Difficulty: Introductory | Prerequisites: None
Biodiversity is the variety of all living things on Earth, and it underpins the food, medicine, and clean air humans depend on. Species interact through webs of interdependence, so losing even one key species can destabilise an entire ecosystem. Six major human-driven factors, from habitat destruction to climate change, are accelerating biodiversity loss in what scientists call the sixth mass extinction.
Biodiversity
The variety of all living things in a given area or on Earth as a whole, encompassing species diversity, genetic diversity, and ecosystem diversity. In simple terms, it is the full range of life, from bacteria to blue whales, and every habitat they occupy.
Species richness
The number of different species present in a particular area. Think of it as a simple headcount of species: more species means higher richness.
Species evenness
A measure of how equal the population sizes of different species are within a community. In simple terms, if one species massively outnumbers all others, evenness is low even if richness is high.
Interdependence
The ecological principle that species rely on each other for survival, forming networks of mutual dependency. Think of it as a web: pull one thread and the whole structure shifts.
Keystone species
A species whose presence and role have a disproportionately large effect on its ecosystem relative to its abundance. Its removal can trigger dramatic changes or collapse. In simple terms, it is the one species holding the ecosystem together.
Indicator species
A living organism, plant or animal, that provides information about the overall health or condition of its environment. Think of it as an early-warning system: if the indicator species disappears, something in the ecosystem is going wrong.
Biomagnification
The process by which toxins become increasingly concentrated as they move up through trophic levels of a food chain. In simple terms, a small amount of pesticide in water can become a lethal dose by the time it reaches top predators.
Habitat fragmentation
The splitting of large, continuous habitats into smaller, isolated patches, typically caused by human activity such as road-building or agriculture. Think of it as cutting a forest into islands: species inside each patch may not be able to reach mates, food, or shelter in other patches.
Invasive species
Non-native organisms introduced to an ecosystem that outcompete, prey on, or otherwise harm local wildlife. In simple terms, they are uninvited guests that take over resources from species that evolved there.
Endangered species
A species at serious risk of extinction in the near future. In simple terms, numbers are critically low and the species may not survive without intervention.
Threatened species
A species likely to become endangered if current pressures continue. Think of it as the step before endangered: the population is declining, but there is still time to act.
Poaching
The illegal hunting, capturing, or killing of wildlife. In simple terms, it is another term for illegal hunting, and it is a major driver of species decline worldwide.
Extinction
The complete removal of a species from an ecosystem, with no surviving individuals anywhere. In simple terms, once a species is extinct, it is gone permanently.
Ecological hot spot
A region with a high level of biodiversity that is simultaneously threatened by human activity. Think of it as a place that contains a lot of life but is losing it fast.
Biodiversity is the variety of living things in an area or across the planet.
Two main metrics:
Species richness: the count of distinct species in a defined area.
Species evenness: how evenly individuals are distributed among those species. A community with 100 deer and 2 wolves has low evenness; one with 50 deer and 48 wolves has high evenness.
Biodiversity increases as you move towards the equator. Tropical regions support more species than temperate or polar ones, largely because of stable climates and high energy input.
Species do not exist in isolation. Interdependence means organisms rely on one another for food, shelter, pollination, seed dispersal, and nutrient cycling.
Keystone species have an outsized role. Remove them and the ecosystem can change dramatically or collapse. Classic example: sea otters control sea urchin populations; without otters, urchins destroy kelp forests.
Indicator species act as biological monitors. Their health signals the condition of the broader ecosystem. Frogs, for instance, are sensitive to water quality, so a frog population crash may indicate pollution or habitat degradation.
If an indicator species goes extinct, environmental problems that would have been caught early may go undetected and worsen.
Biodiversity provides food, medicine, and clean air.
Ecosystem services include water purification, pollination of crops, flood regulation, and carbon storage.
Loss of biodiversity is not only an ecological concern; it is a direct threat to human well-being and economic stability.
Habitat destruction: the outright loss of living spaces for plants and animals, through deforestation, draining wetlands, or converting land to agriculture.
Pollution: contamination of air, water, and soil that harms ecosystems. Three main types:
Air pollution (emissions from industry, vehicles, and burning fossil fuels)
Water pollution (runoff, chemical discharge, plastics)
Soil pollution (pesticides, heavy metals, industrial waste)
Climate change: altered weather patterns, rising temperatures, and shifting seasons that affect species survival and distribution.
Overexploitation: excessive harvesting of resources such as fish, timber, and wildlife beyond what populations can sustain.
Invasive species: non-native organisms that outcompete local wildlife for food, space, and resources. Three reasons they are harmful:
They prey on or outcompete native species.
They can alter habitats (e.g. invasive plants changing soil chemistry).
They often have no natural predators in the new environment, so their populations grow unchecked.
Urbanisation: the expansion of cities and infrastructure, which replaces natural habitats with concrete and reduces the space available for wildlife.
Large habitats can be fragmented when they are split into smaller, isolated areas, often by roads, fences, or development. Fragmented populations may lose genetic diversity, struggle to find mates, and become more vulnerable to local extinction.
