Water Worlds

This post was originally published under the title "Water Worlds" in the December 2016 issue of The Biologist, the Royal Society of Biology's magazine. If you're a member of the RSB you can also view the article on their website here.



Tropical rainforests are known for being full of life. Whether you're captivated by the calls of colourful birds, mesmerised by hypnotically patterned cats, or fascinated by the apparently infinite variety of invertebrate life, you still have to acknowledge that what makes a rainforest are the plants. A forest is by definition a collection of trees, but there is a whole lot more to plant life in a rainforest. Coating every surface, including the trees themselves, are more plants. Some of these we would as surface-covering vegetation in temperate environments as well, like the mosses and liverworts that coat stone walls or the dandelions that poke their sunny faces out between cracks in the concrete. However, there are plants which in the rainforest we see making other plants their hosts that we are far more used to seeing with other life styles. Cacti and ferns, for instance, can be found growing on other plants, as well as aroids (think peace and Easter lillies), bromeliads and orchids.
A pineapple fruit growing from
the centre of an Ananas comosus
bromeliad. © X O'Reilly

A pink bromeliad and other epiphytes,
Ecuadorian cloudforest. © X O'Reilly
Plants which grow on other plants are called epiphytes (from the Greek for "on", epi- , and "plant", phyton) if they do not directly harm their host. Epiphytes, therefore, are not parasites – they do not take advantage of their host's resources. What they do take advantage of is the surface area and, often, height provided by their hosts. Under the canopy of a forest, the higher you are, the more light you'll receive to photosynthesise. There is one particular group of epiphytes that has brought it's rain-catching strategy to the extreme – bromeliads.

A Tillandsia bromeliad with it's silvery leaves,
courtesy of its abundant and absorbent trichomes.
Dublin Botanical gardens. © X O'Reilly.
Bromeliaceae is a large and diverse family of flowering plants native to the neotropics. Most family members are epiphytes, though some – particularly those pertaining to dry environments, like the South American páramo – also grow on the ground or even on rocks. A bromeliad most will probably be familiar with is the delicious pineapple, which is fruit of a bromeliad called Ananas comosus. However, pineapple plants are not epiphytes, even in their native Central America.

In order to live in the nutrient poor rainforest canopy, bromeliads have come up with many of the same solutions as other unrelated epiphytes, such as roots that can cling to vertical surfaces and crevices. An adaptation unique to bromeliads is the ability to absorb moisture through specially adapted trichomes on their leaves. Trichomes are hair-like epidermal structures found on plant surfaces, they come in a variety of shapes and provide a range of functions on different plants. Because the trichomes of some bromeliads provide a direct route through which the plant can obtain water, they can be far less dependent on roots to do this for them and in some bromeliads roots have become little more than a means of attachment 1.



However, what really makes bromeliads stand out amongst the epiphytic crown of the neotropical rainforest is the biodiversity they contain. Most bromeliads have strap-like leaves which grow in a rosette arrangement around an empty centre. This space in the axis of the bromeliad collects rainwater or any debris making its way through the canopy and is called a phytotelm (plural phytotelmata). Phytotelmata can refer to any cavity within a plant that collects water or debris from the outside, whether it's a hole in a tree or the space between leaves. This rosette-and-tank arrangement is by no means unique to bromeliads, but combined with their unique water absorption strategies and their diversity in the Neotropics, it has made them a very successful and ecologically important group of plants.

Anatomy of a tank bromeliad
© The Biologist, Royal Society of Biology
In a rich and competitive environment like a rainforest, any and every still body of water is precious – and bromeliad phytotelmata are no exception. Consequently, rainforest bromeliads can harbour a diverse array of organisms within their phytotelmata. Usually, the most prominent group of organisms inhabiting bromeliads – as is the case in most environments – are insects. Many types of insects have an aquatic larval stage and as such need bodies of water in which to proliferate. Bromeliad phytotelmata provide relatively sheltered aquatic environments compared to streams or lakes and are full of nutrients from the falling debris they accumulate, making them a perfect nursery for things like hoverflies, stoneflies, and dipteran/true flies, including, perhaps unfortunately, mosquitoes. This buffet of juicy young invertebrate life makes it attractive to other, more formidable insect larvae, such as beetle larvae and caterpillars. Terrestrial crawlies creep around the phytotelm and between the dryer bracts of the bromeliads: stunning cockroaches and skittish woodlice, vegetarian millipedes and venomous centipedes, pseudoscorpions and scary scorpions, sneaky spiders and fluffy tarantulas, all take up temporary or permanent residence in these floating puddles in the canopy.

