Many excellent field guide books seek to provide information about the extent of a particular plant species’ actual geographic range. Yet sometimes this information either is not complete, or grows outdated over the course of time. Why?
A number of reasons may potentially account for discrepancies. Two of the most common relate to incomplete initial plant survey information and changing environmental conditions in a local area.
Incomplete plant survey information
Numerous reasons account for incomplete, or even inaccurate, initial plant survey information. Although botanists work hard to accurately describe every species of plant, descriptive categories sometimes change as new information develops over the course of several years.
For instance, just consider that DNA remained unknown two hundred years ago…Many early botanists sought to classify unfamiliar types of plants based mainly on appearances. Today, researchers can sometimes use DNA studies to place plants more accurately within particular families.
Additionally, from time to time, botanists discover entirely new plants. It may become evident that previous generations wrongly classified two separate types of plants as a single species, when a more accurate taxonomic description would regard the samples as members of different species.
Other reasons may account for incomplete initial survey information, too. A previous report may have included incorrect information because someone mistakenly described a plant and the error was not detected at the time. Or perhaps people within a particular local area did not expect to find a particular plant species there, and they overlooked its presence for many years?Throughout history, people from time to time do not correctly identify particular types of plants.
Since no one compiling a field guide can actually survey every single acre within a typical guide’s coverage area personally, many explanations exist for incomplete or even inaccurate data, even in excellent guides.
Changing environmental conditions within a local area
Another set of reasons for inaccurate botanical field guide geographic range information pertains to the dynamic nature of the physical world. The face of the globe changes across the span of time. Field guides only cover specific periods and they cannot always keep up with the natural and man-made environmental transformations occurring in some places.
Just as human and animal populations on occasion move into new locations, for instance, plants may “travel” into additional spots on their own over the course of several generations. For example, many gardeners have experienced the strange sensation of watching some annual flowers (or weeds) gradually expand.
A bed of tulips may thrive in a particular location and come back year after year—eventually occupying eight square yards instead of an original two square yard space. The offspring from a single dandelion in one corner of a yard, unchecked by a homeowner, may explode across the lawn over the course of just a few years. So it does not seem unreasonable that two botanists conducting field surveys in different years in the same location might measure different ranges for some plant species.
Many botanical field guides obtain information from somewhat dated sources. Yet as human society or changing environmental conditions such as floods or fires transform particular locales, an area may become better suited to different sets of plants. Gradually, some species in the location decline in numbers while others become better established.
Two illustrations: “desertification” and “invasive plant species”
One illustration of this situation involves the radical transformation of the landscape in some parts of the world which may occur when livestock overgraze an arid region. Livestock may eat so much of the available forage in a location that a desert-like environment emerges and some native species of plants listed in field guides never return to the area naturally.
Some sources estimate that roughly half of South Africa remains at risk of desertification today. Few field guides can keep pace with such a rapid pace of man-made change.
Additionally, conditions sometimes promote the survival of certain “invasive” plants and enable them to develop flourishing population in unexpected places. For instance, many field surveys do not detect plant movements into quite distant places until the foreign plant’s population numbers in a given area grow widespread.
A beautiful and very hardy green vine native to Japan illustrates this point. Today, people across the southeastern USA often encounter Kudzu clinging to the branches of roadside bushes and trees. Yet this plant species remained unknown in the United States through the Civil War and Reconstruction eras. No early botanical field guide would have listed it despite the plant’s widespread prevalence in many southern states today.
Kudzu first came into the country as a garden exhibit for an exposition held in Philadelphia in 1876 and it was not widely cultivated in the USA until the early 1900s!
Just like other types of reference books, botanical field guides require periodic updating. Even many excellent popular field guides may fail on occasion to accurately list the complete geographic range of a particular plant species.