Chapter 3 Notes: Communities, Biomes, and Ecosystems

Limiting Factors Recall from chapter 2, that all ecosystems have biotic and abiotic factors. In every ecosystem, there are always certain factors that are abundant and other factors that are scarce or hard to come by. Those factors that are important but scarce or hard to come by in an ecosystem are referred to as limiting factors. Especially if those factors are essential for life.

Limiting Factors If you are a plant or animal that lives in a desert, the main limiting factor might be water. If you are a plant in the rainforest that is competing for sunlight, sunlight might be the limiting factor in that ecosystem. If you are a moose that depends on plants that are high in nutrients, then forest fires might be the main limiting factor for a moose, because forest fires increase the amount of nutrients available in a boreal forest ecosystem.

Limiting Factors in Ecosystems

Range of Tolerance All living organisms have a range of tolerance for certain factors that they can withstand. A range of tolerance can be described as a zone within an upper limit and lower limit of environmental characteristics. Moose have a range of tolerance for temperatures down to about -40 and up to about 60 degrees Fahrenheit. Above or below these temperatures, moose may undergo physiological stress.

Range of Tolerance for Organisms

Ecological Succession As ecosystems mature, eventually they reach a stage of maturity called a climax community. Climax communities are not ecologically productive in terms of biomass and diversity. Once an ecosystem reaches the stage of climax community, some type of natural or man-made disturbance must take place in order to restore the climax community to a more productive state.

Climax Community

Ecological Succession Examples of natural disturbances could be volcanic eruptions, forest fires, glaciation, flooding, water and wind erosion, storms, hurricanes etc. Examples of man-made disturbances could be building roads, agriculture, prescribed forest fires, logging, clear cutting, development of land etc. After a disturbance occurs, ecosystems go through a natural progression or a series of changes called an ecological succession.

Ecological Succession One type of ecological succession is called a primary succession. In primary succession, the disturbance causes such a severe change that usually the only thing left is bare rock. In the case of primary succession, the disturbed area must reestablish a new layer of soil before plants can move in and reestablish an area. Plants and animals that are the first to reestablish an area after a disturbance are called pioneer species. lichens are an important pioneer species in primary succession.

Primary Succession

Ecological Succession Lichens are important pioneering species, because they are one of the few examples of living organisms that can survive on bare rock. They are also important because they also help create new soil by producing acids that help chemically break down the rock into soil, so that other species can come in and grow after them. If a disturbance occurs, but the soil in the area is still intact, then another type of succession occurs called secondary succession.

Secondary Succession

Ecological Succession A good example of an event that may cause a secondary succession is a forest fire. In a forest fire, the trees and vegetation are removed, but the soil remains intact. Nutrients from the trees are released back into the soil once the trees are burned. Fireweed, alders and willows are all good examples of pioneer species that help reestablish an area after a forest fire because they are shade-intolerant. These pioneer species help the ecosystem by either nitrogen fixation or they allow shade-tolerant species like birch and aspen to eventually grow and recolonize.

Ecological Succession

Biomes A group of ecosystems in an area with a similar climate makes up a biome. The condition of the atmosphere at any given time or place is the weather. The average weather conditions in an area including its temperature and precipitation is called its climate. Climates are determined by the distance north or south of the equator which is called the latitude.

Latitude, Weather, and Climate

Major Biomes of the World

Biomes Tundra – low precipitation (15-25 cm), cold temps (-34-12 C), low growing vegetation, animals adapted to the cold (caribou, polar bears, arctic fox, arctic ground squirrels), permafrost, cold and dark winters. Boreal forest – moderate prec. (30-84 cm), cold temps (-54-21 C), coniferous trees dominate, animals of the boreal forest (moose, lynx, hare, red squirrel, owls, hawks), short and moist summers, cold and dry winters.

Biomes Tundra Boreal Forest (Taiga)

Biomes Temperate forest – moderate prec. (75-150 cm), moderate temps (-30-30 C), deciduous broadleaf trees dominate, animals of the temperate forest (deer, squirrels, turkeys, hawks, owls, raccoons etc.), well-defined seasons, hot summer, cold winter. Temperate woodland or chaparral – moderate prec. (38-100 cm), warm temps (10- 40 C), shrubs dominant vegetation, animals of the chaparral (coyotes, jackrabbits, lizards, snakes, insects), summers hot and dry, winters cool and wet.

Biomes Temperate Deciduous Forest Temperate Woodland or Chaparral

Biomes Temperate grassland – moderate prec. (50-89 cm), moderate temps. (-40-38 C), grasses are dominant vegetation, animals of the grassland (gazelles, bison, horses, lions, antelope, deer, mice, coyotes, fox, wolves, snakes, insects), summers hot, winters cold, regular fires. Desert – low prec. (2-26 cm), temps vary, cacti and other plants adapted to arid conditions dominate, desert animals (lizards, coyotes, snakes, jackrabbits, sheep, vultures).

