Ecology Lecture No. 3: The Biosphere
Tuesday September 18 , 2012
-The biosphere is the zone of life on Earth and is restricted to thin-crust surface of the Earth. Biomes are
large-scale biological communities shaped by the physical environment, particularly climate. Biomes are
categorized by dominant plant forms, not taxonomic relationships. This is because animals are quite
mobile and can avoid the local conditions, by choosing a favourable microclimate. Plants do not share
-Plants occupy sites for a long time and are good indicators of the physical environment, reflecting
climatic conditions and disturbances. Terrestrial biomes are characterized by growth forms of the
dominant plants, such as leaf deciduousness or succulence.
Plant Growth Forms:
-Deciduous trees – Moist, seasonally warm/cool or cool/cold on fertile soils or warm, seasonally wet/dry.
-Cacti & Shrubs/Succulent stems or leaves – Dry, seasonally hot/cool.
-Needle-leaved Evergreen trees – Moist, seasonally warm/cool or cool/cold on infertile soils.
-Grasses/Sedges – Moist, seasonally warm/cool, with fire.
-Evergreen Broad-leaved trees – Wet, warm year-round.
-Forbs (non-woody leaves) – Seasonally cool/cold.
-Sclerophyllous shrubs – Seasonally dry/moist and warm/cool.
Variation With Temperature & Precipitation:
-Plants have taken many forms in response to selection pressures such as aridity, extreme temperatures,
intense solar radiation, grazing, and crowding. Similar growth forms can be found on different
continents, even though the plants are not genetically related. It is important to note that biomes don’t
imply related plants, but similar growth forms among distant species.
-Convergence - Evolution of similar growth forms among distantly related species in response to similar
selection pressures. Temperature has direct physiological effects on plant growth form. Precipitation
and temperature act together to influence water availability and water loss by plants. Water availability
and soil temperature determine the supply of nutrients in the soil. This is significant because if the soil is
dry, it slows down the growth microbial organisms that the biome depends upon.
-*Variation in temperature and precipitation does not account for seasonal variation in a biome. Global Biome Distributions:
-If one examines the global distribution of biomes across the Earth, the pre-human distribution shows
great inconsistencies with the distribution that is occurring today. Human activities influence the
distribution of biomes. Many changes of land use such as: the conversion of land to agriculture, logging,
resource extraction, and urban development are the result of modern human activity. The potential and
actual distributions of biomes are markedly different.
- There are nine major terrestrial biomes. Climate diagrams show the characteristic seasonal patterns of
temperature and precipitation at a representative location.
Biome #1 (Tropical Rainforests):
- High biomass, high diversity—about 50% of Earth’s species. Light is a key factor—plants must grow
very tall above their neighbors or adjust to low light levels. Smaller plants harvest sun flecks (small gaps
of light between the leaves of the canopy). Emergents (very tall trees) rise above the canopy. Lianas
(woody vines) and epiphytes (plants growing non-parasitically upon other plants) use the trees for
support. Understory trees grow in the shade of the canopy, and shrubs and forbs occupy the forest floor.
- Tropical rainforests are disappearing due to logging and conversion to pasture and croplands. About
half of the tropical rainforest biome has been altered. Recovery of rainforests is uncertain: Soils are
nutrient-poor, and recovery of nutrient supplies may take a very long time.
Biome #2 (Tropical Seasonal Forests & Savannahs):
- Wet and dry seasons associated with movement of the ITCZ (Intertropical Convergence Zone) where
winds origination in the Northern and Southern hemispheres meet near the Equator. The biome
features shorter trees, deciduous trees in dry seasons and more grasses and shrubs. Fires promote the
establishment of savannas; some are set by humans. In Africa, large herbivores (wildebeests, zebras,
elephants, and antelopes) also influence the balance of grass and trees through grazing habits. On the
Orinoco River floodplain, seasonal flooding promotes savannas.
- Less than half of seasonal tropical forests and savannas remain. Human population growth in this
biome has had a major influence. Large tracts have been converted to cropland and pasture.
Biome #3 (Hot Deserts):
-Known for High temperatures, low moisture. Sparse vegetation and animal population are present. Low
water availability constrains plant abundance and influences form. Many plants have succulent stems
that store water and convergence of this form is shown by cacti (Western Hemisphere) and euphorbs
(Eastern Hemisphere). Other plants (forbs) complete their life cycle for a few weeks when rare heavy
rains affect the biome. - Humans use deserts for agriculture and livestock grazing. Agriculture depends on irrigation, and results
in soil salinization. Long-term droughts and unsustainable grazing can result in desertification—the loss
of plant cover and soil erosion.
Biome #4 (Temperate Grasslands):
- Warm, moist summers and cold, dry winters. Grasses dominate and are maintained by frequent fires
and large herbivores such as bison. As grasses invest most of their biomas