The Flight of the Dandelion

The common dandelion (Taraxacum officinale) comes with a collection of traits that make it a very successful weed. Nearly everything about it screams success, from its asexually produced seeds to its ability to resprout from a root fragment. Evolution has been kind to this plant, and up until the recent chemical warfare we’ve subjected it to, humans have treated it pretty well too (both intentionally and unintentionally).

One feature that has served the dandelion particularly well is its wind-dispersed seeds. Dandelions have a highly-evolved pappus – a parachute-like bristle of hairs attached to its fruit by a thin stalk. The slightest breath or puff of wind will send this apparatus flying. Once airborne, a dandelion’s seed can travel up to a kilometer or more away from its mother plant, thereby expanding its territory with ease.

Such a low-growing plant achieving this kind of distance is impressive. Even more impressive is that it manages to do this with a pappus that is 90% empty space. Would you leap from a plane with only 10% of a parachute?

Dandelion flight was investigated by researchers at the University of Edinburgh, who used a wind tunnel along with long-exposure photography and high-speed imaging to observe the floating pappus. Their research was presented in a letter published in an issue of Nature in October 2018. Upon close examination, they observed a stable air bubble floating above the pappus as it flew. This ring-shaped air bubble – or vortex – which is unattached to the pappus is known as a separated vortex ring. While this type of vortex ring had been considered theoretically, this marked the first time one had been observed in nature.

Seeing this type of air bubble associated with the dandelion’s pappus intrigued the researchers. About a 100 filaments make up the parachute portion of the pappus. They are arranged around the stalk, leaving heaps of blank space in between. The air bubble observed was not what was expected for such a porous object. However, the researchers found that the filaments were interacting with each other in flight, reducing the porosity of the pappus. In their words, “Neighboring filaments interact strongly with one another because of the thick boundary layer around each filament, which causes a considerable reduction in air flow through the pappus.”

The pappus acts as a circular disk even though it is not one, and its limited porosity allows just enough air movement through the filaments that it maintains this unique vortex. “This suggests,” the researchers write, “that evolution has tuned the pappus porosity to eliminate vortex shedding as the seed flies.” Fine-tuned porosity and the resultant unattached air bubble stabilizes the floating fruit “into an equilibrium orientation that minimizes [its] terminal velocity, allowing [it] to make maximal use of updrafts.” The result is stable, long distance flight.

Wind-dispersed seeds come in two main forms: winged and plumed. Winged seeds are common in trees and large shrubs. They benefit from the height of the tree which allows them to attain stable flight. While such seeds have the ability to travel long distances, their success is limited on shorter plants. In this case, plumed seeds, like those of the dandelion, are the way to go. As the researchers demonstrated, successful flight can be achieved by bristles in place of wings. The tiny seeds of dandelions seen floating by on a summer breeze are not tumbling through the air haphazardly; rather, they are flying steadily, on their way to spoil the dreams of a perfect lawn.

Further Reading (and Watching):

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Investigating the Soil Seed Bank

Near the top of the world, deep inside a snow-covered mountain located on a Norwegian island, a vault houses nearly a million packets of seeds sent in from around the world. The purpose of the Svalbard Global Seed Vault is to maintain collections of crop seeds to ensure that these important species and varieties are not lost to neglect or catastrophe. In this way, our food supply is made more secure, buffered against the unpredictability of the future. Seed banks like this can be found around the world and are essential resources for plant conservation. While some, like Svalbard, are in the business of preserving crop species, others, like the Millennium Seed Bank, are focused on preserving seeds of plants found in the wild.

Svalbard Global Seed Vault via wikimedida commons

Outside of human-built seed banks, many plants maintain their own seed banks in the soil where they grow. This is the soil seed bank, a term that refers to either a collection of seeds from numerous plant species or, simply, the seeds of a single species. All seed bearing plants pass through a period as a seed waiting for the chance to germinate. Some do this quickly, as soon as the opportunity arises, while others wait, sometimes for many years, before germinating. Plants whose seeds germinate quickly, generally do not maintain a seed bank. However, seeds that don’t germinate right away and become incorporated in the soil make up what is known as a persistent soil seed bank.

