A Paleoecological Perspective on the Assisted Migration Debate

Paleoecology and the Assisted Migration Debate:
Why a Deep-Time Perspective Is Vital
Torreya taxifolia as Exemplar
assembled by Connie Barlow, Torreya Guardian
published 11 February 2010

    BACKGROUND and ABSTRACT
In the six years since the assisted migration (assisted colonization) debate was launched as a pro and con "Forum" in the Winter 2004/2005 issue of Wild Earth journal, the paleoecological perspective has been almost entirely missing from both the academic papers and the media reports on this vital conservation issue. The original PRO-assistance essay by Connie Barlow and (the late) Paul S. Martin was titled "Bring Torreya taxifolia North — Now". The original ANTI-assistance essay by Mark Schwartz was titled "Conservationists Should Not Move Torreya taxifolia".
     The intent of this skeletal essay is to demonstrate, by using the example of "poster plant" Torreya taxifolia, why the assisted migration debate must broaden to encompass a deep-time, paleoecological perspective.
     Since the beginning of the debate, the major advocates in favor of assisted migration for Torreya taxifolia have argued that paleoecological reasoning is exceedingly strong in favor of assisting the migration of this highly endangered conifer tree northward from the panhandle of Florida to the southern Appalachians. Notably, there is no dispute that the highly restricted endemic range of Torreya taxifolia in Florida (along the east shore of the Apalachicola River) served as one of the primary "pocket refuges" for America's rich Appalachian flora during the peak of the last glacial advance c. 18,000 years ago. It is therefore reasonable to suggest that Torreya taxifolia be looked upon as a "glacial relict" that, for one reason or another, failed to make the return trip north when the present interglacial began some 13,000 years ago. It is also plausible to posit that the reason this particular species failed to make the return journey north is that, in contrast to previous interglacial episodes, newly arrived humans obstructed northward migration by overhunting the conifer's obligate seed dispersal partners (squirrels and possibly now-extinct tortoises) in the narrow riverine corridor that contains soils rich enough to support the maturation of this yewlike tree.
1. WHY FOSSIL EVIDENCE IS SPOTTY FOR GENUS TORREYA
Torreya is a very ancient conifer genus, represented today by a half-dozen species and a disjunct range characteristic of many plants affected by what is sometimes called the "Arcto-Tertiary Geoflora. Today the genus is represented by one species in Florida, one in California, and four or five in Japan, Korea, and eastern China. The only fossil evidence of genus Torreya in eastern North America comes from the upper Cretaceous in what is now North Carolina and Georgia. Genus Torreya is represented in the fossil record of western North America as follows: California during the late Pleistocene and the Oligocene, Oregon during the Eocene, Alaska during the Cretaceous, and (forthcoming by Estella Leopold et. al, pers. comm.) in north-central Washington during the Miocene.
     My point is that one should not insist on fossil evidence in order to best discern probable paleo native range. If one did, then one would have to conclude that genus Torreya lived only in the John Day volcanic region of north-central Oregon during the Eocene, and only in Alaska and North Carolina and Georgia during the Cretaceous. (Note: Both the Eocene and the late Cretaceous were very warm times in Earth history.)
1A. MACROFOSSILS of plants are superbly preserved very often in the ashfall of nearby volcanic eruptions — hence the John Day fossils of Torreya. I would suspect all the California occurrences of genus Torreya in the fossil record (and the new discovery in Washington state) also occur in ashfall, though I have not researched that level of detail. (I myself found some fabulous Cretaceous broadleaf fossils in the Cretaceous Chuckanut Formation of NW Washington State, which is volcanic ash.) Overall, Cenozoic fossils of any species are not well represented in the Appalachian region, in contrast to the volcano-rich Pacific Range mountains.
1B. MICROFOSSILS of genus Torreya are simply unavailable because, as my scientific mentor and coauthor, the late Pleistocene paleoecologist Paul S. Martin, explained to me (and as we wrote in our 2005 torreya advocacy paper), Torreya pollen is indistinguishable from the pollen of yews (Taxus), bald cypress (Taxodium), and cypress (Cupressus). Hence it would be inappropriate to even ask for microfossil evidence of Torreya in the Appalachians.

