Ecology: Distribution and Adaptations of Organisms
Introduction
Ecology is the scientific study of the interactions of organisms and their environments. Both abiotic and biotic factors are included in an organism's environment. Organisms are not only effected by their environment, but also organisms can also effect their environment.
I. THE SCOPE AND DEVELOPMENT OF ECOLOGY
Ecology is the study of the distribution and abundance of organisms.
There are four levels of inquiry in ecology:
Organismal ecology studies how individual organisms tolerate environmental stresses that determine where they can live. This includes the study of behavioural, physiological and morphological ways individuals meet environmental challenges.
Population ecology studies groups of individuals of the same species in a particular geographic area.
Community ecology studies all organisms that inhabit a particular area. The questions of community ecology concern predation, competition and other interactions that affect community structure and organisation.
Ecosystem ecology studies all abiotic factors as well as communities in an area. The questions of Ecosystem ecology concern energy flow and chemical cycling though the environment.
B. Ecology as an Experimental Science
Ecology has a long history as a descriptive science but is young as an experimental science. This is partly due to the fact that it is very difficult to conduct experiments and control variables. Some ecologists test hypotheses in lab experiments. These experiments create a very simplified system that can be controlled by the researcher. Others have attempted understand how communities work by manipulating large communities in field experiments. Lab experiments are so simple that they may not represent the real world, but the results are easier to interpret and understand. While field experiments are more representative of real life, they yield results that are very difficult to interpret. In order to be able to manipulate many complex variables at once some ecologists devise mathematical models that include important variables and hypothetical relationships, usually studied with the aid of a computer. This approach is appealing because it might allow ecologists to simulate large-scale experiments that might be impossible to conduct in the field.
C. Ecology and Evolution
Darwin realised that the short-term interactions of organisms with their environments could have long term effects through natural selection. Another way of thinking of this is that momentary episodes enacted in the frame sometimes called “ecological time” translates into effects which extend over much longer time period called “evolutionary time”. For example, predator-prey interactions may affect gene pools where individuals with protective coloration would become more prevalent. Another connection between ecology and evolution is seen in the effects of geological history on the current distribution of species. for example the pattern of continental drift helps explain the geographical distribution of many animal groups seen today.
II. TERRESTRIAL BIOMES
The biosphere is that portion of Earth inhabited by life and represents the sum of all communities and ecosystems. The biosphere is a thin layer consisting of seas, lakes, rivers, streams, the land to a soil depth of a few meters, and the atmosphere to an altitude of a few kilometres.
Biomes are the major types of communities that are typical of broad geographic areas.
Terrestrial biomes are often named for the predominant vegetation but each is also characterised by animals adapted to that particular environment. The species composition may vary from one location to another within a biome, and biomes may grade into each other without sharp boundaries. Biomes may be patchy, with several communities represented in one biome. The prevailing climate, particularly temperature and rainfall, is most important factor in determining what kind of biome develops.
Tropical forest is found near the equator (within 23.5º latitude) where temperature varies little from approximately 23ºC and the length of daylight varies from 12 hours by less than one hour over the whole season. Rainfall is variable and the amount determines what vegetation is found in an area. In lowlands with prolonged dry seasons and scarce rain, tropical thorn forests occur - these are a mixture of thorny trees and shrubs, and succulents. In regions with distinct wet and dry seasons, tropical deciduous forests occur. Trees relief following heavy rains and drop leaves during the dry season. Near the equator where rainfall is abundant (>250 cm/year) and the dry season lasts less than a few months is tropical rain forest (fig. 46.13) These areas harbour more plants and animal species than any other community. The widespread individual plants depend on animals for pollination and dispersal of fruits and seeds. In these areas of dense vegetation competition for light is strong selective force in plant communities. Soils in these areas are typically poor due to rapid nutrient recycling. Animals are typically tree-dwellers, and numerous exothermic animals are found due to warm temperatures.