A seed is a tiny plant encased in a protective layer. Germination is not the birth of a plant; rather, the plant was born when the seed was formed. The dispersal of seeds is both a spatial and temporal phenomenon. First the seed gets to where its going via wind, water, gravity, animal assistance, or some other means. Then it waits for a good opportunity to sprout. A seed lying in wait in the soil seed bank is an example of dispersal through time. Years can pass before the seed germinates, and when it does, the plant joins the above ground plant community.

Because seeds are living plants, seeds found in the soil seed bank are members of a plant community, even though they are virtually invisible and hard to account for. Often, the above ground plant community does not represent the population of seeds found in the soil below. Conversely, seeds in a seed bank may not be representative of the plants growing above them. This is because, as mentioned earlier, not all plant species maintain soil seed banks, and those that do have differences in how long their seeds remain viable. Depending on which stage of ecological succession the plant community is in, the collection of seeds below and the plants growing above can look quite different.

Soil seed banks are difficult to study. The only way to know what is truly there is to dig up the soil and either extract all the seeds or encourage them to germinate. Thanks to ecologists like Ken Thompson, who have studied seed banks extensively for many years, there is still a lot we can say about them. First, for the seeds of a plant to persist in the soil, they must become incorporated. Few seeds can bury themselves, so those with traits that make it easy for them to slip down through the soil will have a greater chance of being buried. Thompson’s studies have shown that “persistent seeds tend to be small and compact, while short-lived seeds are normally larger and either flattened or elongate.” Persistent seeds generally weigh less than 3 milligrams and tend to lack appendages like awns that can prevent them from working their way into the soil.

The seeds of moth mullein (Verbascum blattaria) are tiny and compact and known to persist in the soil for decades as revealed in Dr. Beal’s seed viability experiment. (photo credit: wikimedia commons)

Slipping into cracks in the soil is a major way seeds move through the soil profile, but it isn’t the only way. In a study published in New Phytologist, Thompson suggests that “the association between small seeds and possession of a seed bank owes much to the activities of earthworms,” who ingest seeds at the surface and deposit them underground. Later, they may even bring them back up the same way. Ants also play a role in seed burial, as well as humans and their various activities. Some seeds, like those of Avena fatua and Erodium spp., have specialized appendages that actually help work the seeds into the soil.

Not remaining on the soil surface keeps seeds from either germinating, being eating, or being transported away to another site. Avoiding these things, they become part of the soil seed bank. But burial is only part of the story. In an article published in Functional Ecology, Thompson et al. state that burial is “an essential prelude to persistence,” but other factors like “germination requirements, dormancy mechanisms, and resistance to pathogens also contribute to persistence.” If a buried seed rots away or germinates too early, its days as a member of the soil seed bank are cut short.

The seeds of redstem filare (Erodium circutarium) have long awns that start out straight, then coil up, straighten out, and coil up again with changes in humidity. This action helps drill the seeds into the soil. (photo credit: wikimedia commons)

Soil seed banks can be found wherever plants are found – from natural areas to agricultural fields, and even in our own backyards. Thompson and others carried out a study of the soil seed banks of backyard gardens in Sheffield, UK. They collected 6 soil cores each (down to 10 centimeters deep) from 56 different gardens, and grew out the seeds found in each core to identify them. Most of the seeds recovered were from species known to have persistent seed banks, and to no surprise, the seed banks were dominated by short-lived, weedy species. The seeds were also found to be fairly evenly distributed throughout the soil cores. On this note, Thompson et al. remarked that due to “the highly disturbed nature of most gardens, regular cultivation probably ensures that seeds rapidly become distributed throughout the top 10 centimeters of soil.”

Like the seed banks we build to preserve plant species for the future, soil seed banks are an essential long-term survival strategy for many plant species. They are also an important consideration when it comes to managing weeds, which is something we will get into in a future post.

Seed Oddities: Vivipary

Seeds house and protect infant plants. When released from their parent plant, they commence a journey that, if successful, will bring them to a suitable location where they can take up residence (upon germination) and carry out a life similar to that of their parents. Their seed coats (and often – in the case of angiosperms – the fruits they were born in) help direct them and protect them in this journey. Physical and chemical factors inhibit them from germinating prematurely – a phenomenon known as dormancy. Agents of dispersal and mechanisms of dormancy allow seeds to travel through time and space — promises of new plants yet to be realized.

