Canopy Crane Access System

Biotic interactions

1) Herbivory and seed predation

Courtesy S. Joseph Wright & Mirna Samaniego

Insects, vertebrates and pathogens that consume leaf tissue have a tremendous impact on vegetation. Due to the inaccessibility of the upper forest strata, levels of herbivory in the canopy of tropical forests where more than 90% of forest leaves are concentrated are virtually unknown. A survey of canopy herbivory levels was initiated from the PNM crane beginning in November 1992. Levels of herbivory were unexpectedly low. The mean proportion of leaf area consumed by herbivores averaged just 8.3% over the lifetime of the leaves of the nine most common canopy tree and liana species. The canopy herbivory levels observed here average 67% lower than understorey herbivory levels observed in nearby forests on Barro Colorado Island and elsewhere in the tropics. Future work will explore the reasons for low herbivory levels in the canopy. Hypotheses to explain low canopy herbivory include high levels of plant defenses, high levels of predation on herbivorous insects, and/or an adverse impact of the canopy environment (i.e., low humidity and high temperatures) on herbivorous insects. Predator exclosure experiments and shading experiments to ameliorate the canopy environment have been initiated to test the latter two hypotheses. Plant defenses against herbivores include a wide variety of secondary compounds that are toxic to herbivores but potentially useful to humans. Examples include caffeine, strychnine, pyrethrin, cocaine and morphine. Canopy leaves with low levels of herbivory will be screened for active secondary compounds. The observed canopy herbivory levels will also be incorporated into carbon acquisition models. These models will scale from leaf-level processes to their canopy-level consequences. Herbivory was initially expected to be a critically important phenomenon in these models, however, the low observed rates of herbivory now suggest otherwise.

A different study aimed at (i) identifying the consumers of reproductive structures of canopy trees, lianas and epiphytes at PNM; (ii) assessing the percentage of each crop that is attacked; and (iii) determining the degree of specialization of insects on single host plant species. The fates of reproductive structures of Anacardium excelsum, which is an important timber species throughout its range, are illustrative. Sixty per cent of the flower buds are lost to the larvae of an unidentified species of Lepidoptera (butterfly or moth). Flowers potentially have both male and female functions. However, 89% of the flowers are female sterile due to predation by an unidentified thrip and by the Lepidoptera larvae mentioned previously. Fifty-four per cent of pollen is also killed by thrips and/or environmental conditions such as drought. Eighty-five per cent of the ovules that are fertilized are attacked and killed by the invasive, air-borne fungus Fulvia fulva (Cooke) Ciferri. Thirty-three per cent of the fertilized ovules are also killed by the larvae of an unidentified species of scarab beetle and by the larvae of a second unidentified Lepidoptera. These larvae are in turn parasitized by an unidentified species of wasp. In sum, the larvae of two species of Lepidoptera and one species of beetle, adults of one species of thrip and one species of fungus collectively kill 99.6% of the flower buds initiated by Anacardium excelsum.

More info: S.J. Wright web page

See also: Van Bael, S., Aiello, A., Valderrama Cumbrera, A., Medianero, E., Samaniego, M. & Wright, S.J. (2004)
General herbivore outbreak following an El Niño-related drought in a lowland Panamanian forest. Journal of Tropical Ecology, 20, 625–633.

Examples of leaf damage on A. excelsum in the canopy of Parque Natural Metropolitano

2) Multi-trophic interactions in tropical forest canopies

Courtesy Sunshine Van Bael

For decades, ecologists have debated the circumstances under which a predator limits its prey’s consumption of organisms in lower trophic levels – a set of species interactions referred to as a predator-driven trophic cascade. Experimental tests have shown that insectivorous birds limit arthropod abundances and decrease damage to plants, but the few previous tests have been conducted in relatively low diversity settings such as temperate forests (Atlegrim, 1989; Marquis & Whelan, 1994) or agricultural systems (Greenberg et al., 2000). To address whether trophic cascades occur in diverse, tropical forest canopies, I used canopy access to experimentally exclude birds and bats from canopy branches. I then compared arthropod densities and damage on foliage that was accessible to predators and inaccessible to predators. With the availability of two cranes in Panama, I was able to examine the effects of vertebrate predation across two Neotropical forests that differ in rainfall and species richness. Indeed, the results differed from site to site. At the less diverse, dry forest site, arthropod densities were significantly higher and leaf damage increased by 85% on foliage that was inaccessible to vertebrate predators. In contrast, few effects of a vertebrate-driven trophic cascade were observed in the canopy of the diverse, wet forest. In general, foliage defense via vertebrate predators was greater for fast-growing, sun-loving tree species than for slow-growing, shade-tolerant tree species. These results have conservation implications as projected trends in habitat loss predict the decline and local extirpation of many bird species (Van Bael et al., 2003; Van Bael & Brawn, 2005). With decreased defense from vertebrates, tree crowns will suffer increased damage levels, which may act as a persistent drain on the photosynthetic ability, growth (Marquis & Whelan, 1994), and eventually the fitness of canopy trees.

More info: S. Van Bael web page

3) Pollination in tropical forest canopies

Courtesy Shoko Sakai & David W. Roubik

Formerly, biologists believed that most plants in tropical forests, which typically display high species richness and low population density of each species, reproduced through self-fertilization. In fact, the prevalence and the importance of outcrossing have been only recognized recently, even in tropical forests. In most tropical plants, outcrossing is achieved by animal pollen vectors, such as bees, beetles, birds, bats, which visit flowers for their foods (pollen, nectar) or other resources such as resin, a material used by some bees to make their nests. Since such plant-pollinator interactions are thought to contribute to the increase and maintenance of biodiversity, information on pollination biology is essential for forest conservation.

