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Ecosystems & Food Webs: Comparing land and ocean

Land and ocean ecosystems obey the same basic rules However, there are some key differences between the two Understanding these differences helps us to understand how ecosystems work

Comparing land and ocean ecosystems Examining an ocean ecosystem Summary

Comparing land and ocean ecosystems Examining an ocean ecosystem Summary

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

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

Land covers about 30% of the planet’s surface There are also plants in the oceans, as this satellite image shows Image from the SeaWiFS sensor, courtesy ORBIMAGE Colours show the amount of biomass: Blue and purple = low Yellow and orange = high

Most vegetation in the ocean is microscopic plankton This is a chain-forming diatom - a common type of plankton alga Each cell is about 0.05 mm long

The average depth of the oceans is more than 3.5 kilometres, and 84% of the oceans are at least 2 kilometres deep Enough light for plant growth only reaches to about 100 metres depth This means that rooted plants can only grow in shallow seas - less than 5% of total ocean area

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 are almost completely edible

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

Comparing land and ocean ecosystems Examining an ocean ecosystem Summary

The Southern Ocean provides an example of a simple oceanic ecosystem Food chains tend to consist of only a few levels

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

This simple food chain is an extreme example of an oceanic ecosystem It has only two links from producer to the ‘top-level’ consumer There are huge differences in size between the different components

We can visualise the size differences more easily if we equate each living thing to familiar objects

Making each phytoplankton cell 1000 times bigger means that it is about the size of a squash ball

Making the krill 1000 times bigger means that it becomes as long as a small bus

Making the whale 1000 times bigger means that it is still huge - it would stretch across the Isle of Wight

In a simple ecosystem such as this, plant growth is transferred efficiently from producer to top-level consumer because …. Primary consumers (herbivores) can convert most of their plant food to tissue growth and reproduction There are few steps in the food chain, so most plant biomass can be transferred to the top-level consumer

Comparing land and ocean ecosystems Examining an ocean ecosystem Summary

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

Land ecosystems have large, slow- growing plants It takes a large biomass of plants to support herbivores You have seen that -

Ocean ecosystems have small, fast-growing plants In the oceans, a small biomass of plants supports high herbivore production You have seen that -

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