The Understory


July 31, 2012

Invertebrates are by far the most abundant and most diverse animals in the rainforest. They have invaded nearly every niche imaginable and many unimaginable, and each plays a unique, although still poorly understood, role in the ecosystem. For example, in the soil invertebrates are essential in the process of decomposition. These species feed on broken-down plant plant material and organic particles. Earthworms, termites, and others fragment larger particles into sizes more manageable for bacteria, fungi, and microorganisms.

Most invertebrates in both tropical and temperate regions are small and inconspicuous but rainforests house some of the world's largest. For example, Malagasy planaria and tropical American beetles may reach six inches (15-16 cm), and centipedes may reach eight inches (20 cm). Centipedes are often brightly colored, are carnivorous, and kill their prey with poison claws located under their first body segment. In some centipede species the female carefully guards the young. Millipedes feed on rotting logs. Jewel beetles, scarabs, termites, and earthworms are part of decomposition high in the canopy, in the soil-like debris found on epiphytes. Scorpions are much smaller than their arid and temperate counterparts but often pack a powerful sting.

Leeches are fascinating rainforest dwellers even though their feeding habits repel most people. Rainforest leeches of Southeast Asia, Africa, and Madagascar, unlike the leeches of the United States, do not live in water, but are able to live terrestrially by the humid conditions of the forest. Leeches are blood-suckers that are attracted to their prey by movement, temperature, and carbon dioxide. If one sits in the rainforest of Borneo for a few minutes, leeches, moving like inch worms, can be seen approaching from the forest floor and even dropping from the trees. The victim often does not feel the bite of the leech, which has razor-sharp teeth and releases an anticoagulant into the bite to allow the blood to flow freely. Leeches are amazingly persistent, and once attached they should be removed only by dousing them with salt or shampoo or burning them with a cigarette butt. Leeches are a nuisance, but relatively harmless, since they carry no known diseases. Leeches can ingest as much as fifteen times their body weight at a single feeding, enough to satisfy them for six to twelve months before their next meal.

Leaf-cutter ant in the Amazon. Photo by Rhett A. Butler


One of the most fearsome groups of animals in the rainforest are not jaguars, snakes, or crocodiles, but ants. Many ants in the rainforest can inflict excruciatingly painful bites and stings on the unwary forest visitor. The 24-hour ant of South American is regionally famous for its bite that can leave the victim in terrible pain for hours. However, ants also happen to be one of the most interesting and important animals in the forest as exemplified by two ant types: army ants and leafcutter ants.

Army ants of the New World have long been depicted in fictional movies and books as a marauding force that threatens everything in its path including people and entire villages. This is hardly an accurate scenario. Some rainforest peoples actually welcome the periodic visitations by army ants to clear their hut of unwanted resident pests. In addition, forest peoples have been known to use large soldier ants (also soldier leaf-cutter ants) for medical purposes. The ant is picked up by its body with its powerful mandibles open, and placed over an open wound where it is allowed to clamp the wound closed. The native then twists the head off and the jaws remain as a temporary, natural suture. Although soldier army ants are formidable with their huge jaws, the majority of ants in a given column are medium-sized worker ants. The sheer numbers of these ants enable a column to overtake animals that normal-sized colonies could not. There are reports of tethered animals being devoured, but most of the column's prey consists of other invertebrates. The column scares up many insects that usually remain hidden or camouflaged during the day. Enough of these insects are scared up to support numerous species of birds that follow ant columns and feed exclusively on the insects. Dependent on the antbirds are ithomiine butterflies which feed on their nitrogen-rich droppings. Unlike other butterflies which are restricted to food reserves built up during their caterpillar stage, these ithomiine butterflies are able to live and reproduce for months thanks to the proteins gleaned from the bird droppings. The ithomiines are safe from predation by the antbirds, because as caterpillars they feed on leaves containing poisonous alkaloids and giving the adult butterflies an unpleasant taste. Other moths and butterflies mimic the warning coloration of ithomiine butterflies to afford themselves protection.

Other species benefit from the army ant columns. Trachinid flies wait in vegetation above the ant column for grasshoppers to appear. When one does, the fly lays an egg on its body. The egg hatches into a larva which burrows into the grasshopper and devours the insect from the inside. Certain insects including wasps, beetles, and millipedes are capable of chemically mimicking the odor of army ants so they are undetectable as they move through the column, since most ants have very poor vision and can only really distinguish between light and dark. These creatures are able to get free meals in the form of prey exposed or captured by the column.

The Old World equivalents of army ants are the driver ants, which are blind and move in massive armies (20 million) under the leaf litter.

Another well-known group of ants are the leafcutter ants of the genus Atta. Large columns of these ants are a common sight in tropical regions worldwide. Foragers, are the most apparent, carrying cut pieces of leaves, petals, and fruit from their place of origin back to their nest. Leafcutter ants exist in highly structured communities in which individual size determines the ants' specialized role in the community. The largest ant type is the soldier which may weigh 300 times more than the next largest ant type, the forager. At the bottom of the size scale is a tiny ant type whose function is to ride atop the leaves as they are carried by the foragers, and keep a watch out for species of flies that lay their eggs on the leafcutters' leaves. When the larvae of this fly hatch, they can decimate the entire colony.