There is rarely a need for a seed to germinate immediately upon reaching maturity. In many cases, such as in temperate climates or in times of drought or low light, germinating too soon could be detrimental. The most vulnerable time in a plant’s life comes when it is a young seedling. Thus, finding the right time and space to get a good start is imperative.

The fruits (and accompanying seeds) of doubleclaw (Proboscidea parviflora) are well equipped for long distance dispersal. (via wikimedia commons)

In rare instances, dispersal via seeds offers little advantage; instead, dispersal of live seedlings or propagules is preferable. For this select group of plants, vivipary is part of the reproductive strategy. In vivipary, seeds lack dormancy. Rather than waiting to be dispersed before germinating, viviparous seeds germinate inside of fruits that are still attached to their parent plants.

Occasionally, seeds are observed germinating inside tomatoes, citrus, squash, and other fruits; however, these fruits are usually overripe and often detached from the plant. In these instances, what is referred to as “vivipary” is not a genetic predisposition or part of the reproductive strategy. It’s just happenstance – a fun anomaly. The type of vivipary discussed in this post is actually quite rare, occurring in only a handful of species and prevalent in a select number of environments.

There are three main types of vivipary: true vivipary, cryptovivipary, and pseudovivipary. In true vivipary, a seed germinates inside the fruit and pushes through the fruit wall before the fruit is released. In cryptovivipary, a seed germinates inside the fruit but remains inside until after the fruit drops or splits open. Pseudovivipary is the production of bulbils or plantlets in the flower head. It does not involve seeds and is, instead, a form of asexual reproduction that will be discussed in a future post.

True vivipary is commonly seen among plant communities located in shallow, marine habitats in tropical or subtropical regions, such as mangroves or seagrasses. The term mangrove is used generally to describe a community of plants found in coastal areas growing in saline or brackish water. It also refers more specifically to the small trees and shrubs found in such environments. While not all mangrove species are viviparous, many of them are.

Seedlings of viviparous mangrove species emerge from the fruit and drop from the plant into the salty water below. From there they have the potential to float long or short distances before taking root. They may land in the soil upright, but often, as the tide recedes, they find themselves lying horizontally on the soil. Luckily, they have the remarkable ability to take root and quickly stand themselves up. Doing this allows young plants to keep their “heads” above water as the tides return. It also helps protect the shoot tips from herbivory.

Viviparous seedlings emerging from the fruits of red mangrove (Rhizophora mangle) via wikimedia commons

Another example of vivipary is found in the epiphytic cactus (and close relative of tan hua), Epiphyllum phyllanthus. Commonly known as climbing cactus, this species was studied by researchers in Brazil who harvested fruits at various stages to observe the development of the viviparous seedlings. They then planted the seedlings on three different substrates to evaluate their survival and establishment.

Epiphyllum phyllanthus is cryptoviviparous, so the germinated seeds don’t leave the fruit until after it splits open. In a sense, the mother plant is caring for her offspring before sending them out into the world. The researchers see this as “a form of parental care with subsequent conspecific [belonging to the same species] nursing.” Since the plant is epiphytic – meaning that it grows on the surface of another plant rather than in the soil – local dispersal is important, since there is no guarantee that seeds or propagules dispersed away from the host plant will find another suitable site. That being said, the researchers believe that “vivipary involves adaptation to local dispersal,” since “the greater the dispersal distance is, the higher the risk and the lower the probability of optimal dispersion.”

Epiphyllum phyllanthus via Useful Tropical Plants

While some viviparous seedlings of mangroves can travel long distances from their parent plant and don’t always root into the ground immediately, they maintain their advantage over seeds because they can root in quickly upon reaching a suitable site and lift themselves up above rising tide waters. As the authors of the Epiphyllum study put it, vivipary is “a reproductive advantage that, in addition to allowing propagules to root and grow almost immediately, favors quick establishment whenever seedlings land on suitable substrates.”

