Ecology, Populations & Biodiversity — Print · Physics & Environmental Science, Grade 12

Ecosystem Structure & Energy Flow

Learning goals

• I can trace energy through trophic levels and food webs and quantify the loss at each step.
• I can distinguish gross primary productivity from net primary productivity and use the 10% rule to estimate energy at higher trophic levels.

Readings for this unit

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

Nearly every ecosystem on Earth runs on a single incoming fuel supply: sunlight. Autotrophs — green plants, algae, and photosynthetic bacteria — are the only members of the community that can capture that raw solar energy and lock it into the chemical bonds of sugars through photosynthesis. Because they manufacture food from scratch, autotrophs form the productive base that every other organism ultimately depends on. (A striking exception lives in perpetual darkness: at hydrothermal vents on the deep ocean floor, bacteria run on chemosynthesis, harvesting energy from chemicals like hydrogen sulfide instead of light — a reminder that what an ecosystem really needs is not sunlight specifically, but a steady source of usable energy.)

A heterotroph obtains organic carbon by consuming organic material; not all organisms that lack photosynthesis are heterotrophs, because some autotrophs use chemical energy. Trophic levels describe feeding roles: primary consumers eat producers and higher consumers eat other consumers. Decomposers and detritivores process dead tissue and wastes. Real organisms may use several food sources and span trophic positions, so a food web is often more informative than one chain.

To compare ecosystems quantitatively, ecologists measure how much energy the producers actually capture. Gross primary productivity (GPP) is the total energy fixed by photosynthesis, but producers, like all living things, must spend energy on their own respiration just to stay alive. Subtract that cost and you are left with net primary productivity (NPP) — the energy stored as new plant biomass and, crucially, the energy actually available to consumers: NPP = GPP - respiration. NPP is usually reported in units like kcal/m²/yr or g/m²/yr, and it varies enormously across the planet, from lush tropical rainforests and estuaries at the high end to deserts and the open ocean at the low end.

Trophic-transfer efficiency compares compatible production amounts or rates between feeding levels. A 10% value is a useful classroom estimate, not a constant for every ecosystem. Respiration, uneaten material, unassimilated material, and detrital pathways contribute to differences between levels. State the area, interval, and definition of production before calculating a ratio. Standing biomass and organism numbers are different measures and need separate interpretation.

Energy-production pyramids generally narrow toward higher trophic levels within a compatible account because transfer into new biomass is limited. This does not impose a universal pyramid of organism numbers: a few large trees can support many small herbivores, and standing-biomass patterns can differ from production. Available production, organism size, turnover, and feeding relationships influence food-web structure. There is no fixed universal maximum of four or five trophic levels derived from an exact 10% law.

Food-web production comparisons can inform ecological and resource questions, but no fixed 90% loss applies to every feeding step. Match species, food source, area, interval and production definition. A lower-trophic service can require fewer intermediate production transfers under comparable conditions; actual food systems also differ in land, water, nonhuman-edible inputs, waste and nutritional service. Quantitative conclusions should retain those boundaries rather than turning one classroom efficiency into a universal diet or land-use ranking.

Model check: The 10% transfer rule is an approximate teaching model, not a universal measured constant. First 1000, second 100 units. State that the given efficiency is modeled and energy is not recycled like matter.

Net Primary Productivity (NPP)

NPP = GPP - respiration

GPP is the total energy producers fix by photosynthesis; subtracting the energy they spend on their own respiration leaves the energy stored as biomass and available to consumers (often in kcal/m²/yr or g/m²/yr).

The 10% Rule (ecological efficiency)

Energy at next trophic level ≈ energy at current level × 10%

estimates how much energy transfers from one trophic level to the next; the other ~90% is lost to respiration, heat, and undigested material at each step.

Worked examples

Avoid these mistakes

• The 10% transfer rule is an approximate teaching model, not a universal measured constant.

