I have just posted a video from the scene of the action. I show some clumps of alder (Alnus maritima) growing in the Blue River in south central Oklahoma. One glance and you would say that the river is their habitat. (By the way, that habitat, with its white water emerging from the aquifer, is the nicest place to do field studies in Oklahoma in the summer—in fact, it might be the only nice place to do field studies in an Oklahoma summer.)
But the alders are not just growing in the river. They are creating their own habitat, in four ways.
First, once an alder bush starts growing on a rock in the river, its trunks emerge from the water. (Sometimes floods wash away the trunks; they just grow back, usually within a year.) The trunks slow the water down just a little bit. Dead leaves and stems in the river accumulate at the trunk base, as well as some mineral soil. The alder clumps are creating little islands of soil in the river that would not have been there without them. When these dead tissues decompose, the soil can nurture other plants, such as the sycamore saplings that grow up from the soil.
Second, the roots of the alders have little nodules that are filled with Frankia bacteria. Like the more familiar Rhizobium that grows in legume roots, the Frankia bacteria “fix” nitrogen; that is, they take nitrogen molecules from the air (the little air spaces in the soil) and fix it into ammonium molecules, which act as a fertilizer for the alder. It is a mutually beneficial (symbiotic) arrangement: the alder feeds the bacteria, and even protects them from excessive levels of oxygen gas. In turn, the bacteria produce more ammonium than they need. That is, the alder roots are absorbing nitrogen atoms from the inside, not from the outside. But it goes beyond this: when the alders lose their leaves, stems, and a few roots, the plant tissues have nitrogen atoms in them that came from the bacteria. But when these dead tissues decompose, the nitrogen atoms go into the soil, where it can be used by other plants, such as the walnut saplings that grow up from the soil.
Third, the roots of the alders have mycorrhizal fungi growing in them. This is what makes them fat and orange rather than skinny and brown the way most roots are. The alder feeds the fungi, and in turn the fungi extend their filaments out into the soil and absorb phosphate ions more effectively than could the roots themselves. It is a mutually beneficial (symbiotic) arrangement: the alder feeds the fungi. In turn, the fungi absorb more phosphate than they need. That is, the alder roots are absorbing phosphorus atoms from the inside, not from the outside. But it goes beyond this: when the alders lose their leaves, stems, and a few roots, the plant tissues have phosphorus atoms in them that came from the fungi. But when these dead tissues decompose, the phosphorus atoms go into the soil, where it can be used by other plants, such as the dogwood bushes that grow up from the soil.
But wait, there’s more! Fourth, the mycorrhizae of one alder clump can connect with those of another alder clump, causing the whole alder woodland to form an interconnected whole. They can share nutrients and even send signals to one another. This is the “wood-wide web” that Suzanne Simard first wrote about in 1997. The individual alder clumps create their own individual habitats in three ways, and a collective habitat in a fourth way. How cool is that?
Natural selection does not always favor competition and warfare among species. It can favor cooperation and mutualism. Natural selection favors whatever works: sometimes competition, sometimes cooperation, so long as it enhances the reproduction of the individuals, in this case, the alders. The alders are doing well by doing good.