From one elephant to the biosphere: the levels of life
By the end of this lesson you will be able to
- Define individual organism, population, community, ecosystem, biome and biosphere, and place a named local example at every level.
- Describe the hierarchical structure of an ecosystem and explain how each level depends on the one below it.
- Distinguish the biological community of an ecosystem from its physical environment, and outline how the two interact.
- Construct food chains, food webs and pyramids of numbers from field data, and explain why energy is lost at every step.
- Interpret quadrat data to estimate a population and evaluate the reliability of the estimate.
The puzzle
In the dry season a single elephant walks down to the Chobe River at dusk. Behind it come forty more — a breeding herd. Along the riverfront there are thousands, and they share the water with buffalo, hippo, crocodiles, fish eagles, reeds and the bacteria in the mud. In the wet season most of them are gone, spread across a savanna that stretches into four other countries. Biologists say the Chobe elephants are one population but many communities, part of one ecosystem, one biome and one biosphere. Why do they need six different words for what a tourist calls “the bush”?
Write down everything you can see in a photograph of the Chobe riverfront at dusk: elephants, buffalo, a hippo, a fish eagle, reeds, mopane trees, the river, the mud, the sunlight. Now sort your list into two groups — living and non-living. Predict which group a dead tree trunk belongs to, and whether the bacteria you cannot see are part of the picture.
Compare your answer
Living (biotic): elephants, buffalo, hippo, fish eagle, reeds, mopane trees, the bacteria and fungi in the mud and the algae in the water. Non-living (abiotic): the river water, the mud, sunlight, air, temperature. The dead trunk is tricky: it is no longer alive, but it is made of biological material and is being eaten by termites and fungi, so ecologists treat it as part of the biotic side — dead organic matter. The bacteria are the most numerous organisms in the picture and do the most important job — they recycle every nutrient. The six words in the puzzle exist because these things are organised in nested levels, and a change at one level ripples through the others.
The science — structured notes
1.1 Six levels, one river
Ecology is the study of how organisms interact with each other and with their surroundings. Ecologists organise the living world into a hierarchy — a set of levels in which each level is made of many of the level below. Here it is, with the Chobe riverfront as the example at every step.
| Level | Definition | Chobe example |
|---|---|---|
| Individual organism | One living thing of one species | One elephant — a cow with a torn left ear, known to the guides |
| Population | All the individuals of one species living in one area at one time, able to breed together | The elephants of the Chobe riverfront — several thousand in the dry season |
| Community | All the populations of different species living and interacting in one area | Elephants + buffalo + hippo + fish eagles + reeds + mopane + termites + bacteria along the river |
| Ecosystem | A community together with the non-living environment it interacts with — water, soil, light, temperature, air | The Chobe riverfront: the community plus the river, the floodplain soil, the sunlight and the seasons |
| Biome | A large region of the Earth with a characteristic climate and community type, containing many ecosystems | The southern African savanna — grassland with scattered trees, wet summers and dry winters, from Botswana to Tanzania |
| Biosphere | Every part of the Earth where life exists: all biomes, the oceans, the soil, the lower atmosphere | The thin living skin of the planet — from about 10 km up to a few kilometres down |
1.2 The biological community and the physical environment
Every ecosystem has two kinds of parts. The biotic parts are the living organisms and what they do to each other: eating, competing, pollinating, decomposing, spreading disease. The abiotic parts are the physical and chemical conditions: light, temperature, water (rainfall, soil moisture, flooding), soil (its pH, its minerals, its depth), wind, and the gases in the air and water. The two sides are not separate: the abiotic conditions decide which organisms can live there, and the organisms in turn change the conditions.
Classify each of the following as a biotic or an abiotic factor for a mopane tree on the Chobe floodplain, and state one effect it has on the tree.
The elephant that strips its bark: biotic — the tree loses phloem tissue under the bark, so sugar cannot travel down to the roots; heavy stripping kills it.
The frost in a cold July night: abiotic — ice crystals damage the cells of young leaves; mopane grows only where frosts are rare, which sets its southern limit near Serowe.
The mopane worm (caterpillar of the emperor moth) eating its leaves: biotic — the tree loses leaf area, so photosynthesis falls; it often refoliates, and the caterpillars are a protein-rich harvest for people.
