What is alpha diversity and beta diversity?
Understanding Alpha and Beta Diversity
Alpha and beta diversity are terms used in ecological and conservation biology. A key element about biodiversity is that species are unevenly distributed around the world. Different sites are habitats for different biological communities. Alpha and beta diversities seek to understand how different bio diversities are distributed across the world or different sites. Here we will delve into the two terms to understand what they mean, the differences between them and their relevance in ecological studies.
Alpha Diversity Microbiome
Alpha diversity refers to the number of species in a given area or ecosystem. It has two components which include species richness and equitability richness. Species richness refers to species in a particular landscape, region, or microbial ecosystem.
An equitability index or Shannon evenness index is concerned with the relative abundance of different species in their distribution. For instance, a community with an equal number of certain species will have a higher evenness index than one with a disproportionate number of species. Shannon evenness index looks at the diversity aspect of the species without considering the abundance.
Shannon diversity index looks at both richness and diversity. The index measures the number of species in a given ecosystem and the inequalities in their abundance. Species diversity helps ecologists know the relative abundance of certain species in a given sample
Beta Diversity Microbiome
Whittaker coined the term beta in 1960 to refer to community differentiation. It’s the ratio between gamma and alpha diversities. Beta diversity looks at the differences between microbial composition in a given ecosystem and another. Beta diversity looks at how taxonomic abundance profiles compare with others from different samples.
It utilizes the following measures:
Bray-Curtis Dissimilarities
• Quantifies microbial dissimilarities between two sites
• Measures differences in species between two samples
• The statistic ranges between 0 and 1
• 0 denotes that the two sites have the same species
• 1 denotes that the two sites don’t share any species
• The samples have to be similar in terms of size or volume
Jaccard Distance
• Measures similarities and differences between two sets of samples
• Jaccard coefficient ranges between 0 and 1
• 0 means that the two samples have exactly the same species
• 1 means that the two samples bear no species in common
UniFrac
• UniFrac uses phylogenetic distances
• Unweighted UniFrac is purely sequence-distance based, meaning it does not
incorporate abundance information
• Weighted UniFrac incorporates the relative abundance of taxa between samples.
Importance of Alpha and Beta Diversities
Beta diversities show the degree of similarities or dissimilarities between two biological communities or compositions. A high beta diversity index connotes low similarity, while a low beta diversity index means a high level of similarities of species in a particular ecosystem. Alpha diversities tell us about the species’ abundance, evenness, and distribution in a given ecosystem. Environmentalists and scientists can use alpha vs beta diversity microbiome information to make certain interventions that benefit the human-ecological system. They want to ensure a more sustainable and productive ecosystem so as to support life.