19.1 Habitat
Key Takeaways
A habitat is the place an organism lives, its address, described by abiotic and biotic factors.
Abiotic factors are nonliving conditions: temperature, water, light, salinity, soil, pH, and oxygen.
Biotic factors are living influences: predators, prey, competitors, parasites, mutualists, and vegetation.
The tolerance range runs from too little to too much of a factor, and Liebig's limiting factor is the requirement in shortest supply.
Intertidal barnacles are often limited upward by drying and downward by competition or predation, and a microhabitat can be as small as the moist underside of a log.
19.1 Habitat
The place, not the job
A habitat is the place an organism lives. On CLEP Biology items it is the address, not the organism's job and not a menu of foods or foraging times. Two oak woods on the same map can still be different habitats if one is wet, shaded, and full of predators while the other is dry and open. Name the place at the scale the organism uses, then name the factors that make it usable.
This section stays with place. The next section separates the niche, the organism's role, from this address. A question about where something lives is a habitat question. A question about diet or timing is about that role, which is not defined here.
Every habitat needs two kinds of factors. Abiotic factors are nonliving chemical and physical conditions. Biotic factors are other living things and their effects. A lake is not cold water alone, and it is not fish alone. It is water of a certain temperature, oxygen level, and pH, plus the predators, prey, competitors, and plants that live in it.
Abiotic factors
- Temperature sets the pace of metabolism. Too hot or too cold can exclude a species even when food is abundant.
- Water includes rainfall, soil moisture, humidity, and standing water. Drying kills many shoreline animals that otherwise have enough to eat.
- Light powers photosynthesis and also times daily activity. A dim forest floor can exclude herbs that thrive in a canopy gap.
- Salinity is the salt content of water or soil. Few organisms move freely between fresh water and the open ocean.
- Soil supplies minerals, holds water, and gives roots and burrows a structure. Sand and clay under the same rain support different plants.
- pH, the acidity or alkalinity of water or soil, changes which nutrients dissolve and which animals can keep body chemistry stable.
- Oxygen is critical in water and in packed mud. Warm, still water holds less dissolved oxygen than a cold stream.
Climate is abiotic. Do not call rainfall, heat, light, or salinity biotic.
Biotic factors
- Predators remove individuals and can change where prey dare to feed.
- Prey and other foods decide whether energy is available at that address.
- Competitors use the same limited resource and can push a species out of a site that looks physically suitable.
- Parasites and pathogens can make a place unusable during an outbreak.
- Mutualists, such as pollinators, mycorrhizal fungi, or required gut microbes, can be necessary partners. A site may fail if the partner cannot live there.
- Vegetation is living structure. Plants shade the ground, slow the wind, drop litter, and feed herbivores.
Tolerance range and the limiting factor
The tolerance range is the span between too little and too much of a factor. Shelford's law of tolerance says both ends can exclude a species. Performance is usually best near an optimum and weaker near either edge. Past either limit, survival or reproduction fails. A trout with plenty of food still disappears from water above its thermal limit. A salt-marsh plant can be absent where the water is too fresh as well as where it is too salty.
A limiting factor is the requirement that most restricts growth or survival in a particular place. Justus von Liebig's law of the minimum says the resource in shortest supply relative to need sets the pace. Extra amounts of other resources do not cancel that shortage. A meadow can have a comfortable temperature, plenty of light, and regular rain, and still lack a plant if soil nitrogen is scarce. The temperature looks fine. The population is absent or stunted because a different factor is limiting.
The tolerance range is the window for each factor. Liebig's limiting factor is the essential requirement currently in shortest supply. A site inside the temperature window can still fail for lack of oxygen, a harmful pH, or too little water. One failed requirement is enough to explain an absence.
Microhabitats
A microhabitat is a small patch of conditions inside a larger place. The sunny, dry surface of a fallen log and the moist underside of that same log are different microhabitats. The top may bake and lose water by afternoon. The underside stays cool, dark, and damp. Fungi, isopods, and many salamanders use the moist underside, while animals that need heat use the exposed surface. If the stem contrasts sun and shade, or wet and dry, name that patch rather than the whole forest.
Barnacles on a rocky shore
Intertidal barnacles show the two factor lists drawing different edges of one habitat. High on the rocks, long exposure to air lets drying and heat set an upper limit. Lower on the shore, drying is milder, yet many barnacles are still absent or rare. Competition from other barnacles, or predation by whelks, sets that lower limit. The lower rocks remain a habitat for other organisms. They are outside this species' usable habitat because a biotic factor, not drying, excludes it. Milder physical conditions do not guarantee presence.
| Kind of factor | Examples | What an item is testing |
|---|---|---|
| Abiotic | Temperature, water, light, salinity, soil, pH, oxygen | Nonliving conditions, including climate |
| Biotic | Predators, prey, competitors, parasites, mutualists, vegetation | Living influences, not a synonym for climate |
| Tolerance range | Too little and too much of one factor | Why a species is absent outside its window |
| Limiting factor | The scarce requirement, in Liebig's sense | Why one failed need matters when other factors look fine |
Warning
A habitat is the place an organism lives, not the organism's job. Abiotic factors are nonliving conditions such as temperature, water, light, salinity, soil, pH, and oxygen. Biotic factors are living influences such as predators, prey, competitors, parasites, mutualists, and vegetation. Do not call the climate biotic.
Reading an absence
Name the scale first, including the microhabitat when sun and shade or wet and dry are the contrast. Then test the tolerance range, name Liebig's scarce requirement if one resource has run short, and check predators, competitors, parasites, mutualists, and vegetation. A stream and a pond can share a temperature and still differ in oxygen. That is a difference in place, not a description of feeding role.
Which description fits a habitat?
The place an organism lives, set by abiotic and biotic factors
The organism's job, including its diet and daily activity schedule
Whichever single nutrient happens to be scarcest at the moment
Only the nonliving climate, such as temperature and rainfall
Drying is milder low on a rocky shore, but a barnacle species is still missing there. Which explanation fits the classic pattern?
The upper limit and the lower limit must both be caused by drying
Those lower rocks are not a habitat, because seawater covers them for part of the day
Competition or predation can set the lower limit even when drying is weaker
Biotic factors are nonliving conditions such as salinity, light, and oxygen
Temperature at a site is inside a plant's tolerance range, yet the plant is absent. Which statement explains that absence?
A different requirement, such as water, nitrogen, or pH, can still be the limiting factor
Liebig's idea says the most abundant resource sets the pace of growth
Temperature is the only abiotic factor that can ever limit a plant
A microhabitat is always identical to the whole landscape, so soil cannot matter
Sections you finish are checked off in the contents.