Toxins such as pesticides and heavy metals enter food chains at the lowest trophic level and become more concentrated at each step up. Top predators, including birds of prey and large fish, accumulate the highest doses. This is why DDT famously thinned the eggshells of bald eagles.
Earth has experienced five major mass extinctions in its history.
The sixth mass extinction is ongoing and is being caused primarily by human activities: habitat loss, pollution, climate change, and overexploitation.
Increased human populations lead to increased consumption of resources, compounding the pressure on natural systems.
Poaching (illegal hunting) is a significant driver of species decline, particularly for high-value animals such as elephants, rhinos, and tigers.
In the United States, a hunting or fishing licence is required before you can legally hunt or fish (specifics vary by state; in Virginia, this is mandatory).
Students often confuse species richness with species evenness. High richness (many species) does not guarantee high evenness (balanced populations). An area could have 50 species yet be dominated by just one.
Students sometimes treat endangered and threatened as interchangeable. Endangered means extinction is imminent without intervention. Threatened means the species is declining and could become endangered, but it is not yet at the critical stage.
Biomagnification is sometimes confused with bioaccumulation. Bioaccumulation is the build-up of a substance within a single organism over its lifetime. Biomagnification is the increase in concentration across trophic levels of a food chain.
Students often assume the sixth mass extinction is a future event. It is not. It is happening now, driven by ongoing human activity.
⚠️ Know the difference between species richness and species evenness. This distinction appears frequently on exams.
⚠️ Be able to name and describe all six factors that lead to biodiversity loss. Expect a table or matching question.
⚠️ Understand the distinction between endangered and threatened species. A common exam trap swaps their definitions.
⚠️ Know what keystone and indicator species are and what happens when each is removed. Expect a scenario-based question.
⚠️ Be ready to explain biomagnification with an example (e.g. DDT in food chains).
⚠️ The sixth mass extinction is caused by human activity, not a natural event. Make sure your answer names human drivers specifically.
True or False: Species richness and species evenness measure the same thing. Answer: False. Richness counts species; evenness measures how balanced their populations are.
Fill in the blank: Areas of biodiversity increase as you move towards the ________. Answer: equator.
True or False: The sixth mass extinction is a predicted future event that has not started yet. Answer: False. It is ongoing and driven by human activities.
Fill in the blank: The process by which toxins become more concentrated at higher levels of a food chain is called ________. Answer: biomagnification.
True or False: A threatened species is at greater immediate risk of extinction than an endangered species. Answer: False. Endangered species are at greater immediate risk; threatened species could become endangered if conditions worsen.
Q: Define biodiversity and explain why it is important to humans.
A: Biodiversity is the variety of all living things. It is important because it provides food, medicine, and clean air, and it supports ecosystem services such as water purification and pollination.
Q: What is the difference between species richness and species evenness?
A: Species richness is the number of different species in an area. Species evenness is how equally individuals are distributed among those species.
Q: Name and briefly describe the six factors that lead to biodiversity loss.
A: Habitat destruction (loss of living spaces), pollution (contamination of air, water, and soil), climate change (altered weather patterns), overexploitation (excessive harvesting of resources), invasive species (non-native organisms outcompeting locals), and urbanisation (expansion of cities reducing habitats).
Q: What is a keystone species, and what happens if one goes extinct?
A: A keystone species has a disproportionately large effect on its ecosystem. If removed, the ecosystem can change dramatically or collapse, and other species may suffer or disappear.
Q: Explain biomagnification using an example.
A: Biomagnification is the increasing concentration of toxins at higher trophic levels. For example, small amounts of DDT in water are absorbed by algae, concentrated in fish, and reach dangerous levels in birds of prey, causing eggshell thinning.
Q: What distinguishes an endangered species from a threatened species?
A: An endangered species is at immediate risk of extinction. A threatened species is declining and could become endangered, but is not yet at the critical point.
Q: What is habitat fragmentation and why is it a problem?
A: Habitat fragmentation is the splitting of large habitats into smaller, isolated patches. It reduces genetic diversity, makes it harder for organisms to find mates and resources, and increases vulnerability to local extinction.
Q: What is an indicator species and what does it tell us?
A: An indicator species is an organism whose health reflects the condition of its environment. Its decline or disappearance signals that something in the ecosystem is deteriorating.
Biodiversity, biological diversity, species richness, species evenness, species diversity, ecosystem diversity, genetic diversity, interdependence, ecological interdependence, food web, keystone species, indicator species, bioindicator, biomagnification, bioaccumulation, habitat destruction, habitat loss, deforestation, habitat fragmentation, invasive species, non-native species, introduced species, pollution, air pollution, water pollution, soil pollution, climate change, global warming, overexploitation, overharvesting, overfishing, urbanisation, urbanization, mass extinction, sixth mass extinction, Holocene extinction, endangered species, threatened species, at-risk species, poaching, illegal hunting, extinction, species loss, ecological hot spot, biodiversity hotspot, equator biodiversity gradient, latitudinal diversity gradient