195

15. ECOLOGY

15.1 WHAT IS ECOLOGY?

Ecology is the study of how organisms interact with one another and with their environment. The environment includes all the surroundings of an organism, and any conditions that may affect it. For example, the environment of a particular fish is not just the water that it swims in. The environment also includes the other animals and plants that live in the water, the oxygen dissolved in the water, the temperature of the water, the light that penetrates the water and anything else that may affect the fish, even the wind that makes waves on the surface!

A person who studies ecology is called an ecologist and another way to define ecology is to say that it is the study of ecosystems. An ecosystem is a group of organisms living together in a particular place, together with their environment. Ecosystems come in all sizes! For example, an ecologist might decide to study a small ecosystem like a field. They might start by identifying all

the plants that grow in the field, then study the soil and find out how that affects the plants. Next, they might want to observe and identify all the insects and find out which of them pollinate which flowers, which of them lay their eggs on which plants, which of them eat which other insects, and so on. Then there are the lizards and birds and mice that may eat some the plants and their seeds, and may eat some of the insects too. In ecology there are always more things to observe and discover! The other pictures show three large ecosystems, a savannah grassland in Africa, a tropical forest in Asia and a coral reef in the Pacific.

Ecology focuses attention on the ways in which all living organisms interact with, and depend on, one another. In this chapter we start by defining some of the basic terms used in ecology. Then we look at how living organisms are classified and named, at the ways in which different plants and animals are adapted to live in different habitats, and at how such adaptations occur. Next we consider some close relationships! We look at what constitutes a species, and at organisms that always live together, such as a parasite and its host. After that, we see how simple food chains combine to form complex, delicately balanced, food webs. We learn how information about food webs can be summarised in pyramids of numbers or biomass and about what happens when the balance of a food web is disturbed. Finally, we consider how food webs transmit energy through an ecosystem and how the elements needed by living organisms are recycled. 1. What is an ecosystem? What four ecosystems are

named in this module? Add your own list of four more. 2. Choose a small ecosystem that you are familiar

with and write down all you know about its ecology.

196

15.2 ECOSYSTEM, COMMUNITY, POPULATION AND HABITAT

In this module you will become more familiar with some of the language of ecology. Before you go on, please review Modules 2.4 to 2.7 about the classification of living things.

An ecosystem is a group of organisms living together in a particular place, together with their environment. The environment includes the physical surroundings of the organisms, and all the conditions that affect them. The group of all the organisms that live together in a particular ecosystem is called a community. A community may include organisms from all the different kingdoms; not just animals and plants, but also fungi, monera and protista. These other kingdoms are briefly reviewed in the box on the right.

The simple diagram above shows a few features of a marine ecosystem. In this ecosystem, the community includes all the organisms named in the diagram and many, many others. Phytoplankton are microscopic floating plants and algae that

live near the surface of the water where there is plenty of sunlight for photosynthesis. Zooplankton are very tiny animals that feed on the phytoplankton. Now consider a grassland ecosystem. The community of organisms will include many kinds of grass, other small plants, and probably a few trees. There will also be different kinds of fungi,

worms and snails, and many kinds of insects and other arthropods including spiders, centipedes and maybe scorpions. There will also be amphibians such as frogs and toads, reptiles such as lizards and snakes, birds of many kinds from seed eaters to birds of prey, and mammals of different sizes from mice to antelopes and lions. Every community also includes billions of bacteria of many different kinds.

In ecology, the word population refers to all the individuals of a particular organism that live in a particular community. For example, a marine community may include a population of sharks, and a grassland community may include a population of grasshoppers.

The habitat of an organism is the part of the ecosystem that it lives in. For example, the habitat of a dolphin is the open sea, but the habitat of a starfish is the seabed. The habitat of a worm is the soil and the habitat of most birds is the air and the branches of trees.

1. In ecology, what are: (i) environment, (ii) organism, (iii) community, (iv) population, (v) habitat.

2. In ecological terms, describe your own: (i) habitat, (ii) environment, (iii) community, (iv) population.

3. What are the following? Give examples if possible: (i) plants, (ii) animals, (iii) monera, (iv) protista, (v) fungi, (vi) algae, (vii) molluscs, (viii) arthropods, (ix) amphibians, (x) reptiles, (xi) mammals, (xii) zooplankton, (xiii) phytoplankton.

