Community composition and diversity indices are crucial tools in ecology. They help us understand the richness, evenness, and overall diversity of species in ecosystems. These measures provide insights into community structure, ecosystem functioning, and how different factors shape biodiversity patterns.
From species richness to complex diversity indices, ecologists use various metrics to quantify community structure. These tools help uncover relationships between biodiversity and ecosystem stability, productivity, and resilience. Understanding these connections is vital for conservation and predicting ecosystem responses to environmental changes.
Species richness, evenness, and diversity
Defining key diversity concepts
- Species richness measures the total number of distinct species in an ecological community, regardless of their relative abundances
- Species evenness quantifies the relative abundance distribution of individuals among species in a community
- Higher evenness indicates more equal distribution across species
- Lower evenness suggests dominance by one or a few species
- Diversity encompasses both species richness and evenness, providing a more comprehensive measure of community structure
- Can have high richness but low evenness (many rare species, few dominant)
- Or high evenness but low richness (few species, all similarly abundant)
Types of ecological diversity
- Alpha diversity represents diversity within a particular area or ecosystem
- Typically measured as species richness in that area
- Examples: number of tree species in a forest plot, fish species in a lake
- Beta diversity measures change in species composition between communities or along environmental gradients
- Captures turnover in species identities across space
- Examples: change in bird species between forest and grassland habitats, shift in plant communities along an elevation gradient
- Gamma diversity refers to total species diversity in a landscape or region
- Encompasses multiple communities or ecosystems
- Examples: total mammal diversity across a mountain range, plant diversity of an entire country
Common diversity indices
- Shannon-Wiener Index (H') accounts for richness and evenness
- Calculated as H′=−∑(pi∗lnpi), where pi is proportion of individuals of species i
- Higher values indicate greater diversity
- Example: H' = 2.5 for a forest with many evenly distributed tree species
- Simpson's Diversity Index (D) measures probability two random individuals are different species
- Calculated as D=1−∑(pi2)
- Ranges from 0 (low diversity) to 1 (high diversity)
- Example: D = 0.8 for a diverse coral reef community
- Pielou's Evenness Index (J') quantifies how evenly individuals are distributed among species
- Calculated as J′=H′/ln(S), where S is total number of species
- Ranges from 0 (low evenness) to 1 (perfect evenness)
- Example: J' = 0.9 for a grassland with very similar abundances across plant species
Interpreting diversity indices
- Higher index values do not always indicate better ecosystem health or functioning
- Context-dependent interpretation needed (pristine vs disturbed habitats)
- Indices can be sensitive to sampling effort and presence of rare species
- Requires careful consideration of sampling methods and data quality
- Rarefaction curves allow comparison of richness among communities with different sample sizes
- Standardizes sampling effort for more accurate comparisons
- Example: comparing bird diversity between small and large forest fragments
- Margalef's Richness Index (DMg) measures species richness while accounting for sample size
- Calculated as DMg=(S−1)/ln(N), where N is total number of individuals
- Useful for comparing communities with different abundances
- Example: comparing plant richness in habitats with sparse vs dense vegetation
Community composition and ecosystem function
Biodiversity-ecosystem functioning relationships
- Biodiversity-ecosystem functioning (BEF) relationships link community composition to ecosystem processes and services
- Complementarity effect suggests diverse communities use resources more efficiently
- Results from niche partitioning and facilitation among species
- Example: mixed tree plantations often have higher productivity than monocultures
- Selection effect proposes diverse communities more likely contain highly productive species
- These species disproportionately influence ecosystem functioning
- Example: presence of nitrogen-fixing plants enhancing soil fertility in grasslands
- Functional diversity considers range of functional traits in a community
- Often stronger predictor of ecosystem functioning than species diversity alone
- Example: diversity of leaf traits better predicting forest carbon storage than tree species richness
Community composition and ecosystem stability
- Redundancy in functional roles among species contributes to ecosystem stability and resilience
- Multiple species performing similar functions buffer against disturbances
- Example: diverse pollinator communities maintaining pollination services despite individual species fluctuations
- Keystone species have disproportionate effects on community and ecosystem relative to abundance
- Removal can cause cascading effects through the ecosystem
- Examples: sea otters in kelp forests, wolves in Yellowstone National Park
- Ecosystem engineers modify habitats, influencing community composition and functioning
- Create or maintain habitats for other species
- Examples: beavers creating wetlands, corals building reef structures
- Insurance hypothesis suggests higher diversity buffers ecosystem functioning against environmental fluctuations
- Increases likelihood of compensatory dynamics among species
- Example: diverse plant communities maintaining productivity across varying rainfall conditions
Factors influencing community composition and diversity
- Environmental filtering selects species with traits adapted to local abiotic conditions
- Shapes community composition at local scales
- Examples: soil pH influencing plant communities, temperature gradients affecting insect distributions
- Biotic interactions influence species coexistence and relative abundances
- Competition, predation, mutualism, and parasitism all play roles
- Examples: competitive exclusion in similar plant species, predator-prey cycles in small mammal communities
- Disturbance regimes create temporal and spatial heterogeneity in composition and diversity
- Can increase diversity by preventing competitive exclusion
- Examples: fire regimes in grasslands, treefall gaps in forests
Broader-scale factors shaping communities
- Neutral theory proposes random processes explain patterns of composition and diversity
- Dispersal, speciation, and extinction drive community assembly
- More influential at larger spatial scales
- Example: explaining similar species-area relationships across different tropical forest plots
- Metacommunity dynamics influence composition through spatial connectivity and dispersal
- Source-sink relationships and mass effects link local communities
- Examples: island biogeography in archipelagos, stream networks shaping aquatic communities
- Historical factors shape regional species pools and local community assembly
- Evolutionary history and biogeography play important roles
- Examples: Wallace's Line influencing species distributions in Southeast Asia, post-glacial recolonization patterns
- Anthropogenic impacts alter composition and diversity across multiple scales
- Habitat fragmentation, climate change, species introductions all have effects
- Examples: urbanization homogenizing bird communities, invasive species altering native plant assemblages