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]

NOTE: Guest posts are invite only

Daffodil Insights

Photo by Amanda Slater licensed under CC BY-SA 2.0

Daffodils seem to be everywhere. Their horticultural popularity means that, for many of us, these plants are among the first flowers we see each spring. Daffodils are so commonplace that it's as if they evolved to live in our gardens and nowhere else. Indeed, daffodils have had a long, long history with human civilization, so much so that it is hard to say when our species first started to cohabitate. Our familiarity with these plants belies an intriguing natural history. What follows is a brief overview of the world of daffodils. 

If you are like me, then you may have gone through most of your life not noticing much difference between garden variety daffodils. Though many of us will be familiar with only a handful of daffodil species and cultivars, these introductions barely scratch the surface. One may be surprised to learn that as of 2008, more than 28,000 daffodil varieties have been named and that number continues to grow each and every year. Even outside of the garden, there is some serious debate over the number of daffodil species, much of this having to do with what constitutes a species in this group.

Narcissus poeticus

As I write this, all daffodils fall under the genus Narcissus. Estimates as to the number of species within Narcissus range from as few as 50 to as many as 80. The genus itself sits within the family Amaryllidaceae and is believed to have originated somewhere between the late Oligocene and early Miocene, some 18 to 30 million years ago. Despite its current global distribution, Narcissus are largely Mediterranean plants, with peak diversity occurring on the Iberian Peninsula. However, thanks to the aforementioned long and complicated history in cultivation, it has become quite difficult to understand the full range of diversity in form and habitat of many species. To understand this, we first need to understand a bit about their reproductive habits.

Much of the evolution of Narcissus seems to center around floral morphology and geographic isolation. More specifically, the length of the floral tube or "corona" and the position of the sexual organs within, dictates just who can effectively pollinate these plants. The corona itself is not made up of petals or sepals but instead, its tube-like appearance is due to a fusion of the stamens into the famous trumpet-like tube we know and love.

Variation in corona shape and size has led to the evolution of three major pollination strategies within this genus. The first form is the daffodil form, whose stigma is situated at the mouth of the corolla, well beyond the 6 anthers. This form is largely pollinated by larger bees. The second form is the paperwhite form, whose stigma is situated more closely to or completely below the anthers at the mouth of the corona. This form is largely pollinated by various Lepidoptera as well as long tongued bees and flies. The third form is the triandrus form, which exhibits three distinct variations on stigma and anther length, all of which are situated deep within the long, narrow corona. The pendant presentation of the flowers in this group is thought to restrict various butterflies and moths from entering the flower in favor of bees.

Narcissus tazetta. Photo by Fanghong licensed under CC BY-SA 3.0

The variations on these themes has led to much reproductive isolation among various Narcissus populations. Plants that enable one type of pollinator usually do so at the exclusion of others. Reproductive isolation plus geographic isolation brought on by differences in soil types, habitat types, and altitudinal preferences is thought to have led to a rapid radiation of these plants across the Mediterranean. All of this has gotten extremely complicated ever since humans first took a fancy to these bulbs.

Narcissus cyclamineus. Photo by Francine Riez licensed under CC BY-SA 3.0

Reproductive isolation is not perfect in these plants and natural hybrid zones do exist where the ranges of two species overlap. However, hybridization is made much easier with the helping hand of humans. Whether via landscape disturbance or direct intervention, human activity has caused an uptick in Narcissus hybridization. For centuries, we have been mixing these plants and moving them around with little to no record as to where they originated. What's more, populations frequently thought of as native are actually nothing more than naturalized individuals from ancient, long-forgotten introductions. For instance, Narcissus populations in places like China, Japan, and even Great Britain originated in this manner.

All of this mixing, matching, and hybridizing lends to some serious difficulty in delineating species boundaries. It would totally be within the bounds of reason to ask if some of the what we think of as species represent true species or simply geographic varieties on the path to further speciation. This, however, is largely speculative and will require much deeper dives into Narcissus phylogenetics.

Narcissus triandrus. Photo by Dave Gough licensed under CC BY 2.0

Despite all of the confusion surrounding accurate Narcissus taxonomy, there are in fact plenty of true species worth getting to know. These range in form and habit far more than one would expect from horticulture. There are large Narcissus and small Narcissus. There are Narcissus with yellow flowers and Narcissus with white flowers. Some species produce upright flowers and some produce pendant flowers. There are even a handful of fall-blooming Narcissus. The variety of this genus is staggering if you are not prepared for it.