2. ABSENCE OF EVIDENCE IS NOT EVIDENCE OF ABSENCE
Thus, to insist that one must find direct evidence of genus Torreya inhabiting the southern Appalachians during the Pleistocene (indeed, during any time of the Cenozoic) in order for Torreya Guardians or others to responsibly posit that the southern Appalachians were previously part of the species' native range is not tenable. "Absence of evidence" should not be used as "evidence of absence" in this regard.
3. CURRENT RANGE AS RELICT OF A PEAK-GLACIAL "POCKET REFUGE"
Palynologist Hazel Delcourt, in her 2002 book, Forests in Peril: Tracking Deciduous Trees from Ice-Age Refuges into the Greenhouse World, has well established that the Apalachicola region of the Florida panhandle is one of a handful of primary "pocket refuges" along the Gulf Coast and the southern Atlantic to which the rich flora of the central and southern Appalachians retreated during the peaks of glacial episodes. Indeed, it can be reasonably inferred that had this continent lacked such large riverine environments with banks of rich soil (contrasting to the predominant sandy soils of the region), North America would have lost to extinction genus Liriodendron (tuliptree) — as did Europe. On the one visit I made to Hazel Delcourt before she retired, she told me that she has seen European botanists begin to cry when they visit a magnificent old Liriodendron in a forest here — because they know that, for want of adequate glacial refuges north of the Mediterranean and Carpathians, Europe lost this genus during the Pleistocene.
     The point here is: since we can all agree that the Apalachicola did indeed serve as a peak glacial refuge, we should at least entertain the possibility that any species that is strangely endemic there might have as the primary reason for its endemism the fact that it is a glacial relict. Indeed, would not the best explanation for Florida yew be that it is the remnant of the Taxus population "left behind" when the genus began reinhabiting the Appalachians as Canadian yew? Ditto with the endangered endemic species of genus Croomia there. Surely, no one would look at the remnant American beech population that is still holding on along the Apalachicola River and declare that it is best adapted for exactly the climatic conditions now found there. Indeed, Hazel Delcourt has advocated that this southernmost population of American beech may well contain warm-adapted alleles that will prove vital for ensuring continued representation of this species in the Appalachians and points north a century hence (pers. comm.).
4. ARGUMENTS FOR PREFERRED RANGE TO THE NORTH
Unless it can be established that a particular introduced disease agent is the sole cause of the inability of almost all wild Torreya individuals to reproduce in the range it occupied in 1491 (which is the implicit standard for ecological restoration activities in the USA), then we need to consider the strong possibility that inability to thrive is a sign that the current stage of interglacial climate hit a tipping point for the species back in the 1950s and 1960s (when this once-common Apalachicola species suddenly became uncommon and unreproductive). In contrast, there are semi-wild (planted, but now inhabiting regrowth forest) mature T. taxifolia in North Carolina, and they are still tall and producing seed. Plus our own 2008 plantings of seedlings in North Carolina are doing very well — except where rodents ate the cambium off several during the first winter. I understand a study is ongoing to look for pathogens infecting the soils near various of the North Carolina specimens (pers. comm., Vivian Negron-Ortiz). If evidence of Phytophthora water mold is found in the soil there, but if the plants are nontheless able to withstand this pathogen, then such should be considered strong evidence that the southern Appalachians at rather high elevations are more amenable habitat than northern Florida for Torreya taxifolia in the current climatic regime.
5. ARGUMENTS AGAINST FLORIDA AS ETERNALLY NATIVE RANGE
I do not know exactly when the panhandle of Florida was underwater during the full life-span of genus Torreya, but if today's Torreya taxifolia is a descendent of the Cretaceous species of Torreya fossilized in North Carolina, then there are tens of millions of years during which it had to be living somewhere other than Florida. Certainly, during the past 70 million years, there were times when the panhandle of Florida was fully marine or beach-dune habitat. One must speculate where native range was during those times; the question cannot be avoided.
6. THE POSSIBILITY THAT TORREYA IS A MOUNTAIN-ADAPTED GENUS
For whatever reason (perhaps because its large seed means that it disperses only very slowly) the other four or five species of Torreya all exist in the wild today only in mountainous regions of Asia and California. T. taxifolia is the outlier species, in that it lives nowhere near any mountains. But could that be because it was forced to retreat in glacial times too far south of its nonglacial mountain habitat and simply failed to make the return migration? After all, mountain species can react to climate change much more quickly by curving around a mountain between northern and southern exposures, and by moving altitudinally higher or lower. Perhaps T. taxifolia did successfully migrate back and forth between the Apalachicola and the Appalachians throughout previous climate cycles in the Pleistocene — but human presence (overhunting of seed dispersers) during this current interglacial may have blocked its northward movement for the very first time.
7. A PALEOECOLOGICAL PERSPECTIVE SHIFTS THE BURDEN OF PROOF
I regard all the forgoing arguments not only as scientifically reasonable, but as scientifically necessary. We cannot simply look to where plants were in the year 1491 as the sole, nor even the best, standard for where native range was in millennia past — and where preferred and perhaps required range will likely be in the not-too-distant future. In my view, one cannot serve the interests of this endangered species in a time of rapid climate change without taking a deep-time (that is, paleoecological) perspective. Until a significant minority of botanists and ecologists make the shift to a deep-time perspective, however, those of us advocating assisted migration northward will continue to face an uphill battle. But, if and when the professionals do make the paradigm shift, the burden of proof will reverse for all three species of endangered plants "endemic" to the Apalachicola region. From then on, advocates for the status quo will be the ones with an uphill battle to fight.
8. PUTTING TO REST THE "INVASIVE PLANT" BOGEYMAN FOR INTRA-CONTINENTAL N-S ASSISTED MIGRATION
It is widely recognized that Torreya taxifolia is the "poster plant" for assisted migration advocates in the USA because (a) it no longer reproduces in its "native" range, and (b) no one who knows this plant could responsibly argue that this species poses an ecological threat if moved northward. Granted, there is concern about the spread of disease from nursery stock currently being used for this effort — but so long as the Torreya Guardians are not introducing this plant into an area where the genus already lives, and especially if the pathogen Phytophthora is already present in the region but simply unproblematic there, then is it professionally responsible to even hint that "invasion" might be a reason to continue to delay initiating an official pilot project to assess the possible benefits of assisted migration for this particular endangered species?
     Fellow Torreya Guardian (and caretaker of a dozen T. taxifolia seedlings on his NC property, elevation 3,800 feet) Russell Regnery emphasizes, until regulators start going after homeowners and nurseries that willy nilly plant just about anything anywhere, why single out the Torreya Guardian efforts in ways that incite the uniformed to call us eco-terrorists?
NOTE: A key paper published in 2007 concludes that north-south translocations of land-plant species within a continent pose very little risk of invasion. This paper should be required reading for all participants in the assisted migration debate:
"An Assessment of Invasion Risk from Assisted Migration" by Jillian M. Mueller and Jessica J. Hellmann, Conservation Biology, 28 June 2007.
9. AN "INVASIVE PATHOGEN" ARGUMENT IN FAVOR OF ASSISTED MIGRATION OF TORREYA TAXIFOLIA INTO THE APPALACHIAN MOUNTAINS.
Torreya Guardian Russell Regnery also participates in the American Chestnut Foundation effort to cross-breed American with Asian Chestnut in order to eventually produce a species with only the minimal Asian genes necessary to withstand the exotic chestnut blight disease agent here in America. In a February 2011 email, he writes (pers. comm.):
"It is pretty well accepted by participants in the American Chestnut recovery project that the primary reason that the American chestnut is thought of as a tree of the mountains is that Phytopthora doesn't take freezing well. The American Chestnut Foundation chapter checked my soil for presence/absence of Phytopthora before they let me plant my chestnut orchard. In fact, there are those who hypothesize that before the current forms of Phytopthora were introduced into North America (very early in colonial times) that the range of American Chestnut may have existed well into the Piedmont. Early introduction of Phytophthora may have played a part, too, in the demise of the wild population of Franklinia [along the Altamaha River]. So helping Torreya escape a freeze-intolerant pathogen (which I'm sure is well ensconced along the Apalachicola), is one more reason to consider assisted migration to save the species. No one, after all, can possibly control Phytopthora on an ecological scale [where climatic conditions enable it to thrive].