A massive tarantula found in the centre axis of a bromeliad phytotelm in Ecuadorian Amazon. © X O'Reilly Berkeley

A butterfly checks out some of the minute flowers within a bromeliad's flashy flower bracts in the Quito Botanical Gardens,
Ecuador. © X O'Reilly Berkeley

A baby arboreal tarantula tries to hide in the leaf bracts of a bromeliad, Ecuadorian Amazon. © X O'Reilly Berkeley

A tiny frog sits on a bromeliad leaf in the Ecuadorian cloudforest. © X O'Reilly Berkeley

Even the flower bracts of many bromeliads
can provide a home for an array of invertebrates.
Ecuadorian rainforest © X O'Reilly
Bromeliads not only provide a nursery for invertebrates, but can be home to small vertebrates, such as poisonous frogs and slinky salamanders. Some species of dart frog – such as the strawberry poison dart frog (Oophaga pumilio) – will carry tadpoles up into the canopy and deposit each individual tadpole in a separate bromeliad phytotelm. In Central America, there is an entire genus of salamanders known as the "bromeliad salamanders" for their association with the epiphytes.

The abundance of activity in bromeliads is said to attract larger insectivores which don't actually live in the plants, such as birds or even mammals – in the cloudforests of Ecuador, I have heard locals claim that the rare spectacled bear can at times be seen to forage for insects out of bromeliads! Unfortunately this has not been confirmed officially.

Bromeliads therefore provide a myriad of critters with food and shelter, but this relationship is by no means a one way street – bromeliads, too, benefit from their animal tenants. By harbouring so much animal life, bromeliads can overcome a challenge that all plants, but particularly epiphytes, face – nutrient limitation. In order to absorb and utilise the carbon obtained from photosynthesis, plants need to obtain nitrogen and phosphorus from the environment. These nutrients are often limiting factors for plant growth and are usually absorbed from the soil by a plant's roots. Without access to soil, epiphytes must obtain these essential nutrients by other means. Tank bromeliads get their nitrogen and phosphorus from a combination of falling debris, animal waste, and animal decay. Before the plant can utilise these animal-derived nutrients, however, they are broken down by yet another layer of biodiversity – microorganisms. Much like the soil, bromeliads harbour large numbers of decomposing bacteria and fungi. This means these epiphytes not only contain the macroscopic components of an ecosystem but also the nutrient cycling machinery to sustain it. They are microcosms.

Large bromeliads (and other epiphytes) coating an enormous tree branch in the Ecuadorian cloudforest. The longest of this individual bromeliad's leaves are around 2 metres in length and the flower bract tower far above them. © X O'Reilly
Many factors which are thought to influence the diversity and composition of bromeliad phytotelm communities – including bromeliad size, morphology, host tree, position in the canopy and genetic variation. It is not always entirely clear which factors are most important in structuring these communities, but one thing is for certain: they are very diverse assemblages. Taking into account the 3000+ species of bromeliads in the neotropics and the fact that many of them occur in megadiverse forests, it is easy to see their ecological significance. In addition to their ecological importance, bromeliads are a popular house plants – you may have one yourself or have seen one in a hotel lobby – and produce the much-loved pineapple that is now grown widely and consumed around the world. The enzyme bromelain, used as a meat tenderiser, also comes from bromeliads. Bromeliads are also proving to be enlightening research tools and subjects, with the recent sequencing of the pineapple genome shedding light upon the evolution of CAM photosynthesis 2 and the humble monocots proving effective mesocosms in which to study aquatic ecosystem processes 3.

Next time you look at your strap-leaved house plant or see a collection of bromeliads in a hotel lobby, you will know that these plants are valuable ecological, economic and research assets. You will know that in the wild, these plants cling to trees and play host to mini ecosystems of their own, creating diverse kingdoms in the canopy, without which the rainforest would be poorer.

Epiphyte-heavy secondary rainforest in Payamino, on the Ecuadorian side of the Amazon. © X O'Reilly Berkeley
Glossary
Epiphyte - An organism, usually a plant, that grows attached to the surface of a plant from which it takes advantage of structural support but does not derive its nutrition from its host.
Phytotelm [pl. phytotelmata] - A plant body that holds water.
CAM photosynthesis - A photosynthetic pathway which allows plants in hot and/or dry environments to   close their stomata (pores) during the day in order to avoid water loss due to evapotranspiration. Instead, stomata open at night to fix carbon dioxide and stored until the next day when sunlight can initiate the necessary reactions to convert carbon dioxide and water into sugars.

More bromeliad, epiphyte, and tree-climbing posts:
· Kingdoms in the Canopy (I): Biodiversity within Bromeliads
· Kingdoms in the Canopy (II): Who's Who Up a Tree?
· Kingdoms in the Canopy (III): Climbing trees

Cited literature

1.        Leroy, C., Carrias, J. F., Céréghino, R. & Corbara, B. The contribution of microorganisms and metazoans to mineral nutrition in bromeliads. J. Plant Ecol. 9, 241–255 (2016).
2.        Ming, R. et al. The pineapple genome and the evolution of CAM photosynthesis. Nat Genet 47, 1435–1442 (2015).

3.        Dézerald, O. et al. Environmental determinants of macroinvertebrate diversity in small water bodies: Insights from tank-bromeliads. Hydrobiologia 723, 77–86 (2014).

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