This is a tropical rain forest.
This is a temperate rain forest, like those found on the pacific coast of British Columbia.
Destruction of tropical rain forests is occurring rapidly due to human intervention and this may cause large-scale changes in world climate, as well as destruction of many species.
B. Savanna
Savanna is a grassland community with scattered individual trees. This Biome covers areas of central South America, central and southern Africa and parts of Australia. The climate generally shows 3 distinct seasons: cool and dry, hot and dry, warm and wet, in that order. Soils are generally porous with a thin humus layer; and water drainage is rapid. This biome usually has a simple physical structure but are often rich in species numbers. Frequent fires inhibit invasion by trees, maintaining the grasses (wind pollinated) and forbs (often insect pollinated). Large herbivores (zebras, giraffes) and burrowing animals are commonly most active in the rainy season and many are nocturnal. During the dry season, many small animals are dormant or survive on seeds and dead plant parts. The term savanna also applies to areas where forest and grassland biomes integrate: the climatic conditions and community features are intermediate between grassland and forest.
C. Desert
Desert is characterised by low and unpredictable precipitation (<30 cm/year), not by temperature: both cold and hot deserts exist. Hot deserts occur in SW United States, W. South America, North Africa, the Middle East and Central Australia. Cold deserts occur in E. Argentina, central Asia and west of the Rocky Mountains. The cycles of growth and reproduction are keyed to rainfall. Scattered shrubs, cacti and other succulents that store water are common. Many bloom abundantly after a rainfall. Reptiles and seedeaters such as ants, birds and rodents are common. Many live in burrows, are nocturnal or have other adaptations for conserving water. Light coloration on plants reflects sunlight: some mice derive all of their water from the metabolic breakdown of the seeds they eat and never drink.
These pictures were taken in the Sonoran Desert in southern Arizona.
Leaves of desert trees are small and covered with wax to reduce water loss.
D. Chaparral
Chaparral (scrubland) are regions of dense, spiny shrubs with tough evergreen leaves found along coasts where cool ocean currents circulate offshore to make mild, rainy winters and long, hot dry summers (between 30º and 40º latitude). Occur in Mediterranean and coastlines of California, Chile, SW Africa, and S. W. Australia. This type of habitat is maintained by periodic fires. Many shrubs have root systems and seeds that are adapted for fire; root crowns may be fire resistant and resprout quickly, others have seeds that only germinate after a fire. Other plants are colonial and use asexual reproduction. Browsers such as deer, fruit-eating birds, rodents, snakes and lizards are common.
Chaparral
Temperate grasslands are similar to tropical savanna but occur in regions with relatively cold winters. The veldts of southern Africa, the pampas of Uruguay and Argentina, the steppes of the former Soviet Union and the plains and prairies of the U.S. and Canada are examples of this biome type. Occasional fires and seasonal drought prevent encroachment of trees upon the grassland. Soils tend to be thick and rich, annual rainfall amounts influence the height of vegetation. Large grazing mammals and large carnivores are common. Burrowing rodents and other small mammals are often present in dense populations.
Temperate forests grow throughout midlatitude regions with sufficient moisture to support growth of large, broad-leaved deciduous trees (fig. 46.18). These habitats occur in Eastern U.S., Middle Europe and E. Asia. Temperatures range from very cold in winter to very hot in summer (-30º C to 30º C) with a 5 to 6 month growing season. Precipitation is fairly high and evenly distributed throughout the year. Soil is rich in nutrients, decomposition rates are slow and a thick layer of leaf litter usually accumulates. There are several layers of vegetation, including herbs, shrubs and one or two strata of trees (species composition varies widely). The variety and abundance of food and habitat supports a rich diversity of animal life.
