“No part of the world can be truly understood without a knowledge of its garment of vegetation, for this determines not only the nature of the animal inhabitants but also the occupations of the majority of human beings.” — Ellsworth Huntington

Species exist in places that meet their needs with respect to food, cover, breeding sites, moisture, humidity, and temperature. Those places are that species’ habitat and habitats are defined by the species that occupy those habitats. Any description or mention of a habitat should always with reference to the species occupying that habitat (“fringe-toed lizard habitat," or “habitat for desert tortoises.”) Another way to refer to and describe a place is as a natural community. Natural communities are associations of many species and include relationships and interactions among those species. A natural community is typically defined by dominant vegetation or landforms, with plant species that have evolved together with similar habitat requirements. Those plants can then have specific pollinators and seed dispersers, parasites, and soil fungi that have all evolved together, and so are integral components of that natural community. A natural community provides habitat for dozens, to hundreds, to thousands of species, but you can refer to and describe a natural community without the requirement of referencing a particular species that lives there. Natural communities in our deserts include “sand dunes,” “creosote bush scrub," “alkali sink scrub,” “pinyon-juniper forests," “desert fan palm oases," “Joshua tree woodlands," and “ironwood-paloverde woodlands."

Fredric Clements was a naturalist and what today we would call a plant ecologist, who, working in the early 20^th century, was among the first to try to define and describe just what a natural community was. His view of a community was of a cohesive group of interdependent, coevolved species, distinct from a nearby adjacent community. He viewed the relationships and interactions among the species within a natural community much like a single living entity. This view was widely embraced, particularly in Europe. European landscapes, especially those at lower to mid elevations, have a history of human use going back a dozen or more centuries. In many cases every tree in a European forest was planted at the direction of a feudal king or queen. As a result, those forests have very consistent composition and sharp edges distinguishing them from an adjacent natural community. Those edges between communities are referred to as ecotones, and in a Clementsian view those ecotones were typically sharp boundaries. 

Henry Gleason was also a naturalist and plant ecologist, but he saw natural communities as associations of individual species, with individual habitat requirements. Where those habitat requirements overlapped with those of other species in both time and space, that was then a natural community. This Gleasonian perspective allowed for wide ecotones. Clements’ and Gleason’s hypotheses regarding the nature and formation and existence of natural communities were hotly debated within the academic halls in both Europe and America. Clements’ view was holistic, clean, and sharply defined, whereas Gleason’s was messy, and early on Clements was winning the argument. Frustrated, Gleason abandoned plant ecology to focus on plant taxonomy (like defining species’ boundaries would be any less controversial than defining natural communities!).  However, in more recent years the Gleasonian perspective has become the more accepted hypothesis. Nature is in fact messy.

Still, the Clementsian view can still be seen on maps. One side of a thin mapped line is labeled the Mojave Desert, the other side is the Colorado Desert. The reality is quite different. As an example, Blackbush, Coleogyne ramossisima, is a typical associate of Joshua trees and so with Joshua trees helps define the extent of Joshua tree woodlands as well as the Mojave Desert. But stands of blackbush can be found far from any Joshua tree, and far south of the mapped boundary of the Mojave Desert, all the way into San Diego County. During those glacial maxima of the Pleistocene, not only did northern species and plant communities shift south, but so did desert species. With the return of warming climates those southern-shifted species sometimes became stranded waifs but have nevertheless been able to sustain populations on cooler north-facing slopes or in cold air drainages below high mountain peaks. Similarly, plant species associated with more coastal-Mediterranean climate chaparral communities shifted into our deserts during those glacial maxima. With the return of warm glacial minima, manzanitas, oaks, and others got stranded on desert slopes. One otherwise more coastal lizard, the Blainville’s horned lizard got stranded with them. The result has been “messy” distribution patterns that defy any kind of Clementsian definition.

Even more recently, acknowledging the inherent messiness of nature, plant ecologists are abandoning the concept of a plant or natural community. The trend now is to define “alliances” and “associations." So now there is a creosote bush alliance, with separate associations for the next most dominant plant, such as brittlebush, or burro bush, or galleta grass. There is a parallel with the use of a genus and species name to identify the scientific name for a plant or animal. The alliance is akin to the genus and the association is akin to the species. So, in this example the creosote bush alliance is the “genus," and for instance the brittlebush association is the “species."  The alliances and species are typically reserved for the dominant perennial or woody vegetation growing at a given location. However how then do we include desert sand dunes, which often lack dominant woody plants except for widely spaced creosote bushes, into this scheme? We were able to receive a “rule variance” from the folks in charge of the alliance naming scheme and used dominant annual vegetation. Back when natural communities were in vogue, we previously had gotten an OK to modify “desert sand dunes” to “active dunes," “stabilized sand fields," “mesquite dunes,” and “ephemeral sand fields” to represent the diversity of sand dune types found just in the Coachella Valley. Now what were the active dunes are now the dune primrose association, and the stabilized sand fields are the sand verbena association. 

Unlike physics, which uses the same formulas and notations that Newton developed centuries ago, nature is more complicated. As we better understand the living world, naming schemes change to reflect that better understanding. It is frustrating for those of us who feel our “brains are full” but if new nomenclature reflects a better knowledge of the complex patterns in nature, the changes are worth the effort to learn anew. Alliances and associations are a way to better detect future change; if we map an area as one association and return in future years to find it is now a different association, then we can better understand how our human footprint is affecting the world around us. Naming is a critical step for appreciating and valuing nature, let alone for doing science. Imagine entering a room of strangers and leaving without learning their backgrounds, histories, or names. The next day you may have little or no thoughts of their fates. Then imagine the same room where you learn of their names, their histories, their values. Their future now becomes interesting. It can be so with learning the background, histories and names of species and natural communities (or alliances and associations) in nature.

Nullius in verba

Go outside, tip your hat to a chuckwalla (and a cactus), think like a mountain, and be safe.