Decline of Biological Diversity

What We Mean by “Decline of Biological Diversity”

Biological diversity (biodiversity) is the variety of life at three main levels:

• Genetic diversity – variation of genes within a species (e.g., different crop varieties, wild populations with rich gene pools).
• Species diversity – the number and relative abundance of species in an area.
• Ecosystem diversity – the variety of habitats and ecological communities (forests, coral reefs, wetlands, grasslands, etc.).

The decline of biological diversity means:

• Fewer genes (loss of local varieties, inbreeding in small populations).
• Fewer species (local disappearance, global extinction).
• Fewer and more simplified ecosystems (destruction or fragmentation of habitats, loss of complex food webs and functions).

It is not only about species going extinct, but also about the erosion of variability and the simplification of nature, which reduces the capacity of life to adapt and persist.

Patterns and Dimensions of the Decline

Local Extinction vs. Global Extinction

• Local (regional) extinction: A species disappears from a given area but still exists elsewhere (e.g., wolves extirpated from parts of Europe, later reintroduced).
• Global extinction: The last individual of a species dies; the species is lost forever (e.g., passenger pigeon, dodo).

Many local extinctions can eventually lead to global extinction if remaining populations are small, isolated, and declining.

Declining Populations and “Living Dead”

Before extinction, typical warning signs appear:

• Population decline – fewer individuals over time.
• Range contraction – the species occupies fewer and smaller areas.
• Fragmentation – remaining groups are isolated from each other.

Some populations become so small and isolated that they are demographically and genetically doomed, even if a few individuals still exist. These are sometimes called “living dead” populations: they are unlikely to survive in the long term without human help.

Loss of Genetic Diversity

Even when a species still seems common, it may lose genetic diversity because:

• Many small, isolated populations replace one large connected one.
• Only a few individuals are used for breeding (e.g., in agriculture, livestock, tree plantations).
• Strong selection for uniform traits eliminates rare variants.

Consequences:

• Reduced adaptive potential to new diseases, climate changes, or other stressors.
• Increased inbreeding and expression of harmful recessive traits.
• Higher risk of population collapse during environmental changes.

Biotic Homogenization

As some species disappear and others spread with human help, many regions of the world become more similar in their species composition:

• Invasive species (often human‑introduced) become widespread.
• Generalist species (that tolerate many conditions and use many food types) increase.
• Specialist species (narrow habitat or food requirements) decline or disappear.

This homogenization leads to:

• Loss of unique regional floras and faunas.
• Decline of endemic species (those found only in one area).
• More uniform, less distinctive ecosystems worldwide.

Main Drivers of Biodiversity Decline

Multiple drivers act together. Their relative importance can differ by region and ecosystem, but the following five are often highlighted as central in current biodiversity assessments.

1. Habitat Loss, Degradation, and Fragmentation

Habitat Loss

• Conversion of natural habitats to agricultural land, settlements, roads, mines, dams, and other infrastructure.
• Direct destruction (e.g., clear‑cutting forests, draining wetlands, destroying coral reefs).

This is considered the single most important driver of biodiversity loss in many terrestrial and freshwater systems.

Habitat Degradation

• Habitat is not fully destroyed but its quality declines.
• Examples: pollution, soil compaction, overgrazing, noise and light pollution, altered water flow, logging that removes old trees and dead wood.

Degraded habitats may still support some species, but often fewer, more tolerant, and more generalist species.

Fragmentation

• Remaining habitat is broken into smaller patches separated by roads, fields, settlements, or other barriers.
• Consequences:
• Smaller populations → more vulnerable to random events and inbreeding.
• Reduced dispersal between patches → less gene flow and recolonization.
• More edge effects (see below).

Edge Effects

Where remaining habitat borders human‑altered land:

• Altered microclimate (more light, heat, wind).
• Increased exposure to predators, invasive species, and pollutants.
• Changed species composition near edges compared to interior.

When fragmentation increases, the proportion of edge relative to core habitat rises, which is especially harmful for species that require large, undisturbed areas.

2. Overexploitation of Species

Overexploitation means using wild populations faster than they can renew themselves.

Typical forms:

• Overfishing (decline of large predatory fish, collapse of some stocks).
• Overhunting and poaching (large mammals, birds, reptiles).
• Overharvesting of plants (medicinal plants, timber, ornamental plants).

Consequences:

• Population declines and possible extinction.
• Disruption of trophic relationships (e.g., removal of top predators alters whole food webs).
• “Fishing/hunting down the food web”: as large, profitable species disappear, smaller or less desirable species are targeted.

