1. Chapter 3: Species Populations, Interactions and Communities
Principles of Environmental
Science - Inquiry and Application
by William and Mary Ann Cunningham
Chapter Three considers populations, communities and species interactions.
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2. Chapter 3. Key Terms McGraw-Hill Course Glossary
adaptation
Batesian mimicry
biotic potential
carrying capacity
coevolution
commensalism
complexity
convergent evolution
divergent evolution
diversity
ecological development
ecological niche
ecotones
overshoots
pioneer species
predator
primary productivity
primary succession
r-adapted species
resource partitioning
S-curve
secondary succession
selective pressure
symbiosis
tolerance limits
edge effects
environmental resistance
evolution
exponential growth
habitat
J curve
K-adapted species
keystone species
logistic growth
Mullerian mimicry
mutualism
natural selection
The following are key terms for this chapter. After reviewing these slides, and reading the chapter, please review them. You can also consult the McGraw-Hill Course Glossary if you have a link to the internet.

3. Chapter 3 - Topics Who Lives Where, and Why? Species Interactions
Population Dynamics
Community Properties
Communities in Transition
In this chapter, we consider what kinds of species live where; why they live where they do; interactions among species; population dynamics, or how numbers of species change over time; properties of communities (an assemblage of species and organisms); and how communities of species can change over time and space.

4. The following slides are designed to give you examples of the myriad adaptations of organisms to their environment. Alligators, for example, are organisms that have been living on earth for hundreds of millions of year. Alligators and their relatives in the Crocodilian group existed and flourished well before the age of dinosaurs. Adaptation to the environment and ability to adapt to a change in the environment are among the key criteria that determine the success of organisms.

5. On the other hand, a hundred millions years ago, elephants did not exist. They have evolved only over the last few million years. Before elephants, there were organisms that occupied their position in the environment. These were the Sauropods, the largest of the dinosaurs. The evolution of a great variety of mammal species and their large size is a relatively recent event. Elephants are highly adapted to their environment and even though many are endangered today, they represent a later stage in evolutions and success in adapting to change in their environment.

6. There is strong evidence that birds evolved from dinosaurs
There is strong evidence that birds evolved from dinosaurs. Their structure, anatomy, and other characteristics, such as scales on their legs, lead scientists to this conclusion. Birds are certainly diverse and abundant, and people tend to like them because of their beauty and many interesting behaviors, such as the highly developed courtship rituals among some species.

7. Note the owl's large eyes – an adaptation for seeing at night
Note the owl's large eyes – an adaptation for seeing at night. Its feathers are also highly adapted so that it makes very little noise when in flight. Both of these adaptations are excellent for a predator that uses ambush as the primary means of capturing its prey.

8. Fairly closely related to humans and what many people would consider to be highly evolved are the primates. Primates like these chimpanzees exhibit many human characteristics, including the ability to use tools and learn. Their social structures are quite complicated, and some scientists have been a little disturbed to discover practices such as infanticide among otherwise gentle social behavior. Among many primates, there is a long adaptation period in which young are taught how to survive in the environment and within the social structure.

9. Notice the incredible expression of color in two species here, one a flower and the other a butterfly.

10. If we went back 65 million years ago or so, these lions would be the Tyrannosaurus rex's of their time. Highly evolved predators such as lions sit at the top of the food chain and require large amounts of plant matter to support their size and enormous energy.

11. Part 1: Who Lives Where, and Why?
Generalists vs. Specialists
To understand biology, we must study of characteristics of the organism itself (things like its size, its life span, its potential reproductive rate, what kind of environmental extremes can it live in, and so on). There are two basic strategies for surviving. One is to be reasonably good at exploiting different environments and being capable of sustaining yourself in them. Organisms that use this strategy are termed "generalists". The other strategy is to become very, very good at living within a more narrow set of environmental circumstances so that you can outcompete other organisms that would live in that same environment. Organisms that use this strategy are termed "specialists." In an environment where conditions change a lot, it is better to be a generalist, but in an environment that is stable, a specialist can usually outcompete a generalist.

