An Intruiguing Relationship Between Ants and Cacti

The extrafloral nectaries of  Pachycereus gatesii  appear as tiny red bumps just below the areole.

The extrafloral nectaries of Pachycereus gatesii appear as tiny red bumps just below the areole.

It’s hard to think of a group of plants that are better defended than cacti. Frequently and often elaborately adorned with vicious spines, these succulents make any animal think twice about trying to take a bite. And yet, for some cacti, spines don’t seem to cut it. A surprising amount of species appear to have taken their defense system to a whole new level by recruiting nature’s most tenacious bodyguards, ants.

Plants frequently have a friend in ants. Spend some time observing ants at work and it’s east to see why. These social insects have numbers and strength on their side. Give ants a reason to be invested in your survival and they will certainly see to it that nothing threatens this partnership. For cacti, this involves the secretion of nectar from specialized tissues called extrafloral nectaries.

Extrafloral nectaries are not unique to cacti. A multitude of plant species produce them, often for similar reasons. Ants love a sugary food source and the more reliable that source becomes, the more adamant an ant colony will be at defending it. The odd thing about cacti is that they don’t seem to have settled on a single type of extrafloral nectary to do the trick. In fact, as many as four different types of extrafloral nectaries have been described in the cactus family.

Ants visiting the extrafloral nectaries covering the developing flowers of  Pilosocereus gounellei .

Ants visiting the extrafloral nectaries covering the developing flowers of Pilosocereus gounellei.

Some cacti secrete nectar from highly modified spines. A great example of this can be seen in genera such as Coryphantha, Cylindropuntia, Echinocactus, Ferocactus, Opuntia, Sclerocactus, and Thelocactus. Such spines are usually short and blunt, hardly resembling spines at all. Other cacti secrete nectar from regular looking spines. This adaptation is odd as there does not seem to be anything special about the anatomy of such spines. Examples of this can be seen in genera such as Brasiliopuntia, Calymmanthium, Harrisia, Opuntia, Pereskiopsis, and Quiabentia. Still others secrete nectar from highly reduced leaves that are found at the base of where the spines originate (the areole). Such leaves have been described in Acanthocereus, Leptocereus, Myrtillocactus, Pachycereus, and Stenocereus. They aren’t easy to recognize as leaves either. Most look like tiny scales. Finally, the fourth type of extrafloral nectary comes in the form of specialized regions of the stem tissue. This has been described in genera such as Armatocereus, Leptocereus, and Pachycereus.

Highly modified spines functioning as extrafloral nectaries in  Ferocactus emoryi.

Highly modified spines functioning as extrafloral nectaries in Ferocactus emoryi.

Seemingly normal spines of  Harrisia pomanensis  secreting nectar.

Seemingly normal spines of Harrisia pomanensis secreting nectar.

Regardless of where they form, their function remains much the same. They secrete a form of nectar which ants find irresistible. The more reliable this food source becomes, the more aggressive ant colonies will be in defending it. This is an especially useful form of defense when it comes to small insect herbivores. Whereas spines deter larger herbivores, they don’t do much to deter organisms that can just slip right through them unharmed. Ants also clean the cacti, potentially removing harmful microbes like fungi and bacteria. Though we are only just beginning to understand the depths of this cactus/ant mutualism, what we have discovered already suggests that the relationship between these types of organisms is far more complex than what I have just outlined above.

For instance, it may not just be sugar that the ants are looking for. In arid desert habitats, water may be the most limiting resource for an ant colony and large, succulent cacti are essentially giant water reservoirs. The key is getting to that water. One study that looked at a species of barrel cactus growing in Arizona called Ferocactus acanthodes found that as spring gives way to summer, the concentration of sugars secreted by the extrafloral nectaries decreases. As a result, the nectar becomes far more watery. Amazingly, ant densities on any given barrel cactus actually increased throughout the summer, despite the fact that the nectar was being watered down. Ants are notoriously prone to desiccation so it stands to reason that water, rather than sugar, is the real prize for colonies hanging out on cacti in such hot desert environments.

The incredible floral display of  Ferocactus wislizeni , a species whose reproductive efforts are affected by the types of ants they attract.

The incredible floral display of Ferocactus wislizeni, a species whose reproductive efforts are affected by the types of ants they attract.

Another interesting observation about the cactus/ant mutualism is that it appears that the identity of the ants truly matters. Though defense is the main benefit to the cactus, research suggests that there is a tipping point in how much such defenses benefit cacti. It has been found that although cacti initially benefit from anti-herbivore and cleaning services, extra aggressive ant species can actually drive off potential pollinators. At least one study has shown that when less aggressive ant species tend cacti, they produce more fruits and those fruits contain significantly more seeds than cacti that have been tended by extremely aggressive ant species. This is especially concerning when we think about the growing issue of invasive ants. As more and more non-native ant species displace native ants, this could really tip the balance for some cactus species.

