“There is nothing stable in the world; uproar's your only music.” — John Keats

In his 1798 book, An Essay on the Principle of Population, using an elegant mathematical model, Thomas Malthus showed that if unchecked, populations will grow exponentially until they consume all available resources.

Malthus was focused on humans, but his model applies equally to all species. Both Charles Darwin and Alfred Russel Wallace read Malthus’ before they independently developed their nearly identical Theory on Natural Selection and the Origin of Species. As much as did Darwin’s Galapagos finches and mockingbirds, or Wallace’s Malay Archipelago butterflies and birds of paradise, Malthus’ essay planted a seed in their minds about the often-unseen struggles for existence within and among species. A key was Malthus’ idea what will happen if population growth was “unchecked." The paradox of Malthus’ idea is that we are not overrun by snakes, lizards, finches, butterflies, creosote bushes, cactus, or (although it sometimes seems otherwise) house flies. So, what are the mechanisms that keep these populations in check? The list is long: parasites, disease, drought (famine), predation, wildfires, floods, climate shifts, competition within and between species ... you get the idea. 

We have inadvertently tested this theory many times, introducing species into new regions where they lack the parasites, predators, and diseases that they grew up (evolved) with. Sahara mustard and tamarisk of species were brought to our deserts either by accident or on purpose, but without any of their natural population controls their populations have exploded and degraded and threatened the sustainability of our native species.

A similar “experiment” has occurred in Australia, southern Africa, and Madagascar with the introduction of cactus from our hemisphere. The cactus population explosions have, at least to some extent, been checked by introducing a cochineal scale insect native to our North American deserts. Tamarisk tree explosions may be starting to be checked by the introduction of a leaf beetle otherwise native to Afghanistan, the original home of salt cedar, Tamarix ramossima. In both cases the introduction of a parasite/predator (the distinction seems blurred in these cases) will not eliminate the invasive plants, but it appears to be controlling their growth enough to make coexistence tenable. So far there are no similar “biological controls” identified for Sahara mustard. Take away the controls and Malthus’ predictions ring true.

It is interesting to contemplate the unseen dramas taking place in nature, dramas that can give us the false impression of stability. There are thousands of species of nearly microscopic parasitic flies that keep other insect populations in check, not to forget those macroscopic spiders and assassin bugs that do the same. There are ongoing “chemical wars” between plants and their parasite/predators. Sometimes we can see a hawk or a roadrunner prey on a lizard or a snake. Chuckwallas will sometimes become habituated to hikers along our trails and just watch as those brightly colored nylon packs and Vibram soles pass by. But their eyes are constantly scanning the skies, as their primary predators are red-tailed hawks. If a hawk is seen they immediately seek cover within a deep rock crevice. Then at a landscape scale wildfires and floods can sometimes dramatically change the fates of the species living within their path. Often bemoaned as catastrophes, especially when people who in their hubris or out of economic necessity opt to build their homes in floodplains or fire-prone ecosystems, these are otherwise natural ecosystem processes that pave the way for recycling nutrients and renewing habitats. What at some scales seems to be a stability of nature, is at both larger and smaller spatial and temporal scales rife with change, and dramas of processes that keep the Malthusian paradox from becoming runaway population explosions ending in extinction events.

My own research aims to separate the natural population dynamics that sustain populations from changes that might indicate an unsustainable downward spiral. For most desert species it is straightforward. Droughts equal lower food resources and lower reproduction. Fringe-toed lizards adhere to that pattern; populations decline after dry years and increase after wetter years. I remember early on when, during a multiple year drought period and the lizard populations declined, that some folks worried out loud that the conservation plan was clearly a failure, as evidenced that the lizards were in decline. Apparently they reasoned from a naively Malthusian unchecked point of view, that populations should always increase. I suggested that before erasing all the hard efforts that had gone into creating the conservation areas, they wait for a year or two with average rainfall. The next year turned out to be a good rain year, followed by a bumper crop of hatchling lizards. I never heard from them again.  

However, there are sometimes when the lizard populations do not increase after wetter years, then it can often be a result of Sahara mustard infestations. The mustard out competes the native plants, but the mustard does not support the insects that the native plants do. Wet years with heavy mustard infestations result in insect declines, and then fringe-toed lizard declines. Without the mustard, those occasional wet years allow the fringe-toed populations to increase, which could be critical for keeping their populations from not getting too low during the inevitable dry years. With the mustard, the lizards’ may not build their populations to create buffers against the next drought, droughts that are becoming more frequent and drier.

Another lizard I follow is the flat-tailed horned lizard. This species has lost a huge portion of its original distribution due to home and agriculture development in the Coachella Valley and especially in Imperial County. There is just one location left in the Coachella Valley where they still occur, in a habitat that overlaps in part with where fringe-toed lizards live. So, I would expect that they too would follow the fringe-toed lizards’ pattern. Except they do not. They only, or almost only, eat harvester ants and the ants find and eat plant seeds buried in the sand. The seeds are always there, wet or dry year, so the flat-tail populations do not follow a wet-boon dry-bust cycle, but they nevertheless do fluctuate in numbers from year to year.  I am still trying to figure out why. Maybe it is predation pressures. Interestingly, populations of shovel-nosed snakes fluctuate in parallel with the flat-tails. There is a clue there, I just need to solve it.

Nullius in verba

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