Ecosystems and Ecology

Communities and Ecosystems

Defining Communities and Ecosystems

A community in ecology refers to all the populations of different species living and interacting within a particular area. An ecosystem, on the other hand, encompasses both the community of organisms and their physical environment, including abiotic factors like soil, water, and climate.

Example

The African savanna is an ecosystem that includes communities of lions, zebras, and acacia trees, along with the grasslands, water sources, and climate they inhabit.

Energy Flow Through Ecosystems

Energy flow in ecosystems begins with photosynthesis, where primary producers (mainly plants and some bacteria) convert solar energy into chemical energy stored in organic compounds. This energy then moves through the ecosystem via food chains and food webs.

Photosynthesis and Respiration

The basic equation for photosynthesis is:

$6CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2$

Cellular respiration is essentially the reverse of this process, where organisms break down glucose to release energy:

$C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}$

Food Chains and Food Webs

Food chains represent a linear sequence of energy transfer from one organism to another. For example:

Grass → Grasshopper → Frog → Snake → Hawk

Food webs are more complex and realistic representations of energy flow, showing multiple interconnected food chains within an ecosystem.

Trophic Levels and Ecological Pyramids

Trophic levels represent the feeding position of organisms in a food chain. The main levels are:

1. Producers (autotrophs)
2. Primary consumers (herbivores)
3. Secondary consumers (carnivores that eat herbivores)
4. Tertiary consumers (carnivores that eat other carnivores)
5. Decomposers (break down dead organic matter)

Ecological pyramids visually represent the structure of trophic levels in terms of:

• Numbers: The quantity of organisms at each level
• Biomass: The total mass of organisms at each level
• Energy: The amount of energy available at each level

Note

Energy pyramids are always upright, as energy is lost at each trophic level due to the Second Law of Thermodynamics.

Nutrient Cycling

Nutrient cycling involves the movement and recycling of nutrients within an ecosystem. Two crucial cycles are:

Carbon Cycle

The carbon cycle involves the exchange of carbon between the atmosphere, biosphere, hydrosphere, and geosphere. Key processes include:

• Photosynthesis (carbon fixation)
• Respiration
• Decomposition
• Combustion
• Weathering and erosion
• Ocean-atmosphere exchange

Nitrogen Cycle

The nitrogen cycle involves the transformation of nitrogen between various forms. Key processes include:

• Nitrogen fixation (by bacteria and lightning)
• Nitrification
• Assimilation
• Ammonification
• Denitrification

Common Mistake

Students often confuse denitrification with nitrogen fixation. Remember, denitrification converts nitrates back to atmospheric nitrogen, while nitrogen fixation converts atmospheric nitrogen into biologically available forms.

Ecosystem Productivity

Ecosystem productivity refers to the rate of biomass production within an ecosystem.

• Gross Primary Productivity (GPP): Total rate of organic compound production by producers
• Net Primary Productivity (NPP): GPP minus energy used for the plant's own respiration
• Net Secondary Productivity (NSP): Rate of biomass production by consumers

The relationship can be expressed as:

$NPP = GPP - R_p$

Where $R_p$ is the respiration of producers.

Tip

When studying ecosystem productivity, always consider limiting factors such as light, water, and nutrients that can affect the rate of production.

Species and Populations

Defining Species, Habitat, and Niche

• Species: A group of organisms capable of interbreeding and producing fertile offspring
• Habitat: The physical environment where a species lives
• Niche: The role and position a species has in its environment, including its interactions with other species and its use of resources

Example

The niche of a lion in the African savanna includes its role as a top predator, its hunting behavior, its interactions with other species (like hyenas and vultures), and its use of resources (like water holes and shade trees).

Population Dynamics and Carrying Capacity

Population dynamics refers to how population sizes change over time due to births, deaths, immigration, and emigration. The basic equation for population growth is:

$\frac{dN}{dt} = rN(1 - \frac{N}{K})$

Where:

• $N$ is the population size
• $r$ is the intrinsic growth rate
• $K$ is the carrying capacity

Carrying capacity is the maximum population size that an environment can sustain indefinitely given the available resources.

S and J Population Growth Curves

Population growth curves illustrate how populations change over time:

• S-curve (logistic growth): Shows population growth that levels off as it approaches carrying capacity
• J-curve (exponential growth): Shows rapid, unrestricted population growth

Note

Real populations often show more complex patterns than these idealized curves due to factors like environmental fluctuations and density-dependent effects.

Biomes, Zonation and Succession

Major Terrestrial and Aquatic Biomes

Biomes are large-scale ecosystems characterized by distinct climate patterns and vegetation types. Major terrestrial biomes include:

1. Tropical rainforest
2. Temperate deciduous forest
3. Coniferous forest
4. Grassland
5. Desert
6. Tundra

Major aquatic biomes include:

1. Freshwater (lakes, rivers, wetlands)
2. Marine (oceans, coral reefs, estuaries)

Each biome has unique characteristics in terms of temperature, precipitation, soil type, and dominant plant and animal species.

Zonation in Ecosystems

Zonation refers to the arrangement of plant and animal communities in distinct zones within an ecosystem, often in response to gradients in environmental factors.

