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More & Improved Plant Range Maps

July 19th, 2021 by Alex Zorach

Our range maps use ecoregions rather than political boundaries, which, although more useful for ecological purposes, has made them challenging to build as most sources report plant distribution based on political regions such as states and counties. A little less than two years ago we announced tentative range maps for 5,660 plants native to the continental U.S., but we emphasized that these range maps had numerous limitations and were only a starting point.

We are excited now to announce a major improvement in our range maps. We have now published a total of 16,369 range maps, and of these, 15,232 of the maps are new, with numerous improvements over our first iteration of maps. These maps now cover both native and introduced species in North America. Furthermore, the new maps are more accurate and communicate greater nuance than the old maps.

Limitations of the Old Maps

Our first range maps were derived from the Ecoregional Revegetation Application (ERA) and overestimated plant ranges at the borders of their range, were only available to the resolution of level 3 ecoregions (not the level 4 ecoregions currently used as the finest level of detail on our site), only applied to the continental US, and suffered from limitations inherent in both the source material and methodology used to map them onto ecoregions. On top of this, due to limitations in the ERA data, we were initially missing maps not only on all introduced species, including invasive species, but on many native species as well. The missing maps included many common, widespread, and ecologically-important plants.

Until recently, our progress on range maps had proceeded piecemeal and slowly, with us constructing only isolated, individual range maps.

However, we have been working behind the scenes to improve our tools for researching and constructing maps, and are excited to announce major breakthroughs towards more rapidly improving existing maps and constructing maps for the species that lack them.

Features of the New Maps

New Status Categories / Colors / Legends

Our initial maps had fewer categories for the status of a particular plant in a particular region, mainly native, introduced, and uncertain, with a few other categories that were rarely used. The "uncertain" category was a catch-all designation that could mean either that the presence or absence of a species in a particular region was uncertain, or that its presence was certain, but its establishment method was unknown; in some cases it could signify uncertainty in both the presence and potential establishment method.
LegendColor
Native (Prior to European Colonization)
Introduced (Discontinuously with Native Range)
Extirpated (Native But Locally Extinct)
Expanded (Adjacent to Native Range)
Garden Persistent (Reproduces But Only In Gardens)
Uncertain
Not Present
Introduced but Eliminated
Native or Not Present
Introduced or Not Present
Native or Introduced
Native or Expanded
Expanded or Not Present
Expanded or Introduced
Native or Introduced or Not Present
Native or Expanded or Not Present
Expanded or Introduced or Not Present

Previously, these different scenarios were not distinguished, which made the old "Unknown" designation less useful to gardeners or those doing ecological restoration projects. There is a big difference between a plant that may be present in a region but is definitely native there if present, vs. one that is definitely present but may be introduced (and thus potentially invasive). In the first case, many people would choose to go ahead and plant the plant, whereas in the second case, people would be more cautious about planting it.

Our new maps distinguish between these subtleties by blending together and/or graying out colors. Colors are blended together when the method of establishment (i.e. native vs. introduced) in a region is unknown. When the presence/absence of the plant, on the other hand, is unknown, the color is grayed out, i.e. blended with the light gray color used on the map to denote the absence of the plant.

The result is a color scheme that we hope will be intuitive. However, in case it is less than fully clear, the maps are all accompanied by a legend showing what each color means. In case the above list seems overwhelming, rest assured that an overwhelming majority of maps have only a few colors, and the legend under each map shows only the colors in the map.

More Accurate Distinctions Between Native, Introduced, and Expanded

Our old maps only showed plant ranges, making no distinctions between areas where the plants were native vs. introduced or expanding their ranges. Although all the plants covered in the ERA are native to North America, or at least were thought to be native and marked as such by the USDA PLANTS database, they are not necessarily all native everywhere on the continent, and many of them have either been introduced across geologic divides, such as West Coast plants being introduced in the East or vice-versa, or have expanded their ranges to include areas far from their original ranges prior to European colonization of the continent.

This map shows the range of the Carolina laurelcherry (Prunus caroliniana) to the resolution of level 3 ecoregions; although it is native to the southeast (green), it has also been introduced (red) to the West Coast, and has also expanded its range northward (blue) into Arkansas and Tennessee adjacent to its native range.