12. Tolerance Limits
An environmental gradient is a stepwise increase or decrease in an environmental factor – for example, pH, rainfall, temperature, and so on. Species can tolerate different environmental gradients. For instance, humans have a relatively narrow range of temperatures at which they can survive without clothing and shelter. Similarly, humans can live on relatively few types of unprepared food. Becoming very efficient at living in a narrow environmental range is a type of specialization. Generally, the more highly organisms are adapted to a narrow environmental range, the less adapted they are to changes in the environment. Why would this be true?
Each environmental factor (temperature, nutrient supply, etc.) has both minimum and maximum levels beyond which a species cannot survive or is unable to reproduce.

13. Abundance and Distribution of Species
Liebig - proposed that the single environmental factor in shortest supply relative to demand is the critical determinant in species distribution
Shelford - added to Liebig's work by proposing that the single environmental factor closest to tolerance limits determines where a particular organism can live
Why do species live in different kinds of environments? The chemist Liebig proposed that a single environmental factor (the one in shortest supply) would be the critical determinant in whether or not a species was present, and where it would live. Shelfold added to Liebig's work by focusing on tolerance limits. According to Shelfold, the environmental factors (for example, temperature) closest to the tolerance limits determine where a species lives. This example of a Saguaro cactus shows a species that is highly adapted to living in an environment that is very dry. Also, the temperature must not drop below freezing as the Saguaro would not survive. Thus, an environment that becomes cooler than the tolerance limits would tend to limit the Saguaro more than an environment that is dry but within the Saguaro's tolerance limits. Another species might have different tolerance limits and the dryness would be the major limiting environmental factor.

14. Today we know that for many species the interaction of several factors, rather than a single limiting factor, determines biogeographical distribution.
Sometimes, the requirements and tolerances of species are useful indicators of specific environmental characteristics.
The concepts of Liebig and Shelfold have been useful tools in studying the geographic distribution of a species, but today we believe that interactions of several, or even many, factors determine distribution of a particular species. Sometimes the presence of individual species can tell us lots about the environment itself. The presence of indicator species (species that tell us something specific about the environment) are often very useful to ecologists in determining the properties of an environment. For instance, in the Pacific Northwest, the presence of sword fern and devil's club plants are indicators of very fertile, moist conditions. The presence of salal (which is commonly found in the understory of Pacific Northwest forests) is an indicator of dry and/or very low nutrient conditions. Often, salal indicates that nitrogen is in short supply. Other forest species cannot compete with salal's adaptation to low nitrogen conditions.

15. Adaptation and Natural Selection
Two types of adaptation:
Acclimation - changes in an individual organism due to non-permanent physiological modifications
Evolution - gradual changes in a species due to changes in genetic material and competition
Theory of evolution - developed by Charles Darwin and
Alfred Wallace.
For many millenia, people have noted organisms' abilities to adapt to their environment. For example, mammals may develop thicker coats of fur in colder environments. (The thickness of fuzz on caterpillars (they are not mammals) is used by the Farmer's Almanac to predict the intensity of winter.) After spending over a year trapped in Antarctica (read the book "South"), Ernest Shackleton, complained that trying to remain in a normally heated room caused him pain and that he was much more comfortable in freezing conditions. These are examples of acclimation, or changes in an individual organism due to non-permanent physiological modifications.
Evolution is another mechanism by which life adapts to its environment. However, the changes are passed on through inheritance and offer competitive advantages only to the progeny of organisms, rather than to the organisms themselves. The theory of evolution through natural selection offered by Charles Darwin and Alfred Wallace provides a clear mechanism that can explain much of what is observed in the change of species over time.

16. Natural selection - genetic combinations best adapted
for present environmental conditions tend to become
abundant
Spontaneous, random mutations
Selective pressure - physiological stress, predation,
competition, luck
The impacts of natural selection are such that genetic combinations that help a species survive in its current conditions tend to become more abundant. This increase in adaptive genetic combinations in a population can happen in a relatively short period of time in populations that have a wide range of genetic variability. If populations do not have high genetic variation, they are not as subject to natural selection. For instance, so many cheetahs were killed in the recent past that eventually, just a few individuals remained. Even though their numbers have increased again (in part do to protection of their environment), the genetic variability within the species is very low. Historically, cheetah genetic variability was probably higher. So now the cheetahs don't have as great an opportunity to pass on genetic changes to their offspring as they did in the past.
Genetic mutations can increase an individual's chances of survival and reproductive fitness. Unfortunately, most mutations have negative consequences, though occasionally they offer a competitive advantage.

