Ecosystems: Food Webs and Interdependence

A pond holds fish, plants, insects, and many smaller living things. Water, light, and temperature affect them all. Different words help us describe these parts of the pond.

From one organism to an ecosystem

An organism is one living thing. A species is a group whose members can breed together and produce fertile offspring. All members of one species in one area at one time form a population. The pond's sticklebacks form one population; its frogs form another.

A community includes all the populations living and interacting there. Add the water, mud, sunlight, and other nonliving surroundings, and you have an ecosystem. A list of animals alone describes neither the whole community nor the whole ecosystem: plants and microorganisms belong too.

A habitat is where an organism lives and finds what it needs. A pond is a habitat for a fish. A damp crack beneath a stone is a tiny habitat, or microhabitat, for a small animal.

Living and nonliving factors

Abiotic factors also include water supply, soil type, minerals, and wind. In water, salt content and current matter too. A pH value describes how acidic or alkaline water or soil is. Temperature affects living things, but it is still abiotic. The category depends on what the factor is.

Consider a woodland log. One woodlouse is an organism. All woodlice of the same species beneath it form a population. Add the fungi, moss, beetles, and other species to describe the community. Moisture, darkness, and cool air describe abiotic conditions. The woodlouse's food and its centipede predators are biotic factors.

Large ecosystems contain smaller habitats

Large ecosystem types are often called biomes. Their climates help shape the life found there.

• Tropical rainforests are hot and wet all year. They hold more species than any other land biome.
• Deserts are very dry, usually receiving less than 250 mm of rain yearly.
• Grasslands support grasses; temperate forests have strong seasonal changes.
• Arctic tundra has long, cold winters and short summers. Much has frozen ground beneath the surface.
• Oceans, lakes, and rivers contain many water habitats.

One rainforest contains treetop, trunk, forest-floor, and stream habitats. Conditions differ between them: the treetops receive far more light than the ground below.

A thick coat helps an animal keep warm. Large ears can help another animal lose heat. An adaptation is an inherited feature that helps survival and reproduction in a habitat.

Link a feature to its benefit

A good explanation makes three links: feature → what it does → survival benefit.

The fennec fox has large ears. Their large surface lets heat pass into the air. This helps the fox stay cool in a hot desert. An Arctic fox has small ears, which lose less heat in the cold.

Adaptations fall into three groups:

Surviving difficult conditions

Polar bears have fat and dense fur for insulation. Their large paws spread their weight on snow and ice. Pale fur helps them blend into their surroundings while hunting. Ground squirrels survive winter by hibernating, with a slow heartbeat and low body temperature.

Many cacti have spines in place of leaves. Spines lose little water and help protect the plant. A thick stem stores water; a waxy surface reduces water loss. Wide, shallow roots quickly collect brief rainfall. A camel's hump stores fat, not water. Kangaroo rats gain water from food and from processes that break down food.

Plants in deep shade often have broad, thin leaves to catch dim light. Deciduous trees drop leaves in fall, reducing water loss when frozen soil limits water uptake. Water lilies have air spaces that keep leaves afloat, with gas-exchange pores on the upper surface.

Finding food and avoiding danger

Predators may have sharp claws, forward-facing eyes, or camouflage. Prey may have wide views, spines, speed, or warning colors. Plants also have defenses: thorns, stinging hairs, and bitter chemicals discourage animals from eating them.

Behavior helps too. Migrating birds reach places with food and suitable weather. Schooling fish are harder for predators to single out. Courtship displays help birds attract mates. Flowers use colors, scents, and nectar to attract pollinators.

Adaptations are inherited

An animal does not develop a new inherited feature just because it needs one. Individuals already vary. Over many generations, those with helpful inherited features tend to leave more offspring. Those features become more common. This is natural selection.

A benefit depends on the habitat. A koala is a specialist, relying mainly on eucalyptus leaves. Losing that food is a serious threat. A raccoon is a generalist with many foods and habitats, giving it more choices when conditions change.

A food chain follows one feeding path. A food web joins several paths, showing how organisms share foods and predators.

Name the feeding roles

Producers, including plants and algae, make food using light energy. Consumers eat other organisms. Herbivores eat plants or algae; carnivores eat animals; omnivores eat both.

Consumers also have positions in a chain. A primary consumer eats a producer. A secondary consumer eats a primary consumer. A tertiary consumer eats a secondary consumer. These positions are trophic levels, with producers at level 1.

