Community Ecology and Conservation Biology: Key Concepts
Chapter 15: Community Ecology
15.1: Defining Communities
Communities are groups of interacting species that occur together at the same place and time. They can be defined by their physical environment or by biological characteristics, such as the presence of abundant species.
Due to the impracticality of studying all species within a community, ecologists often focus on subsets like taxonomic groups, guilds, functional groups, and food/interaction webs.
15.2: Community Structure
Species diversity, a key measure of community structure, combines species richness (number of species) and species evenness (relative abundance of each species).
Communities can have the same species richness but different evenness. For example, one might have a few dominant species and many rare ones, while another might have equally abundant species.
Species composition, the identity of species present, is another crucial aspect of community structure.
Sampling effort influences species richness estimates; however, after a certain point, additional sampling yields few new species.
15.3: Species Interactions
Communities are characterized by complex networks of direct and indirect species interactions, varying in strength and direction.
Indirect interactions, where a relationship between two species is mediated by a third (or more), can significantly impact direct interactions.
Dominant species exert strong effects due to their abundance or biomass, while keystone species have significant impacts due to their roles within the community.
Ecosystem engineers create, modify, or maintain physical habitats.
The environmental context can modify species interactions, potentially altering their outcomes.
Chapter 16: Community Dynamics
16.1: Agents of Change
Agents of change, both abiotic and biotic, act on communities across various temporal and spatial scales.
Abiotic agents can be disturbances (injuring or killing organisms) or stresses (reducing growth or reproduction).
Biotic agents include negative interactions like competition, predation, and trampling. Ecosystem engineers and keystone species are common biotic agents of change.
Agents of change vary in intensity, frequency, and extent.
16.2: Succession
Succession is the process of change in species composition over time due to abiotic and biotic agents.
Theoretically, it progresses through stages towards a stable climax stage.
Primary succession occurs in lifeless habitats, while secondary succession happens where most, but not all, organisms or organic matter have been destroyed.
Early ecologists debated whether succession was deterministic or random.
Connell and Slatyer proposed three models: facilitation, tolerance, and inhibition.
16.3: Mechanisms of Succession
Research shows that succession is diverse and context-dependent, with no single model universally applicable.
Elements of all three models (facilitation, tolerance, inhibition) are often observed in various systems.
Generally, facilitation is important in early stages, while competition dominates later stages.
16.4: Alternative Stable States
Communities can follow different successional paths and exhibit alternative stable states, where different communities develop under similar conditions.
Strong interactors typically control succession in such communities.
Human activities can cause potentially irreversible regime shifts.
Chapter 17: Biogeography
17.1: Spatial Scales of Diversity
Species diversity and distribution patterns vary across global, regional, and local scales.
Biogeography studies this variation among geographic locations.
These patterns are hierarchically connected.
The global scale encompasses the entire world, with significant variations in climate, diversity, and composition.
The regional scale covers a smaller, climatically uniform area, limited by dispersal.
The local scale represents the smallest area, essentially equivalent to a community.
Beta diversity measures species turnover across the landscape.
Regional species pools largely determine local diversity, but local conditions also play a crucial role.
17.2: Global Patterns
Global patterns are influenced by geographic area and isolation, evolutionary history, and climate.
Earth’s landmass is divided into six biogeographic regions with distinct diversity and composition.
These biotas reflect historical isolation due to continental drift.
Vicariance provided evidence for early evolutionary theories.
Species diversity peaks in the tropics and declines towards higher latitudes.
Hypotheses explaining this latitudinal gradient involve diversification rate, time, and productivity.
17.3: Regional Differences
Regional diversity is influenced by area and distance, affecting immigration and extinction rates.
Species richness generally increases with area and decreases with distance from the source.
Island biogeography theory predicts that the balance between immigration and extinction controls diversity on islands or island-like areas.
Larger, closer islands have higher diversity due to higher immigration and lower extinction.
Similar patterns exist on mainlands, but the rate of increase with area is lower.
Chapter 18: Community Diversity
18.1: Factors Influencing Diversity
Regional species pools and dispersal abilities influence community membership.
