In this first chapter, we’ll review the first two units covered on the AP Environmental Science exam, which AP calls The Living World: Ecosystems and The Living World: Biodiversity, respectively. Obviously, these two topics are extremely interrelated. According to College Board, about 6 to 8% of the test is based directly on each of these topics. That’s a total of 12-16% off test material.
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The chapter starts with a discussion of what an ecosystem is, a review of the concept of evolution, and the classification of biomes. Next, we’ll discuss the abiotic elements that are essential to life and their natural cycles. Then we’ll examine the biotic components of ecosystems — living systems —and how energy is used among them. To review biodiversity, we’ll start with a discussion of what biodiversity is, what can affect it, and how ecosystems. Next, we’ll review how ecosystems and biodiversity provide humans with essential services. Finally, will review how ecosystems change as a result of disruptions, including the process of ecological succession.
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One of the fundamental concepts over me and I’m at the sizes of interconnected systems. Interconnected system. The resort on the small living in nonliving systems we see every day. To begin to understand this complexity, an easy place to start is with the idea of an ecosystem.
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When a habitat (an organism's physical surroundings) selects certain organisms to live and reproduce and others to die, that population is said to be undergoing natural selection. In natural selection, beneficial characteristics that can be inherited are passed down to the next generation, and unfavorable characteristics that can be inherited become less common in the population
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Any cause that reduces reproductive success (fitness) in a portion of the population is selective pressure, and this is what drives natural selection.
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It is important to remember that natural selection acts upon a whole population, not on an individual organism during its time. What changes during evolution is the total genetic makeup of the population or gene pool, and natural selection is one of the mechanisms by which evolution operates.
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The other way evolution operates is genetic drift. Genetic drift is the accumulation of changes in the frequency of alleles over time due to sampling errors—changes that occur as a result of random chance. For example, in a population of owls, there may be an equal chance of a newly born owlet having long talons or short talons, but due to random breeding variance, a slightly larger number of long-taloned owlets are born. Over many generations, this slight variance can develop into a larger trend, until the majority of owls in that population have long talons.
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These breeding variances could be a result of a chance event--such as an earthquake that drastically reduces the size of the nesting population one year. Small populations are more sensitive to the effects of genetic drift than large, diverse populations. When a population displays small-scale changes over a relatively short period of time, micro-evolution has occurred. Macroevolution refers to large-scale patterns of evolution within biological organisms over a long period of time.
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Just as new species are formed by natural selection and genetic drift, other species may become extinct. Extinction occurs when a species cannot adapt quickly enough to environmental change and all members of the species die.
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Biological extinction is the true extermination of a species. There are no individuals of biologically extinct species left on the planet (for example, the dodo bird or passenger pigeon).
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Ecological extinction is when there are so few individuals of a species that this species can no longer perform its ecological function (for example, alligators in the Everglades in the 1960s or wolves in Yellowstone before re-introduction in the last decade). Commercial or economic extinction is when a few individuals exist but the effort needed to locate and harvest them is not worth the expense (for example, the groundfish population of the Grand Banks off the Maritimes of Canada).
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Let's talk more about how species get along together in ecosystems. You probably recall from your biology class that a group of organisms of the same species is called a population, and when populations of different species occupy the same geographic area, they form a community.
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Every species within a community has an ecological niche. A species' niche is described as the total sum of a species' use of the biotic and abiotic resources in its environment.
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The niche describes where the species lives, what it eats, and all of the other resources the species utilizes in an ecosystem. Another term you should know for the exam is habitat —a habitat is an area or environment where an organism or ecological community normally lives or occurs.
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Some species interact quite a bit with other members of their population; for example, some animals form herds, while other species are loners--like bears. The reasons for these different levels of sociability are largely competition, predation, and a general need to exploit the resources in the environment.
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Direct competition can also be avoided cause of resource partitioning. This occurs when different species use slightly different parts of the habitat but rely on the same resource. For example, there are five species of warblers that can all live in the same pine tree. They can coexist because each species feeds in a different part of the tree: the trunk, at the ends of the branches, and at other sites.
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Keep in mind that many types of species can engage in both short- and long-term migration, for reasons including food and water availability, temperature changes, mating opportunities, and safety from predation. This means that a given species might be part of several different communities at different times and might fill a given niche in each of those communities only some of the time. All right, moving on!
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Although it's relatively easy to observe competition between animals, competition between plants is much more subtle and occurs much more slowly. However, if you have a few years to kill, spend some time in your backyard watching the trees and other plants grow. You’ll see that they compete for sunlight and for ground space; they even produce chemicals that inhibit other plants growth.
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The second important type of interspecies interaction is predation.
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The third type of relationship that exists between organisms is the symbiotic relationship. Symbiotic relationships are close, prolonged associations between two or more organisms of different species that may, but do not necessarily, benefit each member.
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There are three types of symbiotic relationships, and you should be familiar with all of these for exam day.
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Because different geographic areas on Earth differ so much in their abiotic and biotic components, we can easily place them in broad categories. The two largest categories are broken down in this way: ecosystems that are based on land are called biomes, while those in aqueous environments are known as aquatic life zones.
Land environments are separated into biomes based on factors such as climate, geology, and soils. topography, hydrology, and vegetation. Although it might seem that each biome listed in the table on the following page is very distinct, in reality, biomes blend into each other; they do not have distinct boundaries. The transitional area where two ecosystems meet actually has a name- these areas are called ecotones. Another important term that you should be familiar with for the exam is ecozones: ecozones (also called ecoregions) are smaller regions within ecosystems that share similar physical features.
