Physical and Chemical Factors

Tolerance Range
The habitat is the physical portion of the environment. The physical features or abiotic components of the environment that are often of importance in ecology are water availability, temperature range, pH, salinity, wind speed, etc. For any of these factors there is a range over which the species is able to survive. This is known as the tolerance range for that factor and it represents the physiological limits for that species. Each species has it's own tolerance range that affects the type of habitat it can be found in. Goldfish, for example, prefer temperatures between 6 and 34° C while brook trout have a tolerance range between 0.5 and 25° C.

Temperature
Temperature can act on any stage of the life cycle to limit an organisms distribution.   It may affect survival, reproduction, development of young, or the competition with other organisms.  In addition, predation, parasitism, and disease may be affected right temperature extremes. Depending on the species, maximum temperature, mean and temperature, average (or some combination) may be important.

Poikilotherms limited by temperature.

The mosquito Aedes aegypti has a world-wide distribution in tropical and semitropical zones. It is important because it is a vector for yellow fever and dengu fever. The distribution of the mosquito is sharply limited by latitude and temperature:

1. 10° C is lethal to adults and larvae
2. The egg stage is more resistant to low temperatures and in some areas, Aedes aegypti can overwinter as eggs.
3. In the temperate zone eggs can overwinter but relatively low summer temperatures cause the adults to develop slowly and females die before they reach maturity and lay eggs.

Global warming could affect the distribution of these species, allowing them and other tropical diseases to move into the temperate zone.

Homeotherms limited by temperature.
Homeothermy is a type of metabolism where an organism expends energy to keep its core temperature within a limited range. This process of thermoregulation allows the animal to be continuously active with a constant metabolic rate. With a constant rate, the animal can be active at all times of the day and throughout the seasons. It doesn't have to wait to warm up in the sun in order to become active in the morning. Poikilothermic animals, on the other hand, take on the ambient temperature as their core temperature (although they do have some "tricks" listed below). Crickets, for example chirp at a rate that is dependent on temperature. At low temperatures their metabolism is decreased and all their biochemical processes run slower (nervous system, muscles, etc) so the chirp rate is slower. Since females are also cool, there perception is slowed and they don't notice that the males are singing slower (although all the homeotherms around them would appear to be moving faster from the cricket's point of view).

Some poikelothermic animals control their temperature through behavioral means:

1. Bees will clump at the center of a hive beat their wings to build up muscle temperature (similar to shivering) to maintain hive temperature. When it's hot, they collect at the hive entrance and use their wings to create an air flow through the hive to cool it down.
2. Many snakes will shiver to increase muscle temperature when brooding their eggs (boas, for example).
3. Lizards fight viral diseases by inducing a behavioral fever. They sun themselves to increase their body temperature far above normal to destroy the virus. Fever in homeotherms serves the same purpose. One reason why your nose clogs up when you have a cold is to increase the temperature in your nasal cavity to fight off the cold virus.
4. Although homeothermic, birds often will open their wings toward the rising sun to capture heat to warm up in the morning. Similarly, lizards will turn broadside to the sun and lower their dewlaps to collect as much heat energy as possible.

Homeothermy

FACTORS AFFECTING DISTRIBUTION: MOISTURE

For most organisms, temperature and water are the most important factors for limiting their distribution. For terrestrial organisms, the problem is usually straight-forward: they don't have enough water. To conserve water, we see a number of physiological and anatomical adaptations including waxy cuticles, oils, specialized metabolisms and/or kidney structures). Freshwater fish tend to gain water while saltwater fish have similar problems as terrestrial animals.  Plants are often divided into groups that depend on their water needs. Xerophytes have thick cuticles, and prefer dry habitats. Hydrophytes, on the other hand, are water-loving plants. Mesophytes are plants found somewhere in the middle of the moisture scale.

Tree lines (shown above) are a dramatic example of how abiotic factors can affect the distribution of a species. Tree lines are affected by the following:

SOIL CHARACTERISTICS
Soil and dirt are not the same thing. Soil is a complex, living mixture of microorganisms, minerals, organic compounds, air, and water. Dirt is misplaced soil on the bottom of your shoe that no longer has a useful purpose. Soil formation starts when simple organisms, such as moss and lichens settle on a rock. Release of carbonic acid by these organisms begins to dissolve the rock and release minerals. At the same time, the structure of the plants begins to catch wind-blown dust which is added to the soil mixture. The mosses and lichens also begin to attract bacteria, fungi, protists, and animals that begin adding organic materials to the soil through their excretions and deaths. When the mat thickens higher forms of life (both plant and animal) can begin taking advantage of the site which leads to an acceleration in the rate of soil formation.

Soil productivity.

• Nitrogen, potassium, and phosphorus are needed, but they have to be in the proper form or they won't be available to plants. Microorganisms (especially certain bacteria) are necessary to convert these elements into compounds that can be used by plants.
• Trace elements are also needed to support proper plant growth Iron, magnesium, boron, zinc, cobalt, and sulfur are often found in the form of salts.
• pH affects productivity by changing the charge of the soil. Soil charge allows it to hold on to nutrients. If it hangs on too tightly, it won't be available to plants. Too loosely and it'll be washed away when it rains.

TOO MUCH OF A GOOD THING: THE GULF OF MEXICO DEAD ZONE
The growth and distribution of many organisms is normally held at bay because they can't get enough of a particular nutrient. One such limiting factor for both freshwater and saltwater algae is nitrogen. Nitrogen runoff from fertilizer applications on farmlands in the Midwest eventually makes its way to the Mississippi. Off the coast of Louisiana, these nutrients are dumped into the Gulf of Mexico where they encourage the growth of algae. These algal blooms are so huge that the algae remove all the oxygen from the water. Shrimp and fish can move out of the way, but clams and other bottom-dwelling animals die from lack of oxygen. These then begin to rot, encouraging the growth of foul-smelling anaerobic bacteria.

The dead zone in the Gulf is mapped out by a satellite as chlorophyll concentration in the image below. The dead zone can be as large as 7000 square miles.

Dead Zone Link <= REQUIRED!

OTHER PHYSICAL AND CHEMICAL FACTORS THAT CAN AFFECT DISTRIBUTIONS

• Light: Flatworms and pill bugs prefer dark places.
• Fire: Many pines require fire to melt the resin in the cones to release the seeds for growth.
• Oxygen: Anaerobic bacteria are limited to the oxygen-poor muck at the bottom of a pond.
• Salinity: Many shore plants are able to tolerate salty soils.
• pH: Bog plants are capable of withstanding acid conditions that would kill most plants.
• Soil nutrients: Plants differ in their nutrient requirements. Some are capable of living on nutrient-poor soils (serpentine soils).