Topic Introduction

In this topic, we look at two key aspects of plant reproduction, seed set and dispersal and seed dormancy and germination. Both of these are critical to the successful reproduction of plants. They are also important to the explanation of tropical species diversity.

While a tree may have abundant flowers, only a small proportion of the ovules are set as seed. This caused by a variety of factors, including insufficient or inappropriate pollination, selective abortion and pre-dispersal predation. Many flowers are not visited by pollinators, and with those that are, they may be no pollen transfer. This prevents fertilization. Flowers, fruit and seeds are all important foods for a range of species, and constitute a form of predation.

If a seed is successfully is set, it generally needs to move away from the parent tree if the species is to be successful in the long-term. This involves dispersal. Some species have no means of dispersal, but for the majority, the seeds are adapted to being moved. This may occur by wind or water, or with the help of an animal. Animals don’t disperse seeds willingly – either there is some form of a reward for doing so, or it has little choice (e.g. in the case of sticky seeds that cling to an animals feathers or fur). There are some interesting stories surrounding dispersal, although some of these have proven to be just stories, and factually inaccurate. An example is that the Tambalacoque tree of Mauritius co-evolved with the Dodo, such that seeds would only germinate after passing through the digestive system of a Dodo. With the extinction of the Dodo, the Tambalacoque Tree was thought to be doomed, but it has survived, as the idea that it had to be eaten by a Dodo before it would germinate was wrong.

Once dispersed, some seeds germinate very quickly, often within a few weeks. If they do not, the seeds die. Others are designed to survive for much longer, and may exist on the soil seed bank for a year or more until the right environmental conditions trigger their germination. Those conditions are sometimes created by a disturbance to the canopy that allows more light to reach the ground, or changes the humidity levels close to the ground. In other cases, it may be the moisture that triggers that germination (especially in seasonally dry forests).

These are all issues addressed in this topic, so by the end of it, we expect that you will be able to:

• Compare the importance of pre- and post-dispersal seed predation.
• Evaluate the different theories related to density-dependent seed predation.
• Outline how patterns of seed dispersal and predation can lead to the clumped distribution of individual species in tropical forests.
• Describe the processes leading to the successful germination of seeds in the tropical forest environment.
• Analyze the tradeoffs between different seed survival strategies.
• Assess the factors leading to successful seedling emergence.
• Evaluate the importance of mycorrhizal associations in seedling establishment.

Topic Readings

Required Readings

• Ghazoul, J. and Sheil, D. 2010. Tropical Rain Forest Ecology, Diversity, and Conservation. Oxford, Oxford University Press, 516 pp. Sections 12.2 and 12.4.

Optional Readings

• Crawley, M.J. (ed.) 1997. Plant Ecology. 2^nd edition. Oxford, UK: Blackwell Scientific. 717 pp. Chapters 7 and 9. 
• Dennis, A.J., Green, R.J., Westcott, D.A. 2007. Seed dispersal: Theory and its application in a changing world. Wallingford, UK: CABI Publishing. 720 pp.
• Estrada, A. and Fleming, T.H. (eds.) Frugivores and seed dispersal. Dordrecht, Netherlands: Dr. W. Junk Publishers. 406 pp.
• Fleming, T.H. and Estrada, A. (eds.) 1993. Fruiting and seed dispersal: ecological and evolutionary aspects. Dordrecht, Netherlands: Kluwer Academic Publishers. 404 pp.
• Levey, D.J., Silva, W.R., Galetti, M. 2002. Seed dispersal and frugivory: Ecology, evolution and conservation. Wallingford, UK: CABI Publishing. 500 pp.

Fundamental Concepts

PPT Presentations

This topic contains two presentations:

3.2.1 Plant reproduction - Seed set.pptx Download 3.2.1 Plant reproduction - Seed set.pptx

3.2.2 Plant reproduction - Seed dormancy and germination.pptx Download 3.2.2 Plant reproduction - Seed dormancy and germination.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 3.2 to start taking the quiz.

Pre-readings and Discussion (For certificate learning only)

After reading sections 12.2 and 12.4 of Ghazoul, J. and Sheil, D. 2010. Tropical Rain Forest Ecology, Diversity, and Conservation, choose to post one thought-provoking question related to the seed dispersal that you have read, and answer/respond to at least one other student’s question on Module 3 discussion board Module 3 Discussion. 

Graded Assignments (For certificate learning only)

• There is no graded assignment associated with this topic. 

Topic Summary

In this topic, we have examined the many different mechanisms of seed dispersal. Most seeds never germinate as they predated or die before they have chance to do so. Understanding dispersal mechanisms is important, as it helps explain why individuals of a species can be so widely distributed within a particular forest area. Predation plays an important role in this, with theory suggesting that predation should be greatest nearest to a parent tree. The occurrence of monospecific stands suggests that distance from the parent tree is not always a factor. In such cases, the effects of predation may be overcome by the simultaneous production of seeds by large number of trees (the so-called ‘mast’ events found particularly amongst the Dipterocarpaceae, but also present with other families.

The difficulties facing reproduction are not over when a seed has been dispersed and successfully germinates. It still has to survive, and this is what we examine in the next topic.

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:

• How does the importance of pre- and post-dispersal seed predation compare?
• Which of the different theories related to density-dependent seed predation are best supported by the evidence?
• How could seed dispersal and predation lead to the clumped distribution of individual species in tropical forests?
• What processes lead to the successful germination of seeds in the tropical forest environment?
• What tradeoffs are involved in different seed survival strategies?
• What factors lead to successful seedling emergence?
• How do mycorrhizal associations help in seedling establishment?