I (Kem Luther) was contacted recently by a British Columbia provincial biologist about non-native and invasive mosses. The topic is not much discussed in bryological circles. When compared with the impact of non-native vascular plant species, the ecological issues presented by the invasion of non-vascular flora are relatively benign.
The main problem with determining whether a moss is new to a specific area–to British Columbia, let’s say–is that the native ranges of the mosses, liverworts, and hornworts are quite large. The distribution map of a moss species, it has been often pointed out, is similar to the distribution map of a vascular plant genus. Another problem is that bryophytes were less known and studied in the period when vascular plants were being distributed by European colonization. Unless a moss has spread beyond its native range in the last century, we can’t always say whether it might have already been present in small numbers in its newly colonized range and simply overlooked by taxonomists.
There has been some attempt to describe the movement of bryoflora by defining geographic kingdoms. When mosses show up outside of their kingdom, they are candidate invasives. The problem is, these geokingdoms are really big. The land masses of the temperate zones of the Northern Hemisphere are usually assigned to a single kingdom. Many of the mosses and liverworts found in this huge geokingdom, moreover, are widespread, scattered throughout North America, Europe, and Asia. Even among the less widely distributed mosses, rates of endemism–bryophytes confined to just one small region–are extremely low.
Still, the bryokingdom system does give us some insight into the movements of bryophyte species. The successful transplantations of bryophytes, at least the ones we know about, are mostly between kingdoms. A number of mosses that are native to bryofloristic kingdoms in the Southern Hemisphere, for example, have established themselves in the Northern Hemisphere over the last couple of centuries. Campylopus introflexus, Orthodontium lineare and Lophocolea semiteres are mentioned as the usual culprits.
The bryophyte in North America that is nearest to what we might call an “invasive species,” however, is actually an intrakingdom transfer—it has come over to us from Europe. The lawn moss Pseudoscleropodium purum, neat feather-moss, may have been introduced into North America in packing material. East-coasters first began to notice it at the end of the nineteenth century. Elva Lawton, who studied moss species throughout the Pacific Northwest in the middle of the twentieth century, first saw it in 1959, when it was presented to her by a Seattle homeowner. Since then, neat feather-moss has become a common resident of lawns everywhere along the Pacific coast. I’ve got it in my lawn, and if you have a lawn in the region, you probably do too.
The yellow to golden-green mats of Pseudoscleropodium purum (PDF) weave themselves through clumps of lawn grass and forbs, especially in shady areas of the lawn. The individual stems of the moss have pinnate branches, like a feather. The leaves are cupped, overlapping, and lie close to the stem, making the shoots look like stiff green yarn. A hand lens will show that the narrow points at the tops of the leaves curve away from the direction of cupping. The local moss that looks most like neat feather-moss, to my eye at least, is the widespread Pleurozium schreberi. Pleurozium schreberi, however, has red stems, lacks the recurved points, and is usually found in forests and in wilder environs.