Three connected levels

Biodiversity is the total diversity of living systems and it exists at several levels. These are habitat diversity, species diversity and genetic diversity, so biodiversity involves much more than simply counting species.

Habitat diversity refers to the variety of distinct habitats within an ecosystem, landscape or biome. For instance, woodland, marsh and open water each provide different physical conditions and ecological niches.

Species diversity is a community property shaped by the number of species present and their relative abundances. Two communities may contain the same number of species yet have very different species diversity if a single species dominates one of them.

Genetic diversity is the variety of genetic material found within a population or species. Different gene variants can create differences among individuals, giving natural selection variation on which to act.

These levels connect. A range of habitats provides more kinds of niche and may support more species, while genetic variation helps populations respond to different conditions. Biodiversity is therefore best explained at nested scales, rather than through one headline number.

Diversity provides response options

Resilience is a system’s capacity to resist disturbance, recover afterwards or adapt to changing conditions without losing its essential structure and functions. Every component of biodiversity can make a living system more resilient, but high diversity doesn’t make it invulnerable.

• Habitat diversity provides alternative refuges, feeding areas and recolonization sites for organisms when a habitat is disturbed.
• Species diversity maintains multiple ecological interactions and can create functional overlap. When one species declines, another may carry on a similar process, such as pollination or decomposition.
• Genetic diversity allows some individuals to have heritable characteristics suited to a new disease, climate or other selective pressure. If they survive, the population may avoid total collapse.

Together, these mechanisms allow ecological systems to absorb change without crossing a tipping point. Biodiversity therefore supports sustainability: several ecological pathways are safer than reliance on one habitat, one species or one narrow gene pool.

This relationship isn’t automatic. Severe or repeated disturbance can exceed a system’s capacity to recover. A species-rich community may also become destabilized if it loses a keystone species. Diversity increases the range of possible responses, but it doesn’t guarantee recovery.

Change accumulated across generations

Evolution is cumulative change in the heritable characteristics of a population or species. Individuals don’t evolve during their lifetimes. Instead, populations evolve as inherited characteristics become more or less common from one generation to the next.

Evolution explains biodiversity. Populations face different environments, so they accumulate different genetic changes. Given enough time, these changes produce the variety of forms, functions and species found on Earth. Existing biodiversity records past evolutionary change while providing the raw material for future change.

However, not every difference between organisms is evolutionary. A characteristic gained through an individual’s experience won’t pass to its offspring unless it has a heritable genetic basis. The key phrase is heritable characteristics—keep hold of that.

Continuous differential success

Natural selection is a non-deliberate evolutionary process. Individuals with heritable characteristics that give them an advantage in a particular environment tend to survive and reproduce at higher rates than other individuals.

It occurs whenever heritable variation leads to unequal reproductive success. Natural selection doesn’t work towards a planned goal, and organisms don’t develop needed characteristics by trying to adapt. The environment simply favours some existing variants over others.

This process operates continuously. Strong selection pressures may cause detectable change within a few generations. Across billions of years, however, its cumulative effects have contributed to the enormous biodiversity of life on Earth. An advantageous characteristic depends on the environment, so changing conditions can shift the direction of selection.

The sequence of natural selection

Variation refers to the genetic and observable differences between individuals in a population. Genetic diversity creates heritable variation, which means individuals differ in how well suited they are to a particular environment.

Overproduction occurs when a population produces more offspring than can survive and reproduce. Food, space, mates and other resources are limited. As a result, overproduction causes competition, an interaction in which organisms seek the same limited resource.

Adaptation is the evolutionary process through which a population becomes better suited to its environment. An adaptation can also be a heritable characteristic that improves reproductive success under particular conditions.

The mechanism follows this sequence:

1. Individuals have heritable variation.
2. The population produces more offspring than the environment can support.
3. Individuals compete for limited resources and encounter other selective pressures, such as predation or disease.
4. Those with advantageous variations are more likely to survive and reproduce.
5. Survivors pass the advantageous genes to their offspring.
6. Over many generations, these genes become more frequent and the population changes.

