SOIL MITES and Other Animals

Dr. Maria Minor, Ecology Group, Massey University. E-mail: m.a.minor@massey.ac.nz


The Soil Biodiversity Lab

In the Soil Biodiversity Lab at Massey University, New Zealand, we study the taxonomy and ecology of invertebrates, especially free-living mites and other small arthropods that inhabit soil and litter in a variety of habitats. Below is a short introduction to the fascinating world of these animals.

 

 

Note: most images on this site are copyrighted. Please, do not use images without prior permission from the authors.

 

Working with mites requires a microscope!

What's in the soil?

The soil is a unique habitat that supports rich and diverse life. It is a matrix of myriad solid mineral particles and pores filled by water, air, and decomposing organic matter. The penetration of light into the soil is limited. Soil animals generally avoid light; many of them do not possess specialized eyes, using instead well-developed tactile and chemical receptors and communication signals. The soil atmosphere is saturated with water, poor in oxygen and rich in carbon dioxide. Most soil animals absorb and lose water through their integument and depend on water-saturated atmosphere for their existence. If the soil dries out, the animals in soil either die, exist as resistant cysts or eggs, or migrate away from unfavorable conditions. A square foot of forest soil may be inhabited by hundreds of species of invertebrates, many barely visible to the naked eye. These are organisms whose identities and life cycles are often unknown. Their intricate relationships within soil ecosystem and the roles played by individual species remain 'terra incognita'.

Every cubic inch of forest litter hosts hundreds of individuals from a variety of species. A large diversity of microscopic animals can be found in the soil of our back yards.

Berlese-Tullgren apparatus (below) is a great tool for separating microscopic animals from the litter and soil they inhabit. Free of soil debris, they can be counted and identified. A soil sample is placed on the sieve at the top of a funnel. A small lamp with a low-power light bulb heats and dries the soil from above, which stimulates the soil animals to move downward (positive geotaxis in response to dryness). This downward movement eventually causes the soil animals to fall through the sieve into a container with preservative. This method gives a biased sample of soil fauna, because it is based on specific avoidance behavior triggered by dryness and thus best captures animals that are mobile and do not desiccate easily. Immobile larvae, endophagous nymphs and soft-bodied invertebrates such as nematodes are not extracted by a Berlese funnel.

Berlese funnel

 

Berlese funnel - the actual view


Soil microarthropods

Most of soil animals extracted by a Berlese funnel are so-called microarthropods, arthropods with a body size of 0.1-5.0 mm. These animals are unable to dig their own way in soil; they inhabit soil crevices, pores, and hollows created by larger animals. As they consume plant and animal residues, graze on soil fungi and bacteria, and produce fecal pellets, these tiny arthropods promote the formation of humus in the soil and aid in maintaining soil structure. By dispersing microbial propagules, they assist in the microbial colonisation of fresh litter material. After death they leave important nitrogenous waste. Through interactions with fungal and bacterial microflora, these animals regulate decomposition rate, affect nutrient cycling and play an important part in soil fertility. The most numerous microarthropods are collembolans and free-living soil mites.

 

Mites from forest soil sample, Canada. Photo courtesy Valerie Behan-Pelletier and B. Eamer.

Microarthropods extracted from forest soil, New Zealand. Minor.

 

Mites from agricultural soil fertilized with chicken manure, NY, USA. Minor.


Collembola

Collembolans (common name "springtails") are generally 0.2-5.0 mm in length. Most but not all of them are able to jump using forked abdominal appendage, the furca. Collembolans are now not considered to be insects, although they share a common ancestor. Collembolans are omnipresent members of soil fauna, they inhabit both the surface and the depth of soil and usually occur in aggregations. Their diet consists of decaying vegetation and associated bacteria and fungi. They are also known to eat nematodes and dead animal matter. Many collembolans are opportunistic species capable of rapid population growth. Under favorable conditions they can reach the densities of more than 100 individuals per square inch. The commonly seen "snow fleas" are aggregations of millions of collembolans from the family Hypogastruridae. Collembolans are a major food source for a variety of soil predators.

 

Soil-dwelling springtails (Collembola: Hypogastruridae). BugPeople.org Reproduced by kind permission of Eddie Dunbar.

 

Springtail Neanura muscorum. Reproduced by kind permission of Steve Hopkin.

 

 

Surface-dwelling springtail Orchesella villosa. Reproduced by kind permission of Steve Hopkin.

