After almost two decades of gradual growth, the size of the deer population in Doñana National Park now slightly exceeds that before the 2005 drought. The latest data recorded by the Natural Processes and Resources Monitoring Program, carried out by the ICTS Doñana, indicate that the population of this herbivore has gradually recovered until reaching this year the maximum value of the historical series 2005-2024.
Monitoring Program censuses provide a first estimate of relative abundance, the kilometric abundance index (KAI), based on the number of contacts per kilometer. From this data, scientific staff can adjust statistical models to calculate species density (the number of individuals per square kilometer).
Thus, the IKA obtained in 2023 for Doñana deer increased to 4.69 contacts/kilometer, compared to an average of 2.74 in the previous period (2004-2022), as highlighted by the Council of Higher School of Scientific Research (CSIC). ). During this period, abundance increased gradually over the decade following 2005 and has remained stable since then, reflecting population stabilization.
Livestock competition for grazing
In this sense, Luis Santamaría, scientific researcher at the Doñana Biological Station (EBD), who leads the Spatial Ecology Group, explains that there is a continued increasing trend, since “during the drought of 2005, the The abundance decreased significantly, but since then the deer have recovered to reach values slightly higher than those at the beginning of the series.
Thus, since 2014, the deer population in Doñana National Park has remained stable, around 4,000 individuals, without decreasing despite the drought of the last two and a half years, according to data from IREC and ICTS Doñana. For this reason, experts believe that the increase in the deer population is probably favored by “the excellent conditions” offered by Coto del Rey, an area located north of El Rocío (in the municipality of Almonte) and constituted Mediterranean forest.
“During this fall’s census, this farm recorded a significant increase in deer abundance. The cause is surely that Coto del Rey does not have livestock, so there is more grass and the deer have less competition,” explains Francisco Carro, responsible for monitoring deer populations at ICTS Doñana.
Specifically, this is where the second highest abundance of all annual fall routes was detected (25.5 contacts/km). In addition, the species is increasing in all transects, notably in the Doñana Biological Reserve (5.16 contacts/km in 2023 compared to 4.32 in 2022); Strength leg (0.4 versus 0.26); Algaida-Los Sotos (3.35 vs. 2.13); and Marismillas (2 against 1.16). These values reflect both the general increase of the species and its preferences for using different farms, since animals can move between them.
Censuses and population control
The ungulates (wild boar and deer) and lagomorphs (rabbits and hares) of the Doñana Natural Area have been recorded every year since 2005 using a specially adapted vehicle which travels through all the existing habitats. These counts are carried out in March and September, the latter case coinciding with the rut. Compared to other farms on the peninsula, deer population densities in Doñana are moderate.
“Even though in Doñana there are no predators or hunting operations, the environment is less productive, because the grassy areas represent only a fraction of the park and the resources they offer are distributed with difficulty. livestock and other wild ungulate species. In addition, the absence of predators capable of regulating the population increases the risk that the population grows beyond the carrying capacity of the vegetation, especially during years of drought, which can degrade ecosystems,” explains Santamaría .
This is why, according to the expert, it is “very important” to monitor the size of the population and its impact on vegetation, to assess whether it is necessary to regulate it.
International research project
The Spatial Ecology Group is developing the international research project Resilgraze, which analyzes the functioning of the relationship between vegetation and large herbivores in pastoral systems of high natural value in the European Atlantic region. These systems include livestock farming (horse and cow), which coexists with several species of wild herbivores (deer, fallow deer, roe deer). The objective is to search for tools allowing early prediction of possible mismatches between the production of vegetation and its consumption by the herbivore community. This project benefits from the participation of the universities of A Coruña, Groningen (Netherlands) and Edinburgh (Scotland).
“When the usual cycles of drought years and wetter years occur, processes of overharvesting of grass are triggered, which reduces food production in subsequent years. These processes can be moderate and local, thanks to the resilience of these ecosystems and the regulatory processes of herbivore populations; or they may reach thresholds in which vegetation is severely, or even irreversibly, degraded. With climate change, these cycles will become more frequent and more extreme,” says Santamaría.
The Resilgraze project focuses on developing tools to estimate and verify the amount of food available to deer (and other herbivores) using satellite images and GPS collars, analyzing how which they move to find and exploit areas with more abundant and better quality grass. Finally, the impact of these on the composition and productivity of the vegetation is also measured.
Fencing of young plants
The work already developed shows, for example, that the availability of different types of vegetation in Doñana helps to increase resilience in years when it rains little or even when it rains too much.
Additionally, on high-density farms, the effect of livestock on vegetation is “much more severe” than that of deer and fallow deer. However, when densities are moderate, “the presence of livestock favors wild ungulates, by selecting for them higher quality types of grasses.”
“To encourage the regeneration of noble trees and shrubs, park staff must protect young plants with fences. This method may be useful temporarily, but it is not a lasting solution. Our contribution is to try to determine the impact of the two types of herbivores, wild and domestic, and what densities are compatible with vegetation regeneration. And how to adapt these figures to the climate changes that are already occurring,” explains Luis Santamaría.