In "grass → grasshopper → frog → snake → hawk," the hawk is at level 5. It is a fourth-level consumer, because the producer counts as the first level but is not a consumer.

Point arrows toward the eater

Read each arrow as "is eaten by." An arrow carries energy and matter from food to eater. The Sun supplies energy, but it is not an organism being eaten.

Build a grassland web from these facts:

• Rabbits and grasshoppers eat grass.
• Mice eat grass seeds and grasshoppers.
• Frogs eat grasshoppers.
• Snakes eat frogs and mice.
• Hawks eat rabbits, mice, and snakes.

Start with grass. Add an arrow from grass to each eater: rabbit, grasshopper, and mouse. Next add grasshopper → mouse and grasshopper → frog. Add frog → snake and mouse → snake. Finish with rabbit → hawk, mouse → hawk, and snake → hawk. There are ten feeding arrows.

Check every feeding statement against the drawing. An animal placed higher on the page does not necessarily eat everything below it.

Follow more than one route

This web contains six chains ending at the hawk. One is "grass → rabbit → hawk." Another is "grass → grasshopper → mouse → snake → hawk."

The mouse is at level 2 when eating seeds, but level 3 when eating grasshoppers. An omnivore can belong to more than one trophic level. The snake can be at levels 3 and 4; the hawk can be at levels 3, 4, and 5.

Rabbits and grasshoppers share grass as food, so they can compete. Predators hunt and kill prey. A top predator has no natural predators as an adult, but its arrows still show only the prey it actually eats.

Include tiny producers and decomposers

Ocean webs often begin with phytoplankton, tiny drifting algae. Southern Ocean krill eat phytoplankton. Fish, penguins, seals, and baleen whales eat krill. Some seals and penguins are prey for leopard seals or orcas.

Bacteria and fungi are the main decomposers. They break down dead organisms and waste from every level. Draw arrows from dead material toward the decomposers. Earthworms and woodlice eat and fragment dead material, helping decomposition; these animals are called detritivores.

A plant uses sunlight to make sugars. A caterpillar eats the plant, and a bird eats the caterpillar. Energy and matter move through these meals, but their paths eventually differ.

Where energy goes

Producers capture some sunlight during photosynthesis. Energy stored in their food passes to consumers when they eat. Organisms release energy from food through respiration. Their activities eventually release heat into the surroundings.

A consumer receives only part of the energy from the level below. Some parts, such as roots or bones, are not eaten. Some food leaves as feces. Dead material and waste supply energy to decomposers, which also respire and release heat.

Less energy reaches each higher feeding level. This helps explain why chains rarely have more than four or five levels. Top predators usually need large hunting areas and occur in small numbers.

What pyramid bars show

Ecological pyramids stack bars for trophic levels, with producers at the bottom. Check what each bar measures.

Imagine 1 oak tree feeding 2,000 caterpillars, then 20 songbirds and 1 hawk. The tree makes a tiny bar in a numbers pyramid. Yet the tree is much heavier than the caterpillars together. Counting bodies and measuring their mass answer different questions.

In some water ecosystems, tiny producers are eaten and replaced quickly. Their mass at one moment can be smaller than their consumers' mass. Energy pyramids still narrow upward: compare energy over the same area and time, not a snapshot of body mass.

Follow matter back into living things

Matter consists of atoms that can be used again. Decomposers break down a fallen leaf. Some of its carbon leaves as carbon dioxide during respiration. Water and minerals also return to the surroundings. Roots take up minerals for new growth.

The leaf's matter has moved into other substances; it has not vanished. Without decomposers, dead material would pile up and many nutrients would stay trapped inside it.

The carbon cycle can be followed in a few steps:

1. Plants and algae take in carbon dioxide for photosynthesis.
2. They build its carbon into sugars and other substances.
3. Animals gain carbon by eating.
4. Respiration returns carbon dioxide to the air or water. Decomposers respire too.

Burning wood and fossil fuels also releases carbon dioxide. Most of a plant's dry mass is built using carbon dioxide from the air, together with water. Soil supplies water and small amounts of minerals.

Energy leaves as heat, so most ecosystems need a continuing supply of sunlight. Decomposers recycle matter; they do not recycle that lost energy.