17. Speciation
It was once thought that the development of new species was a relatively slow and even process. We are not so certain of that anymore. It seems that in some cases, new species may evolve very quickly under certain conditions (at least in terms of the long history of evolution and life).
Regardless of the time factor involved, we can look at different species and see clearly how they adapted to a successful "life strategy." For example, the bills of these different species of finches show adaptations that vary from very hard bills good at crushing seeds and tearing fruit to very eloquently structured, fine probing bills that are designed to get under bark in order to capture insects. This example shows how a species change is expressed over time. Adaptations in bill structure offer advantages under different conditions of food availability.

18. Types of speciation Allopatric: due to geographic isolation
Sympatric: due to genetic make up differences.
Isolation could result in the change of physical, behavioral and genetic characteristics of individuals.
Based on how it affects populations, environmental pressures could be directional, stabilizing or disruptive.

19. The Taxonomic Naming System
ADD TABLE 3.1
Scientists and non-scientists alike love to classify and categorize. The science of classification is called taxonomy. Scientists have a taxonomic system for species that proceeds from a very broad, general level called a kingdom (animals vs. plants vs. microbes, etc.) down to smaller levels known as genus and species. Classification continues within species to the level of varieties. Different varieties of apples are an example of within-species classification. As we go down the levels from kingdom to species, organisms within groups become more and more alike. For example, domestic or sweet corn (genus Zea) and orchids (many are in the genus Cattlyea) are both in the phylum that includes flowering plants (Anthophyta), even though there are many differences between corn and orchids. When you get down to the level of genus and species, you find that Zea mays and Zea luxurians, sweet and wild corn, are very similar.
The way organisms are classified does not necessarily reflect their function in the environment. For instance, Rob has been in areas of the arctic north of the Brooks Range in Alaska where species of willows function ecologically like grasses. They are only a few inches long, yet they are taxonomically related to other willow trees thousands of times larger. Even though they are closely related taxonomically, they play a very different role than larger willows. So, the classification of a species does not necessarily tell you anything about how it functions in its environment.

20. The
Ecological Niche
Habitat - the place or set of environmental conditions in
which a particular organism lives
Ecological niche - the role played by a species in a biological community
Ecological niche is another aspect of adaptation that helps determine where an organism lives and its abundance. In contrast to habitat, which is the set of environmental conditions in which an organism lives, ecological niche is the role that the organism plays in a biological community – for example, it's relationship with the plants and animals in its community. Pandas, for instance, depend on giant bamboo for food. Giant bamboo has an interesting property in that populations flower and die within a short period of time. The bamboo must then regrow to provide enough food for pandas. The pandas dependent on that bamboo population decline in numbers during the period of regrowth.

21. Resource Partitioning
Over time, niches can
evolve as species develop new strategies to exploit resources.
Law of Competitive
Exclusion:
No two species will
occupy the same niche and compete for the same resources in the same habitat for very long.
According to the law of natural selection, any competitive advantage that a species possesses by virtue of its ability to function better within the environment is advantageous to the survival and reproduction of that species. In the case of stable environments, the best "specialist" tends to benefit at the expense of other species. The law of competitive exclusion dictates that where two species occupy the same niche, the species that is more competitive would eventually take over the other species and eliminate it. Basically, what this means is that no two species can occupy the same niche for very long. Eventually one species adapts better and pushes the other one out.

22. Here is an example of niches that birds might fill
Here is an example of niches that birds might fill. If we consider a tree from the very top of its canopy down to the ground, we find that different species of birds forage within different parts of the canopy. Note that there is some overlap of species in these foraging "partitions," but most species have a niche that is separate from other species. In an aquatic environment, niches might be formed by the increasing depth of water in a bay or a river. You might expect to see different but closely related species of aquatic life filling these niches.

23. Part 2: Species Interactions
Noting, studying and understanding the interactions among and between species is one of the most fascinating aspects of environmental science. One relationship that has been studied a lot is that of predation. Predation means that one species uses another as food. Competition -- for resources such as food, habitat, or the ability to mate -- is also an important interaction.
Intra-specific (within a species) competition is often very intense. Here, you see two snowshoe hares fighting over something… probably these are males fighting over a female. The competition to reproduce and breed is widespread and typically intense. It is not uncommon to see similar interactions within humans where people might fight over a boyfriend/girlfriend …of course all of this competition disappears by the time students reach college-age, like the students in ESC 110.
Most obvious are Predation and Competition - antagonistic relationships

24. How does competition shape population
Intraspecific: Among organisms of the same spp. Dispersal, Territoriality and Generational resource partitioning.
Interspecific: different spp. Resource partitioning
NB. When two species compete, the one in the center of its tolerance limit has the advantage.

