C - Interaction and Interdependence

Community Structure

Limiting Factors and Species Distribution

Limiting factors play a crucial role in determining where species can survive and thrive. These factors can be abiotic (non-living) or biotic (living) and include:

• Temperature
• Water availability
• Soil pH
• Predation pressure
• Competition for resources

Example

In a desert ecosystem, water availability is often the primary limiting factor. Cacti have adapted to survive in these harsh conditions, while many other plant species cannot.

The distribution of species is not random but is shaped by these limiting factors. This concept is fundamental to understanding biogeography and ecological niches.

Keystone Species

Keystone species have a disproportionately large effect on their environment relative to their abundance. They play a critical role in maintaining the structure of an ecological community and affect many other organisms in an ecosystem.

Note

The removal of a keystone species can lead to dramatic changes in ecosystem structure and potentially a collapse of the entire system.

Examples of keystone species include:

1. Sea otters in kelp forests
2. Wolves in Yellowstone National Park
3. Beavers in riparian ecosystems

Species Interactions

Species interactions are classified based on the nature of their relationship:

1. Competition ($-/-$): Both species are negatively affected
2. Predation ($+/-$): One species benefits, the other is harmed
3. Mutualism ($+/+$): Both species benefit
4. Commensalism ($+/0$): One species benefits, the other is unaffected
5. Amensalism ($-/0$): One species is harmed, the other is unaffected
6. Parasitism ($+/-$): Similar to predation, but the relationship is typically long-term

Example

Lichens are an example of mutualism between fungi and algae. The fungus provides structure and protection, while the algae produce food through photosynthesis.

Competitive Exclusion Principle

The competitive exclusion principle states that two species competing for the same limiting resource cannot coexist indefinitely. Over time, the more competitive species will exclude the other.

This principle is expressed mathematically using the Lotka-Volterra competition model:

$$ \frac{dN_1}{dt} = r_1N_1\left(1 - \frac{N_1 + \alpha_{12}N_2}{K_1}\right) $$

$$ \frac{dN_2}{dt} = r_2N_2\left(1 - \frac{N_2 + \alpha_{21}N_1}{K_2}\right) $$

Where:

• $N_1$ and $N_2$ are the population sizes of species 1 and 2
• $r_1$ and $r_2$ are their respective growth rates
• $K_1$ and $K_2$ are their carrying capacities
• $\alpha_{12}$ and $\alpha_{21}$ are the competition coefficients

Common Mistake

Students often confuse the competitive exclusion principle with the idea that species never coexist. In reality, species can coexist through niche differentiation or in non-equilibrium conditions.

Ecosystems

Trophic Levels and Food Webs

Trophic levels represent the feeding position of organisms in a food chain. A typical sequence includes:

1. Producers (autotrophs)
2. Primary consumers (herbivores)
3. Secondary consumers (carnivores)
4. Tertiary consumers (top predators)
5. Decomposers

Food webs are more complex representations of feeding relationships in an ecosystem, showing multiple interconnected food chains.

Energy Flow Through Ecosystems

Energy flow in ecosystems follows the laws of thermodynamics. The primary source of energy is typically sunlight, which is converted to chemical energy by producers through photosynthesis.

Energy pyramids illustrate the decrease in available energy at each trophic level:

Note

Only about 10% of the energy is transferred from one trophic level to the next, due to energy loss through heat, movement, and undigested material.

Nutrient Cycling

Nutrient cycling involves the movement and recycling of nutrients within an ecosystem. Key biogeochemical cycles include:

1. Carbon cycle
2. Nitrogen cycle
3. Phosphorus cycle
4. Water cycle

These cycles ensure that essential elements are continuously available to organisms in the ecosystem.

Succession and Climax Communities

Ecological succession is the process of change in the species structure of an ecological community over time. There are two main types:

1. Primary succession: Occurs in lifeless areas where there is no soil (e.g., after a volcanic eruption)
2. Primary succession: Occurs in areas where an ecosystem has been disrupted but soil remains (e.g., after a forest fire)

Example

After a glacier retreats, primary succession begins with pioneer species like lichens and mosses, gradually leading to more complex plant and animal communities.

A climax community is the final stage of succession, reaching a stable state where species composition remains relatively constant over time.

Disturbances and Ecosystem Changes

Ecosystems are dynamic and can be affected by various disturbances, both natural and anthropogenic:

• Natural disturbances: Fires, floods, hurricanes
• Anthropogenic disturbances: Deforestation, pollution, climate change

These disturbances can reset succession, alter species composition, or lead to regime shifts in ecosystems.

Human Impacts on Ecosystems

Invasive Species

Invasive species are non-native organisms that, when introduced to a new environment, can cause ecological or economic harm. They often lack natural predators in their new habitat, allowing them to outcompete native species.

Example

The introduction of cane toads in Australia has had devastating effects on native wildlife, as the toads are toxic to many predators.