The depth of sand above the clay layer: abiotic — mopane roots need the clay to hold water; on deep Kalahari sand it is replaced by acacia and silver terminalia.
1.3 Food chains: the glue between the levels
What holds a community together is feeding. A food chain shows the direction in which energy and matter move: the arrow means “is eaten by”. Every chain starts with a producer — a plant or alga that makes its own food by photosynthesis from sunlight, carbon dioxide and water — and each following link is a consumer.
grass → impala → leopard algae → tilapia → tigerfish → fish eagle
| Trophic level | Name | Gets its energy from | Chobe examples |
|---|---|---|---|
| 1 | Producer | Sunlight, by photosynthesis | Grasses, reeds, mopane, algae in the river |
| 2 | Primary consumer (herbivore) | Producers | Impala, elephant, buffalo, lechwe, grasshoppers, tilapia grazing algae |
| 3 | Secondary consumer (carnivore) | Herbivores | Leopard, wild dog, tigerfish, bee-eater |
| 4 | Tertiary consumer (top carnivore) | Other carnivores | Lion killing a wild dog; fish eagle taking a tigerfish; crocodile |
| — | Decomposer | Dead organisms and waste at every level | Bacteria, fungi, termites, dung beetles, vultures (a scavenger) |
A savanna food web contains: grass, acacia, impala, giraffe, zebra, grasshopper, francolin (a ground bird that eats insects and seeds), leopard, lion, jackal (eats francolins, insects and carrion), vulture, bacteria. Draw the web, then predict the effect on the francolin population of removing the leopards.
Producers at the bottom: grass and acacia. Arrows: grass → impala, zebra, grasshopper; acacia → giraffe, impala; grasshopper → francolin, jackal; francolin → jackal, leopard; impala → leopard, lion; zebra → lion; giraffe → lion; all dead animals → vulture, bacteria.
Removing leopards: fewer francolins are eaten directly, so at first their numbers rise. But leopards also eat impala; more impala survive, more grass is eaten, and grasshoppers — which compete with impala for grass — may fall, so francolins have less insect food. Jackals, no longer losing kills to leopards, may increase and eat more francolins. The honest prediction is “probably rise at first, then uncertain”, and the reason is that a food web has many routes between any two species — which is why removing one predator has effects nobody can fully predict.
1.4 Pyramids of numbers
A pyramid of numbers draws each trophic level as a horizontal bar whose length is proportional to the number of organisms at that level in a given area. For grass → impala → leopard the pyramid is the expected shape: millions of grass plants, hundreds of impala, a handful of leopards, because each level has less energy available to it.
| Food chain | Level 1 | Level 2 | Level 3 | Shape |
|---|---|---|---|---|
| grass → impala → leopard (per km²) | ~ 10 million plants | ~ 30 impala | ~ 1 leopard | Upright pyramid |
| mopane tree → mopane worms → hornbills | 1 tree | ~ 2000 caterpillars | ~ 5 hornbills | Inverted at the base — one huge producer feeds many small consumers |
| impala → ticks → oxpeckers | 1 impala carries hundreds of ticks; a few oxpeckers eat the ticks | Bulges in the middle — parasites are small and many | ||
Sort the following into levels of the hierarchy, then answer the questions.
(a) “All the tilapia in the Chobe River” — which level, and what property does it have that one tilapia does not?
(b) “The Kalahari” — which level? Give one reason it contains many ecosystems.
(c) A conservation report says “the Okavango community is under stress”. Explain what an ecologist would understand by this, and what the report would say instead if only the water was the problem.
Reveal answer
(a) A population — one species, one place, one time. It has a size, a density (fish per km of river), a birth rate and a death rate; a single fish has none of these.
(b) A biome — a large region defined by its climate (250–450 mm of summer rain, hot days, cold winter nights) and its characteristic community (grasses, acacia and other thorn trees, gemsbok, springbok). It contains many ecosystems because the abiotic conditions vary from place to place: a salt pan, a fossil river valley, a dune crest and the pan margin each hold a different community interacting with different soil and water.
(c) The community is all the interacting populations — fish, birds, hippo, papyrus, lechwe and so on — so “community under stress” means the living things are affected: populations falling, food webs disrupted, invasive species spreading. If only the water were the problem the report would refer to an abiotic factor of the ecosystem — “the flood is failing” or “the water quality is declining” — which may become a community problem later, but is a different claim.
