Emus + Ants = One Heck of a Seed Dispersal Strategy

A guest post by Dr. Scott Zona

The emu is a large, flightless bird, a cousin of kiwis and cassowaries. They range throughout much of Australia, favoring savannah woodlands and sclerophyll forests, where they are generalist feeders, consuming a variety of plants and arthropods. A favorite food of the emu is Petalostigma pubescens, a tree variously known as quinine tree, bitter bark or quinine berry. Petalostigma is in the Picrodendraceae, a family formerly included in the Euphorbiaceae. Quinine trees grow in the same open woodlands favored by emus.

The quinine tree bears yellow fruits, 2.0-2.5 cm in diameter, with a thin layer of flesh. The fruits are divided into six to eight segmented, like a tangerine, and each segment contains a hard endocarp or stone (technically, a pyrene). Each endocarp contains a single seed, 6-8 mm long. Left on the tree, the fruits will eventually dry up and open to release their seeds, but if ripe fruits are discovered by a hungry emu, the feasting begins.

A quinine tree (Petalostigma pubescens) in bloom. Photo by Ethel Aardvark licensed by CC BY 3.0

An emu may eat dozens of fruits in one meal. It swallows fruits whole, digesting the soft, fleshy part and defecating the hard, indigestible endocarps. On an average day, an emu can range over a large territory, spreading endocarps as it goes. In one of science's least glamorous moments, Australian biologists counted by hand as many as 142 endocarps in one emu dropping. If the story ended with Quinine Tree seeds in a pile of emu dung, we would say that the emu provided excellent seed dispersal services for the quinine tree, but the dispersal story is not over.

Quinine tree (Petalostigma pubescens) fruits. Photo by Robert Whyte licensed by CC BY-NC-ND 2.0

The emu dung and endocarps begin to bake in the hot, outback sun. As the endocarps dry, they explode. Just like the pod of a legume, the endocarp has fibers in its tissues oriented in opposing directions.  As the fibers dry, they contract and pull the endocarp apart. The dehiscence is sudden and explosive, sending seeds up to 2.5 m from the point of origin. Launching seeds away from the dung pile is beneficial to seeds: the special separation means that seedlings well be less likely to compete with one another.

But that is not the final disposition of Quinine Tree seeds. Each Petalostigma seed bears a small, oily food body, called an elaiosome, that is attractive to ants. Ants pick up the seed with its attached elaisome and carry it back to their nest. Once at the nest, the ants will remove and consume the elaisome and deposit the inedible seed in midden outside the nest. It is the ants that disperse the seeds to their ultimate site.

The association between emus, exploding endocarps, ants and Petalostigma pubescens probably represents one of the most complicated dispersal scenarios in the Plant Kingdom.

Photo Credits: [1] [2] [3]

Further Reading: [1]

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Of Bladderworts & Birds

Photo by Jean and Fred licensed by CC BY 2.0

Bladderworts are as beautiful as they are deadly. Though they are known the world over for their carnivorous bladder traps, their flowers are something to marvel at as well. Bladderworts flower in a range of colors from yellows to whites, purples to reds. What’s more, the variety of shapes and sizes among bladderwort flowers are incredible. Though the vast majority of bladderwort species rely on insects for pollination, at least one species appears to have co-opted a bird for its reproductive needs.

Red coats (Utricularia menziesii) are endemic to a few coastal regions of western Australia. They are not floating aquatic plants like many of their North American cousins, nor do they grow epiphytically like many tropical bladderworts. Red coats are terrestrial in their habit. Moreover, they live in habitats that dry up for good portions of the year. As the soils dry out, red coats die back into tiny corms in which they store energy during their dry dormancy that will fuel growth as soon as rains return and the surrounding soils are once again saturated.

Photo by Jean and Fred licensed by CC BY 2.0

When conditions are right, red coats produce some of the most spectacular flowers of the entire genus. Though other species also produce red flowers, few produce such outlandishly bright blossoms. Moreover, the flowers themselves are rather robust structures complete with a long, tough nectar spur. Their color, form, and proximity to the ground has led more than one author to suggest that birds, not insects, are the main pollinators of this species.

