The Mighty Saguaro Cactus

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Where does one begin with a plant like the saguaro cactus (Carnegiea gigantea)? It is recognized the world over for its iconic appearance yet its native range is disproportionately small compared to its popularity. It is easily one of the most spectacular plants I have ever encountered and I will never forget the sound the wind makes as it blows over its spiny pleated trunk. It would be impossible to sum up our collective knowledge of this species in one article, however, I feel that some form of an introduction is necessary. Today I want to honor this icon of the Sonoran Desert.

The saguaro is the only member of the genus Carnegiea, which is part of a subtribe of cacti characterized by their columnar appearance. Despite its unique taxonomic affinity, the evolutionary origins of this cactus remains a bit of a mystery. Though it is undoubtedly related to other columnar cacti of the Americas, a proper family tree seems to be just out of our reach. Due to lots of convergent and parallel evolution as well as conflicts between genealogies and species histories, we still aren't sure of its evolutionary origins. What we do know about this species on a genetic level is nonetheless quite interesting. For instance the saguaro has one of the smallest chloroplast genomes of any non-parasitic plant and we aren’t exactly sure why this is the case.

Saguaro are long lived cacti. Estimating age of a cactus can be rather tricky considering that they don’t produce annual growth rings. This is where long term monitoring projects have come in handy. By observing hundreds of saguaro throughout the Sonoran Desert, experts believe that saguaro can regularly reach ages of 150 to 170 years and some individuals may be able to live for more than 200 years. Amazingly, it is thought that saguaro will not begin to grow their characteristic arms until they reach somewhere around 50 to 100 years of age. That being said, some saguaro never bother growing arms. It all depends on where the conditions they experience throughout their lifetime.

Growth for a saguaro depends on where they are rooted. Under favorable conditions, a saguaro can grow to heights of 50 feet or more, with the world record holder clocking in at a whopping 78 feet in height. Such growth becomes all the more impressive when you realize just how agonizingly slow the process can be. Studies have shown that juvenile saguaro only put on about 1.5 inches of growth in their first eight years of life.

Despite preconceived notions about the hardy nature of most cacti, saguaro have proven to be rather specific in their needs. They are limited in their growth and distribution by the availability of water and warm temperatures. Saguaro, especially young individuals, cannot tolerate periods of prolonged frost. Additionally, germination and seedling survival occur most frequently only during the wettest years. In fact, one study showed that successful years for reproduction in these beloved cacti were tied to volcanic eruptions that cooled the climate just enough to allow the young saguaro to become established.

Outside of volcanic eruptions, saguaro appear to have friends in the surrounding vegetation. Studies have shown that saguaro seedlings seem to do best when growing under the shade of trees like the palo verde (Parkinsonia florida), ironwood (Olneya tesota), and mesquite (Prosopis velutina). The microclimates produced by these trees are much more favorable for saguaro growth than are open desert conditions. In essence, these trees serve as nurseries for young saguaro until they are large enough to handle more exposed conditions. Their nursery habits are not mutually beneficial however as research suggests that saguaro eventually compete with the trees that once protected them for precious resources like nutrients and water.

Saguaros outgrowing their palo verde nurse tree. 

Saguaros outgrowing their palo verde nurse tree. 

At roughly 35 years of age, a saguaro will begin to flower. Flowers are small compared to the size of the cactus but they are abundant. Most flowers are produced at the apex of the cactus and it is thought that the growth of saguaro arms is largely a way of increasing the reproductive potential of large individuals. The flowers are cream colored and night scented. They open in the evening but will stay open and continue to produce nectar well into the morning hours.

Though a wide variety of animals will visit these flowers, the main pollinators are bees during the day and lesser long-nosed bats at night. Interestingly, it has been found that certain amino acids within the nectar of the saguaro can actually help female bats sustain lactation while raising their young, making them a valuable food source for these flying mammals. Catering to such a broad spectrum of potential pollinators is thought to have evolved as a means of increasing seed set. Each saguaro ovary contains many ovules and the more pollen that makes it onto the stigma, the more seeds will be produced.

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A lesser long-nosed bat pollinates a saguaro bloom.

A lesser long-nosed bat pollinates a saguaro bloom.