The three other kingdoms of organisms

Fungi obtain their food by breaking down the cells of other living or dead organ-isms. They are usually made of thread-like hyphae instead of cells (exception yeast). Fungi include mushrooms, moulds and yeast.

Monera are single-celled organisms with cells that have a cell wall but no nucleus. Monera covers all the many different kinds of bacteria, including the poisonous, so- called ‘blue-green algae’ that grow in stagnant water.

Protista are organisms that have proper cells with nuclei, but are not true animals or plants. Protista include single celled organisms like amoeba (Module 5.4) that feed like animals; and also algae like seaweed that feed like plants (by photosynthesis) but have a much simpler structure.

starfish coral

zooplankton

fish

phytoplankton squid

seaweed

molluscs

dolphin

sea birds

example of

phytoplankton

example of

zooplankton

197

15.3 TAXONOMY – THE CLASSIFICATION AND NAMING OF ORGANISMS Taxonomy is a branch of biology that classifies and names organisms in a systematic way. Study the diagrams below; they summarise what you learnt about classification in Chapter 2.

The most general level of classification is kingdom. All living organisms belong to one of five kingdoms: plants, animals, fungi, monera or protista. Each kingdom is divided into a number of phyla (or divisions in the case of plants) such as ferns or molluscs. Each phylum is divided into classes such as monocots, insects or fish. The classes are divided into orders, the orders are divided into families, the families are divided into genera (singular genus), and finally each genus is divided into species. The scientific name of an organism has two parts; the genus name (spelt with a capital letter) followed by the species name (with a small letter). Here are some examples:

The coconut tree is the only species that belongs to the genus Cocos in the Palm family. The Palm family is in the order Arecales, in the class Monocots, in the division Flowering-plants in the kingdom Plants! The scientific name of the coconut tree is Cocos cocos!

There are many species of mosquitoes. The ones that spread malaria belong to the genus Anopheles in the family Culicids, in the order Diptera, in the class Insects, in the phylum Arthropods, in the kingdom Animals. The scientific name of the species in the picture is Anopheles albimanus.

Humans belong to the genus Homo in the family Hominids, in the order Primates, in the class Mammals, in the phylum Vertebrates in the kingdom Animals. The scientific name for humans is Homo sapiens which means ‘wise man’. Ancient cave men belonged to the species Homo neanderthalenis.

1. A system for classifying and naming organisms, similar to the one described in this module was first published in 1735. Who published it?

2. In biology, what is meant by: (i) classification, (ii) taxonomy, (iii) kingdom, (iv) family?

3. What are (i) Cocos cocos, (ii) Homo sapiens?

PLANTS

CONIFERS MOSSES FLOWERING PLANTS

KINGDOM

DIVISION

monocots dicots CLASS

FERNS

ANIMALS

MOLLUSCS ANNELIDWORMS ARTHROPODS VERTEBRATES

insects arachnids crustaceans myriapods

KINGDOM

PHYLUM

CLASS fish amphibians reptiles birds mammals

KINGDOM

PHYLUM or DIVISION

CLASS

ORDER

FAMILY

GENUS

SPECIES

198

15.4 ADAPTATION IN PLANTS Do you remember the baobab tree in Module 2.9? All baobabs belong to the genus Adansonia. There are eight species and they all grow in dry habitats where rainfall is uncertain. Baobabs store water in their fat trunks and branches. This enables them to survive long droughts that may last several years. Their fat shape adapts them to the dry habitat.

The cactus family (Cactaceae) are also adapted to dry, desert habitats. Their thick, fleshy stems store water, and their leaves have become spines that protect them from being eaten by animals and also

save loss of water by transpiration.

Plants need sunlight to make their food (by photosynthesis). Different plants are adapted to expose their leaves to the sun in different ways. Trees have long trunks and branches that push their leaves high up towards the light. Vines have flexible stems with tendrils, suckers or roots that adapt them for climbing over trees and other plants to reach the sunlight. Grasses reproduce through runners or underground stolons (Module 5.14). These adapt them for spreading out quickly, to cover large areas and get as much sunlight as possible. Plants that grow in shady places are often

adapted by having large leaves to catch as much light as they can.

