Mutualistic protectors

13.2.1 Cleaner and client fish

'Cleaner' fish, of which at least 45 species have been recognized, feed on ectoparasites, bacteria and necrotic tissue from the body surface of 'client' fish. Indeed, the cleaners often hold territories with 'cleaning stations' that their clients visit - and visit more often when they carry many parasites. The cleaners gain a food source and the clients are protected from infection. In fact, it has not always proved easy to establish that the clients benefit, but in experiments off Lizard Island on Australia's Great Barrier Reef, Grutter (1999) was able to do this for the cleaner fish, Labroides dimidiatus, which eats parasitic gnathiid isopods from its client fish, Hemigymnus melapterus. Clients had significantly (3.8 times) more parasites 12 days after cleaners were excluded from caged enclosures (Figure 13.1a, top panel); but even in the short term (up to 1 day), although removing cleaners, which only feed during daylight, had no effect when a check was made at dawn (middle panel), this led to there being significantly (4.5 times) more parasites following a further day's feeding (lower panel).

Moreover, further experiments using the same cleaner fish, but at a Red Sea reef in Egypt, emphasized the community-wide importance of these cleaner-client interactions (Bshary, 2003). When cleaners either left a reef patch naturally (so the patch had no cleaner) or were experimentally removed, the local diversity (number of species) of reef fish dropped dramatically, though this was only significant after 4-20 months, not after 2-4 weeks (Figure 13.1b). However, when cleaners either moved into a cleanerless patch naturally or were experimentally added, diversity increased significantly even within a few weeks (Figure 13.1c). Intriguingly, these effects applied not only to client species but to nonclients too.

In fact, several behavioral mutualisms are found amongst the inhabitants of tropical coral reefs (where the corals themselves are mutualists - see Section 13.7.1). The clown fish (Amphiprion), for example, lives close to a sea anemone (e.g. Physobrachia, Radianthus) and retreats amongst the anemone's tentacles when ever danger threatens. Whilst within the anemone, the fish gains a covering of mucus that protects it from the anemone's stinging nematocysts (the normal function of the anemone slime is to prevent discharge of nematocysts when neighboring tentacles touch). The fish derives protection from this relationship, but the anemone also benefits because clown fish attack other fish that come near, including species that normally feed on the sea anemones.

13.2.2 Ant-plant mutualisms

The idea that there are mutualistic relationships between plants and ants was put forward by Belt (1874) after observing the behavior of aggressive ants on species of Acacia with swollen thorns in Central America. This relationship was later described more fully by Janzen (1967) for the Bull's horn acacia (Acacia cornígera) and its associated ant, Pseudomyrmex ferruginea. The plant bears hollow thorns that are used by the ants as nesting sites; its leaves have protein-rich 'Beltian bodies' at their tips (Figure 13.2) which the ants collect and use for food; and it has sugar-secreting nectaries on its vegetative parts that also attract the ants. The ants, for their part, protect these small trees from competitors by actively snipping off shoots of other species and also protect the plant from herbivores - even large (vertebrate) herbivores may be deterred.

In fact, ant-plant mutualisms appear to have evolved many times (even repeatedly in the same family of plants); and nectaries are present on

Richness Evenness And SpeciesPlant Leaves Rich Protein

effects at the community level, too do the plants benefit?

Figure 13.2 Structures of the Bull's horn acacia (Acacia cornigera) that attract its ant mutualist. (a) Protein-rich Beltian bodies at the tips of the leaflets (© Oxford Scientific Films/Michael Fogden). (b) Hollow thorns used by the ants as nesting sites (© Visuals Unlimited/C. P. Hickman).

Figure 13.3 (a) The intensity of leaf herbivory on plants of Tachigali myrmecophila naturally occupied by the ant Pseudomyrmex concolor (•, n = 22) and on plants from which the ants had been experimentally removed (•, n = 23). Bottom leaves are those present at the start of the experiment and top leaves are those emerging subsequently. (b) The longevity of leaves on plants of T. myrmecophila occupied by P. concolor (control) and from which the ants were experimentally removed or from which the ants were naturally absent. Error bars ± standard error. (After Fonseca, 1994.)

