Topic No. & Title: Topic 10 Community Development - Ecological Succession & Major Biomes

Transcription

1 [Academic Script] Community Development Ecological Succession & Major Biomes Subject: Course: Paper No. & Title: Zoology B.Sc. 3 rd Year Z-301B Ecology Topic No. & Title: Topic 10 Community Development - Ecological Succession & Major Biomes Lecture Title: Community Development - Ecological Succession & Major Biomes

2 Academic Script 1. Introduction of Succession Ecosystems are dynamic, changing units. On a daily basis, plants grow and die, animals feed on plants and on one another, and decomposers recycle the chemical elements that make up the biotic portion of any ecosystem. Abiotic factors (such as temperature, rainfall, intensity of sunlight, and seasonality) also have a major influence on the kind of community that will be established. Since all organisms are linked together in a community, any change in the community affects many organisms within it. Over long time periods, it is possible to see trends in the way the structure of a community changes. Generally, this series of changes eventually results in a relatively long lasting, stable combination of species that is self-perpetuating. The concept that communities proceed through a series of recognizable, predictable changes in structure over time is called succession. The relatively stable, long-lasting community that is the result of succession is called a climax community. 2. Types of Succession Ecologists have traditionally recognized two kinds of succession. Primary Succession

3 It occurs in an area that has never before been colonized. Generally, the area is nothing but bare rock. This type of environment may come about when Lava flows from a volcano and hardens into rock. A glacier retreats and leaves behind bare rock. A landslide uncovers an area of bare rock. The first species to colonize a disturbed area such as this are called pioneer species. They change the environment and pave the way for other species to come into the area. Pioneer species are likely to include bacteria and lichens that can live on bare rock. Secondary Succession It is a type of ecological succession that occurs on a site where an ecosystem previously existed (soil is present). But due to a disturbance to the ecosystem something upsets the natural balance. Disturbances may be natural or man-made (anthropogenic). Now think of examples of each. Examples of natural disturbances are tornadoes, floods, trees falling, fire, hurricane, disease etc. Examples of man-made (anthropogenic) disturbances are deforestation, pollution, urbanization, harvesting etc. Secondary succession is faster because the soil is already in place. In this case, the pioneer species are plants such as grasses and fireweed. Organic matter from the pioneer species improves the soil. This lets other plants move into the area. 3. Mechanism of succession

4 In 1916, Frederic Clements published a descriptive theory of succession and advanced it as a general ecological concept. His theory of succession had a powerful influence on ecological thought. Clements' concept is usually termed classical ecological theory. According to Clements, succession is a process involving several phases: 1. Nudation: Succession begins with the development of a bare site, called Nudation (disturbance). 2. Migration: It refers to arrival of propagules. 3. Ecesis: It involves establishment and initial growth of vegetation. 4. Competition: As vegetation became well established, grew, and spread, various species began to compete for space, light and nutrients. This phase is called competition. 5. Reaction: During this phase autogenic changes affect the habitat resulting in replacement of one plant community by another. 6. Stabilization: Reaction phase leads to development of a climax community. Example of Terrestrial primary Succession Imagine a lifeless area of bare rock. What will happen to it as time passes? 1. Very few species can live on bare rock since it stores little water and has few available nutrients. The first colonizers are usually lichens, which are a mutualistic relationship between an alga and a fungus. The alga photosynthesizes and makes organic compounds, while the

5 fungus absorbs water and minerals and clings to the rock. Lichens are such good colonizers that almost all "bare rock" is actually covered in a thin layer of lichen. Mosses can grow on top of the lichens. Between then these colonizers start to erode the rock and so form a thin soil. Colonizers are slow growing and tolerant of extreme conditions. 2. Pioneer species such as grasses and ferns grow in the thin soil and their roots accelerate soil formation. They have a larger photosynthetic area, so they grow faster, so they make more detritus, so they form better soil, which holds more water. 3. Herbaceous Plants such as dandelion have small winddispersed seeds and rapid growth, so they become established before larger plants. 4. Larger plants (shrubs) such as hawthorn, broom and rhododendron can now grow in the good soil. These grow faster and so out-compete the slower-growing pioneers. 5. Trees grow slowly, but eventually shade and out-compete the shrubs, which are replaced by shade-tolerant forestfloor species. A complex food web is now established with many trophic levels and interactions. This is called the climax community. Example of Secondary Succession by stages: A stable deciduous forest community. A disturbance, such as a wild fire, destroys the forest. The fire burns the forest to the ground. The fire leaves behind empty, but not destroyed, soil. Grasses and other herbaceous plants grow back first.

