Thoughts on the ecological differences between flies and dung beetles

(writing in progress) 
 
To continue on the topic of the ecological differences between flies and dung beetles, both of which eat faeces:
 
Think of flies as bacteriophages and dung beetles as herbivores. In that sense, they are not in the same guild. So, is the coincidence that they both eat faeces actually misleading?
 
Faeces contain both plant residues and bacteria. The bacteria remain alive in the faeces as long as they remain moist and of suitable temperature. To some extent the bacteria continue to degrade the plant fibre in the faeces.
 
The ecological strategy of flies (Diptera) is for the larvae to select the bacterial material from the faecal mass, leaving the fibre out of their diet. Because bacteria are relatively rich food, the larvae can grow fast.

It is possible for the winged adult to emerge in as few as 10 days after the eggs are laid, because the flies are ‘skimming’ the best food from the faeces, as it were.

Many flies will breed successfully in filth other than faeces, e.g. rotting food or drain sludge. And then of course there are many flies that specialise on animal matter, their larvae consuming carcases as they rot.

I am unsure if the latter again eat mainly bacteria. However, the point is rather academic, because animal matter can be converted to fly biomass quite rapidly, whether the bacteria are used as agents or not.

At the adult stage, flies have diets suited to the lightness and mobility of these winged, fast-flying insects: they restrict their diet to liquids or eat rich foods which they are mobile enough to find. Adult flies lack a gut of mass/capacity sufficient to process plant fibre. A bit like butterflies, flies emphasise the larval stage for eating, and the adult stage for breeding.
 
The ecological strategy of dung beetles (Coleoptera: Scarabaeidae) is different.

The larvae and adults both eat plant fibre as their main diet. Although the adults do choose liquids as far as possible, their diet is not essentially different from that of the larvae. Although most dung beetles fly, they are not in the same league as, well, flies.
 
Dung beetles, depending on their body size, set out to consume dung wholesale, which means that most of what they eat is plant fibre. They process the dung mechanically, to a degree achieved by no fly of similar body size.

This is partly because no fly is as massive as many of the dung beetles. Their larvae grow slowly compared to fly maggots, because the larval diet of fibre is relatively poor.

As adults they continue to eat dung, or juices from dung. Certainly this includes bacteria. However, they are incapable of separating bacteria from fibre, as fly larvae do efficiently.

The life cycle of most dung beetles, even if they are as small as flies, seems to take weeks or months instead of days or weeks.
 
The faeces of bosts (Bos spp.) consist of extremely finely milled fibre, deposited wet and providing a poor substrate for bacteria. They are sufficiently nutritious for large dung beetles, but make a poor diet for flies or even small dung beetles.

Such flies as can breed in them have only superficial effects on the pats. This is because the fly larvae live ‘interstitially’ as it were, and inevitably leave most of the mass of the pat intact.

If there are no large dung beetles in the local fauna, the pat remains (once it dries) like a mat of papier-mache, covering pasture and preventing new growth. This is at least half of the reason why the Australian CSIRO made such an effort to bring in dung beetles from other countries to Australia.

‘Cattle dung’ is a problem because it is eaten by the (larvae of) the bush fly, but by the same token it is a problem because it is not eaten by the bush fly, if you see what I mean (the adults cannot eat bost dung to any extent and even the larvae can only eat so little that they hardly even undermine the structure of the pat).
 
I am drawing a rather long bow here, but I might suggest that the faeces of emus are special in that they provide particularly suitable food for flies, while not providing particularly suitable food for large dung beetles. Emu faeces are rich compared with ‘cattle pats’ despite sharing the sloppy consistency. This is because the difference between emu and bost is that the former has a remarkably superficial digestion (with laughably small gizzard and large intestine), while the latter has remarkably thorough digestion (with a four-chambered stomach, cud-chewing, and a substantial large intestine).

'Cattle dung’ consists essentially of mashed cellulose plus water, with a minimum ‘bacterial soup’ in it. By contrast, emu pats consist of gross fibre (including even some lignin) plus bacterial soup – and of course in many or most cases lots of really gross items such as whole seeds or even whole fruits that look like they have been hardly digested, and may still be green.

In principle, emu faeces are similar to those of elephants, just on a smaller scale and usually sloppier. Because of the difference in body size, the faeces of emus are even less digested than those of elephants. However, both animals are extremely wasteful, in the sense that they eat a lot and just skim the top off the food, as it were. Both also seem to be remarkably willing to eat gross fibre, almost to the degree that they seem to seek it out gratuitously (what on Earth are emus thinking of when they consume whole banksia inflorescences or casuarina cones??).

My point is that emu faeces provide lovely bacterial soup plus lots of fibre so coarse that even those herbivores, the dung beetles, find the fibre daunting. The result, I suggest, is that emu faeces are as suitable for the larvae of the bush fly as human faeces are, but less suitable for dung beetles than human faeces are. Emu faeces are so grossly fibrous and so sloppy that I doubt that many ball-rolling dung beetles could even roll them.
 
(writing in progress)

Publicado el julio 19, 2022 06:39 MAÑANA por milewski