What Role Does Biodiversity Play?

What you will learn

• Explain how biodiversity relates to ecosystem health.
• Give one example in which a declining species leads to changes in others.
• Recognise why a high species count does not guarantee recovery.

Before you start

No specialist background is needed; technical terms are explained as they appear.

How does biodiversity relate to ecosystem health?

Biodiversity can be viewed in two ways: the number of species present in an ecological system, and the extent to which some species are threatened or endangered. Ecosystem health refers to the interrelationships among organisms, including the structure of food webs and the ability of the system to sustain itself over time. Ecosystem health is a dynamic characteristic rather than a fixed state.

Both can influence the functioning and stability of the system. Scientists generally agree that as the number of species in a type of ecological system declines, that system can potentially lose its resilience, meaning its ability to rebound after it has been stressed.

Biodiversity also has value for people. Different species provide different products, such as different tree species for different woods. Genetic diversity within a species is necessary to maintain diversity among species, and it contributes to the food, fibre and medicines available from nature.

How can one declining species affect others?

Because species are linked through food webs, a change in one can lead to changes in many. The clearest example in the source is the sea otter.

When otter numbers declined, the sea urchins they preyed on were no longer held in check. The urchins grazed the kelp down to the sea floor, and marine kelp forests were lost. The chain runs from otter, through urchin, to kelp.

There are other examples. The decimation of grazers such as bison, and predators such as wolves, has had cascading impacts on upland vegetation, wetlands, fish and other species. The introduction of the sea lamprey to the Great Lakes led to sweeping changes in the entire food chain.

Does a higher species count guarantee recovery?

No. Determining the number of species, their types and their distribution that ecosystems need to be resilient has been an exceedingly complex and difficult scientific challenge. A species count alone does not answer that question.

Context matters too. A disturbance that reduces biodiversity on a small scale may or may not affect ecological function on a larger scale. Introduced species, such as plants and plant pathogens, can rapidly transform landscapes.

An overabundance of nutrients in water can also trigger blooms of cyanobacteria. A list of species therefore does not tell you what is acting on the system or what state it is in over time.

Real-world example

Picture a coast with a kelp forest. If you only count species, you see otters, urchins and kelp. A better question is who eats whom, and what happens when one species declines.

If otters decline, urchins eat more kelp, and the habitat of many other species disappears. A map of relationships helps you predict the consequences better than a single number.

Common misunderstandings

Biodiversity is not only a species count. It also covers how threatened species are and the relationships among them.

Losing a species does not always affect only that species. The effects can spread to others through food webs.

More species does not guarantee a system will recover. You need to look at context and the pressures acting on it.

Frequently asked questions

How is biodiversity measured?

By the number of species present and the extent to which species are threatened, together with the relationships in the system.

Why did fewer otters lead to a loss of kelp forests?

Otters prey on sea urchins. When otters declined, urchins grazed kelp down to the sea floor.

What is the resilience of an ecosystem?

Its ability to rebound after stress. A system that loses species can lose part of this ability.

Summary

• Biodiversity can influence how ecosystems function and how stable they are.
• A declining species can set off wide changes through food webs.
• A high species count does not guarantee recovery.