There are more described species of insects than any other animal. Of the hundreds of thousands of described insect species, only a small handful live in or on the ocean. Ecologists and biogeographers ask the question, “Why are insects so successful on land but not in the oceans.” There are a number of possible reasons, but none are entirely convincing. One suggestion is that insects have difficulty adapting to the high salt concentrations. However, insects can be found in salt marshes and saline lakes that have even higher salt concentrations than oceans. Some have suggested that the wave action of the oceans deter insects. However, large lakes, such as the Great Lakes have both waves and plenty of insect species. Could competition be excluding insects from the oceans?
A large part of the success of insects is due to their ability to fly. When animals first moved out of the water and onto land over 400 million years ago, no animals had the ability to fly. Animals that live in oceans would gain little or no advantage by flight. In the oceans, even very small animals can travel great distances by hitching rides on the ocean currents. On land, movement is very different. Small animals can be blown by the wind and occasionally transported, but this method is not very reliable and has a high mortality. Most small animals are restricted to very small territories. Many terrestrial arthropods never travel more than a few meters in their lifetime.
Insects gained an advantage over all the other terrestrial species because they were the first to have powered flight. Recent evidence pushes the evolution of powered flight as far back as 405 million years ago. The oldest known true insect fossil is belongs to a group of that is known to have powered flight. The evidence suggests that insects may have evolved flight within a few tens of millions of years after the first insects evolved. Powered flight is a great advantage to small terrestrial animals. Flight allows small animals to move great distances in a short time. Imagine how long it would take an ant to travel the length of a soccer field. Imagine how much less time an ant reproductive with wings and powered flight can fly the same distance. Flight makes it easier for insects to find mates. Flight makes it easier for insects to migrate from areas of poor resources to areas of better resources. Flight makes it possible for insects to exploit food sources that are scattered about the landscape in very small patches. Flight is an important factor in the diversity and numbers of insect species. Less than one percent of the extant insect species belong to groups that formed before the evolution of flight. Over 99 percent of insects are descendents of insects that evolved flight. Clearly flight is important to the success of terrestrial insects. Insects were the only animals that could fly for well over 100 million years. Having the skies all to themselves gave insects a huge advantage and led to their great diversity.
The oceans, however, are a different story. The oceans were populated with animals long before life moved onto land and the evolution of insects. Flight would give no clear advanatage to ocean dwelling animals. The largest advantage that insects had over their terrestrial competitors (powered flight) gave them no advantage in the ocean. While flight can be an advantage to insects that inhabit rivers and lakes, or small bodies of water that can dry up, insects have no competitive advantage over animals that live in the oceans. To move into the oceans, insects would have to successfully compete with other species of animals that are well adapted to life in the oceans. Without a competitive advantage, we would hardly expect insects to diversify and proliferate in the oceans to a greater extent than the other animals that inhabit the oceans. Competition is a huge limiting factor that prevents insects from successfully colonizing the oceans. On land, insects greatly increased in diversity because they were masters of the air with scant competition.