Plants are adapted in various ways to help them produce and distribute their seeds. Many flowering plants are adapted to attract insects or birds for pollination by their coloured petals, scent and nectar. Their fruits or seeds are also adapted in different ways for dispersal by wind, water or animals (Modules 5.11/12). The picture on the right shows seeds of Bidens tripartite being dispersed by hitching a ride on someone’s clothes!

Adaptation by natural selection. How do organisms come to be so well adapted to their habitats? Adaptation takes place over many generations, sometimes hundreds or thousands, by a process called natural selection. Consider the three facts in the box. Before you read on, see if you can work it out for yourself!

Consider two grass plants of the same species, side by side in a sunny place with no shade. One has slightly narrower leaves than the other. This is an example of natural variation. The narrower leaves lose less water by evaporation so this plant is more vigorous and lives longer and produces more young plants than the other. This is an example of survival of the fittest. Most of the young plants it produces also have narrow leaves. This is an example of inheritance. All these narrow-leaved plants do well in the sunny habitat and have plenty of young of their own. After many generations, all the plants growing in the sunny habitat are the narrow-leaved type because they leave no room for the weaker, wide-leaved plants. However, in a shady place the wide-leaved plants do better. They can absorb more of the dim light for photosynthesis. After many generations, this wide-leaved type takes over the shady habitat. Now there are two different species of grass, a narrow-leaved species that grows in sunny habitats and a wide-leaved species that grows in shady habitats. Natural selection is an ecological process that has been working on the adaptation of organisms for countless generations, ever since life first appeared on Earth about 3.5 billion years ago. And it is still doing so!

1. Explain the meaning of the following: (i) genus, (ii) family, (iii) adaptation, (iv) natural variation, (v) survival of the fittest, (vi) inheritance.

2. Use natural selection to tell the story of how seeds of Bidens tripartite may have become adapted for dispersion by mammals.

Three facts that help to explain adaptation by natural selection

Natural variation – no two individuals of any species are exactly the same.

Survival of the fittest – individuals that are well adapted to their habitats live longer and reproduce more.

Inheritance – parents pass on some of their characteristics to their young.

African baobab (Adansonia digitata)

199

15.5 ADAPTATION IN ANIMALS

Every organism has to be well adapted to its habitat. For example, water is the habitat of fish and trees are the habitat of monkeys. A fish cannot live in trees and monkeys cannot live under the water! Fish are adapted to living in water in many ways. They have special organs called gills instead of lungs. The gills extract oxygen from air that has dissolved in the water, and pass it on to

the blood for respiration. Fish also have a streamlined shape so they can slip through the water easily. Their tail fins and backbones, which can flex from side to side, adapt them for swimming through the water, and their other fins are adapted for control and steering. In similar ways, monkeys are adapted to living in trees. They have strong and flexible hands and feet that can grip the branches, and their prehensile tails are adapted to curl around and grip branches too. Like humans, but unlike most other mammals, monkeys’ eyes are at the front of their heads instead of the sides. This gives them binocular vision (Module 7.12) which helps them to judge distances. Binocular vision adapts them for moving around fast in three dimensions, and especially for jumping from branch to branch.

Birds provide many good examples of adaptation. They are adapted for flying in several ways. They have hollow bones and they are covered with feathers. Both these things help to make them very light for their size. They have wings instead of front legs, and their feathers give the wings a big surface area. This gives them more lift when they fly. Unlike fish, birds have a very inflexible backbone. This provides a rigid framework to support an enlarged breastbone to which enlarged pectoral muscles are attached. The pectoral muscles are the strong muscles that flap the wings. They are also the breast meat that we enjoy when we eat a bird like a chicken.

In Module 2.9 we saw how the beaks of different birds are adapted for eating different kinds of food. Look at the photos of the duck and the falcon below. The broad, flat beak of the duck is adapted for pulling up weeds from the bottom of a pond or river, but the sharp, hooked beak of the falcon is adapted for killing its prey and tearing off strips of flesh. The feet of these two birds are also adapted for different ways of life. The duck has flat, webbed feet that are adapted for walking on soft mud and that act like paddles under the water to

push the duck along when it swims. The feet of the falcon are very different. They have long, strong claws adapted for gripping prey or perching on rocks.

