Biodiversity & Ecosystem Services — Print · Grade 12 Science

Biodiversity & Ecosystem Services

Learning goals

• I can distinguish genetic, species, and ecosystem diversity and use a diversity index to compare communities.
• I can explain how biodiversity supports ecosystem services and resilience, and why keystone species matter.

Readings for this unit

• A food-web arrow and a survey count both need a definition

Biodiversity is the variety of life, and ecologists measure it at three nested scales. Genetic diversity is the variation in genes and alleles within a single population. Species diversity is the variety of species living together in a community. Ecosystem diversity is the range of distinct habitats and ecological communities spread across a landscape or region. A place can be rich at one level and poor at another, so healthy biodiversity really means variety at all three at once.

Genetic diversity is the quietest of the three but arguably the most fundamental, because it is the raw material evolution works with. A population carrying many different alleles is far more likely to contain a few individuals that can survive a new disease, a heat wave, or a novel predator — some will happen to have resistant traits, and the population persists. A genetically uniform population, by contrast, is fragile: the classic cautionary tale is a monoculture crop in which every plant is nearly identical, so a single pest or blight can wipe out the entire field at once. Low genetic diversity is also why very small or inbred populations struggle to recover.

Species diversity has two ingredients that students often blur together. Richness is simply the count of species present. Evenness describes how balanced their abundances are. Two forests can both have four tree species (equal richness), yet one where each species is common is far more diverse than one dominated by a single species with three rare stragglers. To capture both at once, ecologists use a diversity index such as Simpson's Diversity Index, D = 1 - Σ(n/N)², where n is the number of individuals of a species and N is the total; D runs from 0 (one species dominates completely) toward 1 (many species, evenly spread). The index lets scientists put a single, comparable number on how diverse two communities are.

Why should anyone outside biology care? Because functioning ecosystems quietly perform work that human economies depend on, called ecosystem services. These fall into four groups: *provisioning* services supply tangible goods like food, timber, and fresh water; *regulating* services stabilize conditions by purifying water, pollinating crops, controlling pests, and buffering the climate; *supporting* services underpin the rest through soil formation, nutrient cycling, and photosynthesis; and *cultural* services provide recreation, beauty, and scientific and spiritual value. Economists estimate these services are worth tens of trillions of dollars a year — and generally, more diverse ecosystems deliver them more reliably.

Not every species contributes equally to holding a system together. A keystone species exerts an influence out of all proportion to its numbers. Sea otters are the textbook case: by eating sea urchins, a modest otter population protects entire kelp forests that dozens of other species need. Remove the otters and urchins explode, the kelp is grazed to bare rock, and diversity collapses. Wolves in Yellowstone and beavers that engineer wetlands play similar oversized roles. Losing a keystone species can unravel an ecosystem far faster than losing a more abundant but ordinary one.

Biodiversity can support resilience when organisms have differing responses or overlapping functional roles, but recovery also depends on species identity, disturbance, habitat, and time. More species does not guarantee faster recovery from every event. A resilience claim should specify the function being maintained and the disturbance being tested. Conservation reasoning needs measured ecological outcomes and mechanisms rather than treating one diversity count as a universal performance guarantee.

Model check: A service value or diversity index does not represent every ecological or ethical criterion. D=.5. State this version of the index and the sampling effort before comparing sites.

Simpson's Diversity Index

D = 1 - Σ(n/N)²

n is the number of individuals of a given species and N is the total number of individuals; D ranges from 0 (one species dominates) toward 1 (many species, evenly spread), combining richness and evenness.

Worked examples

Avoid these mistakes

• A service value or diversity index does not represent every ecological or ethical criterion.

Words to know

biodiversity

The total variety of life in a place, measured at three levels: genetic, species, and ecosystem diversity.

genetic diversity

The variety of genes and alleles within a population or species, which supplies the raw material for adapting to change.

species diversity

A measure combining species richness (how many species) with evenness (how equally abundant they are).

ecosystem diversity

The variety of different ecosystems, habitats, and ecological communities across a region.

ecosystem services

The benefits people get from functioning ecosystems, grouped as provisioning, regulating, supporting, and cultural services.

keystone species

A species whose effect on its ecosystem is far larger than its abundance would suggest, so its loss triggers disproportionate change.

Two ways to put a number on diversity

Simpson's index is not the only diversity metric. Simpson's Diversity Index emphasizes dominance — it answers 'if I grab two individuals at random, how likely are they to be different species?' The Shannon index instead weighs how much uncertainty there is in predicting the species of the next individual, giving rarer species relatively more influence. Both combine richness and evenness into one comparable number, and ecologists pick between them depending on whether they care more about common or rare species — a reminder that 'biodiversity' only becomes measurable once you define exactly what you mean.

Biodiversity as biological insurance

Why does diversity make ecosystems more stable? The leading answer is the insurance hypothesis (or portfolio effect). When many species can perform a similar role, the failure of any one is cushioned because others carry on — much like a diversified investment portfolio weathers a single stock's collapse. Experiments in grasslands show that more diverse plots resist drought better and rebound faster than species-poor plots. Redundancy that looks wasteful in good years becomes the safety margin that keeps the system functioning in bad ones.

Trophic cascades and the Yellowstone wolves

When wolves were returned to Yellowstone in 1995 after a 70-year absence, they thinned and moved the overabundant elk, which allowed willows and aspen along streams to recover, which in turn benefited beavers and songbirds — a chain reaction called a trophic cascade. It is one of the most famous demonstrations of a keystone species reshaping a whole landscape from the top down. (Popular accounts sometimes overstate it — claiming wolves literally 'changed the rivers' — so scientists urge care in separating the well-documented effects from the poetic ones.)

HIPPO: the drivers of biodiversity loss

Biologist E.O. Wilson summarized the main threats to biodiversity with the acronym HIPPO: Habitat destruction, Invasive species, Pollution, human Population growth, and Overharvesting — with habitat loss the single largest driver. Add accelerating climate change, and current extinction rates run an estimated hundreds to thousands of times above the natural background rate, leading many scientists to argue we have entered a human-caused 'sixth mass extinction.' The unsettling part is that these drivers compound one another — a fragmented habitat is far more vulnerable to an invasive species or a warming climate.

When a species runs out of genes: cheetahs and the banana

Genetic diversity can be quietly lost even while individuals remain plentiful. Cheetahs passed through a severe population bottleneck thousands of years ago and are now so genetically uniform that unrelated individuals can accept skin grafts from one another — leaving the species dangerously exposed to any new disease. The commercial Cavendish banana is a living cautionary tale: it is a single clone, genetically identical worldwide, and a spreading fungal disease now threatens it exactly as an earlier blight wiped out its predecessor. Abundance is not the same as safety.

In the real world

When economists try to price the world's ecosystem services — pollination, water purification, flood control, climate regulation — the estimates run into the tens of trillions of dollars a year, benefits that would be enormously expensive to replace with technology.

Exit check

Two species have fractions.5,.5. UseD=1−Σpi².