There is still much to learn about this unusual and rare botanical feature. The research that does exist is relatively scant, so it will be interesting to see what more we can discover. For now, check out the following resources:

Also, check out this You Tube video of :

The Seed Salting Experiments of Charles Darwin

In the second chapter of his book, The Diversity of Life, Edward O. Wilson describes the massive volcano that sunk a large portion of the island Krakatau in the summer of 1883. Rakata, the remnant that remained, was now “a sterile island” covered in ash. But it didn’t remain sterile for long. This natural disaster offered biologists the opportunity to watch as a fragment of earth, suddenly stripped of life, turned green again.

Life returned pretty quickly, too. In less than 50 years, nearly 300 species of plants had recolonized the charred landscape. Much of this rebirth was thanks to “aeolian plankton” – tiny wind-borne lifeforms that Wilson describes as “a rain of planktonic bacteria, fungus spores, small seeds, insects, spiders, and other small creatures” that fall “continuously on most parts of the earth’s land surface.” The seeds of some plants likely floated or “rafted” over, and still others may have arrived in the stomachs of birds “to be deposited later in their feces.”

Wind, water, and wing. It is well-accepted today that these are natural means by which the seeds of plants make their way to remote islands. However, in Charles Darwin’s day, things were not so settled. Decades before we understood things like plate tectonics and continental drift, there was ongoing debate about how the flora and fauna residing on islands got there. Were there multiple creation events or were there a series of land bridges and continental extensions now sunken in the sea? Unconvinced of one and skeptical of the other, Darwin embarked on a series of experiments to determine the possibility of an alternate hypothesis: long-distance dispersal.

Darwin wasn’t completely opposed to the idea that some species may have reached remote islands by land bridges of some sort; however, as James T. Costa writes in Darwin’s Backyard, his “imagination [ran] wild with scenarios for long-distance transport by floods and currents, whirlwinds and hurricanes, dispersal by birds, rafting quadrupeds carrying seeds in their stomachs or adhering to their fur, floating trees with seeds wedged in root masses, insects with seeds or eggs stuck to their legs, icebergs, and more.” He was convinced, “improbable as it was that, aided by wing or wave, propagules from a mainland could make it to distant islands.” After all, the vastness of geological time allows for chance events despite how improbable they may be. Even more, such events are “testable.”

So test them, he did. Among a series of experiments regarding long-distance dispersal were Darwin’s extensive seed salting trials. He began by using common vegetable seeds: broccoli, cabbage, oats, radish, lettuce, flax, and many others. He placed seeds in small bottles containing 2-3 ounces of salt water. Some bottles were placed outside in the shade where the air temperature varied throughout the day; other bottles were kept in his cellar where the temperature was more stable. He also placed seeds in a tank of salt water made with melted snow. The water in some of the jars, particularly those with brassica and onion seeds, turned foul, and as Darwin writes, “smelt offensive to a quite surprising degree;” however, “neither the putridity of the water nor the changing temperature had any marked effect on their vitality.”

In fact, while a few did quite poorly, the majority of the seeds that Darwin tested germinated just fine after soaking in salt water. At least for a short period anyway. Germination rates tended to decrease dramatically the longer seeds were soaked. For example, “fresh seed of the wild cabbage from Tenby germinated excellently after 50 days, very well after 110 days, and two seeds out of some hundreds germinated after 133 days immersion.” Darwin found that capsicum (i.e. peppers) “endured the trial best, for 30 out of 56 seeds germinated well after 137 days immersion.”

The seeds and dried fruit of Capsicum annuum (via wikimidia commons)

Darwin’s seed salting experiments seemed to be going well until his friend and colleague, Joseph Hooker, pointed out that seeds often sink when placed in water. Darwin wondered if he had been “taking all this trouble in salting the ungrateful rascals for nothing.” Despite the setback, he began another series of tests to determine which seeds sink, which float, and how long they float before they ultimately sink. The results weren’t as bad as expected. A number of species floated for several days, including the seeds of asparagus which were found to float for about 23 days if the seeds were fresh and up to 86 days if they were dried. By his calculations then, ocean currents could carry asparagus seeds over 2800 miles.

While soaking seeds in salt water, Darwin was engaged in a number of other seed dispersal studies, some quite bizarre. In one, he attempted to get goldfish to take mouthfuls of seeds, the idea being that if a fish having recently swallowed seeds was eaten by a seabird which then deposited the undigested seeds on a distant island, those seeds could germinate and establish themselves in a new environment. Unfortunately, Darwin’s subjects wouldn’t oblige: “the fish ejected vehemently, and with disgust equal to my own, all the seeds from their mouths.”