25. How does predation affect spp populations in communities
Adjustments in behavior and body characteristics of predators and prey

26. Defensive Mechanisms
Many organisms develop defensive mechanisms that can protect them from predation. For instance, the poison arrow frog shown in this slide has a strong neurotoxin in its skin. Typically, the frog is only preyed on once by a single predator, and though this doesn't help the frog that gets killed, predators soon learn to leave other frogs alone, which benefits the frogs and their progeny.
Other organisms use the frog's defensive mechanism to their own advantage. Natives in certain parts of Brazil use the frog's skin to tip their arrows for hunting.
There are many kinds of defense mechanisms that aren't toxic. For instance, antelopes are adapted to run very fast, which helps them escape from predators. Camouflage is also a good defense.

27. Notice the unusual coloration of this stonefish – part of it comes from the algae that live on it. The coloration is camouflage, one of two defensive mechanism that the stonefish relies on. The other defensive mechanism is a series of poisonous spines along it's back. There are many stonefish in the Indian Ocean, and unless you've grown up with them, you don't know what to look for. If you're a swimmer or diver who happens to step on a stonefish, you are in a dangerous situation.
When I was in the Peace Corps, I was wading around in a pool in the Indian Ocean with several other volunteers and our teachers during my initial language training in Swahili. We played in the tidal pool until one of the local guys pointed out several stonefish, noting that we had better not step on them. We had no idea that they were there, and even when they were pointed out to us they were hard to make out. Multiple adaptations like those of the stonefish are relatively common, and camouflage and poison are a deadly combination.

28. Batesian Mimicry
Sometimes it is good enough to just look like something that has a good defensive mechanism or a reputation for being nasty and dangerous. For instance, the yellow jacket is highly developed, with a strong sting and good armor. To the right is the picture of a beetle that mimics a yellow jacket. Notice the similarity, even in the head structure. Predators are fooled into thinking that the beetle is a dangerous yellow jacket, and they stay away. Thus, the beetle gets protection without have to devote energy into the production of a real defensive system.

29. Keystone species - species that play essential community
roles (examples: mycorrhizae, giant kelp)
Keystone species are species that are so important in the benefits that they provide to other organisms that they are considered critical to whole communities. Douglas fir is one of the keystone species of the Pacific Northwest coniferous forests.
A good example of an aquatic keystone species is the eel grass in the Puget Sound. I once took a field trip with an elementary school class and in a tidal pool, the kids picked out dozens of species of marine organisms that were living off of the eel grass. Another aquatic keystone species is the giant kelp, which grow in "forests" of California ocean waters. It was impossible to find a bunch of kelp that didn't harbor other species, including tiny crab, snails and sea lice.

30. ADD FIG. 3.18 A-C Three Types of Symbiosis:
Commensalism - one member benefits, while the other is neither benefited nor harmed
Mutualism - both members of the partnership benefit;
Parasitism - a form of predation where one species benefits and the other is harmed
Other kinds of important species interactions are classified as symbiosis. Symbiosis is "the intimate living together of members of two different species." There are three types of symbiosis:
Commensalism is a type of species interaction in which one member of the interaction benefits and the other neither gains nor loses. An example of commensalism is a barnacle riding on a turtle shell. The barnacle benefits because it has a place to anchor and live. The turtle is indifferent to the barnacle, and would do fine whether or not the barnacle was there.
Mutualism is an interaction in which both members of the partnership benefit. An example is the relationship between red alder and the tiny soil bacteria Frankia that forms nodules in the alder's roots. Frankia produces nitrogen that the alder needs, and the bacteria receives food, or photosynthate (reduced carbohydrates), from the alder. Lichens, shown in the slide above, also are an example of a mutualistic relationship. Lichens are formed by algae and fungi growing together as one organism. The alga carries out photosynthesis, making sugars for itself and the fungus. The fungus surrounds and protects the algae from dehydration and injury.
Parasitism is another type of symbioses. In parasitism, one species benefits and the other is harmed. An example of parasitism is an intestinal fluke that lives within the organism using the organism's food and energy to its benefit and the host's detriment. There is definite harm to the host in this case because the parasite is robbing it of food.