G. Taiga
Taiga (coniferous or boreal forest) is characterised by harsh winters and short, wet (and occasionally warm) summers. This biome occurs in North America, Europe, and Asia and at high elevations in more temperature latitudes. There is considerable precipitation in the form of snow that insulates the soil to reduce the permafrost; and also provides a protective layer for small mammals. Conifers like spruce, pine, fir and hemlock and deciduous oak, birch, willow, alder and aspen are common; usually one or very few species are present in dense stands. Animals include: seed eaters such as squirrels, birds and insects; browsers such as elk, moose, deer, beaver, and porcupine; predators such as grizzly bears, wolves and wolverines. The soil is thin, acidic and forms slowly due to low temperatures and slow decomposition of waxy coniferous needles.
H. Tundra
Tundra is at the northern-most limits of plant growth and at high altitudes plant forms are limited to low, shrubby or matlike vegetation (fig. 46.20)
There are two types of tundra:
Arctic tundra encircles the North Pole and is very cold with little light for long periods. It has brief warm summers that are marked by nearly 24 hours of daylight; and plant growth and reproduction occur rapidly during the summer. Some areas of the tundra are characterised by permafrost (permanently frozen ground) which contributes to the absence of taller plant forms. The soil is continuously saturated, further restricting plant forms. Dwarf perennial shrubs, sedges, grasses, mosses and lichens are common.
Here you can see a Muskox on the Artic tundra (summer time)
In the winter the tundra is very cold and very dry.
Alpine tundra occurs at high elevations in all latitudes. When this biome occurs near the equator, daylight varies little from 12 hours throughout the year and vegetation exhibits slow, steady rates of photosynthesis all year round.
In the tundra many animals are migratory and exhibit adaptations to the cold. Most large animals are herbivores like musk oxen and caribou. Smaller animals are lemmings, white fox and snowy owls. Insects are prevalent during the summer but spend the winter as immature stages.
III. FRESHWATER BIOMES
Freshwater biomes have several characteristics that distinguish them from marine biomes. They are "fresh water" with a salt concentration is less than 1%. Freshwater biomes are closely linked to the terrestrial biomes that surround them or through which they flow. Overall characteristics are influenced by the pattern and speed of water flow, and the local climate.
These standing bodies of water vary greatly in size with ponds being smaller than lakes. Ponds and lakes usually exhibit a significant vertical stratification in light penetration and water temperature. Light is rapidly absorbed by the water and micro-organisms in the water resulting in a rapid decrease in light intensity as depth increases. This divides the pond or lake into two layers: the photic zone in the upper layer where light is sufficient for photosynthesis; the lower aphotic zone receives little light and no photosynthesis occurs.
Temperature stratification also occurs in deeper ponds and lakes during summer in temperate zones. Sunlight heats the upper layers of water as far as it penetrates; the deeper water remains cold. As depth increases, the separation point of the warmer upper water form the lower colder water is noticeable as the thermocline; the thermocline is a narrow vertical zone between the warmer and colder waters where a rapid temperature change occurs. The distribution of plants and animals within a pond or lake also shows stratification based on water depth and distance from the shore. The littoral zone is shallow, well-lighted, warm water close to shore. Characterised by the presence of rooted and floating vegetation, a diverse attached algae community, and a very diverse animal fauna including suspension feeders (clams), herbivorous grazers (snails), and herbivorous and carnivorous insects, crustaceans, fishes, and amphibians. Some reptiles, waterfowl, and mammals also frequent this zone.
The limnetic zone is the open, well-lighted waters away from the shore. Occupants of this zone include phytoplankton (algae and cyanobacteria) which are photosynthetic, zooplankton (rotifers and small crustaceans) that grazes on phytoplankton, and small fish that feed on the zooplankton. Occasional visitors to this zone are large fish, turtles, snakes, and piscivorous birds. The profundal zone is the deep, aphotic zone lying beneath the limnetic zone. This is an area of decomposition where detritus (dead organic matter that drifts from above) is broken down. Water temperature is usually cold and oxygen is low due to cellular respiration of decomposers; mineral nutrients are usually plentiful due to decomposition of detritus. Waters of the profundal zone usually do not mix with surface waters due to density differences related to temperature. Mixing of these layers usually occurs twice each year in temperate lakes and ponds; this results in oxygen entering the profundal zone and nutrients being cycled into the limnetic zone.
