Human ecology
Panarchy: Ecosystems developed in adaptive cycles of exploitation, conservation, release and reorganization which could be described in three dimensions - ecological 'wealth', connectedness and resilience. These cycles provide a framework for the opposing forces of growth and stability versus change and variety.
Nutrient cycles
Ecological succession
In the field of ecology, community composition changes over time. The study of succession addresses this change, which can be influenced by the environment, biotic interactions, and dispersal.
frequently disturbed areas full of r-selected indiv. Occasional disturbance: resevoir of rselected able to move in (blackberry).
Communities never disturbed (coral reefs/rainforests) take long time to recover cause no resevoir of opportunistic species.
System behaviour

- environmental resilience


external image forest.jpgThe ecosystem concept has its roots in theoretical concepts regarding the organization and dynamics of natural systems. The word itself is of relatively recent origin; it was initially suggested by a scientist in 1935 as a more abstract replacement for the community concept. In its present usage, however, an ecosystem is generally defined as a community of organisms living in a particular environment and the physical elements in that environment with which they interact. Just as there is an immense diversity of individual species on the planet, so is there a rich diversity of ecosystems ? from the icy arctic zones to tropical forests lush with plants and animals. They occur on many different scales, with smaller systems embedded within larger systems. An ecosystem can be as small as a fallen log or as large as the ocean, depending on the scale that the researcher is examining. But where does one particular ecosystem end and another begin? While the borders of some may be clear, such as a pond; others may be less easy to define, such as marshland that leads into a waterway.
In order to better understand the ecological makeup of the Earth, scientists have proposed many different ways of categorizing terrestrial and marine ecosystems. Most classification systems are defined by the type of plant and animal life living in an area in relation to global climate patterns. The most common system divides the world into biomes based on the dominant plant life that occurs within a certain climate. Other ways of defining ecosystems use more specific classifications incorporating characteristics such as rainfall patterns, type of soil, and particular species. Eco-regions, for example, are nested within biomes and are used by conservationists to define areas of the world which share a majority of their species and ecological dynamics in similar environmental conditions. Newer methods of classifying the world are more human-centered. For instance, anthropogenic biomesare defined by sustained direct human interaction with ecosystems.
Because ecosystems are so interconnected, there have been many efforts by scientists to define the effect of change on ecosystem functioning. Though there may be no true ?balance of nature,? changes to the number or type of species, temperature, soil nutrients, and other factors have all been observed to alter ecosystem functions. Some changes may ultimately lead to species extinctions and eventual ecosystem collapse; though it can be difficult for scientists to determine exactly which factor or combination of factors contribute to any negative outcomes. There are many uncertainties in predicting both ecosystem change and ecosystem functioning, and scientists continue to refine both their method of assessment as well as the definition of a ?healthy? ecosystem.
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Scientific Facts on Ecosystem Change
In 2005, the Millennium Ecosystem Assessment ? the largest assessment of the Earth's ecosystems to date ? was conducted by a team of over 1,000 scientists who concluded that in the past 50 years humans have altered the Earth's ecosystems more than any other time in our history. GreenFacts digests the report?s major findings, including uncertainties in calculating and predicting ecosystem change over time.
World Resources 2000-2001: People and Ecosystems
The World Resources Institute focused on five critical ecosystems shaped by the interaction of the physical environment, biological conditions and human intervention. A Time magazine article and Bill Moyer?s Earth on Edge program broadcast on PBS ? both of which are based on the report ? are also accessible from this site.
Data & Maps
This non-profit association houses detailed information about ecosystems and species on their NatureServe Explorer site, a searchable database with data on more than 50,000 plants, animals, and ecological communities in North America.
For the Classroom
National Geographic Habitats: Home Sweet Home
This National Geographic site for kids focuses on Earth?s habitats: cities and suburbs, deserts and tundra, forests, fresh water, oceans and coasts, and prairies. It includes a photo gallery, games and activities, video clips, maps and lesson plans.
This Access Excellence project by Nora Doerder has students create a sustainable, self-contained ecosystem in a ten-gallon aquarium. [Grades 9-12]
Studying a Piece of an Ecosystem
Teacher Dorothea Sinclair created this Access Excellence project in which students identify the components of an ecosystem and learn how biotic and abiotic factors interact. [Grades 9-12]