Figure 13.3 (a) The intensity of leaf herbivory on plants of Tachigali myrmecophila naturally occupied by the ant Pseudomyrmex concolor (•, n = 22) and on plants from which the ants had been experimentally removed (•, n = 23). Bottom leaves are those present at the start of the experiment and top leaves are those emerging subsequently. (b) The longevity of leaves on plants of T. myrmecophila occupied by P. concolor (control) and from which the ants were experimentally removed or from which the ants were naturally absent. Error bars ± standard error. (After Fonseca, 1994.)

the vegetative parts of plants of at least 39 families and in many communities throughout the world. Nectaries on or in flowers are easily interpreted as attractants for pollinators. But the role of extrafloral nectaries on vegetative parts is less easy to establish. They clearly attract ants, sometimes in vast numbers, but carefully designed and controlled experiments are necessary to show that the plants themselves benefit, such as the study of the Amazonian canopy tree Tachigali myrmecophila, which harbors the stinging ant Pseudomyrmex concolor in specialized hollowed-out structures (Figure 13.3). The ants were removed from selected plants;

these then bore 4.3 times as many phytophagous insects as control plants and suffered much greater herbivory. Leaves on plants that carried a population of ants lived more than twice as long as those on unoccupied plants and nearly 1.8 times as long as those on plants from which ants had been deliberately removed.

Mutualistic relationships, in this case between individual ant and plant species, should not, however, be viewed in isolation - a theme that will recur in this chapter. Palmer et al. (2000), for example, studied competition

competition amongst mutualistic ants

Figure 13.4 (a) Average growth increment was significantly greater (P < 0.0001) for Acacia drepanolobium trees continually occupied by ants (n = 651) than for uninhabited trees (n = 126). 'Continually occupied' trees were occupied by ant colonies at both an initial survey and one 6 months later. Uninhabited trees were vacant at the time of both surveys. (b) Relative growth increments were significantly greater (P < 0.05) for trees undergoing transitions in ant occupancy in the direction of the ants' competitive hierarchy (n = 85) than for those against the hierarchy (n = 48). Growth increment was determined relative to trees occupied by the same ant species when these ants were not displaced. Error bars show standard errors. (After Palmer et al., 2000).

amongst four species of ant that have mutualistic relationships with Acacia drepanolobium trees in Laikipia, Kenya, nesting within the swollen thorns and feeding from the nectaries at the leaf bases. Experimentally staged conflicts and natural take-overs of plants both indicated a dominance hierarchy among the ant species. Crematogaster sjostedti was the most dominant, followed by C. mimosae, C. nigriceps and Tetraponera penzigi. Irrespective of which ant species had colonized a particular acacia tree, occupied trees tended to grow faster than unoccupied trees (Figure 13.4a). This confirmed the mutualistic nature of the interactions overall. But more subtly, changes in ant occupancy in the direction of the dominance hierarchy (take-over by a more dominant species) occurred on plants that grew faster than average, whereas changes in the opposite direction to the hierarchy occurred on plants that grew more slowly than average (Figure 13.4b).

These data therefore suggest that take-overs are rather different on fast and slow growing trees, though the details remain speculative. It may be, for example, that trees that grow fastest also produce ant 'rewards' at the greatest rate and are actively chosen by the dominant ant species; whereas slow growing trees are more readily abandoned by dominant species, with their much greater demands for resources. Alternatively, competitively superior ant species may be able to detect and preferentially colonize faster growing trees. What is clear is that these mutualistic interactions are not cosy relationships between pairs of species that we can separate from a more tangled web of interactions. The costs and benefits accruing to the different partners vary in space and time, driving complex dynamics amongst the competing ant species that in turn determine the ultimate balance sheet for the acacias.

Ant-plant interactions are reviewed by Heil and McKey (2003).

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