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Chapter 17 Communities and Ecosystems
Interactions Among Organisms An exotic species is a species that evolved in one region but has intentionally or inadvertently been introduced to and become established in another. An example would red fire ants that are originally from South America but have spread across the Southeast and Western United States and have even been found as far North and Kansas and Delaware. In their new area, these exotic species are often unaffected by competitors, predators, parasites and diseases that would have their population numbers in check in their native home. As a result, these exotic species often outcompete and displace native species in their new environment. These fire ants have decreased the numbers of the Texas horned horned lizard, who feeds on native ants but can’t eat the fire ants. They have also threatened songbirds, since they feed on nestlings.
Interactions Among Organisms However, scientists have introduced one of the ants’ enemies from South America to try to control their populations in this country. Phorid flies lay their eggs in the body of fire ants and their larvae develop inside, killing the ant. A species takes part in a web of relationships with other species, and interacts with its nonliving environment. All of the species that live and interact in a specific area are called a community. All of the communities in a specific area together with their physical environment makes up an ecosystem. We will consider in this chapter the interactions among species, as well as between species and with their nonliving environment.
Factors That Shape Communities • All communities differ in their species diversity. • Species diversity is important in the health of a community and an ecosystem. • Two components of species diversity: • Species richness: number of species present • Species evenness: relative abundance of each species • For example, a pond with five species of fish that has about equal numbers of each species has more species diversity than a pond with five species of fish where one species is abundant and the other four are rare.
Factors That Shape Communities There are factors that affect the structure of a community, changing the array of species and their relative abundances over time. Communities may change gradually over a long period of time as they form and age. Communities may also change suddenly as a result of natural disturbances or human activity.
Factors That Shape Communities • Non-biological Factors: Geographic and climate variables, including sunlight, nutrient availability, rainfall, temperature, latitude, elevation or depth. • Tropical regions get more sunlight and more even temperatures, resulting in them having the greatest number of species. The same is true for tropical reef communities in the ocean. • Biological Factors: evolutionary history and adaptations of species in a community • Each species is adapted to a different habitat ( or type of place where it occurs) • All species of a community share the same habitat, but each also has its own ecological role (or niche).
Factors That Shape Communities • Species in a community interact in ways that affect community structure. • An increase in the numbers of one species often will affect the abundance of other species, either directly or indirectly. • There are five types of direct interactions among species in a community: • Commensalism • Mutualism • Parasitism • Competition • Predation
Species Interactions in Communities • Direct Interactions: • Symbiotic relationships: Symbiotic species spend most or all of their lives in close association with one another. • Commensalism: one species benefits and the other is unaffected Example: barnacle on a whale • Mutualism: both species benefit Example: flower and its pollinator This is a mutual exploitation by the two species. Mitochondria and chloroplasts probably began as bacteria in a mutualistic relationship with another bacterium. The two, through, evolution, became dependent upon one another for survival and today we have chloroplasts and mitochondria. This is called the Endosymbiotic Theory. • Parasitism: one species (the parasite) benefit and the other species (the host) is harmed. Example: tick on an animal • Parasitoids are free-living insects that lay their eggs in other insects. The eggs then hatch and the larvae devour and kill the host. Because parasatoids are very specific they are often used as biological controls, such as the example of the phorid flies.
Species Interactions in Communities • Direct interactions (cont’d.) • Competition: Individuals of the same species compete for the same resources. • This competition is an important driving force for natural selection. • Interspecific competition refers to the competition between members of different species for the same resources. This kind of competition is generally not as fierce since members of different species are never as similar as members of the same species. • When species compete for a resource, each gets less of that resource than it would if it were alone. In this way, competition negatively affects all competitors. More similar competitors compete more intensely. If two species require the same resource, the better competitor will drive the worse competitor to extinction if they live in the same habitat. This outcome is calledcompetitive exclusion. • However, competitors can live together in the same habitat if the resources they require aren’t exactly the same. The more different individuals are from their competitors, the better they will do. Over time, this leads to evolution of new species to minimize the differences between the competing species. This results inresource partitioning, in which resources are subdivided to reduce competition among the species that use it.
Species Interactions in Communities • Direct Interaction (contd.) • Predation: one free-living species (predator) feeds on and kills another (prey) • Predator and prey exert selective pressures on one another. Predators exert selection pressure that favors better defenses by prey, which in turn exerts selection pressure on predators, and so on.
