CHAPTER 53 COMMUNITY ECOLOGY Section A: What Is

CHAPTER 53 COMMUNITY ECOLOGY Section A: What Is

CHAPTER 53 COMMUNITY ECOLOGY Section A: What Is a Community? 1.Contrasting views of communities are rooted in the individualistic and interactive hypotheses 2.The debate continues with the rivet and redundancy models

Introduction What is a Community? A community is defined as an assemblage of species living close enough together for potential interaction. Communities differ in their species richness, the

number of species they contain, and the relative abundance of different species. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Imagine two small forest communities

with 100 individuals distributed among four different tree species. Species richness may be equal, but relative abundance may be different.

Species Richness Species richness is a measure of the number of species found in a sample. Since the larger the sample, the more species we would expect to find, the number of species is divided by the square root of the number of individuals in the sample. This particular measure of species richness is known as D, the Menhinick's

index. D=s N where s equals the number of different species represented in your sample, and N equals the total number of individual organisms in your sample. Species Diversity

Species diversity differs from species richness in that it takes into account both the numbers of species present and the dominance or evenness of species in relation to one another. As a measure of species diversity the Shannon index is often used. Interestingly Shannon, a physicist, developed the index as a formula for measuring the entropy of matter in the universe. It turns out that the mathematical relationships hold true whether one is dealing with

molecules in solution or species in an ecological community. H = (pl) |ln pl|pl) |ln pl|ln pl|ln pl| Where (pl) |ln pl|pl) is the proportion of the total number of individuals in the population that are in species l. Benthic Index of Biotic Integrity (BIBI, B-IBI) The BIBI scoring system is a quantitative method for determining and comparing the biological condition of streams. We currently use the Puget Sound

Lowlands BIBI, which can be calculated three different ways based on the taxonomic resolution of macroinvertebrate data: Species-Family , Species-Genus, and Family. The debate continues with the rivet and redundancy models The rivet model of communities is a reincarnation of the interactive model.

The redundancy model states that most species in a community are not closely associated with one another. No matter which model is correct, it is important to study species relationships in communities. An interactive hypothesis depicts a community as an assemblage of closely linked species locked in by mandatory biotic interactions.

Fig. 53.1b Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings An individualistic hypothesis depicts a community as a chance assemblage of species found in the same area because they happen to have similar abiotic requirements.

Fig. 53.1a Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Species richness generally declines along an equatorial-polar gradient Tropical habitats support much larger numbers of species of organisms than do temperate and polar regions.

Fig. 53.23 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings What causes these gradients? The two key factors are probably evolutionary history and climate. Organisms have a history in an area where they are adapted to the climate. Energy and water may factor into this

phenomenon. Species richness is related to a communitys geographic size The species-area curve quantifies what may seem obvious: the larger the geographic area, the greater the number of species.

Fig. 23.25 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings There are different interspecific interactions, relationships between the species of a community. Possible interspecific interactions are introduced in

Table 53.1, and are symbolized by the positive or negative affect of the interaction on the individual populations. Mutualism is where two species benefit from their interaction. Commensalism is where one species

benefits from the interaction, but other is not affected. An example would be barnacles that attach to a whale. Fig. 53.9

Coevolution and interspecific interactions. Coevolution refers to reciprocal evolutionary adaptations of two interacting species. When one species evolves, it exerts selective pressure on the other to evolve to continue the interaction.

What are some examples of coevolution? Mechanical defenses include spines. Chemical defenses include odors and toxins Aposematic coloration is indicated by warning colors, and is sometimes associated with other defenses (pl) |ln pl|toxins).

Fig. 53.6 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Mimicry is when organisms resemble other species. Batesian mimicry is where a harmless species mimics a harmful one. Fig. 53.7

Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Mllerian mimicry is where two or more unpalatable species resemble each other. Fig. 53.8 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Interspecific competition for resources can occur when resources are in short supply. There is potential for competition between any two species that need the same limited resource. The ecological niche is the sum total of an organisms use of abiotic/biotic resources in the environment.

