A |
[Latest update: October 31st, 2000]
Welfia georgii, a common
rainforest palm in Central America which supports rich epiphyll communities,
subject to studies of diversity patterns and biochemical interactions in the phyllosphere.
(Photograph by Robert Lücking)
The leaf probably represents the most important part of the plant, being the place of photosynthesis and gas exchange with the atmosphere. It is therefore astonishing that leaves support the often luxurious growth of foliicolous organisms, without apparent damage or negative reaction of the phorophyte. The biologist Volkmar VARESCHI once wrote: "Nirgends offenbart sich die Lebenskraft der Tropen so eindringlich wie in einem von Epiphyllen überwucherten und trotzdem noch immer lebensfähigen Trägerblatt". In fact, the phyllosphere, the living space on the leaf surface, shelters a great variety of different organisms, including lichens, bryophytes, algae, fungi, microorganisms and numerous animals, exhibiting multiple interactions between them and with the phorophyte leaf:
Biochemical or biophysical interactions between epiphylls and the phorophyte leaf.
Competitive interactions between different epiphylls.
Feeding interactions between animals and epiphylls.
Mimetic interactions between animals and the phorophyte leaf or its epiphylls.
Biochemical or biophysical interactions include the most diverse but least known types of interactions between epiphylls and their phorophyte leaf. Light interception caused by epiphylls may account for up to 50 % in the shady forest understory and have influence on the photosynthetic balance of the phorophyte. However, luxuriant colonization is restricted to old leaves which take over other functions such as storage. The photosynthetically most active leaves are usually less than one year old and only scarcely colonized by epiphylls. Epiphyll deterrence by biochemical substances of the phorophyte has not yet been studied but cannot be excluded, since fungal-deterrent substances are produced by plants [CHERRET in LIETH & WERGER, Tropical Rain Forest Ecosystems: 473-488 (1989)]. On the other hand, passive leaching of substances from the leaf [TUKEY in PREECE & DICKINSON, Ecology of Leaf Surface Micro-organisms: 67-80 (1971)] may provide nutrients for epiphylls. In a study on Citrus plants, epiphylls were shown to prevent leaf cutter ants from feeding upon the phorophyte leaves [MUELLER & WOLF-MUELLER, Oecologia 86: 36-39 (1991)]. However, the most obvious advantage of epiphyll growth comes from nitrogen-fixing cyanobacteria which often grow between bryophytes and lichens and may account for up to 20 % of the nitrogen balance of the ecosystem [BENTLEY & CARPENTER, Oecologia 63: 52-56 (1984); FREIBERG, Stickstoffixierung in der Phyllosphäre tropischer Regenwaldpflanzen in Costa Rica (1994)].
Competitive interactions between epiphylls mainly refer to lichens and bryophytes, which require most of the space provided by the leaf surface. Competition is partly avoided by slightly different microhabitat preferences. In cases where both have similar growth conditions, e.g. humid light gaps, bryophytes are usually competitive and overgrow young lichen thalli. However, some lichen species are known to prevent bryophytes from overgrowing them by unknown mechanisms probably related to their surface chemistry, and in a few cases lichens have been found to overgrow and kill small bryophytes.
|
|
Minute snails and psocids are some
examples of invertebrates which feed on foliicolous lichens and bryophytes.
The damage caused by these animals can positively influence epiphyll diversity on
individual leaves.
(Photographs by Robert Lücking)
Feeding interactions between animals and epiphylls are subject of an ongoing research project in Costa Rica [74, 75, 78]. Several groups of invertebrates which can be found regularly in the phyllosphere have been demonstrated to feed on foliicolous lichens, bryophytes, algae and fungi: Annelida, Gastropoda (snails), Lepidoptera (larvae of certain moths), and in particular Psocoptera. While snails and larvae of Lepidoptera seem to specialize either on lichens or bryophytes, Psorocoptera are omnivorous and often occur in large quantities on single leaves. Females place their eggs on the lower leaf surface where up to 25 juveniles might emerge. These are protected by their mother and seem to stay on the same leaf until they reach maturity. Correspondingly, they might cause considerable damage, while in turn positively affecting the diversity of the foliicolous lichen and bryophyte community [57, 78].
Mimetic interactions between animals and the phorophyte leaf or its epiphylls have been discovered in several cases. Perhaps the first documented case was an African frog imitating leaves with patches of epiphylls [FARKAS & PÓCS, Acta Bot. Hung. 35: 73-76 (1989)]. Grasshoppers resembling a leaf covered with epiphylls were documented from tropical Africa as well [SEAWARD in GALUN, Handbook of Lichenology: 107-132 (1988)]. Similar grasshoppers have also been detected in Costa Rica. While some look similar to epiphyllous bryophytes, others perfectly imitate leaves covered with foliicolous lichens, and even the genera of lichens which are imitated can be determined [17].
The catydid Rhossophyllum sp.
mimicks a leaf colonized by the foliicolous lichen genera Calopadia and Gyalectidium.
These abound in illuminated parts of the forest and give hints about the habitat
preferences of the animal.
(Photograph by Andrea Bernecker-Lücking)
