The greasewood (Larrea tridentata) is native to the American Southwest and Mexico; species are also native to southern South America. Recent work shows that the North American Larrea probably is derived from the South American relatives.
There are about 30 species of Navarretia (Polemoniaceae) in western North America, and one in temperate South America. This suggests that that one species reached South America by long-distance dispersal. The leaves and calyx lobes of Navarretia are spine-tipped, adapting them for penetration of skin.
Krameria occurs in dry areas of North and South America and in the Caribbean. The genus seems to have reached suitable habitats by means of long-distance dispersal or at least medium-range dispersal. The curious barbed spikes on Krameria fruits account for dispersal ability.
Rhipsalis is a cactus native mostly to tropical America, but a few species occur in Africa and Madagascar. Because all cacti except for those few species are native to the New World, one suspects that the distribution of Rhipsalis is the result of long-distance dispersal.
The fruit of Rhipsalis cut in half: the fruit, enlarged here, is only about 5 mm in diameter. The seeds are very small and the flesh of the fruit is very viscid. These qualities would suit Rhipsalis for dispersal by birds.
A map of the distribution of Coriaria. If one neglects the occurrence of Coriaria on some volcanic islands, one might be tempted to say that it is an ancient genus, once widely present on all continents. However, one cannot neglect the occurrence of Coriaria on islands such as Tahiti. If long-distance dispersal has carried Coriaria to Tahiti, that could account for at least some of the disjunctions between continents and continental islands. There is no reason to believe that Coriaria is an ancient plant.
Upper elevations of Mt. Kenya, which lies on the equator is host to numerous temperate plants. These plants, like those of other tropical alpine regions, have reached Mt. Kenya by long-distance dispersal from mountains of the temperate zone. Resistance to cold pre-adapts such plants to tropical alpine locations. This photograph of Ranunculus oreophila was taken at about 4,500 m. At these elevations, the temperatures go below freezing at night (note the frost on some of the Ranunculus leaves in this picture). Such a climate can be called “summer every day, winter every night). In this respect, it differs from temperate mountains, where temperatures fluctuate between these extremes seasonally rather than daily. Notice that the flowers of this Ranunculus are borne very close to the ground, where they are not likely to be injured by frost. Ranunculus species in temperate zones bear flowers on stalks well above the ground, because during the flowering season, frost that might damage flowers is unlikely to occur. This photograph was taken in early morning, before the frost melted.
Althought most of the alpine plants on Mt. Kenya are descended from colonizers that originated in the north temperate zone (like Ranunculus), a few, like this Helichrysum, are derived from south temperate ancestors. Notice that this Helichrysum forms a shrub and bears flowers well above the ground surface, in contrast to the Ranunculus. The Helichrysum occurs at elevations lower than those where the Ranunculus grows; Helichrysum is not subject to nightly frost.
Some have thought that the senecios and lobelias of the east African volcanoes are relatively ancient plants. They’re not. They are descended from species of Senecio and Lobelia that reached these volcanoes by long-distance dispersal from temperate areas. The curious form of these plants is the result of their adaptation to the occurrence of warm temperatures during the day but frost at night as this elevation (about 4,100 m) on Mt. Kenya. The Senecio keniodendron shown here attains a treelike shape because it has evolved the ability for the leaves to close over the tips of the branches. The growing point inside stays above freezing all night, although frost forms on the outer surfaces of the leaves. Lobelia teleki is a much lower plant. Its growing point is near enough to the ground surface to escape frost damage, but the plant flowers, as show here, the dense coverings of bristly bracts may help protect the flowers from frost damage. In addition, even in the north temperate zone, some Senecio and Lobelia species experience a little frost during their growing season, so that there may be some resistance to a few degrees of frost related to cell chemistry.
Intercontinental Dispersal [ PDF ]
I can’t remember the year. Let’s say it was 1980. Admittedly, I want to forget it. I was invited to speak at Texas Tech—Lubbock, Texas. The invitation came from Vernon Proctor; I admired greatly his work on how long shorebirds can retain seeds fed to them. What he didn’t tell me was that there would be two other speakers. One was Dan Simberloff, who had done interesting work creating islands (by dredging) in the Florida Keys, defaunating them by fumigation, and then observing which species (insects) reached them. The other speaker was Donn Rosen, who was an ichthyologist from the American Museum of Natural History, New York, who studied primary division freshwater fish—fish that can’t tolerate saltwater, so that their dispersal patterns are very conservative.