“In a food chain, energy is recycled by the decomposers so that the producers can use it again.”
Reveal answer
False. Decomposers recycle matter — the carbon, nitrogen, phosphorus and other elements in dead organisms are returned to the soil, water and air, where plants take them up again. Energy is not recycled: at every step, including decomposition, most of it is transferred to the surroundings as heat by respiration and leaves the ecosystem. That is why every ecosystem needs a continuous supply of sunlight, and why a food chain is a one-way flow with an arrow that never points back to the grass.
The Chobe elephant population has grown from a few thousand in the 1960s to tens of thousands today. Predict, using the loop in 1.2, what has happened to the riverfront woodland, to the abiotic conditions of the riverbank and to the community that depends on the trees — and why the change does not reverse when the elephants move away for the wet season.
Reveal answer
Elephants push over and bark-strip trees faster than seedlings can replace them, so along the river the woodland has thinned to scattered dead trunks with grass and shrubs between. With less shade the soil surface is hotter and drier and the bank is more exposed to trampling and erosion — abiotic changes. Species that need the trees — roosting birds, nesting hornbills, browsers such as bushbuck and kudu — lose habitat; grazers and species of open ground gain. The change persists because the altered conditions (hot bare soil, heavy grazing of any seedling, elephants returning every dry season) prevent tree seedlings from growing: the imbalance has created conditions that stop the system returning to its old state. Ecologists call this a shift to a new stable state, and it is one reason the elephant question in Lesson 3.5 is so difficult.
Try it — sampling a population with quadrats
You cannot count every grass plant on a floodplain, and you cannot count every impala in a reserve. Ecologists sample. Place a 1 m × 1 m quadrat at random, count what is inside, repeat, and scale up: population ≈ mean count per quadrat × number of quadrats that fit in the area. The lab lets you decide how many quadrats to throw and whether the plants are evenly spread or clumped. Watch the estimate settle as the sample grows — and notice that it settles more slowly for the clumped field.
Suppose a student threw five quadrats on a 200 m² patchy field and counted 0, 14, 2, 11 and 3 plants.
(a) Calculate the population estimate, showing the working.
(b) Explain why five quadrats give an unreliable estimate for this field but might be enough for an evenly spread one.
(c) The student placed the quadrats “where the plants looked typical”. Explain why this is a systematic error and state its likely effect on the estimate.
Reveal answer
(a) Total = 0 + 14 + 2 + 11 + 3 = 30 plants in 5 m², so the mean is 30 ÷ 5 = 6 plants per m². The field is 200 m², so the estimate is 6 × 200 = 1200 plants.
(b) The counts range from 0 to 14 — a quadrat lands either in a clump or in a gap. With only five throws, one more clump or one more gap changes the mean by two or three plants per m², which is 400–600 plants in the estimate. In an evenly spread field every quadrat gives a similar count (say 5 to 8), so the mean from five throws is already close to the true value and extra throws change it little.
(c) Choosing “typical-looking” places is not random: the student avoids bare patches and the very densest clumps, so every quadrat is biased the same way. That is a systematic error (all samples pushed in one direction, not cancelled by averaging). Since bare ground was avoided, the estimate is most likely too high. Random coordinates from a random-number table or a phone app remove the bias.
Common mistakes to avoid
- Population = all the animals — one species only; the community is all species, including the plants and microbes.
- Ecosystem = community — an ecosystem includes the non-living environment: the water, soil, light and temperature.
- Arrows the wrong way — a food-chain arrow points from the eaten to the eater: it shows where the energy goes.
- “Energy is recycled” — matter cycles; energy flows through and is lost as heat.
- Pyramid of numbers must be a pyramid — one tree feeding thousands of caterpillars is inverted, and that is correct.
- Quadrats “where it looks good” — random placement is what makes the estimate fair; choosing sites is a systematic error.
How we found out — a word invented in 1935 and a lake in Minnesota
People have always known that living things depend on one another; the idea that they and their surroundings form a single working system is younger than your grandparents. In 1935 the English botanist Arthur Tansley invented the word ecosystem. He was arguing against colleagues who described a forest or a heath as a kind of “super-organism” that was born, grew and died. Tansley insisted that what mattered was the system: organisms plus the physical environment, exchanging energy and materials, and that this system could be studied with the methods of physics — measuring what went in and what came out. The word gave ecologists something to measure.