Charles Darwin was an English biologist. In his famous book, On the Origin of Species (1859), he developed the idea of ‘natural selection’. One of his many observations concerned the beaks of some small birds called finches which he saw in the Galapagos Islands. He noticed that finches from different islands were very similar and he thought that they must have been a single population to start with. On one island, the finches lived in trees and ate small insects. They had small, grasping beaks. On another island, there were fewer insects and the finches had to dig under the bark of trees to find them. They had longer and thinner beaks that were well adapted for this task. And on another island, the finches lived on the ground and ate seeds. They had bigger, stronger beaks that were well adapted for cracking open seeds. From these and many other studies, Charles Darwin developed the idea of adaptation by natural selection.

1. List and explain as many adaptations as you can for one plant and one animal not mentioned in the last two modules.

Whale shark (Rincodon typus)

Spider monkey

(Ateles fusciceps)

Mallard duck (Anas platyrhynchos)

Brown falcon (Falco berigora)

200

15.6 CLOSE RELATIONSHIPS – SPECIES AND SYMBIOTES

This module is about close relationships. Are horses and donkeys the same species? What about lions and tigers? And why do some different species always live together – sometimes actually on or inside each other? But first we need to ask what, exactly, is a species?

Species. In Module 15.3 we learnt that species is the lowest and most exclusive group in the biologists’ taxonomy. Together with Genus, species provides the scientific name for a particular kind of organism. A species can be defined as a group of organisms that interbreed to produce

fertile offspring. For example, two dogs may look quite different, but if they mate they will produce puppies, and when their puppies grow up, they can mate and have puppies too. Therefore the two dogs are the same species. In fact, all dogs belong to the same species, Canis lupus. However, if a horse and a donkey mate, they produce an offspring called a mule. Mules are infertile; they cannot produce young. Therefore horses and donkeys are different species. A lion and a tiger can mate to produce ligers (if the father is a lion) or tigons (if the father is a tiger). But unfortunately

ligers and tigons are infertile. They cannot breed therefore lions and tigers are separate species. Their scientific names are Pantheria leo for the lion and Pantheria tigris for the tiger.

Symbiosis means living together. Biologists recognise three different ways in which two species live very closely together: mutualism benefits both species, commensalism benefits one species and is harmless to the other, and parasitism benefits one species and is harmful to the other.

Mutualism. A good example of mutualism is the clown fish (Amphiprion ocellaris) and the sea anemone (Heteractis magnificus). These two species always live together. The clown fish is immune to the sting of the anemone so it can hide from predators amongst the tentacles of its host. The anemone benefits because the clown fish eats other small fish and

invertebrates that might otherwise harm it. Also, the faeces of the clown fish provide the anemone with nutrients. Cattle and antelopes often have close mutual relationships with various species of birds. The birds feed on ticks and other parasites which infest the skin of the mammal. The bird benefits from a good source of food and the mammal benefits in health and comfort. Some very important mutual relationships are those between insects and flowers. Many insects depend on flowers

for food in the form of nectar, and the flowers depend on the insects for pollination.

Commensalism. On open grasslands, some birds always follow browsing mammals. The mammals disturb insects in the grass and the birds feed on these. The birds benefit from this relationship and the mammals are not harmed. Another example is the bacteria that live all over our skin. The bacteria get a place to live and dead skin cells to feed on, but they do us no harm.

Parasitism. The fleas, ticks and lice that live on the skins of mammals and suck their blood are good examples of external parasites. They often cause disease as well as discomfort for their hosts. The picture (left) is a tick feeding on a human. Skin diseases like ringworm and athletes foot are caused by parasitic fungi (Tinea sp.

– sp. means there are various species). Parasitic fungi also cause diseases in many plants. The picture (right) shows a parasitic plant called dodder (Cuscuta sp.) that is invading and killing a tree. Dodder has special roots that burrow into the xylem and phloem of the tree and steal its sap. Internal parasites are parasites that live inside their hosts. They include various species of worm that live in the intestines of many animals including humans.

1. Look up mutualism and parasitism. List all the new examples you can find and give brief descriptions of two of them. Add pictures if possible.

201

15.7 PRODUCERS, COMSUMERS AND DECOMPOSERS

Organisms can be classified according to their roles as producers, consumers or decomposers.

