When Urban Pollinator Gardens Meet Native Plant Communities

Public concern about the state of bees and other pollinating insects has led to increased interest in pollinator gardens. Planting a pollinator garden is often promoted as an excellent way for the average person to help protect pollinators. And it is! However, as with anything in life, there can be downsides.

In many urban areas, populations of native plants remain on undeveloped or abandoned land, in parks or reserves, or simply as part of the developed landscape. Urban areas may also share borders with natural areas, the edges of which are particularly prone to invasions by non-native plants. Due to human activity and habitat fragmentation, many native plant populations are now threatened. Urban areas are home to the last remaining populations of some of these plants.

Concern for native plant populations in and around urban areas prompted researchers at University of Pittsburgh to review some of the impacts that urban pollinator gardens may have and to develop a “roadmap for research” going forward. Their report was published earlier this year in New Phytologist.

Planting a wildflower seed mix is a simple way to establish a pollinator garden, and such mixes are sold commercially for this purpose. Governmental and non-governmental organizations also issue recommendations for wildflower, pollinator, or meadow seed mixes. With this in mind, the researchers selected 30 seed mixes “targeted for urban settings in the northeastern or mid-Atlantic USA” to determine what species are being recommended for or commonly planted in pollinator gardens in this region. They also developed a “species impact index” to assess “the likelihood a species would impact remnant wild urban plant populations.”

A total of 230 species were represented in the 30 seed mixes. The researchers selected the 45 most common species for evaluation. Most of these species (75%) have generalized pollination systems, suggesting that there is potential for sharing pollinators with remnant native plants. Two-thirds of the species had native ranges that overlapped with the targeted region; however, the remaining one-third originated from Europe or western North America. The native species all had “generalized pollination systems, strong dispersal and colonization ability, and broad environmental tolerances,” all traits that could have “high impacts” either directly or indirectly on remnant native plants. Other species were found to have either high dispersal ability but low chance of survival or low dispersal ability but high chance of survival.

This led the researchers to conclude that “the majority of planted wildflower species have a high potential to interact with native species via pollinators but also have the ability to disperse and survive outside of the garden.” Sharing pollinators is especially likely due to super-generalists like the honeybee, which “utilizes flowers from many habitat types.” Considering this, the researchers outlined “four pollinator-mediated interactions that can affect remnant native plants and their communities,” including how these interactions can be exacerbated when wildflower species escape gardens and invade remnant plant communities.

photo credit: wikimedia commons

The first interaction involves the quantity of pollinator visits. The concern is that native plants may be “outcompeted for pollinators” due to the “dense, high-resource displays” of pollinator gardens. Whether pollinator visits will increase or decrease depends on many things, including the location of the gardens and their proximity to native plant communities. Pollinator sharing between the two has been observed; however, “the consequences of this for effective pollination of natives are not yet understood.”

The second interaction involves the quality of pollinator visits. Because pollinators are shared between native plant communities and pollinator gardens, there is a risk that the pollen from one species will be transferred to another species. High quantities of this “heterospecific pollen” can result in reduced seed production. “Low-quality pollination in terms of heterospecific pollen from wildflower plantings may be especially detrimental for wild remnant species.”

The third interaction involves gene flow between pollinator gardens and native plant communities. Pollen that is transferred from closely related species (or even individuals of the same species but from a different location) can have undesired consequences. In some cases, it can increase genetic variation and help address problems associated with inbreeding depression. In other cases, it can introduce traits that are detrimental to native plant populations, particularly traits that disrupt adaptations that are beneficial to surviving in urban environments, like seed dispersal and flowering time. Whether gene flow between the two groups will be positive or negative is difficult to predict, and “the likelihood of genetic extinction versus genetic rescue will depend on remnant population size, genetic diversity, and degree of urban adaptation relative to the planted wildflowers.”

The fourth interaction involves pathogen transmission via shared pollinators. “Both bacterial and viral pathogens can be transmitted via pollen, and bacterial pathogens can be passed from one pollinator to another.” In this way, pollinators can act as “hubs for pathogen exchange,” which is especially concerning when the diseases being transmitted are ones for which the native plants have not adapted defenses.

photo credit: wikimedia commons

All of these interactions become more direct once wildflowers escape gardens and establish themselves among the native plants. And because the species in wildflower seed mixes are selected for their tolerance of urban conditions, “they may be particularly strong competitors with wild remnant populations,” outcompeting them for space and resources. On the other hand, the authors note that, depending on the species, they may also “provide biotic resistance to more noxious invaders.”