Humans have expanded regional pools by introducing non-native species.
Local abiotic conditions act as a filter.
Species reliant on others for survival require those species’ presence.
Competition, predation, parasitism, and disease can exclude species.
18.2: Resource Partitioning
Resource partitioning reduces competition and enhances diversity.
Species must utilize resources differently to avoid competitive exclusion.
Less resource overlap allows for greater coexistence.
The resource ratio hypothesis suggests species can coexist by using resources in different proportions.
18.3: Promoting Coexistence
Disturbance, stress, predation, and positive interactions can mediate resource availability, promoting coexistence and diversity.
Fluctuations in populations due to these factors free up resources.
The intermediate disturbance hypothesis proposes that intermediate levels of these factors maximize diversity.
The dynamic equilibrium model suggests highest diversity when disturbance and competitive displacement are balanced.
Positive interactions can also promote diversity, especially under intermediate to high disturbance/stress/predation.
The Menge-Sutherland model separates predation from physical disturbance.
Lottery models assume random resource capture by recruits after disturbance/stress/predation.
18.4: Diversity and Function
Studies show a positive relationship between species diversity and community function.
Diversity influences productivity, soil fertility, water quality, gas exchange, and disturbance responses.
Hypotheses explaining this relationship consider functional overlap and variation among species.
Chapter 19: Ecosystem Production
19.1: Primary Production
Ecosystem energy originates from primary production by autotrophs.
Gross primary production (GPP) is the total carbon fixed.
GPP depends on photosynthesis rate and leaf area index.
Net primary production (NPP) is GPP minus autotroph respiration.
NPP changes during succession due to shifts in leaf area index and tissue balance.
Various methods measure NPP across different scales.
19.2: Constraints on NPP
Physical and biotic factors constrain NPP.
Terrestrial NPP varies with temperature and precipitation, affecting resource availability and plant types.
Plant growth rates influence spatial NPP variation and resource response.
Aquatic NPP is controlled by nutrient supply, especially phosphorus and nitrogen.
19.3: Global NPP Patterns
Global NPP patterns reflect climatic constraints and biome types.
Terrestrial and oceanic NPP contribute roughly equally.
Most terrestrial NPP occurs in the tropics.
Biome differences in NPP reflect variations in leaf area index and growing season length.
While upwelling and coastal zones have high NPP, the vast open ocean contributes the most due to its size.
19.4: Secondary Production
Heterotrophs generate secondary production by consuming organic matter.
They derive energy from live or dead matter.
Stable isotope analysis can determine heterotroph diets.
Net secondary production is energy ingested minus respiration and waste.
Chapter 20: Energy Flow
20.1: Trophic Levels
Trophic levels describe organisms’ feeding positions in ecosystems.
They are determined by the number of feeding steps from the first level (autotrophs and detritus).
Omnivores feed at multiple levels.
All organisms eventually become food or detritus.
20.2: Energy Transfer
Energy transfer between trophic levels depends on food quality, consumer abundance, and physiology.
Energy and biomass pyramids depict relative amounts at different levels.
High autotroph turnover in aquatic systems can invert biomass pyramids.
Terrestrial autotroph biomass consumption is generally lower than in aquatic systems.
Transfer efficiency depends on food quality and consumer physiology.
20.3: Trophic Cascades
Changes in abundance at one level can impact energy flow across multiple levels.
Consumer changes at higher levels can influence primary production via herbivore consumption.
Trophic cascades are more apparent in aquatic systems but also occur in complex terrestrial ones.
The number of sustainable trophic levels depends on energy input, disturbance frequency, and ecosystem size.
20.4: Food Webs
Food webs model trophic interactions within ecosystems.
They diagram complex interactions among species.
Simplification can focus on the strongest interactions.
Keystone species have disproportionately large effects on energy flow and composition.
Indirect predator effects can offset or reinforce direct effects, potentially stabilizing food webs.
Chapter 21: Nutrient Cycling
21.1: Nutrient Entry
Nutrients enter ecosystems through mineral breakdown or atmospheric gas fixation.
