SAT Subject Test Biology: Touring the Major Biomes
Surviving the SAT Subject Test means spending time getting familiar with ecology. Ecology covers the real lifestyles and interactions of all the different species on earth. About 23 percent of the E version of the SAT Subject Test in Biology hinges on something within the realm of ecology. If you’re taking the M version, expect around 13 percent of your test to be ecology-based.
The study of ecology starts at home — or rather, in the homes of the earth’s many and varied species. Biomes are the major biological divisions of the earth. Biomes, such as oceans, deserts, and savannas, are characterized by the area’s climate, plus the particular organisms that live there. The living organisms make up the biotic components of the biome while everything else makes up the abiotic components. The most important abiotic aspects of a biome are its amount of rainfall and its amount of temperature variation. More rain and more stable temperatures means more organisms can survive. It turns out that these two abiotic components are usually linked because the wetter a biome is, the less its temperature changes from day to night or from summer to winter. That’s why so many people want to move to southern California and Florida — the weather is never too hot and never too cold. The number of organisms that can survive in a biome is called that biome’s “carrying capacity”
Drying out in the desert
Deserts are areas that get less than about ten inches of rain per year. Although most of the deserts we know of are hot (like the Sahara), some are actually cold (like parts of Antarctica), so the real distinguishing characteristic of deserts is their extreme dryness.
Temperatures in deserts change a lot from day to night and from summer to winter. The organisms that live in a desert need to be able to survive these drastic temperature swings along with dry conditions, so the density and diversity of species are very low. In other words, the desert has a low carrying capacity. Cacti are among the few plants that can survive the hot deserts, because they can store water after a rainstorm in order to have a nice supply during the frequent droughts. Animals in the desert include reptiles like lizards and snakes, along with some arachnids like spiders and scorpions.
Getting drenched in the tropical rainforest
A tropical rainforest is the exact opposite of a desert. Tropical rainforests get lots of rain and the temperatures are very steady and mild all year round. These are perfect conditions for most terrestrial organisms, so the tropical rainforest has by far the greatest density and diversity of life. This means that the tropical rain forest has the greatest carrying capacity of all terrestrial biomes. The best known tropical rain forests are in South America and southeast Asia, and they contain densely packed trees, plants and vines that are home to millions of insect species, along with lots of rodents, reptiles, monkeys, birds, and just about every other kind of terrestrial animal there is.
Enjoying the fall colors of the temperate deciduous forest
The temperate deciduous forest is sort of average, with an average carrying capacity. It gets more rain than the desert, but less than the tropical rain forest. Its temperature changes quite a bit from season to season, but not as dramatically as the desert. Temperate biomes have a medium density and diversity of species, with big trees that lose their leaves in the winter (that’s what deciduous means), and lots of mammals like rodents, deer, and many types of song birds. You find this biome in the eastern U.S. and Europe. The changing of the leaves in autumn can really be a wonderful sight, but the winters can be pretty harsh.
Seeing the taiga for the trees
A taiga is another forest biome, but it has mostly evergreen trees like pine and spruce, along with animals like squirrels, deer, moose, wolves, bears, and birds. The taiga’s carrying capacity is a little lower than the temperate deciduous forest because the taiga is a little colder and it usually gets less rain. Good examples are the forests of northwestern North America and northern Europe and Asia.
Outrunning lions in the savanna
The savanna biome is mostly grasslands with a few trees here and there. It has one good rainy season with long periods of drought every year, so its carrying capacity is below average. Because of all the grass, this biome supports lots of grazing mammals such as antelope, zebra, and bison, along with the famous feline predators such as lions and cheetahs. The best known savanna is in central Africa, but the central prairies of the U.S. count as well.
Freezing fields of the tundra
The main characteristic of the tundra is that the ground stays permanently frozen. The extreme tundras like those near the north and south poles are too cold for almost anything to live there, so the carrying capacity is really low. The less extreme tundras can support lots of mosses and grasses, along with mammals such as caribou and bears. The tundra can actually be pretty nice in the summer, but winter turns the place into a deep freeze.
Splashing around in freshwater
The freshwater biome includes things like rivers, lakes, and ponds. These areas can be affected by temperature swings, the amount of available O2, and the speed of water flowing through. All of these are affected by the larger climate area the freshwater biome is in, which also affects the biotic components. For example, lakes and rivers near the tropics can have really high carrying capacities, while those in the tundra will have very low carrying capacities. Algae, fish, amphibians, and insects are found in freshwater biomes.
Exploring the open oceans
The oceans cover about 70 percent of the earth’s surface, so this is by far the biggest biome. The temperature swings aren’t nearly as big in the oceans as they are on land, and there’s plenty of water to go around (duh, it’s the ocean) so the carrying capacity of the oceans is really huge. The density and diversity of organisms isn’t quite as high as in the tropical rain forest, but the total number of organisms in the oceans is way bigger than all the terrestrial biomes put together.
The ocean biome is divided into the following different regions:
- Intertidal zone: The intertidal zone is the beach. This area is covered by water during high tide and is exposed during low tide. Lots of seaweed, crabs, sea urchins, and starfish tend to live here.
- Neritic zone: The continental shelf extends out pretty far before dropping off to the really deep ocean — that part after the beach but before the drop-off is called the neritic zone. The diversity and density of species here is pretty high, with lots of fish, seaweed, and crustaceans.
- Pelagic zone: The pelagic zone is out in the really deep water and is divided into two layers:
• Photic zone: The photic layer is where sunlight penetrates to promote photosynthesis. Lots of phytoplankton live here, so that’s also where plankton-feeders like blue whales hang out.
• Aphotic zone: The aphotic zone is completely dark because sunlight does not penetrate, so there’s no photosynthesis. Down there live a lot of decomposers that eat the bits and pieces of dead stuff that sink down to the bottom, and there’s also some freakish looking predators that feed on the decomposers.
Now for some sample biome questions.
Which of the following biomes has the lowest carrying capacity?
- temperate deciduous forest
- tropical rain forest
The desert is the driest biome among the choices, plus it has the largest temperature swings, so it definitely has the lowest carrying capacity, so (E) is the correct answer. Some deserts have a decent number of cacti and reptiles, but some, like the Sahara, are almost nothing but sand dunes.
Which part of the ocean biome is likely to have the most decomposers?
- intertidal zone
- aphotic zone
- photic zone
- neritic zone
- continental shelf
When something dies in the ocean, it usually sinks to the bottom. Decomposers feed on dead things, so they probably hang out near the bottom. Aphotic means “no light,” so the aphotic zone is where light does not penetrate, which is in the deepest part of the ocean. That means (B), the aphotic zone, is the correct answer.