• An ecosystem is a complex system of interrelationships between living organisms and their physical environment. For example, a pond's ecosystem has organisms like fish, insects and algae all living together in one body of water. Larger factors, such as sunlight and regional weather conditions, have huge effects on the inhabitants of the pond. The complex system of interrelations between all the components, both living and nonliving, is an ecosystem.
  
  

Internal Factors

• Balanced ecosystems undergo cyclical changes. For example, a predator-prey cycle happens when a dominant species eats a weaker species to the point that there is no more food. The food shortage causes the predator population to decrease. Over time, the decrease in the number of predators allows the prey population to increase. Then the cycle begins all over again. An ecosystem can have numerous dynamic relationships like this, leading to predictable patterns of change.
  
  

Direct Drivers

• A direct driver is any factor that immediately and directly causes the overall ecosystem to change. For example, if the size of the pond were to shrink because of humans draining water out of it, this would affect all the inhabitants of the pond. The organisms would now have to share a smaller area, which would have a huge effect on predator-prey relationships. The overall patterns of the ecosystem would change forever during the re-balancing of its components. In this example, human action was the direct driver, but natural factors, such as weather or seismic activity, can be direct drivers as well.
  
  

Indirect Drivers

• An indirect driver is a distant factor that alters one or more direct drivers. For example, if a region's human population increases over the course of a decade, the number of fish harvested from the pond may increase, causing the fish population to decrease, and eventually leading to an increase in the insect population. Scientists study indirect drivers to understand the chain of effects that distant events can have on ecosystems. According to a report by Millennium Ecosystem Assessment, a scientific advisory committee, the five main indirect drivers are "population change, change in economic activity, sociopolitical factors, cultural factors and technological change."