Our group prepares to leave Lighthouse Cave, San Salvador, Bahamas.
It has been well documented that of all terrestrial environments the plant communities of tropical rainforests are the highest in diversity and abundance (biomass). Epiphyte communities can be held up as a stereotypical example of the efficiency of plants and their adaptations in favorable conditions. The basic description of an epiphyte is a plant that lives on another plant without deriving nutrients (parasitizing) the host plant. Individual epiphyte communities vary throughout the rainforest depending on several parameters, mainly strata and elevation. The epiphyte communities in the canopy, for instance, are distinct from that of the middle or lower portions of the forest. Epiphytes are unique also in that they over time will often produce their own substrate. It would follow that with the increased diversity presented by the epiphyte communities a larger diversity of animals would then be present. In fact other classified organisms such as soil bacteria, soil communities of invertebrates, bryophytes and vascular plants are increased in diversity. Each of these groupings will have a unique relationship with other organisms from occasional visitors to those that form specialized relationships. This review will generally describe epiphytes of the tropics and neotropical birds and discuss relationships between epiphytes and birds of the tropical rainforest with a final focus on conservation efforts in the Cloudbridge Nature Reserve in Costa Rica and general conservation ideas.
What Is An Epiphyte?
Tropical Epiphytes include a wide variety of species of algae, fungi and plants. These organisms occur in abundance within the tropical rainforest. There are in fact up to 25,000 species of vascular epiphytes alone that occur mainly in the tropics (Zotz and Hietz, 2001).
An epiphyte, as defined above, is generally described as a plant that lives attached to another plant without directly parasitizing the host plant. However, the definition might be expanded to include other classified plant-like organisms such as Protista (algae) and Fungi. A classical example of an epiphyte that occurs all over the world would be the algae/fungi complex known as lichen. However they may be described there is a large diversity and abundance of epiphytes well adapted to favorable conditions that exist in the tropics and quite unfavorable conditions such as tundra regions. Tropical epiphytes may exist as the most diverse and abundant of this grouping.
Vascular epiphytes are higher organisms represented by a broad range of taxa, the larger majority of these or up to 80% are represented by monocotyledons (Zotz and Hietz, 2001). These plants exist in much the same form and are in fact from groupings that include non-epiphytic species such as ferns, orchids and woody plants. Their rooting systems often take advantage of what is known as an epiphyte mat formed primarily by non-vascular epiphytes. Something I think of as similar to a bog mat formation in northeast deciduous/boreal transitional zones.
The non-vascular epiphyte community generally begins the formation of the epiphytic mat by capturing wind borne soil and with it, necessary nutrients. This group includes those familiar epiphytic organisms we see in North America, lichens, mosses, liverworts and fungi. Important to many vascular plants, mycorrhizae occur frequently in epiphytic plants (Kricher, 1997). Ideal environmental conditions exist in the tropical rainforest for the attachment and growth of these organisms with the existence of a dense canopy covering the forest and trapping heat as well as water vapor, the effect being a mini water cycle inside the forest (Hauenschild, et al. 1999). In addition many of these groupings are well represented more independently in the middle and lower strata of the forest much the same way as they are in temperate rainforests and mesic deciduous forests (like mosses clinging to the base and sides of trees).
Epiphytes Of The Tropics Occurrence And Distribution
A discussion of tropical epiphyte occurrence cannot be separated from the topic of overall tropical rainforest occurrence. They will occur in the tropical rainforest especially if left undisturbed.
For centuries, rainforests have been utilized by many people for many purposes, from the unique properties of individual plants to clearing for land cover use such as agriculture. Because of this historical harvesting and overall deforestation many species are presumed to have been lost to extinction. How many species is the question as we are unable to keep the pace of identification and classification up with the pace of rainforest destruction. There are remnants of tropical rainforests in areas of historical human impact and climate change that may serve as examples of what occurred naturally in the past (Hauenschild and Smith, 1999). Becoming less and less peripheral to this discussion are the widespread effects of pollution and rapid global climate change (Hietz, 1999). While the earth has experienced climate change in its geologic past, the current increased rate of warming can be placed squarely on the shoulders of humans and our associated activities (anthropogenic effects).
Another aspect to the distribution of tropical epiphytes is the stratification effect. This discussion is addressed below.
THE EPIPHYTE COMMUNITY
Forest Stratification, Nutrient Capture and Plant Interactions
There are three basic layers to a forested community as defined by ecologists, the understory or ground level, the mid-layer and the canopy. Interestingly, while a temperate system would have the understory layer responsible for decomposition of materials and nutrient cycling, the epiphytes of the canopy in a rainforest are capable of containing detritus and “canopy soil” which together is called “crown humus”. Bryophytes and lichen form the base of the crown humus supporting a unique community of invertebrates while vascular epiphytes occupying the crown humus will produce fruits and flowers (Sillett, 1994). It strikes me that this unique interaction contains all the elements of the successional model for terrestrial communities, that is, pioneer organisms consisting of non-vascular organisms that capture nutrients and soil from aerial fallout forming the necessary soil for further successional species to survive and function until a stable or “climax community” is established.
