Download Basic Ecology: Characteristics of the Earth`s Terrestrial Biomes

Basic Ecology: Characteristics of the Earth's Terrestrial Biomes
Introduction Many places on Earth share similar climatic conditions despite being found in
geographically different areas. As a result of natural selection, comparable ecosystems have
developed in these separated areas. Scientists call these major ecosystem types biomes. The
geographical distribution (and productivity) of the various biomes is controlled primarily by the
climatic variables precipitation and temperature.
Most of the classified biomes are identified by the dominant plants found in their communities. For
example, grasslands are dominated by a variety of annual and perennial species of grass, while
deserts are occupied by plant species that require very little water for survival or by plants that have
specific adaptations to conserve or acquire water.
The diversity of animal life and subdominant plant forms characteristic of each biome is generally
controlled by abiotic environmental conditions and the productivity of the dominant vegetation. In
general, species diversity becomes higher with increases in net primary productivity, moisture
availability, and temperature.
Adaptation and niche specialization are nicely demonstrated in the biome concept.
Organisms that fill similar niches in geographically separated but similar ecosystems usually are
different species that have undergone similar adaptation independently, in response to similar
environmental pressures. The vegetation of California, Chile, South Africa, South Australia, Southern
Italy and Greece display similar morphological and physiological characteristics because of
convergent evolution. In these areas, the vegetation consists of drought-resistant, hard-leaved, low
growing woody shrubs and trees like eucalyptus, olive, juniper, and mimosa.
Arctic and Alpine Tundra: Tundra means marshy plain. The geographical distribution of the tundra
biome is largely pole-ward of 60 degrees North latitude. The tundra biome is characterized by an
absence of trees, the presence of dwarf plants, and a ground surface that is wet, spongy, and
hummocky. Soils of this biome are usually permanently frozen (permafrost) starting at a depth of a
few centimeters to meter or more. The permafrost line is a physical barrier to plant root growth.
Within this biome, temperature, precipitation, and evaporation all tend to be at a minimum. Most
tundra locations, have summer months with an average temperature below 10 degrees Celsius.
Precipitation in the wettest month is normally no higher than 25 millimeters. However, despite the low
levels of precipitation the ground surface of the tundra biome is often waterlogged because of low
rates of evapotranspiration.
The species diversity of tundra vegetation is relatively small. Plant communities are usually
composed of a few species of dwarf shrubs, a few grass species, sedges, and mosses. Perhaps
the most characteristic arctic tundra plants are lichens like Reindeer Moss. The principal herbivores
in this biome include caribou, musk ox, arctic hare, voles, and lemmings. Most of the bird species of
the tundra have the ability to migrate and live warmer locations during the cold winter months. The
herbivore species support a small number of carnivore species like the arctic fox, snow owl, polar
bear, and wolves. Reptiles and amphibians are few or completely absent because of the extremely
cold temperatures.
Alpine tundra is quite similar to some arctic tundra but differs in the absence of permafrost and in the
presence of better drainage.
Boreal Coniferous Forest This moist-cool, transcontinental coniferous forest, or taiga lies largely
between the 45th and 57th North latitudes. The climate of this biome is cool to cold with more
precipitation than the tundra, occurring mainly in the summer because of midlatitude cyclones. The
predominant vegetation of boreal biome are needle-leaf evergreen variety tree species. Some
common species include: White Spruce and Balsam Fir east of the Rockies; Red Pine, White Pine,
and Hemlock, in the Great Lakes Region. In British Columbia, dominant boreal trees include: Black
Spruce, White Spruce, Lodgepole Pine, Ponderosa Pine, Douglas Fir, and Alpine Fir. The understory
is relatively limited as a result of the low light penetration even during the spring and fall months.
Understory plants in the deciduous biome take advantage of the leafless condition of trees during
these seasons concentrating there growth during this time period. Common understory species
include orchids, shrubs like rose, blueberry, and cranberry. The understory of boreal forest habitats
is usually poorly developed.
Mammals common to the boreal forest include moose, bear, deer, wolverine, marten, lynx, wolf,
snowshoe hare, vole, chipmunks, shrews, and bats. Reptiles are rare, once
again, because of cold temperatures. Bears are common in the boreal forest ecosystem.