Species Interactions in Communities • Prey adaptations include to help them avoid being detected or to defend them from predators) • Defensive toxins • Hard or sharp parts • Warning coloration (a conspicuous pattern or color that predators learn to avoid) • Mimicry (when a nontoxic species evolves to resemble the appearance of a more toxic, dangerous species) • Eye spots/hissing sounds/foul smelling chemicals • Camouflage (a form, patterning, color, and/or behavior that allows an organism to blend into its surrounding and avoid detection by predators)
Species Interactions in Communities • Predator adaptations: (to help them detect, catch, and kill prey) • Sharp teeth and claws • Camouflage • Sharp eyes • Speed
Homework • Give your own example of: • Mutualism • Commensalism • Parasitism • Competition • Predation • Give your own example of an adaptation evolved by a prey species and the adaptation that its predator has evolved to be able to overcome this prey adaptation.
How Communities Change • Ecological Succession: the process by which the variety of species gradually changes over time as organisms change their own habitat. One species is replaces by another and so on. • Primary succession: succession in habitats that have no soil and few or no existing species 1. No multicellular organisms present (maybe some prokaryotes/algae) 2. Pioneer species (species that colonize new or empty habitats) take hold and begin changing the community. Pioneer species include lichens, mosses, annual plants. 3. Generations of pioneers species live and die, building and improving the soil. Seeds of shrubby species take root. 4. Organic wastes and remains build up, adding volume and nutrients to the soil, allowing tall trees to take hold.
How Communities Change • Ecological Succession (contd.) • Secondary succession: occurs when a natural or human disturbance wipes out the natural array of species but not the soil. For example, what happens in a forest after a fire.
How Communities Change • Random Factors and Disturbances: • Windstorms freezes, droughts, and fires alter community structures. • The frequency and severity of disturbances influences the number of species in a community. • Species diversity is highest in areas where moderate disturbances occur occasionally. • The occasional disturbances provide enough time and opportunities for new species to enter the community, but not enough time for many species to become competitively excluded.
How Communities Change Keystone Species: a species that has a disproportionately large effect on its community relative to its actual abundance in the community. Absence of this keystone species greatly reduces the species diversity in a community. Examples are sea stars/mussels and beavers
How Communities Change • Exotic Invaders • A species from an established community that has dispersed from its home range and become a permanent part of a new community. • In this new community, there are no co-evolved parasites, pathogens, or predators to keep its numbers in check so it may overrun its new community, wiping out or drastically reducing the numbers of native species in that community. • Examples: red fire ants, kudzu vine, nutria, and gypsy moth caterpillars in the U.S.
The Nature of Ecosystems • All ecosystems have a one-way flow of energy and cycling of nutrients as the organisms of a community interact with their environment. • Participants • Producers: capture energy and use it to make food (Usually photosynthetic plants , prokaryotes, and protists) • Consumers: get energy and carbon by feeding on tissues, wastes, and remains of producers and one another (Herbivores, predators, parasites) • Detritivores: eat tiny bits of organic matter or detritus (ie. crabs, earthworms) • Decomposers: break down wastes and remains of organisms into inorganic building blocks (bacteria, protists, fungi)
The Nature of Ecosystems • Food Chains and Webs • Feeding relationships within an ecosystem that all organisms in that ecosystem participate in are organized into a hierarchy called trophic levels. • When an predator eats its prey, energy and nutrients are transferred from the prey to the predator. • All organisms at the same trophic level are the same number of energy transfers away from the energy source in that system (usually the sun). • Afood chain is the sequence of steps by which energy captured by primary producers is transferred to higher trophic levels. • A number of food chains that are cross-connected with one another is called a food web. • Nearly all food webs include two types of food chains: a grazing food chain and adetrital food chain.
Homework In the food web on the previous slide, which organism would be an example of a: Producer? Consumer? Detritivore? Decomposer?
The Nature of Ecosystems When one animal eats another, not only does it obtain energy and nutrients, but it may also receive pesticides and poisons in the prey’s body. This may result in the build-up of the poison as it travels up the food chain from one consumer to another. This is known an bioaccumulationor biomagnification. Essentially, the poison becomes stored in the fat of the consumers’ bodies and it becomes increasingly concentrated as it moves up the food chain. Example: mercury
The Nature of Ecosystems Primary production ( the capture and storage of energy by producers) begins the flow of energy through every ecosystem. Even though primary production on land is generally higher than in the ocean, because more than 70% of the Earth’s surface is covered by ocean, primary production from the ocean makes up about half of Earth’s total primary production.
The Nature of Ecosystems Anenergy pyramid shows the amount of energy from primary production that reaches higher trophic levels. The base of the pyramid is always largest, representing primary production, and then the levels get smaller as you go higher, since only about 10% of the energy from one trophic level passes to the next. The other 90% of the energy at each level is used by the organisms at that level for life processes, is lost as heat, or is tied up in molecules that cannot be used by the trophic level above it (ex. Cellulose). This explains why most food chains do not have more than four or five levels.