Lakes are often classified as oligotrophic or eutrophic, depending on the amount of organic matter produced. Oligotrophic lakes are deep, nutrient-poor lakes in which the phytoplankton is not very productive. The water is usually clear; the profundal zone has a high oxygen concentration since little detritus is produced in the limnetic zone to be decomposed. Eutrophic lakes are shallow, nutrient-rich lakes with very productive phytoplankton. The waters are usually murky due to large phytoplankton populations and the large amounts of detritus being decomposed may result in oxygen depletion in the profundal zone during summer.
oligotrophic lake
Eutrophic pond
Oligotrophic lakes may develop into eutrophic lakes over time. If runoff from surrounding terrestrial habitats brings in mineral nutrients and sediments. Human activities increase the nutrient content of runoff due to lawn and agricultural fertilisers; municipal wastes dumped into lakes dramatically enriches the nitrogen and phosphorus concentrations which increases phytoplankton and plant growth. Algal blooms and increased plant growth results in more detritus and can lead to oxygen depletion due to increased decomposition.
Streams and rivers are bodies of water that move continuously in one direction.
The structure of these bodies of water changes from their point of origin (headwaters) to where they empty into a larger body of water (mouth). At the headwaters, the water is cold and clear, carries little sediment, and has few mineral nutrients. The channel is narrow with a rocky substrate and the water flows swiftly.
Near the mouth, water moves slowly and is more turbid due to sediment entering from other streams and erosion; the nutrient content is also higher; the channel is usually wider with a silty substrate that has resulted from deposition of silt.
The nutrient and oxygen content, turbidity, and rate of flow in rivers and streams are influenced by many factors. Rough, shallow bottoms produce turbulent flow known as riffles; smooth, deep bottoms result in a slower, smooth flow in areas called pools. Nutrient content of the water is higher in streams and rivers flowing through densely vegetated regions (leaves and other vegetation entering the water add organic matter) and where erosion takes place (increases inorganic nutrient content). Oxygen content of the water is affected by the flow rate; turbulent flow constantly oxygenates the water while slow water contains relatively little oxygen. Turbidity reflects the amount of material suspended in the water; streams and rivers flowing through areas of high erosion will have more suspended materials than those surrounded by hard substrates. Large amounts of suspended organic matter also increases turbidity. The biological communities found in rivers and streams differ as you move from headwaters to mouth; they also differ from those found in ponds and lakes. Due to the current, large plankton communities are not found in rivers and streams; photosynthesis which support the food chains is carried out by attached algae and rooted plants. Organic material washed into the system also provides an important food source, especially where dense vegetation along the shore blocks out sunlight or high turbidity prevents light penetration. In upstream areas where water is cool, clear and has high oxygen content, many insects are found which require these conditions; fish such as trout are also found in these areas.
IV. MARINE BIOMES
Ocean covers nearly 75% of the Earth's surface and contributes greatly to conditions on the other 25%. Most of the planet's rainfall is provided by evaporation of seawater. The world's climate and wind patterns are affected by ocean temperatures. Marine algae produce a large portion of the Earth's oxygen and consume large amounts of carbon dioxide. Salinity varies over time and area, but averages 3%. Marine communities are classified based on the depth at which they occur, their distance from shore, and light penetration. A photic zone is present and extends to the depth at which light penetration supports photosynthesis; occupied by phytoplankton, zooplankton and many fish species. The aphotic zone is below the level of effective light penetration and represents a majority of the ocean's volume. The intertidal zone is the shallow zone where the terrestrial habitat meets the ocean's water. The neritic zone extends from the intertidal zone, across the shallow regions, to the edge of the continental shelf. Oceanic zones extend over deep water from one continental shelf to another. Pelagic zones refer to open waters of any depth. Benthic zones refer to the seafloor
A. Estuaries
An estuary is the area where a freshwater stream or river merges with the ocean
Often bordered by salt marshes or intertidal mudflats, Salinity varies within the estuary from nearly fresh water to ocean water; varies daily in areas due to rise and fall of tides. Estuaries are very productive due to nutrients brought in by rivers. Estuaries have a diverse flora and fauna due to their productivity. Salt marsh grasses, algae, and phytoplankton are the major producers. Many species of annelids, oysters, crabs and fish are also present. Many marine invertebrates and fish breed in estuaries. A large number of waterfowl and other semiaquatic vertebrates use estuaries as feeding areas.
