by Brian Tomasik
First written: 4 Nov. 2016; last update: 4 Nov. 2016
Grassland vegetation feeds many invertebrate herbivores and detritivores, including springtails. I estimate that on a typical grassland, each gram of dry matter of vegetation creates roughly 3 to 6 springtail-years, as well as life for many other invertebrates (mites, earthworms, etc.).
Following is a screenshot from this video to illustrate what one gram of (fairly) dry vegetation looks like (the amount on the scale):
I've also drawn yellow dots on the person's thumb fingernail to illustrate the rough size of 6 springtails. A thumb nail is ~1.5 cm (which I verified on myself), while springtails "are normally about 1 mm long." So I drew the yellow dots to be about 1/15 of the width of the person's thumb. The calculation in this piece finds that the mass of vegetation shown, if it were in a grassland ecosystem, would feed, among other animals, the 6 yellow dots for a year (including replacement dots if these die during the year).
Grassland plants feed numerous invertebrates, both while the plants are alive (e.g., grasshoppers, leafhoppers, aphids) and once the plants die and decompose (e.g., earthworms, mites, springtails). Springtails are prominent components of the detritivore community. For example, this page says of springtails: "some estimates suggest they are responsible for up to 20% of litter fall decomposition in some places."
This piece attempts two rough calculations of how many years of springtail life get created by a given dry mass of grassland vegetation. Obviously numerous non-springtail invertebrates are also created by grassland plants, so this calculation is extremely conservative as far as estimating the total number of invertebrates created by grassland primary productivity.
Calculation based on actual springtail densities
This study includes the following map in Fig. 4, showing modeled net primary productivity (NPP) of pastures and hayfields.
The paper says "the grassland process-based ecosystem model PASIM was used to estimate the spatial-temporal distribution of NPP, providing a European average value of 750±150 g C across extensively grazed, intensively grazed pastures, and forage production systems."
This page says:
The carbon content of vegetation is surprisingly constant across a wide variety of tissue types and species. Schlesinger (1991) noted that C content of biomass is almost always found to be between 45 and 50% (by oven-dry mass).
That page recommends using 47.5% as a conversion factor. So, an NPP of 750 g C per m2 per year is roughly 750/0.475 = 1600 g dry matter (DM) of vegetation per m2 per year.
This study reports that 104 to 105 springtails can be found per m2 of European grassland. Let's be conservative and call this an annual average of 104 springtails (probably less than this in winter and more in summer), or roughly 104 springtail-years per year.
Finally, I'll assume that in equilibrium, one year's springtail population on grassland is supported by one year's net primary production. Then, we have
(104 springtail-years per m2 per year) / (1600 g vegetation DM per m2 per year) = ~6 springtail-years per g vegetation DM.
Alternative calculation based on efficiency of converting food to body mass
Suppose that -- just making this up -- 5% of vegetation is eaten by springtails (sometimes indirectly, via springtails eating fungi/etc.).
This study reports that "The body mass of adult springtails ranges between 0.1 and 0.2 mg (Christian, 1979; Verhoef and Witteveen, 1980)" (average = 0.15 mg).
Suppose that the ECI of springtails is -- just making this up -- 4%. I'm setting this number low because some detritus is eaten indirectly through bacteria, fungi, etc., which means the food goes up two trophic levels. The ECI for some insects is actually much higher than 4%:
That said, the mealworms and crickets mentioned in the table were (sadly) being grown as food, and we would expect that insects chosen for this purpose would have high ECIs. Moreover, they were fed high-quality diets. For example, the mealworms were fed rolled oats and given carrots "as a source of water."
Starting with 1 g = 1000 mg of grassland vegetation DM, 1000 * 5% = 50 mg would be eaten by springtails, and it would yield 50 * 4% = 2 mg DM of springtail body mass. Humans are ~1/2 to ~2/3 water by mass, and I'll assume something similar is true for springtails. For simplicity, suppose springtails are 50% water, so that 2 mg DM is 4 mg of wet matter. Assuming that an adult springtail has a mass of 0.15 mg, that's equivalent to ~30 adult springtails.
Now we need to convert springtails into springtail-years. Springtails may mature into adults in as little as 3-5 weeks. Based on this number, let's assume adult springtails live, say, 1/10 of a year.a Then 30 adult springtails is 30 * 1/10 = 3 springtail-years. This is surprisingly close to the previous estimate of 6 springtail-years per g vegetation DM.
This calculation assumed that all springtails live to adulthood. Of course, that's not actually the case; most springtails die young, given that "A female springtail can lay 400 eggs in her lifetime." However, the calculation I just did still works if we make the (unrealistic but not totally wrong) assumption that springtails accumulate body mass at a constant rate (mg per day) throughout life. Given this assumption, 1000 mg vegetation DM can create 30 adult springtails or 2 * 30 springtails that live half as long as adults. Either way, it's the same number of springtail-years. More generally, any distribution of the 1000 mg vegetation DM among springtails of any set of lifespans will produce the same number of springtail-years.
Estimate per gram of detritus
The above calculations estimate springtails on a grassland where grass is eaten both by detritivores and herbivores, possibly including large herbivores like cattle. What if the vegetation is instead eaten only by detritivores, such as is the case in a compost bin?
This book reports on an experiment in which "42% of the energy captured by ryegrass in the leniently grazed pasture entered the detrital food chain". In general, a rough estimate might be that ~half of grassland vegetation is eaten by herbivores and ~half by decomposers like bacteria, fungi, and springtails. In this case, an estimate of springtails per gram of vegetation DM would be ~double the previous estimates if the vegetation is only eaten by decomposers.