Category Archives: Insects and the economy

Unsung Heroes: The Humble Dung Beetle

If I asked someone to make a list of beneficial insects, he or she would most likely think of pollinators like honeybees or maybe predators such as dragonflies.  I suspect that very few people would think to include dung beetles on their list, but these insects are actually very important, ecologically and economically.  Chelse and I were recently at a grassland ecology conference held at the Konza Prairie Biological Station.  We had a very lively and very entertaining discussion with an ecologist from South Africa about whether or not insects were important to ecosystems.  This ecologist maintained that insects could not have strong effects on ecosystems the way that large vertebrates do, however he did make a single exception: dung beetles.  So why are dung beetles so important?  What do they do and why should we care?

Let’s start with some natural history.  There are around 7,000 known species of dung beetles, which belong to the superfamily Scarabaeoidea.  Dung beetles are found on all continents except Antarctica, and occur in a variety of habitat types including grasslands and forests.  Dung beetles are coprophagic, which means they eat poop.  Dung beetles eat poop as adults and as larvae.  Many dung beetles roll the dung into a ball that is used for food or to create a “brooding ball”, in which the females lay eggs.  After the eggs hatch, the larvae feed on the dung.  Dung beetles can be grouped into three main categories.  “Tunnelers” bury their brooding balls in the ground, often near the original dung.  “Rollers” transport the dung balls farther away from the original dung, often in remarkably straight lines, before they bury it.  Dung beetles recently made news when it was discovered that they can use the stars to orient themselves!  Dung beetles are the only species apart from humans known to do this.  “Dwellers” brood their young inside the dung, rather than in separate brooding balls.

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Watching dung beetles is fun! Im always impressed by how fast they move and how symmetrically round they get the dung balls. This beetle is from Konza Prairie, where there is a herd of ~ 300 bison.

Most areas support a diverse community of dung beetles made up of species from each of these groups.  The effects of dung beetles on ecosystem services depend on the composition of the dung beetle community.  Dung beetles with different behaviors (i.e. rollers vs. dwellers) will have different effects on ecosystem services.   The maintenance of ecosystem services by dung beetles depends on a diverse assemblage of dung beetle species.

So how do dung beetles affect ecosystems?  Dung beetles contribute to several ecosystem services.   A nice review paper by Nichols et al (2008) details the many ecosystem services provided by dung beetles.  I’ve summarized the highlights from their paper here.

Dung beetles are important to nutrient cycling.  Poop contains many nutrients.  By breaking poop into smaller pieces and moving it underground, dung beetles facilitate decomposition and make the nutrients available to other organisms.  Studies have found that many soil nutrients are increased when dung beetles are present.  Dung beetles also increase nitrogen mineralization.  Nitrogen mineralization is the process by which nitrogen is converted from an organic to an inorganic form, making it available for use by plants (and subsequently, the rest of the food web).  This is important because nitrogen is often a limiting nutrient in terrestrial systems, which means that adding nitrogen can increase productivity.  Thats why fertilizers contain nitrogen.

Dung beetles are also important for soil bioturbation.  Bioturbation is the mixing and redistribution of sediments.  This is an important process that affects soil moisture and soil aeration.  For comparison, earthworms are also important for bioturbation, which is why gardeners like to see them in their flowerbeds.  Tunnelers are especially important for bioturbation.

By increasing nitrogen availability and by facilitating decomposition, nutrient cycling, and bioturbation, dung beetles can increase plant productivity.   Many studies have found that dung beetles can increase plant biomass, nitrogen content, and grain production.  Some studies have even shown that the positive effects of beetles on plant production are equal to or higher than the effects of chemical fertilizers!   Dung beetles may also contribute to plant productivity by dispersal of seeds found in dung, which can lead to increased plant recruitment.   And some species of dung beetles are important pollinators of decay-scented flowers.

Dung beetles can reduce the abundance of parasites and flies that breed in dung.  For example, fly abundances and parasite loads in livestock decrease when dung beetle abundance is experimentally increased.

The importance of dung beetles to livestock production has been demonstrated in Austraila, where cattle were introduced by Europeans.  Because the local dung beetles were not acclimated to feeding on cattle dung, rangelands became fouled with slowly decomposing cattle dung.  Cattle will not forage near dung, so as the amount cow poop increased, rangeland available for cattle foraging declined.  Pests like flies also increased in abundance, but declined sharply after dung beetles were introduced.

Economic importance.  Losey and Vaughan (2006) published a really cool paper that estimates the economic value of four ecosystem services (dung burial, pollination, pest control, and wildlife nutrition) provided by insects.  They estimated the economic benefit of dung beetles to cattle production in the United States is worth at least $380 million a year!  This is based on the many ecosystem services that dung beetles provide that increase forage production and decrease livestock pests.  It’s important to note that this figure doesn’t include the effects of dung beetle on production of other types of livestock, or on ecosystem services not related to livestock production.  This means that the economic value of the ecosystem services provided by dung beetles in the United States is actually much higher.

Bison at Konza Prairie.  A health dung beetle community contributes to sustainable management of bison and other large herbivore populations.

Bison at Konza Prairie.  A healthy dung beetle community contributes to sustainable management of  ecosystems like grasslands and the animals that live there.

Importance of a diverse dung beetle community.  Dung beetles are important to ecosystems and provide a variety of important ecosystem services. There is still a lot to learn about how dung beetles affect ecosystem processes, especially in forests.  But protecting dung beetle diversity and abundance may be critical to the sustainable management of many ecosystems, including rangelands used for livestock production.  For example, a recent study by Beynon et al (2012) finds that high dung beetle species richness increases dung decomposition rates, and may be especially critical for maintaining ecosystem services when the system is disturbed.  They measured decomposition rates of cattle dung exposed to one, two, or three species of dung beetles.   After 36 weeks, more dung was decomposed when three species of dung beetles were present, than when only one or two species of dung beetles were present.  They also conducted the experiment on dung collected from cattle treated with ivermectin, which is used to reduce parasites.  Dung decomposition in these treatments was lower than dung decomposition of control treatments (no ivermectin), but not when three dung beetle species were used.

So what do these data tell us?  They tell us that ivermectin treatment of cattle can have negative effects on decomposition by dung beetles.  However, when a more diverse assemblage of dung beetles (3 species) was present, this negative effect of ivermectin disappeared.  This is because each of the dung beetle species used responded differently to the ivermectin treatment.   So, having several species available means it is more likely that there is at least one species that can tolerate new or different conditions.  Maintaining high species diversity of dung beetles may be critical to maintaining the ecosystem services they provide.

So give some love to the humble dung beetles!  Not only are they really cool animals, they provide many important ecosystem services that are economically and ecologically valuable.

References:

Beynon, S. A., D. J. Mann, E. M. Slade, and O. T. Lewis. (2012). Species-rich dung beetle communities buffer ecosystem services in perturbed agro-ecosystems. Journal of Applied Ecology 49:1365-1372.

Losey, J. E. and M. Vaughan. (2006). The economic value of ecological services provided by insects. BioScience 56:311-323.

Nichols, E., S. Spector, J. Louzada, T. Larsen, S. Amequita, M. E. Favila, and The Scarabaeinae Research Network. (2008). Ecological functions and ecosystem services provided by Scarabaeinae dung beetles. Biological Conservation 141:1461-1474.

Insect Songsnoise or Music?

You’ve undoubtedly heard many insects making sounds, and maybe you find certain noises of theirs annoying, such as flies and bees buzzing. There are some true musicians in the insect world: namely, orthopterans (grasshoppers, katydids, and crickets) and cicadas. There are countless scientific studies on insect songs, ranging from descriptive studies that detail the particular song of a species, to how temperature affects insect songs. If you’re interested, there is a blog dedicated solely to the music of nature, which discusses insect songs much more in-depth than I will here.

I started thinking more about the songs of these insects when I started working at my current field site at the University of Houstons Coastal Center, a remnant of rare tallgrass coastal prairie along the Gulf Coast of Texas. You can hear different orthopterans (grasshoppers, katydids, and crickets) calling at different times of the day. The softer thrills of southeastern crickets while it’s dark, and loud calls of katydids during the heat of the day, which to me often sound like those sprinklers with rotating heads.

Not knowing too much about these different insect calls, I started looking for a good resource on the subject and stumbled across this wonderful book by Lang Elliott and Wil Hershberger. It describes 72 species of orthopterans and cicadas that produce songs in east/central North America. The best part, for an insect geek like me, is the CD it contains that provides sound clips of many of these insects singing, and if you’re particularly interested, you can learn to identify many species of these singing insects by learning their calls.

Most of these songs are produced to find a mate, and it’s more often the males calling to attract females than visa versa. Sometimes calls let another male, potentially rivaling for females know not to get too close, that this area is claimed. A few band-winged grasshopper species (characterized most simply by bright, colorful wings) produce a clicking sound both in an attempt to scare off a potential predator, and, again, to attract mates.

So, are people affected by these insect musicians? My guess would be if you asked this question to the average urban or suburban American, they would say no. In these settings, we hear fewer insects simply because there isn’t much habitat for them in cities, and even where there is grass, it’s often a lawn that is mowed or treated with pesticides. If they happen to be observant and live in the areas of certain annual cicadas whose calls are harbingers of summer, they may say yes. Maybe folks in rural areas would be inclined to admit a nostalgia for or distinct feeling of summer upon hearing insect songs, as this is when many of these singing insects are active.

David Attenborough talks about what is considered one of the most spectacular gatherings of insect musicians globally, the songs of 17-year cicadas in the midwestern US in the BBCs Life in the Undergrowth (which, as a side note, is a must watch series for anyone that appreciates insects).

The first time I experienced this phenomenon was as a child in 1987, growing up in Northern Kentucky. Once they emerged, cicadas appeared to be dripping off plants. You could not walk down the sidewalk without stepping on one. My mom and I made cobblers from blackberries we’d picked. My aunt bit into something crunchy, and sure enough, a cicada had snuck into the cobbler. Either way, the major thing I remember was people complaining about them. In places with many old trees where the ground has not recently been tilled, the screeching can, as Attenborough says, be deafening. This large emergence of cicadas has other important effects on ecosystems, including effects nutrient cycles (Yang 2004), and providing huge pulses of food for predators, which often causes a boom in predator populations, such as red-winged blackbirds that also have distinctive calls (Strehl and White 1986). Although not everyone appreciates this phenomenal spectacle, it is certainly a wonder of nature.

Apparently, in certain areas of Asia, however, singing insects are revered. In Japan, people sell insects as a profession (and are called mushiya; there is a book chapter about this here).  People keep them in their houses in the winter time so they can hear their songs year-round, and plan vacations to places where certain insects are known to call. Calling insects are also important to people in China: there has even been recent press about this (although, the title is misleading the insects are not singing for their food). One man, who apparently carries around a singing orthopteran in his pocket is quoted as saying, During the winter it is dry and cold and you cannot hear any birdsong. The sound of bush crickets makes me happier. That would make me happy in the cold, gray winter, too!

But now there is evidence that singing insects are affected by our noise as well.  A forthcoming study in Functional Ecology has gotten a lot of public press recently (Lampe et al 2012) provides evidence that male grasshoppers near German roadsides alter their songs to be better heard by potential female mates. This is the first evidence that insect musicians must now compensate for human noise. I wonder what other insects may be changing their songs because were a noisy species?

Human feelings about these singers may range from annoyance to appreciation to reverence. I’d like to do more research but haven’t seen any studies that attempt to quantify how humans feel about the sounds of different insects. Although Japanese and Chinese cultures are certainly influenced by insect songs, their economies must be affected in some small way, I haven’t yet found any studies that quantify this.

I, for one, have gained a fondness for insects songs, and a desire to be able to identify insects by their songs since my time working in prairies. Do you notice these singing insects? Do they affect your life in any appreciable way?