Characteristics of Bioclimatic Zones

World

Climate

There is a temperature gradient from the equator to the poles because of the earth's orientation toward the sun. The rotation of the earth and the inertia of the air above it causes westerly winds at the equator. The Coriolis force deflects winds to the right in the northern hemisphere and to the left in the southern hemisphere, causing wind gyres. Equatorial air rises and, deflected, falls back to the surface in temperate latitudes, where it then flows toward the east. Wind patterns produce major ocean currents revolving counter-clockwise in the northern hemisphere and clockwise in the southern hemisphere in each ocean basin. Current gyres carry cold water toward the equator on east sides of the oceans, warm water to higher latitudes on the west sides. The temperature of water bodies affects nearby land masses greatly, although there is less effect with distance from water. Land masses and underwater topography deflect currents into complex patterns near the continents. These in turn cause upwellings, which affect productivity and thus diversity and abundance of marine organisms, including those such as birds and pinnipeds that are based on land.

The wind blows water vapor from the ocean to the land. If the land is hotter, it evaporates; if cooler, it falls as precipitation. As the air ascends and cools, it loses its capacity to hold water (adiabatic cooling). Thus in hot weather thermals rise, and water condenses and falls as showers; also precipitation increases up mountainsides. Mountains serve as barriers to water vapor, causing rain shadows on their leeward sides. The tilting of the earth, with accompanying migration of the thermal equator, causes seasonal changes of wind direction responsible for the seasonality of precipitation patterns in the tropics; precipitation is also affected by the seasons in temperate latitudes.

Soils

Parent rock erodes physically and chemically into smaller and smaller particles, which become soils as they collect on both coarse and fine scales; the chemical nature of soil reflects the nature of its parent rock. Particles are further sorted by size when deposited by water and wind. Soils are classified by the size of the particles that make them up, from the finest (clay) through increasingly coarse (silt, sand) to the coarsest (gravel), and by organic content (humus). Loam is soil that is well mixed with sands, silts and clays. Soils form horizons, layers that are distinct from one another chemically and physically because of their distance from the surface (which in turn controls the amount of water and organic material present). The overall direction of water movement (up or down) through soil is of some importance in determining its structure. The water-holding capacity of soil is related to the soil type, with the finest soils holding water and coarser ones allowing it to percolate through. The value of soil to plants is related to its water-holding capacity, physical nature (what mixture of sand, silt and clay particles), and chemical content; both inorganic minerals and organic humus are important.

Vegetation

Vegetation patterns are responsive to the interaction of temperature (latitudinal and altitudinal gradients) and precipitation (more complex, often longitudinal gradients because of the east-west tendency of the winds, as well as well-defined latitudinal ones) patterns. The degree of seasonality also has a major effect. The earth's surface is forested with trees except where too cold, too dry, or too wet. Herbaceous vegetation is dominant nearer extremes, with no vegetation at all at the coldest and driest extremes.

Vegetation is the sum of its parts, the parts being individual plants of individual species, and the species present and their relative proportions are important descriptive components of it. Descriptions of vegetation take into account plant species and their abundance and size distribution, life forms (trees, shrubs, herbs, bryoids, epiphytes, lianas), seasonal functions (evergreen vs. deciduous), and leaf shapes, sizes, and textures.

Diversity

There are two major components of diversity: within-habitat (diversity may vary greatly between two forest types, for example) and between-habitat (this covers diversity for an entire area, thus also dependent on the diversity of habitats present). Within-habitat diversity is highest in forests of the equatorial lowlands, and lower with increased latitude, altitude, seasonal variation in climate, simplicity of vegetation, and any environmental extreme, to very low near poles and on high mountains. There are similar latitudinal gradients in fresh and salt water, also decreased aquatic diversity with depth and chemical extremes. There is also lowered diversity on peninsulas and islands because of problems of historical access.