I had prepared a talk on intercontinental dispersal. For decades, people had noticed that there were “amphitropical disjunctions”—Constance, Raven, and others. Such conspicuous shrubs or trees as greasewood (Larrea) and mesquite (Prosopis) were represented in Argentina or Chile by species that weren’t far distant from those in the American Southwest, but not in the areas in between. The list of species of herbaceous genera that are represented in central Chile and then in similar Mediterranean-type climates of California is a long one—Bowlesia, Chorizanthe, Downingia, Elatine, Gilia, Haplopappus, Lasthenia, Madia, Navarretia, and Osmorhiza are genera on the list. In my talk at Texas Tech, I mentioned that in a few cases, the same species is apparently represented in both Chile and California, but in most of the genera, the Chilean species are similar to, but distinct from the Californian species. The Chilean species of a genus could even be hybridized with the Californian species in cases where that had been attempted. My explanation: distributions of these genera in the two hemispheres had never been much different from what they are today. Long-distance dispersal from Chile to California or from California to Chile in recent times accounted for these distributions. I discarded two alternative interpretations. One was that there had been a belt of temperate vegetation that went from near sealevel in temperate regions into the high Andes in equatorial regions and so these genera were once continuous. Another was that there were many steppingstone peaks between the two hemispheres, and that these had once hosted species of these genera. Both of these alternative hypotheses would have required the genera to adapt to tropical alpine conditions—quite different from temperate conditions in the 30—40° latitude range—and then readapt back to temperate conditions. And they must do that without leaving behind a trace in the intervening latitudes! Unlikely evolution, bad biogeography, I thought.
Obviously, events of great long-distance dispersal were involved in the interpretation I favored for the amphitropical disjuncts. After all, quite a number of birds use the Pacific flyway from far north to far south. If shorebirds and marine birds could account for dispersal of seeds from North America to the Hawaiian Islands, surely they could account for dispersal of seeds from North America to Chile (or the reverse). The distance was greater, but the precision of dispersal was enormous: migratory birds don’t get off course much, and they go to areas where they will find food in a very predictable way. California and Chile represent large source areas and target areas, and migratory birds that use the Pacific Flyway are guided by the coastline, where their food sources are likely to be plentiful, they don’t have to navigate from the coast of North America out to Hawaii and back as the Pacific golden plover and a number of other shorebird species do. My Hawaiian experience, and my experience living on the West Coast and seeing most of the genera involved meant everything. It’s easy, I guess, to disbelieve in something that one has never seen or had to come to terms with. I had had to explain the origin of the Hawaiian flora, so explaining the origin of the amphitropical disjuncts was a kind of corollary: what explains one can also account, with minor modifications, to the other situation [ PDF ]. I was probably the first to claim long-distance dispersal as the explanation, without modification, for the distributions of the amphitropical disjuncts.
Imagine my surprise when Donn Rosen, at the end of my talk, immediately and vociferously denounced my ideas as complete fiction. Couldn’t happen, he said. Now remember that Donn Rosen had never heard this talk before, knew nothing about the amphitropical disjunction in temperate plants, and had never seen any of those genera. He denounced my ideas because they conflicted with his concepts of “Vicariance Biogeography.” Biogeography wasn’t an exact science in those days, and there was a great desire on his part to put biogeography on a precise basis: to find a method whereby one could determine where groups of plants or animals had been and how they got to where they are now. In vicariance biogeography, plants and animals are regarded as having very little dispersal ability. They don’t move much, the ground moves: pieces of land separate (tectonic plates, for example), and the plant and animals just sit on the pieces of ground where they always were. A piece of ground moves, the organisms on it move very little. If one can develop a cladistic tree (based on morphology) for a group of organisms and superimpose it onto the distribution of those species and there is a (rough) match, one has a “track,” Rosen said. If one does the same for other organisms in the area, and develops several “tracks,” one gets a “generalized track.” In fact, the “generalized tracks” that have been produced look rather like the maps of tornado or hurricane pathways in the United States: they really don’t produce the coherent picture one might hope, and they lack explanatory and predictive dimensions, although one might see some similarities among some of the component “tracks.” What Donn Rosen wouldn’t say is that his methodology of vicariance biogeography couldn’t tolerate any long-distance-dispersal events, because if one admits them, there is no longer any unity to the tracks: any given dispersal event can differ from others in unpredictable ways if long-distance dispersal works.
In fact, a number of organisms do have very poor dispersal abilities, and their distributions do illustrate movement of land masses by tectonic plates. Among animals, the primary division freshwater fish. Among plants, the conifers with large seeds, such as Araucariaceae. This phenomenon was known a long time ago, when Wallace and others recognized the distinction between continental islands and oceanic islands. What Wallace and his colleagues didn’t know was that continental islands had moved to their present position by tectonic plate movement. They described the biogeography correctly; they just didn’t know the geological background. Araucariaceae are all on continents or continental islands. Why weren’t both kinds of factors operating, I said [ PDF ]? That seemed eminently sensible to me.