All of these interactions require further investigation. In their conclusion, the authors affirm, “While there is a clear potential for positive effects of urban wildflower plantings on remnant plant biodiversity, there is also a strong likelihood for unintended consequences.” They then suggest future research topics that will help us answer many of these questions. In the meantime, pollinator gardens should not be discouraged, but the plants (and their origins) should be carefully considered. One place to start is with wildflower seed mixes, which can be ‘fine-tuned’ so that they benefit our urban pollinators as well as our remnant native plants. Read more about plant selection for pollinators here.

Advertisements

Bumblebees and Urbanization

Urban areas bear little resemblance to the natural areas that once stood in their place. Concrete and asphalt stretch out for miles, buildings of all types tower above trees and shrubs, and turfgrass appears to dominate whatever open space there is. Understandably, it may be hard to imagine places like this being havens for biodiversity. In many ways they are not, but for certain forms of life they can be.

An essay published earlier this year in Conservation Biology highlights the ways in which cities “can become a refuge for insect pollinators.” In fact, urban areas may be more inviting than their rural surroundings, which are often dominated by industrial agriculture where pesticides are regularly used, the ground is routinely disturbed, and monocultures reign supreme. Even though suitable habitat can be patchy and unpredictable in the built environment, cities may have more to offer than we once thought.

Yet, studies about bee abundance and diversity in urban areas show mixed results, largely because all bee species are not created equal (they have varying habitat requirements and life histories) and because urban areas differ wildly in the quality and quantity of habitat they provide both spatially and temporally. For this reason, it is important for studies to focus on groups of bees with similar traits and to observe them across various states of urbanization. This is precisely what researchers at University of Michigan set out to do when they sampled bumblebee populations in various cities in southeastern Michigan. Their results were published earlier this year by Royal Society Open Science.

common eastern bumble bee (Bombus impatiens) – photo credit: wikimedia commons

The researchers selected 30 sites located in Dexter, Ann Arbor, Ypsilanti, Dearborn, and Detroit. Most of the sites were gardens or farms in urban centers. They collected bumblebees from May to September using pan traps and nets. The species and sex of each individual bumblebee was identified and recorded for each site. The percentage of impervious surface that surrounded each site was used as a measurement of urban development. Other measurements included the abundance of flowers and average daily temperatures for each location.

Bumblebees were selected as a study organism because the genus, Bombus, “represents a distinct, well-studied set of traits that make it feasible to incorporate natural history into analysis.” Bumblebees live in colonies – eusocial structures that include “a single reproductive queen, variable numbers of non-reproductive female workers, and male reproductive drones.” They are generalist foragers, visiting a wide variety of flowering species for pollen and nectar, and they nest in holes in the ground, inside tree stumps, or at the bases of large clumps of grass. The authors believe that their nesting behavior makes them “a good candidate for testing the effects of urban land development,” and the fact that members of the colony have “distinct roles, [behaviors], and movement patterns” allows researchers to make inferences regarding “the effects of urbanization on specific components of bumblebee dynamics.”

Across all locations, 520 individual bumblebees were collected. Nearly three quarters of them were common eastern bumblebees (Bombus impatiens). Among the remaining nine species collected, brown-belted bumblebees (Bombus griseocollis) and two-spotted bumblebees (Bombus bimaculatus) were the most abundant.

brown-belted bumblebee (Bombus griseocollis) – photo credit: wikimedia commons

Because bumblebees are strong fliers with an extensive foraging range, impervious surface calculations for each site had to cover an area large enough to reflect this. Results indicated that as the percentage of impervious surfaces increased, bumblebee abundance and diversity declined. When male and female bumblebee data was analyzed separately, the decline was only seen in females; males were unaffected.

Female workers do most of their foraging close to home, whereas males venture further out. The researchers found it “reasonable to hypothesize that worker abundance is proportional to bumblebee colony density.” Thus, the decline in female bumblebees observed in this study suggests that as urban development increases (i.e. percent coverage of impervious surface), available nesting sites decline and the number of viable bumblebee colonies shrinks. Because male bumblebees responded differently to this trend, future studies should consider the responses of both sexes in order to get a more complete picture of the effects that urbanization has on this genus.