Water availability is considered by some to be one of the more important abiotic stresses to epiphytic habitat (Zotz and Hietz, 2001). One group of vascular epiphytes having a significant adaptation that alleviates this stress, to some degree, is referred to as tank bromeliads. The tanks are formed by leaves that overlap at the base. The larger individuals of this grouping can hold water for over a week without an external water supply, while some of the smaller species the supply lasts only about a day. On the flip side of the water availability issue is the consequence of too much moisture. Bromeliads contain trichome shields or structures on the epidermis of the leaves that serve to allow direct moisture and nutrient absorption. When trichomes are continuously covered with moisture, gas exchange may be significantly reduced. Zotz and Hietz suggest that this may be a reason that these species are lower in abundance within the Cloud Forest.
There are several ways in which various epiphytes may obtain nutrients. As mentioned above, aerial fallout or blowing dust, rain and mist can provide nutrients. In addition, the invertebrate community will play a role. One study documents the role of debris left by ants as an important source of nitrogen for an epiphyte (Zotz and Hietz, 2001).
The nature and interactions of epiphytes are so intriguing that I could continue this discussion for the remainder of the paper. However, I must move forward and get to the heart of the topic!
Neotropical birds, simply put, are bird species that exist in the neotropics for all or part of the year. Many of the species that have been studied in North America are neotropical migrants, or those that will winter in the tropics and migrate northward to breeding grounds during the warmer summer season.
Generalists and Specialists
In ecology, species are often described by their adaptability to change and ability to be opportunistic. Those species that are capable of utilizing a wide variety of resources are called generalists. An example of a generalist species in North America would be the American Robin (Turdus migratorius). This species will be found deep within a forested environment as well as, quite abundantly, around homes, parks and lawns.
An example of a recently re-discovered specialist is the Ivory-billed Woodpecker (Campephilus principalis). This species requires a specific habitat type and is unable to adapt to the thinning or clearing of the forests it is adapted to. While this species is not as strict of a specialist as such species as the Kirtland’s Warbler (Dendroica kirtlandii) requiring one species of tree at a certain height in order to nest, C. principalis is enough of a specialist that it succumbed to deforestation and has not been “officially” documented until recently, for over 60 years. This points to the fact that there are degrees of specialization as will be discussed below.
Bird Dependence on Epiphytes
The use of epiphytes by birds has been observed by a number of researchers. The importance of epiphytes to specialists however, is more difficult to assess and study. At least 193 species of neotropical birds have been documented as utilizing the resources of epiphytes. Such use includes nest material and nesting sites, water resources and food, including fruit, nectar and invertebrates (Nadkarni and Matelson, 1989). Nadkarni and Matelson observed bird use of epiphyte resources in the Monteverde, lower montane forest and pastures of Costa Rica. They found in their study area those families that most frequently used epiphyte resources were hummingbirds (Trochilidae), tanagers (Thraupidae) and flycatchers (Tyrannidea). Quantitatively, it is important to note that of all the foraging behavior they observed over 30% of the visits were specific to resources provided by epiphytes. They suggest that this may be an underestimate of bird/epiphyte interaction. They also suggest that the larger diversity of resources provided by epiphytes especially in the canopy, “may enhance opportunities for resource specialization” and that seasonal phenology of epiphytes, often different than the host trees, may be important to bird use. In other words, epiphytes provide resources at different times of the year than host plants.
Foraging, Cover, and Nest Sites
One group of birds in the Tanager family that I only found generally described in our text book is the Euphonias (genus Euphonia). This genus is mentioned as important to mistletoe seed dispersal but also they commonly nest in bromeliads (Kricher, 1997). The Euphonias are also cited as utilizing other epiphytes for nests including mosses and ferns. The Euphonias appear to prefer this resource as cover for their nests since they are covered by a roof with a side entrance hidden within the dense cover (Stiles and Sutch, 1989).
Insectivorous behavior has also been studied to determine the dependence of birds on epiphyte resources, specifically, the epiphyte mat that harbors an invertebrate community (Sillett, 1994). The degree of specialization is often important in determining dependence on a resource. This study defined epiphyte specialists as those birds that used epiphyte resources during a minimum of 75% of the observed foraging attempts. Sillett separated the foraging behaviors of eight species of insectivorous birds into three categories, substrate-restricted, near-surface and prey-specific foraging. Four of the species observed by Sillett fit the definition of epiphyte specialists. These were: Spotted-crowned Woodcreeper (Lepidocolaptes affinis), Ruddy Treerunner (Margarornis rubiginosus), Buffy Tuftedcheek (Pseudocolaptes lawrencii), and Ochraceous Wren (Troglodytes ochraceus). The first three of these are in the family of ovenbirds (Furnariidae).