Producers are organisms that make complex, organic compounds from simple, inorganic molecules. The main producers are green plants and algae, including phytoplankton, which grow in the world’s grasslands, forests, lakes and oceans. In the process called photosynthesis

(Module 6.12), they use energy from sunlight to make complex carbohydrates, such as starch (C12H10O5)n, from carbon dioxide (CO2) and water (H2O). Oxygen is produced as a by-product and released into the atmosphere. Producers use the starch, together with minerals from

the soil or the surrounding water, to make other complex compounds including sugars, cellulose, lipids and proteins (Module 13.11). The elements absorbed from the soil include everything needed to make complex living tissues; nitrogen, phosphorus, sodium, potassium, magnesium, calcium, iron and chlorine are the most important ones. Producers are the basic source of food for all other organisms.

Consumers are organisms that feed on the complex, organic compounds made by producers. They use these compounds for energy and for building tissues. The main consumers are animals and they all need food that contains carbohydrates, lipids and proteins. Animals that feed on plants are called primary consumers or herbivores; they include examples from caterpillars to

cattle! Animals that feed on other animals are called secondary consumers or carnivores; they include examples from ticks to tigers. Animals that eat both plants and animals are called omnivores; they include examples like pigs, rats and crows. Humans are omnivores too. Herbivores have mouth parts and digestive systems adapted for breaking down the tough, cellulose walls of plant cells. Herbivorous mammals have flat

teeth for grinding cellulose and long intestines digesting it. Carnivores have mouths and digestive systems adapted for eating meat or sucking blood! Carnivorous mammals have sharp teeth and need only short digestive systems. Mosquitoes have sharp feeding tubes that pierce skin.

Decomposers are involved in the decay of organic matter. The main decomposers are fungi and bacteria. When organic matter decays, fungi and bacteria break down the complex organic substances into simple inorganic molecules. These are recycled in the soil as minerals and in the atmosphere as carbon dioxide. The chart on the left

summarises the main relationships between producers, consumers and decomposers.

1. What are (i) photosynthesis, (ii) producers, (iii) carbohydrates, (iv) cellulose, (v) minerals, (vi) proteins, (vii) consumers, (viii) organic matter, (ix) decomposers?

2. What shows us that the liger must be a carnivore? 3. Use the chart above to summarise, in 3 sentences,

the roles of producers, consumers and decomposers.

Grassland in Mongolia

Forest in Malaysia

PRODUCERS

make complex organic substances

CONSUMERS

feed on complex organic substances

DECOMPOSERS breakdown complex organic substances

feeding

SOIL or WATER

CO2

H2O minerals

decay

O2 light

energy CO2

CO2

O2

O2

Liger

Cattle

Fungi

202

15.8 FOOD WEBS

In Module 2.8 we summarised the way in which living organisms depend on one another for food by writing simple food chains. Food chains follow a basic pattern like the one below.

In a natural ecosystem, the criss-crossing food chains can be represented by a food web. A simplified example is shown on the next page. Every food chain starts with a producer (a green plant or an alga) and ends with a tertiary consumer. Tertiary consumers (sometimes called top carnivores) feed on secondary consumers (carnivores) and may also eat herbivores. Tertiary consumers include predators that hunt and kill their prey (such as sharks, crocodiles, eagles and the big cats), and scavengers that eat animals, or the remains of animals, that are already dead (such as hyenas and vultures). The photo shows hyenas and vultures feeding on an antelope killed by a cheetah.

In a grassland ecosystem, the producers are mainly grass and other flowering plants, and

the top predators are the big carnivores like tigers, wolves and birds-of- prey. In a marine ecosystem, the main producer is the phytoplankton. Water plants and algae contribute little because they grow only in shallow water. The top predators are sharks, dolphins and killer whales. However, the biggest consumer is the blue whale which filters more than three tonnes of krill through its mouth every day. Krill is a small (1 to 2

cm) crustacean like a shrimp that feeds on plankton.