The grayed-out red, green, and blue signify regions where the plant may or may not be present, but would be respectively introduced, native, or expanded if present. The bold teal color, a blend of green and blue, signifies that the plant is definitely present, and may have both native and expanded populations in that region.

This species is a case where the reported status of the plant differs between different sources; USDA PLANTS reports it (probably erroneously) as native everywhere it is found, whereas BONAP reports it as "adventive" (a non-standard term they often use for plants native to North America but not the locality) in California, Arkansas, and Tennessee, and the Tennessee-Kentucky Plant Atlas reports it as not native, and occurring in counties not reported in either of those two sources.

The new maps generally make these distinctions. To answer questions about a plant's native range and establishment method in a particular site, we have consulted numerous sources. In most cases our starting point is BONAP, as their maps tend to be more accurate than the USDA PLANTS database, and often make such distinctions, although the way they do so is inconsistent. We have also consulted various regional flora and other individual sources. Especially for assessing the ranges of plants outside the continental U.S., we also have been referencing Plants of the World Online (POWO) which is run by the Royal Botanic Gardens in Kew, London, UK. Our methodology for distinguishing introduced vs. expanded status is discussed below.

Limitations of the New Maps

Although the new maps represent a dramatic improvement over our earlier maps, there are still several major limitations.

Limited Data on Canada and Mexico

The construction of detailed plant ranges in Canada and Mexico is considerably more difficult and time-intensive. There is simply no counterpart to the USDA PLANTS database and BONAP for Canada or Mexico; both USDA PLANTS and BONAP show only province-level data for Canada and no data for Mexico. Furthermore, POWO and other sources that show plant range maps in these countries only show the coarsest levels of detail, often coarser than the level of states or provinces, and always much coarser than the scale of the level 3 ecoregions which we are using for our maps.

As such, only a small portion of our maps have been individually reviewed and verified to be accurate into Canada and/or Mexico. Over time, we hope to see the portion of maps that have been completed in this fashion, to increase, and our rate of completing these maps will likely be much faster now that we have new and improved tools, relative to the pace over the past two years.

Subjectivity in Labeling Introduced vs. Expanded

In many cases, such as when a plant has been introduced across a major geographic divide, to a new place far from its native range, it is clear a plant is introduced. In other cases, it is clear that a plant has expanded into new areas contiguously with its native range. However, in other cases, there can be considerable subjectivity in assigning a plant as introduced vs. expanded.

This photo © Sandy Wolkenberg, CC BY 4.0, Source, illustrates bigbract verbena (Verbena bracteata), a plant native, common, and abundant in the West, east to Ohio through Alabama, but now also found in disjoint populations to the East Coast, from Georgia to Maine.

Even though this plant "skipped over" many counties in establishing in new areas, we marked it as Expanded rather than Introduced for several reasons. It is a weedy plant, not generally planted in gardens, but rather, spreading on its own. And it is colonizing new habitats created by humans, such as degraded agricultural land and poorly-maintained urban areas, living in cracks in pavement of roads and sidewalk, and waste areas with sand, gravel, or rocks. It is small and inconspicuous, probably more widely distributed than reported, its populations are not separated by major geographic divides, and it is likely to expand and fill gaps in its range over time.

These most difficult cases consist of plants either whose native population distributions are already scattered, isolated, or disjoint, or ones that have numerous scattered or isolated native populations around the margins of their range, but extending well outside the regions where they are common. For such a species, when a new population is found which is known not to be fully native, it can be hard to know whether to label it as expanded or introduced.

When handling such cases, we made a number of arbitrary judgment calls based on a variety of factors including how far out of its original range the new population was, whether or not the two parts of the range are separated by areas the plant probably could not survive in, whether the plant is widely planted in gardens (and thus likely to escape from them), how the plant tends to spread naturally, and whether the plant has weedy or aggressively-spreading tendencies.

Some of these judgment calls were made hastily and will likely not hold up to scrutiny, so please contact us if you see a designation of a plant as introduced when you think it would make more sense to mark as expanded, or vice-versa, especially if you can provide a source or compelling reasoning explaining why one category makes more sense than the other.

Uncertainty notated on the maps themselves

Most of our new maps display significant uncertainty. The uncertainty has multiple origins.

