The soil in tropical forests lacks the nutrients of temperate forests as the majority of goodness is stored in the trees themselves. The relatively thin layer of soil doesn’t even offer enough support for vast number of magnificent trees. This has possibly lead to the evolution of the buttress root system. Trees are also supported by one of the highest biodiversity components of tropical forest, the vines. The soil still contains a large amount of life like different arthropods, nematodes, earthworms, flat worms, and, of course, bacteria.
Scientists have found a larger fraction of earthworms in Amazon soil than other locations around the globe, but the Amazon soil community was dominated by arthropods and particularly mites. Nematodes and flatworms also took up significant fractions.
Amazon Rainforest Earthworms
A favorite study animal of Charles Darwin, earthworms are more important to the world as we know it than commonly believed. You may have heard about the dark earth found in the Amazon Rainforest. It is unusually high in nutrients, which makes people think large areas of the Amazon Rainforest have grown on land that was artificially fertilized i.e. farmed land. The theory is that ancient civilizations farmed these areas and when their societies collapsed from either introduced diseases or invasion, their land was abandoned allowing forest to regrow. Along with this theory is the idea that Amazon tribes are really the descendants of scattered societies that fled into the wilderness to escape attack, disease, or enslavement. Supporting this bold theory are the remnants of ancient knowledge and tradition in the culture of certain tribal groups.
There are other ideas to the existence of the dark soil, and one revolves around an earthworm by the name of Pontoscolex corethrurus. This earthworm eats a mixture of soil and charcoal, grinds it up in its gizzard, combines it with intestinal mucous, and egests it on the forest floor. This increases the carbon content of the soil meaning plants tend to grow faster and it causes the mysterious dark color. The activity helps the earthworms as it increases the plant matter in the forest, which means there’s more organic matter for the earthworms to eat.
Amazon Rainforest Bacteria
There hasn’t been a lot of research on Amazon Rainforest soil bacteria, but we know the soil contains a diversity of different species and many that don’t fit into known bacterial kingdoms. However, there are also familiar bacterial faces like Streptomyces. Amazon Rainforest soil is more acidic than other soils and this is supposedly linked to a comparative lack of bacterial diversity compared to soils elsewhere.
Flatworms include the not so cute and cuddly tapeworms most people have heard of, but also harmless non-parasitic species like the Pasipha backesi above. Another non-parasitic flatworm is Geoplana, which is more commonly encountered. There are about 100 different Geoplana flatworm in the South American tropics. Geoplana are predators and hunt the forest floor for invertebrate prey like earthworms, snails and insects.
Arthropods make up the largest component of soil animal life and its the mites that dominate this community. Mites are mainly found in the soil and litter of the rainforest floor, and its common for 50,000–250,000 mites to be found in the upper 10cm of a square metre of soil. Not all mites live on the ground, however. One rainforest tree was found to contain over 400,000 mites that lived in the tree’s leaves. Also remember that not all soil in a rainforest is on the ground, as there is a large amount in the canopy environment. For example in tree holes & epiphytes. The most diverse group of soil mites are the beetle or armored mites, the Oribatida. As high as 75% of all the arthropods collected from a tree can be Orbatid mites.
Nematodes are very adaptable animals and live in a range of environments from the ocean to the tropical forest. The nematode in the video (Caenorhabditis elegans) is about 1mm in length and so it’s understandably difficult to study in the wild. This species prefers to live in plant stems and rotting fruits and flowers rather than the rainforest soil. Rainforest soil still contains many species, but there are higher numbers of nematodes in grassland soil rather than forests, and the Amazon in particular seems to have a lower nematode diversity component than many other places.
If you’re unfamiliar with nematodes, it’s simply because our senses are not powerful enough to detect them. For example, Nathan Cobb illustrates in the following passage that nematodes are hardly undetectable because of their low numbers:
“In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites.”