The measuring was done by a graduate student who did not live to see the result. Raymond Lindeman spent five years wading in Cedar Bog Lake in Minnesota, collecting every producer, consumer and decomposer he could find, and calculating how much energy passed from one trophic level to the next. His 1942 paper, published a few months after his death at 27, showed that only about 10 % of the energy at one level reaches the next and that this loss is why food chains are short and pyramids of numbers are the shape they are. Reviewers had rejected the paper as too theoretical; his supervisor, G. Evelyn Hutchinson, persuaded the journal to publish it. Every food-web diagram and every “10 % rule” in this lesson comes from a lake you have never heard of and a student who measured instead of describing.
Tansley said an ecosystem should be studied “with the methods of physics”. Explain what he meant, and why Lindeman’s lake counted as the first real test of the idea.
Reveal answer
Tansley meant treating the living community and its surroundings as a system through which energy and matter flow — quantities that can be measured, added up and compared, as a physicist measures energy in and out of an engine — rather than describing the community as if it had a life of its own. Lindeman did exactly that: he measured the energy captured by the lake’s producers, the energy stored at each consumer level, and the energy lost at each transfer, and found a consistent pattern (about 10 % passed on). A pattern that could be checked in other lakes, and was, made “ecosystem” a testable scientific idea rather than a name.
Where it is used now
- Aerial and ground counts in Botswana. The Department of Wildlife and National Parks and partners survey the north of the country from light aircraft flying strip transects — a quadrat method in the air. The 2022 KAZA survey, covering five countries, estimated about 228 000 elephants, of which about 132 000 — well over half — were in Botswana; each figure is quoted with a confidence range because it is a sample, not a census.
- Fisheries on the Okavango. Fish populations are estimated by catch-per-net-per-night and by mark and recapture; the estimates set the fishing closed season from January to February, when the tilapia breed.
- Mopane worm harvests. Households across Botswana, Zimbabwe and South Africa harvest the caterpillars of Gonimbrasia belina, a trade worth tens of millions of pula a year; the caterpillars are a primary consumer, and a poor mopane leaf flush in a drought year cuts the harvest a year later.
- Global food webs. Fishing quotas for tuna, cod and anchovy are set from food-web models that trace energy from plankton upwards; when the anchovy off Peru collapsed in 1972, seabird populations that ate them crashed within a year.
- Rewilding. Wolves returned to Yellowstone in 1995 changed elk behaviour, which allowed willow and aspen to regrow along rivers, which brought back beavers — a food-web change that altered the abiotic environment of the streams.
Science now — and the scientist you need to be
What is happening in 2026
Counting the living world has become a job for sensors. Camera traps in the Okavango and Chobe now feed millions of images to image-recognition software that sorts them by species in hours instead of the months a team of volunteers once took; the researcher’s job is to check the doubtful identifications and design the sampling. Environmental DNA — the traces of skin, scales and dung that every organism sheds into water — lets a single litre from a river channel list the fish, frogs and mammals that have been upstream that week without anyone seeing them. Satellites map tree cover and grass biomass across the whole Kalahari every few days, so the abiotic side of an ecosystem — rainfall, flood extent, surface temperature — is measured continuously from orbit. What none of this replaces is the question a person asks: which level is changing, what caused it, and what will it do to the next generation.
The mindshift
What you need to be able to do
Criterion A questions in this unit give you an unfamiliar ecosystem — a rock pool, a termite mound, a rice paddy — and ask you to name the levels, build the food web and predict the effect of a change. The marks are for using the terms precisely (population, not “group”; community, not “ecosystem”) and for tracing a change through the web with a mechanism at every step. Criterion C questions give you quadrat or count data and ask for the estimate, its working and its reliability.
See an example
A patch beside a tap in Francistown: grass, a dandelion, ants, a grasshopper, a spider, two kinds of beetle, a gecko on the wall, a mushroom after rain, and (under a stone) woodlice. Producers: grass, dandelion. Primary consumers: grasshopper, woodlice (dead leaves), one beetle. Secondary: spider, ants, gecko. Decomposers: the mushroom, the woodlice (partly). The AI correctly placed the gecko as a secondary consumer but called the spider a “primary predator” — a phrase that is not a trophic level, so the answer was corrected to secondary consumer.
