ecosystems bio10 4/6

In many situations, resource partitioning is accompanied by character displacement, an evolutionary divergence in one or both of the species that leads to a partitioning of the niche. A clear example occurs among two species of seed-eating finches on the Galápagos Islands. On islands where both species live, their beak sizes differ significantly. One species has a deeper beak, better for large seeds, while the other has a shallower beak, better for smaller seeds, and they do not compete. On islands where either species occurs alone, beak size is intermediate between the two sizes. Figure 15-23 Allowing organisms to divide resources.

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Predators, too, make use of mimicry. The angler fish, the tasseled frogfish, and the snapping turtle all have physical structures that mimic something—usually a food item—that is of interest to potential prey. As the prey come closer to inspect, the predator snaps them up. Figure 15-26 A better predator.

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biomass

10% rule Where does the rest go? Expended in cellular respiration (energy loss as heat, movement), lost as feces, or uneaten body parts This means that only about 10% of the biomass—the total weight of all the living organisms in a given area—of plants in an ecosystem is converted into herbivore biomass. So the herbivore consuming five pounds of plant material is likely to gain only about half a pound in new growth, while the remaining 90% of the meal is either expended in cellular respiration or lost as feces. Similarly, a carnivore eating the herbivore converts only about 10% of the mass it consumes into its own body mass. Again, 90% is lost to metabolism and feces. And the same inefficiency holds for a top carnivore, as well. Let's explore how this 10% rule limits the length of food chains and is responsible for the rarity of big, fierce animals outside your window and across the world.

what are ecosystems?

A community of biological organisms (biotic components) plus the non-living (abiotic) components with which the organisms interact Ecosystems are made up of smaller divisions called habitats -San Francisco Bay ecosystem consists of habitats -- open water bay bottom intertidal habitats - mud flats, salt marshes many more

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An ecosystem is all of the living organisms in a habitat as well as the physical environment. Ecosystems are found not just in obvious places such as ponds, deserts, and tropical rainforests but also in some unexpected places, like the digestive tracts of organisms, on their surface, in their orifices

For example, where it is always moist and the temperature does not vary across the seasons, tropical rainforests develop. And where it is hot but with strong seasonality that brings a "wet" season and a "dry" season, savannas or tropical seasonal forests tend to develop. At the other end of the spectrum, in dry areas with a hot season and a cold season, temperate grasslands or deserts develop. Figure 15-3 (Terrestrial ecosystem diversity) shows examples of the nine chief terrestrial biomes; all are determined, in large part, by the precipitation and temperature levels.

Aquatic biomes are defined a bit differently, usually based on physical features such as salinity, water movement, and depth. Chief among these environments are 1) lakes and ponds, with non-flowing fresh water; 2) rivers and streams, with flowing fresh water; 3) estuaries and wetlands, where salt water and fresh water mix in a shallow region characterized by exceptionally high productivity; 4) open oceans, with deep salt water; and 5) coral reefs, highly diverse and productive regions in shallow oceans (Figure 5-4 Aquatic ecosystem diversity).

The Four Most Important Chemical Cycles

Carbon Nitrogen Phosphorus Water

Case 3: Parasites can induce bizarre and risky behavior in their hosts.

Case 3: Parasites can induce bizarre and risky behavior in their hosts. The lancet fluke is a parasitic flatworm. It has also been described as a "zombie-maker." This fluke spends most of its life in sheep and goats, but the fluke's eggs pass into snails that graze on vegetation contaminated by sheep and goat feces. Once inside the snail, the fluke eggs grow and develop, eventually forming cysts that the snail surrounds with mucus and then excretes. Continuing on their complex life cycle, the fluke cysts find their way into ants that eat the snail mucus. The flukes' journey back to a sheep or goat is now expedited by the so-called zombie-making. In an infected ant, the flukes grow into the ant's brain, altering its behavior. Whereas ants normally remain low to the ground, when infected by the lancet fluke, they climb to the tops of grass blades or plant stems and clench their mandibles on leaves. Is this an accident? No. This behavioral change puts the ants much more at risk of being eaten by a grazing mammal—a bad outcome for the ant, but just what the parasite needs to complete its life cycle.

commensalism

Commensalism: an interaction with a winner but no loser Some species interactions are one-sided. The cases in which one species benefits and the other neither benefits nor is harmed are called commensal relationships, or commensalism. Cattle egrets have just such a relationship with grazing mammals such as buffalo and elephants. As the large mammals graze through grasses, they stir up insects. The birds, which feed near the mammals—particularly near the forager's head—are able to catch more insects with less effort this way. The grazers are neither helped nor harmed by the presence of the birds. Figure 15-29 part 2 Not always "red in tooth and claw."