Despite a few botched experiments, Darwin turned out to be correct – long-distance dispersal explains much of the geographical distribution of species. Those who favored ideas of sunken land-bridges and continental extensions weren’t altogether wrong either. Costa writes: “Ironically, there is a kernel of truth behind the old idea of continental extensionism: rearranged and sometime contiguous continents…explain the distribution of some groups. But chance long-distance dispersal has never gone away. Improbable and rare as such events are, they are far from mysterious, and certainly not miraculous.”

Want to carry out your own seed salting experiments?

Darwin’s Backyard by James T. Costa includes detailed instructions, along with instructions for Darwin’s duck feet experiment [Do ducks transport snails, seeds, or other things that get attached to their feet?] and many others. Darwin Correspondence Project is a great resource as well.

Seed Dispersal via Caching – The Story of Antelope Bitterbrush

Generally speaking, individual plants produce an enormous amount of seeds. This may seem like a huge waste of resources, but the reality is that while each seed has the potential to grow into an adult plant that will one day produce seeds of its own, relatively few may achieve this. Some seeds will be eaten before they get a chance to germinate. Others germinate and soon die from lack of water, disease, or herbivory. Those that make it past the seedling stage continue to face similar pressures. Reaching adulthood, then, is a remarkable achievement.

Antelope bitterbrush is a shrub that produces hundreds of seeds per individual. Each seed is about the size of an apple seed. Some seeds may be eaten right away. Others fall to the ground and are ignored. But a large number are collected by rodents and either stored in burrows (larder hoarding) or in shallow depressions in the soil (scatter hoarding). It is through caching that antelope bitterbrush seeds are best dispersed. When rodents fail to return to caches during the winter, the seeds are free to sprout in the spring. Some of the seedlings will dry out and others will be eaten, but a few will survive, making the effort to produce all those seeds worth it in the end.

Fruits forming on antelope bitterbrush (Purshia tridentata)

Antelope bitterbrush (Purshia tridentata) is in the rose family and is often simply referred to as bitterbrush. It occurs in grasslands, shrub steppes, and dry woodlands throughout large sections of western North America. It is a deciduous shrub that generally reaches between three and nine feet tall but can grow up to twelve feet. It has wedge-shaped leaves that are green on top, grayish on bottom, and three-lobed. Flowers are yellow, strongly fragrant, and similar in appearance to others in the rose family. Flowering occurs mid-spring to early summer. Fruits are achenes – single seeds surrounded by papery or leathery coverings. The covering must rot away or be removed by animals before the seed can germinate.

Bitterbrush is an important species for wildlife. It is browsed by mule deer, pronghorn antelope, bighorn sheep, and other ungulates, including livestock. It provides cover for birds, rodents, reptiles, and ungulates. Its seeds are collected by harvester ants and rodents, its foliage is consumed by tent caterpillars and other insects, and its flowers are visited by a suite of pollinators. For all that it offers to the animal kingdom, it also relies on it for pollination and seed dispersal. The flowers of bitterbrush are self-incompatible, and if it wasn’t for ants and rodents, the heavy seeds – left to rely on wind and gravity – would have trouble getting any further than just a few feet from the parent plant.

Antelope bitterbrush (Purshia tridentata) in full bloom – photo credit: wikimedia commons

In a study published in The American Naturalist (February 1993), Stephen Vander Wall reported that yellow pine chipmunks were the primary dispersal agents of bitterbrush seeds in his Sierra Nevada study area. The optimal depth for seedling establishment was between 10-30 millimeters. Seeds that are cached too near the surface risk being pushed out of the ground during freeze and thaw cycles where they can desiccate upon germination. Cached bitterbrush seeds benefit when there are several seeds per cache because, as Vander Wall notes, “clumps of seedlings are better able to push through the soil and can establish from greater depths than single seedlings.”

