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.


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

When Sunflowers Follow the Sun

Tropisms are widely studied biological phenomena that involve the growth of an organism in response to environmental stimuli. Phototropism is the growth and development of plants in response to light. Heliotropism, a specific form of phototropism, describes growth in response to the sun. Discussions of heliotropism frequently include sunflowers and their ability to “track the sun.” This conjures up images of a field of sunflowers in full bloom following the sun across the sky. However cool this might sound, it simply doesn’t happen. Young sunflowers, before they bloom, track the sun. At maturity and in bloom, the plants hold still.

What is happening in these plants is still pretty cool though, and a report published in an August 2016 issue of Science sheds some light on the heliotropic movements of young sunflowers. They begin the morning facing east. As the sun progresses across the sky, the plants follow, ending the evening facing west. Over night, they reorient themselves to face east again. As they reach maturity, this movement slows, and most of the flowers bloom facing east. Over a series of experiments, researchers were able to determine the cellular and genetic mechanisms involved in this spectacular instance of solar tracking.

Helianthus annuus (common sunflower) is a native of North America, sharing this distinction with dozens of other members of this recognizable genus. It is commonly cultivated for its edible seeds (and the oil produced from them) as well as for its ornamental value. It is a highly variable species and hybridizes readily. Wild populations often cross with cultivated ones, and in many instances the common sunflower is considered a pesky weed. Whether crop, wildflower, or weed, its phototropic movements are easy to detect, making it an excellent subject of study.

Researchers began by tying plants to stakes so that they couldn’t move. Other plants were grown in pots and turned to face west in the morning. The growth of these plants was significantly stunted compared to plants that were not manipulated in these ways, suggesting that solar tracking promotes growth.

The researchers wondered if a circadian system was involved in the movements, and so they took sunflowers that had been growing in pots in a field and placed them indoors beneath a fixed overhead light source. For several days, the plants continued their east to west and back again movements. Over time, the movements became less detectable. This and other experiments led the researchers to conclude that a “circadian clock guides solar tracking in sunflowers.”

Another series of experiments helped the researchers determine what was happening at a cellular level that was causing the eastern side of the stem to grow during the day and the western side to grow during the night. Gene expression and growth hormone levels differed on either side of the stem depending on what time of day it was. In an online article published by University of California Berkeley, Andy Fell summarizes the findings: “[T]here appear to be two growth mechanisms at work in the sunflower stem. The first sets a basic rate of growth for the plant, based on available light. The second, controlled by the circadian clock and influenced by the direction of light, causes the stem to grow more on one side than another, and therefore sway east to west during the day.”

The researchers observed that as the plants reach maturity, they move towards the west less and less. This results in most of the flowers opening in an eastward facing direction. This led them to ask if this behavior offers any sort of ecological advantage. Because flowers are warmer when they are facing the sun, they wondered if they might see an increase in pollinator visits during morning hours on flowers facing east versus those facing west. Indeed, they did: “pollinators visited east-facing heads fivefold more often than west-facing heads.” When west-facing flowers where warmed with a heater in the morning, they received more pollinator visits than west-facing flowers that were not artificially warmed, “albeit [still] fewer than east-facing flowers.” However, increased pollinator visits may be only part of the story, so further investigations are necessary.


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Summer of Weeds: Common Mullein

The fuzzy, gray-green leaves of common mullein are familiar and friendly enough that it can be hard to think of this plant as a weed. Verbascum thapsus is a member of the figwort family and is known by dozens of common names, including great mullein, Aaron’s rod, candlewick, velvet dock, blanket leaf, feltwort, and flannel plant. Its woolly leaves are warm and inviting and have a history of being used as added padding and insulation, tucked inside of clothing and shoes. In Wild Edible and Useful Plants of Idaho, Ray Vizgirdas writes, “the dried stalks are ideal for use as hand-drills to start fires; the flowers and leaves produce yellow dye; as a toilet paper substitute, the large fresh leaves are choice.”

Common mullein is a biennial that was introduced to eastern North America from Eurasia in the 1700’s as a medicinal plant and fish poison. By the late 1800’s it had reached the other side of the continent. In its first year it forms a rosette of woolly, oblong and/or lance-shaped leaves. After overwintering it produces a single flower stalk up to 6 feet tall. The woolly leaves continue along the flower stalk, gradually getting smaller in size until they reach the inflorescence, which is a long, dense, cylindrical spike. Sometimes the stalk branches out to form multiple inflorescences.

