Chapter 24

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Biodiversity and Ecosystem Function

Introduction

Human activities alter biodiversity - agriculture, invasive introductions on purpose or via global transport of goods. Global changes in atmospheric chemistry alters regional biodiversity.

Biodiversity is a big topic but this chapter refers just to how it is a measure of ecosystem function.

Defining Biodiversity

Can be defined as both species richness and equitability.

Species richness is simply a measure of how many different species are in an area.

Equitability also takes into account the relative importance of different species by measuring abundance.

Evolution and Extinction

In the Earth's 4 billion year history, land plants did not appear until approximately 400 million years ago. Since then the number of species has increased linearly.

Evolution from one species to two occurs when subpopulations are isolated for long enough for mutations to lead to divergence.

Patterns of Biodiversity in Terrestrial Ecosystems

Species richness tends to increase from poles to tropics and from low productivity ecosystems to high. This tends to be linked with greater structural diversity of habitats.

The age of an ecosystem and the structure of disturbance events is very important in directing biodiversity.

Niche packing is the amount of division of habitats between species. Older ecosystems have had more evolutionary time for species to develop interactions allowing them to finely divide niches.

Island biogeography explains how islands tend to have a high % of endemic species and lower total number of species than similar continental habitats. Resurveying islands over time show that they tend to persistently hold similar numbers of species but that the composition changes regularly - high extinction and recolonisation rates. This theory can be related to isolated ecosystems and used to understand how large areas need to be to support viable populations.

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Human Effects of Biodiversity and Consequences for Ecosystems

Rates of extinction are not constant over time, there have been epochs of large extinction events.

These are generally linked to large natural events, however in more recent times humans have become the dominant force.

Working out how biodiversity affects ecosystem function should follow one of three paths - Path A- EF increasing continuously with increasing biodiversity, Path B- EF increasing at an ever decreasing rate with biodiversity, Path C- EF not increasing with biodiversity. The second pattern is generally shown to be the most likely.

These paths are linked to redundancy. In A, redundancy is very low, in B redundancy is uneven and in C, redundancy is very high.

Several studies show that redundancy may be fairly common in ecosystems. It may however support the system in response to disturbance. Studies show that resilience is higher in more biodiverse systems.

Keystone species are those which have a relatively large role in maintaining ecosystem function.

Invasive Species and Introductions

Textbook lists some key examples.

Environmental Change and Biodiversity

Several studies are listed.

One study shows that increased nitrogen loading of soils leads to the dominance of certain plant species, reduction in biodiversity and loss of C4 photosynthesis plants which usually specialise on low N soils. Species richness tends to be higher in low N soils throughout Europe.

The results are expected to be similar for increased CO2 levels, with C4 pathway plants loosing their advantage over C3 plants.

CO2 will also increase temperature and change local climates. Different models have been used to predict the effects. These models differ slightly in their predictions for changes in temperatures and rainfall. Climate change will have the largest effects when species can not adapt fast enough. This may be because there are physical barriers to migration. Species that naturally disperse slowly will also be effected more than those which can disperse rapidly.