Another study by Vander Wall, published in Ecology (October 1994), reiterated the importance of seed caching by yellow pine chipmunks in the establishment of bitterbrush seedlings. Seed caches, which consisted of anywhere from two to over a hundred seeds, were located as far as 25 meters from the parent plant. Cached seeds are occasionally moved to another location, but Vander Wall found that even these secondary caches produce seedlings. Of course, not all of the seedlings that sprout grow to maturity. Vander Wall states, “attrition over the years gradually reduces the number of seedlings within clumps.” Yet, more than half of the mature shrubs he observed in his study consisted of two or more individuals, leading him to conclude that “they arose from rodent caches.”

A study published in the Journal of Range Management (January 1996) looked at the herbivory of bitterbrush seedlings by rodents. In the introduction the authors discuss how “rodents [may] not only benefit from antelope bitterbrush seed caches as a future seed source, but also benefit from the sprouting of their caches as they return to graze the cotyledons of germinating seeds.”  In this study, Ord’s kangaroo rats, deer mice, and Great Basin pocket mice were all observed consuming bitterbrush seedlings, preferring them even when millet was offered as an alternative. The two species of mice also dug up seedlings, possibly searching for ungerminated seeds. Despite seed dispersal via caching, an overabundance of rodents can result in few bitterbrush seedlings reaching maturity.

A cluster of antelope bitterbrush seedlings that has been browsed. “Succulent, young seedlings are thought to be important in the diets of rodents during early spring because of the nutrients and water they contain.” — Vander Wall (1994)

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Photos of antelope bitterbrush seedling clusters were taken at Idaho Botanical Garden, where numerous clusters are presently on display along the pathways of the native plant gardens and the adjoining natural areas. 

Beavers and Water Lilies – An Introduction to Zoochory

Beavers are classic examples of ecosystem engineers. It is difficult to think of an animal – apart from humans – whose day-to-day activities have more impact on the landscape than beavers. Their dam building activities create wetlands that are used by numerous other species, and their selective harvesting of preferred trees affects species composition in riparian areas. And that’s just the start. Their extensive evolutionary history and once widespread distribution has made them major players in the landscape for millions of years.

Today, the beaver family (Castoridae) consists of just two extant species: Castor fiber (native to Eurasia) and Castor canadensis (native to North America). Both species were hunted by humans to the brink of extinction but, thanks to conservation efforts, enjoy stable populations despite having been eliminated from much of their historical ranges. Before the arrival of Europeans, North American beavers are estimated to have been anywhere from 60 million to 400 million strong. Extensive trapping reduced the population to less than half a million. Today, 10 million or more make their homes in rivers, streams, and wetlands across the continent.

North American beaver (Castor canadensis) - photo credit: wikimedia commons

North American beaver (Castor canadensis) – photo credit: wikimedia commons

Beavers are herbivores, and they harvest trees and shrubs to build dams and lodges. Their interactions with plants are legion, and so what better way to introduce the concept of animal-mediated seed dispersal than beavers. Plants have several strategies for moving their seeds around. Wind and gravity are popular approaches, and water is commonly used by plants both aquatic and terrestrial. Partnering with animals, however, is by far the most compelling method. This strategy is called zoochory.

Zoochory has many facets. Two major distinctions are epizoochory and endozoochory. In epizoochory, seeds become attached in some form or fashion to the outside of an animal. The animal unwittingly picks up, transports, and deposits the seeds. The fruits of such seeds are equipped with hooks, spines, barbs, or stiff hairs that help facilitate attachment to an animal’s fur, feathers, or skin. A well known example of this is the genus Arctium. Commonly known as burdock, the fruits in this genus are called burs – essentially small, round balls covered in a series of hooks. Anyone who has walked through – or has had a pet walk through – a patch of burdocks with mature seed heads knows what a nuisance these plants can be. But their strategy is effective.

The burs of Arctium - photo credit: wikimedia commons

The burs of Arctium – photo credit: wikimedia commons

Endozoochory is less passive. Seeds that are dispersed this way are usually surrounded by fleshy, nutritious fruits desired by animals. The fruits are consumed, and the undigested seeds exit out the other end of the animal with a bit of fertilizer. Certain seeds require passage through an animal’s gut in order to germinate, relying on chemicals produced during the digestion process to help break dormancy. Other seeds contain mild laxatives in their seed coats, resulting in an unscathed passage through the animal and a quick deposit. Some plants have developed mutualistic relationships with specific groups of animals regarding seed dispersal by frugivory. When these animal species disappear, the plants are left without the means to disperse their seeds, which threatens their future survival.

