[PCA] Geography of being native

[Larry Morse]

As shown in the current Mid-Atlantic EPPC discussion of Baccharis distribution, note that saying "native" requires specification of a geographical region. 

All plants are native to Planet Earth, but at any specified point or region, only some are native, and any others are non-native (exotic, alien, nonindigenous, whatever).  For most species, this "origin" (or  "nativity")  status is readily determinable at large scales, such as continents, oceanic islands, and large nations, but is increasingly difficult at more localized scales, ultimately becoming more of a historical question rather than a biogeographic one for very tiny areas (showing that natural history indeed does involve history as well as science).

 For example, the District of Columbia is quite near the eastern limit of natural distribution (native range) of sugar maple.  The species has been widely planted here, and "wild" individuals are occasionally seen nowadays.  However, no convincingly native report is known (specimen, literature, ancient tree, etc.).  It's reasonable that sugar maple did once upon a time grow naturally within the boundaries of D.C., since it is definitely known from old records quite nearby, but that biogeographic conclusion doesn't prove that the species actually did occur here naturally within the timeframe (European contact onward) generally taken as the starting point for most American biogeographic records.

A key part of many definitions of native (such as those used by the National Park Service, The Nature Conservancy, and NatureServe) is a phrase to the effect of "present without direct or indirect human intervention."  The planted sugar maples in D.C., clearly go there with human help, and any sugar maples arising from their seeds, and descendants of such trees, are also present due to the earlier human introduction of their ancestral trees (indirect human intervention).

On the other hand, consider the origin status for D.C.  for the tiny wildflower Floerkia (false mermaid-weed).  This distinctive, although inconspicuous, species is apparently  missing from early specimen records and floristic accounts for D.C. (ignoring reports from "vicinity of ..." which may be from nearby Maryland or Virginia).  It is locally frequent a few miles upriver along the Potomac in Maryland.  A few years ago, I found a small but well-established stand of it at a floodplain site within D.C. itself, at an easily reach locality near public parking and just off a well-traveled trail.  While it could have been there for centuries or longer, the patch is highly localized (no outliers more than a very short distance away seen in fairly intense zig-zag searching), and seems to me to more likely be recently established, within the past few years or decades.  While it's possible that some well-intentioned wildflower enthusiast might have planted it there, that seems unlikely since the plants aren't showy at all, and furthermore aren't visible from any trail or other commonly used area.  Given the frequent major floods on the Potomac (about every 5-10 years lately, although earlier considered more like every 10-30 years), and the presence of the species in riparian floodplain habitat a few miles upstream of the D.C. line, it seems to me much more likely that the D.C. population of Floerkia recently got there naturally, without human intervention.  If so, its D.C origin status is native.

Another kind of indirect human intervention involves human activity that provides an otherwise unavailable habitat within the area of interest.  For example, many plants disperse to places they cannot (or do not) grow.  Fern spores are quite tiny and particularly dispersible, with one biogeographic account (by the Tryons) concluding that prolific ferns readily colonize small, disjunct patches up to about 1,000 km (ca. 600 miles) away downwind from a major source population.  In the mid-Atlantic region, a number of calcareous-substrate (limestone, etc.) fern species are abundant in the Great Appalachian Valley (Shenandoah Valley, etc.), about 100km (60 miles) westward of Washington.  No natural outcrops of limestone or comparable calcareous substrate are recorded within D.C., with few if any such substrate patches in the natural landscape between the District and the Shenandoah Valley.  Yet several species of limestone ferns are known within Washington or in nearby places, generally on lime-based mortar in old masonry.  One such fern has even shown up on the tunnel walls of a deep-underground subway station, as noticed by Julie Moore of USFWS and more recently featured in the Washington Post.  Assuming the fern spores in these cases arrived in the D.C. area by natural dispersal, it  might seem that these ferns are therefore native to D.C.  However, they succeed only on masonry with lime-based mortar or similar artificial substrates, something not naturally present within the District but provided instead through human activity.  Invoking the "indirect human intervention" clause makes these species non-native within D.C., which makes sense when one considers that they could not have grown here in pre-settlement times.

For species such as the Baccharis that have spread from their earlier distributions along roadsides (or railroads, canals or canal banks, powerline or pipeline rights of way, etc.), their status as local natives is not always easily determined in lack of actual historical evidence.  The extent to which the human-altered habitat differs from nearby natural habitat, and the plant species' local degree of dependence on the human-altered habitat, should both be considered, judgement calls invoking experience and expertise as much as hard science.  Most species do change their ranges over time, so recent arrival alone is not the test.  Establishment nearby in natural habitats, especially away from roadsides and other human-maintained areas, could suggest that the species may be arriving on its own as part of natural range expansion.  A pattern of incremental spread over space and time is also characteristic of routine natural range expansion, although occasional establishment of a few more distant, moderately disjunct occurrences should also be expected, if appropriate to the species' dispersal mechanisms.  Again, more an instance of natural history than natural science.

Larry

Larry Morse
larry.morse.dc at earthlink.net
(larry.e.morse at LEM-Natural-Diversity.com)
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