Chapter 3 notes for BIOB50 – 27/01/2013
- Biosphere is the layer of the earth that supports most of the earth’s life forms. It’s
sandwiched between the lithosphere, the earth’s surface crust and upper mantle and the
troposphere, the lowest layer of the atmosphere.
- Biomes are large-scale biological communities shaped by the physical environment in
which they are found. They are characterized by the most common type of plants
distributed across them.
- Terrestrial biomes are classified by the growth form (size and morphology) and other
characteristics, such as deciduousness, (whether they shed leaves in winter) thickness,
succulence etc. some examples are (Fig. 3.3):
o Sclerophyllous shrubs have tough, leathery leaves, grow in dry/moist and
o Deciduous trees, which seasonally shed their leaves, grow in warm-cold on
fertile soils, and seasonally wet/dry
o Grasses and sedges, which grow from the base of their leaves, grow in moist,
seasonally warm/cool environments with fire.
o Cacti and shrubs with succulent stems and leaves store water and grow in
cool/hot and dry areas.
o Evergreen broad-leaved trees carry out photosynthesis year round and grow in
wet, year-round warm areas.
o Evergreen needle-leaved trees retain their photosynthetic tissues year-round and
grow in moist, seasonally warm-cold areas with infertile soil
o Forbs, which are broad-leaved herbaceous (non-woody) plants grow in
seasonally cool/cold areas.
- The emergence of similar growth forms among distantly related animals in response to
similar selection pressures is called convergence.
- The terrestrial earth is divided into several biomes:
o Tropical rainforests are found between 10 N and S latitude where precipitation
They experience warm, seasonally invariant temperatures.
Plants grow throughout the year.
They contain ~50% of the earth’s species in ~11% of its terrestrial
vegetation cover The tallest emergent trees in the rainforest form the rainforest canopy
consisting of tall evergreen trees about 30-40m tall. Below are the shorter
understory trees and below them, on the ground grow shrubs and forbs
that utilize what little flecks of light fall on the ground.
o Tropical seasonal forests and Savannas are north and south of the tropical
rainforests, near the tropics of Capricorn (23.5 S) and Cancer (23.5 N).
The rainfall in this region is primarily seasonal, with large amounts of
rainfall between October and March in the winter months and very low
between March and October (summer) (Fig pg 53).
There is leaf deciduousness as a response to seasonal droughts.
There are more grasses and lesser trees compared to tropical rainforests.
There are three types of vegetation, tropical dry forests, tropical savannas
and thorn woodlands. More frequent fires and grazing herbivores promote
savannas, which have a lot of grass and few trees. Thorn woodlands
have widely spaced trees and shrubs.
o Hot deserts are situated north and south of the tropical seasonal forests and
savannas (30 N and S) corresponding to the high pressure generated by
descending Hadley cells which prohibits precipitation.
They contain sparse populations of plants and animals.
An excellent example of convergent evolution can be found here, as both
euphorbs in the eastern hemisphere and cacti in the western hemisphere
have developed succulent stems and thorns.
The abundance of organisms is low but species diversity can be high.
Agriculture is difficult because of salinization, and rearing grazing animals
is difficult due to the unpredictability of precipitation.
Desertification is caused when the grass etc in an area dies after long
drought periods and unsustainable grazing practices so that the ground
o Temperate grasslands occur between 30 to 50 N and to some extent between
30 to 50 S on the eastern coasts of South America and Africa.
They have greater seasonal temperature variation than tropical climates,
with increasing periods of sub-freezing temperatures near the poles.
Usually associated with warm, moist summers and cold, dry winters. Aleem Ahmad
Grasses keep the soil fertile, so the land is usually cleared to make way
for agricultural plots.
Over-grazing and salinization through irrigation leads to desertification.
The most human-influenced biome on earth.
o Temperate woodlands and shrublands occur between 30 and 40 N and S off
the west coasts of Africa, Australia, Europe and the Americas.
The regions have wet winters and dry summers.
Mediterranean type climates; have synchronicity between precipitation
and growing season.
Vegetation is evergreen shrubs and trees which are active even in winter
and do not waste nutrients growing back leaves every summer.
They have sclerophyllous leaves and well adapted to dry weather
They are fire resistant
Also found in the continental interior of North America.
Increases in fires could lead to replacement by invasive grasses.
o Temperate deciduous forests occur between 30 and 50 N on the eastern and
western edges of Eurasia and eastern North America.
Occur in areas where the soil is fertile enough and the rainfall is high
enough to support tree growth (500-2500 mm/year)
Common trees include Oak, maple and beech trees.
Clearing for farming was very common because of the warm moist
summers and fertile ground but has tapered in preference of temperate
grasslands and tropics.
o Temperate evergreen forests occur between 45-50 N and S and 30 and 50
latitude on the west coasts
Variable precipitation, from 500 to 4000 mm/year.
Grow on poor nutrient soil
Regular fires at 30-60 years
Typical flora includes needle-leaved conifers eg pines, junipers and
douglas firs in the northern hemisphere and beeches, eucalyptus, cedars and podocarps in the southern hemisphere, on the west coast of Chile
and Tasmania and southwestern Australia.
Provides high quality wood; heavily deforested.
Suppression of natural fires recently has led to increased pests and more
o Boreal Forests (far north) are found between 50 and 65 N in continental
locations such as Siberia.
The ground is typically in a state of permafrost, or having a subsurface
layer that stays frozen year round for at least 3 years.
• The permafrost prevents soil drainage so the soil remains moist
even though precipitation is scarce.
Subfreezing temperatures last for up to 6 months.
Common species include spruces, pines and larches (deciduous needle-
leaved trees) but also include deciduous birch forests.
Conifers can resist freezing better than angiosperms and so can maintain
leaves all year round.
It is the largest biome by area and contains 1/3 of the earth’s forests.
Fires are crucial to the life cycles of the trees.
o Tundra occurs above 65 N but also occurs in the Antarctic.
High pressure g