How Fungus Gnats Maintain Jack-in-the-pulpits

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There are a variety of ways that the boundaries between species are maintained in nature. Among plants, some of the best studied examples include geographic distances, differences in flowering phenology, and pollinator specificity. The ability of pollinators to maintain species boundaries is of particular interest to scientists as it provides excellent examples of how multiple species can coexist in a given area without hybridizing. I recent study based out of Japan aimed to investigate pollinator specificity among fungus gnats and five species of Jack-in-the-pulpit (Arisaema spp.) and found that pollinator isolation is indeed a very strong force in maintaining species identity among these aroids, especially in the wake of forest disturbance.

Fungus gnats are the bane of many a houseplant grower. However, in nature, they play many important ecological roles. Pollination is one of the most underappreciated of these roles. Though woefully understudied compared to other pollination systems, scientific appreciation and understanding of fungus gnat pollination is growing. Studying such pollination systems is not an easy task. Fungus gnats are small and their behavior can be very difficult to observe in the wild. Luckily, Jack-in-the-pulpits often hold floral visitors captive for a period of time, allowing more opportunities for data collection.

By studying the number and identity of floral visitors among 5 species of Jack-in-the-pulpit native to Japan, researchers were able to paint a very interesting picture of pollinator specificity. It turns out, there is very little overlap among which fungus gnats visit which Jack-in-the-pulpit species. Though researchers did not analyze what exactly attracts a particular species of fungus gnat to a particular species of Jack-in-the-pulpit, evidence from other systems suggests it has something to do with scent.

Like many of their aroid cousins, Jack-in-the-pulpits produce complex scent cues that can mimicking everything from a potential food source to a nice place to mate and lay eggs. Fooled by these scents, pollinators investigate the blooms, picking up and (hopefully) depositing pollen in the process. One of the great benefits of pollinator specificity is that it greatly increases the chances that pollen will end up on a member of the same species, thus reducing the chances of wasted pollen or hybridization.

Still, this is not to say that fungus gnats are solely responsible for maintaining boundaries among these 5 Jack-in-the-pulpit species. Indeed, geography and flowering time also play a role. Under ideal conditions, each of the 5 Jack-in-the-pulpit species they studied tend to grow in different habitats. Some prefer lowland forests whereas others prefer growing at higher elevations. Similarly, each species tends to flower at different times, which means fungus gnats have few other options but to visit those blooms. However, such barriers quickly break down when these habitats are disturbed.

Forest degradation and logging can suddenly force many plant species with different habitat preferences into close proximity with one another. Moreover, some stressed plants will begin to flower at different times, increasing the overlap between blooming periods and potentially allowing more hybridization to occur if their pollinators begin visiting members of other species. This is where the strength of fungus gnat fidelity comes into play. By examining different Jack-in-the-pulpit species flowering in close proximity to one another, the team was able to show that fungus gnats that prefer or even specialize on one species of Jack-in-the-pulpit are not very likely to visit the inflorescence of a different species. Thanks to these preferences, it appears that, thanks to their fungus gnat partners, these Jack-in-the-pulpit species can continue to maintain species boundaries even in the face of disturbance.

All of this is not to say that disturbance can’t still affect species boundaries among these plants. The researchers were quick to note that forest disturbances affect more than just the plants. When a forest is logged or experiences too much pressure from over-abundant herbivores such as deer, the forest floor dries out a lot quicker. Because fungus gnats require high humidity and soil moisture to survive and reproduce, a drying forest can severely impact fungus gnat diversity. If the number of fungus gnat species declines, there is a strong change that these specific plant-pollinator interactions can begin to break down. It is hard to say what affect this could have on these Jack-in-the-pulpit species but a lack of pollinators is rarely a good thing. Certainly more research is needed.

Photo Credit: [1]

Further Reading: [1]

Deer Skew Jack-in-the-Pulpit Sex Ratios

Photo by Michael Janke licensed under CC BY-ND 2.0.

Deer populations in North America are higher than they have been at any point in history. Their explosion in numbers not only leads to series health issues like starvation and chronic wasting disease, it has also had serious impacts on regional plant diversity. Wherever hungry herds of deer go, plants disappear from the landscape. However, the impacts of deer on plants aren’t limited to species they can eat. Research on Jack-in-the-Pulpit (Arisaema triphyllum) has shown that deer can have plenty of surprising indirect impacts on plants as well.

Though I wouldn’t put anything past a hungry deer, plants like Jack-in-the-Pulpit aren’t usually on the menu for these ungulates. Their leaves, stems, and flowers are chock full of raphide crystals that will burn the mouths and esophagus of most herbivores. Still, this doesn’t mean deer aren’t impacting these plants in other ways. Because deer are congregating in high abundance in our ever-shrinking natural spaces, they are having serious impacts on local growing conditions. Wherever deer herds are at high numbers, forests are experiencing soil compaction, soil erosion, and a disappearance of soil leaf litter (also due in part to invasive earthworms). Thanks to issues like these, plants like Jack-in-the-Pulpit are undergoing some serious changes.