Put another way, an organisms niche is its role in the environment. The competitive exclusion principle can be restated to say that two species cannot coexist in a community if their niches are identical. Classic experiments confirm this. Fig. 53.2

Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Resource partitioning is the differentiation of niches that enables two similar species to coexist in a community. Fig. 53.3 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 53.2 Dominant species and keystone species exert strong controls on community structure Dominant species are those in a community that have the highest abundance or highest biomass (pl) |ln pl|the sum weight of all individuals in a population). If we remove a dominant species from a community, it can change the entire community

structure. A Keystone species is a species that is not necessary the most abundant in a community, yet exerts strong control on community structure by

the nature of its ecological role or niche. Fig. 53.14 If they are removed, community structure is greatly affected. Fig. 53.15

Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings 2. Trophic structure is a key factor in community dynamics The trophic structure of a community is determined by the feeding relationships between organisms. The transfer of food energy from its source in photosynthetic organisms through herbivores

and carnivores is called the food chain. Charles Elton first pointed out that the length of a food chain is usually four or five links, called trophic levels. He also recognized that

food chains are not isolated units but are hooked together into food webs. Food webs. Who eats whom in a community? Trophic relationships

can be diagrammed in a community. What transforms food chains into food webs? A given species may weave into the web at more than one trophic level.

Fig. 53.11 CHAPTER 53 COMMUNITY ECOLOGY Section C1: Disturbance and Community Structure 1. Most communities are in a state of nonequilibrium owing to disturbances 2. Humans are the most widespread agents of disturbance

3. Ecological succession is the sequence of community changes after a disturbance Introduction Disturbances affect community structure and stability. Stability is the ability of a community to persist in the face of disturbance.

Most communities are in a state of nonequilibrium owing to disturbances Disturbances are events like fire, weather, or human activities that can alter communities. Some are routine. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 53.16

We usually think that disturbances have a negative impact on communities, but in many cases they are necessary for community development and survival. Fig. 53.18 Marine communities are subject to

disturbance by tropical storms. Fig. 53.17 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings 2. Humans are the most widespread agents of disturbance Human activities cause more disturbance than natural events and usually reduce species

diversity in communities. 3. Ecological succession is the sequence of community changes after a disturbance Ecological succession is the transition in species composition over ecological time. Primary succession begins in a lifeless area

where soil has not yet formed. Mosses and lichens colonize first and cause the development of soil. An example would be after a glacier has retreated. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 53.19 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Secondary succession occurs where an existing community has been cleared by some event, but the soil is left intact. Example: Grasses grow first, then trees and other organisms.

Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Soil concentrations of nutrients show changes over time. Why would this be important? Fig. 53.20 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

4. Species richness on islands depends on island size and distance from the mainland Because of their size and isolation, islands provide great opportunities for studying some of the biogeographic factors that affect the species diversity of communities. Fig. 53.27

CHAPTER 54 ECOSYSTEMS Section A: The Ecosystem Approach to Ecology 1. Trophic relationships determine the routes of energy flows and chemical cycling in an ecosystem 2. Decomposition connects all trophic levels 3. The laws of physics and chemistry apply to ecosystems

Introduction An ecosystem consists of all the organisms living in a community as well as all the abiotic factors with which they interact. The dynamics of an ecosystem involve two processes: energy flow and chemical cycling. Ecosystem ecologists view ecosystems as energy machines and matter processors. We can follow the transformation of energy by

grouping the species in a community into trophic levels of feeding relationships. 1. Trophic relationships determine the routes of energy flow and chemical cycling in an ecosystem The autotrophs are the primary producers, and are usually photosynthetic (pl) |ln pl|plants or algae). They use light energy to synthesize sugars and other

organic compounds. Heterotrophs are at trophic levels above the primary producers and depend on their photosynthetic

output. Fig. 54.1 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Herbivores that eat primary producers are called primary consumers. Carnivores that eat herbivores are called secondary consumers.

Carnivores that eat secondary producers are called tertiary consumers. Another important group of heterotrophs is the detritivores, or decomposers. They get energy from detritus, nonliving organic material and play an important role in material cycling. 2. Decomposition connects all trophic levels

The organisms that feed as detritivores often form a major link between the primary producers and the consumers in an ecosystem. The organic material that makes up the living organisms in an ecosystem gets recycled. An ecosystems main decomposers are fungi and prokaryotes, which secrete enzymes that

digest organic material and then absorb the breakdown products. Fig. 54.2 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings 3. The laws of physics and chemistry apply to ecosystems The law of conservation of energy applies to

ecosystems. We can potentially trace all the energy from its solar input to its release as heat by organisms. The second law of thermodynamics allows us to measure the efficiency of the energy conversions. CHAPTER 54 ECOSYSTEMS