Recall that aquatic life zones are the equivalents of biomes in aquatic ecosystems categorized primarily by the salinity of their water--freshwater and saltwater ecosystems fall into separate categories.
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In all-natural bodies of water, there exist layers of water that vary significantly in their temperatures, oxygen content, and nutrient levels. These layers are affected differently by seasonal changes and other disturbances, and this also contributes to how they are categorized.
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The layers of freshwater bodies may also be categorized differently, according to the types of organisms that can live in them. You should definitely be familiar with the following terms for the AP Environmental Science Exam, so take note!
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Some of the Earth's most important ecologically diverse ecosystems are wetland areas along the shores of fresh bodies of water, wet inland habitats fed only by rainwater, and ephemeral seasonally temporary) water bodies. Types of wetlands include marshes, swamps, and bogs. prairie potholes (which exist seasonally), and floodplains (which occur when excess water Hows out or the banks of a river and into a flat valley). So, those are the main types of freshwater bodies you’ll need to know. Let’s look more specifically at the mangrove swamp, Mangrove swamps are coastal wetlands (areas of land covered in fresh water, salt water, or a combination of both) found in tropical and subtropical regions, and they are threatened by activities such as shrimp aquaculture and the degradation of the western coastlines.
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A huge diversity of animals is found in mangrove swamps. Because these estuarine swamps are constantly replenished with nutrients transported by freshwater runoff from the land, they support a bursting population of bacteria, other decomposers, and filter feeders. These ecosystems also sustain billions of worms, protozoa, barnacles, oysters, and other invertebrates, which in turn feed fish and shrimp, which support wading birds, pelicans, and, in the United States, the endangered crocodile.
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The importance of mangrove swamps has been well established. They function as nurseries for shrimp and recreational fisheries, exporters of organic matter to adjacent coastal food chains, and enormous sources of nutrients valuable to plants, wildlife, and ecosystem function. Their physical stability also helps to prevent shoreline erosion, shielding inland areas from severe damage during hurricanes and tidal waves.
Before we get into our review of the world's oceans, let’s consider another aquatic ecosystem (besides wetlands and estuaries) that's an essential source of biodiversity.
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This one is a salt-water ecosystem. Certain landforms that lie off coastal shores are known as barrier islands. Because barrier islands are created by the buildup of deposited sediments, their boundaries are constantly shifting as water moves around them.
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These spits of land are generally the first hit by offshore storms, and they are important buffers for the shoreline behind them.
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In tropical waters, a very particular type of barrier island called a coral reef is quite a common.
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Like freshwater bodies, oceans are divided into zones based on changes in light and temperature. Study the following terms, and know them cold for the test!
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Both freshwater and saltwater bodies experience a seasonal movement of water from the cold and nutrient-rich bottom to the surface. These upwellings provide a new nutrient supply for living organisms in the photic regions. Therefore, they are followed by an almost immediate exponential growth in the population of organisms in these zones, especially the single-cell algae, which may form blooms of color called algal blooms. These algae can also produce toxins that may kill fish and poison the beds of filter feeders such as oysters and mussels. One notorious recurring toxic algal bloom is referred to as red tide, this is caused by a proliferation of dinoflagellates.
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Keep in mind that the worldwide distribution of biomes is dynamic; the distribution of both biomes and aquatic life zones across specific places on Earth has changed in the past and may again shift as a result of global climate changes.
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Now that we've outlined what the major biomes and aquatic life zones look like, let's take some time to discuss the abiotic elements of ecosystems: specifically, the elements that bridge the gap between the nonliving and the living—water, nitrogen, carbon, and phosphorus they cycle through the environment, bringing the key components that ecosystems need to function and burgeon with life.
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As you may have learned in your biology class, nutrients such as carbon, oxygen, nitrogen, phosphorus, sulfur, and water all move through the environment in complex levels known as biogeochemical cycles. Well, you'll need to know a bit about these cycles for the exam, so we’ll go through each of them here.
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As you can probably tell from the collective name of these natural cycles, living organisms, geologic formations, and chemical substances are all involved in these compounds and how they move toward their destinations. In other words, you'll need to know that water moves from the atmosphere to the Earth's surface through precipitation either in the form of snow or rainfall.
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But let’s talk about a few things that all of these cycles have in common before we go into each one in detail. First of all, the term reservoir is used to describe a place where a large quantity of a nutrients sits for a long period of time (in the water cycle the ocean is an example of a reservoir). The opposite of a reservoir is an exchange pool, which is a site where a nutrient sits for only a short period of time (in the water cycle, a cloud is an example of an exchange pool). The amount of time a nutrient spends in a reservoir or an exchange pool is called its residency time. In the water cycle, water might exist in the form of a cloud for a few days, but it might exist as part of the ocean for a thousand years! Perhaps surprisingly, living organisms can also serve as exchange pools and reservoirs for certain nutrients; we’ll delve into more about this later.
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Though we won't get into a discussion of trace elements here, you should also know that certain trace elements such as zinc, copper, and iron are necessary for small amounts for living organisms.
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Let's start with perhaps the best-known biogeochemical cycle: the water cycle.
As you might imagine, the water that exists in the atmosphere is in a gaseous state, and when it condenses from the gaseous state to form a liquid or solid, it becomes dense enough to fall to the Earth because of the pull of gravity.
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Water is returned to the atmosphere from both the Earth’s surface and from living organisms in a process called evaporation. Specifically, animals respire and release water vapor and additional gases into the atmosphere.
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In plants, the process of transpiration releases large amounts of water into the air. Finally, other major contributors to atmospheric water are the vast number of lakes and oceans on the Earth's surface. Incredibly large amounts of water continually evaporate from their surfaces.
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