In any ecosystem, the numbers of herbivores are limited by the numbers of producers available for them to eat. In the same way, the numbers of carnivores are limited by the numbers of herbivores. These balances tend to be self-regulating. If the number of carnivores increases, they will eat more herbivores. The number of herbivores will decrease and eventually some of the carnivores will starve. In time, a new balance will establish itself. If the number of carnivores goes down, they will eat fewer herbivores. The numbers of herbivores will go up until there are not enough producers for them to eat, then some of them will starve. In time, a new balance will establish itself. These balances apply not only to the ecosystem community as a whole, but also to the populations of each species within the community.

Natural food webs are complicated and we do not always understand them completely. If disease or human interference disturbs a food web, it may be hard to predict what will happen. For example, if a modern grain crop replaces a natural grassland, the increase in the yield of the producers can lead to an increase in the population of the natural consumers. Especially the consumers that like eating grain, for example mice! And if the wrong chemicals are carelessly used to protect the new grain crop from insect pests, these chemicals can also kill the small birds that eat the poisoned insects, and the top predators that eat the poisoned birds. Without enough predators to keep

the mouse population in check, there may be a sudden plague of mice which destroys the crops and spreads disease. Things like this have happened in many countries.

1. What are (i) food webs, (ii) secondary consumers, (iii) tertiary consumers, (iv) predators, (v) scavengers?

2. Research krill and write a few sentences about it.

3. Many organisms are missing from the food web on the next page. Draw a new food web of your own that includes frogs and lizards.

A basic food chain

eaten by eaten by eaten by PLANT HERBIVORE CARNIVORE CARNIVORE

(producer) (primary consumer) (secondary consumer) (tertiary consumer)

Krill

Plague of mice in Australia

203

PHYTOPLANKTON

ALGA

WATER PLANTS

WATER PLANTS

DEAD PLANTS

WORMS

MOLLUSCS

WATER BIRDS DEAD ANIMALS

VULTURES

SMALL FISH herbivores

SMALL FISH herbivores

LARGE FISH

SMALL FISH carnivores

SHARKS

EAGLES

CROCODILES

GRASS

GRASS

TREES

PRODUCERS

FLOWERING PLANTS

MOLLUSCS

WORMS

EAGLES

FLOWERING PLANTS

ANTELOPES

INSECTS carnivore INSECTS

herbivores

INSECTS herbivores

SMALL BIRDS herbivores

SMALL BIRDS herbivores

SMALL BIRDS carnivores

SPIDERS

LARGE BIRDS

RATS

DEAD PLANTS

SMALL MAMMALS herbivores

SNAKES

CATTLE

SMALL MAMMALS carnivores

LARGE MAMMALS carnivore

TOP CARNIVORES

KEY:

Simplified food web for a lagoon and grassland ecosystem

SNAKES

KRILL

ZOOOPLANKTON

FUNGI

204

15.9 THREE MAJOR ECOSYSTEMS AND PYRAMIDS OF NUMBERS

Aquatic ecosystems include the oceans, together with fresh-water lakes, rivers and even small ponds. Together they cover about three-quarters of the Earth’s surface. The oceans (marine ecosystems) contain a huge variety of organisms from sea shells to sharks, and from coral to killer whales. The producers at the bottom of almost every marine food chain are the microscopic phytoplankton that float near the surface. They use energy from sunlight to change carbon dioxide, water and minerals from the sea around them, into carbohydrates and all the other complex substances that living things depend on. Microscopic zooplankton, and the small shrimps called krill, feed on the phytoplankton, and almost everything else feeds on them! The top carnivores include sharks, dolphins, whales and giant squid.

Fresh-water ecosystems also depend on phytoplankton as the main producers, but the algae and green plants that grow in shallow waters are important in some systems. A pyramid of numbers shows the populations of organisms in a food chain or food web. The pyramid on the right shows a fresh-water food chain involving six levels. The top predator, a bird-of-prey, is supported by a population of ten big fish. This does not mean that one bird eats ten large fish. It has other prey too, but sometimes it will catch one of the big fish. Meanwhile the other nine will hide and breed and grow. In the end, the balance of the populations will remain about 10 big fish to 1 bird-of-prey. The big fish are supported by larger numbers of smaller fish and so on down to the billions of phytoplankton at the bottom of the food chain. In the end, all these tiny organisms help to support the bird-of-prey! The pyramid shape indicates (but not to scale) that the populations supported by the system get smaller as we move up the food chain.