Narcissus viridiflorus - a green, fall-blooming daffodil. Photo by A. Barra licensed under CC BY 3.0

After pollination, the various Narcissus employ a seed dispersal strategy that doesn't get talked about enough in reference to this group. Attached to each hard, black seed are fatty structures known as eliasomes. Eliasomes attract ants. Like many spring flowering plant species around the globe, Narcissus utilize ants as seed dispersers. Ants pick up the seeds and bring them back to their nests. They go about removing the eliasomes and then discard the seed. The seed, safely tucked away in a nutrient-rich ant midden, has a much higher chance of germination and survival than if things were left up to simple chance. It remains to be seen whether or not Narcissus obtain similar seed dispersal benefits from ants outside of their native range. Certainly Narcissus populations persist and naturalize readily, however, I am not aware if ants have any part in the matter.

The endangered Narcissus alcaracensis. Photo by José Luis López González licensed under CC BY-SA 4.0

Despite their popularity in the garden, many Narcissus are having a hard go of it in the wild. Habitat destruction, climate change, and rampant collecting of wild bulbs are having serious impacts on Narcissus numbers. The IUCN considered at least 5 species to be endangered and a handful of some of the smaller species already thought to be extinct in the wild. In response to some of these issues, protected areas have been established that encompass at least some of the healthy populations that remain for some of these species.

If you are anything like me, you have ignored Narcissus for far too long. Sure, they aren't native to the continent on which I live, and sure, they are one of the most commonly used plants in a garden setting, but every species has a story to tell. I hope that, armed with this new knowledge, you at least take a second look at the Narcissus popping up around your neighborhood. More importantly, I hope this introduction makes you appreciate their wild origins and the fact that we still have much to learn about these plants. I have barely scratched the surface of this genus and there is more more information out there worth perusing. Finally, I hope we can do better for the wild progenitors of our favorite garden plants. They need all the help they can get and unless we start speaking up and working to preserve wild spaces, all that will remain are what we have in our gardens and that is not a future I want to be a part of.

Photo Credits: [1] [2] [3] [4] [5] [6] [7]

Further Reading: [1] [2] [3] [4] [5] [6] [7] [8] [9]

The Fetid Adderstongue

Photo by Ken-ichi Ueda licensed under CC BY-NC 2.0

"Fetid adderstongue" seems like a pretty ominous name for such a small and beautiful plant. Hailing from coastal North America, the genus Scoliopus is most at home in the deep shaded forests of California and Oregon. Spring is the best time to see these little lilies and once you know a little bit about their ecology, such encounters are made all the more interesting.

There are two species nestled within this genus - S. bigelovii and S. hallii. Both are similar in that they are plants of deep shaded environments, however, you are more likely to find S. hallii growing along the banks of wooded streams. As is typical of many members of the lily family, their flowers are quite beautiful in appearance. The trick is finding them. Though showy, they are rather small and their dark coloration causes them to blend in well in their shaded environments. That is all fine and dandy for a species that relies more on smell rather than looks to attract pollinators.

As the common name suggests, the flowers of the fetid adderstongues give off a bit of an odor. I have heard it best described as "musty." The flowers of these two species attract a lot of fungus gnats. Although these tiny flies are generally viewed as sub par pollinators for most flowering plants, the fetid adderstongues seem to do well with them. What they lack in robust pollination behavior, they make up for in sheer numbers. There are a lot of fungus gnats hanging around wet, shaded forests.

Photo by Eric in SF licensed under CC BY-SA 3.0

The flowers themselves are borne on tall stalks. Though they look separate, they are actually an extension of a large, underground umbel. Once pollination has been achieved, the flower stalks begin to bend over, putting the developing ovaries much closer to the ground. Each seed comes equip with a fleshy little attachment called an eliasome. These are essentially ant bait. Once mature, the seeds are released near the base of the parent. Hungry ants that are out foraging find the fleshy attachment much to their liking.

They bring the seeds back to the nest, remove the eliasomes, and discard the seed into a trash midden. Inside the ant nest, seeds are well protected, surrounded by nutrient-rich compost, and as some evidence is starting to suggest, guarded against damaging fungal invaders. In other words, the plants have tricked ants into planting their seeds for them. This is a very successful strategy that is adopted by many different plant species the world over.

Though small, the fetid adderstongues are two plants with a lot of character. They are definitely a group that you want to keep an eye out for the next time you find yourself in the forests of western North America. If you do end up finding some, just take some time to think of all the interesting ecological interactions these small lilies maintain.

Photo Credits: [1] [2]

Further Reading: [1] [2] [3]