Despite all of the interesting things we have learned about extrafloral nectaries in the family Cactaceae, there are so many questions yet to be answered. For starters, we still do not know how many different taxa produce them in one form or another. It is likely that closer inspection, especially of rare or poorly understood groups, will reveal that far more cacti produce some type of extrafloral nectary. Also, we know next to nothing about the anatomy of the different types of nectaries. How do they differ from one another and how do some, especially those derived from ordinary spines, actually function? Finally, do these nectaries function year round or is there some sort of seasonal pattern to their development and utility. How does this affect the types of ants they attract and how does that in turn affect the survival and reproduction of these cacti? For such a charismatic group of plants as cacti, we still have to much to learn.

Photo Credits: Thanks to Dr. Jim Mauseth and Dr. John Rebman and Dr. Silvia Rodriguez Machado for use of their photos [1] [2] [3]

Further Reading: [1] [2] [3] [4] [5] [6]

The Ant-Farming Tillandsias

Tillandsias are all the rage. Their relative ease of care has found them included in seemingly every terrarium sold these days; often in very inappropriate circumstances that result in their death. There is no denying that these epiphytic relatives of the pineapple are unique and beautiful plants but I would argue that their ecology is probably the coolest aspect about them. I am particularly fond of the bulbous species because of their relationship with ants.

That's right, there are upwards of 13 species of bulbous Tillandsia that offer up housing for ants. If you look closely at the leaves of these species, you will notice that they roll up to form tubes that lead down into the bulb at the base. The space between the leaves forms a hollow chamber, functioning as a perfect microclimate for ants to nest. In many habitats, these Tillandsia offer better housing than the surrounding environment. One would be surprised at how many ants can fit in there too. Colonies containing anywhere between 100 - 300 ants are not unheard of.

The rewards for the plant are obvious. Ants provide nutrients as well as protection. In return the ants get a relatively safe and dry place to live. Ant domatia have been recorded in roughly 13 different species, many of which are some of the most commonly sold Tillandsias on the market such as T. baileyi, T. balbisiana, T. bulbosa, and T. caput-medusae. If this doesn't make your hanging glass Tillandsia orb even cooler then I don't know what will.

Photo Credits: scott.zona ( and Alex Popovkin (

Further Reading: [1] [2]

Fern Ant Farm

An epiphytic lifestyle is no walk in the park. Baking sun, drying winds, and a lack of soil are the norm. As a result epiphytic plants exhibit numerous adaptations for retaining water and obtaining nutrients. One of the most interesting adaptations to this lifestyle can be seen in plants that have struct up a relationship with ants.

An amazing example of one such relationship can be seen in a genus of epiphytic ferns called Lecanopteris. Native to Southeast Asia and New Guinea, their unique look is equally matched by their unique ecology. Using a highly modified rhizome, they are able to latch on to the branch of a tree. In species such as Lecanopteris mirabilis (pictured here), it's as if the fronds are emerging from a strange green amoeba.

It's whats going on underneath their strange rhizomes that makes this group so fascinating. These ferns literally grow ant farms. Chambers and middens within the amorphous rhizome entice colonies of ants to set up shop. In return for lodging, the ants provide protection. Anything looking to take a bite out of a frond must contend with an army of angry ants. Moreover, the ants provide valuable nutrients in the form of waste and other detritus.

These are by no means the only plants to have evolved a relationship of this sort. Myriad plant species utilize ants for protection, nutrient acquisition, and seed dispersal. It has even been suggested that the unique morphology of Lecanopteris spores is an adaptation for ant dispersal. Certainly one can imagine how that would come into play. Interestingly enough, this group of ferns has attracted the attention of plant enthusiasts looking for a unique plant to grow in their home. As such, you can now find many different species of Lecanopteris being cultivated for the horticultural trade.

Photo Credit: Ch'ien C. Lee (

Further Reading:

On Peonies and Ants


It is just about that time when peony buds burst forth and put on their late spring display. My mother loves her peonies and she gets very excited every year when they bloom. It's adorable. However, she has always been disgusted by the amount of ants the peonies attract. Indeed, many people all over the internet seem to feel the same way. Growing up, I always wondered why the ants seemed to swarm all over peony buds, so I decided to look into it a little deeper.

There are many sources out there that claim that peonies need ants in order to bloom. To me, this seems very maladaptive on the part of the peony. The genus Paeonia is represented in Asia, Southern Europe and parts of western North America. I am going to assume that the ant/peony relationship didn't start in the garden so it's roots have to be somewhere in the evolutionary history of the plant. What sense does it make for a plant to produce flower buds that excrete sticky sugars that keep them from opening until something cleans the sugars off? In fact, despite anecdotal reports, peony buds will open without ants. So then why does the plant bother to produce sugars that attract ants?

Interestingly enough, despite a good amount of searching, there is not a lot of research done on this subject but the answer to this question can come from looking at how ants interact with other plants and animals. Many plant species have special glands on their stems that produce sugary secretions which attract ants. It's not just plants either. Insects such as aphids and leafhoppers famously excrete honeydew that ants can't resist. In each of these cases, organisms are using the ants' natural tendency to guard a food source. The ants will viciously attack anything that threatens this easy meal.