31. Part 3: Population Dynamics
Exponential growth - the unrestricted increase in a population (also called the biotic potential of a population)
Carrying capacity - the maximum number of individuals of any species that can be supported by a particular ecosystem on a sustainable basis
Individual species of organisms together form populations. For instance, all of the students in this class can be considered a population, or all of the people on earth another, larger population that includes the smaller ESC 110 population. A population can grow, shrink or stabilize.
If we introduce organisms into a new, rich environment where they can survive and reproduce but haven't lived before, they typically exhibit exponential growth because they reproduce at the maximum possible rate. In this case, the growth of the population is only limited by the biotic ability of this species to reproduce. At some point, all populations in the ecosystem reach carrying capacity, or the maximum number of individuals in a species that can be supported by an ecosystem. The presence of other organisms may affect the carrying capacity because of competition, predation, or mutualism.

32. Two types of populations
r-selected: grow exponentially, expend energy in giving birth to progeny.
k-selected: grow logistically, expend energy raising progeny

33. ADD FIG. 3.20 Overshoots and Diebacks
This graph depicts the potential growth of a population over time. We see that the carrying capacity may or may not be (typically it is not) well defined because of changes in the environment over time. Variations in the environment – drought, for example, can actually change the carrying capacity. As seen in the graph, a population might overshoot its carrying capacity, drop below the average, and then increase and overshoot it again.

34. ADD FIG. 3.22 Growth to a Stable Population
The J curve (so called because of its shape) is a growth curve that depicts exponential, or maximum, growth. Because of the impact of environmental conditions (environmental resistance) and the limited availability of resources, we typically don't see population growth curves following the J curve for very long. Very quickly, environmental conditions tend to limit not only the absolute growth of populations, but also their growth rates. The S curve indicates how a population grows when it is limited by environmental conditions.
The human population has recently (the last 10,000 years) undergone J curve growth because of the advent of agriculture, disease control, and technological advances such as heating and air conditioning (to allow people to live and reproduce in warm and cold climates). We are starting to see that the rate of increase for the human population may be leveling off, approaching an S curve. This slowing of the rate of increase is not due to starvation, but to social changes. We don't know where the population will level off. Hopefully, the human population won't undergo the "boom and bust" cycle depicted on the previous slide. With the current population of the earth at more than 6.5 billion, and food supplies unevenly distributed, the result would be the death of hundreds of millions of people.

35. Environmental Resistance
Environmental resistance - factors that tend to reduce population growth rates:
Density-dependent
- linked to population size
- disease, lack of food
Density-independent
- often environmental
- droughts, floods, habitat
destruction
Intrinsic
- attributes of a species
- slow reproduction
Extrinsic
- external to a species
- predators, competitors,
environmental risks
Environmental resistance to population growth, which limits growth rates, includes the following:
Within the population, lack of food and the presence of disease can play a role in population size.
External to the population are natural disasters such as floods, drought, tornados and habitat destruction. These factors can indirectly play a role in population size.
3. There are also factors that are external or extrinsic to the species. These include the impact of predators, competitors for habitat and environmental risks. Extrinsic factors can reduce the population growth rate, or (conceivably) if it is a mutually beneficial relationship, increase the growth rate.
4. For any given organism, its reproduction rate and the way it reproduces is a characteristic of the genetic makeup of that organism. These attributes affect whether the species has a long or short time to respond to environmental change. Elephants, for instance, have a long gestation rate and only produce a few individuals, whereas insects often produce thousands of eggs. Their higher reproduction rate makes them able to adapt faster to environmental change.

36. Reproductive Strategies
r-Selected spp
Short life
Rapid growth
Early maturity
Many, small offspring
Little parental care and protection
Little investment in individual offspring
Adapted to unstable environment
Colonizers
Niche generalists
Prey
Regulated mainly by intrinsic factors
Low trophic level
k-Selected spp
Long life
Slower growth
Late maturity
Few, large offspring
High parental care and protection
High investment in individual offspring
Adapted to stable environment
Later stage of succession
Niche specialists
Predators
Regulated mainly by extrinsic factors
High trophic level