 

This giant (8 mm in size!) springtail occurs on the South Island of New Zealand. A group of these springtails was found under the bark of a dead tree. Minor & Robertson.

 

 

Collembolans - alive and dead - are the popular food source for many soil mites. Predatory soil mite (Acari: Labidostomatidae) with a collembolan prey. Reproduced by kind permission of Roy Norton.


Oribatid mites

Mites of the suborder Oribatida (=Cryptostigmata), also called "beetle" or "moss" mites, are the world's most numerous arthropods living in soil. Their density in forest soil can reach hundreds of thousands of individuals per square meter. There are several thousand described species, yet the fauna of much of the tropics is still unknown. Adult oribatid mites usually have strong exoskeleton, hardened by sclerotization, as in other mites, and by mineralization, similar to millipedes and isopodes. These slow moving mites are 0.2 - 1.0 mm in length and occur in the top layer of soil, in litter debris, sometimes also on plants, mosses and lichens. In contrast to many other microarthropods, oribatid mites reproduce slowly. In cold climates, their life cycle can stretch up to seven years (usually one-two years). Females lay few eggs. Many species are obligately parthenogenetic, with no males recorded. With their slow metabolic rate, slow development and low fecundity, Oribatida are not capable of fast population growth and are usually restricted to relatively stable environments, in contrast to more opportunistic groups such as collembolans. Oribatid mites graze on fungi and algae, consume decomposing plant matter and occasionally dead collembolans. Some oribatids feed on live nematodes. For many groups feeding habits are still unknown. Adult oribatid mites experience relatively little predation due to their strong exoskeleton, though they are subject to predation by ants, certain beetles and small salamanders. The soft-bodied nymphs are attacked by many soil predators. Oribatida comprise an important component of soil decomposers; their abundance, species composition and diversity in a particular habitat serve as good indicators of soil "health".

 

Oribatid mite Jacotella quadricaudicula. Reproduced by kind permission of Roy Norton.

 

Immatures of Collohmannia sp. feeding on a dead leaf. Reproduced by kind permission of Roy Norton.

 

Oribatid mite Phthiracarus sp. The family Phthiracaridae - 'box mites' - is interesting because of the adult body shape, which allows them to fold up in defense.

 

 

Parthenogenetic oribatid mites common in agricultural soils in NY, USA - Nothrus sp., Scheloribates sp., Oppiella nova, Rhysotritia ardua. Minor.

Four-toed salamander - the predator of adult oribatid mites. Reproduced by kind permission of Roy Norton.


Gamasid mites

The cohort Gamasina (Acari: Mesostigmata) contains fewer species than oribatid mites. Gamasid mites are universally present in soil, though not as numerous as oribatid mites or collembolans. Almost all Gamasina are predators. Similar to spiders, they inject digestive liquid into the prey they caught, and then suck up dissolved tissues. The abundance and community structure of these mites reflect the availability of their prey. The larger surface-dwelling gamasid mites attack small arthropods (collembolans, soft-bodied mites, insect larvae and eggs). Smaller deep-litter and soil forms are predominantly nematophagous and are the most important predators of nematodes in many habitats. Some species are fungal feeders. Several genera are considered good bioindicators of habitat and soil condition.

Podocinum sp. Reproduced by kind permission of Roy Norton.

 

Large gamasid mite Pergamasus crassipes is a voracious predator. It attacks most soft microarthropods of smaller size. Minor.

Arrhenoseius gloriosus (Mesostigmata: Blattisociidae) from rainforest of Queensland, Australia. Reproduced by kind permission of David Walter.

 

Dinychus sp. Reproduced by kind permission of Roy Norton.


Soil Mites and Collembolans on WWW

Soil biota remains among the unknown on our planet. Our knowledge of distribution, abundance, population structure, and ecological roles and requirements is poor for many microarthropods. You can learn more about soil mites and collembolans at following sites:

Acarology WWW home page

Systematics, ontogeny and ecology of the family Acaridae, by Barry OConnor

The Almanac of Alberta Oribatida, by David Walter

A Pictographic Key to Leaf Litter Arthropods from the Missouri Ozark Forest Ecosystem Project (MOFEP) - Collembola, maintained by the University of Missouri (mite site still under construction)

Check-list of the collembola of the world, maintained by Frans Janssens


September 19, 2011