Bio 1300, Ch 58 Communities and Ecosystems

Definition of Community

Community: a group of different species (populations) living together in a shared environment at the same time, interacting with one another and their surroundings.

Scales of Community

Can occur on various scales, from micro-communities like a single tide pool to larger communities such as forests or grasslands. These scales can include mini-communities within larger communities, highlighting the diversity of interactions at different ecological levels.

Community Ecology

The study of interactions among species and how these interactions affect community structure, dynamics, and functioning. It encompasses various types of interactions, such as competition, predation, mutualism, and parasitism, shaping the composition and organization of communities.

Ecosystem and Ecosystem Ecology

Ecosystem: the combined biotic (living organisms) community and abiotic (non-living, such as sunlight, water, and nutrients) environment affecting the community. Ecosystems can vary significantly in size and type, from small ponds to vast rainforests, and include both terrestrial and aquatic environments.

Ecosystem Ecology: focuses on the flow of energy and materials through these biotic and abiotic components. It investigates cycles such as the carbon cycle, nitrogen cycle, and water cycle, and how energy flows from one trophic level to another.

Scale

Ecosystems can occur at any scale, lacking definitive boundaries, making it challenging to delineate their edges, especially in fluid environments like estuaries or lakes.

Species Richness Patterns

Species Richness: defined as the number of different species in a community, serving as a crucial indicator of biodiversity. It offers insights into the health and stability of ecosystems.

Factors Influencing Species Richness

• Latitudinal Gradient: species richness typically increases towards the tropics. This pattern is attributed to factors such as climate stability, warmer temperatures, and longer growing seasons, with significantly fewer species present in polar regions.

• Topographical Variation: regions with complex terrain (like mountains) tend to have higher habitat diversity, promoting more microclimates and specialized niches, thus increasing species numbers.

• Peninsular Effect: species richness decreases with distance from the mainland, likely due to reduced opportunities for immigration and colonization as well as insular environmental pressures.

Hypotheses Explaining Species Richness

• Time Hypothesis: suggests that communities gain species over time, with older habitats generally showing higher species richness. Examples: Tropical regions are richer due to being less affected by glaciation compared to temperate regions, which have been periodically disturbed.

• Area Hypothesis: larger areas support more species due to larger populations and more environmental niches. Example: Relationship between species diversity and the range of host trees (species-area effect), where larger areas provide a variety of habitats and resources.

• Productivity Hypothesis: higher plant productivity supports greater species richness. Notable factors include warmth and wet conditions that enhance plant growth, yet exceptions exist (e.g., some productive tropical seas have low species richness due to various ecological reasons).

Measuring Species Diversity

Relative Abundance: frequency of occurrence of different species in a community, offering insights into community composition and health.

Species Diversity: considers both the number of species (richness) and their distribution (evenness), essential for understanding ecosystem resilience.

Shannon Diversity Index: quantifies diversity based on species proportions.
Equation: H' = - \sum (pi \cdot \ln(pi))
Indicates diversity level; values typically range from 1.5 to 3.5 in ecological studies; higher values indicate greater diversity and complexity, reflecting a more stable ecosystem.

Stability of Communities

A community is considered stable when there are minimal changes in species composition and abundance over time, offering resilience against disturbances.

Elton's Diversity-Stability Hypothesis: proposes that disturbances have less impact on species-rich communities due to complex interactions among species creating a buffer against changes.

Research Findings: Studies show lower variance in biomass in species-rich plots, highlighting greater stability and a community's ability to withstand environmental stressors and changes.

Succession: Community Change

Succession: gradual transformation in species composition over time, reflecting ecological development and recovery.

• Primary Succession: begins on newly formed areas without soil (e.g., volcanic islands), involving pioneer species that can tolerate harsh conditions.

• Pioneer Species: first organisms to inhabit a barren environment, often lichens or mosses, which contribute to soil formation over time.

• Secondary Succession: occurs in areas where soil is present but life has been disturbed (e.g., after a forest fire), typically resulting in faster recovery due to existing soil and seed banks.

• Climax Community: the final stage of succession characterized by stable communities that remain relatively unchanged until disrupted.

