BIODIVERSITY
The extraordinary diversity of life on Earth is indeed a marvel, and it raises profound questions about its origins, maintenance, and significance. The alien visitor would undoubtedly be perplexed by the vast array of species coexisting on our planet.
Why are there so many species?
- The answer to this question involves various ecological and evolutionary processes. The concept of natural selection, as proposed by Charles Darwin, suggests that species evolve and adapt to specific ecological niches over time. The diversity arises through a combination of genetic variation, environmental changes, and selective pressures.
Did such great diversity exist throughout Earth’s history?
- The fossil record indicates that the diversity of life has changed over geological time. There have been periods of mass extinctions followed by adaptive radiations, during which new species evolved to fill vacant ecological niches. The current level of diversity is the result of millions of years of evolutionary processes.
How did this diversification come about?
- Diversification is driven by mechanisms such as speciation, genetic drift, and natural selection. Geographic isolation, for example, can lead to the formation of new species. Additionally, interactions between species, such as competition, predation, and mutualism, play crucial roles in shaping biodiversity.
How and why is this diversity important to the biosphere?
- Biodiversity is essential for the health and stability of ecosystems. Each species has a unique role or function, contributing to ecosystem services like pollination, nutrient cycling, and disease regulation. Biodiversity enhances ecosystem resilience and adaptability, making it crucial for the survival of life on Earth.
Would the biosphere function any differently if the diversity was much less?
- Reduction in biodiversity can lead to imbalances in ecosystems, making them more vulnerable to disturbances and less resilient to environmental changes. Loss of key species can disrupt ecological processes, affecting the overall functioning of the biosphere.
How do humans benefit from the diversity of life?
- Humans depend on biodiversity for various resources, including food, medicine, and materials. Biodiversity also contributes to cultural and recreational values. Additionally, diverse ecosystems provide essential services like clean air and water, soil fertility, and climate regulation, all of which are vital for human well-being.
Biosphere Diversity:
- Scope: Extensive diversity observed not only at the species level but across all levels of biological organization.
- Terminology: Coined as “Biodiversity” by sociobiologist Edward Wilson to encapsulate diversity at various biological levels.
Crucial Aspects:
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Genetic Diversity:
- Definition: Diversity within a single species at the genetic level.
- Example: Rauwolfia vomitoria, a medicinal plant in the Himalayas, exhibits genetic variation in terms of the potency and concentration of the active chemical (reserpine) it produces.
- Magnitude: India showcases significant genetic diversity, evident in over 50,000 genetically distinct strains of rice and 1,000 varieties of mango.
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Species Diversity:
- Definition: Diversity at the species level within a given area.
- Example: The Western Ghats display higher amphibian species diversity compared to the Eastern Ghats.
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Ecological Diversity:
- Definition: Diversity at the ecosystem level.
- Example: India’s diverse ecosystems include deserts, rain forests, mangroves, coral reefs, wetlands, estuaries, and alpine meadows. This contrasts with a Scandinavian country like Norway, which has comparatively lower ecosystem diversity.
Temporal Accumulation and Threat:
- Time Scale: Accumulation of rich biodiversity over millions of years through the process of evolution.
- Current Challenge: Present rates of species loss pose a significant threat to the accumulated biodiversity.
International Environmental Concern:
- Global Significance: Biodiversity conservation emerges as a critical global environmental concern.
- Awareness: Growing recognition worldwide regarding the pivotal importance of biodiversity for the survival and well-being of life on Earth.
How Many Species are there on Earth and How Many in India?
Known Species: Over 1.5 million plant and animal species have been identified and named.
Unknown Territory: The total number of species on Earth is uncertain, with estimates ranging from 7 million to as high as 50 million.
Discovery Challenges: Discovering species is more complete in temperate regions compared to tropical areas. The majority of undiscovered species are believed to be in tropical regions.
Estimation Methods: Biologists use statistical comparisons between temperate and tropical species richness in certain groups, like insects, to estimate the total number of species globally.
Conservative Estimate: A scientifically sound estimate by Robert May suggests around 7 million total species on Earth.
Animal Dominance: More than 70% of all recorded species on Earth are animals.
Plant Presence: Plants, including various types like algae, fungi, bryophytes, gymnosperms, and angiosperms, make up around 22% of the total species.