Interestingly, results obtained from the study locations in Detroit did not conform to the results found elsewhere. Bumblebee abundance and diversity was not decreasing with urbanization. Unlike other cities in the study, “Detroit has experienced decades of economic hardship and declining human populations.” It has a high proportion of impervious surfaces, but it also has an abundance of vacant lots and abandoned yards. These areas are left unmaintained and are less likely to be mowed regularly or treated with pesticides. Reducing disturbance can create more suitable habitat for bumblebees, resulting in healthy populations regardless of the level of urbanization. Thus, future studies should examine the state of insect pollinators in all types of cities – shrinking and non-shrinking – and should consider not just the amount of available habitat but also its suitability.

two-spotted bumblebee (Bombus bimaculatus) – photo credit: wikimedia commons

Grasshoppers – More Friend Than Foe?

Major outbreaks of grasshoppers can be devastating. A plague of locusts of biblical proportions can decimate crop fields and rangelands in short order. Clouds of grasshoppers moving in and devouring every plant in sight makes it easy to see why these insects are often seen as pests. Even small groups of them can do significant damage to a garden or farm. Yet, grasshoppers and their relatives have great ecological value and are important parts of healthy ecosystems. Love them or hate them, they are an essential piece of a bigger picture.

Grasshoppers are in the order Orthoptera, an order that includes katydids, crickets, wetas, and a few other familiar and not so familiar insects. Worldwide, there are more than 27,000 species of orthopterans. These insects mostly feed on plants; many are omnivorous while others are exclusively herbivorous. They are most commonly found in open, sunny, dry habitats like pastures, meadows, disturbed sites, open woods, prairies, and crop fields. Most insects in this order are fairly large, making them easy to identify; yet they don’t seem to receive the same level of human attention that charismatic insects like bees and butterflies do. In Field Guide to Grasshoppers, Katydids, and Crickets of the United States, the authors defend this diverse group of arthropods: “Grasshoppers often are thought of as modest-looking brown or green insects, but many species in this family are brightly colored, and some of the most dull-colored species rival butterflies in beauty when they spread their wings in flight.”

photo credit: wikimedia commons

photo credit: wikimedia commons

The voracious appetite of grasshoppers and their preference for plants can influence ecosystems in many ways. Certain plants may be favored over others, which affects the diversity and distribution of plant communities. Grasses are a particular favorite, despite being high in hard to digest compounds like lignin, cellulose, and silica. As grasshoppers consume vegetation – up to their body weight per day – digested materials return to the soil where soil dwelling organisms continue to break them down. In this way, grasshoppers and their relatives are major contributors to nutrient cycling. Returning nutrients to the soil results in increased nutrient availability for future plant growth. In fact, one grassland study found that despite short-term losses via grasshopper herbivory, plant growth was enhanced in the long-term due in part to accelerated nutrient cycling.

Because grasshoppers are such prolific consumers, their robust bodies are loaded with nutritious proteins and fats, making them a preferred food source for higher animals. Reptiles, raccoons, skunks, foxes, mice, and numerous species of birds regularly consume grasshoppers and related species. While many adult birds feed mostly on seeds and fruits, they seek out insects and worms to feed their young. Nutrient-packed grasshoppers are an excellent food source for developing birds. Humans in many parts of the world also find grasshoppers and crickets to be a tasty part of their diet.

Grasshoppers provide food for other invertebrates as well. The aforementioned field guide refers to the fate of grasshoppers and certain species of blister beetles as being “intimately linked,” because the larvae of these blister beetles feed exclusively on grasshopper eggs. Several species of flies and other insects, as well as spiders, also feed on grasshoppers and other orthopterans.

grasshopper on blade of grass

In short, grasshoppers play prominent roles in plant community composition, soil nutrient cycling, and the food chain. When grasshopper populations reach plague proportions, their impact is felt in other ways. From a human perspective, the damage is largely economic. However, their ability to thoroughly remove vegetation across large areas can be environmentally devastating as well, particularly when it comes to soil erosion and storm water runoff. The USDA’s Agricultural Research Service considers grasshoppers “among the most economically important pests” and cites research estimating that they are responsible for destroying as much as 23% of available range forage in the western United States annually. A paper published in the journal, Psyche, references a period between 2003-2005 in Africa where locusts were responsible for farmers losing as much as 80 to 100% of their crops.