37. Part 4: Community Properties
Primary productivity - a community's rate of biomass production, or the conversion of solar energy into chemical energy stored in living (or once-living organisms)
Net primary productivity - primary productivity minus the energy lost in respiration
Productivity depends on light levels, temperature, moisture, and nutrient availability.
Now that we have considered some of the characteristics of individual species and populations of that species, we need to start looking at life on a more complex level, particularly the interrelationships among more than one species. In ecological terms, a community is an assemblage of a number of different species; for instance, the different species that inhabit a particular area.
Communities are characterized by a number of elements. These include primary productivity, which is defined as the raw amount of biomass production, or how much solar energy is converted into chemical energy through photosynthesis. For instance, the primary productivity of something like a wheat field would be centered mostly around the growth of the wheat. The primary productivity of a Douglas-fir forest would be primarily centered around the growth rate of the trees.
The basic driving force for productivity is the gross photosynthetic rate, but trees also use some of the energy that they store, so the amount of wood and other biomass stored over time would be a measure of the net primary productivity versus the gross productivity of that ecosystem.

38. ADD FIG. 3.29 Relative biomass accumulation of major world ecosystems.
Some types of ecosystems are more productive than others. The total annual production and/or accumulation of biomass in terms of its raw weight is often used to describe the productivity of the world's major ecosystems. The figure above shows the total accumulation of biomass in several different ecosystem types, ranging from very low productivity in desert ecosystems and tundra to extremely high productivity in tropical rain forests, estuaries, and coral reefs. The accumulation of biomass in these ecosystems does not necessarily reflect their growth rates, but is a balance between photosynthesis (which takes carbon dioxide from the air and turns it into biomass) and respiration, which turns biomass back into atmospheric carbon dioxide.

39. Abundance and Diversity
Abundance - the number of individuals of a species in an area
Diversity - the number of different species in an area
A useful measure of the variety of ecological niches or genetic variation in a community
Decreases as we go from the equator towards the poles
Simply measuring the dry weight or amount of carbon in an ecosystem, though important, doesn't tell us how rich and diverse the ecosystem is. Ecologists place a great deal of value on the variability of that biomass and its properties. The number of species and individual organisms are also considered important characteristics of ecosystems. For instance, the abundance of the population is the number of individuals of a species in a particular area. Obviously, there can be fewer Douglas-fir trees, which may weigh several tons each, than small organisms such as mites and insects. The diversity of an ecosystem is a much used measure of the number of species, or of the number of ecological niches occupied, or the genetic variation within a system. Abundance and diversity depend both on the total resources available in an ecosystem and how variable those resources are within that ecosystem.
Abundance and diversity depend on total resource availability in an ecosystem.

40. Antarctic Marine Food Web
Complexity - the number of species at each trophic level and the number of trophic levels in a community
The figure above shows how complex and highly interconnected an ecosystem such as the Antarctic food web is. Notice that much of this ecosystem depends on a single species, krill, at some point in the food web. Krill are small, shrimp like crustaceans and are extremely abundant in Antarctic waters. There have been proposals to capture and use Antarctic krill as a food source for humans and animals; however, since krill are such an important food source for so many other species, directly or indirectly, the impacts of a large-scale removal of of the Antarctic krill on Antarctic ecosystems is likely to be substantial. Interestingly, if another species, such as the humpback whale, which is generally much more impressive and historically useful, to humans, was removed, the effects on the entire ecosystem are likely to be much less than the removal of krill. Krill are a very good example of a keystone species in Antarctic ecosystems.

41. Stability and Resilience
Stability - a dynamic equilibrium among the physical
and biological factors in an ecosystem or a
community
Resiliency - the ability to recover from disturbance
Three kinds of stability or resiliency in ecosystems:
Constancy - lack of fluctuations in composition or functions
Inertia - resistance to perturbations
Renewal - ability to repair damage after disturbance
The ability of ecosystems to retain their basic characteristics, and to recover from disturbances is also an important. Stability is the equilibrium, albeit sometimes a dynamic equilibrium, among the physical, chemical, and biological characteristics of an ecosystem. Stable ecosystems do not change dramatically over time. The resiliency of an ecosystem is its ability to recover from a disturbance, such as fire. Resilience can be expressed as constancy, or lack of change after a disturbance; inertia, or the resistance to change after a disturbance, and renewal, or the ability of the ecosystem to renew itself following disturbance.
Ecologists have been impressed at how rapidly the Mt. St. Helens ecosystem is recovering from the 1980 volcanic eruption and how quickly the Yellowstone ecosystem is recovering from fires in the 1990's.