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Detritivores and decomposers extract energy from organic waste and the remains of organisms that have died. At each step in a food chain, some usable energy is lost as heat. Herbivores then consume the primary producers, the herbivores are consumed by carnivores, and the carnivores, in turn, may be consumed by top carnivores.

Two general types of parasites make life difficult for most organisms.

Ectoparasites ("ecto," meaning outside) include organisms such as lice, leeches, and ticks. One species of Mexican parrot is all too aware of ectoparasites: It has 30 different species of mites living on its feathers alone. And many of the parasites even have parasites of their own! Endoparasites are parasites that live inside their hosts ("endo," meaning inside). They are equally pervasive. Endoparasites infecting vertebrates include many different phyla of both animals and protists, the single-celled eukaryotes. In all of these parasite-host interactions, as with all predator-prey interactions, the predator or parasite benefits and the prey or host is harmed. Figure 15-27 Parasites: predators dwarfed by their prey. A velvet mite, an ectoparasite on human skin, is shown in extreme close-up. Trypanosome brucei is an endoparasite that invades a human host and is the cause of African sleeping sickness.

All the while, as energy is transformed through the steps of a food chain, organic wastes are produced. Whether in the form of animals' waste products or the dead bodies of plants and animals, organic material accumulates in every ecosystem. But this material does not go to waste. Another component of every food web harnesses the last bits of energy that remain. Decomposers, usually bacteria or fungi, and detritivores, including scavengers such as vultures, worms, and a variety of arthropods, break down the organic material, harvesting energy still stored in the chemical bonds (Figure 15-13). Because the decomposers are able to break down a much larger range of organic molecules, they are distinguished from the detritivores. Both groups, nonetheless, release many important chemical components from the organic material than can eventually be recycled and utilized by plants and other primary producers.

Energy flows from one stop to the next in a food chain, but not in the way that runners pass a baton in a relay race. The difference is that, at every step in the food chain, much of the usable energy is lost as heat. An animal that eats five pounds of plant material doesn't convert that into five new pounds of body weight. Not by a long shot. In the next section, we'll see how this inefficiency of energy transfers ensures that most food chains are very short.

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Energy from sun passes through an ecosystem in several steps known as trophic levels. Energy pyramids reveal that the biomass of primary producers in an ecosystem is far greater than the biomass of herbivores. Biomass transferred at each step of food chain is only 10% of the biomass of the organisms being consumed. As a consequence of this inefficiency, food chains rarely exceed four levels.

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Even though they are considered predators, parasites have some unique features and face some unusual challenges relative to other predators. The most obvious of these features is that, in parasitism, the parasite generally is much smaller than its host and stays in contact with the host for extended periods of time, normally not killing the host but weakening it as the parasite uses some of the host's resources. Being located right on your food source all the time can be advantageous. But this also leads to what is perhaps the greatest challenge that parasites face: how to get from one individual host to another. A parasite can't survive long once its host dies, after all. The methods by which parasites accomplish such dispersal are unexpected and surprising. Many of their complicated life cycles involve passing through two (or more) different species (and could have come about only through coevolution with each of the host species). These life cycles are likely to give us a new appreciation for the ingenuity of these microorganisms—or rather, for the evolutionary process that has produced them. Let's look at a few representative examples.

energy flows from producers to consumers

Everything, all life on that savanna and everywhere else on earth, is made possible because energy flows perpetually from the sun to the earth. This is where our pathway of energy flow begins. Most of the energy is absorbed or reflected by the earth's atmosphere or surface, but about 1% of it is intercepted and converted to chemical energy through photosynthesis. That intercepted energy is then transformed again and again by living organisms, making about four stops as it passes through an ecosystem. Let's examine what happens at each stop, known as trophic levels.

