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.

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.

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.

Ecosystems

• An ecosystem is a system of interconnected elements: a community of living organisms and its environment. It includes both biotic (living) and abiotic (nonliving) components.

• Ecosystems are the result of the biotic and abiotic components interacting. Biotic components – life – require resources to flourish, and the availability of those resources influences the interactions between species and their interactions with the environment over time the most influential fundamental interaction between and among ecosystems is that of evolution – which produces life as we know it.

Evolution

• Biodiversity in all forms is the result of evolution. Evolution is the change in a population’s genetic composition over time.

• We use a figure called a phylogenetic tree to model evolution. Phylogenetic trees can be very broad and encompass many types of species, or they could be very specific and describe the evolutionary relationships that exist between two species (or even the genome of one species!).

• While you don’t need to know much about evolution for this exam, you need to have a rough idea of how and why it takes place, so we’ll run through that now. Without trying to re-create the evolution of all living organisms, we will limit our discussion to a description of how many species are found formed. This process is called speciation. Strictly speaking, a species is defined as a group of organisms that are capable of breeding with one another – and incapable of breeding with other species.

• As you may recall, individual organisms that are better adapted for their environment will live and reproduce, ensuring that their genes are part of their population’s next generation. This is what Charles Darwin meant by evolutionary fitness.

How Evolution Works

• 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

• Any cause that reduces reproductive success (fitness) in a portion of the population is selective pressure, and this is what drives natural selection.

• 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.

• 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.

• 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.

• 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.

• 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).

• 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).

Relationships Between Species

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.

• 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.

• 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.

• Species can be generalists or specialists. A specialist species is one that has a narrow niche and can only live in a certain habitat.

• A generalist species is one that has a broad niche, is highly adaptable, and can live in varied habitats. Specialist species tend to have an advantage when their environments are relatively unchanging, while generalist species have the advantage in habitats that undergo frequent change.

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.

• Competition arises when two individuals of the same species or of different species--are competing for resources in the environment.

• When the two individuals that are competing are of the same species, this is called intraspecific competition, and when they are of different species, it's called interspecific competition. The resources that are competed for can be food, air, shelter, sunlight, and various other factors necessary for life: individuals may be competing to live in a fallen tree, to catch a running rabbit, or to mate with the most desirable female in the population.

• The competitor who is "most fic" eventually wins and obtains the resource. That's right — the others are eliminated by competition.

• One more thing about competition: when two different species in a region compete and the better-adapted species win, this phenomenon is called competitive exclusion.

• Gause's principle states that no two species can occupy the same niche at the same time and that the species that are less fit to live in the environment will relocate, die out, or occupy a smaller niche.

• When a species occupies a smaller niche than it would in the absence of competition, this compromised niche is called its realized niche. (The niche it would have if there were no competition is known as its fundamental niche.)

• 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.

• 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!

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.

The second important type of interspecies interaction is predation.

• Predation occurs when one species (a predator) feeds on another (prey), and it drives changes in population size. For example, in a year in which rainfall is relatively high in some regions, rabbits have plenty of food; this enables them to reproduce very successfully, and the number of rabbits in a population will increase dramatically. In turn, if the coyote is a predator of the rabbit, coyotes will have plenty of food, and their population will also boom.

• However, if the following year the rainfall is below average, there will be less grass. Then the population of rabbits will decline, and this will result in a decline in the population of coyotes. As a final note about predation: while it's tempting to think of predation existing only between animals, remember that herbivores prey on plants and Zooplankton on phytoplankton!

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.

There are three types of symbiotic relationships, and you should be familiar with all of these for exam day.

• In mutualistic symbiotic relationships (mutualism), both species benefit: for example this type of relationship exists between sea anemones and clown fish. The clown fish protects the sea anemone from some of its predators, while the stinging cells of the anemone protect the clownfish; the fish also eats some of the detritus left behind when the anemone feeds.

• In commensalism symbiotic relationships (commensalism), one organism benefits while the other is neither helped nor hurt. One example of this type of relationship exists between trees and epiphytes (bromeliads and some orchids). The trees are not affected by the epiphytes growing in them, and the epiphytes benefit by collecting water running down the bark and get better access to light than they would on the ground.

• Finally, parasitism is a relationship in which one species is harmed and the other benefits; for example, the relationship that exists between fleas and dogs. Now that you've reviewed how the biotic components of ecosystems change, survive, and thrive. let's look at what ecosystems can be found on the planet we call home.

The World’s Ecosystem

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.

Biomes

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.

Aquatic Life Zones

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.

Freshwater Biomes

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.

• In freshwater, the layers are the epilimnion, which is the uppermost and thus the most oxygenated, layer; and the hypolimnion, which is the lower, colder, and denser layer. The demarcation line between these two layers, at which the temperature shifts dramatically, is the thermocline.

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!

• Littoral zone: Begins with the very shallow water at the shoreline. Plants and animals that reside in the littoral zone receive abundant sunlight. These also include turtles, frogs, and other species that travel back and forth from water to land. The end of this zone is defined as the depth at which rooted plants stop growing.

• Limnetic zone: Surface of open water; the region that extends to the depth that sunlight can penetrate, Organisms that are residents in this zone tend to be short-lived and rely on sunlight: photosynthesizing phytoplankton use it directly, and they provide energy to zooplankton, insects, and fish.

• Profundal zone: The depths: water that is too deep for sunlight to penetrate. Because the profundal zone is aphotic (a place where light cannot reach), photosynthesizing plants and animals cannot live here; instead, organisms adapted to little light, colder temperatures, and less oxygen reside in this less populated zone.

• Benthic zone: The surface and sub-surface layers of the river-, lake-, pond-, or stream bed, characterized by very low temperatures and low oxygen levels and inhabited by organisms that live on, in, or below the sediment surface, including bottom-feeders, scavengers, and decomposers (including microorganisms such as bacteria and fungi).

