Topic Introduction

If you did the accompanying course on the structure and composition of tropical forests, you will have had the opportunity to learn about the diversity of tropical forests. If you haven’t done the accompanying course, it is still possible to complete this course, but we will be assuming that you have some familiarity with the organisms that we will be discussing.

In this topic, we will examine some of the trends in species richness that are apparent in the Tropics. For example, how is species richness affected by altitude? Why are there fewer species on the summits of tropical mountains than lower down? How is species richness affected by soil type (and, in turn, by the underlying geology)? Are certain species restricted to certain soil types? We illustrate some of the trends using a particularly species-rich and uniquely tropical family of trees, the Dipterocarpaceae.

Answering these questions assumes that we can measure the number of species present in a forest. This is a lot more difficult than it sounds. Firstly, the species have to be identified. When a 50 hectare area of forest can have upwards of 600 tree species, this is a challenge in itself. Add to this the difficulties of actually seeing the crown of a tree, which may be 50 m or more above the ground with a riot of vegetation obscuring the view from the ground. Assessing what species of epiphytes are present in the crown requires accessing it, and it is only quite recently that people have started accessing tropical forest canopies to do this. When they do, many new and undescribed species have been found – another problem for anyone trying to describe species rich ness in a tropical forest.

There are added difficulties: even if we have good quantitative data, such as the numbers of individuals of every species of tree present, we still have to express this in a way that enables us to compare different sites. Numerous ways to do this have been devised, and we introduce you to some of the more commonly used techniques.

Outcomes

By the end of this topic, you should have a good idea of the sort of variation in species richness that you might encounter in a tropical forest, and the ways that you can use to describe this richness and its variation. You should be able to:

• Describe the types of variation found in the species richness of tropical forests.
• Explain why it is so difficult to provide comprehensive species lists for any group of species in a tropical forest.
• Use the Dipterocarpaceae to demonstrate the extent of species variation with altitude and soil type.
• Analyze the species richness of a site, and express this using different indices of diversity.
• Compare species richness between different sites.

Topic Readings

Required Readings

• Ghazoul, J. and Sheil, D. 2010. Tropical Rain Forest Ecology, Diversity, and Conservation. Oxford, Oxford University Press, 516 pp. Chapter 8.

Optional Readings

• Kricher, J. 2011. Tropical Ecology. Princeton, New Jersey, USA: Princeton University Press, 632 pp. Chapters 3 and 4
• Magurran, A.E. 2003. Measuring biological diversity. Oxford, UK: Blackwell Science. 264 pp.
• Magurran, A.E. and McGill, B.J. 2011. Biological diversity. Frontiers in measurement and assessment. Oxford, UK: Oxford University Press. 345 pp.

Fundamental Concepts

PPT Presentations

This topic contains two presentations, introducing to species richness and its measurement.

1.1.1 Species richness in tropical forests.pptx Download 1.1.1 Species richness in tropical forests.pptx

1.1.2 Measuring species richness.pptx Download 1.1.2 Measuring species richness.pptx

Self-check Quiz (For certificate learning only)

After viewing topic presentations, take this self-check quiz to check your own progress. This quiz is designed to see how well you remember some of the more important issues presented in the topic. There are five questions, and each has a potential of 4 answers, only one of which is correct. When you are ready, click the link Self-check Quiz 1.1 to start taking the quiz.

Pre-readings and Discussion (For certificate learning only)

Once you have read Chapter 8 of Ghazoul, J. and Sheil, D. 2010. Tropical Rain Forest Ecology, Diversity, and Conservation, choose to post one thought provoking question related to the material that you have read, and answer/respond to at least one other student’s question on Module 1 discussion board Module 1 Discussion.

Graded Assignments (For certificate learning only)

• There is no graded assignment associated with this topic.

Topic Summary

In this topic, we introduced to the some ideas about the diversity and species richness of tropical forests. This was intended primarily as an introductory overview, and we will be looking at species richness in a lot more detail in the next two topics.

Assessing the number of species of a particular group of organisms (e.g., birds, trees, or ants) is difficult and time-consuming. In some groups, particularly amongst the invertebrates, there is a strong possibility of encountering new, undescribed, species. The vast majority or birds and animals have already been found and described, although it is still possible to find new species, and many species are being reclassified following genetic analyses. With plants, some areas are relatively well-known, whereas in places such as central Borneo, there are many trees that have yet to be named and described.

Finding good ways to describe and compare the data for species richness is challenging. Many different indices have been proposed, and all have their advantages and disadvantages. However, you will likely need to make use of such indices when making comparisons, such as between unlogged and logged forest, or regenerating forest and undisturbed forest. Making such comparisons requires scientific rigour, and we hope that you are now equipped to do this.

Topic Self-review (For self learning)

To review what you have learned in the topic, you are encouraged to use the following reflection questions as a study guide to do a self-review for the topic:

• Why does the spatial arrangement of species richness vary in tropical forests?
• What sort of problems would you encounter when developing a comprehensive species lists for a group of species in a tropical forest?
• Would the problems be the same for all groups, or would they differ depending on what you were looking at?
• Why are members of the Dipterocarpaceae not randomly distributed with tropical forests?
• Why are there so many different measures of species diversity? Are these the sane as measures of species richness?
• How would you express species diversity at a site in a way that you could compare it with other sites?
• Compare species richness between different sites?