first stop primary producers

First stop: primary producers. When it comes to energy flow, all the species in an ecosystem can be placed in one of two groups: producers or consumers. Plants (along with some algae and bacteria) are, as we noted earlier, the primary producers. They convert light energy from the sun into chemical energy through photosynthesis. We use another word to describe that chemical energy: food.

carbon

For carbon, the most important reservoir is the atmosphere, where carbon is in the form of carbon dioxide (CO2). As we saw in Chapter 4, plants and some microorganisms utilize carbon dioxide in photosynthesis, separating the carbon molecules from CO2 and using them to build sugars. Carbon then moves through the food chain as organisms eat plants and are themselves eaten (Figure 15-16 Element cycling: carbon). A secondary reservoir of carbon is in the oceans. Here, many organisms utilize dissolved carbon to build shells (which later dissolve back into the water after the organism dies). Most carbon returns to its reservoir as a consequence of organisms' metabolic processes. Organisms extract energy from food by breaking carbon-carbon bonds, releasing the energy stored in the bonds, and combining the released carbon atoms with oxygen. They then exhale the end product as CO2.

fourth stop: tertiary consumers - the top carnivores

Fourth stop: tertiary consumers—the "top" carnivores. In some ecosystems, energy makes yet another stop: the tertiary consumers, or "top carnivores." These are the "animals that eat the animals that eat the animals that eat the plants." They are several steps removed from the initial capture of solar energy by a plant, but the general process is the same. A top carnivore, such as a tiger, eagle, or great white shark, consumes other carnivores, breaking down their tissue and releasing energy stored in the chemical bonds of the cells. As in each of the previous steps, the top carnivores harness this energy for their own physiological needs.

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Global patterns of weather are largely determined by the earth's round shape. Solar energy hits the equator at a more direct angle than at the poles, leading to warmer temperatures at lower latitudes.

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Hiding or escaping. Anti-predator adaptations need not involve toxic chemicals, physical structures, or special coloration. Many species excel at hiding and/or running. With vigilance, it is possible to get advance warning of impending predator attacks, and then quickly and effectively avoid predators. A variation of this strategy comes from safety in numbers: Many species, including schooling fish and emperor penguins, travel in large groups to reduce their predation risk. Alarm calling and fighting back. In many species, especially birds and mammals, individuals warn others with an alarm call. Although risky for the caller, such alarm calling can give other individuals—often close kin that are nearby—enough advance warning to escape. Some prey species also turn the tables, mobbing predators to keep them from successfully completing their task. This category might also include the fulmar, a seabird that defends its nest from attacks with projectile vomiting aimed at the intruder. Figure 15-25 Prey defenses: behavioral means for avoiding predation.

15-15. Many communities change over time.

Human "progress" and development can completely transform an environment—turning a patch of desert into Las Vegas, for example. Urban landscapes, too, can obliterate any signs of the nature that was once there. This is why it can be surprising (and heartening) to observe what happens when humans abandon an area. Little by little, nature reclaims it. The area doesn't necessarily recover completely, and change is slow. Still, this process is almost universal and virtually unstoppable. Nature responds similarly to other disturbances, too, from a single tree falling in a forest, to a massive flood or fire, to massive volcanic eruptions. The process of nature reclaiming an area and of communities gradually changing over time is called succession. It is defined specifically as a change in the species composition over time, following a disturbance. There are two types of succession. Primary succession is when the process starts with no life and no soil. Secondary succession is when an already established habitat is disturbed but some life and some soil remain.

Physical Defenses

Include Mechanical, Chemical, Warning Coloration, and Camouflage Mechanisms

Behavioral Defenses

Include both seemingly passive and active behaviors: hiding or escaping, or alarm calling or fighting back

15.14. Not all species interactions are negative: mutualism and commensalism.

It's easy to get the idea that all species interactions in nature are harsh and confrontational, marked by a clear winner and a clear loser. That is largely the case when it comes to competition and predation. Nonetheless, not all species interactions are combative. Every flower you see should be a reminder that evolution produces beneficial species interactions as well. These types of interactions fall into two categories: mutualism and commensalism.

