Life at High Temperatures
by Thomas D. Brock
Although the upper temperature limit for animal life is about 45 degrees C (113 degrees F), there are vast areas where the thermal waters have cooled enough so that animals can live. In fact, in many of the hot spring run-off channels, certain specialized animals actually thrive, developing in surprisingly large numbers.
The commonest animals of the hot springs are ephydrid flies, small nonbiting insects that are often found in profusion on the microbial mats. These flies mate in the hot spring environment and when mating usually occur in large clusters. These insects are best seen in winter, when they are unable to live away from the mats because of the cold. In summer the adult flies can best be seen at night (with a flashlight) or early in the morning, when they congregate on the warm mat; during the daytime they are likely to be on the mat only in cloudy weather when the air is cool and humid.
One type of ephydrid fly, Ephydra bruesi, lays bright orange-pink eggs in masses on top of twigs, stones, or other projections above the mat (see photo below). These eggs are most obvious in the winter, when they contrast against the dark-green of the mat. In the summer, when the mats are more orange in color, the eggs are more difficult to see.
Ephydrid flies on mat (National Park Service)
The maximum temperature at which the adult flies can live is about 43 degree C (109 degrees F). However, the adults can go partway underwater surrounded by a bubble of air which acts as insulation, so that they can feed in hotter water.
The ephydrid flies are strict vegetarians, but other animals associated with hot springs are carnivores and eat the ephydrids. One common carnivore is the spider. These animals live at the edges of springs, making mad dashes out onto the mat to catch adult ephydrids. As long as the spider keeps moving, it can traverse hotter water than it can otherwise stand.
The wolf spider, a voracious predator of Ephydrid flies
Ephydrid flies, both larvae and adults, feed on the cyanobacteria. These flies are themselves fed on by other insects, including dolichopodid flies (a vicious predator of the ephydrids), spiders, dragonflies, beetles, and wasps. In addition, the ephydrids are parasitized by red mites, which attach themselves to the bodies of the adult flies, or feed on the pink egg masses. There is also a wasp which parasitizes the pupae of the ephydrids!
These various parasites and predators are themselves fed upon by larger carnivores, of which the most important are killdeers, birds that are often seen stalking their dinners on the geyser basins.
This whole array of organisms constitutes the hot spring food chain, a surprisingly complex, albeit specialized, ecosystem.
The Hot Spring Food Chain
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