First year seedlings of common mullein (Verbascum thapsus)

The inflorescence doesn’t flower all at once; instead, a handful of flowers open at a time starting at the bottom of the spike and moving up in an irregular pattern. The process takes several weeks to complete. The flowers are about an inch wide and sulfur yellow with five petals. They have both female and male sex parts but are protogynous, meaning the female organs mature before the male organs. This encourages cross-pollination by insects. However, if pollination isn’t successful by the end of the day, the flowers self-pollinate as the petals close. Each flower produces a capsule full of a few hundred seeds, and each plant can produce up to 180,000 seeds. The seeds can remain viable for over 100 years, sitting in the soil waiting for just the right moment to sprout.

Common mullein is a friend of bare, recently disturbed soil. It is rare to see this plant growing in thickly vegetated areas. As an early successional plant, its populations can be abundant immediately after a disturbance, but they do not persist once other plants have filled in the gaps. Instead they wait in seed form for the next disturbance that will give them the opportunity to rise again. They can be a pest in gardens and farm fields due to regular soil disturbance, and are often abundant in pastures and rangelands because livestock avoid eating their hairy leaves. Because of its ephemeral nature, it is generally not considered a major weed; however, it is on Colorado’s noxious weed list.

Several features make common mullein a great example of a drought-adapted plant. Its fleshy, branching taproot can reach deep into the soil to find moisture, the thick hairs on the leaves help reduce water loss via transpiration, and the way the leaves are arranged and angled on the stalk can help direct rain water down toward the roots.

Common mullein has an extensive history of ethnobotanical uses. Medicinally it has been used internally to treat coughs, colds, asthma, bronchitis, and kidney infections; and as a poultice to treat warts, slivers, and swelling. The dried flower stalks have been used to make torches, and the fuzzy leaves have been used as tinder for fire-making and wicks in lamps.

The hairy leafscape of common mullein (Verbascum thapsus)

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From Gaia’s Garden by Toby Hemenway

Here’s why opportunistic plants are so successful. When we clear land or carve a forest into fragments, we’re creating lots of open niches. All that sunny space and bare soil is just crying out to be colongized by light- and fertlity-absorbing green matter. Nature will quickly conjure up as much biomass as possible to capture the bounty, by seeding low-growing ‘weeds’ into a clearing or, better yet, sprouting a tall thicket stretching into all three dimensions to more effectively absorb light and develop deep roots. … When humans make a clearing, nature leaps in, working furiously to rebuild an intact humus and fungal layer, harvest energy, and reconstruct all the cycles and connections that have been severed. A thicket of fast-growing pioneer plants, packing a lot of biomass into a small space, is a very effective way to do this. … And [nature] doesn’t care if a nitrogen fixer or a soil-stabilizing plant arrived via continental drift or a bulldozer’s treads, as long as it can quickly stitch a functioning ecosystem together.


Maize Anatomy and the Anatomy of a Maze

Commonly known as corn throughout much of North America, maize is a distinctive emblem of the harvest season. It is one of the most economically important crops in the world (the third most important cereal after rice and wheat) and has scads of uses from food to feed to fuel. The story of its domestication serves as a symbol of human ingenuity, and its plasticity in both form and utility is a remarkable example of why plants are so incredible.

The genus Zea is in the grass family (Poaceae) and consists of five species: Z. diploperennis, Z. perennis, Z. luxurians, Z. nicaraguensis, and Z. mays. Maize is the common name of Zea mays subsp. mays, which is one of four Z. mays subspecies and the only domesticated taxon in the genus. All other taxa are commonly and collectively referred to as teosintes.

The domestication of maize, apart from being an impressive feat, has long been a topic of research and a challenging story to tease apart. The current understanding is that maize was first domesticated around 9000 years ago in the Balsas River valley in southern Mexico, the main progenitor being Zea mays subsp. parviglumis. It is astonishing how drastically different in appearance teosintes are from modern day maize, but it also explains why determining the crop wild relative of maize was so difficult.