Ecosystem services -
The ecosystems of planet Earth are coupled to human environments. Ecosystems regulate the global geophysical cycles of energy, climate, soil nutrients, and water that in turn support and grow natural capital (including the environmental, physiological, cognitive, cultural, and spiritual dimensions of life). Ultimately, every manufactured product in human environments comes from natural systems.[25] Ecosystems are considered common-pool resources because ecosystems do not exclude beneficiaries and they can be depleted or degraded.[41] For example,green space within communities provides sustainable health services that reduces mortality and regulates the spread of vector borne disease.[42] Research shows that people who are more engaged with regular access to natural areas have lower rates of diabetes, heart disease and psychological disorders.[43] These ecological health services are regularly depleted through urban development projects that do not factor in the common-pool value of ecosystems.[44][45]
The ecological commons delivers a diverse supply of community services that sustains the well-being of human society.[46][47] The Millennium Ecosystem Assessment, an international UN initiative involving more than 1,360 experts worldwide, identifies four main ecosystem servicetypes having 30 sub-categories stemming from natural capital. The ecological commons includes provisioning (e.g., food, raw materials, medicine, water supplies), regulating (e.g., climate, water, soil retention, flood retention), cultural (e.g., science and education, artistic, spiritual), and supporting (e.g., soil formation, nutrient cycling, water cycling) services.[48][49]


Primary productivity: rate of biomass production is an indication of the rate of solar energy conversion to chemical energy.
  • The energy left after respiration is the net primary production.external image fig4_18.jpg
  • Photosynthetic rates are regulated by many factors.
    • Light levels
    • Temperature
    • Moisture
    • Nutrient availability
Tropical forests, coral reefs, and estuaries have high levels of productivity because they have abundant supplies of all of the above resources.
Other systems do not have sufficient levels of the necessary resources.
  • Lack of water in deserts limits photosynthesis.
  • Cold temperatures in Arctic tundra or high mountains inhibit plant growth.
  • Lack of nutrients in the open ocean reduces the ability of algae to make use of plentiful sunshine and water.
Even in the most photosynthetically active ecosystems, only a small percentage of the available sunlight is captured and used to make energy-rich compounds.
  • Much of the light reaching plants is reflected by leaf surfaces
  • Most of the light that is absorbed by leaves is converted to heat is either radiated away or dissipated by evaporation and water.
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Abundance and Diversity

Abundance: expression of the total number of organisms in a biological community

Diversity: measure of the number of different species, ecological niches, or genetic variation present.

  • Abundance of a particular species often is inversely related to total diversity of the community.
  • Communities with a very large number of species often have only a few members of any given species in a given area.
  • Climate and history are important factors that dictate the abundance and diversity in a biological community.
  • Productivity is related to abundance and diversity, both of which are dependent a several factors.
    • Total resource availability in an ecosystem
    • Reliability of resources
    • The adaptations of the member species
    • Interactions between species.
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Complexity and Connectedness

Complexity: number of species at each trophic level and the number of trophic levels in a community.
  • Diverse community may not be very complex if all species are clustered in only a few trophic levels.
  • Diverse community may be complex if it has many interconnected trophic levels that can be compartmentalized into subdivisions.
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Resilience and Stability

Three types 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
In 1955, Robert McArthur proposed that the more complex and interconnected a community is, the more stable and resilient it will be in the face of disturbance
Minnesota ecologist David Tilman has found that plots of native prairie and recovering farm fields with high diversity were better able to withstand and recover from drought than those with only a few species.
In highly specialized ecosystems, removal of a few keystone species can eliminate many other associated species.

Ecosystem changes during succession include increases in biomass, primary production, respiration, and nutrient.

As succession changes the diversity and composition of communities, ecosystem properties change as well. In the last section, we saw how plant and animal community structure changes during primary and secondary succession. In this section, we review evidence that many ecosystem properties also change during succession. For instance, many properties of soils, such as the nutrient and organic matter content, change during the course of succession.

