Measure species diversity and explain genetic diversity, evaluate why populations with low genetic diversity are more likely to become extinct, and advise on the trade-offs of crop varieties and wildlife management

Learning goal: Explain biodiversity at two levels, genetic diversity within a population and species diversity within a community; identify organisms with high and low genetic diversity and recognize that species with low genetic diversity are more likely to become extinct; evaluate how specific genetic traits relate to a population's survival; compare the factors that influence growing heirloom and genetically modified varieties and advise stakeholders on the trade-offs; and use a model to analyze the impact of an environmental stressor on the genetic diversity and long-term survival of a population.

Where diversity comes from and what it does. The great diversity of organisms on Earth, and the range of ecological niches they occupy, is the product of more than 3.8 billion years of evolution. Diversity exists at several levels. Genetic diversity is the variety of alleles within a population or species. Species diversity is the number of species in a community and how evenly individuals are spread among them. Ecosystem diversity is the variety of communities across a region. Populations of individual species and groups of species make up a vast reserve of genetic diversity, and losing it is not merely a loss of variety: a loss of diversity alters energy flow, the cycling of matter and the persistence of biological communities, because each species does work in the community, whether as producer, decomposer, pollinator, predator or prey.

Genetic diversity and survival. A population with many alleles is more likely to contain individuals that survive a new disease, a new predator or a changed climate, so it can adapt; a population with little genetic variation is less likely to contain variants suited to a new stressor, so natural selection has less to work with. Loss of genetic diversity in a population increases its probability of extinction. Cheetahs passed through a severe population bottleneck in the past and are genetically so uniform that skin grafts between unrelated cheetahs are often not rejected; they suffer from low fertility and high vulnerability to disease. The northern white rhinoceros has been reduced to a very small number of individuals with no chance of natural recovery, and even intensive management cannot restore alleles that are already gone. The vaquita, a small porpoise of the Gulf of California, has been reduced by drowning in fishing nets to a population so small that a 2023 survey in its core area estimated 8 to 13 animals and the international recovery committee's report put the minimum alive in 2024 at about 10 (these are estimates, not counts, and carry uncertainty); its genetic diversity was naturally low long before the recent decline, and its survival depends on removing the nets, not on genetics alone. Monitoring genetic diversity means sampling DNA from many individuals and counting alleles; management to increase it means enlarging the population, connecting isolated groups so that gene flow resumes, and, in captive breeding, choosing mates to avoid inbreeding. By contrast, tomatoes and beans as crops exist in thousands of varieties carrying many alleles, a reserve breeders draw on whenever a new pest or climate appears.

Traits, diseases and history. The relation between genetic traits and survival shows up in human history. In Ireland in the 1840s much of the population depended on potatoes, and in many regions largely on one variety, the Lumper, propagated as clones, so huge areas carried the same susceptible alleles; when a fungus-like blight arrived, those crops had little resistance, the crop failed for several years running and about a million people died while a million more emigrated. Poverty, land tenure and government policy turned the crop failure into famine, so genetics is only part of the story, but the uniform crop shows what low genetic diversity does under a new stressor. The sickle cell allele shows a different relationship: it harms homozygotes but protects heterozygotes from severe malaria, so in regions with malaria the population's survival is served by keeping the allele. Hemophilia, an X-linked recessive disorder, spread through several European royal families in the 1800s and 1900s from a single carrier, an example of how one allele can move through a small, intermarrying group. Modern data sets from public health agencies and genome databases let students examine allele frequencies within populations directly.

Measuring species diversity. Species diversity has two parts: richness, the number of species, and evenness, how equally individuals are spread among them. A worked model: two study plots of 100 plants each. Plot A has four species with 25 individuals each; plot B has four species with 85, 5, 5 and 5. Both have a richness of four, but B is dominated by one species. Simpson's index of diversity captures the difference: for each species, square its share of the total, add the squares and subtract from one. Plot A: 4 × (0.25)² = 0.25, so the index is 0.75. Plot B: (0.85)² + 3 × (0.05)² = 0.7225 + 0.0075 = 0.73, so the index is 0.27. The index is the probability that two individuals drawn at random (with replacement) belong to different species; three times out of four in plot A, but only about one in four in plot B. Species diversity tends to increase toward the equator and decrease with altitude, a pattern seen from biomes to mountainsides that correlates with energy input and climate stability.

