Rainforests are arranged in horizontal layers, and many animals spend their whole lives in a single layer, never venturing up from the forest floor or down from the canopy, for example. The ultimate source of energy for the whole system is the same (sunlight), and everything is in the end competing for sunlight, or to eat the things that captured the sunlight, or to eat the things that ate the things that ate the sunlight, or just to eat wastes and decayed remains of one kind or another. But while the source of energy is all the same, the physical structure of the system, combined with its year-round warmth and year-round rainfall, allows for a near-infinity of niches for organisms to inhabit (particularly small organisms like beetles), with the result of the famously huge number of species in a single rainforest.
Surprisingly enough, while tropical rainforests have very high primary productivity (the amount of sunlight energy trapped through photosynthesis and made into living things or used for their life processes), they aren’t uniquely high. Salt marshes, for example, the endless ugly wet stands of partially submerged grass near the mouths of rivers into the ocean (you see them near East Coast airports very often), beloved by biting bugs and birdwatchers and not many else, have essentially the same primary productivity as tropical rainforests.
Salt marshes, despite being very good at capturing sunlight and turning it into living stuff, tend to have quite low species diversity. The reason, as I understand it, is that the estuaries and brackish water present a problem that only a small number of organisms can solve– surviving in a wide range of temperatures, moisture levels, and especially salinities. (Often salt marshes attract a wider diversity of migratory birds, but like the old joke about Hell, things look different when you’re a tourist.) For organisms stuck there year round, the number of different obstacles and constraints that the salt marsh imposes mean that the number of potential biological solutions is much smaller, yielding lower species diversity. A stand of marsh grass benefits from the harsh conditions of its sometimes salty, sometimes freshwater, sometimes warm, sometimes freezing, sometimes dry and often inundated environment; it can solve the problem that few other organisms can solve, and it benefits from the lack of competition. Moreover, the physical homogeneity of the environment leaves few niches for organisms to inhabit- it’s all just one big mess of muck, without the individual microhabitats that distinguish the rainforest.
When we think about the forces that would contribute to intellectual or cultural pluralism versus monotony, we might think about both the rainforest and the salt marsh. The salt marsh makes everyone play by the same rules, and they are a very challenging set of rules, with multiple competing simultaneous problems to solve. The result is that only a small set of solutions can survive- the same sea grasses and same few species of animals over and over again. The rainforest both allows more independent subenvironments to persist, and allows each of those subenvironments to impose its own rules on its inhabitants. An organism struggling for survival in each of those niches does not experience its life as easy or forgiving– nature is red in tooth and claw in the Amazon every bit or more as in the marshlands– but the threats that nature imposes are more local, specific, and varied from place to place, with the result that far more varied solutions can result.
We should think about if we are making a forest or a marsh.