Beavers rely on woody vegetation to get them through the winter, but in warmer months, when herbaceous aquatic vegetation is abundant, such plants become their preferred food source. Water lilies are one of their favorite foods, and through both consumption of the water lilies and construction of wetland habitats, beavers help support water lily populations. This is how John Eastman puts it in The Book of Swamp and Bog: “Beavers relish [water lilies], sometimes storing the rhizomes. Their damming activities create water lily habitat, and they widely disperse the plants by dropping rhizome fragments hither and yon.”

Fragrant water lily (Nympaea odorata) - photo credit: wikimedia commons

Fragrant water lily (Nymphaea odorata) – photo credit: wikimedia commons

The seeds of water lilies (plants in the family Nymphaceae) are generally dispersed by water. Most species (except those in the genera Nuphar and Barclaya) have a fleshy growth around their seeds called an aril that helps them float. Over time the aril becomes waterlogged and begins to disintegrate. At that point, the seed sinks to the bottom of the lake or pond where it germinates in the sediment. The seeds are also eaten by birds and aquatic animals, including beavers. The aril is digestible, but the seed is not.

In her book, Once They Were Hats, Frances Backhouse writes about the relationship between beavers and water lilies. She visits a lake where beavers had long been absent, but were later reintroduced. She noted changes in the vegetation due to beaver activity – water lilies being only one of many plant species impacted.

Every year in late summer, the beavers devoured the seed capsules [of water lilies], digested their soft outer rinds and excreted the ripe undamaged seeds into the lake. Meanwhile, as they dredged mud from the botom of the lake for their construction projects, they were unintentionally preparing the seed bed. Seeing the lilies reminded me that beavers also inadvertantly propagate willows and certain other woody plants. When beavers imbed uneaten sticks into dams or lodges or leave them lying on moist soil, the cuttings sometimes sprout roots and grow.

Other facets of zoochory include animals hoarding fruits and seeds to be eaten later and then not getting back to them, or seeds producing fleshy growths that ants love called elaiosomes, resulting in seed dispersal by ants. Animals and plants are constantly interacting in so many ways. Zoochory is just one way plants use animals and animals use plants, passively or otherwise. These relationships have a long history, and each one of them is worth exploring and celebrating.

Harvester Ants – Seed Predators and Seed Dispersers

“The abundance of ants is legendary. A worker is less than one-millionth the size of a human being, yet ants taken collectively rival people as dominant organisms on the land. …  When combined, all ants in the world taken together weigh about as much as all human beings.” – Journey to the Ants by Bert Hölldobler and Edward O. Wilson

Considering how abundant and widely distributed ants are, it is easy to imagine the profound role they might play in the ecosystems of which they are a part. In fact, in the epilogue to Hölldobler and Wilson’s popular book about ants (quoted above), they conclude that in a world without ants, “species extinction would increase even more over the present rate, and the land ecosystems would shrivel more rapidly as the considerable services provided by these insects were pulled away.” It is no doubt then that ants, through their myriad interactions with their surroundings, are key players in terrestrial ecosystems.

photo credit: www.eol.org

photo credit: www.eol.org

Harvester ants offer a prime example of the important roles that ants can play. In the process of collecting seeds for consumption, harvester ants can help shape the abundance and distribution of the plants in their immediate environment. They do this by selecting the types and amounts of seeds they collect, by abandoning seeds along their collection routes, and by leaving viable seeds to germinate in and around their nests. Hölldobler and Wilson have this to say about harvester ants:

[The] numerical success [of ants] has allowed them to alter not just their nest environments, but the entire habitats in which they live. Harvesting ants, species that regularly include seeds in their diet, have an especially high impact. They consume a large percentage of the seeds produced by plants of many kinds in nearly all terrestrial habitats, from dense tropical forests to deserts. Their influence is not wholly negative. The mistakes they make by losing seeds along the way also disperse plants and compensate at least in part for the damage caused by their predation.