Biomes Temperate Grassland or Prairie Desert

Biomes Tropical savanna – moderate prec. (50-130 cm), high temps (20-30 C), grasses and scattered trees, savanna animals (lions, hyenas, cheetahs, elephants, giraffes, zebras etc.), hot and rainy summers, cool and dry winters. Tropical seasonal forest – high prec. (more than 200 cm per year), high temps (20-25 C), forest vegetation, tropical seasonal forest animals (elephants, tigers, monkeys etc.), deciduous trees drop their leaves in the dry season, rainfall is seasonal.

Biomes Tropical Savanna Tropical Seasonal (Deciduous) Forest

Biomes Tropical rain forest – extremely high prec. (200- 1000 cm per year), high temps. (24-27 C), large diversity of plant life, tropical rain forest animals (chimpanzees, Bengal tigers, orangutans, toucans, sloths, cobras, jaguars, insects), high humidity, no seasons, hot and wet all the time. Mountains – mountainous terrain varies with elevation, changes in elevation resemble changes in latitude from an ecological standpoint. Polar – cold all year, covered in ice, no vegetation, animals highly dependant on the sea for food. (polar bears, penguins, krill, seals)

Biomes Tropical Rainforest Polar

Mountain Biomes

Aquatic Ecosystems A large portion of the earth is water. It should be no surprise that many ecosystems in the world are aquatic ecosystems. Approximately 97.5% of the water in the world is saltwater (oceans). Roughly 2.5% of the water is freshwater. How much of the 2.5% freshwater is found in... Glaciers? Groundwater? Lakes and rivers?

Distribution of water graph

River and Stream Ecosystems As streams travel from their headwaters to their destination (ocean, stream confluence, lake etc.) the character of the stream changes creating different habitats. Stream headwaters have a steep gradient, low volume (discharge), waters are cold and rich in oxygen but poor in nutrients. Stoneflies, caddisflies, mayflies, rainbow trout, grayling. Streams closer to their destination have a shallow gradient, high volume, warm waters that are low in oxygen but rich in nutrients.

River Continuum Concept

Lake Ecosystems Lakes are either Eutrophic (shallow, warm, more sunlight, nutrient rich, more vegetation) or Oligotrophic (deep, cold, less sunlight, nutrient poor, less vegetation). Most of the life in a lake is found close to the shoreline (littoral zone). More light means more plants. More plants means more food. The open water area in the middle of the lake is called the limnetic zone. The food chain in the limnetic zone is dependent on algae and plankton.

Lake Ecology

Lake Ecology

Lake Zones

Lake Zones

Lake Ecosystems The bottom of a lake is the benthic zone. The zone in a lake below light is the profundal zone, which is generally the most unproductive. No sunlight means no plants. No plants means no food. Not much life on the bottom except mostly decomposers and detritivores. Seasonal changes in the water temperature and water chemistry cause changes in lake ecology. 4 degrees Celsius (39-40 F) is when water is most dense. This causes thermal stratification.

Thermal Stratification in Lakes During the summer the top layer of the lake (the epilimnion) warms up faster than the bottom. The bottom of the lake stays at 4 Celsius because the water at 4 Celsius has the highest density. As autumn approaches the epilimnion cools off and eventually reaches 4 Celsius. When this occurs the density throughout the lake is the same and wind causes the water throughout the lake to mix nutrients. Dissolved oxygen is replaced and carbon dioxide is removed.

Thermal Stratification in Lakes As winter approaches, the temperature of the epilimnion lowers until it reaches 0 Celsius. At 0 Celsius the top of the lake freezes and the bottom of the lake (the hypolimnion) stays at a constant temperature of 4 Celsius. As spring approaches, the ice melts and begins warming until it reaches 4 Celsius. Once it reaches 4 Celsius, once again the wind causes the lake to mix and turnover. The name of these critical time periods are called spring and fall turnover.

Thermal Stratification Spring and Fall Turnover is important to lake ecosystems, because during summer and winter the amount of carbon dioxide increases, the pH lowers (becomes acidic), and the amount of dissolved oxygen decreases. Spring and Fall Turnover helps to replenish the lake ecosystem with new nutrients and dissolved oxygen, and it helps to eliminate the amount of carbon dioxide that has accumulated by cellular respiration of animals. It also helps regulate the pH levels of the lake.

Thermal Stratification

Other Important Aquatic Ecosystems Wetlands – marshes, swamps, and bogs. Estuaries – Freshwater and saltwater mix. Intertidal zones – Land meets the ocean. Open Ocean (Pelagic) – similar in many ways to lake ecology only bigger. There are different zones of the open ocean like the photic zone, aphotic zone, benthic zone and abyssal zone. Coastal Ocean and Coral Reefs – the most diverse ecosystem of the ocean.

Marine Ecosystems

Marine Ecosystems