Key terms
| Individual organism | One living thing of one species. |
| Population | All the individuals of one species living in one area at one time. |
| Community | All the populations of different species living and interacting in one area. |
| Ecosystem | A community together with the non-living (abiotic) environment it interacts with. |
| Biome | A large region with a characteristic climate and community type — savanna, desert, rainforest, tundra. |
| Biosphere | Every part of the Earth where life exists. |
| Biotic factor | A living part of an ecosystem or its effect: feeding, competition, disease, decomposition. |
| Abiotic factor | A non-living condition: light, temperature, water, soil pH, wind. |
| Producer | An organism that makes its own food by photosynthesis; the first link in every food chain. |
| Consumer | An organism that gets its energy by eating others: primary (herbivore), secondary, tertiary. |
| Decomposer | An organism — bacterium, fungus, some invertebrates — that breaks down dead material and returns nutrients to the soil. |
| Trophic level | The position of an organism in a food chain, counted from the producer. |
| Pyramid of numbers | A diagram whose bars show the number of organisms at each trophic level, to scale. |
| Quadrat | A square frame of known area placed at random to sample a population. |
Think like a scientist
Criterion A A pan in the Makgadikgadi fills with rain for three months every summer. While it holds water it supports: blue-green algae; brine shrimp (which filter the algae); tadpoles (which graze algae); dragonfly larvae (which eat tadpoles and shrimp); lesser flamingos (which filter algae and shrimp); a pied kingfisher (which eats tadpoles and dragonfly larvae); and bacteria in the mud. Around the pan grows salt-tolerant grass grazed by zebra, which are hunted by lions and whose dead are eaten by jackals and bacteria. When the pan dries the shrimp leave eggs in the mud that survive until the next rains.
(a) Define community and identify the community described. [2]
(b) State two abiotic factors of this ecosystem that change between the wet and the dry season, and describe one effect of each on the organisms. [4]
(c) Draw a food web for the pan (water and grassland together). Identify one producer, one primary consumer, one secondary consumer that feeds at two trophic levels, and one decomposer. [5]
(d) Sketch a pyramid of numbers for the chain algae → tadpole → kingfisher, and explain its shape using energy transfer. [3]
(e) A wet year is followed by two dry years in which the pan does not fill. Explain, with reference to at least two levels of the hierarchy, why the community that returns in the third year may differ from the one described. [4]
Model answer
(a) A community is all the populations of different species living and interacting in one area at one time [1]. Here it is the algae, brine shrimp, tadpoles (and the frogs they become), dragonfly larvae, flamingos, kingfisher, bacteria, salt grass, zebra, lions and jackals of the pan and its margin [1].
(b) Water: in the wet season the pan holds water, so the algae can photosynthesise and the shrimp and tadpoles can live; when it dries, every aquatic organism dies or leaves except the shrimp eggs and the bacteria, which survive dormant in the mud [2]. Salinity (or temperature): as the water evaporates the salt concentration rises, which favours the salt-tolerant algae and brine shrimp and kills less tolerant species; the hot surface of the dry pan in October reaches temperatures at which only dormant eggs survive [2]. (Also acceptable: light reaching the mud; oxygen in the water.)
(c) Web with arrows pointing from eaten to eater: algae → brine shrimp, tadpole, flamingo; brine shrimp → dragonfly larva, flamingo; tadpole → dragonfly larva, kingfisher; dragonfly larva → kingfisher; salt grass → zebra; zebra → lion; dead zebra → jackal, bacteria; all dead organisms → bacteria [2 for a correct, complete web with arrows the right way]. Producer: algae (or salt grass) [1 for both producer and primary consumer, e.g. brine shrimp or zebra]; secondary consumer at two levels: the flamingo, which eats algae (level 1) and brine shrimp (level 2), or the kingfisher, which eats tadpoles (level 2) and dragonfly larvae (level 3) [1]; decomposer: bacteria in the mud [1].
(d) An upright pyramid: a very long bar for algae (millions of cells), a much shorter bar for tadpoles (thousands), a tiny bar for the kingfisher (one or two) [1]. At each transfer most of the energy the organism took in is used for respiration, movement and keeping warm and is lost as heat, or is in parts that are not eaten or not digested; only about 10 % becomes tissue available to the next level [1], so each level can support far fewer organisms than the one below, especially when the organisms are larger [1].