Words to know

autotroph

A producer that builds its own food from inorganic sources, usually sunlight (photosynthesis) or, at deep-sea vents, chemicals (chemosynthesis).

heterotroph

A consumer that obtains energy by eating other organisms, since it cannot make its own food.

trophic level

A feeding position in a food chain — producers, then primary consumers, secondary consumers, and so on up to top predators.

gross primary productivity (GPP)

The total amount of energy producers capture and fix into organic matter over a given time and area.

net primary productivity (NPP)

The energy producers have left after their own cellular respiration (NPP = GPP - respiration); it is the energy actually available to the rest of the food web.

ecological efficiency (10% rule)

The general finding that only about 10% of the energy at one trophic level is stored as biomass the next level can eat; the rest is used or lost as heat.

The second law of thermodynamics, hiding in every food web

The 10% rule is really biology obeying physics. The second law of thermodynamics says that every time energy changes form, some of it is dispersed as unusable heat and total disorder (entropy) rises. When a rabbit converts grass into rabbit, most of the grass's energy is spent on respiration and radiates away as body heat that can never be eaten again. So the steady loss up a food chain is not wasteful sloppiness — it is the unavoidable cost of doing biological work, and it is why energy must keep flowing in from the sun rather than simply cycling like matter.

The lake study that founded ecosystem ecology

In 1942, a young ecologist named Raymond Lindeman published a study of energy moving through the food web of a small Minnesota lake, Cedar Bog Lake. Instead of just describing who ate whom, he measured how much energy passed between trophic levels — pioneering the 'trophic-dynamic' view of ecosystems as energy-processing systems. His paper, published shortly before his early death at 27, is often credited with launching modern ecosystem ecology and provided some of the first hard data behind the 10% figure.

Why biomass pyramids sometimes flip upside down

An energy pyramid is always widest at the base, but a biomass pyramid — the total mass of living tissue at each level at one instant — can occasionally look inverted. In the open ocean, tiny phytoplankton reproduce and are eaten so fast that at any single snapshot their standing biomass can be smaller than that of the zooplankton grazing on them. The phytoplankton still produce far more energy over time; they just don't accumulate, because they are consumed almost as quickly as they grow. It's a classic reminder to distinguish a rate (productivity) from a standing stock (biomass).

Is it really exactly 10%?

The '10% rule' is a convenient average, not a natural constant. Measured ecological efficiencies actually range from under 1% to over 40% depending on the organisms. Warm-blooded animals (endotherms) are especially inefficient transfer points because they burn enormous energy maintaining body temperature, so a food chain of mammals may pass along only a few percent per step. Cold-blooded invertebrates can be far more efficient. Ecologists use 10% because it is roughly right across many systems — but the real number is worth questioning in any specific food web.

Poisons that grow stronger up the pyramid

Some pollutants do the opposite of energy — they concentrate as they climb. Fat-soluble toxins like mercury and the pesticide DDT are not broken down or excreted, so they accumulate in an organism's tissues over its lifetime (bioaccumulation). Because each predator eats many contaminated prey, the concentration multiplies at each higher trophic level (biomagnification). This is why top predators such as swordfish, eagles, and orcas carry the heaviest toxic loads, and why health advisories warn against eating large predatory fish too often.

In the real world

Satellites measure the net primary productivity of ecosystems across the whole planet, letting scientists track how much carbon the biosphere is absorbing and estimate how much food and fuel Earth's ecosystems can sustainably provide.

Exit check

Producer energy 10000 units and supplied efficiency 10% per step. Find second consumer level.

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.

Population Ecology

Learning goals

• I can model population change with exponential and logistic growth and locate the carrying capacity.
• I can distinguish density-dependent from density-independent limiting factors and interpret survivorship curves.

Readings for this unit

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

A population is a group of one species in one place, and its size is bookkeeping: it rises with births and immigration and falls with deaths and emigration. Ecologists summarize the balance with a per-capita growth rate, r. When r is positive the population grows, when it is negative it shrinks, and the beauty of the model is that the same small handful of numbers describes a colony of bacteria, a herd of deer, or the human species.

