Living things survive within a specific range of environmental conditions.

Each biome required a mind-boggling array of considerations—not only how diverse plant and animal species would interact within and across biomes but also the nutrient requirements of each organism they planned to include. Termites, for example, would need enough dead wood at the beginning of closure to sustain them until some of the larger plants began dying off. Termites live in the soil, stirring it and allowing air to penetrate soil particles. If the termites ran out of dead wood and starved to death, organisms living in the soil would not get enough oxygen, and the entire desert would be jeopardized. Hummingbirds, on the other hand, would need nectar-filled flowers. “Try figuring out how many flowers a day a hummingbird needs,” says Tony Burgess, a University of Arizona ecologist who helped design the biomes in Biosphere 2 and remained involved until 2004. “From there you need to know what the blooming season is, and then what the nectar load per flower is. And then you have to translate all of that into units of hummingbird support. Now imagine doing that sort of thing about 3,000 times.”

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Biome-level planning was equally complicated. A rain forest would need consistently warm temperatures, but most desert biomes fluctuate wildly between day and night. In the summer, the grassland biome might literally starve the rain forest by gobbling up all the CO2 needed for photosynthesis.

KEY CONCEPT 8.4

Populations are able to survive and thrive in habitats with the needed resources and environmental conditions. Individual variability increases a population’s range of tolerance, expanding its distribution within the habitat as well as increasing the chance that a population will be able to adapt to changing conditions.

In this context, dead wood, flowers, and CO2 are all examples of limiting factors—resources so critical that their availability controls the distribution of species and thus of biomes. The principle of limiting factors states that the critical resource in least supply is what determines the survival, growth, and reproduction of a given species in a given biome. Living things can only survive and reproduce within a certain range (between the upper and lower limits that they can tolerate) for a given critical resource or environmental condition, referred to as their range of tolerance. INFOGRAPHIC 8.6

limiting factor

The critical resource whose supply determines the population size of a given species in a given ecosystem.

range of tolerance

The range, within upper and lower limits, of a limiting factor that allows a species to survive and reproduce.

RANGE OF TOLERANCE FOR LIFE

Populations have a range of tolerance for any given environmental factor (such as temperature). Every species has an upper and a lower limit beyond which it cannot survive (in this example, temperatures that are too cool or too hot). Most individuals, like the butterflies in this population, can be found around the optimum temperature, though what is “optimum” for each individual may differ slightly because of genetic variability. Some individuals may find themselves in areas of the habitat that are warmer or colder than the optimum. They can tolerate these conditions but may be physiologically stressed and not grow well or successfully breed. Genetic differences that allow some individuals to tolerate or even thrive at the edges of the population’s tolerance offer the population a chance to adapt to changing conditions (such as a warmer climate) if needed. The more narrow the range of tolerance and the less genetically diverse the population, the less likely it will survive a change in conditions.

Which population would have the greatest chance of adapting to an increase or decrease in the average temperature—one with a broad range of tolerance for temperature or one with a narrow range of tolerance?

The one with the broader range of tolerance; this simply increases the chance that some individuals present would be able to tolerate the new climatic conditions, even if others died out. In a population with a narrow tolerance for temperature, it would not take as big a change to increase or decrease the temperature such that it was outside of their ability to survive or reproduce and produce a new generation.

Ecologists routinely monitor a variety of limiting factors as a way of assessing ecosystem health, but anticipating what each individual organism would need to survive before the fact proved daunting.

An all-star team of scientists—oceanographers, forest ecologists, and plant physiologists—spent 2 years sorting through these challenges. Drawing on their combined expertise, they set about choosing the combination of soils, plants, and animals that seemed most capable of working together to re-create the delicate balances that had made Biosphere 1 such a spectacular success. A summer-dormant desert, like the ones found in Baja California, was chosen because it would reduce the desert’s CO2 demands when the savanna’s productivity was at its highest. The ocean was situated between the desert and rain forest so that it could serve as a temperature buffer between the two. And each biome was created from a carefully selected array of species: The marsh biome was composed of intact chunks of swampland harvested from the Florida Everglades, and the savanna was composed of grasses from Australia, South America, and Africa. Well water mixed with aquarium salt filled the ocean, to which were added coral reef sections culled from the Caribbean.

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But it wasn’t long before the rigor and pragmatism of good science began to clash with the idealism of Biosphere 2 financiers. And when that happened, critics say, science lost out.

Some scientists worried that the ocean wouldn’t get enough sunlight to support plant and animal life. Others opposed the use of soil high in organic matter; soil microbes decompose the soil’s organic carbon and release it into the air as CO2. Although this organic soil might eliminate the need for chemical fertilizers, there were concerns that it would provide too much fuel for the soil microbes and would thus send atmospheric CO2 concentrations through the roof. Despite these concerns, scientific advisors to the project were overruled.

The first few months of Mission 1 went smoothly enough, but eventually, plants and animals started dying. Humans grew hungry and mysteriously sleepy. And before long, tempers flared.

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Like Earth, Biosphere 2 was designed as a materially closed and energetically open system: Plants would conduct photosynthesis fueled by sunlight that streamed through the glass, but no biomass would enter or leave. Temperature, wind, rain, and ocean waves would be controlled mechanically. “But the facility would have to be self-sustaining,” says Burgess. “Everything would die, unless the biota met its most fundamental purpose—using energy flow for biomass production.” Humans would have to survive exclusively on what they could grow or catch under the dome. The system as a whole would have to continuously recycle every last bit of nutrient that was in the soil on day one.

KEY CONCEPT 8.5

The matter cycles that move nutrients through ecosystems depend on living organisms and abiotic reservoirs of those resources.

At first, the carefully constructed agricultural biome seemed well suited to the challenge. Carrots, broccoli, peanuts, kale, lettuce, and sweet potatoes were grown on broad half-acre terraces that sat adjacent to the sprawling six-story human habitat. A bevy of domestic animals also provided sustenance—goats for milk, chickens for eggs, and pigs for pork. Indeed, eating only what they could grow made the biospherians healthier. Bad cholesterol and blood pressure went down; so did white blood cell counts. And slowly, the biospherians say, their relationship to food changed. “Inside, I knew exactly where my food came from, and I totally understood my place in the biosphere and how it impacted the food I ate,” says Jayne Poynter, the biospherian responsible for tending the farm. “When I breathed out, my CO2 fed the sweet potatoes that I was growing. When I first got out, I lost sense of that. I would stand for hours in the aisles of shops, reading all the names on all the food things and think ‘where does this stuff come from?’ People must have thought I was nuts.”

But the glazed glass of the dome admitted less sunlight than had been anticipated. Less sunlight meant less biomass production. And that meant less food. Mites and disease also cut crop production. Biosphere 2’s size precluded the use of pesticides and herbicides: In that small an atmosphere, the toxins would build up rapidly and could have had a deleterious impact on air quality and human health.

Now, a few months in, food was starting to run out. And that wasn’t the only problem.