Key concept of facilitation: early species modify the environment, making it more suitable for later species, leading to a diverse and complex community over time.

Island Biogeography

Studies species richness on islands, influenced by size and distance from species pools (mainland), providing crucial insights into conservation and biodiversity.

Equilibrium Model: developed by MacArthur and Wilson, predicts that species numbers on islands tend toward an equilibrium based on immigration and extinction rates, integrating both ecological theory and practical conservation strategies.

Predictions: Species numbers should increase with island size (providing more niches) and decrease with distance from the mainland (reducing access to new immigrant species).

Energy Flow and Food Webs

Food Chain: simple linear path of energy flow through trophic levels, illustrating direct energy transfer between species.

Trophic Levels

Feeding levels in a food chain:

• Producers → Primary Consumers → Secondary Consumers → Tertiary Consumers, showcasing the roles different organisms play in energy transfer.

Food Web

Complex interconnections of multiple food chains showing the flow of energy through an ecosystem, emphasizing the importance of biodiversity for stability.

Second Law of Thermodynamics

Energy transfers are inefficient, leading to energy loss at each trophic level, usually expressed as a 90% loss, affecting the energy available to higher trophic levels.

Productivity and Biomass

Gross Primary Productivity (GPP): total carbon fixed through photosynthesis, serving as the foundation of energy flow in ecosystems.

Net Primary Productivity (NPP): energy available to consumers; calculated as NPP = GPP - R (where R = energy lost during respiration), crucial for understanding the energy budget of ecosystems.

Factors influencing primary production include precipitation, temperature, and nutrient availability (e.g., nitrogen and phosphorus), which together determine the productivity of different ecosystems.

Liebig’s Law of the Minimum: productivity is limited by the scarcest resource, highlighting the importance of key nutrients in determining ecosystem health and productivity.

Lecture Questions

58.1

•What is a community? What does a community ecologist study?

• Community: A community is a group of different species (populations) that live together in the same area and interact with one another.

• Community Ecologist: Studies how species interact (e.g., competition, predation, mutualism), how those interactions affect community structure, and how communities change over time (succession, disturbances, etc.).

•What is an ecosystem? What does an ecosystem ecologist study?

• Ecosystem: An ecosystem includes all the living organisms (biotic) in a given area plus the non-living (abiotic) factors (climate, water, soil, nutrients) they interact with.

• Ecosystem Ecologist: Focuses on energy flow and nutrient cycling—how energy enters (e.g., via photosynthesis), moves through (via food webs), and exits an ecosystem, and how matter (like carbon or nitrogen) cycles through systems.

•What are the 3 main hypotheses to explain changes in species richness? What does each hypothesis predict? Support/problems with each hypothesis?

Time Hypothesis

• Prediction: Older habitats have had more time for species to evolve and diversify, so they have higher species richness.

• Support: Tropics (older, less disturbed) have high species richness.

• Problems: Some younger ecosystems (e.g., coral reefs) are also very rich.

2. Productivity Hypothesis

• Prediction: More energy (e.g., via sunlight or primary productivity) = more resources = supports more species.

• Support: Tropical rainforests and coral reefs have high productivity and richness.

• Problems: Not all productive systems (like some agricultural fields) have high species diversity.

3. Intermediate Disturbance Hypothesis

• Prediction: Moderate disturbance levels lead to highest species richness (not too stable, not too chaotic).

• Support: Some experimental and natural studies show peak diversity at intermediate disturbance.

• Problems: Can be difficult to define or measure what counts as "intermediate"; doesn’t apply everywhere.

•What is the difference between species abundance and species diversity? What is the Shannon index? What does the value of the index indicate?

• Species Abundance: The number of individuals per species in a community.

• Species Diversity: Combines richness (number of species) and evenness (how evenly individuals are distributed among species).

Shannon Index (H')

• A mathematical measure of species diversity that accounts for both richness and evenness.

• Formula:
  H′=−∑(pi⋅ln⁡pi)H' = -\sum (p_i \cdot \ln p_i)H′=−∑(pi​⋅lnpi​)
  where pip_ipi​ = proportion of individuals in the ith species

Interpretation:

• Higher H' = more diversity (more species and/or more even distribution)

• Lower H' = less diversity (fewer species and/or one dominant species)

58.2

•What indicates that a community is stable? What is Elton's diversity-stability hypothesis? What is the relationship between biomass and species richness?

