CSET Practice Test Subtest II Science

Modeling Earth Systems

Convection
Inside the Earth motion of heat by radiation is very 
ineffective because rocks are opaque to the near 
infrared and visible areas of the electromagnetic 
spectrum, conduction is extremely slow process in the 
mantle because rocks are poor thermal conductors, 
taking billions of years for heat to go through even a 
fraction of the mantle's thickness. So, the only 
mechanism that can efficiently evacuate heat from the 
Earth's interior is convection. Convection results from 
the fact that, when heated, most solids and fluids 
expand, thus decreasing in density which makes them 
buoyant. Inside the Earth it might appear that 
convection would not be possible since the mantle is 
solid. However, although the mantle appears solid to 
us and it is indeed solid on short time scales such as 
those of seismic waves, in a scale of millions of years 
the mantle behaves like a viscous fluid, able to flow 
at relatively high speeds (tens of centimeters per year) 
making convection possible. Not so the lithosphere, 
which is nearly rigid and therefore releases its heat 
mostly by conduction. The flow of the mantle over 
geologic time is driven by gravity, which acts on the 
density differences created by the loss of heat at the 
Earth's surface.

Food Webs - A Study of Environmental Energy Flow

Almost all animals must eat other organisms to obtain 
energy to live. Plants and algae make their own food 
energy using light from the sun in a process called 
photosynthesis. Organisms that make their own food 
energy are called autotrophs (self feeders). All other 
organisms must gather their food energy by eating 
other organisms or organic matter (like dead stuff 
decaying on the ocean floor). These types of 
organisms are called heterotrophs. All animals are 
heterotrophs and may either eat plants or animals to 
obtain energy. Animals that hunt and eat other 
animals are predators. The animal which is hunted is 
called the prey. Some biologists consider herbivores 
to also be a type of predator. Some animals eat both 
plants and other animals (like bears), and they are 
called omnivores.

Animals do not generally eat just one thing, nor are 
they eaten by only one thing. For example, a seal 
may eat many different kinds of fish, and seals are 
eaten by both orcas (killer whales) and sharks. This 
means that each organism, through feeding, is 
interconnected to may other organisms. This 
interconnection is called a food web. Food webs are 
the energy relationships between organisms within a 
community. It is important to stress here the difference 
between a food web and a food chain. Each of the 
members of a food web eat other organisms, and so a 
food web is made up of may food chains. Food chains 
have different levels, which describe the hierarchy of 
the food/energy relationships. For example, microscopic 
algae in the ocean are primary producers--they produce 
their own food using sunlight. All autotrophs are primary 
producers. Small crustaceans, such as copepods, eat 
these microscopic algae and are called primary consumers 
because they consume the primary producers. All 
herbivores (plant eaters) are primary consumers. Small 
fish in turn eat these copepods and are referred to as 
secondary consumers. Larger consumers which eat the 
small fish are termed tertiary consumers, and so on. 
When a food web is written out on a piece of paper, an 
arrow is drawn from the prey, to the predator. This arrow 
indicates the direction of the flow of food energy in the 
environment (because the predator receives the energy 
from the prey).