Ecosystem Changes at Glacier Bay

Stuart Chapin and his colleagues (1994) documented substantial changes in ecosystem structure during succession at Glacier Bay. They focused their studies in four areas of approximately 2 km2 each. Their first site had been deglaciated about 5 to 10 years and was in the pioneer stage. Their second site had been deglaciated 35 to 45 years and was dominated by a mat ofDryas. Dryas was just beginning to invade this site when it was studied by Reiners' group more than 20 years earlier. The third site had been deglaciated about 60 to 70 years and was in the Alnus stage. This site had been studied by Reiners when it was a young thicket of Alnus and by Cooper when it was in the pioneer stage. The fourth site studied by Chapin and his colleagues had been deglaciated 200 to 225 years earlier and was a forest of spruce, Picea, as it was when studied by Reiners and Cooper.

Chapin and his research team measured changes in several ecosystem characteristics across these study sites. One of the most fundamental characteristics was the quantity of soil. Total soil depth and the depth of all major soil horizons all show significant increases from the pioneer community to the spruce stage (fig. 20.10).

FIGURE 20.10 Soil building during primary succession at Glacier Bay, Alaska (data from Chapin et al. 1994).

Several other ecologically important soil properties also changed during succession at the Glacier Bay study sites. As figure 20.11 shows, the organic content, moisture, and nitrogen concentrations of the soil all increased substantially. Over the same successional sequence, soil bulk density pH, and phosphorus concentration all decreased. Why are these changes in soil properties important? They demonstrate that succession involves more than just changes in the composition and diversity of species. Terrestrial succession changes key ecosystem properties. Changes in soil properties are important because soils are the foundation upon which terrestrial ecosystems are built.

FIGURE 20.11Changesin soil properties during succession at GlacierBay, Alaska (data from Chapinetal.1994).

We can also see from these ecological studies that the physical and biological properties of ecosystems are inseparable. Organisms acting upon mineral substrates contribute to the building of soils upon which spruce forests eventually grow aroundGlacier Bay. Soils, in mm, strongly influence the kinds of organisms that grow in a place.

As we saw in chapter 19, disturbance of vegetation significantly increases the loss of nutrients from forest soils. As we shall see in the next section, succession appears to increase the retention of nutrients by forest ecosystems.

Recovery of Nutrient Retention Following Disturbance

In chapters 1 and 19, we saw how felling trees in the Hubbard Brook Experimental Forest substantially increased nutrient losses. Let's look again at this experiment by Bormann and Likens (1981) to see what it showed about succession and nutrient retention.

Briefly, Bormann and Likens monitored a control and an experimental stream catchment for 3 years prior to their experimental treatment. They then cut the forest on their experimental catchment and suppressed regrowth of vegetation with herbicides for 3 years (Likens et al. 1978). By suppressing vegetative growth, they delayed succession.

When herbicide applications were stopped, succession proceeded and nutrient losses by the forest ecosystem decreased dramatically. As you can see in figure 20.12, the herbicide suppressed vegetative growth on the experimental catchment for at least 3 consecutive years. It was during this period that the experimental catchment lost large quantities of nutrients, including calcium, potassium and nitrate.

FIGURE 20.12 Succession following deforestation and nutrient retention (data from Likens et al. 1978).

When herbicide applications stopped in 1969, Likens's group observed simultaneous increases in primary production and decreases in nutrient loss. However, the researchers point put that uptake by vegetation cannot account completely for reduced nutrient loss and that losses of calcium, potassium, and nitrate all peaked during the time when herbicide was still being applied. They suggest that some of the reduced losses during this period can be attributed to reduced amounts of these nutrients in the ecosystem. In other words, nutrient losses were reducing nutrient pools. However, vegetative uptake is clearly implicated since once succession was allowed to occur, nutrient losses from the experimental catchment declined rapidly. Though losses of nitrate returned to predisturbance levels within 4 years, calcium and potassium losses remained elevated above predisturbance levels even after 7 years of forest succession.

