How important are competitive interactions to the functioning of communities and ecosystems? There has been a long tradition of theory, laboratory studies, and field studies that have emphasized the potential importance of competition in population regulation and in the shaping of population and community relationships among species. Darwin, Liebig, Tansley, Lotka, Volterra, Gause, Park, Connell, MacArthur, Tilman, and many others have contributed to the rich literature on competitive interactions. The competitive-exclusion principle established an expectation that competitive interactions not only shape present-day ecological interactions, but also have directed past evolutionary events (the so-called "ghost of evolution past"). Resource-based competition theory has also laid out certain ground rules (the R*-rule) for coexistence or exclusion in resource-limited environments. On the other hand, the competition-colonization trade-off concept sets up conditions whereby inferior competitors may persist in communities. For example, ant species known as "opportunists" and "insinuators" are able to coexist with the aggressive dominant competitor species, known as "extirpators," based on their ability to find food sources first, or by their stealthy foraging style. And even among the dominant competitors, the species that finds a food resource first and dominates it through recruitment of nestmates is the winner at that resource. Therefore, as suggested by Tilman (1994) and Yu and Wilson (2001), coexistence of inferior competitors should be expected in a spatially diverse environment.
Some ecologists have complained that the fascination with competition has resulted in an underestimation of the importance of parasitic, predatory, or even mutualistic relationships in shaping ecological communities. American politicians have even turned economic competition into a kind of religion. Let us now, therefore, turn to other kinds of interactions.
8.1 Introduction Mutualism or parasitism?
As outlined in the introduction to Part II, mutualism is an interaction in which both species benefit. In facultative mutualism individuals in a population are able to survive and reproduce without their presumptive mutualist, although their fitness is enhanced when they participate in the mutualism. By contrast, in obligatory mutualism, individuals in a species are unable to survive without their mutualistic partner. As pointed out by Vandermeer and Goldberg (2003), mutualistic relationships are complex, and do not necessarily fit into the two categories of facultative and obligatory. For example, a mutualism may be obligatory for one partner, but not for the other. The mutualism may be very weak (provide few benefits) and therefore may only be found in very specific environments. In fact, many types of mutualisms are much more common in tropical environments. For example, Neotropical ants and African termites both raise mutualistic fungi in "gardens" within their nests. Though fungus gardening does occur outside of the tropics, it is much more common and conspicuous in the humid tropics. In ant-plant mutualisms, ants defend plants from herbivores or perform other services in exchange for nest sites and nutrition provided by the plants. Although these mutualisms exist in the temperate zone, almost all of the obligatory ant-plant mutualisms are found within the tropics. The number of plant species providing extra-floral nectar, a low-cost method of attracting ants to plants, declines with increasing latitude and altitude, and is rare in the North Temperate zone.
Animal pollination and fruit dispersal are also much more common in tropical latitudes. For example, bats that provide pollination and fruit dispersal for higher plants are only found south of 33° N latitude. No bees that are obligatory pollinators of orchids are found north of 24° latitude. At Monteverde, a cloud forest in Costa Rica, animals as opposed to wind pollinate more than 90% of the dicots and 88% of the monocots. A study by Murray et al. (2000) of fruit dispersal at Monteverde showed that more than 81% of tree species are adapted for seed dispersal by vertebrates. This compares well with 89% at Alto Yunda, Colombia, 92% at La Selva, Costa Rica, and 92% at Rio Palenque, Ecuador. At Monteverde about 80% of animal-dispersed trees are specifically adapted for bird dispersal. Adaptations for bat and ant dispersal are less common. Not all life forms, however, are adapted for animal dispersal of their fruit at Monteverde. A majority of epiphytes (66%) and herbs (73%) are adapted for wind or other abiotic means of dispersal. Most of the wind-dispersed seeds, however, are orchids (among epiphytes) and weedy species of Asteraceae (among herbs). Lianas and shrubs are intermediate: the fruits of the majority of species are bird-dispersed while 35-45% is abiotically dispersed.
Mutualistic relationships are wonders of natural history and prime examples of co-evolution. Yet, as Bronstein (2000) put it, "Mutualism is the most poorly understood form of interspecific interactions." Others such as Law (1988) and Watkinson (1997) have made the same point. Furthermore, Watkinson (1997) lamented that "There is not even a sound theoretical framework for the treatment of mutualistic interactions." Recent research, however, has begun to emphasize the fact that in most mutualisms, the relationship is simultaneously beneficial and harmful to one or both participants. Rather than thinking of the mutualistic species as happily entering a partnership, Bronstein (2002) has asserted that mutualism is more likely a "reciprocal parasitism" in which each partner obtains what it can at the lowest possible cost to itself.