By excluding the possibility of long-distance dispersal, vicariance biogeography in the limited methodology that Donn Rosen proposed was wrong. What proved decisive in the shift away from his thinking was the development of DNA-based phylogenetic trees using cladistic methodology, trees which mostly have high degrees of statistical probability. Routinely now, DNA-based phylogenetic trees include not only species or genera, they also show the geographical distribution of the component species. These trees provide high-likelihood interpretations not only of the evolutionary history of a group but also the group’s geographical distribution. The papers in which these trees appear do not always attempt to explain the method of dispersal of the group, but often nowadays, the probabilities of either long-distance dispersal or tectonic-plate dispersal are suggested (sometimes the word vicariance is still used for the latter, but not in the Donn Rosen sense). Field studies that show how particular groups disperse, their approximate dispersal capabilities and their dispersal vectors are much needed.
About that talk in Lubbock, Texas. At an open forum with the students after the three talks were given, one student asked how two seemingly intelligent scientists (meaning Donn Rosen and me) could reach such contradictory conclusions. I felt like answering (but of course did not) that one of us was obviously wrong. I knew that ultimately, long-distance dispersal would be proven as operative for species such as those involved in the amphitropical disjunction patterns. I didn’t know at the time that DNA evidence would prove to be the tool that would prove the existence of long-distance dispersal, of course. As it was, I felt discredited and humiliated because Rosen had been so vociferous in denouncing me: I took a taxi to the airport and got an earlier plane back to Los Angeles. I have always been intimidated by biologists who are vociferous, but I suppose I should realize that they really are afraid that they might be wrong.
The mechanisms for long-distance dispersal of the amphitropical disjuncts are mostly not obscure. Most of the genera involved in these patterns have dry seeds that can adhere to feathers or skin by means of bristles or sharp hairs.
Intercontinental dispersal isn’t confined to the New World amphitropical disjuncts. There are many patterns that illustrate the operation of long-distance dispersal between continents. For example, Krameria is a genus (and family) of the New World. It has species scattered from Patagonia to the U.S—some in tropical latitudes as well as in temperate localities. The habitats are dry, sunny habitats. There are reasons to believe (from the work of Beryl Simpson) that Krameria has moderately good dispersal, and may have traveled between the two continents more than once. Habitats suitable for Krameria are more varied than those of the amphitropical disjuncts. If one looks at the fruits of Krameria, one sees that they have anchor-like hooks on them, a mechanism that looks good for dispersal for short distances and longer distances as well.
Groups of plants disjunct between the New World (mostly South America) and West Africa form a fascinating case. Everyone has seen the maps that fit the eastern tip of Brazil into the Gulf of Guinea of Africa, suggesting the separation of the two continents from each other. And some groups of organisms may owe their distribution to that geological event. Soon after the existence of tectonic plates was demonstrated clearly and adopted (early 1970s), many South America—West Africa disjunctions were attributed to that cause. But do all of them result from that? Several species of the cactus Rhipsalis are in West Africa, but most species of Rhipsalis and all of the remaining Cactaceae are in the New World. The most likely explanation appears to be long-distance dispersal, but one cannot rule out some role of humans in the distribution patterns in Africa. Probably a number of disjuctions between northern South America and West Africa are the result of long-distance dispersal, although others may not be.
Astelia, Coriaria, and Fuchia, all occur on New Zealand, but also on Tahiti and in Chile. Coriaria extends to Japan. Genera such as Sanicula and Gunnera reach Hawaii but also occur on continental areas. Long-distance dispersal must be suspected for at least the insular parts of such distributions—and if these genera reach islands, the probability is that dispersal followed by colonization from one continent to another is possible. DNA evidence will illuminate such distributions. Unfortunately, where material from two or more continents is required, field work to obtain material suitable
for DNA analysis becomes more expensive.
Another group of fascinating distributions that demonstrate intercontinental dispersal are represented by tropical mountaintops—“sky islands” of the tropics. Some of these are volcanoes. When one sees such plants as gentians and buttercups on tropical peaks (east Africa, New Guinea, Mt. Kinabalu), one realizes that these peaks are islands of cool temperatures. Resistance to frost requires numerous adaptations, so that tropical floras surrounding these peaks are not likely to be the source for plants of the peaks. The adaptations to grow in cool conditions pre-adapt temperate montane plants for high-elevation tropical peaks. Not surprisingly, most of the genera in the alpine zone of the east African volcanoes (Mt. Kenya, Mt. Kilimanjaro, etc) are those found in the north temperate zone. Most mountains are in the north temperate zone. A few genera on those volcanoes have south temperate relationships (Gladiolus, Helichrysum). Unraveling the source areas for colonization of tropical “sky islands” will be a very interesting enterprise. Although DNA will tell us the likely source areas, the mechanisms for these colonizations will require more study. We see that DNA-based evidence tells us that long-distance dispersal does, in fact, exist. Consequently, it gives us motivation to find out how it works, motivation that did not exist in the days when many scientists were skeptical about long-distance dispersal.