In arid ecosystems, cacti are primary resources for insects and arthropods that depend on dead or decaying wood. A new research review published in August in Annals of the Entomological Society of America explores this relationship and its ecological implications. Shown here are cactus forests in the Barranca de Metztitlán Biosphere Reserve in Hidalgo, Mexico (a) and Tehuacán-Cuicatlán Biosphere Reserve in Puebla, Mexico (b). Photos by Ana Paola Martínez-Falcón, Ph.D.)

By Carolyn Bernhardt

Cacti stand as stoic, resilient icons against a harsh backdrop of soaring temperatures, sparse vegetation, and limited water. Despite their prickly appearance, cacti help forge ecosystems by offering habitat and sustenance to various organisms in the arid and semiarid landscapes that punctuate our planet. And, beneath the surface, a quiet partnership between cacti and an intricate network of insects and arthropods churns away, keeping these ecosystems in balance—or so researchers suspect.

In a new research review published in August in Annals of the Entomological Society of America, a team of multidisciplinary scientists from Mexico explore cactus decomposition and its interplay with insects and arthropods that depend on dead or decaying wood. The study illuminates a previously underexplored relationship, revealing its ecological implications and the role of cactus species as wood resources for creatures uniquely adapted to life amid decomposing and decayed tissues. The article also delves into the complex journey of cacti as they transform into woody structures, creating intricate habitats for insects and other organisms.

Ultimately, the review captures a rich diversity of entomofauna (insects and related arthropods) associated with decaying and decayed cacti, with a significant prevalence of beetles and flies. In so doing, the research shows that cacti serve as fundamental wood resources for saproxylic insects (i.e., those that depend on dead or decaying wood). Revealing such a profound interconnectedness between these enigmatic plants and the specialized insect communities they serve offers a poignant reminder of the pivotal role cacti play in their ecosystems and could help inform management tactics for ecosystem health.

Decomposition and Soil Health

As wood goes through advanced decomposition, it releases organic matter that encourages the creation of soil aggregates. This makes the soil more porous, which means it can hold water and oxygen better. The decomposition process also affects how carbon and micronutrients move around in the soil. When larger pieces of organic material break down, they add more of these valuable substances to the soil.

Cacti decompose as they mature. However, human management, climate, and other abiotic factors can also drive decomposition in some parts of the cactus, according to Alfredo Ramírez-Hernández, Ph.D., researcher in the Environmental Sciences division at the San Luis Potosí Institute of Scientific Research and Technology in Mexico and principal investigator of the study.

Previous studies have focused more on the cacti, but the researchers say more information is needed to know the effect of biotic and abiotic factors on the dynamics of insect communities. “It takes more to know the wood of the cacti. In fact, that was one of the objectives of the article,” Ramírez-Hernández says, “to demonstrate that they are a source of lignin and cellulose and that the associated entomofauna, together with their associated microorganisms, could be specialized in the use of lignified tissues or cellulose.”

Deciphering the Dominant Players

The review assessed 117 papers, with 49 sources ultimately fitting the team’s selection criteria. “We were astonished by the scarce literature found, because there are 1,851 species of cactus in the world and 850 species in Mexico,” says Ramírez-Hernández. Yet, the entomofauna associated with decayed cactus have been studied only in 92 cactus species.

Among the insect orders reviewed in the study, beetles and flies emerged as significant players, constituting 85 percent of the entomofauna studied. This finding clarifies these insects as major contributors to cactus decomposition, highlighting their evolutionary adaptations to exploit this specialized niche.

The review also showed that flies in the family Drosophilidae (vinegar flies) demonstrated a close relationship with cacti and yeast, amplifying their impact on decomposition. According to the researchers, the concept of decomposed cactus stems acting as “fertility islands” in dry soils holds promise, as the nitrogen released during cactus breakdown could potentially enrich the soil and impact nutrient availability.

Protecting Cacti Safeguards Insects

The review’s efforts to unravel the intricate dynamics of the ecological collaboration between cacti and insects lend insights into the sustenance of specialized insect communities, helping to clarify what elements lead to ecological equilibrium in these contexts.

This information can, the team says, help hone key management tactics. “Today, we are facing an insect decline,” says Ramírez-Hernández. “It is important to know the insects associated with this ephemeral habitat to improve understanding about insect extension—i.e., are the most studied insects disappearing or is it a generalized pattern? Concerning management practices, we consider it important to keep rotting cactus logs on the ground to allow for natural degradation and nutrient cycling. By protecting the cactus, insects are protected.”

However, decomposition involves other insect groups besides the well-known insect orders of Coleoptera (beetles) and Diptera (flies). Further investigation and study into additional groups would help researchers better understand their role in the decomposition process.

Since the study centered mainly on dry and partially dry areas, its findings might not reflect the variety of cacti in different places. The complex classification of insects also makes it hard to correctly identify them, which could affect the types of species included in the review. The research team also says, “Despite the existing literature, [past studies] only provide lists of species, and few works have focused on the biology of the species and their role in decomposition.”

So now, Ramírez-Hernández and colleagues are working to develop a project that explores the diversity of arthropods in rotten cacti in other cactus species and in other arid regions in Mexico and beyond. They also hope to make experimental designs to measure changes in micronutrients derived from decomposition and learn more about the biology of the associated species, the relationships among them, and their associated organisms.

Still, their review highlights the resilient life surviving in the some of the world’s harshest environments and inspires a commitment to safeguard these delicate interactions. The complexity of interactions between insects, cacti, and microscopic organisms offers a promising view into the orchestration of decomposition. “Until now, there is scarce information about few insects [in this context]. The entomofauna in dry areas have been poorly studied,” says Ramírez-Hernández. “Studying the insects on decayed cacti [helps us understand] their role in the decomposition processes and improves our knowledge about the ecosystem functioning in the drylands.”

Carolyn Bernhardt, M.A., is a freelance science writer and editor based in Portland, Oregon. Email: carolynbernhardt11@gmail.com.