When resources are effectively unlimited, growth is exponential: each individual reproduces at the same rate, so the more individuals there are, the faster the total climbs, tracing a J-shaped curve that gets ever steeper. Written as a rate, dN/dt = rN. Bacteria in fresh nutrient broth, an invasive species arriving in a new land, and the recent explosion of the human population all show this runaway pattern. But no J-curve can last: multiply anything long enough and it collides with the finite limits of the real world.

The continuous logistic-growth model dN/dt=rN(1−N/K) reduces per-capita growth as N increases toward a fixed conditional carrying-capacity parameter K. It approaches K smoothly under its simple assumptions; overshoot, oscillations and collapse require changing conditions, delays or other model extensions. Real carrying capacity can change with resources and habitat. A measured population trajectory is not required to match one ideal S-curve, and K is not an eternal fixed biological maximum.

What actually applies the brakes are limiting factors, and sorting them into two kinds is one of the most useful ideas in population ecology. *Density-dependent* factors bite harder as a population gets crowded — competition for food, predation, and the rapid spread of disease all intensify with density, so they naturally pull a population back toward K. *Density-independent* factors strike regardless of how many individuals are present: a hurricane, a hard freeze, a drought, or a wildfire can flatten a sparse population and a dense one alike. Real populations are shaped by both, which is why their numbers fluctuate rather than holding perfectly steady.

Species also differ in their whole life strategy, and survivorship curves make the differences visible by plotting how many of a cohort remain alive across the lifespan. *Type I* species (humans, elephants, other large mammals) invest heavily in a few offspring, so most survive to old age and mortality is late. *Type III* species (many fish, insects, oysters, plants) flood the environment with huge numbers of offspring and provide little care, so the vast majority die young while a lucky few survive. *Type II* species (many birds and small mammals) fall in between, facing a roughly constant risk of death at every age.

For the ideal fixed-parameter logistic model, maximum instantaneous total growth occurs at N=K/2. That mathematical result does not by itself establish a safe real harvest limit: variable recruitment, observation error, age structure, ecosystem effects and delayed management responses matter. Use the model as a conditional baseline, compare evidence and uncertainty, and avoid treating maximum modeled production as a guaranteed sustainable policy.

Model check: rN(1−N/K) is an instantaneous rate in the continuous model, not an exact finite-year population increase. dN/dt=.4×500×(1−.2)=160individuals/year at that moment; future N changes the rate.

Exponential growth

dN/dt = rN

N is the population size and r is the per-capita growth rate; produces a J-shaped curve when resources are unlimited.

Logistic growth

dN/dt = rN((K - N)/K)

K is the carrying capacity; the (K - N)/K term shrinks toward 0 as N approaches K, slowing growth into an S-shaped curve.

Population growth rate

growth rate = (births - deaths) / N

the per-capita rate of change (also add immigration and subtract emigration when present); multiply by 100 to express it as a percent.

Rule of 70 (doubling time)

doubling time (years) ≈ 70 / (percent growth rate)

a quick estimate of how long a steadily growing population takes to double; e.g., a 2%/yr rate doubles in about 35 years.

Worked examples

Avoid these mistakes

• rN(1−N/K) is an instantaneous rate in the continuous model, not an exact finite-year population increase.

Words to know

population

A group of individuals of the same species living in the same area, whose size changes with births, deaths, immigration, and emigration.

exponential growth

Growth at a constant per-capita rate when resources are unlimited, producing a J-shaped curve (dN/dt = rN).

logistic growth

Growth that slows as a population nears the limits of its environment, producing an S-shaped curve that levels off at carrying capacity.

carrying capacity (K)

The maximum population size an environment can sustain over the long term, given its resources and limiting factors.

limiting factor

A resource or condition that restricts population growth; density-dependent factors intensify with crowding, while density-independent factors act regardless of density.

survivorship curve

A graph of how many individuals of a cohort survive over their lifespan, classed as Type I, II, or III.