Teosinte, teosinte-maize hybrid, and maize - photo credit: wikimedia commons

Teosinte, teosinte-maize hybrid, and maize – photo credit: wikimedia commons

Teosintes and maize both have tall central stalks; however, teosintes generally have multiple lateral branches which give them a more shrubby appearance. In teosinte, each of the lateral branches and the central stalk terminate in a cluster of male flowers; female flowers are produced at the nodes along the lateral branches. In maize, male flowers are borne at the top of the central stalk, and lateral branches are replaced by short stems that terminate in female flowers. This is where the ears develop.

Ears – or clusters of fruits – are blatantly different between teosintes and maize. To start with, teosinte produces a mere 5 to 12 fruits along a short, narrow cob (flower stalk). The fruits are angular and surrounded in a hard casing. Maize cobs are considerably larger both in length and girth and are covered in as many as 500 or more fruits (or kernels), which are generally more rounded and have a softer casing. They also remain on the cob when they are ripe, compared to teosinte ears, which shatter.

Evolutionary biologist, Sean B. Carroll, writes in a New York Times article about the amazing task of “transform[ing] a grass with many inconvenient, unwanted features into a high-yielding, easily harvested food crop.” These “early cultivators had to notice among their stands of plants variants in which the nutritious kernels were at least partially exposed, or whose ears held together better, or that had more rows of kernels, and they had to selectively breed them.” Carroll explains that this “initial domestication process which produced the basic maize form” would have taken several hundred to a few thousand years. The maize that we know and love today is a much different plant than its ancestors, and it is still undergoing regular selection for traits that we find desirable.

Female inflorescence (or "ear") of Zea mays subsp. mays - photo credit: wikimedia commons

Female inflorescence (or “ear”) of Zea mays subsp. mays – photo credit: wikimedia commons

To better understand and appreciate this process, it helps to have a basic grasp of maize anatomy. Maize is an impressive grass in that it regularly reaches from 6 to 10 feet tall and sometimes much taller. It is shallow rooted, but is held up by prop or brace roots – adventitious roots that emerge near the base of the main stalk. The stalk is divided into sections called internodes, and at each node a leaf forms. Leaf sheaths wrap around the entirety of the stalk, and leaf blades are long, broad, and alternately arranged. Each leaf has a prominent midrib. The stalk terminates in a many-branched inflorescence called a tassel.

Maize Anatomy 101 - image credit: Canadian Goverment

Maize Anatomy 101 – image credit: Canadian Government

Maize is monoecious, which means that it has separate male and female flowers that occur on the same plant. The tassel is where the male flowers are located. A series of spikelets occur along both the central branch and the lateral branches of the tassel. A spikelet consists of a pair of bracts called glumes, upper and lower lemmas and paleas (which are also bracts), and two simple florets composed of prominent stamens. The tassel produces and sheds tens of thousands of pollen grains which are dispersed by wind and gravity to the female inflorescences below and to neighboring plants.

Female inflorescences (ears) occur at the top of short stems that originate from leaf axils in the midsection of the stalk. Leaves that develop along this reduced stem wrap around the ears forming the husk. Spikelets form in rows along the flower stalk (cob) within the husk. The florets of these spikelets produce long styles that extend beyond the top of the husk. This cluster of styles is known as the silk. When pollen grains land on silk stigmas, pollen tubes grow down the entire length of the silks to reach the embryo sac. Successful fertilization produces a kernel.

The kernel – or fruit – is known botanically as a caryopsis, which is the standard fruit type of the grass family. Because the fruit wall and seed are fused together so tightly, maize kernels are commonly referred to as seeds. The entire plant can be used to produce feed for animals, but it is the kernel that is generally consumed (in innumerable ways) by humans.

There is so much more to be said about maize. It’s a lot to take in. Rather than delve too much further at this point, let’s explore one of the other ways that maize is used by humans to create something that has become another feature of the fall season – the corn maze.

Entering the corn maze at The Farmstead in Meridian, Idaho

Exploring the corn maze at The Farmstead in Meridian, Idaho





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Bat Pollinated Flowers of a Mexican Columnar Cactus

Pollination syndromes – suites of floral traits used to determine potential pollinators and routes of pollination – have been informative in studying plant-pollinator interactions, but are generally too simplistic to tell the full story. Most flowering plants are generalists when it comes to pollinators, whereas pollination syndromes imply specialization. Not all pollinators are created equal though, and some may be more effective at pollinating particular plants than others. In fact, occasionally pollination syndromes ring true and a predicted plant-pollinator combination turns out to be the most effective and reliable interaction.