It would seem to me that the peonies are doing just that with their flower buds. By secreting a sugary substance during their development, the plant are likely recruiting ants to protect the flowers, which in the plant kingdom, are the most precious part of the plant. It takes a lot out of a plant to flower and the threat of herbivory is ever present. If an insect tries to take a bite out of a bud, the ants quickly swarm and drive it off. It's a win win situation. The ants get an easy, high-energy food source and the plant suffers less damage to its reproductive organs.

The scary part to me in researching all of this is plethora of information out there on how to get rid of the ants. People go through chemical after chemical to rid their peonies of ants when, in reality, the ants are some of the best friends a peony could have! So leave those ants alone and enjoy the free pest removal services they provide every spring!

Photo Credit: [1]

Further Reading:


Let's hear it for ants! 

Thats right, ants. Without ants I would venture to stay that a lot of life as we know it would be radically different. One of the many ways in which ants fill important niche roles is as seed dispersers. 

Known as myrmecochory, many species of plants rely on ants to move their seeds from place to place. They encourage the ants to do this by attaching appendages to their seeds called elaiosomes. Elaiosomes are little fleshy structures that are packed full of lipids and proteins. Foraging ants take these seeds back to their colonies where the elaiosome is eaten and the seed is then discarded. Ants have special chambers in their colonies for trash. They are basically little underground compost heaps. 

When the seeds are thrown away, they suddenly find themselves in a very stable, nutrient rich area where they can safely germinate. It makes so much sense. The ants get a little meal and the plant has provided its offspring with one of the safest storage and planting environments. Next time you are hiking in the woods and see a population of plants that evolved this method, there is a good chance that an ant colony is near by. 

There is also some evidence to suggest that the seeds gain a cleaning benefit from the ants as well. Living in close quarters and in such high numbers as ants do, disease is a particularly prevalent issue. Because of that, ants have evolved specialized glands that secrete a liquid with antimicrobial properties. It is possible that ants may inadvertently clean seeds that enter their nest with this fluid. Since diseases, especially certain types of fungi, are one of the leading causes of seedling mortality, it is very possible that this is yet another added benefit of having ants as your seed dispersal agent. More research is necessary to see if this is truly what is going on. 

The sheer number of plants species that utilize ants in this way is staggering. Here in North America, the majority of myrmecochorous plants are spring ephemerals. There is a lot less food available to ants in the spring, making these seeds very appealing. Once summer hits, scavenging ants are less likely to pay attention to seeds in lieu of more nutritious food available. Here are just a few examples you may be familiar with:

Wild ginger
Dutchman's breeches
Trout lily

Photo Credit: Cotinis

Further reading:

Carnivorous Bromeliads

Brocchinia reducta

Brocchinia reducta

I would like to introduce you to quite possibly the strangest members of the Bromeliad family - those of the genus Brocchinia. Aside from the odd appearance and habits of this particular group, researchers have learned quite a bit about the Bromeliad family as a whole from studying this group. From their origins to their impressive radiation, Brocchinia offers us a window into the history of this charismatic family.

Brocchinia are considered sister to all other bromeliads. They were the first genus to diverge some 70 million years ago. Their center of origin can be traced back to the Guayana Shield, a region in the northeast corner of what is now South America. The earliest members of this group were likely terrestrial plants growing in nutrient poor areas. Surprisingly, the epiphytic nature of many bromeliad species we know and love today evolved more recently.

Brocchinia reducta

Brocchinia reducta

Since this time, Brocchinia has undergone an impressive adaptive radiation. Because they have remained specialists on nutrient poor soils, much of this radiation has to do with the evolution of nutrient acquisiton. Like its cousins, Brocchinia utilize a "tank" formed by their tightly clasped leaves.

Interestingly, at least two species of Brocchinia, (B. reducta and B. hechtioides) have taken this to the extreme and have adopted a carnivorous lifestyle. Their tall, columnar growth form coupled with slick, waxy leaves means insects can't keep a foothold and fall down into the tank. The plants sweeten the deal by luring them in with sweet secretions.


Whether or not this was a case of true carnivory was highly debated until 2005 when a group of researchers analyzed the chemical makeup of the liquid inside the tank. They discovered that they plant was secreting an enzyme called phosphatase, which actively digest hapless insects that fall in. A true carnivore indeed!

Others have even more peculiar evolutionary adaptations for nutrient acquisition. B. tatei, for example, was discovered to house nitrogen-fixing cyanobacteria within its tank. Another species, B. acuminata, produces hallow chambers at the base of its leaves that house ant colonies. The ants pay rent via their nutrient-rich waste and voracious defense of their bromeliad home.

Brocchinnia acuminata

Brocchinnia acuminata

In total, this group is quite amazing. The amount of information we have been able to glean from studying Brocchinia has allowed us to shine a light on the whole bromeliad family. As we have also seen, the species within this group have quite the evolutionary history to tell in their own way. Brocchinia serves as a reminder to researchers blind to organismal study. We shouldn't have to study ecology at the expense of individual organisms. There is plenty to learn from both avenues of research.

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

Further Reading: [1] [2]