Insect Overload: Within the animal kingdom, insects steal the show as the most species-rich group, constituting over 70% of the total. In simpler terms, for every 10 animals, 7 are insects. Now, that’s an insect invasion of a different kind!
Fungi Fascination: The number of fungi species outnumbers the combined total of fishes, amphibians, reptiles, and mammals. Fungi are like the hidden superheroes of biodiversity.
The struggle to catalog Earth's biodiversity continues, and here's a glimpse of the challenges and wonders:
Prokaryotic Mystery: Prokaryotes, the microscopic world, pose a challenge for estimation due to unsuitable taxonomic methods. Their diversity, if assessed by biochemical or molecular criteria, could be in the millions.
India’s Rich Diversity: Despite occupying only 2.4% of Earth’s land area, India boasts an impressive 8.1% of global species diversity, earning its place among the 12 mega-diverse countries.
Huge Undiscovered Potential: In India alone, with 45,000 recorded plant species and twice as many animals, it’s estimated that over 1,00,000 plant species and 3,00,000 animal species are yet to be discovered and described, based on global estimates by Robert May.
Race Against Extinction: The urgency is real. With a large fraction of these undiscovered species already facing the threat of extinction, there’s a race against time to document and preserve nature’s treasures. The analogy of a burning biological library emphasizes the urgency to catalog and protect Earth’s biodiversity before it’s too late.
Patterns of Biodiversity
Latitudinal Gradients:
- Diversity of plants and animals isn’t evenly spread across the globe.
- A well-known pattern is the latitudinal gradient: species diversity generally decreases moving from the equator towards the poles.
- Tropics (23.5° N to 23.5° S) stand out as biodiversity hotspots, hosting more species compared to temperate or polar regions.
- Example: Colombia (near the equator) boasts 1,400 bird species, while New York (41° N) has 105, and Greenland (71° N) only 56.
- India, situated in tropical latitudes, boasts over 1,200 bird species.
Tropical Marvels:
- Tropical regions, like the Amazonian rainforest in South America, showcase unparalleled biodiversity.
- The Amazon rainforest is a haven: 40,000+ plant species, 3,000 fishes, 1,300 birds, 427 mammals, 427 amphibians, 378 reptiles, and over 1,25,000 invertebrates.
- Scientists speculate that these rainforests may hide around two million insect species yet to be discovered.
The tropics hold the secret to Earth’s biodiversity, and scientists have some compelling theories to explain this phenomenon:
Time for Evolutionary Masterpieces:
- Hypothesis (a): Speciation, the process of forming new species, often requires time. Unlike temperate regions that faced frequent glaciations, the tropics have enjoyed stability for millions of years. This prolonged period of calm allowed for extensive species diversification through evolution.
Consistent Tropical Havens:
- Hypothesis (b): Tropical environments are less seasonal and more constant compared to temperate ones. The stability allows for niche specialization, where species adapt to specific roles within their ecosystems. This specialization contributes to a greater overall species diversity.
Solar Energy Boost:
- Hypothesis (c): The tropics receive more solar energy. This abundance of energy fuels higher productivity in ecosystems. Greater productivity, in turn, indirectly supports a more diverse array of species.
Species-Area relationships
The species-area relationship, as observed by Alexander von Humboldt and explored by ecologists, provides intriguing insights into biodiversity patterns.
Humboldt’s Observation:
- Humboldt noticed that as the explored area in a region increases, species richness also increases, but only up to a certain limit.
Rectangular Hyperbola:
- The relationship between species richness (S) and area (A) follows a rectangular hyperbola.
- On a logarithmic scale, this relationship can be expressed as: log S = log C + Z log A.
Universal Pattern:
- Ecologists discovered that the slope (Z) of the log-log relationship, regardless of the taxonomic group or region, typically falls in the range of 0.1 to 0.2.
- This consistency is remarkable, whether studying plants in Britain, birds in California, or molluscs in New York state.
Continent-Level Analysis:
- When analyzing species-area relationships across entire continents, the slope (Z) becomes steeper, ranging from 0.6 to 1.2.
- For example, frugivorous birds and mammals in tropical forests show a slope of 1.15.
Interpreting Steeper Slopes:
- Steeper slopes in larger areas suggest a more rapid increase in species richness with additional explored area.
- In practical terms, this could mean that as you move across larger geographical scales, the diversity of species increases at a faster rate.