Section B: Primary Production in Ecosystems 1. An ecosystems energy budget depends on primary production 2. In aquatic ecosystems, light and nutrients limit primary production 3. In terrestrial ecosystems, temperature, moisture, and nutrients limit primary production Introduction The amount of light energy converted to

chemical energy by an ecosystems autotrophs in a given time period is called primary production. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings 1. An ecosystems energy budget depends on primary production Most primary producers use light energy to synthesize organic molecules, which can be

broken down to produce ATP; there is an energy budget in an ecosystem. The Global Energy Budget Every day, Earth is bombarded by large amounts of solar radiation. Much of this radiation lands on the water and land that either reflect or absorb it. Of the visible light that reaches photosynthetic organisms, about

only 1% is converted to chemical energy. Although this is a small amount, primary producers are capable of producing about 170 billion tons of organic material per year. Gross and Net Primary Production. Total primary production is known as gross primary production (GPP). This is the amount of light energy that is converted into chemical energy.

The net primary production (NPP) is equal to gross primary production minus the energy used by the primary producers for respiration (pl) |ln pl|R): NPP = GPP R Primary production can be expressed in terms of energy per unit area per unit time, or as biomass of vegetation added to the ecosystem per unit area per unit time.

This should not be confused with the total biomass of photosynthetic autotrophs present in a given time, called the standing crop. Different ecosystems differ greatly in their production as well as in their contribution to the total production of the Earth. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 54.3 2. In aquatic ecosystems, light and nutrients limit primary production Production in Marine ecosystems. Light is the first variable to control primary production

in oceans, since solar radiation can only penetrate to a certain depth (pl) |ln pl|photic zone). Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings We would expect production to increase along a gradient from the poles to the

equator; but that is not the case. There are parts of the ocean and in the tropics and subtropics that exhibit low primary production. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Why are tropical and subtropical oceans less productive than we would expect?

It depends on nutrient availability. Ecologists use the term limiting nutrient to define the nutrient that must be added for production to increase. In the open ocean, nitrogen and phosphorous levels are very low in the photic zone, but high in deeper water where light does not penetrate.

Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Nitrogen is the one nutrient that limits phytoplankton growth in many parts of the ocean. Fig. 54.6 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Nutrient enrichment experiments showed that iron availability

limited primary production. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Evidence indicates that the iron factor is related to the nitrogen factor. Iron + cyanobacteria + nitrogen fixation phytoplankton

production. Marine ecologists are just beginning to understand the interplay of factors that affect primary production. Fig. 54.7

Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Production in Freshwater Ecosystems. Solar radiation and temperature are closely linked to primary production in freshwater lakes. During the 1970s, sewage and fertilizer pollution added nutrients to lakes, which shifted many lakes from having phytoplankton communities to those dominated by diatoms and green algae.

This process is called eutrophication, and has undesirable impacts from a human perspective.

Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Controlling pollution may help control eutrophication. Experiments are being done to study this process. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

3. In terrestrial ecosystems, temperature, moisture, and nutrients limit primary production Obviously, water availability varies among terrestrial ecosystems more than aquatic ones. On a large geographic scale, temperature and moisture are the key factors controlling primary production in ecosystems.

On a more local scale, mineral nutrients in the soil can play key roles in limiting primary production. Scientific studies relating nutrients to production have practical applications in agriculture. Fig. 54.9 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

CHAPTER 54 ECOSYSTEMS Section C: Secondary Production in Ecosystems 1. The efficiency of energy transfer between trophic levels is usually less than 20% 2. Herbivores consume a small percentage of vegetation: the green world hypothesis

Introduction The amount of chemical energy in consumers food that is converted to their own new biomass during a given time period is called secondary production. 1. The efficiency of energy transfer between trophic levels is usually less than 20% Production Efficiency.

One way to understand secondary production is to examine the process in individual organisms. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

If we view animals as energy transformers, we can ask questions about their relative efficiencies. Production efficiency = Net secondary production/assimilation of primary production Net secondary production is the energy stored in biomass represented by growth and reproduction. Assimilation consists of the total energy taken in and used for growth, reproduction, and respiration.

