Borrego Desert: Geographic Distribution Maps of GPS Points for Plant Species
In November, 2015, we have begun putting distribution maps online for some species, whenever they pique our interest enough that we wanted to see a map for ourselves. We had intended to wait until we had completely digitized our GPS records, but even our half-digitized database is complete enough to show the basic distribution of most species in the Borrego Desert.
The purpose of this page is to explain some of the properties of those distribution maps, the species GPS points, and to provide maps of all GPS points for all species to be used as a comparison to the distribution maps for individual species.
Our distribution maps have a number of advantages over the species distribution maps made from vouchers:
- In general, we have many more locations of most species than there are locations from vouchers. For example, we have 44 GPS locations for Matelea parvifolia, from roughly 11 distinct populations, compared to nine vouchers from five populations.
- Our locations are much more accurate than the locations from most vouchers, and hence our maps are not contaminated by incorrectly-georeferenced vouchers.
Most vouchers, until recently, have only vague locations, and their georeferenced position, if there is one, is just someone's best guess as to where to place the locality given on the voucher label. In a number of cases, the location is just something like East of Sentenac Canyon, Colorado Desert, and no one knows where exactly to place such a voucher. Worse, some old vouchers just state the nearest named place, which leads to desert floor species sometimes georeferenced at Warners Springs! Many desert vouchers are just georeferenced at the town of Borrego Springs, even though many species georeferenced there do not grow anyplace in that vicinity.
Our locations were all done with a GPS receiver that had been on long enough to recover from initialization problems. The locations were then checked on a topo map to check for multipath error, or any other problem that occasionally makes the GPS points inaccurate. The elevation was interpolated by a human from the topo map, which is more accurate than a computer interpolation from a Digital Elevation Model (DEM), or that of the GPS receiver itself.
- Our determinations are much more reliable than the determination of vouchers.
Voucher distribution maps are often contaminated by misdetermined vouchers, which can result from a number of causes, such as old species names that were never updated after an older species was split into two or more species; taxonomic problems; misdeterminations on vouchers never reviewed by an expert; and errors in entering the data or making the label.
In contrast, after ten years of intensive work in this area, including well over 300 full-day surveys in this area, we are very familiar with the species of the Borrego Desert, and hence what we call a given species is quite uniform.
Just to be clear, voucher distribution maps for species are wonderful things, especially on larger scales than considered here, and we use them all the time as an invaluable tool. For more information, see Some Things You May Not Have Known About Vouchers.
As of 29 November 2015, we have digitized 9,473 GPS points from our surveys. The records range from -9 feet elevation near the Salton Sea to 5,349 feet at Whale Peak.
There are a number of things to be kept in mind about these points:
- Not all areas have been surveyed, and not all surveyed areas have yet had their GPS points processed. As of 29 November 2015, about half of our survey records have been processed.
- In general, in a given survey, which can cover anywhere from a linear distance of just 0.3 miles to a loop trip of ten miles, only the first occurrence of a species was GPS'd.
- Infrequently, multiple points were taken of an individual species in a given survey, for various reasons. When the number of points for an individual species is much larger than for the average species, we break those numbers out as separate "targeted" records.
- Some of the points may be so closely spaced in the distribution maps that they lie on top of each other. Hence the number of dots on the map may be fewer than the number of GPS points.
- In general, the number of points for each species is directly correlated with the abundance of each species in a geographic sense. Species with many points are often found in our surveys of different areas, and hence are geographically widespread species that are commonly encountered.
- The number of points for each species do not give information about the local abundance, since each point could reflect only a single plant, or it could reflect hundreds of plants, at or near that point. However, we do have the abundance of each species in each survey associated with one of the points in a survey, so we can also make maps of just where each species is abundant.
In the geographic distribution maps for each species, we also give the number of GPS points for each species, and the elevation ranges in our records for the Borrego Desert area. Since we have surveyed a number of areas just outside our definition of the Borrego Desert area, we include elevations of points from those areas.