Grassland ecosystems cover more than 30% of the Earth’s land surface. The producers are mainly various kinds of grass, with many herbs (non-woody flowering plants) and a few small

trees. The primary consumers are invertebrates (including worms, molluscs and many insects and other arthropods), and herbivorous mammals from rodents to buffalos. Important secondary consumers include frogs, lizards, birds and small carnivorous mammals. Top consumers include large snakes, birds-of-prey, and the big cats; also scavengers such as vultures and jackals. The diagram on the left shows a pyramid of numbers for a grassland food chain. A different format has been used for this pyramid. A hundred-thousand grasses and herbs support a population of ten-thousand caterpillars

(larvae of butterflies and moths), which support 50 mice, which support one snake.

Forest ecosystems include tropical and temperate forests which used to cover up to 50% of the Earth’s land surface. Human activity has now reduced this to about 30%. The main producers are trees, but vines are also important in tropical forests. The main primary consumers (herbivores) include insects and other invertebrates, nectar- and seed-eating birds, and fruit-eating mammals including rodents and monkeys. Secondary and tertiary consumers (carnivores) include insect-eaters such as spiders, frogs, lizards and birds, and flesh-eaters including snakes, birds- of-prey such as owls, hawks and eagles, and carnivorous mammals such as bears, leopards and jaguars. The diagram on the right shows a pyramid of numbers for a forest food chain. You will notice that the pyramid of numbers is not pyramid-shaped when the producer is a single huge organism like a tree!

1. What are (i) birds-of-prey, (ii) herbs, (iii) molluscs, (iv) invertebrates, (v) big cats, (vi) larvae, (vii) rodents? List three examples of each.

2. List three top carnivores in a fresh-water community. 3. What is a pyramid of numbers, and what does the

width of the pyramid at each level correspond to?

billions of phytoplankton

10 000 fresh-water shrimps

1000 small carnivorous fish

100 medium carnivorous fish

10 big

carnivorous fish

bird-of-prey 1

10 000 caterpillars

(primary consumers)

50 mice (secondary consumers)

1 snake (tertiary consumer)

100 000 grasses & herbs (producers)

10 000 insects

(primary consumers)

80 lizards

(secondary consumers)

1 owl (tertiary consumer)

1 tree

(producer)

205

15.10 ENERGY IN ECOSYSTEMS AND PYRAMIDS OF BIOMASS

Before you go on, revise Modules 14.2 to 14.4 about different kinds of energy, energy conversions, and energy efficiency. The flow of energy in an ecosystem is summarised below.

Producers convert light energy from the sun into chemical energy, which is stored in the form of carbohydrates, mainly starch and sugars (Module 13.11). Primary consumers (herbivores) obtain this stored chemical energy by eating plants, while secondary and tertiary consumers (carnivores) obtain it by eating animals. All living organisms, including plants, release the energy stored in the carbohydrates by respiration. They use this energy to power all their life processes. These processes include the renewal and growth of tissues in all organisms, as well as movement and the maintenance of body temperature in most animals.

The transfer of energy from the sun, and then along the food chain, is not very efficient. In a grassland, most of the solar energy that reaches the ground is either reflected, or absorbed in heating soil and evaporate-ing water. Only about 1% of the energy is absorbed by photosynthesis. Forest and aquatic ecosystems are somewhat more efficient but the proportion of solar energy absorbed by photosynthesis is still only a few percent. Organisms use most of the energy they obtain from carbohydrates to power their own life processes. Some energy is also wasted as undigested food (faeces) and as heat lost to the environment. At each step in the food chain, only about 10% (on average) of the total energy taken in, is used to grow new tissue that is passed on to the next consumer in the food chain. Primary consumers get about 10% of the total energy stored by the producers, secondary consumers get only about 1%, and tertiary consumers get only about 0.1%!

The transfer of energy in an ecosystem can be shown in a pyramid of biomass. The example below represents the food web for a whole grassland community, not just a single food chain. It is

approximately to scale. The width of the pyramid at each level corresponds to the biomass at that level. The biomass is the dry mass of all the organisms at a given level in a given area. The dry mass is the mass after the removal of all water; it is a good measure of the stored chemical energy available. Biomass is often measured

in grams per square metre (g/m2). This example shows that only about 10% of the total energy

produced or consumed at each level is actually passed on to the next level.

