"I will go root away the noisome weeds which without profit suck the soil's fertility from wholesome flowers." — William Shakespeare

"What is a weed? A plant whose virtues have never been discovered." — Ralph Waldo Emerson 

Webster defines a weed as “a plant that is not valued where it is growing and is usually of vigorous growth especially one that tends to overgrow or choke out more desirable plants.” Oxford calls a weed as “a wild plant growing where it is not wanted." 

My problem with both definitions is that they rely on value judgements – “not valued”, “not desirable," or “not wanted." By relying on value judgements, we tread on a slippery slope of becoming species-ist, categorizing things in nature as “good” or “bad” from the perspective of humans simply on the basis of where that species is from. It's wrong to categorize people that way and wrong to do the same with non-human species.

The California Invasive Plant Council (Cal-IPC) defines weeds as “Invasive species that threaten wildlands that 1) are not native to, yet can spread into, wildland ecosystems, and that also 2) displace native species, hybridize with native species, alter biological communities, or alter ecosystem processes." Now we have a definition that is based on what a species does, not their apparent value. The key in terms of how or if we should endeavor to control the spread of a “weed” is not whether it is native or non-native, but the degree to which it has a positive, benign, or negative impact on otherwise native species assemblages. If positive or benign, that species could increase biodiversity, which at least potentially could create greater redundance, and so increase ecosystem resilience. However, if negative, such species have the potential to create cascades of local extinctions, reduce biodiversity, and reduce ecosystem resilience. 

When I first arrived this desert, in 1986, I was hired to coordinate the management of the “Coachella Valley Preserve," which at the time was a 13,000-acre island of still natural habitat in the middle of this valley. Landowners included the U.S. Fish and Wildlife Service – National Wildlife Refuges, the U.S. Bureau of Land Management (BLM), California State Parks, California Department of Fish and Wildlife – Ecological Reserves, and The Nature Conservancy (my employer at the time). The focus of the preserve was to ensure the protection of the endangered Coachella Valley fringe-toed lizard on what was one of its last remaining habitats. Job one was to stop the illegal off-road recreation on the sand dunes. Fencing and diligent efforts on the part of the BLM rangers eventually got that under control. Job two was to control an invasive, non-native plant known as Russian thistle, or “tumbleweed."  Fans of western movies may be dismayed to know that those dusty scenes of tumbleweeds blowing through abandoned towns would have never happened. Those tumbleweeds are native to the arid Russian steppe and arrived in North America probably in the wool of sheep brought from Europe. Once here it rapidly expanded across western North America. Within their native range most plants are constrained by insects or pathogens or soil nutrients. Sometimes, when introduced into a new region and if those constraints are no longer present, that species can rapidly expand its range, as if like a tsunami wave, sometimes at the expense of the native species.

When I looked across the Coachella Valley Preserve’s landscape in 1986, it was a “sea” of Russian thistle. My question was how to get rid of it. I spent a lot of hours wandering through that sea of tumbleweeds trying to come up with a reasonable strategy to make it go away. I often carried a wooden staff that I could then wedge beneath the tumbleweed and pry it out of the sand, often with a very satisfying “pop” as the roots broke free. Curiously, there was always a little “ecosystem” of native ants and beetles beneath each plant, and it was not at all uncommon to have a juvenile fringe-toed lizard hiding there, and more rarely a sidewinder. My epiphany was slow in coming, but I finally realized I was asking the wrong question. My first question should not be how to get rid of it, it should have been what was this weed’s impact on this otherwise natural landscape? I did the science and discovered that the fringe-toed lizards were more abundant in areas with moderate densities of Russian thistle; the lizards found greater food, better thermal refuges, and better protection from predators when the thistle was present. In a world where nearly 95% of the lizard’s range had been lost to golf courses, freeways and railroad corridors, leaving the thistle in place made sense. As it turned out, over the years the density of tumbleweeds ebbed and flowed with the occurrence of rare, once every few decades, flood events. Between floods the tumbleweeds nearly disappeared on their own. When present, at least at those moderate densities, Russian thistle adds resilience to a fragile ecosystem, something I could not have predicted a priori without doing the science.

Later, in the mid-1990s and since, another weed showed up on the sand dunes. Actually, it was around since at least the 1920s, but its tsunami wave hit in just the last decade of the 20^th century. The weed was Sahara mustard, inadvertently introduced here with the early date palms to establish the Coachella Valley’s date palm industry. This time I did the science first when the mustard’s second wave hit in 2005. This time the data told a different story. Mustard populations ebb and flow, not with floods, but with typical rainfall fluctuations: more rain = more mustard. The presence of higher mustard densities nearly eliminated native annual plants, reduced insect populations, and resulted in declines in fringe-toed lizards when we would otherwise expect their populations to be on the rise due to higher rainfall. We now know we need to control Sahara mustard, but the problem is how. How to control a plant without having similarly negative impacts on native plants? Hand pulling can work but the effort has not kept up with the magnitude of the problem. 

Given the extreme heat and aridity of deserts, it seems like deserts should be weed-free. Only those species who have evolved to tolerate these extreme conditions should be able to live here, but common sense aside, weeds are an increasing problem. Invasive non-native grasses are particularly troubling because at high densities that have the ability to fuel wildfires. Deserts would be historically fire-free, or nearly so, because by mid-summer, when thunderstorms provide an ignition source, the native annuals will have mostly dried up and blown away providing little or no fuel between shrubs to carry a fire. The non-native grasses are dense and persistent so do not blow away, and so easily carry wildfires. Due to a historical lack of wildfire, desert shrubs lack adaptations to survive fires, adaptations that are typical of plants in fire-prone forests and shrublands. Native shrubs die in wildfires and can take decades and perhaps centuries to become re-established. Mediterranean split grass, red brome and cheat grass are all non-native grasses that are implicated in the now increasing wildfire frequency in the Mojave Desert. Interestingly, there is an insidious link between the increases in those grasses and nitrous oxides blowing eastward in Los Angeles-created smog. The nitrogen adds an essential nutrient that, if absent, would limit the grass’ density. 

Another non-native grass, cape needle grass, has recently invaded the Santa Rosa and San Jacinto National Monument. This grass now is abundant on the hillsides above Palm Springs and is rapidly moving eastward.  It has the same capacity to carry wildfire as those previously mentioned grasses, putting hillside homes and native habitat at risk. Our community science surveys have shown a more immediate impact on biodiversity. We found that the hillsides above Palm Springs have especially rich and dense lizard populations and plant communities. In the face of increasing heat and aridity (aka climate change) this area appears to be acting like a climate refugia, enabling lizard species to exist even at the lowest elevations at higher densities than they occur father east where it is hotter and drier. Except where cape needle grass occurs. In those areas lizard and native plant richness is declining, fast. The native plants and the insects associated with those native plants are being squeezed out by the new invader. 

Some of these insults to biodiversity have solutions on the horizon. As we adjust to a lower carbon future (electric cars and trucks) the smog from Los Angeles will disappear and so will its fertilizing effect on the Mojave Desert and those invasive grasses. Since none of these non-native invasive species reach densities in their native homelands that result in affronts to biodiversity there, we need to spend time doing the science there and identify what the differences are. In some cases, there may be a biological control that is so species-specific that it might be safe to introduce here. Such has been the case with a beetle species that eats only tamarisk. Without tamarisk it starves. Otherwise, it will take vigilance and education. Stop planting fountain grass. Identify new invasions when they are small and controllable. Join a community science team to become part of the solution.

Go outside, tip your hat to a lizard, and be safe.