Nutrient requirements are specific to organism physiology.
Autotrophs absorb simple, soluble nutrients, while heterotrophs obtain complex forms through consumption.
Weathering releases soluble nutrients.
Soils comprise mineral particles, detritus, dissolved organic matter, water, and organisms.
Carbon and nitrogen enter via fixation by autotrophs and bacteria, respectively.
21.2: Nutrient Transformations
Chemical and biological transformations alter nutrient forms and supply.
Decomposition releases nutrients in usable forms.
Microbial modification of nutrient forms, especially nitrogen, affects availability and loss.
Plants recycle nutrients through reabsorption and remobilization.
21.3: Nutrient Cycling
Nutrients cycle repeatedly through ecosystem components.
Cycling rates depend on decomposition, influenced by climate and litter chemistry.
Nutrient loss from terrestrial systems can be estimated via stream water output.
The balance between weathering and decomposition-derived nutrients determines which limit primary production.
21.4: Aquatic Nutrient Inputs
Freshwater and marine systems receive nutrients from terrestrial sources.
Stream and river nutrient cycling resembles a spiral of uptake, incorporation, and release.
Lake nutrients cycle between water and sediment.
River and terrestrial inputs support marine production.
Chapter 22: Conservation Biology
22.1: Introduction
Conservation biology applies ecological principles to protect biodiversity.
It studies factors affecting biodiversity maintenance, loss, and restoration.
Biodiversity is crucial for human society due to our reliance on natural resources and ecosystem services.
Growing awareness of biodiversity loss led to the emergence of this applied discipline.
Conservation biology values biodiversity.
22.2: Biodiversity Decline
Global biodiversity is declining at an accelerating rate due to human impact.
Extinction is the endpoint of biological decline.
Earth’s biota is becoming homogenized due to the rise of generalists and decline of specialists, along with genetic diversity loss.
22.3: Threats to Biodiversity
Major threats include habitat loss, invasive species, overexploitation, pollution, disease, and climate change.
Habitat degradation, fragmentation, and loss are the most significant threats.
Invasive species disrupt habitats and native species.
Overexploitation impacts communities and ecosystems.
Pollution, disease, and climate change further erode population viability.
22.4: Managing Declining Populations
Conservation biologists employ various tools and work across multiple scales.
Genetic analyses inform management, identify units, and aid forensic investigations.
Population viability analysis (PVA) assesses extinction risks and evaluates management actions.
Ex situ conservation is a last-resort measure for critically endangered species.
Laws, policies, and treaties are essential for protecting species and habitats.
22.5: Prioritizing Species
Prioritization helps maximize protection with limited resources.
Rarest and fastest-declining species are prioritized based on population size, range, decline rate, and threats.
Surrogate species can indirectly protect others with similar habitat needs.
Chapter 23: Landscape Ecology
23.1: Introduction
Landscape ecology examines spatial patterns and their relationship to ecological processes.
A landscape is a heterogeneous area composed of interacting components.
Landscapes are characterized by their composition and structure.
Landscape patterns influence organism movement, ecosystem properties, and disturbance regimes.
23.2: Habitat Loss and Fragmentation
Habitat loss and fragmentation reduce area, isolate populations, and alter edge conditions.
Fragments are less diverse than the original habitat.
Isolation can restrict organism movement.
Fragment edges have different abiotic conditions and population dynamics compared to interiors.
Fragmentation impacts the evolutionary process.
23.3: Sustaining Biodiversity
Large, connected reserves buffered from human use are best for sustaining biodiversity.
Ideal core areas are large, compact, and connected or close to other protected areas.
Buffer zones allow compatible human use around core areas.
Habitat corridors facilitate movement between natural areas.
Ecological restoration helps degraded areas support native species and processes.
23.4: Ecosystem Management
Ecosystem management prioritizes long-term ecological integrity through collaboration.
Stakeholder collaboration is crucial for effective plans.
It involves setting goals, implementing policies, monitoring effectiveness, and adapting plans.
Recognizing humans as integral parts of ecosystems is key for long-term success.