Introduction

Species interactions can take many forms, and a variety of terms have been coined for these. To simplify the nomenclature, Shorrocks and Bates (2015) describe these interactions as: in the presence of species A (+), does species B increase its numbers (+), not change its numbers (0), or decrease its numbers (-) relative to when species A is absent (-)? The same questions can be asked of species A in the presence of species B. This can be summarized in a 3x3 table, although the symmetry in the table means that there are only 6 types of interaction. These six types of interactions can be described as

• Neutralism (0 0)
• Commensalism (+ 0)
• Predator–prey, parasite–host, herbivore–plant interactions (+ -)
• Amensalism (0 -)
• Competition (- -)
• Mutualism (+ +)

Of these, neutral interactions, although very common, do not contribute to the dynamics of the ecosystem. Commensalism can be viewed as an extreme example of mutualism, where the beneficial impact of one species on the other is very weak. Amensalism can be seen as an extreme example of competition, with the adverse effect of one species on another being much greater than the reciprocal effect.

Apparent competition is not accounted for in the previous examples, and occurs when two species appear to show reciprocal negative (- -) effects that appear to be attributable to competition but are actually the result of predation by a third species. If both species are prey, then the benefits that that predator gains from preying on one species will adversely affect the other prey species.

Predator – prey interactions encompass three types of interactions that are very different, but have the same basic outcome: one species benefits and another loses (+ -). Three different types include:

• Herbivore – plant interactions, with the host rarely killing its host, and the eaten parts usually regenerating
• Interactions whereby a predator kills (and usually eats) its prey (the “classic” form of predator–prey interaction
• Parasite – host interactions, with the host sometimes being killed and sometimes recovering

In topic 1, we have developed four presentations that cover most of these different types of interactions: herbivory, predation, parasitism and competition.

In topic 2, we look at two specific cases. Epiphytes, which are very common in tropical forests, represent a special situation. Some, such as the mistletoes, are true parasites, feeding on the tree. Others are simply using the tree as support, with apparently minimal effect on the tree. However, as the number of epiphytes increases, so does their weight, especially when they and the accumulating litter is saturated. The weight is often sufficient to break the branches, causing obvious harm to the tree. Similarly, when a tree falls, lianas connecting it to other trees may pull down nearby trees with it.

The second case involves ant-mediated interactions. These are so important in tropical forests that we decided to devote an entire presentation to ants. Ants are also discussed elsewhere, reflecting their importance in tropical ecosystems.

In the third topic, we examine two further forms of interaction. The first involves

facilitation, especially what has been termed ‘ecosystem engineering’. In such cases, the activity of one species unintentionally creates opportunities (or problems) for one or more other species. The complex insect faunas that develop in the pools formed by elephant footprints provides an example of this. In the second topic, we examine interactions across multiple trophic levels.

Shorrocks, B. and Bates, W. 2015. The biology of African savannahs. 2^nd edition. Oxford, UK: Oxford University Press. 330 pp.