Live fast or live long: r- and K-selection

Ecologists describe two broad life strategies. r-selected species (many insects, weeds, bacteria) reproduce early and prolifically with little parental care; they thrive in unstable or empty habitats and show boom-and-bust, near-exponential growth — the Type III survivorship pattern. K-selected species (elephants, whales, humans, oak trees) produce few offspring, invest heavily in each, mature slowly, and live near carrying capacity with Type I survivorship. The labels are two ends of a continuum, but they neatly connect growth curves, survivorship, and reproductive strategy into one story.

The lynx and the hare: a century of coupled cycles

Some of the best long-term population data on Earth came from an unlikely source: the fur-trapping records of the Hudson's Bay Company, which logged snowshoe hare and Canada lynx pelts for nearly two centuries. The counts reveal a striking pattern — hare numbers rise and fall on a roughly 10-year cycle, and lynx numbers follow a step behind, a real-world example of coupled predator-prey cycles. More hares feed more lynx; more lynx then crash the hare population; fewer hares then starve the lynx, letting hares rebound. (The cycle is also driven partly by the hares' own food supply, showing that nature is rarely a clean two-species model.)

Does the human population have a carrying capacity?

For most of history human numbers crept along, then exploded into a textbook J-curve after the Industrial and Agricultural Revolutions, reaching 8 billion. Yet growth rates peaked in the 1960 s and have been falling since, as countries pass through the demographic transition from high birth-and-death rates to low ones. Demographers now project the curve bending toward an S-shape, leveling off later this century. Whether Earth's carrying capacity for humans is 9 billion or far more is fiercely debated — because, uniquely, humans can raise K with technology like fertilizer and irrigation, and lower it by degrading the very resources they depend on.

Overshoot and collapse on St. Matthew Island

Populations do not always glide gently to carrying capacity. In 1944, 29 reindeer were introduced to remote St. Matthew Island with abundant lichen and no predators. Freed of limits, the herd grew explosively to about 6,000 by 1963 — then, having stripped the slow-growing lichen bare, it crashed catastrophically to fewer than 50 animals in a single brutal winter. It is a stark real-world lesson that a population can overshoot its carrying capacity, degrade the resource base, and collapse far below where it started, rather than settling smoothly at K.

Why maximum sustainable yield sits near half of K

The logistic model has a practical payoff. Because the growth term rN((K - N)/K) is a downward parabola in N, a population adds the most new individuals per year when N is about K/2 — that is where the curve is steepest. Harvesting a fishery or forest down to roughly half its carrying capacity therefore lets it regrow fastest, defining the maximum sustainable yield. In practice, managers treat this as a ceiling, not a target: uncertainty in the data means aiming exactly at MSY has repeatedly tipped real fisheries, like the Atlantic cod, into collapse.

In the real world

Fisheries managers use logistic-growth models to set catch limits near a stock's maximum sustainable yield (around N = K/2, where the population grows fastest), harvesting enough to feed people while leaving the population able to rebuild.

Exit check

N=500,r=.4/year,K=2500. Find instantaneous growth rate.

Biomes & Aquatic Systems

Learning goals

• I can explain how temperature and precipitation determine which terrestrial biome forms in a region.
• I can describe the major freshwater and marine systems and the physical factors that shape aquatic life.

Readings for this unit

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

A biome is a large region unified by its climate and by the community of life adapted to that climate. Move across the globe and you pass through tropical rainforest, savanna, desert, temperate grassland, temperate deciduous forest, boreal forest (taiga), and tundra — each a recognizable package of vegetation and wildlife that reappears wherever similar conditions occur. The single best predictor of which biome you are standing in is climate, and more specifically two variables: average temperature and annual precipitation.

Plotting biomes against temperature and precipitation (a Whittaker diagram) reveals a clean logic. Hot and wet year-round yields tropical rainforest, the most biodiverse and productive biome on land. Hot with a pronounced dry season gives savanna; hot and dry gives desert. Cooler, moderate climates support temperate grasslands where rainfall is low and temperate forests where it is higher. Colder still lies the vast boreal forest of conifers, and at the cold, dry extreme sits the tundra, where a layer of permanently frozen soil called permafrost and a short growing season allow only low, hardy plants. Latitude and elevation shape all of this because both change temperature: climbing a tall tropical mountain takes you through a sequence of biomes much like traveling from the equator to the poles.