Overexploitation often interacts with other drivers. For example, a species with shrinking habitat becomes especially vulnerable to any additional mortality from harvesting.

3. Pollution and Chemical Stressors

While pollution is covered elsewhere in more detail, here the focus is on how it affects biodiversity.

Direct Toxic Effects

• Pesticides and herbicides can kill non‑target insects, birds, aquatic organisms, and plants.
• Industrial pollutants (heavy metals, persistent organic pollutants) accumulate in food chains and may cause infertility, malformations, or death.

Nutrient Enrichment (Eutrophication)

• Excess nitrogen and phosphorus from agriculture and wastewater lead to algal blooms in water bodies.
• When algae die, their decomposition consumes oxygen → oxygen‑poor (hypoxic) or anoxic “dead zones” where many organisms cannot survive.
• Species that tolerate low oxygen or high nutrient loads may dominate, reducing diversity.

Plastic and Microplastic

• Large plastic items: entanglement, ingestion by marine and terrestrial animals.
• Microplastics: can be ingested by many organisms, potentially causing physical and chemical stress and transporting other pollutants or invasive organisms.

Pollution often combines with habitat degradation, making environments hostile to sensitive species while favoring a smaller number of tolerant species.

4. Invasive Alien Species

Alien (non‑native) species are those introduced outside their natural range by humans, intentionally or accidentally. Only a subset become invasive, meaning:

• They establish self‑sustaining populations.
• They spread widely.
• They cause ecological, economic, or health damage.

Mechanisms by which invasive species reduce biodiversity:

• Competition: They outcompete native species for food, light, space, or other resources.
• Predation: New predators can devastate native prey that have no evolved defenses.
• Disease: Introduced pathogens and parasites can cause epidemics in native species.
• Hybridization: Cross‑breeding with native relatives can dilute or erase unique gene pools.

Islands, freshwater systems, and isolated ecosystems are especially vulnerable.

5. Climate Change

Climate change (addressed more fully under climate chapters) is here considered as a driver of biodiversity loss.

Connections to biodiversity:

• Range shifts: Species move toward poles, higher elevations, or deeper water. Those unable to move or adapt (e.g., high‑mountain, polar, and many island species) face strong extinction risks.
• Phenological changes: Timing of life cycle events (flowering, breeding, migration) shifts. If interacting species change at different speeds, mismatches arise (e.g., pollinators and flowers, predators and prey).
• Extreme events: Heatwaves, droughts, storms, and fires become more frequent or severe, causing sudden mortality and habitat destruction.
• Ocean changes: Warming and acidification of oceans threaten coral reefs, shell‑forming organisms, and many other marine species.

Climate change also intensifies other drivers: it can make invasive species more competitive, weaken already stressed populations, and alter habitats.

Ecological Consequences of Biodiversity Decline

Loss of Ecosystem Functions and Services

Ecosystems provide many functions (processes) that lead to services for humans. The decline of biodiversity can weaken:

• Primary production – production of biomass by plants and algae.
• Pollination – carried out by insects, birds, bats, etc.
• Seed dispersal – by animals, wind, and water; many plant species depend on specific animals.
• Decomposition and nutrient recycling – by microbes, fungi, and detritivores.
• Biological control – natural enemies of pests and diseases.
• Water purification and regulation – wetlands, riparian vegetation, soil organisms.
• Climate regulation and carbon storage – forests, peatlands, seagrass beds.

When species disappear, these functions may become less efficient, less stable, or fail altogether, affecting both nature and human societies.

Reduced Stability and Resilience

Biodiversity often contributes to:

• Resistance – the ability of an ecosystem to withstand disturbances.
• Resilience – the ability to recover after disturbances.

With more species:

• There is usually functional redundancy: several species can perform similar roles. If one disappears, others can partly compensate.
• Different species respond differently to stress, so some continue functioning under changed conditions.

When diversity is low:

• Ecosystems may cross thresholds and shift to alternative states (e.g., clear lake → turbid, algae‑dominated; coral reef → algal field).
• Recovery from disturbance can be slow or incomplete.

Cascading Effects in Food Webs

Loss of certain species can trigger trophic cascades:

• Removal of top predators can lead to increases in herbivores, which may then overeat vegetation, altering habitats for many other species.
• Loss of key herbivores or detritivores can change plant communities and nutrient cycling.

Some species are considered keystone species: their impact on ecosystem structure is disproportionately large relative to their abundance. Loss of such species is often particularly damaging.

