Farmers, fishermen, and scientists have all learned that the environment and ecosystems change through time. This change is called variability and variability can occur over many time scales. A good example is the stock market. However, over the last 12 years the stock market has increased from around 2,000 to almost 10,000. So, on the short term stocks are down, but over the longer term stocks are way up.
Similar changes occur in the marine environment to numbers of plants and animals.
One of the jobs of marine scientists is to try to separate human-caused changes to marine life from the natural variability that occurs over many time scales. A couple of these time scales may be familiar to you. The first is seasonal, with which we are all familiar. We know when to expect to see strawberries at the store, or albacore at the offshore fishing grounds. Environmental changes that occur over time scales such as seasons and El Ninos can have dramatic effects on our coastal wildlife populations, and to really know if we are impacting our coastal ecosystems we need to understand this natural variability.