A change in one population can reach organisms that never eat it. Follow each feeding link and explain what changes at the next step.

Look both ways from the change

First find the population that changes. Check its consumers: will they have more food or less? Then check its food: will more of it be eaten or less? Look for shared foods and alternative prey before writing a prediction.

A direct effect passes between linked organisms. An indirect effect reaches a population through other organisms. Write the reason for each link using "because."

In the grassland web, suppose disease kills most frogs. Grasshoppers are likely to increase because fewer frogs eat them. More grasshoppers may eat more grass, leaving less for rabbits. The change in rabbits would be indirect: frogs do not eat rabbits.

Snakes also lose a food source. They may eat more mice, increasing pressure on the mice. If other food does not meet their needs, snake numbers may fall. A food web can support more than one reasonable prediction when animals can change their diets.

Check alternative foods

Now suppose rabbits decline. Hawks lose one prey species. They may hunt more mice and snakes. Grass is likely to increase because fewer rabbits graze it.

A consumer with several food sources has more choices than a specialist. Still, counting the total links in a web is not enough to judge its stability. The strength of each feeding link and the organisms involved matter too.

A small population can have a large effect

On the Pacific coast of North America, sea otters eat sea urchins. Urchins graze on kelp, a large seaweed. When otters were heavily hunted, urchins increased in many places and stripped away kelp forests. Fish lost shelter as the kelp disappeared. Where otters returned, fewer urchins allowed kelp to recover.

Sea otters are a keystone species: their effects are much larger than their numbers suggest. Protecting them can also protect the habitat used by many other species.

Predictions describe likely changes

A web leaves out weather, disease, and many feeding links. Use "likely" or "may" for predictions, followed by a clear biological reason.

Removing a predator does not mean its prey will grow forever. More prey can use up their food. Shortage then lowers survival and reproduction, and the population may crash. To judge what happened, measure the populations and their resources over time.

Organisms need resources to live and reproduce. When one resource runs short, it can limit the size of a population.

Find what limits growth

Animals need food, water, oxygen, space, shelter, and mates. Plants need light, water, carbon dioxide, minerals, and space. A limiting factor keeps a population from growing larger. Food shortage is one example; extreme temperatures, predators, and disease can limit populations too.

Adding an already plentiful resource will not solve a shortage of another. A shaded plant with enough water still lacks light. Extra watering does not remove that limit.

When resources are plentiful, more individuals survive and reproduce. Births can exceed deaths, and the population grows. Shortages can slow growth, reduce births, cause deaths, or make organisms leave. Carrying capacity is the largest population an environment can support over a long time. It changes when resources and conditions change.

Use numbers to support an explanation

Consider example deer counts. The population rises from 1,200 to 2,000 during several mild winters with plentiful food. That is 800 more deer, about a 67% increase. After a drought reduces plants, the count falls from 2,000 to 1,400: 600 fewer deer, a 30% decrease.

The food explanation links survival and reproduction to the counts. A good investigation also checks hunting and disease. Counts show the change; evidence about food helps explain its cause.

Competition can happen without a fight

Competition occurs when organisms need the same limited resource. It can happen within a species or between species.

Members of one species often have very similar needs, so competition can be strong. Organisms do not have to meet: one tree can take light from another by shading it. A better competitor gains more of the resource; others may grow less, leave, or decline.

Organisms of one species can also cooperate. Wolves hunt together and can catch larger prey. Meerkats act as lookouts. Worker honeybees share jobs in a hive. The same population can show both cooperation and competition.

Organisms affect each other's survival in several ways. Some eat others; some live in close partnerships. Classify a relationship by what happens to each partner.

Why predator peaks follow prey peaks

A predator kills and eats its prey. With plentiful prey, predators have more food. More survive and raise young, so their population grows. More predators then eat more prey. As prey become scarce, predator survival and reproduction fall. Fewer predators let prey recover.

This can create a predator–prey cycle. The predator peak usually follows the prey peak because reproduction takes time. Canadian records of snowshoe hare and lynx pelts show roughly ten-year cycles. Pelt counts are indirect evidence of population size; hare food supply also affects the cycle.

On a graph, read the axes and units first. Peaks can be compared in time even when two populations use different scales. A taller curve alone does not show that more animals were present.

Three kinds of symbiosis

Symbiosis is a close, lasting relationship between different species.