Social, Economic, and Ethical Dimensions

Dependence of Human Societies on Biodiversity

Human societies depend in many ways on rich biological diversity:

• Food security – diverse crops, livestock, wild foods, and fish stocks reduce risk of failure.
• Medicine – many pharmaceuticals originate from wild organisms; biodiversity is a reservoir of potential new substances.
• Materials – wood, fibers, dyes, resins, etc.
• Cultural values – spiritual significance, traditional knowledge, recreation, and tourism.

As diversity declines:

• Resilience of food systems decreases.
• Options for future uses are lost (e.g., undiscovered medicinal compounds).
• Cultural practices linked to certain landscapes or species may disappear.

Irreversibility and Option Value

Extinction is irreversible on human time scales. When a species disappears:

• Its unique genetic information and evolutionary history are lost.
• Potential future uses (e.g., in medicine, agriculture, technology inspiration) are eliminated.
• Its role in ecological networks may not be fully replaced.

The concept of option value emphasizes that biodiversity preserves future possibilities, even if we do not yet know their importance.

Ethical and Aesthetic Considerations

Besides practical benefits, many people see intrinsic value in:

• The mere existence of other species.
• The beauty and diversity of living forms and landscapes.
• The right of future generations to experience rich, functioning ecosystems.

Different ethical perspectives (e.g., anthropocentric, biocentric, ecocentric) all influence arguments and decisions about biodiversity protection, but the observed decline gives them a shared urgency.

Measuring and Monitoring Biodiversity Decline

To recognize and address the decline, various approaches are used.

Species‑Level Assessment

• Red Lists (e.g., IUCN Red List) classify species into categories such as “Least Concern,” “Vulnerable,” “Endangered,” and “Critically Endangered,” based on:
• Population size.
• Rate of decline.
• Geographic range.
• Degree of fragmentation or fluctuation.

These assessments highlight which species are at greatest risk and where conservation efforts are most needed.

Habitat and Ecosystem Assessment

• Mapping changes in land cover (forest, grassland, wetland, etc.) using remote sensing and field surveys.
• Evaluating ecosystem integrity, such as the presence of key structures (old trees, dead wood, coral cover) and processes.
• Classifying and monitoring threatened ecosystem types (e.g., certain grasslands, peatlands, coral reef systems).

Indicators and Long‑Term Monitoring

• Indicator species or groups (e.g., amphibians, birds, freshwater invertebrates) can be used to infer broader environmental conditions.
• Long‑term monitoring programs track:
• Species richness and abundance in standard plots or transects.
• Genetic diversity in selected populations.
• Changes in community composition over time.

These data are essential for detecting trends, identifying drivers, and evaluating the success of conservation measures.

Interactions With Climate Change

Although climate change has its own section, it is tightly linked to biodiversity decline and deserves emphasis here.

Feedbacks Between Biodiversity and Climate

• Intact, diverse ecosystems (forests, wetlands, oceans) help regulate climate by storing carbon and influencing water and energy fluxes.
• As biodiversity declines and ecosystems degrade:
• Carbon storage can decline (e.g., deforestation, peatland drainage).
• More greenhouse gases may be released, accelerating climate change.

This creates feedback loops: climate change harms biodiversity, and biodiversity loss in turn can worsen climate change.

Climate‑Smart Conservation

Efforts to counter biodiversity decline increasingly consider climate change:

• Protect climate refugia: areas expected to remain relatively stable and suitable for many species.
• Maintain or create connectivity (corridors, stepping‑stone habitats) to allow species to shift their ranges.
• Use diverse and native species in restoration projects to increase resilience.

These approaches aim to reduce the combined risks from both biodiversity loss and climate change.

The Decline of Biodiversity as a Global Challenge

Scientific assessments now describe biodiversity loss as a global crisis, comparable in significance to climate change. Important characteristics:

• It occurs at local, regional, and global scales.
• Many changes are rapid compared to past natural variations.
• Extinctions and ecosystem changes are often irreversible on human time scales.
• Causes are strongly tied to human activities, including land use, resource extraction, trade, and consumption patterns.

Recognizing the magnitude and causes of the decline is a prerequisite for:

• Setting conservation priorities (which species, habitats, and regions are most urgent).
• Designing effective policy and management responses (addressed under laws, measures, and international efforts in subsequent chapters).
• Understanding why protecting nature and the environment is not only an ethical choice but also a practical necessity for long‑term human well‑being.