Crop varieties: heirloom and genetically modified. An heirloom variety is an older, open-pollinated variety maintained by saving seed; a genetically modified variety carries one or more inserted or edited genes. Many modern commercial varieties of some crops, modified or not, are hybrids bred for uniformity, but that differs from crop to crop, and three questions must be kept apart: whether a variety is genetically modified, whether it is a hybrid, and how genetically diverse the plants in a field or a region are. Genetic modification by itself does not decide how uniform a crop is. The factors that influence growing each differ. Heirlooms: seed can be saved and replanted, flavors and forms are varied, a locally saved variety often carries alleles adapted to local conditions and contributes to the species' genetic reserve, but yields are often lower, plants may be less uniform and less resistant to certain pests, and they may not suit mechanized harvest. Modified varieties: depending on the trait, often higher or more reliable yield, built-in resistance to a pest or tolerance of a herbicide, uniformity for processing, but seed usually must be bought each year under the seller's terms, a region planted to one or a few commercial varieties of any kind is genetically uniform and so vulnerable to any stressor that defeats them, resistance can spread to pests and weeds by selection, and gene flow to neighboring fields can be a legal and ecological concern. Advice to a community weighs food security now against resilience later; a common recommendation is to grow the productive variety on most land while maintaining heirloom plots and seed banks as insurance.

Stocking, reintroduction and management. Wildlife management alters diversity deliberately. Ohio's Division of Wildlife stocks fish for angling and for restoration; from 1996 through 2003, for example, it released hatchery-reared native brook trout into 15 northeast Ohio streams. Benefits of stocking include recreation and support for local economies, while potential problems include competition with native fish, spread of disease and, where stocked fish can interbreed with wild relatives, dilution of locally adapted alleles. Species removed from endangered lists after recovery, such as the river otter and bobcat that Ohio's model curriculum names, are managed under plans that set monitoring and harvest rules, and evaluating a plan means asking whether the population's size, range and genetic diversity are tracked and what triggers a change in policy. Reintroducing wolves to Yellowstone in the 1990s changed elk behavior and numbers and, according to many studies, allowed streamside vegetation to recover in places, though the size of the effect is debated; a cautious statement is that a top predator can alter a whole community. Historical and current population data from state agencies let students correlate population size with management actions over decades.

Misconceptions. "More individuals means more diversity" confuses abundance with diversity; a million clones have almost no genetic diversity. "Any four species give the same diversity" ignores evenness. "Extinction is only about numbers" misses that a population can be large and yet genetically fragile, like the Irish potato. "A modified variety is always better or always worse" ignores that the trade-offs depend on the stakeholder, the pest and the place.

Investigation you can do. Survey species diversity on the school grounds or in a local park. Mark out five 1 m × 1 m quadrats in a mowed lawn and five in an unmowed edge or field, using a string frame. In each, identify every plant species you can distinguish (label unknowns as species A, B and so on, consistently) and count the individuals or estimate percent cover for each. Compute richness and Simpson's index for each quadrat and average by habitat. A result in which the unmowed habitat has higher richness and a higher index supports the claim that regular disturbance lowers species diversity. Then use a simple model of a stressor: for a hypothetical population of 20 individuals, one card each, labeled with one of 6 alleles of a gene (A to F) in roughly equal numbers, remove half the individuals at random by drawing cards and count how many alleles remain, repeating for a population of 200 with the same 6 alleles; the smaller population loses alleles more often. To follow long-term survival, carry the model through generations: let the survivors rebuild the population to its original size by drawing a survivor's card at random, writing a copy and returning the card, until the deck is full again (numbers recover; lost alleles do not), repeat the random stressor for three more generations, and then apply a selective stressor, a disease that only carriers of allele F survive, and record whether any individuals are left. Run each population size ten times and tabulate the alleles remaining after every generation and the number of runs that end in extinction. What the models cannot show: a quadrat survey counts plants only and misses insects, fungi and soil life, and the card model has one gene, no mutation or immigration to restore alleles and only the selection you script, so it cannot show how long a real population takes to lose or regain diversity.

