Resilience of European Brown Hare Populations to Lagovirus Outbreaks at the Southern Edge of Their Range

In the wild, animal populations are rarely static; they breathe, expand, and contract in response to a complex web of life. For species living at the very edge of their geographic range, where the climate is harsher and the habitat less perfect, these fluctuations can be a matter of life and death. Scientists have long understood that when a population becomes too crowded, resources like food and shelter become scarce, causing the group to shrink back—a natural brake known as density dependence. Conversely, when numbers are low, the population often has room to grow again. But what happens when a new threat, such as a deadly virus, enters this delicate balance? Does the disease act as a second, crushing weight that pushes the edge populations over the brink, or do other forces hold the line? This question is particularly urgent for the European brown hare, a familiar sight across much of the continent that has seen its numbers dwindle in many places, raising concerns about its future in the warming, changing landscapes of southern Europe.

To answer this, a team of researchers turned their attention to northeastern Spain, a region that marks the southernmost limit of the brown hare's European range. Here, the landscape is a patchwork of Mediterranean forests, dry farmlands, and mountainous terrain, a place where the hare struggles to find the ideal conditions it enjoys further north. The scientists wanted to know if the hare populations in this marginal zone were being kept in check by the usual rules of nature, or if they were being hammered by recurring outbreaks of a specific virus called the European brown hare syndrome virus. This virus, which circulates in hare populations, can cause severe illness and death, leading many to suspect it is a primary driver of population declines. The researchers also wondered if the quality of the land itself—how suitable the habitat was for the hare—was the main factor determining whether the animals thrived or struggled from year to year.

Over a period of twelve years, from 2013 to 2025, the team conducted a massive, systematic survey of the region. They drove along 27 fixed routes through the countryside at night, using powerful spotlights to spot the hares as they moved through the darkness. By counting the animals they saw and measuring how far they were from the road, the researchers could calculate exactly how many hares lived in each area. They paired these counts with official records of disease outbreaks and data on the hunting seasons to build a complete picture of the hare's life in this challenging environment. They also created detailed maps of the landscape to understand which areas offered the best food and shelter, and which were more difficult for the animals to survive in.

The results painted a surprisingly steady picture of a population that is low in numbers but remarkably stable. Across the entire study area, the average number of hares was just 1.79 per square kilometer, a figure that is quite small compared to the dense populations found in the fertile farmlands of central Europe. Despite this low density, the population did not collapse. Instead, it fluctuated gently, rising and falling in a pattern that suggested a strong internal self-regulation. The most powerful force the researchers found was not the virus, nor the quality of the land, but the number of hares already present. When the population was slightly higher in one year, it tended to grow more slowly or even shrink the next year. When the numbers were lower, the population had more room to recover. This pattern of density dependence, where the population naturally checks its own growth to avoid running out of resources, was the dominant engine driving the hare's numbers.

Perhaps most striking was what the virus did not do. The study identified several years when the European brown hare syndrome virus caused outbreaks, yet these events did not cause a measurable drop in the overall population growth. In years when the virus was active, the hare population grew at the same rate as in years when the virus was absent. The researchers suggest that because the hare population is so sparse in this southern region, the virus simply cannot spread easily enough to cause a widespread crash. For a virus to burn through a population and cause a major decline, it usually needs a certain density of hosts to jump from one animal to another quickly. In the scattered, low-density groups found in Catalonia, the virus likely fizzles out before it can do significant damage to the population as a whole.

The quality of the habitat also played a less dramatic role than expected. While the maps showed that some areas, particularly in the mountains and certain inland valleys, were much more suitable for hares than the coastal plains, this difference did not translate into big swings in population growth from year to year. Once the hares were established in an area, their short-term ups and downs were driven more by their own numbers than by the landscape itself. The hunting pressure, which remained steady and moderate throughout the study period, also did not appear to be a major factor in these fluctuations.

These findings offer a reassuring glimpse into the resilience of species living at the edge of their world. The European brown hare in southern Spain is not a fragile population teetering on the edge of extinction due to disease or habitat loss. Instead, it is a population that has found a way to persist at low but stable densities, governed by the ancient, self-correcting rhythm of nature. The virus, while dangerous to individual animals, does not seem capable of wiping out the group in this specific environment. This suggests that these southern populations may serve as important reservoirs of genetic diversity and adaptability, capable of surviving the environmental changes that are reshaping the continent. By understanding that these edge populations are held in check by their own numbers rather than by external catastrophes, conservationists can better focus their efforts on maintaining the diverse landscapes that allow these resilient animals to continue their quiet, enduring existence.