A stable community resists change or quickly returns to its original state after a disturbance. Stability includes two key components:

1. Resistance – the ability of a community to remain unchanged when under stress (e.g., drought, fire, invasion).

2. Resilience – the ability to bounce back after a disturbance.

Indicators of stability:

• Species composition stays relatively constant over time.

• Energy flow and nutrient cycling remain balanced.

• Recovery after disturbances is quick and complete.

Charles Elton (1958) proposed that:

The idea:

• Communities with more species have more complex food webs and more functional redundancy (i.e., if one species declines, others can fill its role).

• This buffers the ecosystem against disturbances, preventing collapse.

Support:

• Some experimental studies (e.g., grassland plots) have shown that more diverse systems resist pest outbreaks or recover faster after drought.

Criticisms:

• Not always true: in some ecosystems, more diversity can lead to more fluctuation, depending on species interactions.

• Stability depends on which species are present, not just how many.

Generally:

Why?

• Niche complementarity: More species = better use of resources (e.g., deep-rooted plants + shallow-rooted ones use more soil nutrients).

• Facilitation: Some species improve conditions for others (e.g., nitrogen-fixing plants).

• Sampling effect: Higher diversity = greater chance of having a very productive "superstar" species.

Real-world example:

• In plant communities, more diverse plots usually produce more biomass than monocultures (single-species plots).

58.3

•What is succession? What is the difference between primary and secondary succession? Examples?

Succession is the natural, gradual process of change in the structure and composition of a community over time, especially after a disturbance.

It involves changes in species composition, biodiversity, and ecosystem function as communities progress from simpler to more complex stages.

🔹 Primary Succession:

• Starts from bare, lifeless areas where there’s no soil (just rock, lava, or glacial retreat).

• Organisms must build soil before other life can establish.

• Takes a long time (hundreds to thousands of years).

• Pioneer species: lichens, mosses, microbes.

Examples:

• After a volcanic eruption (e.g., Mount St. Helens, 1980)

• Glacial retreat zones

🔹 Secondary Succession:

• Occurs after a disturbance, but soil is still intact.

• Faster than primary succession.

• Vegetation regrows, often from seeds, roots, or nearby sources.

Examples:

• After forest fires

• Abandoned farmland (old-field succession)

• Hurricane or flood recovery areas

•What is a climax community?

A climax community is a stable, mature ecological community that has reached the final stage of succession.

• Species composition is stable over time.

• Still experiences change (seasons, small disturbances), but overall structure remains.

• Example: A mature oak-hickory forest in the eastern U.S.

•What are the 3 theories for how succession can take place? What is the difference between each theory? Where is each theory typically seen/examples?

These theories explain how new species interact with existing ones during succession:

1. Facilitation Model

• Earlier species make the environment more suitable for later species.

• Succession is a one-way path toward a climax.

• Common in primary succession.

Example:

• Lichens break down rock, creating soil → grasses grow → shrubs → trees.

2. Inhibition Model

• Early species prevent or slow the establishment of later species.

• Succession only continues when disturbance or death removes earlier species.

• Seen in rocky intertidal zones or places with intense competition.

Example:

• Algae or barnacles dominate a surface and block others from colonizing until they die.

3. Tolerance Model

• All species can colonize early, but later species are better competitors and gradually dominate.

• Early species don’t help or hurt later ones—they just coexist until outcompeted.

• Seen in secondary succession or forests.

Example:

• Fast-growing weeds appear after a disturbance, but over time, slower-growing trees outcompete them for light.

🔄 Summary Chart:

58.4

•How do we use island biogeography in succession?

Island Biogeography Theory isn’t just about literal islands—it helps us understand how species colonize new or disturbed areas, including successional landscapes (like after a fire or glacier retreat).

Here’s how it ties into succession:

• Newly disturbed areas (bare soil, lava flows, etc.) act like “islands” of available habitat in a “sea” of established ecosystems.

• Colonization and extinction rates affect how species arrive and establish during succession.

• Just like islands, distance from source populations and patch size affect species richness.