In other words production efficiency is the fraction of food energy that is not used for respiration. This differs between organisms. Trophic Efficiency and Ecological Pyramids. Trophic efficiency is the percentage of production transferred from one trophic level to the next. Pyramids of production represent the multiplicative loss of energy from a food chain.

Fig. 54.11 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Pyramids of biomass represent the ecological consequence of low trophic efficiencies. Most biomass pyramids narrow sharply from primary producers to top-level carnivores because energy transfers are inefficient.

Fig. 54.12a Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings In some aquatic ecosystems, the pyramid is inverted. In this example, phytoplankton grow, reproduce, and are consumed rapidly. They have a short turnover time, which is a comparison

of standing crop mass compared to production. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Pyramids of numbers show how the levels in the pyramids of biomass are proportional to the number of individuals present in each trophic level. Fig. 54.13

Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings The dynamics of energy through ecosystems have important implications for the human population. Fig. 54.14 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

2. Herbivores consume a small percentage of vegetation: the green world hypothesis According to the green world hypothesis, herbivores consume relatively little plant biomass because they are held in check by a variety of factors including:

Plants have defenses against herbivores Nutrients, not energy supply, usually limit herbivores Abiotic factors limit herbivores Intraspecific competition can limit herbivore numbers Interspecific interactions check herbivore densities

CHAPTER 54 ECOSYSTEMS Section D: The Cycling of Chemical Elements in Ecosystems 1. Biological and geologic processes move nutrients between organic and inorganic compartments 2. Decomposition rates largely determine the rates of nutrient cycling 3. Nutrient cycling is strongly regulated by vegetation

Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Introduction Nutrient circuits involve both biotic and abiotic components of ecosystems and are called biogeochemical cycles. 1. Biological and geologic processes move

nutrients between organic and inorganic compartments A general model of chemical cycling. There are four main reservoirs of elements and processes that transfer elements between reservoirs. Reservoirs are defined by two characteristics, whether it contains organic or inorganic materials, and whether or not the materials are directly

usable by organisms. Fig. 54.15 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Describing biogeochemical cycles in general terms is much simpler than trying to trace elements through these cycles. One important cycle, the water cycle, does not fit

the generalized scheme. The water cycle is more of a physical process than a chemical one. Fig. 54.16 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings The carbon cycle fits the generalized scheme of

biogeochemical cycles better than water. Fig. 54.17 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings The nitrogen cycle. Nitrogen enters ecosystems through two natural pathways. Atmospheric deposition, where usable nitrogen is added to the

soil by rain or dust. Nitrogen fixation, where certain prokaryotes convert N2 to minerals that can be used to synthesize nitrogenous organic compounds like amino acids. Fig. 54.18 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings In addition to the natural ways, industrial

production of nitrogen-containing fertilizer contributes to nitrogenous materials in ecosystems. The direct product of nitrogen fixation is ammonia, which picks up H + and becomes ammonium in the soil (pl) |ln pl|ammonification), which plants can use. Certain aerobic bacteria oxidize ammonium into nitrate, a process called nitrification.

Nitrate can also be used by plants. Some bacteria get oxygen from the nitrate and release N2 back into the atmosphere (pl) |ln pl|denitrification). The phosphorous cycle. Organisms require phosphorous for many things. This cycle is simpler than the others because phosphorous does not come from the atmosphere. Phosphorus occurs only in phosphate, which plants absorb

and use for organic synthesis. Humus and soil particles bind phosphate, so the recycling of it tends to be localized. Fig. 54.19 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Figure 54.20

reviews chemical cycling in ecosystems. Fig. 54.20 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings 2. Decomposition rates largely determine the rates of nutrient cycling

The rates at which nutrients cycle in ecosystems are extremely variable as a result of variable rates of decomposition. Decomposition can take up to 50 years in the tundra, while in the tropical forest, it can occur much faster. Contents of nutrients in the soil of different ecosystems vary also, depending on the rate of absorption by the plants. 3. Nutrient cycling is strongly

regulated by vegetation Long-term ecological research (LTER) monitors the dynamics of ecosystems over long periods of time. The Hubbard Brook Experimental Forest has been studied since 1963. Fig. 54.21 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Preliminary studies confirmed that internal cycling within a terrestrial ecosystem conserves most of the mineral nutrients. Some areas have been completely logged and then sprayed with herbicides to study how removal of vegetation affects nutrient content of the soil. In addition to the natural ways, industrial

production of nitrogen-containing fertilizer contributes to nitrogenous materials in ecosystems. CHAPTER 54 ECOSYSTEMS Section E: Human Impact on Ecosystems and the Biosphere 1. The human population is disrupting chemical cycles

throughout the biosphere 2.Combustion of fossil fuels is the main cause of acid precipitation 3. Toxins can become concentrated in successive trophic levels of food webs 4. Human activities may be causing climate change by increasing carbon dioxide concentration in the atmosphere 5. Human activities are depleting atmospheric ozone

