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Structure & function of food chains

We need a more complete understanding of ecosystems for a number of reasons. Examples include ….. Managing resource species, such as fisheries or timber Understanding the role of biological production in climate change

Introduction - land and oceanOcean food chainsSummaryAre there pyramids in the ocean?

Introduction - land and ocean Ocean food chains Summary Are there pyramids in the ocean?

First, let’s start with a familiar ecosystem

Grass - producer Wildebeest - herbivore (consumer) Lion - carnivore (consumer)

In the ocean, the food chain is built the same way, but the plants and animals look different

ALGAE Producer COPEPOD Herbivore FISH Carnivore

This food chain might not stop here...

…. or it it might be surprisingly short ….

Plants are the ‘producers’ in nearly all ecosystems The amount of food for plant-eating consumers - ‘herbivores’ - is determined by... The amount of plants growing in a habitat - their ‘biomass’ The addition of new plant growth - their ‘production’

Plants on land

On land, large plants are the dominant part of most ecosystems

Much of this biomass consists of woody tissue Wood is more or less indigestible, and persists for a long time

Because so much land plant material is difficult to digest, herbivores need to eat very large amounts A four-tonne elephant will eat plants equivalent to nearly 10% of its body weight per day This means that you need a lot of plants to feed one elephant

There are a few marine habitats where living things form the framework for the ecosystem

Kelp Coral

There are obvious differences in plant biomass on land Plant biomass in the ocean is much lower, and also variable Image from the SeaWiFS sensor, courtesy ORBIMAGE Colours show the amount of biomass: Blue and purple = low Yellow and orange = high

Too dark for growth surface 100 m 500 m 1000 m 3500 m Enough light here for growth In the open ocean, algal plankton grow only in the top 100 metre-thick layer

Plankton algae can grow rapidly Unlike trees, they contain little inedible material

There is a small mass of algae per unit area but this is relatively productive

Land and ocean ecosystems have very different plants at the base of the food web We can summarise the main differences ….

30% of the planet’s surface 70% of the planet’s surface 99% of all living material Less than 1% of all living material 54% of the annual growth of new living material 46% of the annual growth of new living material Very big plants Very small plants LAND OCEAN

30% of the planet’s surface 70% of the planet’s surface 99% of all living material Less than 1% of all living material 54% of the annual growth of new living material 46% of the annual growth of new living material Very big plants Very small plants LAND OCEAN

Introduction - land and ocean Ocean food chains Summary Are there pyramids in the ocean?

Not all food eaten by an animal is turned into new body tissue - ‘growth’- or is devoted to reproduction

In fact, for most animals, very little food contributes to growth Some is indigestible, and is lost as faeces Much food is digested, but then burnt up to provide energy

An example: krill feeding on plankton algae 70 milligrammes of algae eaten per day 10 milligrammes - about 15% - is indigestible and lost as faeces A 1-gramme krill needs about 5% of its body mass per day as an energy supply, so it ‘burns up’ 50 milligrammes of food This leaves 10 milligrammes of food that can be used for growth and reproduction

70 milligrammes ‘in’ 50 milligrammes plus 10 milligrammes ‘to waste’ 10 milligrammes to growth = 15% of food eaten

A growth rate that is 15% of the food eaten is high To achieve this, the krill is adding 1% of its body mass per day

More typically, the ratio between the amount of food eaten and the amount of growth is around 10% The ratio is less for animals which use a lot of energy to stay alive Warm-blooded animals such as mammals typically have ratios less than 5%

The loss of food material (to provide energy and as indigestible faeces) means that only part of the growth of plants can be eaten by secondary consumers - ‘predators’

As the food web becomes more complicated, with longer food chains, less plant production reaches the top-level predators

We can contrast two food chains in the this Southern Ocean food web

Krill converts 15% of algal food to growth Blue whale converts just 4% of krill to growth

For 100 units of plankton algal growth... … there are 15 units of new krill growth... … and there are only 0.6 units of new whale growth

Now a more complex food chain...

Copepod converts 15% of algal food to growth Krill converts 15% of copepod food to growth Fish converts 10% of food to growth Squid converts 10% of food to growth Seal converts 4% of food to growth

Now, for 100 units of plankton algal growth... … and there are only units of new seal growth % of the algae eaten by the copepods has been lost

Short food chains give an efficient transfer between producers and top-level consumers Long food chains are inefficient

Introduction - land and ocean Ocean food chains Summary Are there pyramids in the ocean?

This ecosystem demonstrates a clear pyramid of biomass

Producer Secondary consumer - predator Primary consumer - herbivore

What about the open ocean?

Plankton algae: Producers Baleen whale: Consumer - predator Krill (crustacea): Consumers - herbivores

Blue whale eats about 3 tonnes of krill per day

It takes about 120 tonnes of krill growing at their maximum rate to support one whale This means that the whale needs to hunt over an area of between 5 and 40 square kilometres

Algae produce about 20% growth per day, so it takes about 125 tonnes of algae to support the krill to support one whale The krill need to eat 25 tonnes of algae per day

Biomass in this system is similar at all three levels in the food chain

125 tonnes120 tonnes100 tonnes BIOMASS

Production is a different story

25 tonnes per day3 tonnes per day120 kilogrammes per day PRODUCTION

25 tonnes per day3 tonnes per day120 kilogrammes per day PRODUCTION

So, a pyramid of production is real in both ocean and land ecosystems On the other hand, a pyramid of biomass (or numbers) is not a useful representation of ecosystem function in either environment

Introduction - land and ocean Ocean food chains Summary Are there pyramids in the ocean?

You have seen that - There are similarities and contrasts between ocean and land ecosystems Relationship between plant biomass and production is different in the two systems

30% of the planet’s surface 70% of the planet’s surface 99% of all living material Less than 1% of all living material 54% of the annual growth of new living material 46% of the annual growth of new living material Very big plants Very small plants LAND OCEAN

Large amount per unit area Very small amount per unit area Grow slowly - double biomass in months to years Grow very fast - double biomass in days to weeks Difficult to digest Easy to digest Very big plants Very small plants LAND OCEAN

You have seen that - Very little plant production contributes to consumer growth Short food chains are efficient, long food chains are ineffficient

You have seen that - Ocean food webs rely on the high production rate of plankton algae Only a pyramid of production provides a real picture of ocean or land ecosystems

NOTES for USERS The material in this slide show is designed to support the teaching of science at Key Stage 4 A full description of the slide show, and linked activities for students, can be found on the National Marine Aquarium (NMA) web-site: Teachers are free to amend the slide show in whatever way they feel fit, or to use slides in other contexts. However, please note that neither the NMA nor the designers will accept responsibility for modifications, and original material remains copyright of the NMA Individual images used in the slides are copyright of NMA or STEP, except where acknowledged separately The slides have been set up to display as A4 landscape format. If they are incorporated into other slide sequences with different display settings, change in aspect ratio and text location will occur The slide sequence contains the minimum of effects and transitions. However, there are some automated animations, and teachers will wish to make sure that they are familiar with the sequence before use in class Use the PowerPoint notes viewer to obtain additional information for some slides T raining & S cience E ducation P artnership