1. What is a pyramid of biomass? How could you try to measure the biomass of producers in a grassland?

2. What is the source of energy in ecosystems? What % of this energy is passed on to tertiary consumers?

PRODUCERS (plants & algae)

PRIMARY CONSUMERS

(herbivores)

SECONDARY CONSUMERS

(carnivores)

TERTIARY CONSUMERS

(carnivores)

food chain

(chemical energy)

photosynthesis (light energy)

food chain (chemical energy)

food chain (chemical energy)

In photosynthesis, producers absorb light energy from the sun and store it as chemical energy in carbohydrates (made from carbon dioxide and water).

In food chains, consumers obtain this stored chemical energy by eating the producers or other consumers. The energy is released during respiration. <10 %

~10 % ~10 % ~10 %

producers

primary consumers

secondary consumers

tertiary consumers

1% absorbed by photosynthesis

100%

20% reflected

40% heats soil

39% evaporates

water

206

15.11 THE RECYCLING OF ELEMENTS

Living organisms are composed mainly of water and complex organic compounds of carbon, hydrogen and oxygen. They also contain small amounts of a few other elements. A natural ecosystem recycles all these elements so they can be used over and over again.

Oxygen is recycled by producers when they use photosynthesis to store light energy from the sun. In this process, carbon dioxide (from the air) combines with water (from the soil) to make carbohydrates such as starches and sugars. Oxygen is a by-product which is released into the air, where it is available for all living organisms to use in respiration (see the oxygen cycle in Module 6.13).

6CO2 + 6H2O + energy C6H12O6 (glucose) + 6O2

Carbon is recycled by all living organisms when they use respiration to release the energy stored in carbohydrates. In this process, oxygen from the air reacts with carbohydrates to make water and carbon dioxide. Similar reactions occur when organic materials are burned. The carbon dioxide and water vapour are released back into the air where the carbon can be used again by plants for photosynthesis (see the carbon cycle in Module 6.13).

C6H12O6 (glucose) + 6O2 6CO2 + 6H2O + energy

Decomposers recycle all the other elements needed by living organisms. They also recycle some of the carbon. The main decomposers are bacteria and fungi, including mould. In the photos below, the bacteria (top) are magnified 5000 times but the fungi (bottom) are only half life size.

Scavengers, which feed on dead organic matter, often start the process of decay by breaking dead organisms and organic matter (such as leaves and faeces) into smaller pieces. On land, scavengers include worms, molluscs, and insects and their larvae. In aquatic ecosystems, the

main scavengers are zooplankton and krill. Decomposers break down the complex organic compounds of which living things are

made, (mainly compounds of carbon, hydrogen and oxygen), and convert them into simple, inorganic molecules. The respiration of the

decomposers releases carbon as carbon dioxide and hydrogen as water.

Other elements, that are present in much smaller quantities, are released as minerals and recycled through the soil or water.

Minerals are inorganic substances found in rocks and soils. The most important minerals recycled by decomposers include the following.

Nitrates (containing NO3 ions) recycle the nitrogen needed for making proteins (see the nitrogen cycle in Module 8.9).

Phosphates (containing PO4 ions) recycle the phosphorus needed for many vital chemical processes in living organisms, and for making bones in vertebrates.

Compounds containing the Na, K, Mg, Ca, Fe, Cl and SO4 ions that all living organisms need.

1. What are (i) organic matter, (ii) bacteria, (iii) fungi, (iv) minerals, (v) aquatic ecosystems?

2. Why do ecosystems need to recycle elements?

3. Explain the roles of scavengers and decomposers in recycling elements.

4. Why do all living things need nitrogen?

PRODUCERS

make complex organic substances

CONSUMERS

feed on complex organic substances

DECOMPOSERS

breakdown complex organic substances

feeding

SOIL or WATER

CO2

H2O minerals

decay

O2

light energy CO2

CO2

O2

O2

Soil is essential for the growth of many plants. It is a mixture of sand (large grains from the weathering of quartz), clay (tiny gains from the weathering of feldspars) and humus (bits of decaying organic matter). It also contains air, water and minerals dissolved in the water. It is the home of many organisms including bacteria, fungi and scavengers such as worms and insect larvae. Look at soil with a hand lens and see what you can find.

x 5000

x ½