Due to their size and abundance, it is easy to understand why the saguaro is such an ecologically important species in the Sonoran Desert ecosystem. In essence, they function similar to trees in that they serve as vital sources of shelter and food for myriad desert animals. Woodpeckers, especially the gila and the gilded flicker, regularly hollow out and build nests in saguaro trunks. These hollows are subsequently used by many different bird, mammal, and reptile species. The flowers and fruits are important sources of food for wildlife.

Gila woodpecker with its nesting hole.

Gila woodpecker with its nesting hole.

Gila woodpecker holes become homes for other birds like owls. 

Gila woodpecker holes become homes for other birds like owls. 

On rare occasions, woodpecker holes can even become home to other cacti!

On rare occasions, woodpecker holes can even become home to other cacti!

I sincerely hope that this brief introduction does at least some justice to the wonderful organism that is the saguaro cactus. The Sonoran Desert would be a shell of an ecosystem without its presence. What’s more, it has played a significant role in the culture of this region for millennia. Though it appears quite numerous on the landscape, the long-term status of the saguaro is cause for concern. Numerous declines have been reported throughout its range. With its slow growth rates and infrequent recruitment events, the saguaro can be quite sensitive to rapid changes in its environment. Luckily it has received special protection laws throughout its US range.

Photo Credits: [1] [2] [3] [4]


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

A Bat-Pollinated Passion Flower From Ecuador

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Say "hello" to one of Passiflora's most recent additions, the bat-pollinated Passiflora unipetala. The first specimens of this vine were discovered back in 2009 by Nathan Muchhala while studying flower visiting bats in northern Ecuador. It is a peculiar member of the genus to say the least. 

One of the most remarkable features of this plant are its flowers. Unlike its multi-petaled cousins, this species stands out in producing a single large petal, which is unique for not only the genus, but the whole family as well. The petal is quite large and resembles a bright yellow roof covering the anthers and stigma. At the base of the flower sits the nectar chamber. The body of the plant consists of a vine that has been observed to grow upwards of 6 meters up into the canopy.

Flowering in this species occurs at night. Their large size, irregular funnel shape, and bright yellow coloring all point to a pollination syndrome with bats. Indeed, pollen of this species has been found on the fur of at least three different bat species. Multiple observations (pictured here) of bats visiting the flowers helped to confirm. Oddly enough for a bat-pollinated plant, the flowers produce no detectable odor whatsoever. However, another aspect of its unique floral morphology is worth noting. 

The surface of the flower has an undulating appearance. Also, the sepals themselves have lots of folds and indentations, including lots of dish-shaped pockets. It is thought that these might help the flower support the weight of visiting bats. They may also have special acoustic properties that help the bats locate the flowers via echolocation. Though this must be tested before we can say for sure, other plants have converged on a similar strategy (read here and here).

As it stands currently, Passiflora unipetala is endemic to only a couple high elevation cloud forests in northern Ecuador. It has only ever been found at two locations and sadly a landslide wiped out the type specimen from which the species description was made. As such, its introduction to the world came complete with a spot on the IUCN Redlist as critically endangered. Luckily, the two localities in which this species has been found are located on privately protected properties. Let's just hope more populations are discovered in the not-too-distant future.

Photo Credits: [1] 

Further Reading: [1]

Shape Shifting Flowers That Attract Bats

For nectar feeding bats, finding food in a dense tropical rainforest is a complex task. For many years it was thought that nectar feeding bats relied solely on scent to find their food. Unlike insects that move around, flowers are stationary. That all changed once we developed microphones sensitive enough to pick up bat calls.

It turns out, nectar feeding bats utilize different frequencies for echolocation that are primed for resolving finer details than those of their insect feeding cousins. Obviously this works quite well for the bats as they are very important pollinators. Whereas many bat pollinated plants position their flowers in easy to reach places for bats, others take it a step further.

Some plant species couple their floral displays with specialized structures that are prime reflectors for bat sonar. Recently, a species of vine known scientifically as Marcgravia evenia (http://bit.ly/2h358BF) was made famous the world over for the satellite dish shaped leaves it produces just above its inflorescence. However, this species is not alone. Other plants have managed to tap into bat sonar in some very interesting ways. Take, for instance, the bizarrely beautiful blooms of the sea bean.

A member of the legume family, Mucuna holtonii is neotropical in its distribution. It is a vining species that snakes its way up into the canopy. When in flower, the plant produces a stunning pendulum-shaped inflorescence, the end of which is ringed in flowers. Although highly derived for this group, the flowers are nonetheless representative of the family. Their secret to success lies in the single large petal known as the banner or vexillum coupled with some explosive power.