According to a study published in American Journal of Botany by Ibarra-Cerdeña, et al., Stenocereus queretaroensis, a species of columnar cactus endemic to western Mexico, adheres to this scenario. Stenocereus is a genus in a group of columnar and tree-like cacti called the Pachycereeae tribe. Cactus in this group are generally bat pollinated; however, their flowers are typically visited by various species of birds and insects as well, and in some cases, bats are not the primary pollinator. In their introduction, the authors note that specialization appears to be more common in tropical latitudes, and chiropterophilic (bat pollinated) columnar cacti that occur in temperate regions can be comparatively more generalized. This is because “extratropical chiropterophilic cacti appear to be faced with unpredictable seasonal year-to-year variation in pollinators,” while “cacti in tropical regions” experience “highly reliable seasonal availability of nectar-feeding bats, thereby leading to a temporally stable pollination system.”

Stenocereus queretaroensis is a massive cactus, reaching up to ten meters tall. Several vertical stems rise from a short, stocky, central trunk. Each stem has up to eight distinctive ribs and averages around 15 centimeters in diameter. Groupings of white to grey spines up to four centimeters long appear along the ribs. Flowers are light-colored, around 10 to 14 centimeters in length, and occur along the upper half of the stems, extended well beyond the spines. Flowers open at night – producing abundant nectar – and close by the afternoon the following day. Floral characteristics led the authors of this study to predict bats to be the main pollinator, and they set up a series of experiments to test this.

Stenocereus queretaroensis - photo credit: wikimedia commons

Stenocereus queretaroensis – photo credit: wikimedia commons

Part of their experiment consisted of five treatments involving 130 flowers on 75 plants. One group of flowers was bagged and allowed to self-pollinate naturally, while another group was bagged and self-pollinated manually. A third group was left exposed during the night but bagged in the morning, while a fourth group was bagged during the night and exposed during the daytime. The final group was left alone. For each of these five treatments, aborted flowers and mature fruits were counted and seed set was determined. Nectar samples were taken from a separate group of flowers at two hour intervals from 8:00 PM to 8:00 AM, after which no nectar was produced. A camera was also used to document floral visits. Visits were deemed “legitimate” when the “visitor’s body came in contact with anthers and/or stigma” and “illegitimate” when “no contact with anthers or stigma” was made.

The researchers found S. queretaroensis to be “incapable of self-pollination,” as no fruit set occurred for the first two treatments. The control group and the nocturnally exposed group had nearly identical results, producing significantly more fruits with greater seed set compared to the nocturnally bagged group. During the day, flowers were visited by four species of birds (two hummingbirds, a woodpecker, and an oriole) and several species of bees (mainly honey bees). During the night, apart from illegitimate visits from a nectar robbing hawkmoth, one species of bat was the dominant floral visitor, and the majority (93.8%) of the visits were legitimate. This bat species was Leptonycteris curasoae, the southern long-nosed bat.

Leptonycteris curasoae - photo credit: wikimedia commons

Leptonycteris curasoae – photo credit: wikimedia commons

The abundance of nectar-feeding bats was monitored in the study area over a four year period, and L. curasoae was by far the most abundant species throughout the study period. Nectar produced in the flowers of S. queretaroensis was at its maximum around midnight, which seemed to correlate with observations of bat visits. Even though daytime visitors appeared to contribute to fruit and seed set, the nocturnal treatment produced significantly more fruit with significantly higher seed set, suggesting that bats are the more efficient pollinator. Insects visiting during the daytime, when nectar was decreasingly available, were most likely robbing pollen.

The authors acknowledge that for most plant species, “a wide array of taxonomically diverse fauna such as insects, birds, and mammals usually serve as potential pollinators,” and that “generalized pollination systems are more frequent than specialized ones.” However, in this case, “a close association between L. curasoae and S. queretaroensis [suggests] that the chiropterophilic syndrome is still a useful model.”

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Bats As Pollinators – An Introduction to Chiropterophily

Most plants that rely on animals to assist in pollination look to insects. In general, insects are abundant, easy to please, and efficient at transferring pollen. Because insect pollination is such a common scenario, it is easy to overlook pollination that is carried out by vertebrates. Birds are the most prominent pollinator among vertebrates, but mammals participate, too. The most common mammal pollinator is the bat.