A Model of Ecosystem Recovery

As a result of their observations on the Hubbard Brook Experimental Forest, Bormann and Likens proposed a model for recovery of ecosystems from disturbance (fig. 20.13). Their "biomass accumulation model" divides the recovery of a forest ecosystem from disturbance into four phases: (1) a reorganization phase of 10 to 20 years, during which the forest loses biomass and nutrients, despite accumulation of living biomass; (2) an aggradation phase of more than a century, when the ecosystem accumulates biomass, eventually reaching peak biomass; (3) a transition phase, during which biomass declines somewhat from the peak reached during the aggradation phase; and (4) a steady state phase, when biomass fluctuates around a mean level.

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FIGURE 20.13 The biomass accumulation model of forest succession (data from Bormann and Likens 1981).

How well does the biomass accumulation model represent the process of forest succession? Does a similar sequence of stages occur during succession in other ecosystems? For instance, do ecosystems eventually reach a steady state? The generality of the biomass accumulation model can be tested on ecosystems, such as Sycamore Creek, Arizona, that undergo rapid succession. Such ecosystems give the ecologist the chance to study multiple successional sequences. As we will see in the following section, the patterns of ecosystem change during succession on Sycamore Creek suggest that several ecosystem features eventually reach a steady state.

Succession and Stream Ecosystem Properties

Patterns similar to those proposed by the biomass accumulation model were recorded by Fisher's research group during just 63 days of postflood succession in Sycamore Creek, Arizona. Algal biomass increased rapidly for the first 13 days following disturbance and then increased more slowly from day 13 to day 63 (fig. 20.14). Sixty-three days after the flood, algal biomass showed clear signs of leveling off. The biomass of invertebrates, the chief animal group in Sycamore Creek, increased rapidly for 22 days following the flood and then, like the algal portion of the ecosystem, began to level off.

FIGURE 20.14 Changes in biomass during stream succession (data from Fisher et al 1982).

Ecosystem metabolic parameters showed even clearer signs of leveling off before the end of the 63-day study (fig. 20.15). Gross primary production (see chapter 18), measured as grams of O2 produced per square meter per day, increased very rapidly until day 13, increased more slowly between days 13 and 48, and then leveled off between days 48 and 63. Total ecosystem respiration, measured as oxygen consumption per square meter per day, increased quickly for only 5 days after the flood and then began to level off. Respiration by invertebrates, which at its maximum represented about 20% of total ecosystem respiration, leveled off by day 63.

FIGURE 20.15 Ecosystem processes during succession in Sycamore Creek, Arizona (data from Fisher et al. 1982).

Nancy Grimm (1987) studied nitrogen dynamics in Sycamore Creek following floods that occurred from 1981 to 1983. As in the earlier studies by Fisher and his colleagues (1982), Grimm found that during succession, algal biomass and whole ecosystem metabolism quickly reached a maximum and then leveled off, as did the quantity of nitrogen in the system.

In addition, however, Grimm examined patterns of nitrogen retention during stream succession. She estimated the nitrogen budget in each of her study reaches by comparing the nitrogen inputs at the upstream end to nitrogen outputs at the downstream end. Each 60 to 120 m study reach began where subsurface flows up welled to the surface and ended downstream, where water disappeared into the sand. Grimm used the ratio of dissolved inorganic nitrogen entering the study reach in the upwelling zone to the amount leaving at the lower end as a measure of nitrogen retention by the stream ecosystem.

Figure 20.16 shows that in the early stages of succession, approximately equal amounts of dissolved inorganic nitrogen entered and left Grimm's study reaches. What do equal levels of input and output indicate regarding nutrient retention? A balance between input and output means that the most retention by the Sycamore Creek ecosystem to uptake by algae and invertebrates, since levels of nitrogen retention are consistent with the rates at which nitrogen was accumulated by algal and animal populations. What causes the stream reaches to eventually export nitrogen? Grimm suggested that at 90 days postfloodher study sites may have stopped accumulating biomass or may have even begun to lose biomass. A loss of biomass in the later stages of succession is consistent with the predictions of the Bormann and Likens biomass accumulation model.

FIGURE 20.16Nitrogen retention during stream succession (data from Grimm 1987).

The major point here is that succession, which produces changes in species composition and species diversity, also changes the structure and function of ecosystems ranging from forests to streams. However, we are left with a major question concerning this important ecological process. What mechanisms drive succession? Ecologists have proposed that the mechanisms underlying succession may fall into one of three categories.