Homework Consider an energy pyramid. Why would being a vegetarian be more energy efficient than being a meat eater?
Nutrient Cycling in Ecosystems • Abiogeochemical cycle involves ions of molecules or essential nutrients flowing among the environment and into and out of the world of life as producers take up these nutrients from the air, soil, and water and then consumers take in water or these producers. • There are several biogeochemical cycles in an ecosystem, including: • Water cycle • Phosphorus cycle • Nitrogen cycle • Carbon cycle
The Water Cycle Water moves from oceans to atmosphere to land/freshwater ecosystems back to ocean. Solar energy drives evaporation. Precipitation drops water from atmosphere to land. Runoff carries carries water to streams, rivers, etc. Water is also taken up on land by plants and animals. 97% of water on Earth is saltwater. Of the 3% freshwater, surface water (streams, rivers, lakes, freshwater marshes) make up less than 1%.
The Water Cycle Oceans: main water reservoir on Earth
The Phosphorus Cycle Weathering and erosion carry phosphate from rocks and sediments into soil, lakes, and rivers. Runoff carries phosphate to the ocean. Phosphorus settles at the bottom along the edges of continents. Movements of Earth’s crustal plates uplift this newly formed rock to land. Phosphorus enters living things when roots of land plants take up phosphorus from water in the soil. Animals eat plants or one another to get phosphorus. Animal remains and wastes return the phosphate to the soil so that it can runoff into the oceans eventually again.
The Phosphorus Cycle Phosphates can be used as fertilizer. Lack of phosphorus limits the growth of aquatic plants. However,too much phosphorus can pollute aquatic environments, causing algal blooms that can wipeout other species that exist in a body of water.
The Phosphorus Cycle Rocks and sediments: Main reservoir for phosphorus on Earth
The Nitrogen Cycle 80% of Earth’s atmosphere is made up of nitrogen. Plants cannot use this atmospheric nitrogen. Bacteria living in the roots of plants called legumes (clover, soybeans, etc.) can do nitrogen fixation , converting atmospheric nitrogen to ammonia , which then dissolves in the soil forming ammonium that can be taken up by plants. Consumers then eat plants to get nitrogen. Bacteria and fungi in the soil can also break down wastes and form ammonium. Bacteria in the soil called nitrifying bacteria can obtain energy by converting ammonium into nitrates (called nitrification). These nitrates can also be taken from the soil and used by plants. Other bacteria called denitrifying bacteria get energy by converting nitrates into nitrogen gas, which then returns to the atmosphere.
The Nitrogen Cycle Humans alter the nitrogen cycle when they use synthetic fertilizers containing large amounts of nitrogen. Scientists estimate that use of synthetic fertilizers has double the amount of nitrogen that enters land ecosystems.
The Nitrogen Cycle Air: Main reservoir for nitrogen on Earth
The Carbon Cycle After water, carbon is the most abundant substance in living things. Plants take up carbon dioxide from the atmosphere and convert it to carbohydrates to be used by all living things for energy. As a result of respiration, carbon dioxide is then retuned to the atmosphere. Dissolved carbon in water forms bicarbonate ions that can be converted to carbon dioxide by aquatic plants for photosynthesis. A large reservoir of carbon exists in rocks at the bottom of the ocean, but this is not readily accessible to plants and animals.
The Greenhouse Effect and Global Climate Change By burning fossil fuels and wood, humans put carbon dioxide and nitrogen oxides into the atmosphere. We are also cutting down forests, reducing the use of carbon dioxide by producers. The result is a change in the composition of Earth’s atmosphere. A build-up of greenhouse gases is slowing the loss of heat from Earth into space. The process of the warming of the Earth’s atmosphere due to emission of heat on Earth is called the greenhouse effect.
The greenhouse effect is what keeps Earth from being cold and lifeless. Scientists estimate that the level of carbon dioxide in the Earth’s atmosphere is at its highest level in 650,000 years. In the past 100 years, the Earth’s average temperature has risen about 0.74°C and the rate of warming is accelerating. But too much warming of the Earth’s surface (even by an increase of as little as a degree or two) is enough to increase the rate of glacial melting, raise sea level, alter wind patterns, shift distribution of rain and snowfall, and increase the frequency and severity of hurricanes, affecting all life on Earth. The many climate-related effects due to the rise in greenhouse gases is called global climate change.
Homework • Some scientists say that Global Warming is just a figment of other scientists’ overactive imagination. • Do a little of your own research into the evidence for/against global warming and give your own opinion about whether it is really happening. Be sure to support your opinion with at least a couple of FACTS that you found during your research. • (Remember critical thinking and bias!!)