EPIPHYTE DEPENDENCE ON BIRDS
Pollination and Seed Dispersal
Angiosperms are well known as being dependent on pollinators for fertilization and fruit production. Pollinators are largely insects, however a few species of birds and bats are also co-adapted as primary pollinators. While specific information about the dependence of epiphytic angiosperms on bird pollinators has eluded me, certain species of birds that occupy the tropical rainforest are important pollinators. One study focused on the relationship between pollinators and Palicourea padifolia (Rubiaceae) which is a shrub in the understory of middle-elevation cloud forests. This species exhibits distylous polymorphism (some having long styles and short stamens and others have the reverse) and is self-incompatible (individual plants are unable to self pollinate). P. padifolia appears to depend on hummingbirds as pollinators and their reproductive success has been correlated to hummingbird selection based on different nectar “rewards” displayed by the different morphologies. Hummingbirds were also observed to defend their feeding territories which consisted of 1-3 plants. It may follow that hummingbird selection of nectar rewards in epiphytic angiosperms could be similar and they may exhibit similar territoriality. How the territoriality relates to reproductive success of the angiosperm needs further study.
Another study on bird-pollination was done in the tropical rainforest of Malaysia (Yumoto, 2000). This study focused on three species of Durio all of which are also self-incompatible. An interesting aspect of this study is that they found two of the three species were dependent on bird pollination specifically spiderhunters (Nectariniidae) while the third was pollinated by honey bees and bats in addition to birds.
In a study conducted in the montane forests of Argentina, birds were captured (mist nets) and patterns of abundance were compared to two parameters, elevation and seasonal differences (wet or dry seasons). Fruit production was highest in each of the study sites during the wet season (November – January) and they found the highest capture rates during this time (Blake and Rouges, 1997). They also studied the fecal contents of species encountered and found fruit seeds or pulp to be clearly highest in the Andean slaty-thrush (Turdus nigriceps). Few other fecal samples were found to have fruit seeds or pulp. It would be interesting to see the correlation of seed dispersal and the occurrence of T. nigriceps. Although that correlation has not been made (that I know of) it appears that fruit bearing plants in season are important to bird abundance in these forests. Epiphytes are noted as abundant in the study areas as well.
A few of the papers I read complained of the lack of information related specifically to epiphyte and bird interactions. While many researchers have noted occurrences coincidental to their research, specific relationships have yet to be studied in detail. I found this to be frustrating as well. Most of the specific interdependence between these two groupings might be pieced together with existing data but this has yet to be confidently shown.
Threats to the Epiphyte Community and Associated Bird Species
Epiphytes that are adapted to the tropical rainforests utilize resources unique to this type of vegetation cover and the associated microclimates it provides. It follows then that tropical epiphyte survival is directly linked to survival of tropical rainforests. Some epiphyte species have been found on plants in a plantation close to an old growth cloud forest, however the abundance and diversity were greatly reduced (Hietz, 1998). Proximity to the old growth rainforest is presumed to have an effect on the appearance of epiphytes in the plantation as reproductive success in the species of the plantation was also greatly reduced. Other epiphyte species typically occupying the middle range of the forest where shade and moisture are abundant, were completely absent in the plantation. Hietz suggest that the shade species will be most seriously affected by deforestation and disturbance.
Habitat destruction is the main cause of extinction today, however other anthropogenic effects have been noted as important to the tropical rainforest. One of these effects is air pollution which is also linked to global warming. It seems obvious that plants deriving nutrients from aerial fallout would be affected by pollutants. Once the bases of epiphyte communities are affected, species such as lichens and mosses, the whole community will suffer as well. This will invariably affect the bird species dependent on such resources.
Introduction of exotic plants and animals to the rainforest can also affect the rainforest ecosystem. In many areas of the world, introduced species are often able to out-compete for resources and eventually lead to the reduction or extinction of certain native species and in some cases, native communities.
What can be done?
Hietz studied the practice of coffee plantations established under the canopy of an old growth rainforest and found epiphyte abundance to be about the same as in the natural forest. He suggests that these types of plantations can maintain epiphyte populations if the forest strata is maintained. That is, the old growth trees are literally left alone and those trees with significant epiphyte growth be avoided altogether, leaving pockets of undisturbed areas within the plantation.