“Survival of the fittest” refers to the survival and reproduction of individuals best fitted to the current environment. Fitness means reproductive success, not just physical strength. Natural selection changes populations; it doesn’t transform an individual during its lifetime.

From one species to two

Speciation is the generation of new species through evolution. It happens when one part of a population becomes isolated, then evolves differently from the remaining populations of the original species.

Isolation reduces or stops gene flow. Once separated, populations may face different climates, food supplies, predators or competitors. Natural selection therefore favours different characteristics in each population, while mutations and inherited variation accumulate independently. Eventually, they can no longer interbreed successfully to produce fertile offspring. At that stage, they are separate species.

The sequence is isolation, different selection pressures, genetic divergence and, finally, loss of the ability to interbreed. When individuals continue to mix and reproduce freely, gene flow tends to stop the populations from becoming genetically distinct.

Two variables, not one

Richness is the number of species in a community; evenness is how similar the population sizes of each species are. These two variables determine species diversity.

Many species give a community high richness. High evenness occurs when individuals are distributed relatively equally between those species. For example, a community with five similarly abundant species is more diverse than one with five species where almost every individual belongs to a single species.

Richness reflects biological variety and the range of ecological roles because it increases as more species are present. Evenness shows how balanced the community composition is. If one species dominates heavily, that balance falls, which can indicate disturbance or environmental stress. On its own, neither variable fully describes species diversity.

Quantifying and comparing species diversity

Simpson’s reciprocal index is a quantitative measure of species diversity that combines richness with evenness. It can be used to compare similar ecosystems or track changes in one ecosystem over time.

The formula is:

$$D=\frac{N(N-1)}{\sum n(n-1)}$$

Here, $D$ is Simpson’s reciprocal index (dimensionless). $N$ is the total number of individuals in the sample (a count, dimensionless), while $n$ is the number of individuals of one species (a count, dimensionless). The symbol $\sum$ shows that $n(n-1)$ must be calculated for every species and the results added together.

The lowest possible value is 1. Higher values usually indicate greater richness, greater evenness or both. Suppose, for example, that a sample contains four species with counts 8, 6, 4 and 2. In this case, $N=20$ and:

$$\sum n(n-1)=8(7)+6(5)+4(3)+2(1)=100$$ $$D=\frac{20(19)}{100}=3.8$$

Collecting suitable data

A fair comparison requires the same type of organism to be sampled in the same kind of ecosystem. Use the same method, sampling effort, area, season and identification procedure. Randomly positioned quadrats are suitable for plants or slow-moving organisms. To investigate change along an environmental gradient, place quadrats along a transect. Mobile organisms need an appropriate standardized capture or observation method.

Take enough replicated samples to reduce the effect of chance. Don’t select only convenient or visibly rich locations. Organisms should be identified to a consistent taxonomic level, and every individual must be recorded. Pool the counts only when the sampling design supports this. Then calculate $n(n-1)$ for each species before adding the values.

An index allows comparison; it isn’t an absolute verdict on ecosystem health. Compare like with like. A rise in $D$ through time may indicate recovery. A fall may point to disturbance or increasing dominance, though contextual evidence is needed to investigate the cause. Quantification can reveal changes that a simple species list misses.

Evidence before intervention

Conservation works best when managers know which organisms and habitats are present, where they occur, how their abundance is changing and what pressures affect them. Regional and global databases help reveal broad priorities. Local surveys then supply the detail required for site-specific management.

Researchers can gather local knowledge using standardized quadrats, transects, acoustic surveys, camera traps, species records and habitat mapping. Repeating these surveys shows trends and helps managers judge whether a policy or restoration project is working.

Citizen science is scientific research in which members of the public contribute observations, measurements or classification under an organized method. This approach can produce records across wide areas and over long periods. Voluntary organizations may run specialist recording networks, while government-funded agencies maintain monitoring programmes, legal records and national databases.