6 Small bushes and trees begin to colonize the area. Fast growing evergreen trees develop to their fullest, while shade tolerant trees develop in the understory. The short-lived and shade intolerant evergreen trees die as the larger deciduous trees overtop them. The ecosystem is now back to a similar state to where it began. These stages are called seral stages, or seral communities, and the whole succession is called a sere. Each organism modifies the environment, so creating opportunities for other species. As the succession proceeds the community becomes more diverse, with more complex food webs being supported. The final seral stage is stable (assuming the environment doesn t change), so succession stops at the climax stage. Succession will not go any further than the climax community. This is the final stage. This does not however, imply that there will be no further change. When large organisms in the climax community, such as trees, die and fall down, then new openings are created in which secondary succession will occur. The most often quoted examples of succession deal with plant succession. It is worth remembering that as plant communities change, so will the associated microorganism, fungus and animal species. Succession involves the whole community, not just the plants. 4. Major biomes

7 Biomes are defined as "the world's major communities, classified according to the predominant vegetation and characterized by adaptations of organisms to that particular environment" (Campbell). Biomes are classified in various ways. Here we will group biomes into five major types: 1. Aquatic 2. Deserts 3. Forests 4. Grasslands 5. Tundra 1. THE AQUATIC BIOME Water is the common link among the five biomes and it makes up the largest part of the biosphere, covering nearly 75% of the Earth s surface. Aquatic regions house numerous species of plants and animals, both large and small. The aquatic biome can be broken down into two basic regions, freshwater (i.e, ponds and rivers) and marine (i.e, oceans and estuaries). Freshwater Regions Freshwater is defined as having a low salt concentration usually less than 1%. Plants and animals in freshwater regions are adjusted to the low salt content and would not be able to survive in areas of high salt concentration (i.e, ocean). There are different types of freshwater regions:

8 ponds and lakes, streams and rivers, and wetlands. The following sections describe pond and lake ecosystem. Ponds and Lakes These regions range in size from just a few square meters to thousands of square kilometers. Scattered throughout the earth. Many ponds are seasonal; lasting just a couple of months (such as sessile pools) while lakes may exist for hundreds of years or more. Ponds and lakes may have limited species diversity since they are often isolated from one another and from other water sources like rivers and oceans. Lakes and ponds are divided into three different zones which are usually determined by depth and distance from the shoreline. The topmost zone near the shore of a lake or pond is the littoral zone. It is shallow, warmest and can absorb more of the Sun s heat. It sustains a fairly diverse community, which can include several species of algae (like diatoms), rooted and floating aquatic plants, grazing snails, clams, insects, crustaceans, fishes, and amphibians. The vegetation and animals living in the littoral zone are food for other creatures such as turtles, snakes, and ducks. The near-surface open water surrounded by the littoral zone is the limnetic zone. It is well lighted (like the littoral zone) and is dominated by plankton, both phytoplankton and zooplankton. A variety of freshwater fish also occupy this zone. Profundal zone is much colder and denser than the other two. Little light penetrates here. The fauna are heterotrophs,

9 meaning that they eat dead organisms and use oxygen for cellular respiration. Temperature varies in ponds and lakes seasonally and there is thermal stratification with a constant mixing of warm and cold water. Marine Regions Marine regions cover about three-fourths of the Earth s surface and include oceans, coral reefs, and estuaries. Marine algae supply much of the world s oxygen supply and take in a huge amount of atmospheric carbon dioxide. The evaporation of the seawater provides rainwater for the land. Here we discuss oceans. Oceans The largest of all the ecosystems, oceans are very large bodies of water that dominate the Earth s surface. Like ponds and lakes, the ocean regions are separated into separate zones: intertidal, pelagic, abyssal, and benthic. All four zones have a great diversity of species. Some say that the ocean contains the richest diversity of species even though it contains fewer species than there are on land. The intertidal zone is where the ocean meets the land sometimes it is submerged and at other times exposed, as waves and tides come in and out. Because of this, the communities are constantly changing. On rocky coasts, the zone is stratified vertically. Where only the highest tides reach, there are only a few species of algae and molluscs. In those areas usually submerged during high tide, there is a more diverse array of algae and small animals, such as

10 herbivorous snails, crabs, sea stars, and small fishes. At the bottom of the intertidal zone, which is only exposed during the lowest tides, many invertebrates, fishes, and seaweed can be found. The intertidal zone on sandier shores is not as stratified as in the rocky areas. The fauna include worms, clams, predatory crustaceans, crabs, and shorebirds. The pelagic zone includes those waters further from the land, basically the open ocean. The pelagic zone is generally cold though it is hard to give a general temperature range since, just like ponds and lakes, there is thermal stratification with a constant mixing of warm and cold ocean currents. The flora in the pelagic zone includes surface seaweeds. The fauna include many species of fish and some mammals, such as whales and dolphins. Many feed on the abundant plankton. The benthic zone is the area below the pelagic zone, but does not include the very deepest parts of the ocean (see abyssal zone below). The bottom of the zone consists of sand, slit, and/or dead organisms. Here temperature decreases as depth increases toward the abyssal zone, since light cannot penetrate through the deeper water. Flora are represented primarily by seaweed while the fauna, since it is very nutrient-rich, include all sorts of bacteria, fungi, sponges, sea anemones, worms, sea stars, and fishes. The deep ocean is the abyssal zone. The water in this region is very cold (around 3 C), highly pressured, high in oxygen content, but low in nutritional content. The abyssal zone supports many species of invertebrates and fishes. Mid-ocean ridges (spreading zones between tectonic plates), often with

