As always there are many factors, but there is a factor that they share with many other large organisms, whales, sharks, elephants, dinosaurs, and even tarantulas. They have relatively few smaller relatives in their environment. In fact, neither the coastal redwood nor the Sequoia have any close relatives in their environment.
The coastal redwood and the Sequoia are the closest relatives of one another, but the natural range of the coastal redwood and Sequoia are not close to one another. The coastal redwood's natural range is, as the name suggests, close to the coast. The Sequoia lives in the Sierra Nevada mountain range. Their next closest relative, the dawn redwood, is in China, while the coastal redwood and Sequoia are mostly in California. In their natural range, the closest relatives of both trees diverged from their line in Mesozoic, the age of the dinosaurs, or earlier, probably more than a hundred million years ago.
White-tailed deer, the most numerous terrestrial mammal greater than a 50 kilograms, 100 pounds, full grown, provide another example. The white-tailed deer carries a brain worm that can infect and is generally fatal to other deer, including the larger Moose, and Elk. The white-tailed deer survives the brain worm, but other deer find it difficult to impossible live in the range of the white-tailed deer. This is because of the brain worm and several other diseases the white-tailed deer carries.
On the other hand with a large population, it is more likely that some individual within that population will be immune. This will allow that individual to survive and reproduce and through natural selection the whole population is likely to eventually have the immunity.
So living in the range of a close relative that is numerous is a disadvantage, both because they are likely to have many dangerous diseases and because they are more likely to be immune to a disease that is deadly to you.
Smaller organisms are generally more numerous. This is true if we look at the population at any one instant in time, but it is even more true over time. There are likely to be more individuals of a species of mouse than a species of elephant. But if we compare all the individual mice of one species living over the course of a century to the elephants that lived in that same century the difference is likely to be even more dramatic because the larger organism usually has a longer life span. As coastal redwoods and Sequoias can live for thousands of years they are a good example of this rule.
If two similar groups of organisms compete, for example, marsupials and placental mammals, the one which is more fit will tend to take over the niche that has the most individuals. For example, the niches where individuals are small, like mice. The more fit or advanced group will then have trouble taking niches where there are fewer individuals, for example, large animals, like elephants. The less fit group will then win that niche. Niches with fewer individuals like large and predator are consolation or booby prizes that evolution gives to less fit groups. Note the less fit individual does not win, and the less fit species does not win, it is the less fit group of species that wins the booby prize.
People naturally tend to see large animals and predators as winners and not losers. Biologists do not seem to have escaped this bias. If the biologist is sophisticated enough they might say size is not a measure of biological success. But even this is wrong. Size is an indicator of success, it is just that the fit are small, and the less fit are large.
The largest freshwater fish in South America is the arapaima. Its closest relative is the African arowana, from which it diverged 220 million years ago, in the Triassic, the earliest of the three major divisions of the Mesozoic. But as the African arowana is on a different continent it will never infect the arapaima. The arapaima's closest relative in South America is the South American arowana. The arapaima's ancestors diverged from the South American arowana line 275 million years ago in the Permian, which ended when the Mesozoic began. The closest relative an arapaima will ever meet, and perhaps eat, diverged from the arapaima long before the evolution of the first mammal. So all mammals are more closely related to each other than an arapaima is to its closest relative in its environment, the South American arowana.
In North America we find much the same pattern. The largest purely freshwater fish is the alligator gar. Gars are like the arapaimas and arowanas considered primitive. The various species of gars and a distant relative, the bowfin diverged from other bony fish 320 to 330 million years ago, in the Carboniferous period that preceded the Permian.
In Europe the largest freshwater fish is the Wels catfish, which is also the only catfish in most of its range. So once again it does not have close relatives to infect it with a disease. I believe that catfish diverged from other fish in the Mesozoic so in most of its range it will encounter no relative with a common ancestor more recent than the Mesozoic.
In the sea whale sharks are the largest fish, and great white sharks are the largest fish which prey on other fish. Sharks are only distantly related to the predominate bony fish.
Sharks are one example of a simple principle if one is at the top of a bony fish food chain it is best not to be a bony fish, because a disease is likely to move up the food chain, much like mercury. But diseases are even worse because mercury does not reproduce, and diseases do. Whales, seals, and sharks are at the top of that food chain and none are bony fish. This is true now, but it has also been true in the past. In the Mesozoic various reptiles, most famously ichthyosaurs, plesiosaurs, and mosasaurs were often among the largest animals in the Mesozoic oceans. None are bony fish, all are like today's marine mammals descended from land animals.
Coelacanths are still another example. They are living fossils unrelated to other fish in their deep sea environments, and they are giants.
So again and again with fish arapaima, gars, wels catfish, sharks, coelacanths, we see the same pattern we found in giant redwoods and Sequoias, giant living fossils with relatively few is any relatives in their environment.
Many other examples could be given outside of fish and redwoods. Elephants, ostriches, and tarantulas are also examples, and there are many more.
The fact that giant redwoods and Sequoias follow the pattern show how general it is. We can find a huge number of examples among animals but even among plants the pattern still holds.
So while there are many factors that allow giant redwoods and Sequoias to grow so large their isolation from smaller disease carrying relatives is likely to be an important factor.
There are a couple of other pages on this web site dealing with this general idea: whales, and quetzalcoatlus the giant Mesozoic flying reptile.
Last Updated June 17, 2015