Like many aroids, sex expression in the genus Arisaema is fluid and relies on energy stores. Smaller plants store less energy and tend to only produce male flowers when they bloom. Pollen, after all, is cheap compared to eggs and fruit. Only when a plant has stored enough energy over the years will it begin to produce female flowers in addition to males and only the largest, most robust plants will switch over entirely to female flowers. As you can imagine, the ability of a plant to acquire and store enough energy is dependent on the quality of the habitat in which it grows. This is where deer enter into the equation.

High densities of deer inevitably cause serious declines in habitat quality of plants like Jack-in-the-Pulpit. As leaf litter disappears and soil compaction grows more severe, individual plants have a much harder time storing enough energy each growing season. In places where deer impacts are heaviest, the sex ratios of Jack-in-the-Pulpit populations begin to skew heavily towards males because individual plants must grow much longer before they can store enough energy to produce female flowers. It doesn’t end there either. Not only does it take longer for a plant to begin producing female flowers, individual plants must also reach a much larger size in order to produce female flowers than in areas with fewer deer.

Photo by Charles de Mille-Isles licensed under CC BY-ND 2.0.

As mentioned, seed production takes a lot of energy and any plant that is able to produce viable fruits will have less energy stores going into the next season. This means that even if a plant is able to produce female flowers and successfully set seed, they will have burned through so much energy that they will likely revert right back to producing only male flowers the following year, further skewing the sex ratios of any given population towards males. Interestingly, this often results in more individuals being produced via clonal offshoots. The more clones there are in a population, the less diverse the gene pool of that population becomes.

Without actually eating the plants, deer are having serious impacts on Jack-in-the-Pulpit population dynamics. I am certain that this species isn’t alone either. At least Jack-in-the-Pulpit is somewhat flexible in its reproductive behaviors. Other plants aren’t so lucky. I realize deer are a hot button issue but there is no getting around the fact that our mismanagement of their natural predators, habitat, and numbers are having serious and detrimental impacts on wild spaces and all the species they support.

Photo Credits: [1] [2]

Further Reading: [1]

The Arisaema Complex

If you live in the east, Jack-in-the-pulpit or Arisaema triphyllum, is most likely an unmistakable part of late spring. Being a member of the arum family, the bracts of the plant form a tube and hood over the spadix and flowers. This is a highly variable species, in fact, there are at least 4 recognized subspecies that make up the Arisaema complex, A. triphyllum ssp. pusillum, A. triphyllum ssp. quinatum, A. triphyllum ssp. stewardsonii, and A. triphyllum ssp. triphyllum.

Interestingly enough, each subspecies seems to be reproductively isolated from the others. Each also seems to prefer its own habitat. For instance, triphyllum, a denizen of rich woods, blooms after the last frosts while stewardsonii, a denizen of swamps and bogs, blooms a few weeks later. Another interesting aspect of this complex is that pusillum and stweardsonii are both diploid plants, having 28 sets of chromosomes each, whereas triphyllum, our most common subspecies, is believed to be a hybrid of the two and is tetraploid and thus has 56 sets of chromosomes. Some would argue that these plants should be treated as distinct species since the characteristics that designate each subspecies seem rather specific but all across their range, there are many plants that seem to blur the lines. This is a debate that is only going to be solved by more accurate DNA analysis. However, nature doesn't seem to be reading any science texts and therefore rarely falls into our neat, clear-cut mindsets.

Being an arum, this species does produce some heat as well as an odor. The flowers produce a smell reminiscent of mushrooms and indeed, this is to attract their main pollinators, fungus gnats. Next time you come across a blooming Jack-in-the-pulpit, get down and take a whiff. It isn't necessarily good or bad but either way it is an experience. This species is gaining some traction in the gardening community as well due to its ease of care and unique appearance. It is also easy to establish from seed, however, make sure to wear gloves and avoid any skin contact while de-fleshing the seeds because being that it is a member of the arum family, this species produces calcium oxalate crystals that can cause severe burning.

Further Reading:
http://www.efloras.org/florataxon.aspx?flora_id=1&taxon_id=222000013

http://www.theprimrosepath.com/Featured_Plants/Arisaema_triphyllum/triphyllumcomplex.htm

http://plants.usda.gov/java/profile?symbol=ARTR

http://www.amjbot.org/content/90/12/1729.full.pdf+html?sid=5babeddb-908d-4782-a866-6e0938b93580

http://www.amjbot.org/content/91/6/881.full.pdf+html?sid=5babeddb-908d-4782-a866-6e0938b93580