Worked Simpson's index: Plot A, 100 plants, four species with 25 each: shares 0.25 each, squares 0.0625 each, sum 0.25, index 1 − 0.25 = 0.75. Plot B, 100 plants, shares 0.85, 0.05, 0.05, 0.05: squares 0.7225, 0.0025, 0.0025, 0.0025, sum 0.73, index 0.27. Both plots have richness 4; plot A's evenness gives it nearly three times the diversity of plot B. Interpretation: in plot A two randomly chosen plants are different species 75 percent of the time; in plot B only 27 percent, because most draws produce two of the dominant species.

Worked analysis of a bottleneck's effect on alleles: A hypothetical population of 200 birds carries six alleles of a gene at frequencies 0.30, 0.25, 0.20, 0.15, 0.07 and 0.03. A storm kills 190 birds at random, leaving 10 (20 gene copies). The expected number of copies of the rarest allele among the survivors is 20 × 0.03 = 0.6, so it is more likely lost than kept; the allele at 0.07 is expected at 1.4 copies and is often lost too. Adding up the chances that each allele survives, the population most likely emerges with five alleles (about half the time), keeps all six only about a third of the time and ends with four or fewer about one time in seven; and no amount of later growth restores the lost ones unless mutation or immigration reintroduces them. That is why a species that has passed through a bottleneck stays genetically poor long after its numbers recover.

Worked advice on crop varieties: A hypothetical community grows maize on 1,000 hectares. Option 1: a modified pest-resistant hybrid on all land, expected yield 20 percent higher, seed purchased yearly, uniform genotype. Option 2: local heirloom varieties on all land, seed saved, diverse genotypes, lower and more variable yield. Advice: plant the hybrid on about 800 hectares for yield and income, keep 200 hectares in several heirloom varieties with a community seed bank, and monitor pest populations for resistance. Reasoning: the heirloom plots preserve alleles that may be needed when a new pest or drought arrives, while the hybrid meets present food needs; the trade-off accepted is a slightly lower total yield now for insurance against a total failure later, as happened with the Irish potato.

Worked evaluation of a management plan: A state removes the river otter from its endangered list after reintroductions. A good plan should include population surveys on a fixed schedule, a map of occupied watersheds, a harvest limit tied to survey results, and a trigger, such as a decline of a set percentage, that suspends harvest. Predicted effects: a stable or growing otter population, some reduction in fish and crayfish in occupied streams, and possible conflict with pond owners; genetic diversity should be checked because reintroduced populations descend from a limited number of founders.

Supported record: Make a table of five organisms or populations, labeling each high or low genetic diversity with the reason (crop varieties, bottleneck, clonal propagation, small population) and stating one consequence for survival. Under it, work Simpson's index for two hypothetical communities of your own design with the same richness but different evenness, showing every step.

Independent performance: Carry out the quadrat survey in two habitats and write a report that also advises a stakeholder. Success criteria: (a) a data table lists every species and its count or percent cover in all ten quadrats; (b) richness and Simpson's index are computed for each quadrat and averaged by habitat with the arithmetic shown; (c) the explanation states what the difference between habitats shows about disturbance and diversity and what the survey could not detect; (d) a section explains the difference between species diversity and genetic diversity with one example of each; (e) the report closes with advice to a hypothetical farming community on growing an heirloom or a genetically modified variety, naming at least two factors on each side and a recommended trade-off; (f) it states why a species with low genetic diversity is more likely to become extinct, using one real example.