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 Shelfords 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 organisms 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.
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.
The abiotic input in an ecosystem includes:
The abiotic components of an ecosystem include things like:
The biotic components of an ecosystem include:
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.
|Photoperiodism refers to an organisms 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 organisms internal biological clock and photoreceptors and by the organisms response to the actual photoperiod. Most organisms have one of three basic kinds of cycles, depending on when they are most active:||
An organisms 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.
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.
Much like a heat pump for your house or your refrigerator coils, an animals 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 animals 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.
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.
|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.
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 flourishing.||
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 falcons 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:
Food webs consist of the interactions among several food chains. These can be diagramed as pyramids.
Many of the various minerals and other nutrients needed by living organisms can be remembered by the aid:
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:
||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|
Earths 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,
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:
|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 hosts 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.|
Copyright © 1999 by J. Stein Carter. All rights reserved.
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