Plant Adaptations

The basic needs of plants are homeostasis, space, food, predator avoidance, and reproduction. There is no locomotion, thus plants are subject to vicissitudes of the environment, so they are often narrowly restricted to particular combinations of temperature, humidity, salinity, soil water, and chemistry (where homeostasis is thus assured). Plants need space for branches and stems for support of light-gathering, photosynthetic, and respiratory leaves; elaboration of woody tissue provides greater support for big plants. They also need space underground for roots for gas and chemical exchange with the substrate, and for nutrient and water gathering. Competition for space is intense among plants, at leaf and root level. Plant nutrition comes from a combination of leaf photosynthesis and root gathering. Predator avoidance is by physical (spines, hairs, sticky resins) and/or chemical means (there is an almost infinite number of chemicals in plants that lower their palatability to animals) or even by rarity of the plant, its flowers or fruits.

Reproduction is by flowers and fruits. As locomotion is lacking, reproduction depends on other features of the environment--wind, water, or animal pollination and seed dispersal. Animals are often important for dispersal of pollen and seeds. Reproductive structures vary widely: inflorescence type, flower sizes, shapes and colors; infructescence type, fruit sizes, shapes and colors; seed sizes and numbers. Most of these are clearly correlated with the mode of pollination/seed dispersal.

Animal Adaptations

Animal basic needs include homeostasis, space, refuge, food, predator avoidance, and reproduction. Homeostasis needs lead to occurrence only in a restricted range of temperature, humidity, salinity, oxygen level, and other features that affect the animal's internal environment. There is great variation in tolerance to environmental extremes, maximal in endothermal birds and mammals. Land and water are two basic environments, and many animals move between them or secondarily into the air for relatively brief periods. Locomotion is significant in animals (lacking in plants for most part) to allow these movements, also movement about home range and long-distance movements that may be for dispersal or may involve seasonal back-and-forth migration. Some major groups of animals are sessile (fixed in place) and like plants move only in certain life-history stages. Space needs lead to fixed or dynamic home ranges, which are defended in some sessile or territorial motile animals. The size of the home range or territory may change seasonally or be different in different age/sex classes. Refuges are needed for hiding from predators, sleeping, storing food, and having and rearing young.

Food sources are plants (herbivores) or other animals (carnivores), and diets vary from very generalized to very specialized. Many animals are omnivores (plant and animal diets). Herbivores specialize on leaves, seeds, fruits or nectar, with different arrays of adaptations for each type. Leaves are easy to capture but difficult to digest, so leaf-eaters have special adaptations to process cellulose and antiherbivore compounds. Seeds are locally and seasonally abundant, and are often stored by caching for leaner periods. Special adaptations (rodent teeth, finch bill) are needed to crack seed coats. Color vision is an important adaptation to locate flowers and fruits for both nectar- and fruit-eating, with much coevolutionary fine tuning for these pollinating and seed-dispersing interactions. There are just as many specialized sets of adaptations for carnivory, including those for finding and capturing plankton, mollusks, worms, termites, butterflies, fish, snakes, birds, and other morphologically distinctive taxonomic groups, even carrion. Memory and intelligence are characteristic components of some of these predator adaptations, but there are many other special attributes, such as the use of tools or venom.

There are many predator-avoidance adaptations, including hiding out of sight or in sight (camouflage); escaping by speed or stealth or immersion in a group; fighting or threat with weapons (teeth, horns); physical protection such as spines or shells; chemical protection such as unpalatability or poisonous bite or sting; and mimicry of unpalatable/dangerous animals. There is a wide variation in reproductive strategies: clutch size, size of offspring at birth, eggs vs. live birth, and degree of parental care.

Most types of adaptations are clearly attributable to one of these basic needs. For example, coloration may be an adaptation for species recognition (need for conspecific territory defenders to recognize each other involves space; need for mates to recognize one another involves reproduction), to look like the environment (camouflage for predator avoidance or prey capture) or like some other animal (mimicry for predator avoidance) or to warn of its dangerousness (aposematic coloration for predator avoidance) or to increase or decrease heat load by absorbing or reflecting sunlight (homeostasis).

Human Effects

Those that are negative to the environment include habitat destruction, chemical pollution, predator/parasite introduction, competitor introduction, and active persecution for food, skin, sport, etc. Those that are positive to the environment include conservation by habitat preservation and captive breeding. Those that are positive to humans and may or may not be harmful to the environment include animal domestication and horticulture.