Species diversity Knowledge-Based Study

Study notes

1. What is Species Diversity?

   In community ecology, a biological community consists of all the interacting populations of different species inhabiting a defined geographic area at the same time. One of the foundational attributes used to describe community structure is species diversity.

   Species diversity refers to the variety and relative abundance of different species living within a given community. Unlike simple counts of individuals, species diversity integrates both the number of distinct evolutionary lineages present and the way individuals are distributed across those lineages.

   Levels of Biodiversity

   Ecologists distinguish species diversity from other levels of biological organization:

   • Genetic diversity: Variation in genes within a single species or population.
   • Species diversity: Variety and balance of different species within an ecological community.
   • Ecosystem diversity: Range of distinct biomes, habitats, and ecological processes across a broader landscape.

   The Two Key Components

   To accurately evaluate and quantify species diversity, ecologists break the concept into two measurable parameters: species richness and species evenness.

   1. Species Richness ($S$)

   Species richness is simply the total count of distinct species present in a specified community. For example, if a forest plot contains 15 different species of trees, its species richness is 15. However, richness alone does not reveal how common or rare each species is.

   2. Species Evenness (Relative Abundance)

   Species evenness describes how equally individuals are apportioned among the existing species. Evenness measures whether a community is dominated by one or two species or if all species share similar population sizes.

   Consider two hypothetical forest plots, each with 100 total trees representing 4 species:

   • Community A: 25 individuals of Species 1, 25 of Species 2, 25 of Species 3, and 25 of Species 4.
   • Community B: 91 individuals of Species 1, 3 of Species 2, 3 of Species 3, and 3 of Species 4.

   Both communities have the exact same species richness ($S = 4$). However, Community A has substantially greater species evenness, making it more diverse overall than Community B, which is heavily dominated by a single species.

   Measuring Species Diversity

   Because richness and evenness interact, ecologists use mathematical diversity indices to calculate a single metric that balances both factors.

   Common Diversity Indices

   • Shannon Diversity Index ($H'$): This index calculates diversity based on the proportion of individuals belonging to the $i$-th species ($p_i$). Higher values of $H'$ indicate both greater richness and more equitable evenness.
   • Simpson's Diversity Index ($D$): This index measures the probability that two randomly selected individuals from a community belong to the same species. Lower values of $D$ (or higher values of $1 - D$) indicate greater diversity.

   These indices allow ecologists to compare communities across environmental gradients, assess habitat degradation, and monitor recovery following disturbances.

   Why Does Species Diversity Matter?

   High species diversity is closely associated with key community properties and ecosystem functions:

   • Community Stability and Resilience: Communities with higher diversity tend to exhibit greater stability when subjected to environmental fluctuations, such as droughts or disease outbreaks. When multiple species perform overlapping ecological roles (functional redundancy), the loss or decline of one species is less likely to collapse the entire ecosystem.
   • Resource Utilization: Diverse communities display greater niche differentiation, utilizing available resources—such as light, soil nutrients, and moisture—more thoroughly and efficiently than species-poor systems.
   • Resistance to Invasive Species: Communities with high species richness and evenness leave fewer unutilized niches, making it significantly harder for non-native invasive species to establish and spread.
   • Productivity: Numerous ecological studies demonstrate a positive correlation between species diversity and primary productivity, often referred to as the diversity-productivity relationship.