An Intruiguing Relationship Between Ants and Cacti

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.

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.

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. Photo by Joseph j7uy5 licensed under CC BY-NC-SA 2.0

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]

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

Toxic Nectar

I was introduced to the concept of toxic nectar thanks to a species of shrub quite familiar to anyone who has spent time in the Appalachian Mountains. Locals will tell you to never place honeybee hives near a patch of rosebay (Rhododendron maximum) for fear of so-called "mad honey." Needless to say, the concept intrigued me.

A quick internet search revealed that this is not a new phenomenon either. Humans have known about toxic nectar for thousands of years. In fact, honey made from feeding bees on species like Rhododendron luteum and R. ponticum has been used more than once during times of war. Hives containing toxic honey would be placed along known routs of Roman soldiers and, after consuming the seemingly innocuous treat, the soldiers would collapse into a stupor only to be slaughtered by armies lying in wait.

Rhododendron luteum. Photo by Chrumps licensed under CC BY 3.0

The presence of toxic nectar seems quite confusing. The primary function of nectar is to serve as a reward for pollinators after all. Why on Earth would a plant pump potentially harmful substances into its flowers?

It is worth mentioning at this point that the Rhododendrons aren't alone. A multitude of plant species produce toxic nectar. The chemicals that make them toxic, though poorly understood, vary almost as much as the plants that make them. Although there have been repeated investigations into this phenomenon, the exact reason(s) remain elusive to this day. Still, research has drummed up some interesting data and many great hypotheses aimed at explaining the patterns.

Catalpa nectar has been shown to deter some ants and butterflies but not large bees. Photo by Le.Loup.Gris licensed under CC BY-SA 3.0

The earliest investigations into toxic nectar gave birth to the pollinator fidelity hypothesis. Researchers realized that meany bees appear to be less sensitive to alkaloids in nectar than are some Lepidopterans. This led to speculation that perhaps some plants pump toxic compounds into their nectar to deter inefficient pollinators, leading to more specialization among pollinating insects that can handle the toxins.

Another hypothesis is the nectar robber hypothesis. This hypothesis is quite similar to the pollinator fidelity hypothesis except that it extends to all organisms that could potentially rob nectar from a flower without providing any pollination services. As such, it is a matter of plant defense.

The nectar of Cyrilla racemiflora is thought to be toxic to some bees. Photo by Koala:Bear licensed under CC BY-SA 2.0

Others feel that toxic nectar may be less about pollinators or nectar robbers and more about microbial activity. Sugary nectar can be a breeding ground for microbes and it is possible that plants pump toxic compounds into their nectar to keep it "fresh." If this is the case, the antimicrobial benefits could outweigh the cost to pollinators that may be harmed or even deterred by the toxic compounds.

Finally, it could be that toxic nectar may have no benefit to the plant whatsoever. Perhaps toxic nectar is simply the result of selection for defense compounds elsewhere in the plant and therefore is expressed in the nectar as a result of pleiotropy. If this is the case then toxic nectar might not be under as strong selection pressures as is overall defense against herbivores. If so, the plants may not be able to control which compounds eventually end up in their nectar. Provided defense against herbivores outweighs any costs imposed by toxic nectar then plants may not have the ability to evolve away from such traits.

Where Spathodea campanulata is invasive, its nectar causes increased mortality in native bee hives. Photo by mauro halpern licensed under CC BY 2.0

So, where does the science land us with these hypotheses? Do the data support any of these theories? This is where things get cloudy. Despite plenty of interest, evidence in support of the various hypotheses is scant. Some experiments have shown that indeed, when given a choice, some bees prefer non-toxic to toxic nectar. Also, toxic nectar appears to dissuade some ants from visiting flowers, however, just as many experiments have demonstrated no discernible effect on bees or ants. What's more, at least one investigation found that the amount of toxic compounds within the nectar of certain species varies significantly from population to population. What this means for pollination is anyone's' guess.

It is worth noting that most of the pollination-related hypotheses about toxic nectar have been tested using honeybees. Because they are generalist pollinators, there could be something to be said about toxic nectar deterring generalist pollinators in favor of specialist pollinators. Still, these experiments have largely been done in regions where honeybees are not native and therefore do not represent natural conditions.

Simply put, it is still too early to say whether toxic nectar is adaptive or not. It could very well be that it does not impose enough of a negative effect on plant fitness to evolve away from. More work is certainly needed. So, if you are someone looking for an excellent thesis project, here is a great opportunity. In the mean time, do yourself a favor and don't eat any mad honey.

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

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