The importance of Species Diversity to the Ecosystem
The intricate dance of species within an ecosystem is more crucial than it might seem at first glance.
Community Stability:
- Ecologists have long believed that communities with more species tend to be more stable. Stability, in this context, means less variation in productivity over time, resistance or resilience to disturbances, and resistance to invasions by alien species.
Tilman’s Insights:
- David Tilman’s experiments show that plots with more species exhibit less year-to-year variation in total biomass. Increased diversity in these experiments contributes to higher productivity.
Ecosystem Health:
- While the precise link between species richness and ecosystem attributes isn’t fully understood, it’s clear that rich biodiversity is essential for ecosystem health.
Human Survival Connection:
- The well-being of the human race is intertwined with biodiversity. The loss of species at an alarming rate raises questions about the impact on our planet’s ecosystems and, consequently, on our own survival.
Rivet Popper Analogy:
- Paul Ehrlich’s ‘rivet popper hypothesis’ offers a vivid analogy. In an ecosystem (airplane), species (rivets) play a crucial role. Initially, losing a few might not impact the ecosystem’s proper functioning, but as more are removed, the system weakens over time.
Key Species Importance:
- The analogy emphasizes that not all species are equal. Losing key species that drive major ecosystem functions is a more serious threat than losing others.
Loss of Biodiversity
The reality of biodiversity loss is stark, and human activities are the leading cause.
Human Impact on Extinctions:
- Human activities, from colonization to modern practices, have led to the extinction of numerous species. The colonisation of tropical Pacific Islands alone is linked to the extinction of over 2,000 species of native birds.
IUCN Red List:
- The IUCN Red List documents the extinction of 784 species in the last 500 years, including vertebrates, invertebrates, and plants. Recent examples include the dodo, quagga, thylacine, Steller’s Sea Cow, and various tiger subspecies.
Vulnerable Groups:
- Extinctions are not random; certain groups, like amphibians, appear more vulnerable. Currently, over 15,500 species worldwide face the threat of extinction, with 12% of all bird species, 23% of mammals, 32% of amphibians, and 31% of gymnosperms at risk.
Historical Extinctions:
- The study of fossil records reveals that large-scale species loss occurred even before humans. Five episodes of mass extinctions happened over the past 3 billion years.
Unprecedented Rates:
- The ‘Sixth Extinction’ underway is different in its speed. Current extinction rates are estimated to be 100 to 1,000 times faster than pre-human times, with human activities being the primary driver.
Grim Future Projection:
- If current trends persist, ecologists warn that nearly half of all species on Earth could vanish within the next 100 years.
Consequences of Biodiversity Loss:
Decline in Plant Production (a):
- The variety of plants within an ecosystem contributes to overall productivity. A loss of biodiversity can lead to a decline in plant production, affecting the abundance and health of plant species.
Lowered Resistance to Environmental Perturbations (b):
- Biodiversity acts as a buffer against environmental disturbances like drought. A decrease in biodiversity can lower the ecosystem’s ability to resist and recover from such perturbations.
Increased Variability in Ecosystem Processes (c):
- Biodiversity plays a crucial role in stabilizing ecosystem processes. Reduction in biodiversity can lead to increased variability in processes such as plant productivity, water use, and cycles of pests and diseases.
The threats to biodiversity are varied and impactful, and here’s a closer look at some of the key causes:
(i) Habitat Loss and Fragmentation:
- Primary Cause: The most significant driver of extinction.
- Example: Tropical rainforests, once covering 14% of Earth’s land, now reduced to 6%.
- Impact: Destruction for agriculture and cattle ranching, leading to loss of biodiversity. Pollution further degrades habitats.
(ii) Over-exploitation:
- Human Impact: Dependence on nature turns into overexploitation.
- Examples: Historical extinctions like Steller’s sea cow and passenger pigeon.
- Current Threat: Overharvesting marine fish populations worldwide, endangering some commercially important species.
(iii) Alien Species Invasions:
- Cause: Introduction of alien species unintentionally or deliberately.
- Impact: Invasive species can cause the decline or extinction of native species.
- Example: Introduction of Nile perch in Lake Victoria led to the extinction of over 200 cichlid fish species.
(iv) Co-extinctions:
- Definition: When a species goes extinct, other species associated with it also face extinction.
- Examples: Obligatory associations like host and parasite, or coevolved mutualisms like plant-pollinator relationships.
