, and

Many of the various minerals and other nutrients needed by living organisms
can be remembered by the aid:

                         C HOPKINS CaFe Mighty good
but a few other important ones, like sodium (Na), are not included in that list.
For these nutrients, the amounts needed relative to each other are important to
life, as is the state or condition of each. Soil pH can influence solubility and
usability by affecting the number of valence electrons (for example, Fe++ vs
Fe+3). Keep in mind that too much is harmful, too. Macronutrients, such as
Ca, P, and N, are required and found in relatively high amounts in organism’s
bodies. Micronutrients or trace minerals such as Mn, I, or Co, are definitely
needed but are required and found in relatively smaller/lower amounts in
organisms’ bodies. Even though very little of these is needed, a dietary shortage
can be a serious problem. Too much of these can also be bad — we need cobalt
(Co) in vitamin B12, but too much cobalt in one’s diet is toxic.

Plants absorb these nutrients from the soil and pass them on to herbivores,
which are then eaten by carnivores, etc.Humus is incompletely decomposed
organic material in the soil (a stage in the breakdown of materials into minerals,
salts, etc.), and provides a rich source of nutrients for growing plants. To
maintain a constant level, organic material must be added. Normally this occurs
through the death of organisms in the ecosystem and through the annual fall of
leaves from deciduous trees. In the “good old days,” farmers plowed cornstalks
and other plant parts into the soil after harvest and fertilized their soil with
manure, thereby replacing the humus layer in their soil. However, most farmers
no longer use their manure as fertilizer, and often plant stubble is removed from
the fields due to concerns about remaining insects (a problem caused by
monoculture), thus the humus is not replaced, and the soil becomes less and
less fertile. Usually, then, the farmer resorts to strong, chemical fertilizers
which have the side effect of killing any “good” microbes and earthworms in
the soil, essentially sterilizing it. Once the soil is totally depleted and
abandoned, it takes years for the soil to recover.

However, it is not only possible, but better (for the soil, the earthworms, the
environment in general, the plants, and the cattle or people who eat those
plants) to return the “compost” to the soil, to rotate crops, and to manage one’s
fields in a manner that does not require reliance on concentrated, toxic,
synthetic fertilizers, herbicides, and pesticides. For example, the Hartzler
family has been successfully growing crops this way on their northern-Ohio
farm since the 1950s, with the results that their soil is “healthy” and full of
earthworms (a good indicator of soil conditions)
and that their farming methods have been studied
by ecologists from OSU and around the world.

Levels/horizons of the soil profile (from the top
down) include: litter, duff, leaf mold, humus,
leached humus, accumulation of minerals in subsoil, rocky material, and

Some soil types include:

      alluvial soil, which is water-deposited, such as in delta areas
      glacial till, which is glacier-deposited
      loess, which is wind-deposited, such as in dunes, or a dust bowl
      chernozem, a rich, black topsoil with a lower layer of lime, typically
       occurring in an area with a small amount of rain so the rain doesn’t leech
       away Ca++ which stays in the humus
      podzol, an ash-like, gray layer over red, acidic humus, typically
       occurring in forested areas with significant rainfall so the rain washes
       minerals deeper (the terms “podzolic soils,” and “podzolization” are also

                                    laterite, which is red, porous deposits containing
                                    large amounts of aluminum and iron hydroxides
                                    (laterilization) — extremely leeched, acid soil,
                                    weathered to a great depth, low in nutrients,
                                    reddish because of iron oxides, found in tropical
                                    and subtropical areas and southeastern United

      pedocal, which is formed by calcification (cal refers to calcium)
      pedalfer, which is formed by podzolization and contains oxides, etc.
       below or in the bottom layer of the soil, forming a hard, crusty layer
       called hardpan (al refers to aluminum and fer refers to iron)