There are more than 150 species of harvester ants, spanning at least 18 genera. They are found throughout the world (except extreme cold locales) and are particularly common in arid to semi-arid environments. Pogonomyrmex is one the largest genera of harvester ants with nearly 70 species occurring throughout North, Central, and South America. Messor is another large genus of harvester ants that mainly occurs in Europe, Asia, and Africa. Both of these genera build large nests and move massive amounts of soil in the process.

Seed dispersal by harvester ants (also known as diszoochory) is a type of secondary (or Phase II) seed dispersal. It is a case of serendipity, as the dispersal occurs largely by accident. Some plants, on the other hand, have developed a mutualistic relationship with ants, enlisting them to disperse their seeds by way of an elaiosome – a fleshy, nutritious structure attached to seeds that attracts ants. Seeds with such structures are picked up by ants and brought to their nests where the elaiosome is consumed and the seed is left to germinate. This form of ant-mediated dispersal is called myrmecochory and is typically not carried out by harvester ants.

photo credit: wikimedia commons

photo credit: wikimedia commons

Harvester ant colonies have both direct and indirect influences on their surrounding environments; however, there is a dearth of research elucidating the exact details of such influences. A paper published in the Annual Review of Ecology and Systematics in 2000 by MacMahon et. al. reviewed available studies concerning harvester ants and explored our current understanding of the influences that harvester ants (particularly those in the genus Pogonomyrmex) can potentially have on community structure and ecosystem functions. Following are some of the direct influences the authors listed:

  • Removal and consumption of seeds and other materials – The relative abundance of plant species can be affected by the selective removal of seeds. Harvester ants also collect leaves, twigs, pollen, flowers, vertebrate feces, and arthropod body parts.
  • Storage and rejection of seeds – Collected seeds can be dropped during transport, rejected after arriving at the nest, or abandoned in nest granaries. All result in the transport of seeds away from the parent plant and dispersal beyond the plants’ primary dispersal mechanisms.
  • Construction and maintenance of nests – All vegetation and debris is removed from the area immediately surrounding the nest including mature and emerging plants. This area is kept clear for the duration of the life of the colony and, in some cases, can be quite extensive.

Harvester ants can also influence soil properties and soil food webs within and in the vicinity of their nests. They bring large amounts of organic matter down into the soil and redistribute vast amounts of soil particles. Their actions also influence the amount of moisture in the soil surrounding their nests.

This is a mere distillation of the influences that harvester ants might have; see the paper by MacMahon et al. to learn more.

In an effort to better understand how the seed predation and seed dispersal behaviors of harvester ants might influence plant population dynamics, a research team in Spain used data obtained from field research to build a computer model that would predict changes over time. The study site was described as “open and heterogeneous shrubland” and the vegetation was stated to be in “a very early stage in the secondary succession” after being subject to “recurring fires.” The harvester ant colonies involved in the study consisted of three species in the genus Messor. The plant species selected for the study were three native shrubs whose seeds were known to be collected by the harvester ants. Each plant species differed slightly in the amount and size of seeds it produced and in its primary seed dispersal mechanism, which is important because the researchers hypothesized that “the effect of seed predation and seed dispersal may depend on plant attributes.”

Messor bouvieri (photo credit: www.eol.org)

Messor bouvieri (photo credit: www.eol.org)

Data obtained from simulated scenarios and field observations appeared to support this hypothesis; each shrub species interacted differently with the harvester ants. Coronilla minima benefited from “accidental” seed dispersal. Comparatively, it produces a high amount of large seeds, which are primarily dispersed by gravity. Despite predation, ant-mediated dispersal was an advantage. Dorycnium pentaphyllum produced the highest amount of seeds among the three shrub species; however, seed predation was found to have negative effects on its population dynamics. Its primary seed dispersal mechanism involves ballistics (the mechanical ejection of its seeds), so ant-mediated dispersal may not offer an advantage. Finally, Fumana ericoides, despite its limited primary seed dispersal and its comparatively low production of seeds was not affected by the actions of the harvester ants. The authors concluded that “some unknown factor is driving the population dynamics of this species, more than the action of ants.”

Studies such as this, while leaving many unanswered questions, help us understand the important role that harvester ants play in our world. Harvester ants, and ants in general, are truly among Earth’s most enthralling and influential creatures. Learn more about their complex behaviors and countless interactions with flora and fauna by checking out these three documentaries recommended by ANTfinity.