B. The Intertidal Zones
Intertidal zones where land and sea meet, are alternately submerged and exposed by the daily tide cycles. Organisms in this zone are exposed to greater variations in availability of seawater and temperature. These organisms are also subjected to the mechanical forces of wave action.
Rocky intertidal zones are vertically stratified and inhabited by organisms that possess structural adaptations that allow them to remain attached in this harsh environment. The uppermost zone is submerged only by the highest tides and is occupied by relatively few species of algae, grazing mollusks, and suspension-feeding barnacles; these organisms have various adaptations to prevent dehydration. The middle zone is exposed at low tide and submerged at high tide; many species of algae, sponges, sea anemone, barnacles, mussels, and other invertebrates are found in this area. The diversity is greater here due to the longer time spans this area is submerged. Tide pools are often found in the middle zone. These are depressions which are covered during high tide and remain as pools during low tide; tidepool organisms face dramatic salinity increases as water evaporates at low tide. The low intertidal zone is exposed only during the lowest tides and shows the greatest diversity of invertebrates, fishes and seaweeds.
Wave action on rock shore.
Rocky shore at low tide.
Tidal mud flats at low tide.
C. Coral Reefs
Coral reefs are found in warm tropical waters of the neritic zone where sunlight penetrates to the ocean floor. Sunlight penetration permits photosynthesis and a constant supply of nutrients is provided by currents and waves. Coral reefs are dominated by the structure of the coral. This diverse group of cnidarians secretes a hard, calcium carbonate external skeleton that provides a substrate on which other corals and algae grow. The cnidarian coral animals feed on microscopic organisms and organic debris even though they are dependent on the photosynthetic products of their symbiotic dinoflagellates. Coral reefs are very diverse and productive; a large variety of microorganisms, invertebrates, and fish are found among the corals and algae. Many herbivorous snails, sea urchins, and fish are present along with predators such as the octopus, sea stars, and many carnivorous fish. Although many coral reefs are very large, they are delicate and can be severely damaged or destroyed by pollution, human induced damage, or introduced predators.
V. The Environmental Diversity of the Biosphere
Abiotic factors such as temperature, precipitation, and light influence the distribution of organisms. The patchiness of the global biosphere illustrates how the different physical environments produce a mosaic of habitats.
Some of the important abiotic factors that affect distribution of species include:
Temperature affects biological processes and the ability of most organisms to regulate their body temperature. Temperature affects metabolism: few organisms have active metabolisms at temperatures close to 0º C and temperatures above 45º C denature most essential enzymes. The actual body temperature of ectotherms is affected by heat exchange with the environment; most animals maintain a body temperature only a few degrees above or below ambient temperature. Even endotherms function best within the temperature range to which they are adapted.
Water is essential and adaptations for water balance and conservation help determine a species habitat range. Marine and freshwater animals face the problems of regulating intracellular osmolarity; terrestrial animals face the problem of desiccation.
Sunlight provides the energy that drives nearly all ecosystems although only photosynthetic organisms use it directly as an energy source. In aquatic environments, water selectivity reflects and absorbs certain wavelengths; therefore, most photosynthesis occurs near the water surface. The physiology, development, and behaviour of many animals and plants are often sensitive to photoperiod.
Rocks and soil. The physical structure, pH, and mineral composition of soil limit distribution of plants and hence animals that feed on those plants. The composition of the substrate in a stream or river greatly influences the water chemistry, which in turn influences the plants and animals. The type of substrate influences what animals can attach or burrow in intertidal zones.
Wind amplifies the effects of temperature by increasing heat loss by evaporation and convection; wind also increase the evaporation rate of animals and transpiration rate of plants, resulting in more rapid water loss. Mechanical pressure of wind can affect plant morphology (for example, inhibiting growth of limbs on windward side of trees).
Periodic disturbances such as fire, hurricanes, typhoons, and volcanic eruptions can devastate biological communities, after which the area is recolonised by organisms or repopulated by survivors. May go through a succession of changes. Those disturbances that are infrequent (volcanic eruptions) do not illicit adaptations. Adaptations do evolve to periodically recurring disturbances such as fires.
Climate and the Distribution of Biomes
Climate is the prevailing weather conditions at a locality. The major components of climate are temperature, water, light, and wind.
Global Climate Patterns
Solar energy input and the Earth's movement in space determine the planet's global climate patterns. About 50% of the solar energy that reaches the atmosphere's upper layers are absorbed before it reaches the surface. Ultraviolet and certain other wavelengths are more readily absorbed by oxygen and ozone than by other molecules. Some of the solar energy that reaches the Earth's surface is reflected back into the atmosphere; large amounts are absorbed by land and water. The atmosphere, land, and water are heated when they absorb solar energy; this heating establishes the temperature variations, air movement cycles, and evaporation of water responsible for the latitudinal variations in climate.
Latitudinal variation in the intensity of sunlight results from the Earth's spherical shape; seasonal variation in solar radiation in the Northern and Southern Hemispheres are due to the Earth's tilt of 23.5º relative to its plane of orbit. Only the tropics receive sunlight from directly overhead year round; the tilt causes solar radiation to change daily as the Earth rotates around the sun. The tropics receive the greatest annual input of solar radiation and show the least seasonal variation; only small variations in daylength and temperature occur. Seasonal variation in light and temperature increases steadily toward the poles; polar regions have long, cold winters with periods of continual darkness and short summers with periods of continual light.
A global circulation of air which creates precipitation and winds result from the intense solar radiation near the equator. Evaporation of surface water due to high tropical temperatures causes warm, wet air masses to rise near the equator; this rising air creates an area of light, shifting winds (doldrums) along the equator. As these warm air masses rise, they expand and cool; cool air can hold less water vapour so the rising air masses drop large amounts of rain in the tropics. The cool, dry air masses flow toward the poles at high altitudes; they continue to cool as they move farther from the equator. Air mass density increases as they become cooler; they begin to descend toward the surface as cool, dry air masses at about 30º latitude. The air masses absorb water as they descend, thus creating arid climates around 30º N and 30ºS. Some of the descending air masses flow toward the poles at low altitudes, the rest flows toward the equator. The air masses flowing toward the poles is warmed and rises again at about 60º N and 60º S; as these masses of air begin to cool with increased altitude, the water vapour is lost as precipitation in this region. As air from this second cell reaches the higher altitude and cools, it flows toward the poles where it descends and flows back toward the equator.
The Earth's predictable wind patterns are established by air flowing in the circulation cells. The rotation of the Earth deflects the winds from a vertical path since land near the equator is moving faster than at the poles. Tropical and subtropical tradewinds blow from east to west. In temperate zones, the predominating winds flow from west to east.
Local and Seasonal Effects on Climate
Although global climate patterns explain the geographic distribution of major biomes, local variations due to bodies of water and topographical features crate a regional patchiness in climatic conditions. The local variations in climate and soil have a major influence on less widely distributed communities and individual species.