Leaf Cutters in Perspective

The leaves that the foragers bring back to the colony are not eaten, at least not in the conventional sense. The leaves are taken into a chamber some 15-20 feet (4.5-6 m) underground where the leaves are cut into smaller pieces by smaller worker ants. The fragments are taken to another chamber where they are chewed up by smaller ants into a leaf paste. This leaf paste provides sustenance for a certain type of fungus, which grows and is farmed by even smaller ants. This fungus is fed upon by the ants. There is no central coordination of leafcutter activities: each ant simply carries out its job based on its size and age. Communication, as for most other ants, comes in the form of chemical pheromones which cause ants to react in a certain way (though audio signals are also important in ant communication). Even the harvested plant species benefit from the work of the leaf cutters; studies have found that growth rates increase for many plant species after "pruning" by leaf cutters.

Amazonian wax-tailed fulgorid (Lystra lanata) in the Amazon. Photo by Rhett A. Butler

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Continued: Rainforest waters

More reading: News articles about rainforest insects

Insects decline on land, fare better in water, study finds
- A meta-analysis has found that land-dwelling insect populations are decreasing by about 0.92% per year, which amounts to 50% fewer insects in 75 years.
- The numbers of insects that live in the water are on the rise by about 1.08% per year, a figure scientists attribute to effective water protection measures over the past 50 years.
- Habitat loss is associated with the decline of insects in this study.

Costa Rica caterpillar decline spells trouble for ecosystems
- A new study in Scientific Reports suggests declines in caterpillar richness in a protected Costa Rican tropical rainforest, as well as in the parasite species that live off them.
- Researchers examined data from 1997 to 2018 to identify long-term patterns of extreme weather events and the impact these have on insect diversity.
- More than 40% of the 64 common caterpillar genera decreased, suggesting the decline of entire groups of caterpillars.

The Great Insect Dying: How to save insects and ourselves
- The entomologists interviewed for this Mongabay series agreed on three major causes for the ongoing and escalating collapse of global insect populations: habitat loss (especially due to agribusiness expansion), climate change and pesticide use. Some added a fourth cause: human overpopulation.
- Solutions to these problems exist, most agreed, but political commitment, major institutional funding and a large-scale vision are lacking. To combat habitat loss, researchers urge preservation of biodiversity hotspots such as primary rainforest, regeneration of damaged ecosystems, and nature-friendly agriculture.
- Combatting climate change, scientists agree, requires deep carbon emission cuts along with the establishment of secure, very large conserved areas and corridors encompassing a wide variety of temperate and tropical ecosystems, sometimes designed with preserving specific insect populations in mind.
- Pesticide use solutions include bans of some toxins and pesticide seed coatings, the education of farmers by scientists rather than by pesticide companies, and importantly, a rethinking of agribusiness practices. The Netherlands’ Delta Plan for Biodiversity Recovery includes some of these elements.

The Great Insect Dying: The tropics in trouble and some hope
- Insect species are most diverse in the tropics, but are largely unresearched, with many species not described by science. But entomologists believe abundance is being impacted by climate change, habitat destruction and the introduction of industrial agribusiness with its heavy pesticide use.
- A 2018 repeat of a 1976 study in Puerto Rico, which measured the total biomass of a rainforest’s arthropods, found that in the intervening decades populations collapsed. Sticky traps caught up to 60-fold fewer insects than 37 years prior, while ground netting caught 8 times fewer insects than in 1976.
- The same researchers also looked at insect abundance in a tropical forest in Western Mexico. There, biomass abundance fell eightfold in sticky traps from 1981 to 2014. Researchers from Southeast Asia, Australia, Oceania and Africa all expressed concern to Mongabay over possible insect abundance declines.
- In response to feared tropical declines, new insect surveys are being launched, including the Arthropod Initiative and Global Malaise Trap Program. But all of these new initiatives suffer the same dire problem: a dearth of funding and lack of interest from foundations, conservation groups and governments.

The Great Insect Dying: Vanishing act in Europe and North America
- Though arthropods make up most of the species on Earth, and much of the planet’s biomass, they are significantly understudied compared to mammals, plants, birds, amphibians, reptiles and fish. Lack of baseline data makes insect abundance decline difficult to assess.
- Insects in the temperate EU and U.S. are the world’s best studied, so it is here that scientists expect to detect precipitous declines first. A groundbreaking study published in October 2017 found that flying insects in 63 protected areas in Germany had declined by 75 percent in just 25 years.
- The UK Butterfly Monitoring Scheme has a 43-year butterfly record, and over that time two-thirds of the nations’ species have decreased. Another recent paper found an 84 percent decline in butterflies in the Netherlands from 1890 to 2017. Still, EU researchers say far more data points are needed.
- Neither the U.S. or Canada have conducted an in-depth study similar to that in Germany. But entomologists agree that major abundance declines are likely underway, and many are planning studies to detect population drops. Contributors to decline are climate change, pesticides and ecosystem destruction.

The Great Insect Dying: A global look at a deepening crisis
- Recent studies from Germany and Puerto Rico, and a global meta-study, all point to a serious, dramatic decline in insect abundance. Plummeting insect populations could deeply impact ecosystems and human civilization, as these tiny creatures form the base of the food chain, pollinate, dispose of waste, and enliven soils.
- However, limited baseline data makes it difficult for scientists to say with certainty just how deep the crisis may be, though anecdotal evidence is strong. To that end, Mongabay is launching a four-part series — likely the most in-depth, nuanced look at insect decline yet published by any media outlet.
- Mongabay interviewed 24 entomologists and researchers on six continents working in over a dozen nations to determine what we know regarding the “great insect dying,” including an overview article, and an in-depth story looking at temperate insects in the U.S. and the European Union — the best studied for their abundance.
- We also utilize Mongabay’s position as a leader in tropical reporting to focus solely on insect declines in the tropics and subtropics, where lack of baseline data is causing scientists to rush to create new, urgently needed survey study projects. The final story looks at what we can do to curb and reverse the loss of insect abundance.