Predator adaptations for enhancing predation

Just as prey use physical and behavioral features to reduce their risk of predation, predators evolve in parallel ways to increase their efficiency. In milkweed plants, as toxic chemicals have evolved to kill their predators, so in milkweed bugs, toxic-avoidance methods have evolved that allow the bugs to eat the toxic plants without suffering harm. And as prey have become better at hiding and escaping, predators have developed better sensory perception to help them detect hiding prey, and faster running ability to catch them.

mutualism

Mutualism: everybody wins Coral that gain energy from photosynthetic algae living inside their tissues. Termites capable of subsisting on wood, only with the assistance of cellulose-digesting microbes living in their digestive system. Flowers pollinated by animals nourished by nectar. Each of these relationships is an example of mutualism, an interaction in which both species gain and neither is harmed. Mutualism is common in virtually all communities. Plants, particularly, have numerous such interactions: with nutrient-absorbing fungi, with nitrogen-fixing bacteria, and with animals that pollinate them and disperse their fruits. Figure 15-29 part 1 Not always "red in tooth and claw."

nitrogen

Nitrogen is necessary to build all amino acids, the components of every protein molecule, as well as the precursors of other nitrogen-containing molecules—all critical to life. Like carbon, the chief reservoir of nitrogen is the atmosphere. But even though more than 78% of the atmosphere is nitrogen gas (N2), for most organisms, this nitrogen is completely unusable. The problem is that atmospheric nitrogen consists of two nitrogen atoms bonded tightly together, and these bonds need to be broken to make the nitrogen usable for living organisms. Only through the metabolic magic (chemistry, actually) of some soil-dwelling bacteria, the nitrogen-fixers, can most nitrogen enter the food chain. These bacteria chemically convert or "fix" nitrogen by attaching it to other atoms, including hydrogen, producing ammonia and related compounds. These compounds are then further modified by other bacteria into a form that can be taken up by plants and used to build proteins. And once nitrogen is in plant tissues, animals acquire it in the same way they acquire carbon: by eating the plants. Nitrogen returns to the atmosphere when animal wastes and dead animals are broken down by soil bacteria that convert the nitrogen compounds in tissue back to nitrogen gas. Figure 15-17 Element cycling: nitrogen.

Some species are more important than others within a community.

Not all species are created equal. In this chapter, we have examined the ways in which species interact with one another and with their habitats, noting the dependence of species on one another. But not all species have equal dependence on and influence over others. Within a community, the presence of some species, called keystone species, greatly influences which other species are present and which are not. That is, if a keystone species is removed, the species mix in the community changes dramatically. The removal of other species, conversely, causes relatively little change. Bison are a keystone species. Sea stars, too, are a keystone species. Figure 15-31 Preserving biodiversity. Keystone species can keep aggressive species in check, allowing more species to coexist in a community.

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Not all species interactions are combative. Evolution produces beneficial species interactions as well, including mutualism, in which both species benefit from the interaction, and commensalism, in which one species benefits and the other is neither harmed nor helped.

Parasite Predators

Parasites have some unique features and face some unusual challenges: The parasite generally is much smaller than its host and stays in contact with the host for extended periods of time. Complicated life cycles as means of getting from host to host.

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Parasitism is a symbiotic relationship in which one organism benefits while the other is harmed. Parasites face some unusual challenges relative to other predators, particularly how to get from one individual host to another, and some complex parasite life cycles have evolved.