However, studies on pollination biology in tropical forest canopies are limited because of difficulty of access. Studies using canopy cranes have discovered interesting pollination mutualisms, in which plants provide brood sites for their pollinators. Castilla elastica (Moraceae) at PNM is pollinated principally by thrips. The thrips reproduce and increase rapidly on the plants, and are dispersed by wind from canopy branches (Sakai, 2001). Aristolochia spp. (Aristolochiaceae) at the same site have similar mutualistic relationships with dipteran pollinators (Sakai, 2002). On the other hand, many other plant species have less specialized relationships with their pollinators, resulting in a complicated network of interactions. Certain canopy studies focus on factors affecting this network and their outcomes for the genetics and species diversity of plants.

Recently, stability and specificity of relationships between plants and pollinators, and close adaptation of plants to particular pollinators, has been called into question (e.g. Johnson & Steiner, 2000). Ecological relationships seem rather specialized and stable among understorey plants of tropical forests (e.g. Kress & Beach, 1994; Kato, 1996; Sakai et al., 1999). On the other hand, the relationships are looser in upper layers. Roubik et al. (2003) studied ecological certainty using forest canopy observations of insects visiting flowers, and collections of thrips in flowers fallen on the ground, at four lowland sites in central Panamanian forests, including Parque Natural Metropolitano and Parque Nacional Soberania. Thrips and bees were the principal flower visitors and potential pollinators for 30 plant species monitored for two or three sequential flowering seasons. Thrips in general shifted greatly in abundance from year to year for half the plant species, but were (as a group) consistently associated with host plants. For example, Frankliniella parvula (Thripidae) remained the dominant species on Gustavia superba (Lecythidaceae). For bees and other visitors, the observed species remained as dominant flower visitors for two or three seasons in half of the study plants. In contrast, many plants possessed loose pollination niches and the naturalised African honeybees had a prominent role in their visitation. Specialized flowers pollinated by large bees, however, had the least variable pollinator species and the largest variance among individual species, that is, the highest individual bee species dominance. The study concluded that loose niches were a quantitative and common phenomenon, and they often involved generalist plant-pollinator relationships.

More info: D.W. Roubik's publications available in pdf

See also: Sakai, S., Momose M., Yumoto T., Nagamitsu T., Nagamasu H., Hamid, A.A. & Nakashizuka, T. 1999.
Plant reproductive phenology over four years including an episode of general flowering in a lowland dipterocarp forest,Sarawak, Malaysia. American Journal of Botany, 86, 1414-1436.

Rainfall and pollination studies at the Malaysian canopy crane

D.W. Roubik’s home pages

Entomologists netting insects in blooming trees at San Lorenzo

References cited

Atlegrim, O. (1989)
Exclusion of birds from bilberry stands: impact on insect larval density and damage to the bilberry. Oecologia, 79, 136-139.

Greenberg, R., Bichier, P., Angon, A. C., MacVean, C., Perez, R. & Cano, E. (2000)
The impact of avian insectivory on arthropods and leaf damage in some Guatamalan coffee plantations. Ecology, 81, 1750-1755.

Kato, M. (1996)
Plant-pollinator interactions in the understory of a lowland mixed dipterocarp forest in Sarawak. American Journal of Botany, 83, 732-743.

Kress, W. J. & Beach, J. H. (1994)
Flowering plant reproductive systems. La Selva, Ecology and Natural History of a Neotropical Rain Forest. L. A. McDade, K. S. Bawa, H. A. Hespenheide and G. S. Hartshorn. Chicago, University of Chicago Press: 161-182.

Johnson, S. D. & Steiner, K. E. (2000)
Generalization versus specialization in plant pollination systems. Trends in Ecology and Evolution, 15, 140-143.

Marquis, R. J. & Whelan, C. J. (1994)
Insectivorous birds increase growth of white oak through consumption of leaf-chewing insects. Ecology, 75, 2007-2014.

Roubik, D. W., Sakai, S. & Gattesco, F. (2003).
Canopy flowers and certainty: loose niches revisited. Arthropods of Tropical Forests. Spatio-temporal Dynamics and Resource Use in the Canopy. Y. Basset, V. Novotny, S. E. Miller and R. L. Kitching. Cambridge, Cambridge University Press: 360-368.

Sakai, S. (2001)
Thrips pollination of androdioecious Castilla elastica (Moraceae) in a seasonal tropical forest. American Journal of Botany, 88, 1527-1534.

Sakai, S. (2002)
Pollinators of Aristolochia spp. (Aristolochiaceae) breeding on decomposing flowers. American Journal of Botany, 89, 527-534.

Sakai, S., Kato, M. & Inoue, T. (1999)
Three pollination guilds and variation in floral characteristics of Bornean gingers (Zingiberaceae and Costaceae). American Journal of Botany, 86, 646-658.

Van Bael, S.A., Brawn, J.D. & Robinson, S.K. (2003)
Birds defend trees from herbivores in a Neotropical forest canopy. Proceedings of the National Academy of Sciences, 100, 8304-8307.

Van Bael, S.A. & Brawn, J.D. (2005)
The direct and indirect effects of insectivory by birds in two contrasting Neotropical forests. Oecologia, 143, 106–116.

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