🦉Intro to Ecology Unit 6 – Community Structure and Diversity

Key Concepts and Definitions

• Community refers to all the interacting populations of different species living in a specific area or habitat
• Biodiversity encompasses the variety of life at all levels of biological organization, including genetic, species, and ecosystem diversity
• Species richness measures the number of different species present in a community, while species evenness describes the relative abundance of each species
• Interactions between species in a community can be classified as competition, predation, parasitism, mutualism, or commensalism
• Niche refers to the role and position of a species within its environment, including its relationships with other species and its use of resources
• Keystone species have a disproportionately large influence on the structure and function of their communities relative to their abundance (sea otters in kelp forests)
• Foundation species play a critical role in creating and maintaining the physical structure of a community (corals in coral reefs)

Community Composition and Interactions

• Community composition describes the species present in a community and their relative abundances
• Interspecific interactions shape community structure and dynamics, influencing species coexistence, diversity, and ecosystem function
• Competition occurs when species vie for limited resources, leading to reduced growth, survival, or reproduction (two plant species competing for light)
  • Competitive exclusion principle states that two species with identical niches cannot coexist indefinitely
  • Resource partitioning allows species to coexist by utilizing resources differently, reducing direct competition
• Predation involves one species (predator) consuming another (prey), affecting population dynamics and community structure (lions preying on zebras)
  • Predators can regulate prey populations and maintain species diversity by preventing any one species from dominating the community
• Parasitism is a relationship in which one species (parasite) benefits at the expense of another (host), often resulting in reduced host fitness (tapeworms in the intestines of mammals)
• Mutualism is an interaction in which both species benefit (pollination of flowers by bees)
• Commensalism is an interaction in which one species benefits while the other is unaffected (barnacles growing on whales)

Measuring Biodiversity

• Alpha diversity refers to the diversity within a single community or habitat, often measured by species richness and evenness
• Beta diversity describes the change in species composition between communities along an environmental gradient or across different habitats
• Gamma diversity represents the overall diversity of a region, considering both alpha and beta diversity
• Species richness can be quantified using various methods, such as quadrat sampling, transect sampling, or species-area curves
  • Rarefaction curves help compare species richness between communities with different sampling efforts
• Species evenness is commonly measured using indices like the Shannon diversity index or the Pielou evenness index
  • These indices account for both species richness and the relative abundance of each species
• Functional diversity considers the range of ecological roles and traits present in a community, rather than just species numbers
• Phylogenetic diversity measures the evolutionary history and relatedness of species within a community

Factors Influencing Community Structure

• Abiotic factors, such as temperature, precipitation, soil type, and topography, shape community composition and structure
  • Environmental gradients often lead to predictable changes in community composition (altitudinal zonation on mountains)
• Biotic interactions, including competition, predation, and facilitation, play a crucial role in determining community structure
  • Herbivory can alter plant community composition by selectively consuming certain species
• Historical factors, such as past disturbances, dispersal events, and evolutionary history, influence current community structure
• Habitat heterogeneity and complexity promote species diversity by providing a greater variety of niches and resources
• Productivity-diversity relationships suggest that species diversity often peaks at intermediate levels of productivity
  • The intermediate disturbance hypothesis proposes that moderate levels of disturbance maintain higher species diversity
• Island biogeography theory predicts that larger islands and those closer to the mainland will have higher species richness due to increased immigration and reduced extinction rates

Succession and Community Development

• Succession refers to the sequential changes in community composition and structure over time following a disturbance or the formation of a new habitat
• Primary succession occurs on newly formed or exposed substrates, such as volcanic islands or glacial moraines, where no soil or organic matter is initially present
  • Pioneer species, often lichens and mosses, are the first to colonize these bare areas, initiating soil development
• Secondary succession follows a disturbance that removes or alters the existing community without destroying the soil, such as after a wildfire or abandoned agricultural land
  • Remnant species and soil seed banks contribute to the regeneration of the community
• Facilitation occurs when early successional species modify the environment in ways that enable the establishment of later successional species
• Succession typically progresses from communities dominated by fast-growing, opportunistic species to those characterized by slower-growing, more competitive species
• Climax communities represent the final, relatively stable stage of succession, determined by the prevailing environmental conditions and species interactions
  • The concept of climax communities has been debated, as many ecosystems experience ongoing disturbances and may not reach a true equilibrium state

Disturbance and Community Dynamics

• Disturbances are events that disrupt community structure and alter resource availability, such as fires, storms, floods, or human activities
• The frequency, intensity, and scale of disturbances shape community composition and dynamics
  • Frequent, low-intensity disturbances often maintain higher species diversity than rare, high-intensity disturbances
• Resistance refers to a community's ability to withstand disturbance and maintain its structure and function
• Resilience describes a community's capacity to recover and return to its pre-disturbance state following a perturbation
• Disturbance can create a mosaic of patches in different stages of succession, enhancing landscape-level diversity
• Intermediate disturbance hypothesis suggests that moderate levels of disturbance promote species coexistence and diversity by preventing competitive exclusion
• Community stability can be influenced by species diversity, with more diverse communities often exhibiting greater resistance and resilience to disturbance
• Anthropogenic disturbances, such as habitat fragmentation, pollution, and climate change, can have profound impacts on community structure and biodiversity

Ecological Niches and Resource Partitioning

• An ecological niche encompasses all the biotic and abiotic factors that a species needs to survive, grow, and reproduce
• The fundamental niche represents the full range of conditions and resources a species could potentially utilize in the absence of competition or other biotic interactions
• The realized niche is the actual portion of the fundamental niche that a species occupies, as constrained by biotic interactions such as competition and predation
• Niche breadth refers to the range of resources and conditions a species can utilize, with generalist species having broader niches than specialist species
• Niche overlap occurs when two or more species use similar resources or occupy similar ecological roles
  • High niche overlap can lead to strong interspecific competition and potential competitive exclusion
• Resource partitioning allows species to coexist by dividing resources along different dimensions, such as food type, habitat, or time of activity
  • Character displacement is an evolutionary process in which competing species diverge in their resource use or morphology to minimize niche overlap (Darwin's finches)
• The competitive exclusion principle states that two species with identical niches cannot coexist indefinitely, as one will eventually outcompete the other
• Niche differentiation and resource partitioning are key mechanisms promoting species coexistence and diversity in communities

Applications and Case Studies

• Understanding community structure and dynamics is crucial for conservation and ecosystem management
  • Identifying keystone and foundation species can help prioritize conservation efforts and predict the consequences of their loss
• Ecological restoration aims to restore degraded communities to a more natural, diverse, and functional state
  • Knowledge of succession and community assembly processes informs restoration strategies and species selection
• Invasive species can disrupt community structure and ecosystem function, often leading to reduced native biodiversity
  • Studying the traits and impacts of invasive species helps develop effective control and eradication strategies
• Biodiversity assessments and monitoring programs track changes in community composition and species populations over time
  • These data inform conservation planning, habitat management, and environmental policy decisions
• Climate change is altering community structure and species distributions, with far-reaching consequences for biodiversity and ecosystem services
  • Predicting and mitigating the impacts of climate change requires a deep understanding of community ecology and species responses to environmental change
• Agroecology applies ecological principles to design sustainable agricultural systems that maintain biodiversity and ecosystem services
  • Strategies such as intercropping, agroforestry, and conservation biological control draw on knowledge of community interactions and niche partitioning
• Urban ecology investigates the structure and function of ecological communities in cities and towns
  • Insights from community ecology inform urban planning and green infrastructure design to support biodiversity and human well-being in urban environments