If the records show a fairly-continuous distribution in elevation, a single range is given. If the lowest elevation records, and/or the highest elevation records, do not show a fairly-continuous distribution, those outlying records are given in parentheses, so that one knows the elevations at which a given species is commonly seen.
Fig. 1 shows maps of where we have digitized records as of 29 November 2015. Due to limitations of the points to Berkeley Mapper software used to produce these maps, only a representative one fourth of all our GPS points are in the linked maps.
Fig. 2 shows more detailed maps of where we have digitized records as of 7 January 2016, this time with all points used in the map.
These maps are useful in understanding the maps of individual species, since one can use them to see at a glance where we have surveyed and have not found a given species. When our database is complete, they will also be useful for seeing what areas have never been surveyed. Some of those areas can be targeted for future surveys, but some areas are so inaccessible that they might only be surveyed by drones.
Conventional maps showing species distributions only show where a species has been found. Other areas in such maps may simply not have been surveyed, so that one doesn't know if a species occurs in those areas. The maps in Fig. 1 explicitly show where we have surveyed for all species. Any areas shown on those maps, that do not have corresponding points shown on the map for an individual species, are areas where that species is likely not to exist, at least if it is a species whose occurrence can be noted during the time of the survey. Annuals that leave no trace in their dead form are the major exceptions here, since some areas have not had surveys done at prime time in a good rainfall year.
See also Preliminary Analysis of Elevations of the Species in the Flora, done on 5 February 2014 when the database contained 6,106 points, about 2/3 of the number of points now in our database.
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| Fig. 1. Geographic distribution maps for areas where we have digitized our GPS points from our surveys as of 29 November 2015. Due to limitations of the "points to Berkeley Mapper" software used to produce these maps, only a representative one fourth of all our digitized GPS points are shown. | |
Fig. 2. Geographic distribution map for areas where we have digitized our GPS points from our surveys as of 7 January 2016.
More detailed scale maps of the GPS points as of 7 January 2016 are linked below; these maps also show routes we have surveyed but for which we have not yet digitized our GPS points. There are 11,521 digitized GPS points in this data set, from 104 full surveys.
- Agua Caliente area
- Borrego Badlands area
- Borrego Valley area
- Culp Valley area
- Granite Mountain / Whale Peak area
- Harper Flat Area
- Mescal Bajada area
Some surveyed routes not yet digitized in each of those areas are:
- Agua Caliente area. The Potrero; Smuggler Canyon / Bisnaga Alta Wash; Canyon 41.
- Borrego Badlands area. Smoke Tree Canyon; Coachwhip Canyon; Clark Valley routes; Rattlesnake Canyon.
- Borrego Valley area. Coyote Mountain; Borrego Spring; Borrego Sand Dunes; Borrego Palm Canyon; Coyote Creek Wash.
- Culp Valley area. Wilson Trail; Wilson Trailhead area; north of Chimney Rock; Dry Canyon area.
- Granite Mountain / Whale Peak area. Most Granite Mountain surveys; Blair Valley; Little Blair Valley; Ghost Mountain; Rainbow / Box Canyon.
- Harper Flat Area. All routes have been digitized.
- Mescal Bajada area. Mine Canyon; Bitter Creek.
Areas to survey in future
The following are some areas where we will try to get GPS points in the future, to make the coverage of the Borrego Desert more complete (only the Borrego Desert areas below 3000 feet elevation are listed below):
- Borrego Badlands area. The lengths of Inspiration Wash, Fonts Point Wash; Inspiration Point area; middle of the Badlands; Smoke Tree, Palo Verde Wash.
- Borrego Valley area. Area north of Henderson Canyon Road; area north and south of ABDSP Visitor Center.
- Harper Flat Area. Quartz Vein Wash; Sunset Wash; canyons between Harper Canyon and the Elephant Tree Area.
- Mescal Bajada area. The Mescal Bajada proper (areas below the canyons); San Felipe Creek below Mescal Bajada; Chuckwalla Wash and Canyon.