Clivia Foodweb: Part II

Simple Food Chain: bulb - mite - mite

A few weeks ago I introduced the actors in the unfortunate demise of one of my favourite bulbs, a hybrid Clivia. Above is a simple food chain showing the relationships between the three most important species. Red arrows point to the consumer from the organism being consumed: what is eating what. The primary miscreant in this story is the Bulb Mite Rhizoglyphus robini Claparède, 1869. Bulb Mites are major pests of stored bulbs and of field crops such as onions. The predatory mite Gaeolaelaps aculeifer (Canestrini, 1884) is known to be an effective biological control agent. Well, the most abundant predatory mite in my Clivia pot turns out to be this same predator. So why did my bulbs still rot and die?

Bulb Mites don't eat just bulbs

Well, one reason is that Bulb Mites don't just eat bulb tissue. In fact, they would probably starve if all they had to feed on was healthy bulbs. Instead, they vector microbes that rot the bulbs and are attracted to small wounds on a bulb which they inoculate with disease causing microbes. Bulb Mites get most of their nutrition from these microbes. The bad (and opportunistic) microbes eat the bulbs. The predatory mites can do little to save a bulb that is already being eaten by microbes, but they may reduce the population of mites enough so that other healthy bulbs do not get infected.

A more representative food web

A second reason that the Predatory Mites may not have saved my Clivia is that they are not specialist predators on Bulb Mites. Gaeolaelaps aculeifer are very aggressive and will attack and eat any small arthropod or worm that they can overcome. The soil extracted from the Clivia pot had lots of tiny worms and tiny arthropods to distract the Predatory Mite from doing its job. Soil has many bacteria and fungi that live off of nutrients that leak out of roots or are present in humus and these microbes are necessary to healthy soil. These microbes are the primary foods of most of the small mites, springtails, and worms that live in pots with healthy plants.

Actors in the Clivia soil food web

Each of the three representations above of what is eating what have gotten more and more complex. The first is a simple food chain that represents the most important elements of the story if your primary interest is growing healthy bulbs. The first level of the chain is a primary producer (Clivia) that captures sunlight and makes plant matter. The second a primary consumer, the Bulb Mite. The third a secondary consumer, the Predatory Mite. Each of the next two representations of the feeding interactions adds more reality and complexity: a simple chain becomes more like a web of interactions. But the Clivia extractions had a dozen species of arthropods: 8 species of mites, 2 of springtails, and 2 of rove beetles.

A soil food web based on Clivia bulbs

This is the best that I can do to enumerate the feeding interactions in one pot of soil with a couple rotting Clivia bulbs. The good news is that except for the two species of bulb mites, the other animals are harmless or beneficial. The Rove Beetles are probably the top predators in this system - munching on all the smaller arthropods and worms. But I haven't found any papers that give details of their diets other than that Anotylus insecatus is known too feed on bulb maggots. Besides, if I had to draw in arrows linking the Rove Beetles to their probable prey, then things would really get messy. But, that's the way Nature operates.