Consider, for example, the fig-wasp pollination system. Figs (species of Ficus) have an obligatory pollination system with wasps (Hymenoptera: Chalcidoidea: Agaoninae). There are five species of figs at Monteverde in Costa Rica, but the reproductive biology of only one of them has been worked out (Bronstein 2000). The female pollinators (Pegoscapus silvestrii) are drawn to volatile odors released by the female florets of figs. These females will already have mated with males and will also have ripped open the anthers of male flowers in their natal fig. They pack pollen into pockets in their abdomens. Thus, inseminated and pollen laden, they arrive at a new tree where they squeeze into the 1 cm fig flower. Once within a fig, the wasp deposits pollen on the stigma of a floret. She then lays a single egg in each of the ovaries she can reach with her ovipositor. The female is fatally trapped inside the fig flower, but her offspring will develop in the fig ovaries, feeding on the seeds as they develop. Some seeds escape wasp predation because the flower's style was too long for the female's ovipositor. The seeds and seed-eating fig-wasp larvae develop over a two-month period. Mature males eventually emerge and search out females, still developing inside the growing figs, for mating. The male then chews an exit hole through the wall of the fig, through which the females can also depart their natal fig. Thus, in payment for pollination the figs lose a large number of potential seeds to wasp larvae. The female wasps would lay eggs on all of the ovules if they could. If they succeeded they would indeed be parasites.
Or consider the relationship between East African whistling thorn acacias (Acacia drepanolobium) and ants. These acacias are some of the most common plants of the savannas of central Kenya. Almost all of these trees host thousands of ants. The ants provide defense against herbivores in exchange for food and shelter. A pair of thorns lies at the base of every leaf cluster, and each branch is lined with two types of thorns. The slender, white, needle-sharp thorns, which may be 75 mm in length, are the most abundant. Intermingled with these thorns are pairs of thorns with bulbous, hollow bases. These "swollen thorns" house the ant colonies; each thorn can harbor hundreds of ants. As in Central American acacias (Janzen 1966), when an herbivore disturbs the tree, the ants stream out of the thorns and bite the intruder, and when Stanton and Young (1999) experimentally removed ants from trees, they found that herbivore damage by both browsing mammals and insects increased significantly.
The acacia provides the ants with nectar from glands along the leaves. These extrafloral nectaries are particularly abundant on new leaves. However, Acacia drepanolobium does not provide lipid or protein food sources for these ant colonies. The ants must forage for insects and other protein-rich foods. Therefore the mutualism is weaker than the Acacia-Pseudomyrmex relationship described by Janzen (1966) in Central America, where the acacias provide lipid- and protein-rich structures, known as Beltian bodies, on the tips of the leaflets.
There are four different ant species that colonize A. drepanolobium. The red-and-black cocktail ant (Crematogaster mimosae) and the black-and-white cocktail ant (C. nigriceps) are most effective against herbivores. Intruders are immediately attacked by a horde of biting ants. The ants race around emitting alarm pheromones with their abdomens held high (hence the term "cocktail ant"), and this recruits more workers to the scene. Herbivores such as goats, which are bitten while attempting to browse on a defended tree, refuse to approach those trees again. Trees defended by these two ant species are rarely damaged by herbivores.
A third species, the slender black acacia ant (Tetraponera penzigi) has a nasty sting, but is more passive and only attacks if the swollen thorns themselves (the home of larvae, pupae, adults, and winged reproductive ants) are attacked by monkeys or other animals. On the other hand, these ants patrol leaves day and night, removing pollen and probably fungal spores. Therefore these workers may provide the acacias protection from disease.
The fourth species, the black cocktail ant (C. sjostedti), provides no services to the trees whatsoever. Stanton and Young (1999) found that long-horned beetles could girdle stems and kill entire sections of the tree while this ant was present. The reason for this may be that this ant species does not even live in the swollen thorns. Instead it nests in hollow spaces within dead and drying branches. The beetles actually provide this ant with nest space, and the mutualistic relationship has shifted to one in which these two insect species "cooperate" in exploiting the acacias.
Although all four species of ants may occupy trees on a given hectare of land, an individual tree is almost never occupied by more than one species of ant. The species are intolerant of each other and engage in aggressive, mortal combat. Experiments have shown that the fights continue until one species has wiped out the second on a given tree. Unfortunately for the acacias, the black cocktail ant, which is functionally a parasite, is the dominant competitor, winning most battles with the other three species.
The red-and-black cocktail ant, as well as the black cocktail ant, tend scale insects that feed on the phloem of the acacias. Thus, both species have found an additional method of draining energy from the trees. Worse still is the habit of the black-and-white cocktail ant workers of removing the tips of most growing shoots. These workers also remove stem tissue containing leaf and flower buds. New branches are only allowed to grow in proximity to swollen thorns. These black-and-white cocktail ants have therefore changed the architecture of these trees from one with large open canopies to one of compact masses of branching stems. In addition, by chewing off its flower buds, it prevents these trees from reproducing.
In summary, the ant-acacia symbiosis in Africa, although potentially mutually beneficial, appears to have tipped in favor of the ants, which function more like parasites than mutualists. As we examine mutualisms, we should ask ourselves how these mutualisms differ from host-parasite relationships.
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