(e) At the population level: the brine shrimp survive only as eggs, and two dry years kill a fraction of the eggs, so the shrimp population that hatches is smaller and takes longer to build up; the frogs that bred in the pan may have died out locally, so tadpoles are absent until adults recolonise from another pan [2]. At the community level: with fewer shrimp and tadpoles the flamingos and kingfishers will not stay, the dragonfly larvae lack prey, and the food web is shorter; species that arrive first — the algae and the shrimp — dominate. At the ecosystem level: the altered abiotic conditions (salt concentrated by evaporation, mud baked hard) change which species can establish, so the community that returns is a different balance, and the imbalance can persist for several seasons [2].
Probe deeper — exam-style questions
Attempt every question before opening the answer. Marks in brackets show what a full answer is worth.
Distinguish between a population, a community and an ecosystem, using the impala of the Moremi reserve as your example.
Reveal answer
Population: all the impala in Moremi — one species in one place [1]. Community: the impala together with every other species they interact with — the grasses and shrubs they eat, the leopards and wild dogs that eat them, the ticks on them, the bacteria that decompose them [1]. Ecosystem: that community plus the non-living environment it exchanges energy and matter with — the floodwater, the soil, the sunlight, the seasonal temperature [1].
Explain why the arrow in a food chain points from the grass to the impala and not the other way.
Reveal answer
The arrow shows the direction in which energy and matter are transferred [1]: the impala eats the grass, so energy stored in the grass moves into the impala; “impala → grass” would claim that the grass gets its energy from the impala, which it does not — it gets it from sunlight [1].
A student writes: “In the Chobe ecosystem the community is made of the animals, the plants are the environment, and energy is recycled from the lions back to the grass by the decomposers.” Identify three errors.
Reveal answer
Plants are living, so they are part of the community, not the environment [1]; the environment (abiotic part) is the non-living conditions — water, soil, light, temperature [1]; energy is not recycled — decomposers recycle matter (nutrients), while energy flows one way and is lost as heat, so the grass gets new energy only from sunlight [1].
A pyramid of numbers for oak tree → aphids → ladybirds → blue tits has a tiny bottom bar and a huge second bar. Explain the shape and state why a pyramid of biomass for the same chain would be upright.
Reveal answer
One oak tree is a single individual, so the producer bar shows 1, while it feeds many thousands of tiny aphids [1]; the bars then narrow again through ladybirds to a few blue tits as energy is lost at each level [1]. Biomass measures the mass of living material: the single oak has far more mass than all its aphids together, so the base is the widest bar and the pyramid is upright [1].
True or false, with a reason: “A biome is another word for an ecosystem.”
Reveal answer
False [1]: a biome is a much larger unit — a region of the Earth defined by its climate and characteristic community type, such as savanna or desert — and it contains many different ecosystems, each with its own community and local conditions [1].
Ten quadrats of 0.25 m² on a 400 m² lawn gave a mean of 3.2 dandelions per quadrat. Calculate the population estimate, and state two conditions the sampling must meet for the estimate to be valid.
Reveal answer
Density = 3.2 ÷ 0.25 = 12.8 plants per m²; population ≈ 12.8 × 400 = 5120, about 5100 dandelions [1]. Conditions: quadrats placed at random (coordinates from a random number generator, not chosen by eye) so that no part of the lawn is favoured [1]; enough quadrats to cover the variation — more if the plants are clumped — with the sample large enough that the mean has stopped changing when more quadrats are added [1].
Summary
- Life is organised in a hierarchy: individual → population (one species) → community (all species) → ecosystem (community + abiotic environment) → biome (climate region) → biosphere.
- Biotic and abiotic factors interact both ways: conditions decide which organisms live, and organisms change the conditions — sometimes in a loop that keeps a change going.
- Food chains and webs show the one-way flow of energy from producers through consumers; about 10 % passes each level, which keeps chains short. Decomposers recycle matter, not energy.
- Pyramids of numbers are drawn to scale; a single large producer can make the base narrow, and that is correct.
- Populations are estimated by random sampling — quadrats — and the estimate is only as good as its sample size and placement.
Next time → Climate makes the biome: why rainfall and temperature decide whether a place is desert, savanna, wetland or forest — and what happens when the climate shifts.