Epiphytes, Birds, and Coffee: More Tropical Connections
Epiphytes are plants that spend much or all of their lives attached to other plants. Some of our favorite ornamental plants are epiphytic, including some bromeliads, cacti, and orchids.
True epiphytes should not be confused with parasitic plants, such as mistletoes, which absorb water and minerals from the tree they live on. While epiphytes are generally benign (using the tree for support only), parasitic plants may eventually weaken or even kill their host.
Epiphytes are particularly prominent in moist tropical forests and have long fascinated tropical biologists. As an ornithologist, my research focuses on the resources that the epiphytes provide to birds and how their presence contributes to avian diversity.
For example, hummingbirds (and other birds) pollinate many epiphytic bromeliads when visiting them for nectar. Birds, such as the band-backed wren of central Veracruz, Mexico, use bromeliad seeds, which have silky filaments designed for wind dispersal, to line their nests. Some birds build their nests inside epiphyte clumps, which may give them extra concealment from predators. Other birds eat the fruits of epiphytic cacti, aroids and bromeliads, or look for insects inhabiting these plants.
Epiphytes are not only very common plants in humid tropical forests, but they can also be abundant in some agroecosystems, such as shade coffee farms. Because many birds that migrate to the tropics spend the winter in tropical habitats such as forests and coffee plantations, the livelihood of many of our favorite local birds can be linked to the presence and health of tropical epiphytes.
However, because of the increasingly common practice of epiphyte removal in agroforestry systems such as coffee farms, this link is threatened. Early in our research on the use of coffee farms by birds, we discovered that farmers commonly remove epiphytes from the shade trees because they believe that all epiphytes harm their host tree. They also think that if they remove epiphytes, the coffee shrubs might get more sunlight and therefore bear more fruit.
Based on studies correlating the presence of epiphytes with local bird species richness, we believed that removal of epiphytes could have a large negative affect on overall bird diversity. Also, the distribution of epiphytes might be an indicator of general microclimatic conditions or management practices. Obviously, a true assessment of the effect of epiphyte removal on bird diversity requires direct experimentation.
The most feasible approach is the direct removal of epiphytes from experimental plots within the coffee agroecosystem, a process that could require a tremendous amount of labor. For this reason, this approach had never been tried before in any tropical habitat.
We were able to accomplish this daunting task by taking advantage of the desire of a coffee farmer in Coatepec, Mexico, to remove epiphytes on his farm and the availability of relatively inexpensive labor. We needed only to convince the farmer to carry out his epiphyte removal in a pattern consistent with our experimental design, to compensate the workers, and to provide ladders and a small chain saw for them to complete the work.
These farm workers then proceeded to scramble through all of the shade trees and remove all epiphytes in two three-hectare plots (a little more than seven acres). We then compared these plots with other control plots, in which epiphytes were not removed.
Within these epiphyte and non-epiphyte plots, we monitored the number of bird species (diversity) and the number of individual birds (abundance). At first, we were surprised to find about the same number of species in plots with and without epiphytes. But upon closer inspection, we found profound differences in the patterns of abundance in the common species.
Plots with epiphytes (right) and without (left).
© Andrea Cruz-Angon
For example, 18 species of birds were more abundant in the plots with epiphytes, while only three species were more abundant in the plots without epiphytes. When we examined the ecological characteristics of the species associated with epiphytes, we discovered that these species used epiphytes for nesting or feeding and thus were greatly affected by their removal.
This was the case, for example, among some resident birds—two bromeliad-pollinating hummingbirds (White-bellied Emerald and Wedge-tailed Hummingbirds) and three epiphyte-nesting tanagers (The Common Bush-tanager, White-winged Tanager, and Yellow-throated Euphonia).
But upon further inspection, we found a more puzzling result. Some species that do not use epiphytes in any way turned out to be more common in the plot with epiphytes. This is true for some of the migratory species from the north, such as the Olive-sided Flycatcher, Solitary Vireo, Blackburnian and Tennessee warblers, Summer Tanager, and Baltimore and Orchard orioles.
We also found this phenomenon in certain understory breeding birds (Spot-breasted Wren, Golden-crowned Warbler, and Rusty Sparrow) and two resident non-breeding flycatchers (the Greater Pewee and Tufted Flycatcher).
In all, 11 common species showed a statistical association with epiphytes yet there was no evidence that they directly use epiphytes for feeding or nesting.
Benjamin Lorr from Columbia University also worked on our experimental plots. He found that epiphyte removal reduced canopy cover and soil moisture and increased stem-flow and through-fall. Thus, more water was getting to the ground in less time, but it evaporated or ran overland faster. This might translate into sudden changes in the microclimate of the coffee plantation, which could affect birds as well.
This change in microclimate and run-off would affect ground nesters differentially and, hence, would explain the declines in Golden-crowned Warblers and Rusty Sparrows. The change in microclimate could have other indirect effects on the abundance and diversity of arthropods, such as insects and spiders, which could in turn affect some insectivorous birds.
In a parallel study, we fogged canopy trees with insecticides and found that epiphyte removal caused a marked decline in some groups of insects heavily used by birds (such as Lepidoptera - moths and butterflies). These overall changes in insect abundance could account for declines in birds that do not forage in epiphytes such as Olive-sided Flycatcher, Solitary Vireo, Blackburnian and Tennessee warblers, Summer Tanager, Baltimore and Orchard orioles.
In addition, greater canopy openness as a consequence of epiphyte removal may increase the ability of potential predators to detect birds. Finally, some other birds may have preferred plots with epiphytes simply because they more closely resemble a forest in their structure.
One other indirect consequence of epiphyte removal is that the canopy becomes demonstrably more open, allowing more light to reach ground level. In areas where epiphytes have been removed, weeds become more abundant.
This could explain why granivorous (seed eating) species such as Indigo Bunting and Painted Bunting were more common in the plots without epiphytes. However, weeds are a temporary resource in coffee plantations, since they are removed quite often from the understory.
With this experiment we learned that epiphytes are an important resource for birds. They not only provide a wide variety of resources such as food and nest materials, but also play a role in microclimate regulation, and offer refuge and cover to the inhabitant fauna. It is worth noting that we have found no published data to support the supposed agronomic benefits of removing epiphytes.
It appears that farmers could save time, money, and birds by not removing epiphytes from the shade trees on their coffee farms.
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