Most sources, including both BONAP and USDA PLANTS, as well as many local and regional herbaria and plant atlases, use political boundaries such as states and counties, which intersect irregularly with ecoregions, making it hard to know for certain in which ecoregions a plant occurs. Whereas the ERA marked a plant as found in a region if it was reported in any county intersecting that region, we adopted a more conservative approach of marking it as present only if the county was entirely contained within the region, and, in the absence of other evidence supporting its presence, marking it as uncertain in the case of incomplete overlap.

Additionally, the records reported by various sources may be disputed or questionable. BONAP tends to notate such uncertainty in their maps, but does not always do so consistently. Books of regional flora nearly always describe this uncertainty verbally. The USDA, on the other hand, includes numerous dubious records without warning or explanation.

We also found some cases where the reported pattern of distributions of a plant looked unnatural and likely reflected an inconsistency in reporting, such as plants reported as present in nearly all counties of one state, but absent from most counties in an adjacent state. This might reflect different individuals or organizations with widely disparate resources or skill for locating plant populations, or it could reflect reports from some states considering disputed taxa as part of one particular species, while those gathering data in another state might report them under another species.

Our hope is that the reporting of uncertainty on our maps can achieve several goals, including:
  • To give greater confidence in our maps by us avoiding reporting something as definitely native to or present in a particular area when it is not.
  • To make others aware of the limitations of human knowledge in the area of plant's ranges and histories of expansion or human introduction.
  • To guide our further research, as well as encouraging others to contact us and share information with us, to resolve questions about a plant's status in the regions where our maps show uncertainty.

There Will Still Be Errors

As much as we have tried our hardest to be accurate and thorough, and notate uncertainty where warranted, these maps have been a massive undertaking, and it is inevitable that we make mistakes. All our sources have errors in them, and on top of these, we have introduced new errors of our own in our analysis of other sources and construction of our own maps, sometimes due to sloppy data entry or hitting a wrong key, but other times due to systematic errors. We have caught and corrected many of these errors, but it is likely that countless errors still persist.

If you see anything on our maps that looks questionable, or that you know to be false, please get in touch, especially if you can point us to sources or data to help refine or correct our maps. We strive to make these maps as accurate as possible, and the best way for us to do this is to get good feedback to help us refine them.

Checking Other Sources Yourself

We encourage you to use our maps only as one source among many. Our pages for each plant link to other databases, in most cases both USDA PLANTS and BONAP, and in many cases, numerous others. These two sites, with county-level data, can help answer fine-tuned questions about plant ranges not visible on our range maps. We hope to continue interlinking with other sites in the future as well, including POWO, iNaturalist, and more local and regional sources. You can read more about our interlinking with other databases in this previous blog post.

How to Tell Which Pages Have Which Maps

Because of inherent limitations and challenges in constructing these maps, our site is, and probably always will be, in varying stages of completeness. Currently, 1,121 articles still have the old ERA-based maps. 26 articles have new, individually-researched maps that have already addressed some of the shortcomings described above. Most of the maps are somewhere in-between, improved, but still with numerous limitations.


When you use the Search box or view the published plant articles on the Plants tab, the results display a map column to denote the status of the map for each article. Tentative maps that have not yet been constructed for Canada and Mexico, and that may have more errors, are shown grayed out. Over time, as we review these to a higher standard and complete the portions of the range in Canada and Mexico, we will mark them as verified (solid green). Plants for which only the old, ERA-based maps exist, have "ERA" listed in the map columns, and plants with no map show the empty set symbol (∅).

We are also planning to adjust the color of this slider to reflect whether the plant is native or introduced in North America, but we have yet to do this so presently all the sliders are green. There is a legend at the bottom of each search and plant listing page that explains what each designation means.

When you are viewing the article for a particular plant, you will also find a blurb under each map describing its accuracy and limitations.

Thank you for your interest and support!

To everyone who read to the end of this post, thank you so much for your interest, and we hope you enjoy the new maps and find them useful!

If you wish to support our continuing work, please consider becoming a donor. We accept recurring donations through Liberapay, and you can specify both the amount and frequency with which you will be charged, as well as whether it renews automatically or manually. Donations are anonymous, so be sure to get in touch with us if you want to be recognized for your contribution. And thank you to all those who are already supporting us, whether financially or otherwise!