Earth’s atmosphere is about 21% O2, about 19% N2, 0.03% CO2, plus other
gases. Recall that at standard sea-level pressure, 1 m of any gas fills 22.4 L of
space, but (remember PV=nRT?) at 18,000 ft, the pressure is ½ and volume is 2
× per mole. The partial pressure of O2 is different at different altitudes, and
since animals must get O2 to all their body tissue, terrestrial animals which
breathe “air” must be able to acclimate to the local O2 concentration. Humans
in Chile can live permanently up to 17,000 ft, and can work temporarily up to
18,000 ft. At 19,000 ft, the liver, etc. start to deteriorate. Supposedly, Chilean
women who live high in the mountains must go to lower altitudes to give birth.
Also, apparently at one time, some men in a balloon went up to 26,000 ft and
Different animals have different means of getting O2 to their body tissue.
Insects have a finely-divided tracheal system that transports air directly to their
body organs. Fish and some other aquatic animals have gills which allow air
from the water to diffuse into their bloodstreams. We have lungs containing
many tiny alveoli (sacks for air exchange), which collectively have a
tremendous surface area (greater than the surface area of our skin).

O2 is used as the final electron
acceptor in the electron transport
chain during cellular respiration.
Various respiratory pigments in
animals’ blood help to carry O2 to
their body tissues and
include hemoglobinwhich contains
a porphyrin ring with iron (Fe) in
the center, andhemocyanin which
contains a porphyrin ring with copper
(Cu) in the center. In organisms with
hemoglobin, the amount of
hemoglobin per RBC is fixed, so at
higher altitudes,

      more RBCs are formed,
      more oxidative enzymes are found in the tissue of people/animals, and
      there are more capillaries per area of tissue

The opposite is true in diving animals such as porpoises and seals. They
concentrate their blood in the center of their bodies, and because their blood is
in a smaller area of their bodies, their heart rates can be slower and their hearts
do not have to work as hard. They have more myoglobin in their muscles to
store O2. So that the gases in their blood don’t come out of solution during a
dive and so that lungs full of air don’t make them more bouyant, many diving
animals exhale before a dive and depend on circulation and metabolism to
provide the needed oxygen. In humans, stressed muscles do lactic acid
fermentation, and the build-up of lactic acid in muscle tissue causes sore, stiff
muscles, but diving animals such as seals do lactic acid fermentation while
diving, then take in O2 when they surface and re-convert the lactic acid that has
built up in their bodies to pyruvic acid, which is then sent through the Krebs
cycle and electron transport chain to finish aerobic respiration. Remember that
plants also do cellular respiration and need O2, too. If there is too much water
in the soil, a plant’s roots can’t get O2, and the plant “drowns” and dies.
Similarly, earthworms need the high humidity of damp soil because they
“breathe” through their skin, but they will drown in totally water-logged soil.
Thus, in a heavy rain, many earthworms come to the surface so they can get
sufficient oxygen. Unfortunately, many of them end up on our sidewalks where
they dehydrate if they can’t find a way back into the soil.

As mentioned above, CO2 is incorporated into plant tissue via photosynthesis
(carbon fixation) and released from the bodies of those plants and the animals
which eat them as a waste product of cellular respiration. CO2 can also be
incorporated into limestone rocks via both biotic and abiotic processes. The
chemical reactions involved in this are:

                      CO2 + H2O H2CO3 2H+ + CO3-2
                           Ca++ + CO3-2 CaCO3
The White Cliffs of Dover are a build-up of limestone
“shells” of formerly-living plankton. Salmon recognize
“their” stream by its CO2 content and return there to mate
and lay their eggs. Female mosquitoes zero in on
CO2 (and moisture) released from a potential host’s body
(sweat) to find a blood meal to provide the protein
needed for their eggs to develop.

Somewhat similarly, N2 is absorbed from the air and turned into organic
compounds (nitrogen fixation) by bacteria in genus Rhizobium which are
found in root nodules on clover and other legumes.

                         It has been noted that the ratio of “regular” hydrogen (1H) to
                         “heavy” hydrogen (2H) in rainwater (H2O) varies with and can
                         be correlated with location. This knowledge has been used to
                         track the migration of Monarch butterflies. Milkweed plants in a
                         given area absorb the local rainwater, and as they do
                         photosynthesis, that hydrogen is incorporated into their bodies.
                         As the Monarch caterpillars in that location feed on that
                         milkweed, that hydrogen is incorporated into their bodies. Thus,
                         the ratio of 1H to 2H in the bodies of adult Monarchs collected in
                         the overwintering areas in Mexico also varies and can be used to
                         determine from where those Monarchs migrated.

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