1. The human population is disrupting chemical cycles throughout the biosphere Human activity intrudes in nutrient cycles by removing nutrients from one part of the biosphere and then adding them to another. Agricultural effects of nutrient cycling. In agricultural ecosystems, a large amount of

nutrients are removed from the area in terms of standing crop biomass. After awhile, the natural store of nutrients can become exhausted. Fig. 54.22 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Recent studies indicate that human

activities have approximately doubled the worldwide supply of fixed nitrogen, due to the use of fertilizers, cultivation of legumes, and burning. This may increase the amount of nitrogen oxides in the atmosphere and contribute to atmospheric warming, depletion of ozone and possibly acid rain.

Critical load and nutrient cycles. In some situations, the addition of nitrogen to ecosystems by human activity can be beneficial, but in others it can cause problems. The key issue is the critical load, the amount of added nitrogen that can be absorbed by plants without damaging the ecosystem. Accelerated eutrophication of lakes.

Human intrusion has disrupted freshwater ecosystems by what is called cultural eutrophication. Sewage and factory wastes, runoff of animal wastes from pastures and stockyards have overloaded many freshwater streams and lakes with nitrogen. This can eliminate fish species because it is difficult for them to live in these new conditions.

2. Combustion of fossil fuels is the main cause of acid precipitation The burning of fossil fuels releases sulfur oxides and nitrogen that react with water in the atmosphere

to produce sulfuric and nitric acids. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 54.23a These acids fall back to earth as acid precipitation, and can damage ecosystems greatly. The acids can kill plants, and can kill aquatic

organisms by changing the pH of the soil and water. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings 3. Toxins can become concentrated in successive trophic levels of food webs Humans produce many toxic chemicals that are dumped into ecosystems. These substances are ingested and metabolized by the

organisms in the ecosystems and can accumulate in the fatty tissues of animals. These toxins become more concentrated in successive trophic levels of a food web, a process called biological magnification. The pesticide DDT, before it was banned, showed this affect. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 54.25 Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings 4. Human activities may be causing climate change by increasing carbon dioxide concentration in the atmosphere Rising atmospheric CO2. Since the Industrial Revolution, the concentration

of CO2 in the atmosphere has increased greatly as a result of burning fossil fuels. Measurements in 1958 read 316 ppm and increased to 370 ppm today Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings The greenhouse effect.

Rising levels of atmospheric CO2 may have an impact on Earths heat budget. When light energy hits the Earth, much of it is reflected off the surface. CO2 causes the Earth to retain some of the energy that would ordinarily escape the atmosphere. This phenomenon is called the greenhouse effect. The Earth needs this heat, but too much could be disastrous.

Global warming. Scientists continue to construct models to predict how increasing levels of CO2 in the atmosphere will affect Earth. Several studies predict a doubling of CO2 in the atmosphere will cause a 2 C increase in the average temperature of Earth. Rising temperatures could cause polar ice cap melting, which could flood coastal areas.

It is important that humans attempt to stabilize their use of fossil fuels. 5. Human activities are depleting the atmospheric ozone Life on earth is protected from the damaging affects of ultraviolet radiation (pl) |ln pl|UV) by a layer of O3,

or ozone. Studies suggest that the ozone layer has been gradually thinning since 1975. Fig. 54.27a Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 54.27b

Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings The destruction of ozone probably results from the accumulation of chlorofluorocarbons, chemicals used in refrigeration and aerosol cans, and in certain manufacturing processes. The result of a readuction in the ozone layer may be increased levels of UV radiation that reach the surface of the Earth.

This radiation has been linked to skin cancer and cataracts. The impact of human activity on the ozone layer is one more example of how much we are able to disrupt ecosystems and the entire biosphere. Copyright 2002 Pearson Education, Inc., publishing as Benjamin Cummings

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