A parabiologist is a locally based person trained in biological field and identification skills who gathers data for professional scientists and conservation managers. By training Indigenous people and other local participants, projects can combine standardized methods with detailed knowledge of seasonal patterns, species behaviour and remote places. Participation should respect data ownership, consent and the value of Indigenous knowledge, rather than viewing local people simply as inexpensive labour.

Combining these sources gives the strongest evidence. Large citizen-science datasets may suffer from uneven sampling or identification errors. Professional surveys tend to be more standardized, but they are expensive and cover a limited area. Training, verification and shared protocols improve reliability. With this evidence, managers can identify threatened populations, locate protected areas, regulate harvesting, target habitat restoration and monitor outcomes.

New variants and new combinations

A mutation is a change in the sequence of bases in DNA. It can produce a new variant of a gene, known as an allele. When the affected genetic material can be inherited, the mutation adds new variation to the population. In a particular environment, most mutations are neutral or harmful. A small proportion may become advantageous under particular selective pressures.

Sexual reproduction involves the fusion of gametes from two parents. A gamete is a reproductive cell that contains one parental set of genetic information. Fertilization is the fusion of two gametes. As gametes are produced and then combined randomly during fertilization, new combinations of existing alleles arise.

Keep the difference clear. Mutation generates new gene variants, whereas sexual reproduction rearranges variants already present into new combinations. Both increase the variation on which natural selection can act. Only mutation, though, is the ultimate source of genuinely new alleles.

Preventing gene flow

Reproductive isolation separates populations and prevents gene exchange through successful reproduction. A physical barrier may cause it, or it may develop through differences between populations that live in the same area.

• Rivers, mountain ranges, ocean channels or other barriers cause geographical separation by dividing populations.
• Ecological separation develops when populations in the same area occupy different habitats or use different resources. As a result, they rarely meet to reproduce.
• Differences in mating seasons, signals, songs, courtship displays or host preferences cause behavioural separation.

Contemporary examples

Bonobos and common chimpanzees show speciation linked to geographical separation. The Congo River system divided an ancestral ape population. With gene flow restricted, populations on opposite sides evolved independently under different conditions and eventually became reproductively distinct species.

The apple maggot, Rhagoletis pomonella, shows separation within the same geographical area. Some populations switched from mating and laying eggs on native hawthorn fruit to introduced apple fruit. Adults tend to mate on their preferred host, and the fruits ripen at different times. Host choice and timing therefore reduce interbreeding, with ecological and behavioural separation reinforcing divergence.

These examples show speciation or ongoing reproductive divergence—not simply changes in body form within one freely interbreeding species.

Why isolated islands have high endemism

Endemism describes a species that is native to and restricted to one geographical area. Remote islands are hard to colonize, leaving founding populations small and isolated from mainland gene flow. Each island may offer different foods, climates and competitors. Independent mutation and natural selection can then drive divergence, while continued isolation lets locally evolved forms persist. The result is many endemic species, which occur naturally nowhere else.

Biodiversity hotspots

Biodiversity is spread unevenly across the planet. Certain regions hold an exceptionally large share of global species, including many that are endemic, rare or endangered. A biodiversity hotspot is a geographical area with unusually high species diversity and concentrations of endemic and threatened species.

Many hotspots lie within tropical biomes. Warm temperatures, high rainfall and strong year-round insolation drive high rates of photosynthesis and productivity. Because these regions have also experienced long periods without widespread freezing, evolutionary lineages have had time to accumulate. In tropical mountains, steep environmental gradients create many habitats as altitude changes, increasing opportunities for isolation.

Greater habitat diversity generally supports greater species diversity. Barriers such as mountain chains, islands and tectonically complex landscapes restrict gene flow, encouraging speciation. These conditions often work together: a productive tropical region with varied topography may contain many niches as well as numerous isolated populations.

Hotspots help guide conservation planning because limited resources can protect a large amount of irreplaceable biodiversity. However, concentrating only on hotspots may overlook less species-rich ecosystems that provide vital services or contain unique adaptations.

