Ecosystems IV: Terrestrial Biomes and Aquatic Life Zones



I. Biome Controls


The most important factors in determining the type of terrestrial ecosystem or biome found in a given area are climate, particularly the temperature and precipitation, and soil type (also climate controlled in part).


Biome distribution closely follows global climate patterns. Precipitation is most important in deciding whether a desert (low), grassland, or forest (high) develops. Climate controlled by a combination of latitude and altitude.


Plant forms respond to the prevailing climate by trying to maximize photosynthesis and minimize moisture loss, while not losing or gaining too much heat.


Succulents: Fleshy plants, such as cactus, which store water internally. Often have no or small leaves and a stick-like shape. They are well adapted to desert environments.


Broadleaf evergreens: Never drop their broad leaves so they can radiate heat all year. They are well adapted to tropical rain forests.


Broadleaf deciduous trees: Drop their leaves to reduce moisture and heat loss during cold and dry winters. They are well adapted to temperate forests.


Coniferous Evergreens: Waxy, narrow, pointed leaves that never drop. This preserves heat and moisture and no energy expended to grow new leaves. They are well adapted to cold and dry climates.



II. Major Biomes


A. Deserts


A desert, by definition, receives less than 25 cm (10 inches) of rain/year. There are three types depending on temperature: tropical deserts, temperate deserts (with greater species diversity), and Cold or polar deserts.


Precipitation in desert and semi-desert environments often comes in a few, short, but intense rain events. Dormant plants and seeds bloom at these times, creating new seeds and shoots that lie dormant until the next rain (often years away).


Desert animals often are small to increase heat loss. They often are nocturnal or burrowers to keep cool. Well adapted for retaining as much moisture as possible.


Human impacts last a long time due to slow growth and productivity. Impacts include cities expanding into desert environments, deposition of salts due to irrigation, and depletion of groundwater resources. Desert areas are preferred sites for storage of toxic and radioactive wastes and may be used for collection of solar energy.



B. Grasslands


Have enough rainfall for grasses to become established, but drought and fire destroy most trees, although a few isolated stands may survive. Grasses renewed even when grazed because they continue to grow as long as their roots are not destroyed.


Tropical grasslands (savanna): Often have pronounced and distinct wet and dry seasons. Often populated by large herds of grazers (wildebeests, zebra) and browsers (giraffe). They often eat specific plants to reduce interspecific competition by resource partitioning. An example is the Serengeti Plain of East Africa.


Temperate grasslands: Cover vast areas of continental interiors (i.e. the Great Plains of North America). Typically have cold winters and hot summers. Their nutrient-rich soils make them excellent for agriculture.


Polar grasslands (tundra): Very cold climates with a very short summer. They often have a permafrost layer which keeps melted water at the surface during the summer. This forms bogs, swamps, and ponds that support hordes of insects and the migratory birds that feed on them.


Humans have turned large areas of grassland into virtual desert (desertification) by the overgrazing of livestock, poor agricultural practices, and the cutting down of trees for fuel and shelter. Without roots to hold topsoil in place, it blows away in the wind. This creates a ³dust bowl² situation, as is occurring in much of Africa today.



C. Forests.


Moderate to high rainfall allows trees to predominate.


Tropical rain forest: Extremely high rainfall with constant high temperatures. They have a large biodiversity (50%-80% of terrestrial species) with stratification at various levels of the tree canopy. Due of rapid decay and nutrient recycling, most nutrients are stored in the biomass itself and not the soil. Amazon is an example.


Tropical deciduous (monsoon) forests: Have distinct wet and dry seasons. South East Brazil and India are examples.


Temperate deciduous forests: Moderate temperature with significant seasonal changes and abundant rainfall. These are typical of the eastern U.S.


Evergreen or Boreal coniferous forests (taiga): Sub-artic climates with long, very cold winters. Dominated by evergreens. An example is northern Canada


Coastal coniferous or temperate rain forests: Coastal areas with very high rainfall. Dominated by spruce, fir, and redwoods. An example is the northwest U.S.


Human impacts have largely been due to clearing of forest for agriculture, grazing, and logging. Causes rapid soil erosion and habitat loss. Also increases CO2 in the atmosphere.



D. Mountain Ecosystems.


Often are very diverse ecosystems in a relatively small area. May contain many of the ones described above. The particular one present depends on altitude. Often these areas are ³island² habitats surrounded by lower elevation environments affected by humans. These isolated habitats may become sanctuaries for species driven out of other areas. Unfortunately, because of their isolated, restricted nature, they may be very susceptible to climate change and extinctions.


Being affected by human development, resource exploitation, damming of rivers, air pollution carried by high altitude winds, and increased UV due to ozone destruction.



III. Major Aquatic Life Zones


Major controlling factors are salinity, temperature, sunlight, nutrients, and dissolved oxygen. Two major divisions based on salinity: Marine (saltwater) and freshwater. Marine further divided into coastal zone or open ocean environments.



A. Coastal Zone Environments


Found in relatively warm, shallow (less than 100 m), nutrient rich waters that extend from the high-tide mark to the edge of the continental shelf. They represent only 10% of the ocean's area, but 90% of the species. They typically have very high productivity and include the following specific environments:


Estuaries: These are partially enclosed bodies of water where saltwater and freshwater mix. Chesapeake Bay is an example.


Coastal Wetlands: Also known as saltwater marshes. They are nurseries for many marine organisms and important habitats for migratory birds.


Mangrove Swamps: Mangrove roots help stabilize coastal sediments.


Coastal shores may be rocky and have many tide pools or may be sandy and be part of barrier beaches. Shorelines are divided into layered habitat zones, depending on the position relative to sea level and the tidal range.


Areas inland from beaches are protected by sand dunes. Dunes are often destroyed during development, leading to increased coastal flooding and erosion.


Coral Reefs: Found in shallow tropical waters. Coral polyps secrete calcium carbonate and over time build up huge platforms. Corals live in a mutualistic relationship (both benefit) with zooxanthellae algae in their tissue. Coral reefs are home to 25% of marine species and protect many coastlines from wave energy.


Human impacts on coastal environments are large because 2/3 of worldıs population lives within 100 miles of the coast. Impacts include waste pollution, wetland loss (50% since 1900), coral reef loss (10% with another 60% threatened or endangered), landfill, and agricultural runoff (pesticides and herbicides).


Because of bioaccumulation, many marine organisms have high levels of toxic metals and chemical in their tissue, either killing them or making them unhealthy to eat. May ruin commercial fishing and clamming industries.



B. Open Sea Environments


Three zones are defined: Euphotic (shallow), bathyal, and abyssal (deep). Division is based on the amount of sunlight reaching those depths. Represents 90% of the area of the worldıs oceans, but only 10% of the marine species. Except where upwelling of nutrient-rich, deep water occurs, open sea productivity is generally low. However, because of the size of the size, it is the world's largest contributor to total net primary productivity (just very dispersed).



C. Freshwater Environments


Lakes: Standing bodies of water divided into an upper limnetic zone, a deeper profundal zone, and a bottom benthic zone. A shallow littoral zone found near shore. Lakes can be classified as two extreme types.


Oligotrophic Lakes: Typically nutrient-poor. Often newly formed with step banks and clear, sediment-free water.


Eutrophic Lakes: Typically nutrient-rich. Often these are older lakes with gentler banks and murky sediment-rich waters. Usually have high net primary productivity. Nutrients may be derived from human activities and cause algal blooms and low dissolved oxygen (cultural eutrophication). Intermediate conditions between the two extremes are known as mesotrophic.


Streams: Drain all surface runoff and dissolved substances from a watershed or drainage basin. Three defined zones: source, transition, and flood plain.


Source: Swift, cold, and clear mountain (headwater) streams with high dissolved oxygen.


Transition: Warmer, deeper, and broader streams with higher productivity.


Flood plain: Wide, deep, and slow rivers with broad, flat valleys. Water is warmer and has lower dissolved oxygen. Rivers often form broad meanders with many, nearly stagnant backwaters, which may be eutrophic.


Freshwater Wetlands: Covered with water for part or all of the year. They are often seasonal. Defined by plant species and soil, not presence of water. Provide valuable habitats, natural water filtration, and floodwater storage capacity. Increase infiltration and replenishment of groundwater.


They are filled in and drained, largely for agriculture and development (80% in the U.S.). Now protected, although not always effectively. Can be remade, although expensive and not always successful.