Example

In a lake ecosystem, you might observe the following zones:

1. Littoral zone (shallow water near shore)

1. Limnetic zone (open water)

1. Profundal zone (deep water)

1. Benthic zone (lake bottom)

Ecological Succession

Ecological succession is the process of change in species composition of an ecological community over time.

Primary Succession

Primary succession occurs in areas where no soil or previous ecosystem exists, such as on newly formed volcanic islands or retreating glaciers. The stages typically include:

1. Pioneer species (like lichens and mosses)
2. Early colonizers (grasses and small plants)
3. Shrubs and small trees
4. Climax community (stable, self-sustaining ecosystem)

Secondary Succession

Secondary succession occurs in areas where an ecosystem has been disrupted but soil remains, such as after a forest fire or agricultural abandonment. It follows similar stages to primary succession but progresses more quickly.

r and K Species Strategies

r-selected species and K-selected species represent two ends of a reproductive strategy spectrum:

• r-selected species: Produce many offspring, short-lived, rapid maturation (e.g., bacteria, insects)
• K-selected species: Produce few offspring, long-lived, slow maturation (e.g., elephants, whales)

Tip

Remember "r" for "rapid reproduction" and "K" for "carrying capacity" to help distinguish these strategies.

Investigating Ecosystems

Sampling Techniques and Strategies

Ecologists use various sampling techniques to study ecosystems:

1. Quadrat sampling: Using a square frame to sample a small area
2. Transect sampling: Sampling along a line to study changes across gradients
3. Mark-recapture: Estimating population size by capturing, marking, releasing, and recapturing

Example

To study the distribution of plant species on a hillside, an ecologist might use a transect line running from the bottom to the top of the hill, placing quadrats at regular intervals along the line.

Measuring Abiotic and Biotic Factors

Abiotic factors that are commonly measured include:

• Temperature
• Light intensity
• Soil pH
• Humidity
• Wind speed

Biotic factors include:

• Species diversity
• Population density
• Biomass
• Growth rates

Estimating Population Size and Biodiversity

Population size can be estimated using methods like:

• Total counts (for small or easily visible populations)
• Sampling and extrapolation
• Mark-recapture method

Biodiversity is often measured using indices such as:

1. Species richness: The number of different species in an area
2. Simpson's Diversity Index: $D = 1 - \frac{\sum n(n-1)}{N(N-1)}$ Where $n$ is the number of individuals of each species and $N$ is the total number of individuals.
3. Shannon-Wiener Index: $H' = -\sum p_i \ln p_i$ Where $p_i$ is the proportion of individuals belonging to species $i$.

Use of Environmental Indicators and Indices

Environmental indicators are species or groups of species that reflect the quality of the environment. For example:

• Lichens as indicators of air pollution
• Macroinvertebrates as indicators of water quality

Indices combine multiple indicators to give an overall measure of environmental quality. For example, the Biotic Index uses the presence and abundance of certain aquatic invertebrates to assess water quality.

Note

When using environmental indicators, it's important to consider that they may be affected by factors other than the one being studied, so multiple lines of evidence are often necessary.

Flows of Energy and Matter

Solar Radiation and the Hydrological Cycle

Solar radiation drives the Earth's climate and the hydrological cycle. The hydrological cycle involves:

1. Evaporation from water bodies and transpiration from plants
2. Condensation in the atmosphere
3. Precipitation
4. Runoff and infiltration

The energy balance of the Earth can be expressed as:

$R_n = H + LE + G$

Where:

• $R_n$ is net radiation
• $H$ is sensible heat flux
• $LE$ is latent heat flux (evaporation)
• $G$ is ground heat flux

Energy Flow and the Laws of Thermodynamics Applied to Ecosystems

The First Law of Thermodynamics (conservation of energy) applies to ecosystems: energy is neither created nor destroyed, but transformed from one form to another.

The Second Law of Thermodynamics explains why energy transfer in ecosystems is inefficient: in any energy transformation, some energy is lost as heat.

Biomass Transfer Efficiency Between Trophic Levels

Biomass transfer efficiency between trophic levels is typically around 10%. This means that only about 10% of the energy available at one trophic level is transferred to the next.

This can be represented in an energy flow diagram:

Common Mistake

Students often forget that not all biomass consumed is assimilated - some passes through the organism as waste.

Carbon and Nitrogen Cycles

The carbon and nitrogen cycles, as discussed earlier, are crucial for understanding the flow of matter in ecosystems.

In the carbon cycle, key reservoirs include:

• Atmosphere (as CO₂)
• Biosphere (in living organisms)
• Hydrosphere (dissolved in water)
• Lithosphere (in rocks and fossil fuels)

In the nitrogen cycle, key processes include:

• Nitrogen fixation (atmospheric N₂ → biologically available N)
• Nitrification (NH₄⁺ → NO₂⁻ → NO₃⁻)
• Denitrification (NO₃⁻ → N₂)

Tip

When studying these cycles, focus on understanding the processes that move elements between reservoirs, and how human activities are affecting these cycles.

By understanding these fundamental concepts and processes in ecosystems and ecology, students can begin to appreciate the complexity and interconnectedness of the natural world, as well as the impacts of human activities on ecological systems.