It seems logical that if habitat destruction is the primary cause of species extinction today then to solve this problem habitat must be preserved. It is well known that tropical rainforests are being destroyed faster than any other terrestrial biome in the world today. So why do we not just stop this destruction? Easier said than done! Significant efforts have been under way for some time now and one of the most hopeful examples is the Monteverde Cloud Forest of Costa Rica. The Tropical Science Foundation administers this 12000 acre private preserve and encourages scientific research which has been going on for over 30 years. It has been suggested that even with this high level of long term protection the cloud forest could be susceptible to other environmental concerns such as global warming and deforestation which are thought to be responsible for raising the height of the cloud layer (Larsen, 1993).
Beyond complete habitat protection and management (ownership of the land), it is a difficult task to simply stop destruction of the rainforest or any ecosystem especially if we consider that often people are dependent on this resource for their very lives and those of their families. Links can be made between deforestation and the secondary impacts to the tropical rainforests to nearly any place in the world. There are clear indications that the people of North America can have a hand in reducing this impact. If pollution and CO2 production are impacting the tropical rainforests then reducing this impact can have world wide implications. Simple ways to do this would be reducing electrical use (burning of coal is our primary source of electricity). Turn off the lights! Of course, drive fewer miles and more fuel efficient cars, car pool or even public transportation. Reduce potable water use to only the essentials (sorry golfers!). Buy environmentally friendly products, shade grown coffee for example. And finally, pass on the message to others, especially children who will inherit all of this and more.
In broader terms it must be recognized that the size of the human population cannot be ignored. From a strictly ecological perspective, as a species continues to grow exponentially, resource use will also grow exponentially until a limiting factor is applied. Humans rely on food as all other heterotrophs do and continually eliminating this as a limiting factor will only continue the rate of growth until another limiting factor kicks in. This could be any number of things that I will not go into here, but suffice to say it cannot be pretty. That said, who would like to volunteer and help reduce the world population? Ideally and my hope is that humans will voluntarily reduce population by reducing reproductive rate. This can only be done if and when a significant majority of people recognize this as a problem and take personal responsibility to act. Getting into the whole realm of personal responsibility brings up many of the questions we grapple with throughout our lives, social, political, economic and yes, even spiritual. I am convinced that this can only be done if viewed as a deeply personal issue and not solely a societal issue. There are other ways of course like imposing laws, but ultimately this does not work in the face of human choice which we must avoid restricting at all cost as it degrades the human condition even further. As may be apparent I could go on and on in this line of thought. I also recognize the possible debate that these suggestions might inspire. The summarizing point is that once we can recognize our connection to the tropical rainforests, tank bromeliads, and spidercatcher birds then we can see that what we do does matter after all. We do have an impact and we can help to reduce that here in our own world and our own homes. We cannot look at it as a hopeless endeavor, working on what is occurring hundreds of miles away where our futile attempts appear fruitless. It is essential that we act and that we know this makes a difference, whether we realize that change on a global level in our life time or not does not matter, we each can make the difference.
Hietz, Peter, 1999. Diversity and Conservation of Epiphytes in a Changing Environment. Conference Presentation, International Conference on Biodiversity and Bioresources: Conservation and Utilization, 23-27 November 1997, Phuket, Thailand. Http://www.iupac.org/symposia/proceedings/phuket97/hietz.html.
Kricher, John 1997. A Neotropical Companion. Pp. 32.
Larsen, Jessica 1993. The Monteverde Cloud Forest. http://jrscience.wcp.miamioh.edu/fieldcourses02/TropEcoCostaRicaArticles/TheMonteverdeCloudForest-.html
Marcy, N. 2004. Avifauna at Cloudbridge. Ian Giddy, http://cloudbridge.org/avifauna.htm
Nadkarni, N. M. and Matelson, T.J. 1989. Bird Use of Epiphyte Resources in Neotropical Trees. The Condor 91:891-907.
Ornelas J.F., Gonzalez C., Jimenez L., Lara C., and Martinez A.J. 2004. Reproductive Ecology of Distylous Palicourea Padifolia (Rubiaceae) in a Tropical Montane Cloud Forest. II. Attracting and Rewarding Mutualistic and Antagonistic Visitors. American Journal of Botany 91(7): 1061-1069.
Sillett, T. S. 1994. Foraging Ecology of Epiphyte-Searching Insectivorous Birds in Costa Rica. The Condor 96: 863-877.
Stiles, F. G. and Skutch, A. F. 1989. A Guide to the Birds of Costa Rica. Cornell University Press. Pp. 417-421.
Yumoto, T. 2000. Bird-Pollination of three Durio Species (Bombacaeae) in a Tropical Rainforest in Sarawak, Malaysia. American Journal of Botany 87(8): 1181-1188.
Zotz, G. and Hietz, P. 2001. The Physiological Ecology of Vascular Epiphytes: Current Knowledge, Open Questions. Journal of Experimental Botany, Vol 52, No. 364, 2067-2078.
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