Altering the environment alters selection

Human activities change the selective forces that act on populations. Burning fossil fuels alters the climate. Hunting, poaching and harvesting remove particular individuals, while cities produce novel habitats. Habitat fragmentation affects movement and food supply, as well as exposure to predators. When survival or reproductive success is linked to a heritable characteristic, these pressures can drive evolutionary change.

Gorongosa, Mozambique, provides a striking example. During armed conflict from 1977 to 1992, heavy ivory poaching removed roughly 90% of the elephant population. Poachers were much more likely to kill tusked elephants, which gave tuskless elephants a survival advantage. Before the conflict, about 18.5% of adult females lacked tusks. Among females that survived into the early 1990s, the proportion was about 33%.

Although humans created the selective pressure, this is still natural selection: the poachers did not deliberately breed elephants. Heritable variation in tusk development was already present. Selective killing then changed differential survival and reproduction, so tusklessness became more frequent.

Unsustainable resource use can alter more than population size. Size-selective fishing may favour earlier reproduction or a smaller mature size. Repeatedly harvesting conspicuous individuals can favour forms that are less visible. Such evolutionary responses may have ecological consequences and can persist even after harvesting pressure declines.

Deliberate selection by people

Artificial selection occurs when people deliberately choose individual plants or animals with desired heritable characteristics and breed them. They select the parents, keep offspring that show the preferred trait, then repeat the process over many generations.

Natural selection isn't deliberate. Environmental conditions create differences in survival and reproduction. By contrast, artificial selection follows human objectives such as yield, appearance, temperament or uniform ripening. Traits desired by humans may offer no advantage to an organism living in the wild.

Breeding repeatedly from a limited group of parents narrows the gene pool. Unselected alleles may disappear, while related individuals may breed more often. As a result, the crop variety or livestock breed becomes genetically uniform. With low genetic diversity, there is less chance that some individuals will survive a new pathogen, pest, drought or temperature extreme. Artificial selection can therefore make populations less resilient.

Environmental and economic value of genetic diversity

In environmental terms, diverse populations have more capacity to adapt and face a lower risk of collapse when conditions change. Their continued survival also supports food webs and ecosystem processes.

From an economic perspective, genetic diversity acts as biological insurance. Different crop varieties and livestock breeds carry genes that breeders may need later for disease resistance, heat tolerance or changing consumer needs. Uniform stocks can produce predictable yields and allow efficient processing. However, a single vulnerability may affect a whole harvest or herd. Such losses can threaten food security, rural livelihoods, export earnings and public finances.

Conserving wild relatives, traditional varieties and multiple breeding lines therefore brings both environmental and economic value. Any short-term gains from uniformity must be weighed against long-term systemic risk.

Deep time

Earth’s history stretches back approximately 4.5 billion years. Mutation, natural selection, speciation and extinction occur at very different rates, as do climate change and plate movement. Across deep time, the combined effects of these processes have produced the evolution of life on Earth.

A fossil is preserved evidence of past life in geological material. It may be a body remain or impression, a trace, or a chemical signature. Fossils show which organisms existed at different times and reveal sequences of appearance, change and extinction.

A single fossil can’t record an evolutionary process from start to finish. Patterns emerge instead from fossils found across many dated rock layers. Older layers contain earlier forms, while transitional characteristics connect some groups. Changes in fossil assemblages also show diversification and extinction. The record has gaps because fossilization requires unusual conditions, and many organisms leave little durable material.

The fossil record is incomplete, but it provides powerful evidence for evolution over geological time. Its broad sequence can be cross-checked between locations and supported by the relative positions and ages of rock layers.

Organizing Earth history

The geological timescale provides a chronological framework for Earth’s history. It draws on evidence from rocks and fossils, along with major geological and biological events. The divisions fit inside one another: eons are divided into eras, eras into periods, and periods into epochs.

An eon is the largest formal division of geological time. Within an eon is an era; each era is divided into a period, and each period into an epoch.

Scientists use the fossil record to identify boundaries. A major environmental change may trigger widespread extinction, after which different organisms appear and diversify. A marked change in fossil assemblages can therefore signal a transition between time intervals, including the boundary from one epoch to the next.

