Crowberries (Empetrum) are found only in the northern latitudes of the northern hemisphere and in the very southern regions of the southern hemisphere (page 174).The crowberry is another fascinating case. As Sarfati states, the crowberry is found only across the north of the northern hemisphere and in southern South America (and some offshore islands). Sarfati (and Statham, whom Sarfati is summarising) provides no references on crowberries.
I could only find one paper examining the crowberry's distribution, published in the Proceedings of the National Academy of Sciences in 2011 (The Greatest Hoax on Earth was published in 2010).[footnote 1] The paper finds the evidence (molecular analysis and pollen records) doesn't support a gradual migration explanation for the crowberry's presence in South America, and concludes instead "a single dispersal by a bird from northwestern North America to southernmost South America, taking place in the Mid-Pleistocene, is sufficient to explain the disjunction in crowberries." While that explanation has an aspect of deus ex machina to it, the paper points out modern-day arctic birds are known to feed on crowberries before migrating to breeding grounds in South America.
At this point it's worth commenting on Sarfati's use of evidence. Sarfati is focusing on the few exceptions (some of which can be easily explained anyway) to generally cohesive explanations of biogeography. There are plant species, such as the crowberry, that have a puzzling geographical distribution; there are also hundreds of thousands of other plant species that don't present such a puzzle. Likewise, Sarfati mentions Rhipsalis baccifera but not the 1,500 other cactus species that don't have an anomalous distribution. On to Sarfati's next example:
About 150 seed plant genera are common to eastern Asia and eastern North America but are not found in the intervening western North America. Many of these are even of the same species, showing they must have diverged recently, not millions of years ago. Significantly, some of the plants (and animals) found in eastern Asia and eastern North America are identical at the species level, indicating that the disjunctions occurred very recently (that is, within the last few thousand years).This is a rather selective and misleading presentation of evidence. Sarfati supports his statements with a reference to a paper by botanist Hong Qian.[2] Sarfati is correct in his comment that about 150 seed plant genera are common to Eastern Asia and Eastern North America but are not found in Western North America. However, some context (so often missing from Sarfati's summaries) is useful.
First, Qian found:
- Eastern Asia shares 627 genera with Eastern North America (151 of which aren't found in Western North America)
- Eastern Asia shares 520 genera with Western North America (44 of which aren't found in Eastern North America)
- Eastern North America shares 765 genera with Western North America (289 of which aren't found in East Asia)
Second, Sarfati gives no reference to back up his claims about similarities at the species level: that some of the plants from Qian's study "are even of the same species" and therefore "must have diverged recently, not millions of years ago"; and, in the next sentence, that "some of the plants (and animals) found in eastern Asia and eastern North America are identical at the species level" so the disjunctions must have occurred "within the last few thousand years". There is no support for these assertions in Qian's paper. Qian compared the relationships between the floras of Eastern Asia, Eastern North America and Western North America at the genus level. He was comparing genera, not species. This is stated clearly in the abstract ("floristic relationships based on genera shared among EAS, ENA, and WNA were examined") and the body ("this study focuses on the genus level") of his paper.
The wider literature further undermines Sarfati's statements. A 1999 paper by botanist Jun Wen also examined the similarities between the floras of Eastern Asia and Eastern North America.[3] Wen notes that botanists originally thought East Asia and Eastern North America shared many identical species of plant. For example, in the mid-1800s botanist Asa Gray listed 134 identical species. (It may be from such early work that Sarfati got his mistaken idea that many of the disjunct flora of Eastern Asia and Eastern North America are of the same species.) But now they are understood to be largely congeneric (belonging to the same genus) and not conspecific (belonging to the same species). Botanists had narrowed the conspecific list to eight examples by 1992, and to only one example by 1999.
Third, Qian found the similarities between Eastern Asia's and Eastern North America's genera are greater among basal (earlier or more 'primitive') lineages and they decrease among more modern lineages. This isn't the case for the similarities between Eastern North America and Western North America – the similarities between these two tend to increase as you get more modern. This suggests the connection between Eastern Asia and Eastern North America that resulted in their similar floras occurred some time ago (that is, at least several million years ago).
Fourth, Qian describes how Eastern Asia and Eastern North America were connected some time ago, which (partly) explains the similarities in their disjunct floras:
The observed similarities between the floras of EAS, ENA, and WNA resulted, to a large extent, from largescale historical processes such as species movements in the geological past when Eurasia and North America were geographically connected to each other. According to geological and paleobotanical evidence, two major land bridges have connected Eurasia and North America: the Bering land bridge across the north end of the Pacific Ocean connected Siberia and Alaska throughout most of the Tertiary, and the North Atlantic land bridge across the north end of the Atlantic Ocean linked northern Canada to Europe via Greenland in the early Tertiary. It is widely accepted that the intercontinental connections via these two land bridges significantly contributed to the floristic similarities between EAS and North America....
During the Eocene, climates were warm enough to support tropical vegetation at high latitudes in the Northern Hemisphere from which many fossils of tropical taxa have been recovered. The existence of the Bering and North Atlantic land bridges and warm climates at high latitudes during the early Tertiary facilitated the interchange of both temperate and tropical floristic elements between Eurasia and North America.Wen's paper reaches a similar conclusion:
Phylogenetic, molecular, geologic, and fossil data all support the hypothesis that the eastern Asian and eastern North American disjunct distributions are relicts of the maximum development of temperate forests in the northern hemisphere during the Tertiary. Fossil and geologic evidence supports multiple origins of this pattern in the Tertiary, with both the North Atlantic and the Bering land bridges involved.The prevailing explanation for the similar floras of Eastern Asia and Eastern North America is that they are remnants of genera once widely distributed across a temperate zone in the northern hemisphere during the Tertiary Period (which began around 65 million years ago). A wide distribution (including Asia and North America) was possible because of land bridges: at certain times the Bering land bridge connected Asia to North America and the North Atlantic land bridge connected North America to Europe via Greenland. After the establishment of a flora across the northern temperate zone, the continued formation of the Rocky Mountains changed the climate and rainfall patterns, causing the genera to disappear from Western North America. And changes in the last two million years also caused the genera to disappear from Western Europe.[4]
[Discussion on biogeography will continue in the next post]
References:
[1] Magnus Popp, Virginia Mirré, and Christian Brochmann, 'A single Mid-Pleistocene long-distance dispersal by a bird can explain the extreme bipolar disjunction in crowberries (Empetrum)', Proceedings of the National Academy of Sciences, Vol 108, No 16 (2011), pp 6520-6525; available online at: http://www.pnas.org/content/
[2] Hong Qian, 'Floristic relationships between eastern Asia and North America: test of Gray's hypothesis', The American Naturalist, Vol 160, No 3 (2002), pp 317-332; available in part online at: http://business.highbeam.com/
[3] Jun Wen, 'Evolution of Eastern Asian and Eastern North American Disjunct Distributions in Flowering Plants', Annual Review of Ecology and Systematics, Vol 30 (1999), p 421-55.
[4] David Yih, 'Land Bridge Travelers of the Tertiary: The Eastern Asian-Eastern North American Floristic Disjunction', Arnoldia, Vol 69, No 3 (2012), pp 14-23; available online at: http://arnoldia.arboretum.
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