In short: Island biogeography helps predict:

• Which species arrive first

• How many species establish

• How quickly biodiversity builds in a successional area

Example:

• After a volcanic eruption, plant species colonizing the lava field follow similar patterns as species colonizing a remote island.

•What is the equilibrium model? What are the 3 predictions from the equilibrium model regarding species populations on islands? Examples?

1. Larger islands have more species

• Why? Lower extinction rates due to more resources, habitats, and populations.

• Example: Big islands like Borneo have more biodiversity than small nearby islands.

2. Islands closer to the mainland have more species

• Why? Higher immigration rates—it's easier for species to arrive.

• Example: Caribbean islands closer to South America tend to have more species than farther ones.

3. Species richness reaches a stable equilibrium over time

• Even though species may change (turnover), the total number of species balances out due to constant immigration and extinction.

58.5

•What is the difference between a food chain and a food web?

🔍 Summary:

• Food chain = simple, direct line

• Food web = realistic, complex map of ecosystem feeding relationships

•What is the difference between an autotroph and a heterotroph? What position does each fill in a food chain/web?

Autotroph (Producer)

• Makes its own food, usually via photosynthesis

• Doesn’t rely on other organisms for energy

• Position: Base of food chain/web

Examples: Plants, algae, phytoplankton

✅ Heterotroph (Consumer)

• Gets energy by eating other organisms (can’t make its own food)

• Position: Above producers in the food chain/web

Types:

• Herbivores – eat plants (primary consumers)

• Carnivores – eat other animals (secondary/tertiary consumers)

• Omnivores – eat both

• Decomposers/Detritivores – break down dead material (fungi, bacteria, earthworms)

•What is the second law of thermodynamics? How does it come into play in food webs?

"Every energy transfer increases the entropy (disorder) of the universe."
In simpler terms: Energy is lost as heat every time it's transferred or transformed.

• Only ~10% of energy is transferred from one trophic level to the next (this is the 10% rule).

• The rest is lost as heat through metabolism, movement, reproduction, etc.

This explains why:

• Food chains/webs rarely go beyond 4–5 levels

• Top predators are few in number (limited energy supports them)

•What are the different types of ecological pyramids?

These are visual models that represent different aspects of trophic levels:

58.6

•What is gross primary productivity? How does it differ from net primary productivity?

Gross Primary Productivity (GPP) is:

🟡 Think of GPP as the total paycheck that plants earn from the sun.

Net Primary Productivity (NPP) is:

📊 Formula:

NPP=GPP−Respiration (R)

🟢 NPP is the energy available to consumers (herbivores, decomposers, etc.).

🧠 Analogy:

• GPP = your total salary

• Respiration = bills

• NPP = what’s left to spend or save (what supports the rest of the food web)

•What factors influence primary production in terrestrial systems? What about in aquatic systems?

✅ In Terrestrial Systems:

Key limiting factors include:

• Light – available solar radiation (varies with latitude, canopy cover)

• Temperature – warmer temps boost enzymatic activity (to a point)

• Water – more moisture = more photosynthesis

• Nutrients – especially nitrogen and phosphorus

🌿 Most productive: Warm, wet, nutrient-rich areas (e.g., tropical rainforests)
🌵 Less productive: Dry or cold areas (deserts, tundra)

✅ In Aquatic Systems:

Main limiting factors:

• Light penetration – photosynthesis only occurs in the photic zone

• Nutrient availability – especially nitrogen and phosphorus

• Temperature – influences metabolic rates and water mixing

🌊 Most productive: Coastal zones & upwelling areas (nutrient-rich)
💧 Less productive: Open ocean (low nutrients despite large area)

•Where does primary productivity tend to be the greatest on earth? The lowest?

🔼 Highest Primary Productivity:

• Terrestrial:
  🌴 Tropical rainforests – high rainfall, warm temps, year-round growing season

• Aquatic:
  🌊 Estuaries, coral reefs, and upwelling zones – high nutrients and light

🔽 Lowest Primary Productivity:

• Terrestrial:
  ❄ Tundra (cold, short growing season)
  🏜 Deserts (low water availability)

• Aquatic:
  🌐 Open ocean – low nutrients despite its massive area