Indeed, it appears that birds are what these flowers are attracting. Not just any bird will do either. It seems that the western spinebill (Acanthorhynchus superciliosus) is wonderfully primed to pollinate this lovely little carnivore. Red is a major attractant for birds and the fact that red coat flowers are presented so close to ground level places at the perfect height for ground-foraging spinebills. Also, the length, curvature, and nectar content of the nectar spur fits the spinebill beak nicely. Birds approach the plants on the ground and dip their long, curved beaks into the flower, picking up and depositing pollen as they go.

The western spinebill (Acanthorhynchus superciliosus). Note the curved beak. Photo by Jean and Fred licensed by CC BY 2.0

This isn’t the only bladderwort to be suspected of bird pollination. At least two others (Utricularia quelchii & Utricularia campbelliana) have been hypothesized to utilize hummingbirds for pollination. However, there is scant evidence for this. Pollination studies can be tricky like that. Without proper observation and study, one simply can’t confirm a particular pollination syndrome.

Photo Credits: [1] [2] [3]

Further Reading: [1]

Meet the Pygmy Clubmoss

Photo by Leon Perrie licensed under CC BY 4.0

No, these are not some sort of grass or rush. What you are looking at here is actually a member of the clubmoss family (Lycopodiaceae). Colloquially known as the pygmy clubmoss, this odd little plant is the only species in its genus - Phylloglossum drummondii. Despite its peculiar nature, very little is known about it.

The pygmy clubmoss is native to parts of Australia, Tasmania, and New Zealand but common it is not. From what I can gather, it grows in scattered coastal and lowland sites where regular fires clear the ground of competing vegetation. It is a perennial plant that makes its appearance around July and reaches reproductive size around August through to October.

Reproduction for the pygmy clubmoss is what you would expect from this family. In dividual plants will produce a reproductive stem that is tipped with a cone-like structure. This cone houses the spores, which are dispersed by wind. If a spore lands in a suitable spot, it germinates into a tiny gametophyte. As you can probably imagine, the gametophyte is small and hard to locate. Indeed, little is known about this part of its life cycle. Nonetheless, like all gametophytes, the end goal of this stage is sexual reproduction. Sperm are released and with any luck will find a female gametophyte and fertilize the ovules within. From the fertilized ovule emerges the sporophytes we see pictured above.

As dormancy approaches, this strange clubmoss retreats underground where it persists as a tiny tuber-like stem. Though it is rather obscure no matter who you ask, there has been some scientific attention paid to this odd little plant, especially as it relates to its position on the tree of life. Since it was first described, its taxonomic affinity has moved around a bit. Early debates seemed to center around whether it belonged in Lycopodiaceae or its own family, Phylloglossaceae.

Recent molecular work put this to rest showing that genetically the pygmy clubmoss is most closely related to another genus of clubmoss - Huperzia. This was bolstered by the fact that it shares a lot of features with this group such as spore morphology, phytochemistry, and chromosome number. The biggest difference between these two genera is the development of the pygmy clubmoss tuber, which is unique to this species. However, even this seems to have its roots in Lycopodiaceae.

If you look closely at the development of some lycopods, it becomes apparent that the pygmy clubmoss most closely resembles an early stage of development called the “protocorm.” Protocorms are a tuberous mass of cells that is the embryonic form of clubmosses (as well as orchids). Essentially, the pygmy clubmoss is so similar to the protocorm of some lycopods that some experts actually think of it as a permanent protocorm capable of sexual reproduction. Quite amazing if you ask me.

Sadly, because of its obscurity, many feel this plant may be approaching endangered status. There have been notable declines in population size throughout its range thanks to things like conversion of its habitat to farmland, over-collection for both novelty and scientific purposes, and sequestration of life-giving fires. As mentioned, the pygmy clubmoss needs fire. Without it, natural vegetative succession quickly crowds out these delicate little plants. Hopefully more attention coupled with better land management can save this odd clubmoss from going the way of its Carboniferous relatives.

Further Reading: [1] [2] [3] [4] [5] [6]

The Grasstree of Southwestern Australia

Taken by John O'Neill licensed under CC BY-SA 3.0

Southwestern Australia is home to a wonderful and unique flora. A combination of highly diverse, nutrient-poor soil types, bush fires, and lots of time have led to amazing adaptive radiations, the result of which are myriad plant species found nowhere else in the world. One of the most incredible members of southwestern Australia's flora is the grassplant (Kingia australis). Like all plants of this region, it is one hardy species.