To classify a relationship, name each benefit or harm first. A remora eats scraps from a shark's meals while the shark is unaffected. The signs are +/0, so it is commensalism. If evidence later shows a benefit or harm to the shark, the classification changes.

Different partners, the same pattern

Bees gain food from flowers while carrying pollen between them. Clownfish gain shelter in sea anemones and chase away some anemone-eating fish. Bacteria in bean and clover roots supply nitrogen compounds and receive sugars. In a lichen, a fungus lives with an alga or cyanobacterium. Each is a mutualistic partnership, without needing either organism to plan help.

Fleas and ticks take blood from hosts. Mistletoe takes water and minerals from trees. These are parasites: they usually live with a host for some time. A predator kills its prey as part of feeding, so predation differs from parasitism even though one gains and one loses.

These patterns apply across habitats. A coyote hunting a jackrabbit and an orca hunting a seal both show predation. Competition has −/− effects because both competitors have less access to a shared resource than they would without the other.

A flower visitor may gain a meal and help a plant reproduce. This partnership supports wild plants and many of the crops people eat.

How pollen travels

Pollination moves pollen from an anther to the stigma of a flower of the same species. It happens before fertilization, allowing seeds and fruits to form. Wind or animals can carry the pollen.

Animals visit flowers for nectar, a sugary liquid, and protein-rich pollen. Pollen sticks to the visitor and rubs onto another flower's stigma. Both partners benefit, so this is mutualism.

Bees are major pollinators, including more than 20,000 wild bee species. Flies, beetles, butterflies, moths, wasps, some birds, and some bats also pollinate. Honeybees are one part of this wider group.

Why pollinators matter

A 2016 international assessment found that more than three quarters of leading food-crop types benefit at least partly from animal pollination. It valued the yearly crop production directly affected at 235–577 billion US dollars. Crop types are not the same as the amount of food produced.

Apples, cherries, almonds, squash, coffee, and cocoa benefit from animal visits. Tiny flies called midges pollinate cocoa. Nearly 90% of wild flowering plant species depend at least partly on animal pollination. Their fruits and seeds also feed other animals.

Losing pollinators would reduce many harvests and make diets less varied. It would not remove all food. Wheat and rice mostly self-pollinate; corn uses wind.

Investigate a flower visitor's effect

In example data, open apple branches produce a mean of 48 apples. Branches covered by fine net bags produce 6. The difference is 42 apples; open branches produce eight times as many.

The results support the idea that insect visits increase fruit production. Compare similar branches on the same trees and repeat across trees. Bags can also change light, temperature, and airflow, so check those effects before attributing the whole difference to insect access.

Keep food and nesting sites available

Habitat loss removes flowers and nesting places. Pesticides, diseases, pests such as the Varroa mite, invasive species, and climate change also threaten pollinators. Warmer conditions can shift flowering away from the time pollinators are active.

Native flowers blooming from spring to fall provide food over many months. Uncut patches, bare ground, hollow stems, and dead wood provide nesting places. Farm wildflower strips and reduced pesticide use can help protect these partnerships.

Counting every plant in a field takes too long. A sample counts a small part of the habitat. It should be representative: it should give a fair picture of the whole area.

Choose a suitable frame

A quadrat is a square frame marking a known area. A 0.5 m × 0.5 m frame covers 0.25 m². A 1 m × 1 m frame covers 1 m². Small plants suit small frames; tree surveys need much larger plots, such as 10 m × 10 m.

Quadrats suit plants and animals that move little, such as limpets. Fast-moving animals can enter or leave during a count, making a frame less useful for them.

Choose positions without bias

For random sampling, lay two tape measures at right angles along the habitat's edges. Use random numbers for a pair of coordinates. For example, (7, 12) places a chosen frame corner 7 m along one direction and 12 m along the other.

Place the frame at each chosen position, including bare patches. Picking the most colorful patches would overestimate flowers. Throwing the frame is unsafe and does not give all positions a fair chance.

Record the frame number, coordinates, species, and result in a table. Count plants rooted inside the frame. Choose one edge rule and keep it: for example, count top and left edge individuals, but not bottom and right edge individuals. Identify organisms with a guide or key. An unknown species can receive a consistent label such as "species A."

Decide what to record

Cover works well for grass or moss whose individuals are hard to separate. In a frame divided into 100 squares, count squares more than half covered. If 64 squares qualify, cover is 64%. Species can overlap, so their combined covers can exceed 100%.