This level of devastation is relatively rare. In Garden Insects of North America, Whitney Cranshaw states that of the more than 550 species of grasshoppers that occur in North America, “only a small number regularly damage gardens…almost all of these are in the genus Melanoplus.” Like most large, diverse groups of organisms, many grasshopper species are abundant and thriving while others are rare and threatened. Human activity has benefited certain species of grasshoppers while jeopardizing others. In general, grasshopper populations vary wildly from year to year depending on a slew of environmental factors.

Differential grasshopper (Melanoplus differentialis) - one of the four grasshoppers that Whitney Cranshaw lists as "particularly injurious" in his book Garden Insects of North America. (photo credit: www.eol.org)

Differential grasshopper (Melanoplus differentialis) – one of the four grasshoppers that Whitney Cranshaw lists as “particularly injurious” in his book Garden Insects of North America. (photo credit: www.eol.org)

A plague or outbreak of grasshoppers is a poorly understood phenomenon. It seems there are too many factors at play to pin such an occasion on any one thing. Warm, sunny, dry weather seems to favor grasshopper growth and reproduction, so drought conditions over a period of years can result in a dramatic increase in grasshopper populations. But drought can also limit plant growth, reducing the grasshoppers’ food supply. Natural enemies – which grasshoppers have many – also come into play. It seems that just the right conditions have to be met for an outbreak to occur – a seemingly unlikely scenario, but one that occurs frequently enough to cause concern.

Grasshoppers and fellow orthopterans are fascinating insects, and their place in the world is worth further consideration. For an example of just how compelling such insects can be, here is a story about crickets from Doug Tallamy’s book, Bringing Nature Home:

“Male tree crickets in the genus Oecanthus attempt to lure females to them by making chirping songs with their wings. The loudest male attracts the most females, so males often cheat a bit by positioning themselves within a cup-shaped leaf that amplifies the song beyond what the male can make without acoustical help. Each male chews a hole in the center of his cupped leaf that is just large enough to accommodate his raised wings during chirping. This ensures that the sound projects directly from the center of the parabolic leaf for maximum amplification.

Related Awkward Botany Posts:

The Making of a Kill Jar

I often hear stories from plant lovers about their initial nonchalance concerning plants. The common refrain seems to be that they were fascinated by wildlife and largely ignored plant life until they came to the realization that plants were integral in the lives of animals and play a major role in shaping the environments that support all life. Such an epiphany spawns an insatiable obsession with botany, at least for some people.

I seem to be on the opposite trajectory. It’s not like I have ever really been disinterested in animals; I’ve just been significantly more interested in plants and haven’t bothered to learn much about the animal kingdom (with the exception of entomology). My growing fascination with pollination biology (see last year’s Year of Pollination series) isn’t much of a stretch because insects have always appealed to me, and their intimate interactions with plants are hard to ignore. Ultimately, it is my interest in urban ecology and wildlife friendly gardening that is driving me to learn more about animals.

I started this year off by finally reading Doug Tallamy’s popular book, Bringing Nature Home. Tallamy wrote a lot about birds in his book, which got me thinking more about them. I then discovered Welcome to Subirdia, a book by John Marzluff that explores the diversity of birds that live among us in our urban environments. I then found myself paying more attention to birds. Many bird species rely on insects for food at some point in their lives. Plants regularly interact with insects both in defending themselves against herbivory and in attracting insects to assist in pollination. It’s all connected, and it seems I wouldn’t be much of a botanist then if I didn’t also learn about all of the players involved in these complex interactions.

So, now I’m a birdwatcher and an insect collector. Or at least I’m learning to be. Insects are hard to learn much about without capturing them. They often move quickly, making them hard to identify, or they go completely unnoticed because they are tiny and so well hidden or camouflaged. With the help of a net and a kill jar, you can get a closer look. This not only allows you to determine the species of insects that surround you, but it can also help give you an idea of their relative abundances, their life cycles, where they live and what they feed on, etc.

insect net 2_bw

As the name implies, if you’re using a kill jar, your actions will result in the death of insects. Some people will be more pleased about this than others. If killing insects bothers you, don’t worry, insect populations are typically abundant enough that a few individuals sacrificed for science will not hurt the population in a serious way.