42. ADD FIG. 3.25 A-C Community structure Distribution of members of a
population in a given space can be:
Random - individuals live wherever resources are available
Ordered - often the result of biological competition
Clustered - individuals of a species cluster together for protection, mutual assistance, reproduction, or to gain access to a particular environmental resource
ADD FIG A-C
The distribution of individuals with an ecosystem is also an important characteristic of an ecosystem. Distribution can be random, or lacking any particular order; ordered, such as trees planted in rows; or clustered, such as the distribution of wetland species in areas where where conditions are good.

43. Edges and Boundaries
Ecotones- the boundaries between adjacent habitats
Often rich in species diversity
Example: the boundary between a forest and a meadow
Another aspect of community structure include edges and boundaries between habitats. Ecotone is the term given to distinct changes over of a short distance between habitats; for instance, if you walk from a forest into a meadow you cross an ecotone. Some species are highly adapted to living in the edge systems; for instance, whitetail deer browse on shrubs and young trees associated with forest patches, and then use the denser cover of the forest for protection.

44. Edge vs. Core Edge effects - the environmental and biotic conditions
at the edge of a habitat
Temperature, moisture levels, predator species, etc.
Edge effects associated with habitat fragmentation are generally detrimental to species diversity.
Core habitat - the interior area of a habitat
Habitat not impacted by edge effects
Some species avoid edges and ecotones and prefer interior environments.
Since organisms require different types of inputs to complete their entire life cycle, some organisms move through different habitat types, either in the short term or the long term. For instance, as a deer moves from a meadow into a forest, it changes habitat types quickly. However, the juvenile salmon that migrates from a stream to the ocean changes habitat over a much longer time period.

45. Part 5: Communities in Transition
Ecological succession - the process by which
organisms occupy a site and gradually change
environmental conditions by creating soil, shelter,
shade, or increasing humidity
Primary succession - occurs when a community begins to develop on a site previously unoccupied by living organisms
Secondary succession - occurs when an existing community is disrupted and a new one subsequently develops at the site
Ecological succession is the term given to the process where the species on a particular site change over time, as one species makes way to another. For instance, the Pacific Northwest coniferous forest is undergoing succession processes constantly. Following the eruptions of Mount St. Helens, the land was occupied by primarily geologic materials, the rock and ash from the eruption. Only species that could exist under very low nutrients and water availability conditions could survive, and they rapidly occupied this stark landscape. These species of are turned pioneer species. As these organisms grow and die they eventually build up the organic matter and nutrient levels of a developing soil. As they site conditions change, they become a habitat for new, different species, including Douglas-fir. Eventually, given continual development of succession without more disturbance, a particular type of ecosystem will develop, in most cases in the Seattle region this would be a cedar/hemlock forest. However, in this region another disturbance is just around the corner. Secondary succession occurs when an existing community is disturbed, and a new one develops on the same site, but that site contains a developed soil and the dead organisms from the previous community. For instance, one of the major ways that a secondary succession occurs in this region is by the cutting of forests, after which new forests develop. Nearly all of the forests in the Seattle region are secondary forests.

46. Primary Succession on Land
This figure shows primary succession on a brand new landscape of exposed rocks such as might occur after a volcanic eruption or landslide. Abundant chronosequences (examples of the same ecological succession at different ages) are depicted. In the region around Seattle, chronosequencesare common because of the abundance of natural and man-made disturbances.

47. Exotic Species
Sometimes communities can be completely altered by the introduction of exotic species.
Exotic species are often introduced by humans.
Successful exotics tend to be prolific, opportunistic species, such as goats, cats, and pigs.
Many ecologists consider exotic species invasions the most pressing hazard for biological communities in the coming century.
Patterns of ecological succession that have taken place for thousands of years or more may be disrupted by the introduction of a species that was previously not present. Many ecologists consider the introduction of an exotic species to be the single largest problem for the maintenance of existing biological communities in the future.

48. Introduced Species and Community Change
For instance, the introduction of the mongoose into Hawaii has had a dramatic negative impact on the native bird population, as native birds did not have defense mechanisms against this aggressive predator. Introduction of exotic species, including the Spartina grass into Pacific Northwest coastal marshes, purple loosestrife and reed canary grass into freshwater wetlands, and house cats into native bird populations, have greatly altered the normal patterns of succession of different ecosystems.