• Another important type of freshwater body that you should know about is the estuary. An estuary is a site where the "arm" of the sea extends inland to meet the mouth of a river. Estuaries are often rich with many different types of plant and animal species because the freshwater in these areas usually has a high concentration of nutrients and sediments.

• The waters in estuaries are usually quite shallow, which means that the water is fairly warm and that plants and animals in these locations can receive significant amounts of sunlight. Subcategories of estuarine environments that you should know for the exam include saltwater marshes, mangrove forests, inlets, bays, and river mouths.

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.

• Mangroves are characterized by trees, shrubs, and other plants that can grow in brackish tidal waters and are often located in estuaries, which, as you learned earlier, are areas where freshwater meets salt water. In North America, mangrove swamps are found from the southern tip of Florida along the entire Gulf Coast to Texas; Florida’s southwest coast supports one of the largest mangrove swamps in the world.

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.

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.

The World’s Oceans

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.

• 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.

• These spits of land are generally the first hit by offshore storms, and they are important buffers for the shoreline behind them.

In tropical waters, a very particular type of barrier island called a coral reef is quite a common.

• These barrier islands are formed not from the deposition of sediments, but from a community of living things. The organisms responsible for the creation of coral reefs are cnidarians, which secrete a hard, calciferous shell; these shells provide homes and shelter for an incredible diversity of species, but they are also extremely delicate and thus very vulnerable to physical stresses as well as changes in light intensity, water temperature, ocean depth, and pH.

• The increase in Ocean temperatures and dissolved CO, due to climate change is resulting in more acidic waters resulting in coral bleaching. Coral bleaching occurs when acidic conditions cause the coral to expel the colorful algae which provided them with food.

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!

• Coastal zone: This zone consists of the ocean water closest to land. Usually, it is defined as being between the shore and the end of the continental shelf (the edge of the tectonic plate). Life thrives here due to abundant sunlight and oxygen and the proximity of the sediment surface, allowing for varied niches. In addition, coastal zones border and extend into estuaries, beaches, and marshes, which have their own varied populations of organisms adapted to their conditions.

-

• Euphotic zone: The photic, upper layers of water. The euphotic zone is the warmest region of ocean water; this zone also has the highest levels of dissolved oxygen. Much like the limnetic zone in freshwater biomes, it supports algae as well as fish. Algae in marine biomes supply a large portion of the Earth's oxygen, and also take in carbon dioxide from the atmosphere. Since most red light is absorbed in the top 1 meter of water, and blue light doesn't usually penetrate deeper than 100 meters, photo-synthesizers have adapted mechanisms to address the lack of visible light. How deep the euphotic zone extends depends on the turbidity of the water in a given area.

• Bathyal zone: The middle region; is colder and darker and does not receive enough light to support photosynthesis, so the density of organisms that live there is less. It’s difficult for many fish to live there because of the lack of nutrients, and those that do often lack eyes since there's so little sunlight. It is populated by organisms such as sponges and sea stars, as well as larger predators such as squid, octopus, sharks, and large whales.

• Abyssal zone: This is the deepest region of the ocean. This zone is marked by extremely cold temperatures and low levels of dissolved oxygen, but high levels of nutrients because of the decaying plant and animal matter that sinks down from the zones above. Without plants, the base level of the food chain in this aquatic zone is decomposers. Many of the creatures adapted to live here produce bioluminescence in order to attract prey or mates, and most are adapted to the cold, low oxygen, and intense pressure, using slower metabolisms and the ability to eat more when food is available to help them survive.

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.

• Water is densest at 3.98*C or 39°F. In non-tropical regions of the Earth, after ice melts in spring, the water-surface temperature of lakes and ponds will rise from 0°C to 4°C. whereupon this dense surface water will sink to the bottom of the lake or pond. This will displace water at the bottom of the lake or pond to the surface. This overturn brings oxygen to the bottom and nutrients to the top of the lake or pond and occurs during spring and fall as the temperature of the ecosystem changes from cold to warm or the reverse.

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.

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.

Cycles In Nature

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.

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.

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.

• The energy that drives these biogeochemical cycles in the biosphere comes primarily from sources: the sun and the heat energy from the mantle and core of the Earth. The movements of nutrients in all of these cycles may occur via abiotic mechanisms, such as wind through biotic mechanisms, such as through living organisms (as we mentioned earlier).

• Another important fact to note is that while the Law of Conservation of Matter states that matter can neither be created nor destroyed, nutrients can be rendered unavailable for cycling through certain processes-for example, in some cycles, nutrients may be transported to the deep ocean sediments where they are locked away interminably.

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.

• Trace elements can cycle in conjunction with the major nutrients, but there's still much to be discovered about these elements and their biogeochemical cycles. For this exam, know that there are certain trace elements required by living things that cycle, along with the major elements, through the biosphere.

Let's start with perhaps the best-known biogeochemical cycle: the water 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.

• This process is formally known as precipitation. When precipitation falls onto the Earth, it may infiltrate the surface and percolate through soil and rock until it reaches the water table to become groundwater, or it may travel across the land's surface as runoff and enter a drainage system, such as a stream or river, which will eventually deposit it into a body of water such as a lake or an ocean.

• Lakes and oceans are reservoirs for water. In certain cold regions of the Earth, water may also be trapped on the Earth’s surface as Snow or ice; in these areas, the blocks of snow or ice are reservoirs.

• Water is also cycled through living systems. For example, plants consume water (and carbon dioxide) in the process of photosynthesis, in which they produce carbohydrates and oxygen. Because all living organisms are primarily made up of water, they act as exchange pools for water.

• 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.

• 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.