This banner petal acts as a nectar guide, though not in a strictly visual sense. These flowers open at night when nectar feeding bats are out and about. They emit a scent, which likely lets the bats know roughly when and where a meal is available. Unlike bees, however, these bats have no interest in pollen. Instead, they are after the energy-rich nectar reserves that only virgin flowers produce. And they produce quite a bit (relative to their size), upwards of 100 microliters per flower.

Bats that have honed in on the scent are further attracted to the flowers by the shape of the vexillum. Supremely adapted to the specific frequency of these nectar feeding bats, the vexillum of each virgin flower reflects sound waves over a greater range of directions than the clutter of the surrounding forest, thus helping the bats zero in on exactly where they need to be.

Unlike many other bat pollinated flowers, those of M. holtonii cannot be accessed simply by hovering. A bat must land on them in order to access the nectar within. The weight of the bat is what the flowers require to complete the process. When a visiting bat lands on the flower, it triggers an explosive mechanism that snaps the anthers outward, causing pollen to explode from the flower, spattering it all over the bat's back.

Once the bat drinks its fill, nectar production ceases. However, the flowers don't senesce at this point. They still provide a calling card for any other flowers yet to be visited. Despite the fact that they stick around, it has been found that bats are significantly less likely to visit spent flowers. How do they know?

The answer again lies in that banner petal. Once the flowers have been triggered, its shape changes. This results in a change in the way in which bats perceive the flowers via echolocation. Bats soon learn that flowers with this altered shape no longer offer a nectar reward. The plant benefits from this because it reduces the chance that a bat will end up depositing its pollen right back on the flower it came from. It's win win when you think about it. Bats maximize their meals and the plant maximizes its chances of cross pollination.

Photo Credit: Merlin Tuttle

Further Reading: [1] [2] 

On the Wood Rose and its Bats

New Zealand has some weird nature. It is amazing to see what an island free of any major terrestrial predators can produce. Unfortunately, ever since humans found their way to this unique island, the ecology has suffered. One of the most unique plant and animal interactions in the world can be found on this archipelago but for how much longer is the question.

The story starts with a species of bat. In fact, this bat is New Zealand's only native terrestrial mammal. That's right, I said terrestrial. The New Zealand lesser short-tailed bat spends roughly 40% of its time foraging for insects on the ground. It has lots of specialized adaptations that I won't go into here but the cool part is they forage in packs, stirring up insects from the leaf litter until they reach a level of feeding frenzy that I thought was only reserved for sharks or piranhas. Along with using echo location, they also have a highly developed sense of smell. This is important for our second player in this forest floor drama.

Enter Dactylanthus taylorii, the wood rose. This plant is not a rose at all but rather a member of the tropical family Balanophoraceae. More importantly, it is parasitic. It produces no chlorophyll and lives most of its life wrapped around the roots of its host tree underground. Every once in a while a small patch of flowers break through the dirt and just barely peak above the leaf litter. This give this species it's Māori name of "pua o te reinga" or "pua reinga", which translates to "flower of the underworld." The flowers emit a musky, sweet smell that attracts these ground foraging bats. The bats are one of the only pollinators left on the island. They sniff out the flowers and dine on the nectar, all the while being dusted with pollen. Recently, it has been found that New Zealand's giant ground parrot, the kakapo, is also believed to have been a pollinator of this plant. Sadly, today the kakapo exists solely on one small island of the New Zealand archipelago.

Both the wood rose and the New Zealand lesser short-tailed bat are considered at risk for extinction. When modern man came to these islands they brought with them the general suite of mammalian invasives like rats, mongoose, cats, and pigs, which are exacting a major toll on the local ecology. The plants and animals native to New Zealand have not shared an evolutionary history with such aggressive mammalian invaders and thus have no adaptations for coping with their sudden presence. The future of the wood rose, the New Zealand lesser short-tailed bat, and the kakapo, along with many other uniquely New Zealand species are for now uncertain.

Photo Credits: Joseph Dalton Hooker (1859) and Nga Manu Nature Reserve (http://www.ngamanu.co.nz/)

Further Reading:

http://bit.ly/2bBw8FT

http://bit.ly/2bKRY90

http://bit.ly/2bKpxfE