Temperate Deciduous Forest As its name indicates, this biome is characterized by a
moderate climate and deciduous trees. It once occupied much of the eastern half of the United
States, central Europe, Korea, and China. This biome has been very extensively affected by human
activity, and much of it has been converted into agricultural fields or urban developments. Dominant
plants include trees like Maple, Beech, Oak, Hickory, Basswood, Cottonwood, Elm, and Willow. The
understory of shrubs and herbs in a mature deciduous forest is typically well developed and richly
diversified. Many different types of herbivores and carnivores, and some reptiles and amphibians
exist here. Brown forest soils characterize temperate deciduous forest ecosystems. The surface litter
layer in these soils is thin due to rapid decomposition.
Grassland In central North America are the grasslands, the tall grass prairie toward the east and the
short grass prairie westward. In Europe and Asia some grasslands are called Steppes. In South
America, grasslands are known as Pampas. Prior to modern man, the tall grass prairie was
dominated by species of Bluestem. This particular species dominated much of the tall grass prairie
forming dense covers 1.5 to 2.0 meters tall. In the western end of the prairie, where precipitation is
less, Buffalo Grass and other grasses only a few inches above the soil surface are common in this
habitat. Flowering herbs, including many kinds of sunflower-like composites and pea and bean-like
legumes, are common but much less important than grass species. Trees are limited to low lying
areas and the narrow zone immediately adjacent to streams. Natural grassland ecosystems are
dominated by various species of grass. Much of the prairies is now cultivated to grow grain crops.
Most grassland ecosystems have been modified by humans to grow grain and other dryland crops
Grassland mammals are dominated by smaller burrowing herbivores (prairie dogs, jack
rabbits, ground squirrels, and gophers) and larger running herbivores such as bison,
pronghorn antelope, and elk. Carnivores include badger, coyote, ferret, wolf, and cougar. The
population size of many of these species has been drastically reduced because of habitat destruction.
Some of these species are on the edge of extinction.
Desert In its most typical form, the desert consists of shrub-covered land where the plants are
spatially quite dispersed. The major desert biomes of the Earth are geographically found at between
25 to 25 degrees North and South latitude, in the interiors of continents. Climatically, deserts are
influenced by descending air currents that limit the formation of precipitation. Many desert areas have
less than 25 millimeters of precipitation annually. Dominant plants include drought resistant shrubs
like the Creosote Bush and Sagebrush water storing succulents like cactus and many species are
short lived annuals that complete their life cycles during infrequent and short rainy periods. Desert
habitats can be devoid of vegetation if precipitation is in very short supply. Cactus are a common type
of drought resistant plant found in deserts.
Most desert mammals tend to be nocturnal to avoid the high temperatures. Desert habitats have a
rich lizard and snake fauna because high temperatures promote the success of cold blooded life
forms. Lizards are quite common in desert habitats.
Mediterranean Shrubland Mediterranean shrubland has a very specific spatial distribution. It is
found in a narrow zone between 32 and 40 degrees latitude North and South on the west coasts of
the continents. This area has a dry climate because of the dominance of the subtropical high
pressure zone during the fall, summer, and spring months. Precipitation falls mainly in the winter
months because of the seasonal movement of the polar front and its associated mid-latitude
cyclone storms. Annual averages range from about 300 to 750 millimeters and most of this rain falls
in a period between 2 to 4 months long. As a result of the climate, the vegetation that inhabits this
biome exhibits a number of adaptations to withstand drought and fire. Trees and shrubs living in this
zone tend to be small with hard evergreen leaves. Plants in the Mediterranean shrubland do not drop
their leaves during the dry season because of the expense of replacement. The plants of the
Mediterranean shrubland develop leaves that can withstand arid conditions.
Representative species of the Mediterranean shrubland include cork oak, olive, eucalyptus, arbutus,
acacia, maritime pine, shrub oak, and live oak. Many of the plant species have thorns to protect them
from herbivore damage. This biome is sometimes also called chaparral or forest.
Tropical Savanna Tropical savannas are grasslands with scattered drought-resistant
trees that generally do not exceed 10 meters in height. Tree and shrub species in the savanna
usually shed their leaves during the dry season. This adaptation reduces water loss from the plants.
Climatically, these biomes are characterized by distinct wet and dry seasons. Temperatures are hot
all year long. Savanna vegetation is typical composed of a mixture of grass and trees. The savanna
biome constitutes extensive areas in eastern Africa, South America, and Australia. Savannas also
support the richest diversity of grazing mammals in the world. The grazing animals provide food for
a great variety of predators Zebras are a common grazer of the African savanna.
Savannas are also home to a number of predator species that prey on grazing animals.
Tropical Rainforest Tropical rainforests occur in a broad zone outside the equator. Annual
rainfall, which exceeds 2000 to 2250 millimeters, is generally evenly distributed throughout the year.
Temperature and humidity are relatively high through the year.
Flora is highly diverse: a square kilometer may contain as many as 100 different tree species as
compared to 3 or 4 in the temperate zone. The various trees of the tropical rain forests are closely
spaced together and form a thick continuous canopy some 25 to 35 meters tall Every so often this
canopy is interrupted by the presence of very tall trees (up to 40 meters) that have wide buttressed
bases for support. Epiphytic orchids and
bromeliads, as well as vines (lianas), are very characteristic of the tropical rainforest biome. Some
other common plants include ferns and palms. Most plants are evergreen with large, dark green,
leathery leaves.
The tropical rainforest is also home to a great variety of animals. Some scientists believe that 30 to
50 % of all of the Earth's animal species may be found in this biome.
The tropical rain forest is home to many different species of amphibians.
Aquatic Biomes
Water is the common link among the five biomes and it makes up the largest part of the biosphere,
covering nearly 75% of the Earth’s surface. Aquatic regions house numerous species of plants and
animals, both large and small. In fact, this is where life began billions of years ago when amino acids
first started to come together. Without water, most life forms would be unable to sustain themselves
and the Earth would be a barren, desert-like place. Although water temperatures can vary widely,
aquatic areas tend to be more humid and the air temperature on the cooler side.
The aquatic biome can be broken down into two basic regions, freshwater (ponds and rivers)
and marine (oceans and estuaries).
Freshwater Regions
Freshwater is defined as having a low salt concentration—usually less than 1%. Plants and animals
in freshwater regions are adjusted to the low salt content and would not be able to survive in areas of
high salt concentration (i.e, ocean). There are different types of freshwater regions: ponds and lakes,
streams and rivers, and wetlands. The following sections describe the characteristics of these three
freshwater zones.
Ponds and Lakes
These regions range in size from just a few square meters to thousands of square kilometers.
Scattered throughout the earth, several are remnants from the Pleistocene glaciation. Many ponds
are seasonal, lasting just a couple of months (such as sessile pools) while lakes may exist for
hundreds of years or more. Ponds and lakes may have limited species diversity since they are often
isolated from one another and from other water sources like rivers and oceans
The topmost zone near the shore of a lake or pond is the littoral zone. This zone is the warmest since
it is shallow and can absorb more of the Sun’s heat. It sustains a fairly diverse community, which can
include several species of algae (like diatoms), rooted and floating aquatic plants, grazing snails,
clams, insects, crustaceans, fishes, and amphibians. In the case of the insects, such as dragonflies
and midges, only the egg and larvae stages are found in this zone. The vegetation and animals living
in the littoral zone are food for other creatures such as turtles, snakes, and ducks.
The limnetic zone is well-lighted (like the littoral zone) and is dominated by plankton, both
phytoplankton and zooplankton. Plankton are small organisms that play a crucial role in the food
chain. Without aquatic plankton, there would be few living organisms in the world, and certainly no
humans. A variety of freshwater fish also occupy this zone.
Plankton have short life spans—when they die, they fall into the deep-water part of the lake/pond.
This zone is much colder and denser than the other two. Little light penetrates all the way through the
limnetic zone into the profundal zone. The fauna are heterotrophs, meaning that they eat dead
organisms and use oxygen for cellular respiration.
Temperature varies in ponds and lakes seasonally. During the summer, the temperature can range
from 4° C near the bottom to 22° C at the top. During the winter, the temperature at the bottom can be
4° C while the top is 0° C (ice). In between the two layers, there is a narrow zone called the
thermocline where the temperature of the water changes rapidly. During the spring and fall seasons,
there is a mixing of the top and bottom layers, usually due to winds, which results in a uniform water
temperature of around 4° C. This mixing also circulates oxygen throughout the lake. Of course there
are many lakes and ponds that do not freeze during the winter, thus the top layer would be a little
warmer.
Streams and Rivers
These are bodies of flowing water moving in one direction. Streams and rivers can be found
everywhere—they get their starts at headwaters, which may be springs, snowmelt or even lakes, and
then travel all the way to their mouths, usually another water channel or the ocean. The
characteristics of a river or stream change during the journey from the source to the mouth. The
temperature is cooler at the source than it is at the mouth. The water is also clearer, has higher
oxygen levels, and freshwater fish such as trout and heterotrophs can be found there. Towards the
middle part of the stream/river, the width increases, as does species diversity—numerous aquatic
green plants and algae can be found. Toward the mouth of the river/stream, the water becomes
murky from all the sediments that it has picked up upstream, decreasing the amount of light that can
penetrate through the water. Since there is less light, there is less diversity of flora, and because of
the lower oxygen levels, fish that require less oxygen, such as catfish and carp, can be found.
Wetlands
Wetlands are areas of standing water that support aquatic plants. Marshes, swamps, and bogs are all
considered wetlands. Plant species adapted to the very moist and humid conditions are called
hydrophytes. These include pond lilies, cattails, sedges, tamarack, and black spruce. Marsh flora also
include such species as cypress and gum. Wetlands have the highest species diversity of all
ecosystems. Many species of amphibians, reptiles, birds (such as ducks and waders), and furbearers
can be found in the wetlands. Wetlands are not considered freshwater ecosystems as there are
some, such as salt marshes, that have high salt concentrations—these support different species of
animals, such as shrimp, shellfish, and various grasses. They are called brackish because they
contain both salt and fresh water.
Marine Regions
Marine regions cover about three-fourths of the Earth’s surface and include oceans, coral reefs, and
estuaries. Marine algae supply much of the world’s oxygen supply and take in a huge amount of
atmospheric carbon dioxide. The evaporation of the seawater provides rainwater for the land.
Oceans
The largest of all the ecosystems, oceans are very large bodies of water that dominate the Earth’s
surface. Like ponds and lakes, the ocean regions are separated into separate zones: intertidal,
pelagic, abyssal, and benthic. All four zones have a great diversity of species. Some say that the
ocean contains the richest diversity of species even though it contains fewer species than there are
on land.
The intertidal zone is where the ocean meets the land—sometimes it is submerged and at other times
exposed, as waves and tides come in and out. Because of this, the communities are constantly
changing. On rocky coasts, the zone is stratified vertically. Where only the highest tides reach, there
are only a few species of algae and mollusks. In those areas usually submerged during high tide,
there is a more diverse array of algae and small animals, such as herbivorous snails, crabs, sea
stars, and small fishes. At the bottom of the intertidal zone, which is only exposed during the lowest
tides, many invertebrates, fishes, and seaweed can be found. The intertidal zone on sandier shores is
not as stratified as in the rocky areas. Waves keep mud and sand constantly moving, thus very few
algae and plants can establish themselves—the fauna include worms, clams, predatory crustaceans,
crabs, and shorebirds.
The pelagic zone includes those waters further from the land, basically the open ocean. The pelagic
zone is generally cold though it is hard to give a general temperature range since, just like ponds
and lakes, there is thermal stratification with a constant mixing of warm and cold ocean currents. The
flora in the pelagic zone include surface seaweeds. The fauna include many species of fish and
some mammals, such as whales and dolphins. Many feed on the abundant plankton.
The benthic zone is the area below the pelagic zone, but does not include the very deepest parts of
the ocean (see abyssal zone below). The bottom of the zone consists of sand, slit, and/or dead
organisms. Here temperature decreases as depth increases toward the abyssal zone, since light
cannot penetrate through the deeper water. Flora are represented primarily by seaweed while the
fauna, since it is very nutrient-rich, include all sorts of bacteria, fungi, sponges, sea anemones,
worms, sea stars, and fishes.
The deep ocean is the abyssal zone. The water in this region is very cold (around 3° C), highly
pressured, high in oxygen content, but low in nutritional content. The abyssal zone supports many
species of invertebrates and fishes. Mid-ocean ridges (spreading zones between tectonic plates),
often with hydrothermal vents, are found in the abyssal zones along the ocean floors.
Chemosynthetic bacteria thrive near these vents because of the large amounts of hydrogen sulfide
and other minerals they emit. These bacteria are thus the start of the food web as they are eaten by
invertebrates and fishes.
Coral Reefs
Coral reefs are widely distributed in warm shallow waters. They can be found as barriers along
continents (e.g., the Great Barrier Reef off Australia), fringing islands, and atolls. Naturally, the
dominant organisms in coral reefs are corals. Corals are interesting since they consist of both algae
(zooanthellae) and tissues of animal polyp. Since reef waters tend to be nutritionally poor, corals
obtain nutrients through the algae via photosynthesis and also by extending tentacles to obtain
plankton from the water. Besides corals, the fauna include several species of microorganisms,
invertebrates, fishes, sea urchins, octopuses, and sea stars.
Estuaries
Estuaries are areas where freshwater streams or rivers merge with the ocean. This mixing of waters
with such different salt concentrations creates a very interesting and unique ecosystem. Microflora
like algae, and macroflora, such as seaweeds, marsh grasses, and mangrove trees (only in the
tropics), can be found here. Estuaries support a diverse fauna, including a variety of worms, oysters,
crabs, and waterfowl