Kill jars can be purchased or they can be made very simply with a few easy to find materials. Start with a glass jar with a metal lid. Mix up a small amount of plaster of paris. Pour the wet plaster in the jar, filling it to about one inch. Allow the plaster to dry completely. This process can be sped up by placing the jar in an oven set on warm. When the plaster is dry, “charge” the jar by soaking the plaster with either ethyl acetate, nail polish remover, or rubbing alcohol. I use nail polish remover because it is cheap and easily accessible. It doesn’t work as quickly as pure ethyl acetate, but it is less toxic. Place a paper towel or something soft and dry in the jar. This keeps the insects from getting beaten up too much as they thrash about. Once the insect is dead, it can be easily observed with a hand lens or a dissecting microscope. It can also be pinned, labeled, and added to a collection.

There are several resources online that describe the process of collecting and preserving insects, including instructions for making an inexpensive kill jar, which is why I am keeping this brief and will instead refer you to a couple of such sites. Like this one from Purdue University’s extension program. It’s directed toward youth, but it includes great information for beginners of any age. This post by Dragonfly Woman is a great tutorial for making a kill jar, and there are several other posts on her blog that are very informative for insect collectors of all experience levels.

I guess you could consider this part of my journey of becoming a naturalist. Perhaps you are on a similar journey. If so, share your thoughts and experiences in the comment section below.

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.

Year of Pollination: Figs and Fig Wasps

This post originally appeared on Awkward Botany in November 2013. I’m reposting an updated version for the Year of Pollination series because it describes a very unique and incredibly interesting interaction between plant and pollinator. 

Ficus is a genus of plants in the family Moraceae that consists of trees, shrubs, and vines. Plants in this genus are commonly referred to as figs, and there are nearly 850 described species of them. The majority of fig species are found in tropical regions, however several occur in temperate regions as well. The domesticated fig (Ficus carica), also known as common fig, is widely cultivated throughout the world for its fruit.

common fig

Common Fig (Ficus carica) – photo credit: wikimedia commons

The fruit of figs, also called a fig, is considered a multiple fruit because it is formed from a cluster of flowers. A small fruit develops from each flower in the cluster, but they all grow together to form what appears to be a single fruit. The story becomes bizarre when you consider the location of the fig flowers. They are contained inside a structure called a syconium, which is essentially a modified fleshy stem. The syconium looks like an immature fig. Because they are completely enclosed inside syconia, the flowers are not visible from the outside, yet they must be pollinated in order to produce seeds and mature fruits.

This is where the fig wasps come in. “Fig wasp” is a term that refers to all species of chalcid wasps that breed exclusively inside of figs. Fig wasps are in the order Hymenoptera (superfamily Chalcidoidea) and represent at least five families of insects. Figs and fig wasps have coevolved over tens of millions of years, meaning that each species of fig could potentially have a specific species of fig wasp with which it has developed a mutualistic relationship. However, pollinator host sharing and host switching occurs frequently.

Fig wasps are tiny, mere millimeters in length, so they are not the same sort of wasps that you’ll find buzzing around you during your summer picnic. Fig wasps have to be small though, because in order to pollinate fig flowers they must find their way into a fig. Fortunately, there is a small opening at the base of the fig called an ostiole that has been adapted just for them.

What follows is a very basic description of the interaction between fig and fig wasp; due to the incredible diversity of figs and fig wasps, the specifics of the interactions are equally diverse.

First, a female wasp carrying the pollen of a fig from which she has recently emerged discovers a syconium that is ready to be pollinated. She finds the ostiole and begins to enter. She is tiny, but so is the opening, and so her wings, antennae, and/or legs can be ripped off in the process. No worries though, since she won’t be needing them anymore. Inside the syconium, she begins to lay her eggs inside the flowers. In doing so, the pollen she is carrying is rubbed off onto the stigmas of the flowers. After all her eggs are laid, the female wasp dies. The fig wasp larvae develop inside galls in the ovaries of the fig flowers, and they emerge from the galls once they have matured into adults. The adult males mate with the females and then begin the arduous task of chewing through the wall of the fig in order to let the females out. After completing this task, they die. The females then leave the figs, bringing pollen with them, and search for a fig of their own to enter and lay eggs. And the cycle continues.

But there is so much more to the story. For example, there are non-pollinating fig wasps that breed inside of figs but do not assist in pollination – freeloaders essentially. The story also differs if the species is monoecious (male and female flowers on the same plant) compared to dioecious (male and female flowers on different plants). It’s too much to cover here, but figweb.org is a great resource for fig and fig wasp information. Also check out the PBS documentary, The Queen of Trees.

