It took 45 years of constant record-keeping, year-in-year-out, to determine why tenebrionid beetles have such extraordinarily high diversity in the Namib Desert. My recent analyses of these records revealed a cryptic complexity of time niches whereby a seemingly plain gravel plain undergoes a series of subtle changes that favour different species over time. 

Since the 1950s, scientists have been wondering how so many beetle species can coexist in the middle of southern Africa’s driest desert, an area of low primary productivity. To cut a long story short: nearly all are ultimately dependent on irregular pulses of rainfall, but different species navigate differently through time – months, years, decades – since these singular, rare events.

Diligent fieldwork and data collection 

The breakthrough was made thanks to diligent fieldwork and data collection by numerous interns and field technicians at the Gobabeb Namib Research Institute in Namibia at 15 pitfall traps installed two kilometres from the institute in March 1976 and monitored every two to three days ever since. An astonishing 54 tenebrionid species were recorded in these traps, 26 species being sufficiently abundant to analyse population trends over time.

Rain seldom falls, annually only 25 mm on average, ranging from zero to 172 mm per year. Effective rains, driving green flushes when grasses germinate and seed, occurred in 19 of the 45 study years, sometimes eight years apart.

These were the important events that added fresh detritus (litter) to the pool. Detritus comprises dead organic matter mostly coming from plants, such as dry leaves, stalks and husks. The quantities of detritus in the desert are akin to footprints of rainfall events, dwindling over the years. Detritus is the staple food for omnivorous tenebrionids, so the detritus pool and its gradual decay is critical for their populations over the years.

Over 20 000 species of the beetle family, Tenebrionidae, also known as darkling beetles or toktokkies, occur in temperate or tropical terrestrial biomes worldwide. They are abundant and conspicuous in arid lands, renowned for their extreme tenacity, coping with dehydration, heat and hunger, and getting moisture by unconventional means such as fog basking or direct uptake of humidity from the atmosphere.

These beetles lack developmental dormancy, but adults are long-lived, with some lifespans exceeding six years and generations overlapping. Desert populations go through periodic bottlenecks, bridging lean times with extreme ecophysiological resilience and bet-hedging reproduction whereby females lay eggs whenever they have gained enough resources to do so, every day in ideal conditions.

Unusually heavy rainfalls in 1976 (106 mm) and 1978 (85 mm) gave the Namib study a booming kick-off. Two decades of much drier conditions followed (annual rainfall 0–23 mm) when detritus receded, as did beetle numbers, bottoming at 0.18% of boom abundance.

In the years following several effective rainfalls between 1997 and 2005, beetle abundance rose to 38% of the previous peak. Years with unusually high rainfalls averaging 50 mm followed from 2006 onwards, with beetle abundance climbing to 279% of the first peak. This roller-coaster pattern of abundance was driven by the moisture regime and detritus availability, confirming the first main hypothesis.

However, within the envelope of general abundance, the 26 examined species displayed somewhat differing trends. With correlation and cluster analyses, species were assigned to six patterns, which they followed with variations.

While populations of three species quickly irrupted after effective rainfalls and crashed a year later, the five most abundant species tracked the quantity of detritus over several years. Eight species took their time, their populations increasing unhurriedly after heavy rainfalls and declining even more slowly over the following decade. Two species cued to winter rainfalls with prolonged soil moisture, which occurred at bidecadal intervals. These two species were best at avoiding the narrowest bottleneck and were recorded in every study year.

Population trends for another species correlated negatively to all other species, increasing during rain-dry years and declining during years with rain. Finally, seven species with no permanent resident populations in the study area occasionally expanded their ranges from an adjacent habitat.

The overall effect of these six population patterns with variations yielded different beetle community compositions year on year. The seriation usually progressed steadily, sometimes meandering around a certain condition, but there was a run of punctuated shifts between 1990 to 1996, reflecting a sequence of considerable changes in the community during this extremely dry period.

In 2006, 30 years into the study, the community surprisingly returned to its initial condition. Even though the years that followed were moister than three decades ago, the seriation pathway was in roughly the same direction, indicating community cyclicity.