Go to a nursery here, in La Jolla, and you will see species of plants that got their start all over the world. Most of the cacti originated in Mexico and Latin America. On the other hand, most of the vibrant succulents evolved in Africa and Madagascar. My neighbor, Charlie, will have nothing to do with such exotica. He has fanatically devoted himself to rendering his garden as an exemplar of plants native to La Jolla and its environs. But “native” in what sense?
Charlie’s principle of inclusion for a plant is that it pre-dates the arrival of the White Man. It makes for an austere tableau, except in the Spring, which arrives in early January and lasts through April. Although his yard includes 40 native species, it is dominated by a few varieties of coastal sage brush.
The language of nature is infused with relativism. Charlie’s idea of the native marks a boundary that walls off invasives from Europe, but as native tribes migrated they too brought alien species with them. Aside from managing his yard, Charlie is one of a team of para-botanists, each assigned to keep an eye on sections of the backcountry in San Diego. They inventory plants in their respective domains, thereby making it possible to monitor changes. Charlie’s assigned area is nine square miles, and so far he has logged over 200 species of native plants. Imagine a new plant makes its appearance in the space that Charlie tracks, but it does so by means of three different vectors. The first is the result of hikers, who picked up seeds of the plant elsewhere on their gear. The second is the result of migrating birds that flew over Charlie’s patch and excreted seeds from the same plant that they ate elsewhere. The third is the result of climate change. San Diego county ranges from sea level to Mount Palomar at 6,500 feet. Plants that heretofore could not survive at higher altitudes are gradually propagating further afield.
Does the vector by means of which the introduction of the new plant occurred make a difference as to its status as an invasive? If one of them is an invasive, then why not the others? Like the hikers, when tourists visit the Galapagos Islands, they are cautioned to take care not to inadvertently introduce non-native species. This seems pretty straightforward – the boundary is a matter of causal history. If the same plant, introduced by the hikers, was a mutation of an indigenous plant, then we would treat it as native. It’s the external origin that marks it as non-native. But the same seed, excreted by a bird flying overhead is just as external as the hikers. Well, is there a problem in treating it as an invasive as well? Consider Kauai, which, when it was created by erupting magma about 5 million years ago it was, obviously, completely barren. On this view all of its current lush flora count as invasives, whether they originated with humans, birds, or seeds carried by wind or ocean currents.
In the end, perhaps native comes down to nothing more than being “first” and their (causally related) descendants – be they native plants or native peoples, whatever their origins. But even this rough and ready idea does not quite work as is because it implicitly treats ecology as static. External life populated Kauai over millions of years, filling in unoccupied niches. Can we say “first in its niche” then? This is only slightly better since it treats the idea of a niche as static, as if niches are a set of cubbyholes, some of which are filled and some of which are empty, waiting for new occupants. It also treats the idea of a niche as if it is defined independent of living things. Of course, all of this is fanciful. Unoccupied niches are defined contrastively by existing life forms, in the sense that they are where there is less competition for resources.