 

 

Year of Pollination: Mosquitoes as Pollinators

It is difficult to have positive feelings about mosquitoes, especially during summer months when they are out in droves and our exposed skin – soft, supple, and largely hair-free – is irresistible to them. We are viewed as walking blood meals by female mosquitoes who are simply trying to produce young – to perpetuate their species just like any other species endeavors to do. Unfortunately, we are left with small, annoying bumps in our skin – red, itchy, and painful – risking the possibility that the mosquitoes that just drew our blood may have passed along any number of mosquito-borne diseases, some (such as malaria) that potentially kill millions of people every year. For this, it is okay to hate mosquitoes and to long for the day of their complete eradication from the planet. However, their ecological roles (and yes, they do have some) are also worth considering.

There are more than 3,500 species of mosquito. Luckily, only 200 or so consume human blood. Mosquitoes go back at least 100 million years and have co-evolved with species of plants and animals found in diverse habitats around the world. Adult mosquitoes and their larvae (which live in standing water) provide food for a wide variety of creatures including birds, bats, insects, spiders, fish, frogs, lizards, and salamanders. Mosquito larvae also help break down organic matter in the bodies of water they inhabit. They even play an important role in the food webs found inside the pitchers of northern pitcher plants (Sarracenia spp.). Interestingly enough, Arctic mosquitoes influence the migration patterns of caribou. They emerge in swarms so big and so voracious that they have been said to kill caribou through either blood loss or asphyxiation.

However, blood is not the main food source of mosquitoes; flower nectar is. Males don’t consume blood at all, and females only consume it when they are producing eggs. Any insect that visits flowers for nectar has the potential to unwittingly collect pollen and transfer it to a nearby flower, thereby aiding in pollination. Mosquitoes are no exception. They have been observed acting as pollinators for a handful of species, and could be acting as pollinators for many more.

Bluntleaved orchid (Platanthera obtusata) is pollinated by mosquitoes. phot credit: wikimedia commons

Bluntleaved orchid (Platanthera obtusata) is pollinated by mosquitoes. photo credit: wikimedia commons

The scientific literature describes the pollination by mosquitoes of at least two plant species: Platanthera obtusata (syn. Habenaria obtusata) and Silene otites. P. obtusata – bluntleaved orchid – is found in cold, wet regions in North America and northern Eurasia. It is pollinated by mosquitoes from multiple genera including several species in the genus Aedes. Mosquitoes visit the flowers to feed on the nectar and, subsequently, pollinia (clusters of pollen) become attached to their eyes and are moved from flower to flower. This scenario likely plays out in other species of Arctic orchids as well*.

S. otites – Spanish catchfly – is a European species that is pollinated by mosquitoes and moths. Researches have been studying the floral odors of S. otites that attract mosquitoes, suggesting that determining the compounds involved in these odors “might lead to the development of new means of pest control and mosquito attractants and repellents.”

Northern House Mosquito (Culex pipiens) - one of the species of mosquitoes that has been observed pollinating Silene otitis. photo credit: www.eol.org

Northern House Mosquito (Culex pipiens) – one of the species of mosquitoes that has been observed pollinating Silene otites. photo credit: www.eol.org

Despite the list of functions that mosquitoes serve in their varied habitats, an article that appeared in Nature back in 2010 argues for wiping mosquitoes off the Earth, stating that “the ecological scar left by a missing mosquito would heal quickly as the niche was filled by other organisms.” And even though “thousands of plant species would lose a group of pollinators,” mosquitoes are not important pollinators of the “crops on which humans depend,” nor do they appear to be the sole pollinator of any single plant species [the species mentioned above are pollinated by other insects as well]. Eliminating mosquitoes, however, is more of a pipe dream than a realistic possibility as our “best efforts can’t seriously threaten an insect with few redeeming features.”

*Speaking of orchids and pollination, endless posts could be written about this incredibly fascinating and diverse group of plants and their equally fascinating and complex mechanisms surrounding pollination. There will be more to come on such topics. Meanwhile, it should be noted that orchids are also a notoriously threatened group of plants. To learn more about orchids and orchid conservation in North America, visit North American Orchid Conservation Center.

Read more about mosquito pollination here.

And now for your listening pleasure: