We've considered some of the general characteristics of ecosystems. Today, I'd like to talk in some detail about the characteristics of specific ecosystems found on earth.
To understand why a specific ecosystem has the characteristics it does, we must first understand some basic concepts about climate -- with the exception of those in the deep ocean, ecosystems are all at least partly shaped by climate.
Climate: prevailing weather conditions at a location -- e.g. temperature, humidity, wind speed and direction, cloud cover and rainfall.
Climate is the outcome of a number of factors:
1. variations in the amount of incoming solar radiation.
2. the earth’s daily rotation and its path around the sun.
3. distribution of continents and oceans.
4. elevation of land masses.
These factors interact to produce prevailing winds, ocean currents, and seasonal patterns of temperature and rainfall.
Let’s look at how global air currents influence patterns of temperature and rainfall.
1. Sun’s rays more concentrated at the equator -- as air near the equator heats up, it picks up moisture and begins to rise.
2. As the moist air rises, it cools and releases much of its moisture as rain -- high rainfall in the tropics.
3. This drier air moves north and south away from the equator -- begins to descend at about 30 degrees north and south latitude and is heated adiabatically as it descends by increasing atmospheric pressure -- these descending masses of hot, dry air produce deserts at these latitudes.
4. some of the air from these descending air masses is diverted back towards the equator, while part continues to move away from equator.
5. airs masses pick up moisture and are heated at 60 degrees latitude -- rise and release moisture as rain -- e.g. moist climate of Pacific northwest.
some of this air moves south and some moves north, were it cools and descends on the polar regions -- note: this is dry, cold air and polar regions are actually deserts.
Figure 42.4.
The movement of these air masses away from the equator are also influenced by the earth’s rotation and the formation of high and low pressure centers over continents and oceans -- these factors interact to produce the directions of regional prevailing winds.
Prevailing winds, the earth’s rotation and variations in ocean water temperature produce ocean currents which also have distinct regional patterns.
The topography of continents (e.g. mountains, valleys, etc.) influence regional climatic patterns -- the regional climate of California is a good example &emdash; topography creates a rain shadow effect.
1. Westerly winds bring warm, moist air masses inland over California
2. air masses run into the mountain ranges (e.g. Sierra Nevada) which run north-south.
3. as air mass ascends to go over mountains, it cools and drops most of its moisture as rain or snow.
as the now dry air crests the mountains, it begins to sink and is heated adiabatically -- hot, dry air draws water out of the plants and soil east of mountains producing desert conditions -- e.g. Mojave desert is in the "rain shadow of the mountains.
Figure 42.7
Now, let’s talk about the various kinds of ecosystems found in the world
Like the organisms that inhabit them, ecosystems can also be classified. The most obvious groupings are based on whether or not the ecosystem is on land (terrestrial) or in the water (aquatic).
Terrestrial ecosystems -- regional ecosystem types having similar communities are sometimes called biomes.
Biomes arise through a process called succession = changes in community composition through time.
Primary succession -- complete community development from a bare rock surface -- involves formation of soil from rock and may take thousands of years to complete -- e.g. lava flow to tropical rainforest.
Secondary succession -- existing plant community is removed by a disturbance (e.g. fire) -- doesn't include soil formation -- time scale is 10-100 years.
In succession, the first community is called the pioneer community and it consists of weeds that are able to colonize disturbed areas -- gradually each plant community modifies the environment, making it unsuitable for itself, but suitable for another community -- gradually each community is replaced by another -- process culminates when they reach the climax community which is the final stage in succession.
Climax communities: persistent, stable collections of species that typify particular regions of the world -- climax communities are called biomes by ecologists.
Characteristics of biomes are determined by regional climate and by the types of soils available for plants to grow on. Here’s what the global distribution of biomes looks like.
Figure 42.10.
Let’s look at some of the major biomes on earth.
1. Tundra &emdash; cold temperatures for much of year -- short growing season (60 days) -- may be alpine or arctic -- low productivity because of short growing system -- decomposition slow because of cold temperatures -- plant litter accumulates and soils have high organic content -- low plant species diversity -- grasses, dwarf shrubs.
2. Forests -- tremendous above-ground biomass -- about 40X as productive as tundra -- competition for light important to plants -- only 1% of sunlight hitting canopy reaches forest floor--
A. coniferous (evergreen) forests -- cool, wet climates -- pines, firs, spruces are common -- found in northern latitudes.
B. temperate deciduous forest -- e.g. NE U.S. -- e.g. beech, maple, oak, hickory -- deciduous trees lose and re-grow leaves yearly.
C. tropical rainforest -- most productive of all ecosystems -- moderate temperatures and high rainfall (e.g. > 200 cm. per year) -- decomposition rates high -- low organic matter in soil -- soils have low fertility because nutrients are tied up in tremendous biomass of forest -- highest plant and animals species diversity of any biome-- e.g. a single acre may contain over 400 different species of trees -- rainforests are being destroyed and an estimated 1 species/hour is going extinct due to disturbance of rainforest.
3. Grasslands -- 10"-30" rainfall/year -- not enough rainfall to support trees --droughts are common -- soils very fertile and high in organic matter -- grasses are dominant plants and animal community dominated by grazers -- prairie of North America -- once supported 65 million bison and probably an equal number of pronghorn antelope.
4. Savannas -- essentially grassland with scattered trees and shrubs -- Africa, Australia and South America -- animals consist of grazers (e.g. zebras, rhinos, antelopes) and browsers (elephants, giraffes) -- trees deeply rooted and act as nutrient pumps -- leaves fall off trees and decompose to make nutrients available to grasses -- trees are being used for fuel and this is drastically decreasing soil productivity.
5. Shrublands -- dominated by shrubs 15'-20' -- example is the chaparral of southern California -- found around Mediterranean and west coast of South America -- common feature is prolonged summer drought -- shrubs are evergreen with thick, waxy leaves to prevent water loss -- fire is an important part of this biome -- 40 year cycle -- fire suppression has resulted in more fuel accumulating so that fire are often larger and more destructive when the occur.
6. Deserts -- less than 10" rainfall/year -- high temperatures increase evaporative water loss -- least productive terrestrial ecosystem -- sensitive to disturbance -- plant reproduction may be a rare event -- e.g. stand of mesquite in which all individuals are 250 years old -- little or no reproduction in 250 years -- desert plants and animals display numerous water conserving adaptations.
Aquatic ecosystems can be classified as freshwater or marine.
Freshwater ecosystems include streams (lotic ecosystems), lakes and ponds (lentic ecosystems) and marshes and bogs (wetlands).
Lotic ecosystems -- moving water -- rate of water current important because it 1) determines rate at which materials such as nutrients are flushed out of the system and it 2) determines the type of bottom (e.g. sandy, gravel, or rock), which, in turn, determines the kinds of plants and animals that can live there.
Lotic systems are largely open systems as water is constantly moving -- nutrients such as N and P are limiting because they are flushed out -- major input of carbon is in the form of leaf litter from nearby terrestrial ecosystems.
Lentic systems -- lakes and ponds -- standing water -- more closed than lotic systems and have lower rates of nutrients loss.
Lakes and ponds can also be divided into three life zones
1. littoral zone -- closest to shore -- contains rooted aquatic plants.
2. limnetic zone -- sunlit body of lake
3. profundal zone -- below level of light penetration -- no photosynthesis -- decomposition is major process.
Figure 42.21
Lakes in temperate zones of the world display thermal stratification in the summer-- surface layer (epilimnion) is heated by sun -- temperature drops quickly in middle layer (thermocline) -- bottom layer (hypolimnion) -- these layers do not mix -- epilimnion becomes nutrient poor as phytoplankton photosynthesize and use up nutrients -- detritus (organic matter) sinks to bottom of lake -- hypolimnion becomes oxygen-depleted as organic matter is decomposed.
Figure 42.22
Turnover occurs in fall as epilimnion cools, gets heavier and sinks -- results in redistribution of oxygen and nutrients.
Lakes classified according to their nutrient status:
1. oligotrophic -- nutrient poor -- low organic matter -- low phytoplankton productivity.
2. eutrophic -- nutrient rich -- high organic matter, high phytoplankton productivity -- oligotrophic lakes can become eutrophic through inputs of nutrients from outside by human activity -- e.g. pollution.
Marine Ecosystems -- begin at ocean's edge.
Estuaries -- formed where large river flows into ocean -- mixing of freshwater and saltwater -- organisms must be adapted to changing salinity -- act as a nutrient trap -- trap nutrients brought in by tides and carried down by river -- because of their rich nutrients, estuaries have very high primary productivity -- estuaries are often called "nurseries of the sea" because they are the place where many larval and immature marine fish grow up and develop before moving out into the ocean as adults.
Seashores -- exposed to waves and tides -- classified as rocky or sandy --
Rocky shore -- offer something solid for plants and animals to attach to -- seaweed (algae) are primary producers -- animals include barnacles, oysters, sea urchins.
Sandy beach -- most organisms dwell under the sand -- e.g. ghost crabs, ghost shrimp, sandworms.
Coral reefs -- diverse, productive ecosystems -- found in shallow, tropical waters -- reefs formed from calcium carbonate (limestone) exoskeletons of corals -- crevices and caves provide shelter from many different plants and animals -- sponges, echinoderms, fishes.
Ocean -- covers 3/4 of earth's surface -- can be divided into 3 life zones:
1. neritic zone -- shallow coastal waters -- sunlight -- seaweeds and phytoplankton abundant.
2. pelagic zone -- open sea -- includes sunlit waters -- water is nutrient poor and productivity is comparable to that of the terrestrial deserts -- phyto- and zooplankton base of trophic structure -- fed upon by fish and whales.
3. benthic -- ocean floor -- temperature about 2oC and pressures are intense -- total darkness -- bombarded by dead, organic matter sinking from above -- instead of ecosystems based on photosynthesizing autotrophs, benthic ecosystems are based decomposers and organisms that feed on decomposers (detritivores).
Benthic habitats are also the location of a unique, recently discovered ecosystem.
The earth’s surface consists of large tectonic plates -- in areas of the ocean bottom where these plates are formed, heated water is released from cracks in the earth’s crust -- these areas are called hydrothermal vents -- ecosystem associated with these vents are based not on photosynthetic plants (no light), but on chemosynthetic bacteria that are able to obtain energy from hydrogen sulfide (H2S) released at the vents &emdash; these bacteria are the primary producers are fed upon by various consumers (worms and other invertebrates).
Next time: Human Impacts on the Environment.