11 hydrothermal vents, are found in the abyssal zones along the ocean floors. Chemosynthetic bacteria thrive near these vents because of the large amounts of hydrogen sulfide and other minerals they emit. These bacteria are thus the start of the food web as they are eaten by invertebrates and fishes. 2. THE DESERT BIOME A lack of water is the primary factor that determines that an area will be a desert. Deserts receive less than 25cm precipitation per year and have large temperature variation with very hot climate. It is the driest biome, which supports little plant life. Plants that can store water or live with little water and have roots that extend great distances to reach water live well in this area e.g. cactus, short grasses etc. The desert has many kinds of animals. Numerous species are of small size, and many are inactive during the hot part of the day. All the animals that live in deserts are able to survive with a minimal amount of water. e.g. kangaroo rat, camels, reptiles, Gila monster, iguana, scorpion etc. 3. THE FOREST BIOME Today, forests occupy approximately one-third of Earth s land area, account for over two-thirds of the leaf area of land plants, and contain about 70% of carbon present in living things. Forest biomes are biological communities that are dominated by trees and other woody vegetation can be classified according to numerous characteristics, with seasonality being the most widely used. Distinct forest types also occur within each of these broad groups. There are three major types of forests, classed according to latitude:

12 Tropical, Temperate and Boreal forests or Taiga. Here we discuss Taiga. Boreal forests or Tiaga It represents the largest terrestrial biome. Seasons are divided into short, moist, and moderately warm summers and long, cold, and dry winters. The length of the growing season in Tiaga is 130 days. Temperatures are very low. Precipitation is primarily in the form of snow, cm annually. Soil is thin, nutrientpoor, and acidic. Canopy permits low light penetration, and as a result, understory is limited. Flora consists mostly of cold-tolerant evergreen conifers with needle-like leaves, such as pine, fir, and spruce. Fauna include woodpeckers, hawks, moose, bear, fox, wolf, deer, hares, chipmunks, shrews, and bats. 4. THE GRASSLAND BIOME Grasslands are widely distributed over temperate parts of the world. As with deserts, the major factor that contributes to the establishment of grassland is the amount of available moisture. Grasslands generally receive between 25 to 75 centimeters of precipitation per year. These areas are windy with hot summers and cold-to mild winters. In many types of grassland, fire is an important force in preventing the invasion of trees and releasing nutrients from dead plants to the soil. Grasses make up 60 to 90 percent of the vegetation. Animals like giraffe, zebra, African elephant, kangaroo, lion,

13 gazelle, bison, prairie dog, ostrich, rhinoceros, wildebeest, deer, rabbits etc. are common. Tropical parts of Africa, South America, and Australia have extensive grasslands spotted with occasional trees or patches of trees. This kind of a biome is often called a savanna 5. THE TUNDRA BIOME North of the taiga is the tundra, a biome that lacks trees and has a permanently frozen subsurface soil. This frozen soil layer is known as permafrost. Because of the permanently frozen soil and extremely cold, windy climate (up to 10 months of winter), no trees can live in the area and hence having low biotic diversity. Although the amount of precipitation is similar to that in some deserts -less than 25 centimeters per year, the short summer is generally wet because the winter snows melt in the spring and summer temperatures are usually less than 10 C, which reduces the evaporation rate. Since the permafrost does not let the water sink into the soil, waterlogged soils and many shallow ponds and pools are present. Many animals like ducks and geese migrate to the tundra in the spring; there, they mate and raise their young during the summer before migrating south in the fall. Animals such as insects, ducks, geese, other birds, mice, arctic hares, reindeer, polar bears, caribou, snowy owls etc. are found in this region. When the top few centimeters of the soil thaw, many plants grasses, dwarf birch, dwarf willow, mosses and lichens grow. The plants are short, usually less than 20 centimeters tall. Scattered patches of tundra like

14 communities also are found on mountaintops throughout the world. These are known as alpine tundra. 6. Summary Ecosystems change as one kind of organism replaces another in a process called succession. Ultimately, a relatively stable stage is reached, called the climax community. Succession may begin with bare rock or water, in which case it is called primary succession, or may occur when the original ecosystem is destroyed, in which case it is called secondary succession. The stages that lead to the climax are called successional stages. Major regional terrestrial climax communities are called biomes. The primary determiners of the kinds of biomes that develop are the amount and yearly distribution of rainfall and the yearly temperature cycle. Major biomes are Aquatic, Deserts, Forests, Grasslands, and Tundra. Each has a particular set of organisms that is adapted to the climatic conditions typical for the area.