Alpha, Beta, Gamma Biodiversity

There are different facets of biological diversity across spatial scales and organizational levels—everything from genetic variance within species to biome diversity across continents. Ecologists have created a framework to measure biodiversity across spatial scales from genes to biomes to gain a comprehensive understanding of the living world. They are as follows:

• Alpha (α) diversity refers to the diversity within a singular habitat or ecosystem – the variety of species found coexisting in one locality. It measures local richness.
• Beta (β) diversity quantifies the change in species composition between different ecosystems. It compares community turnover across habitats.
• Gamma (γ) diversity evaluates regional species richness by assessing total species diversity across multiple interconnected ecosystems or habitats in a landscape.
• Delta (δ) diversity accounts for ecological roles by tallying the diversity of functional niches occupied rather than just species counts.
• Epsilon (ε) diversity appraises evolutionary heritage by examining phylogenetic relationships between species to gauge the genetic breadth encompassed in an assemblage.

Alpha Diversity: Variety Within Communities

Alpha (α) diversity refers to the diversity within a particular habitat or ecosystem, focusing on the variety of species coexisting in one locality. Higher alpha diversity indicates greater richness of species in that habitat. Environments like tropical rainforests and coral reefs tend to exhibit the highest alpha diversity globally. Here are some examples of alpha biodiversity.

• The Amazon rainforest – Scientists have catalogued over 40,000 plant species, 427 mammals, 1,300 birds, 378 reptiles, and around 400 amphibian species coexisting in Amazon rainforest habitats within the Amazon River basin. This incredibly high species richness represents immense alpha diversity.
• Coral reefs – Coral reef ecosystems harbor extremely high alpha diversity, with just one reef home to over 25% of all known marine species across 100,000+ species of fish, invertebrates, and algae living symbiotically in close quarters.
• Tropical pitcher plant bogs – Unique carnivorous pitcher plant wetlands contain a diversity of specialized insects, spiders, amphibians and other species adapted to thrive in the nutrient-poor, waterlogged bogs where the pitcher plants capture prey.
• Mangrove forests – Intertidal mangrove trees provide vital habitat for diverse marine and terrestrial flora and fauna. A single mangrove forest can contain over 200 tree species along with multitudes of birds, fish, algae, crabs, snakes and other mangrove denizens constituting its high alpha diversity.
• Mountain cloud forests – Tropical and temperate cloud forests cloaked in heavy fog exhibit exceptional alpha diversity in the form of thousands of endemic vascular plants, insects, amphibians, fungi and microorganism species packed into these rare island-like cloud forest habitats isolated by altitude and moisture.

Beta Diversity: Turnover Between Communities

Beta (β) diversity quantifies the change or difference in community composition between ecosystems. It assesses how much species vary from one system to another across space and environment. For instance, beta diversity between an alpine meadow ecosystem and an adjacent montane forest ecosystem would measure the degree of change in plant and animal species between those distinct mountain habitats. High beta diversity points to more rapid shifts in biodiversity between habitats. Some examples are as follows:

• Amazon to Andes – The floral and faunal composition changes dramatically between the flooded Amazon rainforest lowlands, steep Andean cloud forests, and high-altitude alpine tundra ecosystems in South America, reflecting very high landscape-level beta diversity.
• African Savanna Mosaic – Heterogeneous patches of forest, grassland, swamp, and bushland scattered throughout African savannas contain distinct, varied assortments of large mammals occupying different habitats. This complex habitat mosaic drives high beta diversity.
• Madagascar Forest Types – Madagascar features substantial beta diversity between its array of forest ecosystem variants like tropical humid forests, spiny thickets, and dry deciduous forests harboring differing endemic lemur and plant species compositions.
• Hawaiian Island Archipelago – Volcanic Hawaiian islands vary markedly in age, size, elevation and isolation, creating diverse island-specific ecosystems inhabited by different assemblages of native birds, insects and plants manifesting as steep beta diversity gradients.
• Intertidal Zonation – The species makeup of marine flora and fauna shifts noticeably between the splash zone, barnacle line, and subtidal habitats of rocky intertidal shores based on tolerance of tidal exposure, fueling heightened beta change over small scales.

Gamma Diversity: Regional Species Richness

Gamma (γ) diversity measures the overall species richness for a large region encompassing multiple ecosystems, such as the diversity across various habitats in a broad geographic area like a mountain range or large island. For example, surveying gamma diversity in a biodiversity hotspot like Madagascar would involve cataloguing all wildlife across the country’s array of forest, grassland, wetland and desert habitats. Gamma diversity aggregates alpha and beta diversity across the landscape. Some examples of Gamma Diversity are as follows:

• Madagascar – The island of Madagascar off the southeast coast of Africa harbors exceptional gamma diversity across its numerous unique forest, grassland, wetland and desert ecosystems and endemic radiations. Over 200,000 identified animal and plant species make their home among the varied Malagasy habitats.
• Atlantic Forest – This narrow coastal rainforest extending along the Brazilian coastline contains some of the highest gamma diversity on Earth, with around 20,000 plant species, over 850 bird species, 200+ mammals, and 170 reptiles recorded across its fragmented forest habitats and ecosystems.
• Cape Floristic Region – The Cape Floristic region located at Africa’s southern tip encompasses staggering plant gamma diversity with over 9,000 vascular plant species packed within several distinct vegetation types spanning its geography. The Mediterranean climate fuels this concentrated botanical abundance.
• Queensland Wet Tropics – Situated along Australia’s northeastern coast around the Great Barrier Reef, this UNESCO World Heritage site constituting complex rainforest habitats and islands encapsulates extremely high gamma diversity in the form of thousands of plant, mammal, bird and frog species documented across its forests, woodlands and scrublands.
• Philippines – Over 52,000 described terrestrial and marine species have been documented throughout the vast Philippine archipelago, with its array of tropical rainforests, coral reefs and diverse volcanic islands manifesting exceptional gamma diversity.

Delta Diversity: Functional Roles

Delta (δ) diversity moves beyond species counts and acknowledges the diversity of ecological niches and functional roles filled by organisms in an ecosystem. It examines community diversity through the lens of functional traits rather than strict taxonomic diversity. For instance, delta diversity may distinguish four species that occupy separate niches in a forest environment based on traits like nitrogen fixation, canopy height, leaf type, or flowering time that enable resource partitioning and community richness.

Epsilon Diversity: Evolutionary Legacy

Epsilon (ε) diversity considers the genetic breadth and evolutionary legacy encompassed by an ecological community. It scrutinizes phylogenetic relationships and genetic distinctiveness between coexisting species inherited over evolutionary timescales. For example, a habitat with species stemming from vastly different evolutionary lineages represents considerably higher epsilon diversity than a set of closely related species. Protecting epsilon diversity ensures conservation of more total evolutionary pathways.