Sex, Lies and Pollination: Tropical Diversity in Plant-Pollinator Interactions (Fin)

This topic submitted by Caleb Slemmons ( at 7:22 PM on 5/15/04.

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Sex, Lies and Pollination:

Tropical Diversity in Plant-Pollinator Interactions

Caleb Slemmons

May 15, 2004

Tropical Ecosystems of Costa Rica

The vast diversity of plant-pollinator interactions is staggering, from insect pollinators such as moths, bees and ants to birds and even opossums. The tropics host many plant species and thus provide a diverse collection plant pollinating mechanisms and interactions. Plant-pollinator interactions are often regarded as a mutualistic arrangement, the pollinator receives food and the plant receives the benefit of pollen transfer. However, there are relatively few studies that have confirmed how truly beneficial interactions are and some have documented cases of pollinators tipping the odds in their favor. Pollinating species considered to be nectar robbers seem to give little, or no benefit to the plant and may actually harm plant fitness overall. This can be manifest in reduction of seed set and reduced availability of nectar for legitimate pollinators. Plants are not blameless though as many “deceptively” advertise food and sex but deliver little or no reward to their pollinator. More research is needed to further elucidate the importance and abundance of these types of interactions, but in the tropics nectar thievery and plant deceptions are likely a common evolutionary “cheat”.

The tropics contain a seemingly limitless diversity of plant and animal species. In fact, tropical lowland forests are likely the most speciose of all terrestrial ecosystems (Turner, 1996). Therefore interactions among species, such as plant-pollinator interactions, are impressively diverse and numerous in the tropics. Plants have coevolved many close, specific interactions with other organisms to aid in pollen dispersal. Plants, being relatively immobile, depend somewhat on other more mobile organisms both in terms of seed and pollen dispersal. The paucity of information regarding many of these interactions are a direction for future study. However, some plant-pollinator interactions have been extensively studied and the variety in pollinator mechanisms are remarkable. Whether spread by insects, birds or mammals the relationships between plants and their pollinators make for incredible stories and are both vital to ecosystem functioning and the maintenance of biodiversity.

Insect Pollinators

When considering plant pollination, insects are often the first to come to mind. Many insects such as moths, bees and ants pollinate the vast majority of plants. A variety of mechanisms are utilized by plants to entice their insect pollinators – flower colors, scents and shapes all play an important role in the seduction game. For example, in South-eastern Brazil the epiphytic orchid Trigonium obtusum (pictured above) utilizes the sugar pentadecane and morphological adaptations to attract bees that effect pollination (Singer, 2002). Male bees are attracted to the volatile sugar and petals of the flower (or sepals, depending on the morphotype) and attempt copulation with flower parts. Bees, landing on the waxy surface of the sepals, fall into the cavity of the flower. In the process of leaving the orchid, pollen is deposited on the upper thorax, or scutellum, of the bee. Pollination is complete when the bee again falls victim to the seductive cues of T. obtusum and the pollenaria (clumps of pollen) are caught in the concave stigma of the orchid. In addition, the pollenaria must dry out for approximately 40 minutes for transfer to be successful – a remarkable adaptation to ensure cross-pollination! Orchids, the most numerous and diverse group or flowering plants, are outstanding examples of highly evolved pollination mechanisms. Another orchid species (Chiloglottis trapeziformis) attracts its wasp pollinators by producing the same pheromone emitted by female wasps (Schiestl, 2003). The pollination of T. obtusum and C. trapeziformis are compelling examples but there are countless variations on the theme. While bees, wasps and other winged insects provide much of the pollination services for orchids and other plants, birds may have an important role for pollination of other plants.

Bird Pollinators

Ornithophilous plants, or those pollinated by birds, may have other pollinators but birds carry out much of the work of pollination for these species. Hummingbirds are a relatively well-studied example.Lasso and Naranjo (2003) found that eight hummingbird species visited the shrub Hamelia patens during a 54-hour period near the La Selva field station in Costa Rica. Temporal patterns of flower visitation by avian nectivores closely matched the pattern of nectar production of the shrub. Like many hummingbird-pollinated species, H. patens has a tubular flower which hummingbirds can easily access with their long, curved beaks. Hummingbird pollinators are often attracted to flowers with mechanisms that utilize volatile chemical compounds and attractive coloration, similar to insect attractants. However pollinator attraction can also differ in important ways for birds. In fact, researchers have found that hummingbird pollinated species prefer flowers that reflect light at longer wavelengths compared to insect pollinated species (Altschuler, 2003). Pollen transfer can be relatively specific to certain species of hummingbirds as well. In Costa Rica two types of Heliconia plants, with differing flower morphology, are pollinated by two distinct types of hummingbird – those with long,, curved beaks and those with short, straight beaks. The physical structure of plant parts in the two types of Heliconia regulate where pollen is deposited. For plants pollinated by birds with long, curved beaks (hermits) pollen is most often deposited on the head or base of the beak (Stiles, 1975). Much work has been done to understand hummingbird pollination but even less is known about mammal pollination systems.

Mammal Pollinators

Mammals tend to reap the benefits of insect and bird pollinators rather than assisting in pollination itself; as many frugivorous mammals depend on the fleshy parts of plant fruits for food. Still, some mammals such as opossums, lemurs and bats(particularly at tropical latitudes) also act as pollinators (Tschapka and Von Helversen, 1999). The neotropical herb, Phenakospermum, is pollinated by several bat species and there is evidence that a nocturnal marsupial (Calruomys philander) (pictured above) robs nectar from this species (Kress and Stone, 1993). Like many nocturnally pollinated species, Phenakospermum has large white flowers, hexose-based nectar and are receptive to pollination on a single night. Bracts cover the flower and open at sunset allowing pollinators access to the faint, sweet-smelling nectar. As the bat grasps the flower with its teeth or claws, the nectar chamber opens and the anthers, held by closed petals, spring loose depositing pollen onto the bat. The flower does not continue to produce nectar and is rarely visited again during the night, this mechanism is hypothesized to decrease the chances of self-pollination. Furthermore, the stamen of the flower is cast-aside after the flower is tripped so that if visited, the stamen remains out of contact with the pollinator. The bare-tailed wooly opossum (C. philander) was found to gain access to the nectar via the base of the corolla and as such did not effect pollination. It is possible that many other mammals also engage in nectar thievery in other plant-pollinator systems but further detailed observations will need to be made to catch such subtle interactions.

The tropics have a wide diversity of plants and the examples of plant-pollinator interactions are endless. As researchers become increasingly aware of the delicacy of these interactions, the implications to conservation are substantial. The tropics are biodiversity hotspots and rapid conversion of forests to other land-uses undoubtedly has serious deleterious effects on the integrity of plant-pollinator interactions (Borgella et al, 2001). Plants cannot survive without their pollinators and likewise pollinators will suffer in absence of plants on which they are dependent. Pollination is an important ecosystem service that helps to maintain both plant and animal diversity and greater understanding of plant-pollinator interactions will help both to value and conserve them.



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