The taxonomic history of the grassplant has been a bit muddled. As its common name suggests, it was once thought to be a type of grasstree (genus Xanthorrhoea), however, its resemblance to this group is entirely superficial. It has since been placed in the family Dasypogonaceae. Along with three other genera, this entire family is endemic to Australia. Growing in southwestern Australia presents lots of challenges such as obtaining enough water and nutrients to survive and for the grassplant, these have been overcome in some fascinating ways.

The way in which the grassplant manages this is incredible. Its trunk is not really a true trunk but rather a dense cluster of old leaf bases. Within this pseudotrunk, the grassplant grows a series of fine roots. Research has shown this to be an adaptation to life in a harsh climate. Because water can be scarce and nutrients are in short supply, the grassplant doesn't take any chances. Water hitting the trunk is rapidly absorbed by these roots as are any nutrients that come in the form of things like bird droppings.

Photo by Casliber licensed under CC BY-SA 3.0

Coupled with its underground roots, the grassplant is able to eek out a living in this dry and impoverished landscape. That being said, its life is spent in the slow lane. Plants are very slow growing and estimates place some of the larger individuals at over 600 years in age. Its amazing how some of the harshest environments can produce some of the longest lived organisms.

As you can probably imagine, reproduction in this species can also be a bit of a challenge. Every so often, flower clusters are produced atop long, curved stems. Their production is stimulated by fire but even then, with nutrients in poor supply, it is not a frequent event. Some plants have been growing for over 200 years without ever producing flowers. This lifestyle makes the grassplant sensitive to disturbance. Recruitment is limited, even in good flowering years and plants take a long time to mature. That is why conservation of their habitat is of utmost importance.

Photo Credits: [1] [2]

Further Reading: [1] [2]

An Underground Orchid

Photo by Jean and Fred licensed under CC BY 2.0

Are you ready to have your mind blown away? What you are looking at here is not some strange kind of mushroom, though fungus is involved. What you are seeing is actually the inflorescence of a parasitic orchid from Australia that lives and blooms underground!

Meet Rhizanthella gardneri. This strange little orchid is endemic to Western Australia and it lives, blooms, and sets seed entirely underground. It is extremely rare, with only 6 known populations. Fewer than 50 mature plants are known to exist. This is another one of those tricky orchids that does not photosynthesize but, instead, parasitizes a fungus that is mycorrhizal with the broom honey myrtle (Melaleuca uncinata). To date, the orchid has only been found under that specific species of shrub. Because of its incredibly unique requirements, its limited range, and habitat destruction, R. gardneri is critically endangered.

The flowers open up a few centimeters under the soil. They are quite fragrant and it is believed that ants, termites, and beetles are the main pollinators. The resulting seeds take up to 6 months to mature and are quite fleshy. It is hypothesized that some sort of small marsupial eats them and consequently distributes them in its droppings. Either way, the chances of successful sexual reproduction for this species are quite low. Because of this, R. gardneri also reproduces asexually by budding off daughter plants.

Despite not photosynthesizing, this orchid is quite unique in that it still retains chloroplasts in its cells. They are a very stripped down form of chloroplast though, containing about half of the genes a normal chloroplast would. It is the smallest known chloroplast genome on the planet. This offers researchers a unique opportunity to look deeper into how these intracellular relationships function. The remaining chloroplast genes code for 4 essential plant proteins, meaning chloroplasts offer functions beyond just photosynthesis.

I am so amazed by this species. I'm having a hard time keeping my jaw off the ground. What an amazing world we live in. If you would like to see more pictures of R. gardneri, please make sure to check out the following website:
http://www.arkive.org/underground-orchid/rhizanthella-gardneri/

Photo Credit: Jean and Fred Hort

Further Reading:
http://www.sciencedaily.com/releases/2011/02/110208101337.htm

http://www.eurekalert.org/pub_releases/2011-02/uowa-wai020711.php

http://www.environment.gov.au/cgi-bin/sprat/public/publicspecies.pl?taxon_id=20109