10. AN INVITATION TO BEGIN ASSESSMENT OF THE EXISTING SPECIMENS OF T. TAXIFOLIA IN NORTH CAROLINA AS THE FIRST PHASE OF A PILOT PROJECT TESTING THE MERITS OF ASSISTED MIGRATION.
The 2010 revisions to the Endangered Species management plan for Torreya taxifolia state:
"Foster a working partnership between the Torreya Guardians, the Service, and other interested parties to help direct their managed relocation efforts." (p. 18)

Although the revised plan does not specifically call for a pilot project, it doesn't prohibit team members from initiating one. We at Torreya Guardians have done the work to pinpoint the existing Torreyas found in North Carolina and are happy to share that information with those working in accordance with the ESA. In addition to visiting the existing mature specimens that private individuals planted decades ago, we keep excellent records of the nearly 50 seedlings planted on a total of three mountain properties, beginning in 2008. Meanwhile, Atlanta Botanical Garden has an abundance of healthy little seedlings with nowhere, really, to go. (See for example, a photo-essay of a site visit Torreya Guardians made to Atlanta Botanical Garden to learn best how to propagate and nurture this species.
     So let us all combine our efforts to officially begin a multi-location and multi-phase pilot project for testing assisted migration of T. taxifolia. And let us do so in ways that demonstrate the positive benefits (both in data acquisition and economic efficiency) of having paid professionals include (and help direct), wherever possible, the volunteer efforts of citizen naturalists.
11. "ASSISTED MIGRATION" — NOT "ASSISTED COLONIZATION"
Two supplementary webpages speak to the (a) history of the terminology discussion (which began with "assisted migration" and then moved to "assisted colonization"), and then (b) a 2011 advocacy piece by Torreya Guardian Connie Barlow on why "assisted migration" should be restored as the term of use. Barlow writes:
We Torreya Guardians are branded as vigilantes, but helping Florida Torreya find its way northward into the mountains of North Carolina is pretty much like deciding where to go birdwatching to see Arctic Terns in January. Every amateur birder knows that the place to find Arctic Terns when it is cold and dark in the Arctic is to take a cruise to the Antarctic waters of the Southern Hemisphere. You won't find a single "Arctic" Tern in the Arctic during the Northern Hemisphere winter. They've all gone fishing in the south polar seas. Similarly, Florida Torreya is no more native to Florida in today's climate regime than an Arctic Tern is native to the Arctic in January.

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