Thermodynamics and Entropy Made Easy
The first law of thermodynamics (conversion of energy) is a beautiful thing in that it allows us to have a simple equality equation: Energy in = Energy out
Whatever form the energy is in; work, heat, kinetic energy, potential energy… in a closed system the sides must be equal.
The second law (entropy) comes in when you realize that all of the neat and tidy ingredients going into a system are not so neat and tidy out the other side. For instance, a jet engine takes in liquid fuel and cold air but out the other end you get a turbulent mixture of exhaust gases and motion that could never be restored into liquid fuel and air.
Since we can often measure all of a closed system's inputs and outputs (temperature, pressure, flow rate, work, etc.), this entropy difference can be calculated as an actual measure of energy (Joules) just by solving the rest of the equation. Simply put, if we have numbers for nine of the variables in an equation we can solve for the tenth.
The ability to actually calculate and place a number on such a seemingly diffuse and random concept as entropy is the magic of the second law of thermodynamics.
What all this means for ecosystems is that a very real amount of energy is lost at every transformation, not just to heat but also to entropy (disorder). This happens up the food chain (trophic pyramid), during primary production, and even when you eat a twinkie.
Another interesting point is that we and other organisms are more ordered (low entropy) in comparison to our less ordered (high entropy) environment. Since entropy is always trying to move from low to high, we are out of energy-equilibrium with our environment, requiring a constant input of energy to maintain us.