Scientists kept rabbits out of experimental plots in New Mexico's desert for 50 years; by 1989, one grass had 30 times more basal area in protected plots

In New Mexico’s Chihuahuan Desert, keeping rabbits and other small herbivores away from vegetation revealed just how slowly grazing pressure can shape a dryland ecosystem. Some changes were not immediately obvious, but decades of observation eventually exposed a striking difference in the growth of a native grass. The experiment showed that the influence of plant-eating animals can extend beyond the loss of individual shoots, affecting how vegetation develops over time.

According to the study published in Oecologia, researchers established experimental plots in 1939 to examine the effects of excluding lagomorphs, a group that includes rabbits and hares, from a landscape dominated by creosotebush. By 1989, spike dropseed had 30 times more basal area in plots protected from these animals than in plots where they could continue feeding. The scientists also tested shrub removal, furrowing and seeding, allowing them to compare how different treatments affected the desert’s vegetation over the long term.

A desert experiment built to last


The report states that the experiment was set up in the Jornada Experimental Range in southern New Mexico in 1939. Scientists designed 16 plots, each about 21.3 meters by 21.3 meters, arranged in four rows. Buffer areas separated the plots, allowing scientists to observe the effects of rabbit exclusion and other treatments. Chicken wire fencing was used to exclude lagomorphs. Lagomorphs are an order of mammals that includes rabbits, hares and pikas, but this desert experiment focused on rabbits and hares.

A separate barbed-wire fence protected the rest of the experimental plot from cattle, allowing the scientists to distinguish between the impact of small native herbivores and domestic animals. Vegetation was surveyed several times: first in 1938, before the experiments began, and then in 1947, 1956, 1960, 1967 and 1989. Such a large time span helped the researchers follow all changes that could be easily overlooked in a regular experiment.

One grass showed a dramatic response

The most obvious effect was in 1989, when spike dropseed (Sporobolus contractus), which is a perennial grass indigenous to the area, had 30 times greater basal area on plots free of lagomorphs compared to those where they were allowed. Basal area refers to the area covered by the base of the plants. It differs from height and total biomass, so the result indicates a large difference in how much of the plant covered the ground. However, the effect was not consistent across all plants. Exclusion of lagomorphs also affected the canopy cover of honey mesquite, tarbush and mariola, which are woody plants native to the area.

The researchers also noted that seeding and furrowing did not produce similar results. It showed that browsing by rabbits and similar animals can affect the abundance of certain plants, even in a water-limited ecosystem. However, the researchers did not interpret the fluctuations as evidence of ecological succession. The changes were attributed to a species responding to the absence of the main herbivore.

A later study published in Journal of Arid Environments, which examined the experimental plots over 56 years, reinforced the importance of looking beyond a single intervention. It found that shrub removal increased the basal cover of black grama and spike dropseed, although these differences took about 50 years to become evident. The effects of excluding lagomorphs over time were less pronounced than those of shrub removal.

Why decades of observation mattered

This experiment taught people something very valuable about restoring degraded drylands: the removal of just one type of pressure does not change everything. Vegetation was not equally restored, even after so many years of conservation. Some species were positively affected more than others; some methods of treatment did not show any improvement at all.

That complexity remains relevant to modern restoration efforts. A 2019 study in Ecology and Evolution examined how native lagomorphs affected grass establishment in a shrub-dominated, semi-arid grassland in south-eastern Arizona. The researchers noted that native mammals were able to inhibit the growth of grass despite the fact that the livestock were gone and attempts at restoration had taken place. The study showed that native grazers, not just cattle, can influence grassland restoration.