A species present in 7 of 10 frames has 70% frequency. One plant and twenty plants both count as "present," so frequency cannot give the number of individuals.

Match placement to the question

To follow a change from shade into sunlight, place frames at regular intervals along a line, called a transect. This is systematic sampling. Measure light at each position too. To compare two separate areas, use random samples in each.

Add samples until the running mean changes little. More samples help capture patchiness. Avoid trampling before counting. Keep frame size and counting rules consistent; compare means or densities, not totals from different amounts of sampling. Use equal effort when comparing the numbers of species found.

A quadrat count describes one small patch. To estimate a whole population, combine the average count with the areas of the frame and habitat.

Work through an estimate

In an example field, ten 0.25 m² quadrats contain 3, 0, 5, 2, 4, 1, 6, 3, 2, and 4 daisies. The field measures 30 m by 20 m.

1. Add the counts: the total is 30 daisies.
2. Divide by ten quadrats: the mean is 3 daisies per quadrat.
3. Find the field area: 30 × 20 = 600 m².
4. Find how many quadrat areas fit: 600 ÷ 0.25 = 2,400.
5. Scale up: 3 × 2,400 = about 7,200 daisies.

Estimated population = mean per quadrat × (habitat area ÷ quadrat area).

The ten sampled frames help find a mean. They are not the number of frames that fit across the whole field. Multiplying the mean by ten would only return the sample total.

Another route uses density, the number per unit area. Here, 3 ÷ 0.25 = 12 daisies per m². Multiplying 12 by 600 gives the same estimate. Remember to multiply both side lengths for quadrat area: 0.5 × 0.5 = 0.25 m².

Decide how much to trust the estimate

Daisies grow in clumps. A different sample would probably give another mean. Random placement, more frames, and repeat surveys improve the estimate. Keep zero counts: empty patches belong to the habitat too.

The range shows the spread from smallest to largest count. Here it is 6 − 0 = 6. Wide variation means the sample mean may change more when another frame is added. Check an unusual count for an identification or recording error before deciding what to do with it.

Compare habitats fairly

Example clover counts are 9, 7, 11, 8, and 7 in a sunny area. In equal-sized shaded frames they are 1, 0, 2, 3, and 0. The means are 42 ÷ 5 = 8.4 and 6 ÷ 5 = 1.2. Their ranges are 4 and 3.

Clover is more common in the sunny sample. Brighter measured light gives a possible explanation because plants need light for photosynthesis. Soil moisture, trampling, and competing plants may differ too. Measure these and repeat the survey to test the explanation.

When frame sizes differ, compare counts per square meter. A mean percentage cover can also be compared: covers of 64%, 70%, 55%, 80%, and 61% average 66%. Cover cannot be changed into a count of plants.

Compare the variety of life

Species richness is the number of species found. Compare it using the same sampling effort. Finding twelve species after a large search and five after a brief search is not a fair comparison.

Evenness describes how individuals are shared among species. Two meadows may each hold four species and 100 plants. Counts of 25, 25, 25, and 25 are more even than 97, 1, 1, and 1.

A plant frame will miss many moving animals. Choose equipment that matches where the organisms live and how they move.

Match the method to the animals

A raised lid helps keep rain and large animals out of a pitfall trap. A pooter draws a small insect into a jar through a tube. A filter protects the mouth tube. Light traps sample moths; camera traps record larger animals.

Every method favors particular animals. A pitfall catch depends on how actively animals walk, as well as how many live nearby. Combine methods for a fuller survey. For a fair comparison, match the number of sweeps, trap times, or other sampling effort in each area.

Measure the conditions beside the organisms

Record abiotic conditions at the same place and time as each biological sample.

• Use a thermometer or temperature probe for temperature in °C.
• Use a light meter for light intensity in lux.
• Use a moisture meter for soil wetness.
• Use a pH meter, indicator, or soil kit for pH.
• Use an anemometer for wind speed.
• Use an oxygen meter for dissolved oxygen in water.

Data loggers record readings automatically over time. They can reveal changes that a single midday reading misses.

Suppose moss cover falls along a line from shade to open lawn while measured light rises. The paired readings suggest a link. Measurements taken elsewhere, or on a different day, would give weaker evidence about those plants.