Because climate draws the map, changing the climate redraws it — the core idea behind this unit's standard. As the planet warms, the conditions that define each biome are shifting toward the poles and up mountainsides. Biologists already observe treelines creeping higher, tundra shrinking as boreal forest advances, and the ranges of countless species migrating to track the climate they are adapted to. Where species cannot move fast enough, or run out of cooler ground to move into, the result can be a wholly reorganized community — a new ecosystem in place of the old one.

Water covers most of Earth, and aquatic systems split first by salinity. Freshwater systems include standing water (lakes and ponds) and flowing water (streams and rivers). A lake is layered into zones: the shallow, plant-rich littoral zone near shore; the open, sunlit limnetic zone where plankton drive photosynthesis; the dark, cold profundal zone below the reach of light; and the benthic zone on the bottom where decomposers work. Flowing water is shaped by current, which mixes in oxygen but carries organisms downstream. And wetlands — marshes, swamps, and bogs where the land stays saturated — are among the most productive and valuable ecosystems anywhere, filtering pollutants, recharging groundwater, and buffering coasts and rivers against floods.

Marine systems, holding about 97% of Earth's water, are organized largely by light and depth. The sunlit surface layer, the photic zone, is where nearly all ocean photosynthesis happens, powered by microscopic phytoplankton that form the base of marine food webs. Along coasts, shallow, nutrient-rich waters host coral reefs — dazzlingly diverse 'rainforests of the sea' — and estuarys, where rivers meet the ocean and the mix of freshwater nutrients with sunlit shallows makes them exceptional nurseries for fish and shellfish. Farther out, the open ocean is a vast but comparatively unproductive desert per square meter, because nutrients sink into the lightless depths; only where currents force deep water back up (upwelling) do the surface waters bloom with life.

Step back and one theme unites land and water: a small set of physical factors — temperature, water availability, light, nutrients, and salinity — decides which organisms can thrive where, and therefore which biome or aquatic system develops. That is also why these systems are vulnerable. Warming and drought shift terrestrial biomes; rising temperatures, acidification, nutrient pollution that triggers oxygen-starved dead zones, and the draining of wetlands all stress aquatic systems. The most diverse of these communities — rainforests, reefs, estuaries — tend to be the most resilient, but even they have limits when the underlying climate and chemistry change too fast.

Model check: Biome labels summarize patterns; productivity varies by location and area basis. Low local nutrients can limit per-area production, but its enormous total area matters for aggregate output.

Worked examples

Avoid these mistakes

• Biome labels summarize patterns; productivity varies by location and area basis.

Words to know

biome

A large region defined by a characteristic climate and the community of plants and animals adapted to it, such as tundra or tropical rainforest.

climate

The long-term pattern of temperature and precipitation in a region — the master control over which biome forms there.

estuary

A partly enclosed coastal area where fresh river water mixes with salt water, creating brackish, nutrient-rich, highly productive habitat.

photic zone

The sunlit upper layer of a body of water where there is enough light for photosynthesis.

wetland

Land saturated with water at least part of the year (marshes, swamps, bogs); among the most productive ecosystems, it filters water and buffers floods.

salinity

The amount of dissolved salt in water, a key factor that determines which organisms can live in freshwater, brackish, or marine systems.

Whittaker's map of the world's biomes

In the 1970 s ecologist Robert Whittaker distilled the planet's terrestrial biomes into a single elegant graph, plotting them against just two axes: mean annual temperature and mean annual precipitation. The resulting Whittaker diagram shows biomes occupying predictable zones — hot and wet gives rainforest, hot and dry gives desert, cold gives tundra — with fuzzy transition areas where soil, fire, and grazing tip the balance. It remains the clearest illustration that climate, above all, decides which biome develops in a place.

