Biological diversity refers to the vast array of life on Earth, often explored at three distinct, yet interconnected, levels.
The most fundamental level is genetic diversity, which is the variety of genes within a single species. Each individual in a species possesses a unique genetic composition, which accounts for the differences we see among them.
The next level is species diversity, referring to the variety of different species within a particular habitat or region. Some ecosystems, like rainforests, have high species diversity, while others may have fewer species. Finally, ecosystem diversity is the variety of habitats, from deserts to coral reefs, that exist in a given area. This level considers the intricate web of interactions between living organisms and their non-living environments.
The Genetic Basis for Diversity
All new biological traits originate from changes at the genetic level. The source of this novelty is mutation, which creates new versions of genes, called alleles. These spontaneous changes in the DNA sequence are the raw material for all evolution.
This newly created genetic variation is then shuffled and distributed throughout populations. During sexual reproduction, a process called genetic recombination mixes alleles from both parents into new combinations, much like shuffling a deck of cards. This process ensures that offspring are genetically different from their parents and from each other.
Gene flow, which occurs when individuals migrate and breed with new populations, is another method for distributing genetic material. This movement introduces new alleles into a population’s gene pool, increasing its overall diversity.
How New Species Emerge
The accumulation of genetic differences can eventually lead to the formation of new species through a process called speciation. This occurs when populations become so distinct that they can no longer successfully interbreed. A common mechanism for this is allopatric speciation, which involves a geographic barrier physically separating a once-unified population.
A classic example involves the squirrels of the Grand Canyon. When the canyon formed, it isolated a population of Abert’s squirrels. Over thousands of years, the population on the north rim evolved independently from the one on the south rim, facing different environmental pressures. This led to the emergence of the Kaibab squirrel, a distinct species that cannot breed with its southern counterparts.
New species can also arise without a physical barrier in what is known as sympatric speciation. This can happen when a subset of a population begins to exploit a new resource or habitat within the same area as the parent population. Over time, genetic differences accumulate between the two groups until they become reproductively isolated, creating a new species.
Drivers of Diversification in Ecosystems
When a species colonizes a new environment with many unoccupied roles, or niches, it can lead to adaptive radiation. Darwin’s finches in the Galápagos Islands are a prime example of this phenomenon.
From a single ancestral finch species, numerous new species evolved, each with a specialized beak shape suited to a different food source. Some developed thick, strong beaks for cracking seeds, while others evolved slender beaks for probing for insects. This diversification allowed them to take full advantage of the available resources without directly competing with one another.
In established ecosystems, a process called niche partitioning allows for an increase in species diversity. Species evolve to use different resources or the same resources in different ways to minimize competition. For instance, various species of warblers can coexist in the same spruce tree by feeding in different zones—some on the outer branches and others near the trunk.
Biodiversity and Ecosystem Function
An increase in biological diversity has tangible effects on how ecosystems operate. Ecosystems with a greater variety of species are often more stable and better able to withstand environmental disturbances, such as droughts or the introduction of a disease. This is because a diverse community is more likely to contain species that can tolerate the new conditions.
This stability leads to greater resilience, which is the ability of an ecosystem to recover its structure and function after a disturbance. If one species is lost, a diverse ecosystem has a higher chance of having another species that can perform a similar role, preventing a cascade of negative effects. This functional redundancy helps maintain healthy, biodiverse systems.
Higher biodiversity is frequently linked to increased productivity. A diverse plant community, for example, can utilize resources like sunlight, water, and nutrients more efficiently, leading to a greater overall production of biomass. This enhanced productivity forms the base of the food web, supporting a wider array of organisms at higher trophic levels.