15.12 Predation produces adaptation in both predators and their prey.

Prey Adaptations for Reducing Predation -There are two broad categories of defenses against predators: Physical Behavioral

primary succession

Primary succession Primary succession can take thousands or even tens of thousands of years, but it generally occurs in a consistent sequence. It always begins with a disturbance that leaves an area barren of soil and with no life. Frequently, the disturbance is catastrophic. The huge volcanic eruption on Krakatoa, Indonesia, in 1883, for example, completely destroyed several islands and wiped out all life and soil on others. Primary succession has also begun, in a less dramatic fashion, in regions where glaciers have retreated, such as Glacier Bay, Alaska. Although succession does not occur in a single, definitive order, several steps are relatively common. An important feature of the colonizers seen in the earliest stages of primary succession is that, while they are all good dispersers, able to move away from their original home (hence their early arrival to a newly available locale), they are not particularly good competitors. That's why they are gradually replaced. Ultimately, it is the longer-living, larger species that tend to outcompete the initial colonizers and persist as a stable and self-sustaining community, called a climax community. The specific species present in the climax community depend on physical factors such as temperature and rainfall. Figure 15-30 Species composition of a community changes over time.

second stop primary consumers -the herbivores

Second stop: primary consumers—the herbivores. Cattle grazing in a field, gazelle browsing on herbs, insects devouring the leaves of a crop plant—these are the primary consumers in an ecosystem, the animals that eat plants. Plant material such as cellulose can be difficult to digest. Consequently, most herbivores—animals that eat plants—need a little help in digesting the plants they eat. Primary consumers, from termites to cattle, often have symbiotic bacteria living in their digestive system. These microorganisms break down the cellulose, enabling the herbivore to harness the energy held in the chemical bonds of the plants' cells.

15.11 Competition can be hard to see, but it still influences community structure.

Some species, particularly closely related species, have similar niches. This can lead to conflict as they try to exploit the same resources. Almost invariably, when the fundamental niches of two species overlap, competition occurs. This competition doesn't last forever, though. Inevitably, one of two outcomes occurs: competitive exclusion or resource partitioning. 1) In competitive exclusion, two species battle for resources in the same niche until the more efficient of the two wins and the other species is driven to extinction in that location ("local extinction"). In the 1930s, this was demonstrated in simple laboratory experiments using Paramecium, a single-celled organism. Populations of two similar Paramecium species were grown either separately or together in test tubes containing water and their bacterial food source. When grown separately, each species thrived. When grown together, though, one species always drove the other to extinction. 2) Resource partitioning is an alternative outcome of niche overlap. Individual organisms and species can adapt to changing environmental conditions, and resource partitioning can result from an organism's behavioral change or a change in its structure. When this occurs, one or both species become restricted in some aspect of their niche, dividing the resource. In other experiments with Paramecium, for example, one of the two species was replaced with a different species. As in the initial experiment, either species thrived when grown alone. But when the two species were grown together in the same test tube, they ended up dividing the test tube "habitat." One species fed exclusively at the bottom of the test tube, and the other fed only at the top. Simple behavioral change made coexistence possible. Figure 15-22 Overlapping niches: competitive exclusion and resource partitioning.

Biomes - a variety environments occuring around the world, each determined by physical factors.

Specifically, when defining terrestrial biomes, we ask four questions about the weather: What is the average temperature? What is the average rainfall (or other precipitation)? Is the temperature relatively constant, or does it vary seasonally? Is the rainfall relatively constant, or does it vary seasonally?

In humans, why is vegetarianism more energetically efficient than meat-eating?

The 10% Rule in Application Given the 10% efficiency with which herbivores convert plant biomass into their own biomass, how much plant biomass is necessary to produce a single 1200-pound (500 kg) cow? On average, that cow would need to eat about 12,000 pounds (5000 kg) of grain in order to grow to weigh 1200 pounds. But that 1200-pound cow, when eaten by a carnivore, could only add about 120 pounds of biomass to the carnivore, and only 12 pounds to a top carnivore. That's a huge amount of plant biomass required to generate only a very tiny amount of our top carnivore, which explains why big, fierce animals are so rare (and why vegetarianism is more energetically efficient than meat-eating). After all, multiply that 5000 kg of grain by several hundred (or thousands, more appropriately) to get an idea of how much plant matter would be necessary to support even a small population of top carnivores: Millions of kilograms of grain can support only a few top carnivores.

What is important is that the two essential elements of an ecosystem are present: the biotic environment and the physical (abiotic) environment (Figure 15-2 What makes up an ecosystem?).