Care for the habitat and yourself

Plan collection with a teacher. Check traps often enough for the weather and animals; release catches promptly where found. Handle animals gently and keep them in conditions suited to their needs. Replace turned stones or logs and avoid trampling. Wash hands after handling soil or pond water, and stay with an adult near deep or fast water. Use a pooter only with an effective filter and supervision; keep unknown, biting, or stinging animals out.

Watch changes over time

A mesocosm is a small model ecosystem, such as a managed tank. It allows one factor, such as light, to be changed while others stay similar. Its simpler community may respond differently from the habitat outdoors. Animal models need a teacher's plan for their continuing care.

Repeat the same outdoor survey over months or years to track change. The results can show an invasive plant spreading or animals returning to a restored pond. Citizen-science surveys combine repeated observations from many volunteers.

Some pollutants break down slowly and leave bodies slowly. Repeated feeding can make their concentrations especially high in top predators.

Separate two kinds of build-up

Bioaccumulation is build-up within one organism over its lifetime. It happens when intake is faster than removal. Biomagnification is an increase in concentration between feeding levels.

Persistent pollutants include DDT, PCBs, and mercury compounds. DDT and PCBs collect in fatty tissue. Mercury in fish builds up mainly in muscle. A pollutant that breaks down quickly or is easily removed does not build up in the same way.

A predator eats many prey during its life. It gains their retained toxin while removing it slowly. Much of the food is used and its matter leaves through respiration and waste. The toxin stays longer, increasing the amount per kilogram of the predator's body. Energy losses also mean a predator needs a large supply of prey.

Read concentration data

Concentration tells how much toxin occurs in a given mass. One part per million, or ppm, means 1 milligram per kilogram.

Example lake data show:

To compare birds with plankton, divide 25 by 0.04. The birds' concentration is 625 times as high. Compared with the water, it is 25 ÷ 0.00005 = 500,000 times as high.

This compares concentrations, not the total toxin in the whole lake. Birds can gain a high concentration from many contaminated meals without being sprayed directly.

Effects can reach whole ecosystems

DDT built up in fish-eating birds and birds of prey. A breakdown substance weakened eggshells, causing many eggs to break during incubation. Fewer young survived. The United States banned most DDT uses in 1972, and affected birds recovered. The bald eagle was removed from the US list of threatened and endangered wildlife in 2007.

Large, long-lived predatory fish can accumulate mercury. Humans eating contaminated fish can receive it too. Food-choice guidance from health agencies favors lower-mercury fish for children and pregnant people. This follows the same feeding-chain process.

If top predators decline, their prey may increase and trigger further changes in the web. Reducing releases of persistent chemicals helps prevent this. Other approaches include monitoring fish and eggs, cleaning polluted sites, and controlling crop pests with fewer chemicals, such as through crop rotation or carefully chosen natural predators.

A species may live naturally in one place and be introduced somewhere else. Origin alone does not tell us whether it will cause harm.

Native, non-native, and invasive

Many introduced crops and garden plants stay contained. Harmful spread makes an introduced species invasive. Harm can affect native species, habitats, buildings, farming, or health.

In a new place, a species may leave predators, parasites, and diseases behind. Fast reproduction and a broad diet can help it spread. Some native animals lack effective defenses against an unfamiliar predator or toxin.

Three invasions and their effects

Cane toads in Australia: In 1935, 102 cane toads were collected in Hawaii for introduction to Australia to control sugar-cane beetles. All but one survived the journey. They failed to control the beetles but spread widely. Their skin glands produce poison that can kill native predators, including quolls, goannas, and snakes that try to eat them. Introducing a pest's enemy requires careful testing.

Zebra mussels in North America: Found in Lake St. Clair in 1988, they probably arrived in ship ballast water. They spread to all five Great Lakes in less than ten years. They remove plankton that other animals need, grow over native mussels, and clog water pipes.

Gray squirrels in Britain: Introduced from North America from the 1870s onward, they compete with red squirrels for food. They also carry squirrelpox, which usually causes them little harm but kills red squirrels. Red squirrels have disappeared from most of England and Wales.

Trace the harm through the web

Ask what the newcomer eats, what eats it, and what shares its resources. An introduced fish that eats tadpoles may reduce frogs. Animals that eat frogs then lose food. Insects eaten by frogs may increase. Competition, predation, disease, and habitat changes provide different routes for damage.