Biomagnification of Pollutants

Biomagnification is the process by which pollutants, particularly persistent organic pollutants (POPs), become more concentrated as they move up the food chain.

This process is particularly problematic for top predators and humans, as we accumulate high levels of these toxins.

Plastic Pollution

Plastic pollution has become a major global issue, affecting terrestrial, freshwater, and marine ecosystems. Microplastics (plastic particles

< 5mm in size) are of particular concern due to their ability to enter food chains.

Note

Recent studies have found microplastics in human blood and organs, highlighting the pervasive nature of this pollution.

Conservation Efforts

Conservation efforts aim to protect biodiversity and ecosystem functions. These can be categorized into:

1. In situ conservation: Protecting species in their natural habitats (e.g., national parks, marine protected areas)
2. Ex situ conservation: Protecting species outside their natural habitats (e.g., zoos, seed banks, botanical gardens)

Tip

When studying conservation efforts, consider both their ecological impacts and socioeconomic factors that may affect their success.

Indicator Species and Biodiversity Indices

Indicator species are organisms whose presence, absence, or abundance reflects a specific environmental condition. They are used to monitor ecosystem health.

Biodiversity indices are mathematical measures of species diversity in a community. Common indices include:

1. Shannon-Wiener Index: $H' = -\sum_{i=1}^{R} p_i \ln p_i$
2. Simpson's Index: $D = 1 - \sum_{i=1}^{R} p_i^2$

Where $p_i$ is the proportion of individuals belonging to species $i$, and $R$ is the number of species.

Population Ecology

Sampling Techniques

Various sampling techniques are used to estimate population size:

1. Quadrat sampling: Used for sessile organisms or slow-moving species
2. Transect sampling: Used for studying distribution along environmental gradients
3. Mark-recapture: Used for mobile animals

The Lincoln-Petersen method for mark-recapture:

$$ N = \frac{MC}{R} $$

Where:

• $N$ is the estimated population size
• $M$ is the number of individuals marked in the first capture
• $C$ is the total number of individuals in the second capture
• $R$ is the number of marked individuals in the second capture

Population Growth Patterns

Population growth can follow two main patterns:

1. Exponential growth: $\frac{dN}{dt} = rN$
2. Logistic growth: $\frac{dN}{dt} = rN\left(1 - \frac{N}{K}\right)$

Where:

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

Example

Bacterial growth in a petri dish often follows exponential growth initially, before transitioning to logistic growth as resources become limited.

Factors Affecting Population Growth

Population growth is influenced by:

1. Natality (birth rate)
2. Mortality (death rate)
3. Immigration (individuals entering the population)
4. Emigration (individuals leaving the population)

These factors are summarized in the equation:

$$ \Delta N = (B - D) + (I - E) $$

Where $\Delta N$ is the change in population size.

Carrying Capacity

Carrying capacity ($K$) is the maximum population size that an environment can sustain indefinitely. It is determined by resource availability and environmental conditions.

Top-down and Bottom-up Limiting Factors

Population growth can be limited by:

1. Top-down factors: Predation, disease
2. Bottom-up factors: Resource availability (food, water, space)

Note

The relative importance of top-down vs. bottom-up factors can vary between ecosystems and may change over time.

Nutrient Cycles

Nitrogen Cycle

The nitrogen cycle involves the transformation of nitrogen between various chemical forms:

1. Nitrogen fixation (N₂ → NH₃)
2. Nitrification (NH₃ → NO₂⁻ → NO₃⁻)
3. Assimilation (incorporation into organic compounds)
4. Ammonification (organic N → NH₃)
5. Denitrification (NO₃⁻ → N₂)

Phosphorus Cycle

The phosphorus cycle is primarily sedimentary, with the main reservoir being rock and sediment. Key processes include:

1. Weathering of rocks
2. Uptake by plants
3. Transfer through food chains
4. Decomposition and return to soil
5. Sedimentation in aquatic systems

Human Impacts on Nutrient Cycles

Human activities have significantly altered nutrient cycles, particularly through:

1. Fertilizer use: Increasing nitrogen and phosphorus inputs to ecosystems
2. Fossil fuel combustion: Releasing reactive nitrogen into the atmosphere
3. Deforestation: Disrupting nutrient storage and cycling in terrestrial ecosystems

Common Mistake

Students often overlook the interconnectedness of nutrient cycles. For example, increased nitrogen inputs can lead to phosphorus limitation in some ecosystems.

Eutrophication, the excessive enrichment of water bodies with nutrients, is a major consequence of human alterations to nutrient cycles. It can lead to:

• Algal blooms
• Oxygen depletion (hypoxia)
• Fish kills
• Loss of biodiversity

Tip

When studying human impacts on ecosystems, always consider both direct effects and potential cascading effects through food webs and biogeochemical cycles.