The biotic environment consists of all the living organisms within an area and is often referred to as a community. The physical environment, often referred to as the organisms' habitat, consists of: the chemical resources of the soil, water, and air, such as carbon, nitrogen, and phosphorus, and the physical conditions, such as the temperature Biologists view communities of organisms and their habitats as "systems" in much the same way that engineers might, hence the term ecosystem. Biologists monitor the inputs and outputs of the system, tracing the flow of energy and various molecules as they are captured, transformed, and utilized by organisms and later exit the system or are recycled. They also study how the activities of one species affect the other species in the community—whether the species have a conflicting relationship, such as predator and prey, or a complementary relationship, such as flowering plants and their pollinators. Regardless of size, the same principles of ecosystem study apply: observe and analyze organisms and their environments, while monitoring everything that goes into and comes out of the system: salinity (salt level), moisture, humidity, and energy sources.

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These parasite-induced behavior modifications are among the most dramatic, but there are many others that are more subtle but equally effective at allowing parasites to thrive attached to or within the body of their hosts. This subset of predation is an active and exciting area of ecological and physiological investigation.

third stop secondary consumers the carnivores

Third stop: secondary consumers—the carnivores. Energy originating from the sun is converted into chemical energy within a plant's tissue. The herbivore that eats the plant breaks down the chemical bonds, releasing the energy. This energy fuels the herbivore's growth, reproduction, and movement, but the energy doesn't remain in the herbivore forever. Carnivores, such as cats, spiders, and frogs, are animals that feed on herbivores. They are also known as secondary consumers. As they eat their prey, some of the energy stored in the chemical bonds of the carbohydrate, protein, and lipid molecules is again captured and harnessed for their own movement, reproduction, and growth.

Symbiosis

Two organisms living together with one dependent on the other Parasitism Commensalism Mutualism

energy pyramid inefficient energy flow

We can illustrate the path of energy through the organisms of an ecosystem with an energy pyramid in which each layer of the pyramid represents the biomass of a trophic level. In Figure 15-14 (Inefficiencies in the transfer of biomass), we can see that, for terrestrial ecosystems, the biomass (in kilograms per square meter) found in the photosynthetic organisms, at the base of the pyramid, is reduced significantly at each step, given the incomplete utilization by organisms higher up the food chain.

Section 15-2 Opener Monsoon clouds forming over the Indian Ocean and Madagascar. If the terrestrial biomes of the world are determined by the temperature and rainfall amounts and seasonality, what determines those features? In other words, what makes the weather? We investigate next how the geography and landscape of the planet—from the shape of the earth and its orientation to the sun to patterns of ocean circulation—cause the specific patterns of weather that create the different climate zones and the biomes characteristic of each. Then, we'll see how energy and chemicals are made available for life to flourish in these biomes.

Wherever you are, begin walking toward the equator. As you get closer, does it get hotter or colder? Hotter, of course. What is responsible for the increased warmth at the equator? This is most easily explained with a drawing. The sun shines most directly on the equator. That means that a given amount of solar energy hitting the earth at the equator is spread out over a relatively small area. Away from the equator, the earth curves toward the north and south poles. Because of this curvature, the same amount of solar energy hitting the earth at the poles is spread out over a much larger area and also travels a greater distance through the atmosphere, which absorbs or reflects much of the heat. Because the energy is spread out over a larger area, there is less warmth at any one point on the earth's surface. It's similar to the fact that, at noon, the sun's rays hit the earth at a more direct angle than they do later in the day. That is why the sun provides less warmth late or early in the day. It is also why the risk of sunburn and skin cancer is greatest around noon and the nearer you are to the equator. Figure 15-5 The curvature of the earth. The sun shines more directly on the equator. Closer to the poles, the curvature of the earth causes sunlight to be spread out over a larger area, reducing the heart in any one spot.

Why are big, fierce animals so rare? And why are there so many more plants than animals?

the answers are closely related to our earlier observation that an animal consuming five pounds of plant material does not gain five pounds in body weight from such a meal. The actual amount of growth such a meal can support is far, far less. And when that herbivore is consumed by a carnivore, the carnivore, too, can convert only a small fraction of the energy it consumes into its own tissue. The fraction turns out to be about 10%, and it is fairly consistent across all levels of the food chain.