Convergent evolution: same biome, same solutions

Biomes are so shaped by climate that unrelated organisms in the same biome on different continents often evolve to look remarkably alike — a phenomenon called convergent evolution. American cacti and African euphorbs both evolved fat, spiny, water-storing stems to survive desert life, despite being only distantly related. Likewise, the fast-running grazers and streamlined predators of grasslands recur across continents. It's powerful evidence that a biome's climate poses a specific set of survival problems, and natural selection keeps arriving at the same answers.

Dead zones: when nutrients become pollution

Aquatic productivity usually needs nutrients — but too many can be catastrophic. When fertilizer and sewage wash into water, they trigger eutrophication: explosive algae blooms that later die and are decomposed by bacteria, which consume the water's dissolved oxygen. The result is a hypoxic 'dead zone' where fish and shellfish suffocate. A vast seasonal dead zone forms every summer where the nutrient-laden Mississippi River empties into the Gulf of Mexico, showing how a change in one factor — nutrients — can unravel an aquatic ecosystem.

Coral reefs: the rainforests of the sea, under siege

Coral reefs cover under 1% of the ocean floor yet shelter roughly a quarter of all marine species, earning them the nickname 'rainforests of the sea.' Their secret is a mutualism: coral animals host photosynthetic algae (zooxanthellae) that feed them and give them color. But when water grows too warm, stressed corals expel their algae and bleach, starving unless conditions ease; ocean acidification simultaneously makes it harder for them to build their calcium-carbonate skeletons. Reefs are a vivid case of how a highly diverse, productive system can also be dangerously sensitive to a changing climate.

Biomes have blurry edges, not hard borders

It is tempting to picture biomes as sharply bounded regions on a map, but in reality one grades into the next through broad transition zones called ecotones — the belt where forest thins into grassland, or marsh meets open water. Ecotones are often especially rich in species because organisms from both neighboring communities overlap there. They also act as early-warning sensors for climate change: because they sit right at the tipping point between two biomes, ecotones are usually the first places where a shifting climate visibly pushes one biome to advance and another to retreat.

In the real world

As the climate warms, ecologists are already documenting biomes on the move — treelines climbing higher up mountains and tundra giving way to shrubs and forest — which forces the animals adapted to those biomes to relocate or decline.

Exit check

Why can open ocean have low production per area yet large total production?

Ecological Sampling & Diversity Data

Learning goals

• I can plan representative independent ecological samples.
• I can calculate density and a specified diversity index while checking assumptions.

Readings for this unit

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

An ecological claim needs a sampling plan. A quadrat is a bounded area for counting relatively stationary organisms; a transect samples along a line or gradient. Choose sampling locations randomly or systematically to address the question and avoid selecting only visually interesting sites.

Population density is count divided by sampled area. Extrapolate to a habitat area only if the sampled portion represents that habitat and the organism was counted consistently. Patchy distributions, season, detectability, and boundary effects can make a small sample unrepresentative.

Richness counts species; evenness describes how individuals are distributed among them. The simple diversity index used here is D=1−Σpi², where pi is each species’ fraction of counted individuals. State this version because several Simpson-index conventions exist; do not compare numbers from incompatible formulas.

Independent quadrats reveal spatial variability. Report the count, sampled area, sampling effort, mean density, and range. Recounting one quadrat can test observer agreement but does not measure variation across the habitat. Equal sampling effort helps compare sites.

For mobile organisms, mark–recapture can estimate abundance using N≈MC/R with M initially marked, C caught later, and R previously marked among them. The simple model assumes mixing, recognizable marks, comparable capture probability, and little demographic change. It is a paper model here, not an instruction to capture wildlife.

A restoration comparison is stronger with before/after data and a comparison site, ideally with replication. Different biodiversity measurements can support different conclusions. A higher index does not alone prove a habitat meets every conservation goal; consider native species, functions, and threats.

Worked examples

Avoid these mistakes

• State the exact diversity formula; richness is not the same as evenness.

Words to know

quadrat

A bounded area used to sample organisms.

richness

Number of species detected under specified effort.

evenness

How evenly individuals are distributed across species.

In the real world

Use the chapter investigation to connect this unit to measured evidence and a practical decision.

Exit check

What would make a site comparison more convincing?