About a fifth of all mammal species on the planet are bats, with species estimates numbering in the 1200-1300 range. Bats are the only mammals that can truly fly. They are not blind, nor are they flying rodents, and they are not going to suck your blood (except in very rare cases!). Most bats eat insects, but a small, significant group of them are nectarivorous. Their main food source is the nectar produced within flowers. In the process of feeding, these bats pollinate plants.

Out of 18 families in the order Chiroptera, only two include species with morphologies that set them apart as nectar-feeders. The family Pteropodidae, known commonly as Old World fruit bats or flying foxes, occurs in tropical and subtropical regions of Africa, Asia, Australia, Papa New Guinea, and the Pacific Islands. The family Phyllostomidae, known commonly as American leaf-nosed bats, occurs in tropical and subtropical regions of the Americas. For simplicity’s sake, the former are referred to as Old World bats, and the latter as New World bats. While both groups are similar in that they consist of species that feed on nectar, they are only distantly related, and thus the nectar feeding species in these families have distinct behavioral and morphological differences.

Grey headed flying fox photo credit: wikimedia commons

Grey headed flying fox (Pteropus poliocephalus), a floral visiting bat from Australia (photo credit: wikimedia commons)

More than 500 species of plants, spanning 67 plant families, are pollinated by bats. This pollination syndrome is known as chiropterophily. In general, flowers that use this approach tend to be white or dull in color, open at night, rich with nectar, and musty or rotten smelling. They are generally tubular, cup shaped, or otherwise radially symmetrical and are often suspended atop tall stalks or prominently located on branches or trunks. In a review published in Annals of Botany, Theodore Fleming, et al. state “flower placement away from foliage and nocturnal anthesis [blooming] are the unifying features of the bat pollination syndrome,” while all other characteristics are highly variable among species. The family Fabaceae contains the highest number of bat-pollinated genera. Cactaceae, Malvaceae, and Bignoniaceae follow closely behind.

The characteristics of bat pollinated flowers vary widely partly because the bats that visit them are so diverse. Between the two bat families there are similarities in their nectar-feeding species, including an elongated rostrum, teeth that are smaller in number and size, and a long tongue with hair-like projections on the tip. Apart from that, New World bats are much smaller than Old World bats, and their rostrums and tongues are much longer relative to the size of their bodies. New World bats have the ability to hover in front of flowers, while Old World bats land on flowers to feed. Old World bats do not have the ability to use echolocation to spot flowers, while New World bats do. Fleming, et al. conclude, “because of these differences, we might expect plants visited by specialized nectar-feeding [New World bats] to produce smaller flowers with smaller nectar volumes per flower than those visited by their [Old World bat] counterparts.”

Pollination by bats is a relatively new phenomenon, evolutionarily speaking. Flowers that are currently pollinated by bats most likely evolved from flowers that were once pollinated by insects. Some may have evolved from flowers that were previously bird pollinated. The question is, why adopt this strategy? Flowers that are bat pollinated are “expensive” to make. They are typically much bigger than insect pollinated flowers, and they contain large amounts of pollen and abundant, nutrient-rich nectar. Due to resource constraints, many plants are restricted from developing such flowers, but those that do may find themselves at an advantage with bats as their pollinator. For one, hairy bat bodies collect profuse numbers of pollen grains, which are widely distributed as they visit numerous flowers throughout the night. In this way, bats can be excellent outcrossers. They also travel long distances, which means they can move pollen from one population of plants to an otherwise isolated neighboring population. This serves to maintain healthy genetic diversity among populations, something that is increasingly important as plant populations become fragmented due to human activity.

Pollinating bats are also economically important to humans, as several plants that are harvested for their fruits, fibers, or timber rely on bats for pollination. For example, bat pollinated Eucalyptus species are felled for timber in Australia, and the fruits of Durio zibethinus in Southeast Asia form after flowers are first pollinated by bats. Also, the wild relatives of bananas (Musa spp.) are bat pollinated, as is the plant used for making tequila (Agave tequilana).

Durio sp. (photo credit: wikimedia commons)

The flowers of durian (Durio sp.), trees native to Southeast Asia, are pollinated by bats (photo credit: wikimedia commons)

There is still much to learn about nectarivorous bats and the flowers they visit. It is clear that hundreds of species are using bats to move their pollen, but the process of adopting this strategy and the advantages of doing so remain ripe for discovery. Each bat-plant relationship has its own story to tell. For now, here is a fun video about the bat that pollinates Agave tequilana: