Elemental 1: Critical Thermal Limits

I’ve been thinking about a scientific term recently: critical thermal limits. Scientists use it to understand the exact temperatures and habitat conditions that render a species vulnerable to collapse.

Through acclimation, human bodies are adaptable, but only to a point. The proposed limit of human physiological adaptability is a wet-bulb temperature (temp with 100% relative humidity) of 35°C.

Critical thermal limits apply to all species. As species move close to these thresholds, there are novel, unexpected opportunities for adaptation. And new perils. Take spiders.

Spiders will likely become more aggressive in the heat. Like honey giving up its steady state on the stove, as temperatures rise a spider’s blood will course thinner, more rapidly, increasing its agility and speed.

Yet in extreme heat, spiders will also be clumsier, more likely to drop from trees. Their eggs, if exposed to extreme heat, are less likely to hatch. Many species will soon reach a 'fertility thermal limit'.

There is a phenomenon in some spider species: gigantism. Females spiders can be fifty times larger than males, the largest difference in the animal kingdom. Males and females will become vulnerable to heat in different ways. Smaller males have a higher surface to volume ratio, making them more susceptible to over-heating. But they are mobile, can seek the shelter of rocks, and the cool leafy hummus of ground.

Female spiders often have a smaller range. They remain stationary for long stretches, waiting in their webs for prey. They must be careful to spin their webs in areas of cool air and shade.

Finding cool shady places is harder in the creeping biological deserts that we humans create. Human-made green spaces - gardens, parks, golf course, crop fields – are commonly monocultural, containing only a few plant species. Here, plants are clipped, mowed, separated, controlled. Such spaces offer spiders fewer places of respite. This type of greenery warms more rapidly than wild vegetation with its crosshatch of scruffy canopies and leaves.

Thinking about thermal limits helps me see the resilience and vulnerability of life around me. Spiders help me remember that while resiliency and vulnerability appear as opposites, they often go hand in hand.

Image by: Luc Viatour