Prevent spread and act early

Prevention is usually cheaper and more effective than removing a widespread invader. Clean boats, boots, and fishing gear between water bodies. Never release pets or aquarium plants into the wild. Border controls help prevent species traveling with plants, animals, or soil.

Regular surveys can find new arrivals while numbers are small. Control may use traps, barriers, or removal of invasive plants. Biological control uses a living enemy of the pest; it needs careful tests to avoid harming other species. Monitor native species afterward to see whether the action helped.

Storms, droughts, fires, and human actions can change habitats. A disturbance changes the living or nonliving parts of an ecosystem, sometimes affecting many populations.

Follow the first effect

Drought reduces water. Plants may grow less, leaving less food for herbivores and then predators. A new disease starts with a living population, but its effects can also spread through food links.

Natural disturbances include floods, volcanic eruptions, wildfires, and disease outbreaks. Human disturbances include land clearing, dams, pollution, overfishing, and introduced species.

Use three parts to explain a change:

1. Claim: state the proposed cause.
2. Evidence: give measurements that support it.
3. Reasoning: explain the biological link between cause and measurements.

In example wetland surveys, frog counts fall from 320 to 40 over four years after drainage. Heron counts fall from 12 to 3. A claim is that draining the wetland contributed to both declines. The counts are evidence. Frogs need water for eggs and tadpoles; herons lose food when wetland prey decline. Checking disease and changes at undrained wetlands would strengthen the argument.

Habitat loss and fragmentation

Habitat loss is the greatest threat to species on land. Clearing a forest removes food, shelter, and breeding places. Fragmentation breaks remaining habitat into separate patches. Small, isolated populations can disappear more easily. A wildlife corridor joins patches so organisms can move between them.

Climate change can shift suitable conditions toward the poles or uphill. Flowers and pollinators may become active at different times. Long periods of warm seawater cause coral bleaching and can kill corals.

Change does not always mean lasting damage

Some ecosystems depend on occasional disturbances. Heat opens the cones of many lodgepole pines, releasing seeds. After fire, sunlight reaches bare ground where seedlings can grow. Carefully managed burns can reduce fuel for larger fires.

Recovery depends on how large and frequent a disturbance is. Repeated severe changes may leave too little time for populations to recover. Ecosystems change naturally; their populations do not stay at fixed numbers.

An endangered species faces a high risk of extinction. A species is extinct when no individuals remain anywhere. Habitat loss, invasive species, pollution, excessive hunting or fishing, and climate change can drive declines.

Protecting a habitat can help plants, animals, fungi, and microorganisms together. It also protects benefits that people receive from the ecosystem.

What biodiversity includes

Biodiversity is the variety of life. It includes different species, their evenness, genetic differences within species, and different habitats. Protecting only a few large animals misses much of that variety.

Diverse ecosystems tend to recover better after disturbance. Other species may provide food or carry out a role when one declines. A field with one crop is more exposed to a pest that attacks that crop.

Ecosystem services are benefits people receive from ecosystems. They include food, fresh water, timber, fibers, and medicines. Wetlands help clean water and reduce flooding. Roots hold soil in place; decomposers recycle nutrients; animals pollinate crops. Natural places also support recreation and well-being. Willow substances helped lead to aspirin, and the Pacific yew supplied the cancer drug paclitaxel.

Compare actions against a clear problem

A solution needs criteria, the results it should achieve. It must also fit constraints, such as cost, space, time, and people's needs.

Imagine a farm stream with eroding banks and few fish. Criteria include less muddy water and better wildlife habitat. Compare two approaches:

The planted strip meets more wildlife and water-quality criteria at lower cost. It brings a trade-off: the farmer loses some grazing space and must wait for growth. A recommendation should explain why the benefits outweigh that cost. Later measurements of erosion, water quality, and wildlife show whether it worked.

Match protection to the threat

Protected areas conserve habitats on land and at sea. Restoration repairs damaged places through actions such as replanting or re-flooding wetlands. Corridors connect separated patches. Seed banks store plant variety, while breeding and release programs can support endangered populations.

Limits on fishing, controls on invasive species, reduced pollution, and laws against harmful hunting or trade address particular causes of decline. Choose actions using evidence about the threat. Involving people who live and work nearby helps a plan meet both ecological goals and practical needs.