Topic Introduction

In this topic, we introduce two very different types of interaction, although there are clear relationships, as many myrmecophytic plants are epiphytic. Epiphytes exist in a number of forms, including strict epiphytes (plants that spend their entire lives on other plants), primary hemiepiphytes (that start as epiphytes but then send roots down to the ground) and secondary hemiepiphytes (that start in the ground, grow up another a plant, then sever their connection to the ground).

Some epiphytes, such as mistletoes (Loranthaceae), are parasitic epiphytes, drawing nutrients and water from their hosts. However, the majority of epiphytes simply use the host as support for growth. As they grow, debris can collect, and this humus-rich substrate provides opportunities for a range of epiphytic species. It also provides an important habitat for a wide variety of organisms.

Questions have been raised about the relationship between epiphytes and their hosts. While they do not draw nutrients or water from their host, over time, their weight (and the associated humus that accumulates) exerts increasing pressure on the branches of the host. The increased weight can be substantial, especially when the epiphytes and humus are saturated, and the sound of branches crashing to the ground is relatively common in most tropical moist forests. This only occurs because of the weight of the epiphytes, so they could be considered to have a negative effect on the host.

In the second presentation, we look at interactions mediated by ants. These take a number of different forms, but usually seem to involve (+ +) relations between ants and their host plants. The first type of interaction involves myrmecotrophic plants. These are plants that are able to obtain nutrients from debris created by ants. They have special structures that encourage ants and are quite common amongst epiphytes. The ants live within the plants, and the debris that they leave is utilized by the plants. In many cases, the ants may defend the plants against herbivores, particularly insects.

Ant gardens occur when ants bring the seeds of specialized epiphytes into their nests. These germinate, and grow on the debris in the ants’ nest. Ant gardens are developed by a number of different species of ants, including Crematogaster carinata, Camponotus femoratus and Solenopsis parabioticus. The plants come from a number of families including Araceae, Bromeliaceae, Cactaceae, Gesneriaceae, Moraceae, Piperaceae and Solanaceae. Epiphytic plants in the Neotropical genus Codonanthe (Gesneriaceae) appear to be restricted to ant gardens.

Another type of interaction involving ants is the creation of “Devil’s Gardens”. These are created by a single species, the Lemon Ant (Myrmelachista schumanni), found in the Amazon. The ants create a patch of forest that is dominated by the tree Duroia hirsuta (Rubiaceae), poisoning other plant growth by injecting and spraying formic acid. Duroia also has allelopathic properties which help maintain its dominance. The relationship appears to be a (+ +) situation, but the tree leaves are subject to increased herbivory by the ants.

By the end of this topic, you should be able to:

• Distinguish between epiphytes, hemiepiphytes and parasites.
• Describe the characteristics and life history of lianas.
• Explain why epiphytes are important in forest ecosystems.
• Describe the multiple ways in which ants interact with plants.
• Explain how ‘Devils’ Gardens’ are formed.
• Analyze the ecological effects of leafcutter ants.

Topic Readings

Required Readings

• Ghazoul, J. and Sheil, D. 2010. Tropical Rain Forest Ecology, Diversity, and Conservation. Oxford, Oxford University Press, 516 pp. Section 13.2

Optional Readings

Ant-mediated Interactions

• Rico-Gray, V. and Oliveira, P.S. 2007. The ecology and evolution of ant-plant interactions. Chicago, Illinois, USA: University of Chicago Press. 331 pp.

Epiphytes

• Benzing, D.H. 1990. Vascular epiphytes: General biology and related biota. Cambridge, UK: Cambridge University Press.
• Lüttge, U. 1989. Vascular plants as epiphytes: Evolution and ecophysiology. Heidelberg, Germany: Springer Verlag.
• Schnitzer, S.A., Bongers, F., Burnham, R.J. and Putz, F.E. (eds.) 2015. Ecology of lianas. Colchester, UK: John Wiley & Sons. 481 pp.
• Zotz, G. 2016. Plants on plants – The biology of epiphytes. Switzerland: Springer International Publishing. 282 pp.

Fundamental Concepts

PPT Presentations

There are two presentations in this topic:

4.2.1 Interactions - Epiphytes.pptx Download 4.2.1 Interactions - Epiphytes.pptx

4.2.2 Ant-mediated interactions.pptx Download 4.2.2 Ant-mediated interactions.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 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 4.2 to start taking the quiz.

Pre-readings and Discussion (For certificate learning only)

Once you have read Sections 13.2 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 4 discussion board Module 4 Discussion.

Graded Assignments (For certificate learning only)

Six Sentence Answer #9

Please refer to the course pages on graded assessments to see details of the Six-sentence Answers (6SA) exercise. You should not attempt to do this assessment until you have read and fully understood the requirements. 

The Case

You are a member of a major international forest consultancy firm that specializes in doing surveys of forests around the world. Your company is working on a contract looking at the effects of past and present management practices on the forests and savannahs of East Africa. This area is very diverse, including many different forest types. They range from grass and tree savannahs through seasonally dry woodlands and forests to Afromontane rain forest.

In an area of seasonally dry woodland that you have been monitoring, a major reduction in the numbers of Wildebeest has been noted. Using the 6SA approach, argue why this is unlikely to be due to increased predation by large mammalian predators.

Remember to follow the structure of the 6SA outlined in the section on graded assessments.

Formatting:

• Double-space the text.
• 12-point font in New Times Roman or Calibri. 
• Put your name (Family or last name, followed by your first name), student number, the title “Topic 1.1 assignment”, and the date at the top of the first page.
• Name the e-file: your Family or last name_your first name_ Topic 1.1_Assignment.doc (or .docx). Example: Innes_John_Topic 1.1_Assignment (or .docx).
• Standard English usage, clear writing style.
• All assignments should be submitted through Canvas before the deadline.

Topic Summary

In this topic, you have examined two very different types of species interaction. In the first example, we looked at epiphytes. These are abundant in many tropical forests, especially cloud forests where this is much atmospheric moisture. Some epiphytes start on the ground and work up into the crowns of trees. Others start in the crown or elsewhere in the tree, and send roots down to the ground. And yet others exist entirely above ground through their lives.

Ant interactions are particularly interesting and very varied. New information is constantly being generated, and new forms of relationships between ants and plants discovered. In some cases, ants defend the host in return for shelter and, sometimes, food. In other cases, while ants may help remove competition from other species, it comes at the cost of increased herbivory on the favoured species. As ants are social insects, their impact can be significant.

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 do you distinguish epiphytes, hemiepiphytes and parasites?
• What are the characteristics and life history of lianas?
• Why are epiphytes important in forest ecosystems?
• How many ways can you describe that ants use to interact with plants?
• How are ‘Devils’ Gardens’ are formed?
• What are the ecological Impacts of leafcutter ants?