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.

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 called rheotaxis if the organism moves in response to the current or rheotropism 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 be positively 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 community, for example: a whole forest.	A minor community is dependent on another community for its energy input, for example: a rotting log or a dead crow

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:
  • primary consumers or herbivores which eat plant material
  • secondary and tertiary consumers (or higher) or carnivores which eat other animals
  • 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:

DAILY LIGHT CYCLE

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 daytime. Nocturnal organisms are active during the nighttime. 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 phototropic.

YEARLY SUN CYCLE

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.

HEAT CYCLES

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 energy.

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 flowers.

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 feels warmer 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 CYCLE

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 herbivores carnivores 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.

FIRE CYCLES

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 flourishing.

LUNAR/TIDAL CYCLE

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.

NUTRIENT, MINERAL, GAS CYCLES

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 chickens!

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 bedrock.

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 used)

• 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 States

• 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 ˝ 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 died.

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 hemoglobin which contains a porphyrin ring with iron (Fe) in the center, and hemocyanin 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.