The boundaries aren’t equally spaced. Rather than occurring after a fixed number of years, they’re placed where evidence shows important geological or biological change. Using this framework, scientists can compare events and evolutionary patterns preserved in rocks from different regions.

Ecological loss followed by opportunity

Extinction means the permanent loss of a species from Earth. A mass extinction is a geologically brief interval in which a very large proportion of species becomes extinct across multiple regions and taxonomic groups.

The fossil record shows five past mass extinctions. They had different, sometimes interacting causes: tectonic plate movements, super-volcanic eruptions, climatic changes, sea-level changes and meteorite impact. Such processes can quickly change temperature, atmospheric chemistry, ocean conditions and the availability of habitats.

A mass extinction leaves many ecological niches vacant. With less competition, surviving populations may gain access to resources and habitats that have become available. As different populations encounter different selection pressures, speciation can occur rapidly through adaptive radiation. Recovery still takes geological time. Here, “rapid” is measured against the ordinary pace of major evolutionary change.

The proposed sixth mass extinction differs from the previous five because its main pressures are anthropogenic. Habitat destruction, overharvesting, invasive species, pollution and human-driven climate change are all operating simultaneously and at a planetary scale. Unsustainable resource use removes species directly while disrupting the ecological and evolutionary systems that future diversity depends on.

A debated geological boundary

The Anthropocene is a proposed geological epoch characterized by rapid environmental change and species extinction due to human activity. It would be distinct from the currently recognized Holocene, the geological epoch covering the interval since the end of the last major glacial period.

Scientists disagree about whether to recognize the Anthropocene formally. They also debate where to place its lower boundary. A golden spike is an internationally agreed reference point in a specific geological section that marks the lower boundary of a unit of geological time. To be suitable, the marker must be identifiable, widespread, datable and likely to remain preserved.

Three starting signals have been proposed:

• 1610: a dip in atmospheric $CO_2$ linked to European arrival in the Americas, population decline, reduced cultivation and forest regrowth.
• 1950: the widespread appearance of spherical fly-ash particles produced by high-temperature fossil-fuel combustion.
• 1964: a peak in Carbon-14 associated with atmospheric nuclear testing.

Each alternative reflects a different interpretation. The 1610 proposal focuses on early global ecological and colonial change. By contrast, the mid-twentieth-century proposals offer sharper, more globally synchronized industrial or nuclear markers. The debate isn’t about whether human impacts exist. It concerns whether they meet formal geological requirements and when their planetary signal became sufficiently clear.

A planetary signature

Human activity is changing sediments, chemistry and the distribution of organisms. These changes are likely to remain visible in the future geological record, supporting the argument that the Anthropocene could be designated as an epoch separate from the Holocene.

At least five lines of evidence support this argument:

1. Chemical pollutants build up in soils, lake beds, marine sediments and other strata. Persistent compounds and unusual concentrations of elements may survive as recognizable chemical signatures.
2. By transporting species, humans have mixed native and non-native organisms across continents and islands. Future fossil assemblages may show organisms appearing abruptly far beyond their previous ranges, alongside growing biological similarity between regions.
3. Nuclear testing spread distinctive radioactive materials around the planet. Their unusual isotopic signatures act as closely dated markers.
4. Dams, agriculture, mining, dredging, coastal engineering and trawling have altered the movement of sediment on land and in the oceans. They change erosion, deposition rates and the structure of sedimentary layers.
5. Human industry has created minerals and mineral-like materials that are extremely rare or absent in nature. Finding them can identify strata shaped by technological activity.

Rapid species extinction creates another biological signal. Fly ash and other combustion products provide an industrial one. Because these markers are independent, together they make a stronger case than any single marker could.

The evidence has limitations. Signals differ between locations, and very recent deposits haven’t yet become rock. Their preservation is uncertain, while formal geological units need a precise boundary that can be correlated globally. Human influence is clearly planetary and will probably be detectable in the geological record, but the Anthropocene’s formal status and starting date remain matters of scientific classification and debate.