Deer population in Doñana reaches pre-drought abundance levels of 2005
Deer in the Doñana National Park / Photograph: Rubén Olivares
After almost two decades of progressive growth, the size of the deer population in Doñana National Park is now slightly larger than before the 2005 drought. The latest data recorded by the Monitoring Programme of Natural Processes and Resources, carried out by the ICTS Doñana (Doñana Singular Scientific-Technical Infrastructure), indicate that the population of this herbivore has been recovering progressively until reaching this year the maximum value of the historical series 2005-2024.
The Monitoring Programme censuses provide a first estimate of relative abundance, the Kilometric Abundance Index (IKA), based on the number of contacts per kilometre. From these data, the scientific staff can fit statistical models to calculate the density of the species (the number of individuals per km2). The IKA obtained in 2023 for deer in Doñana has increased to 4.69 contacts/km, compared to an average of 2.74 in the previous period 2004-2022. During that period, abundance increased progressively during the decade following 2005 and has been stable since then, reflecting a stabilisation of the population.
Luis Santamaría, researcher at the Doñana Biological Station (EBD-CSIC), where he leads the Spatial Ecology Group, explains that there is a continuous upward trend: ‘In the 2005 drought, abundance dropped a lot, but since then the deer has recovered to slightly higher values than at the beginning of the series’. Since 2014, the deer population in Doñana National Park has remained stable at around 4,000 specimens, without declining despite the drought of the last two and a half years, according to IREC and ICTS Doñana data.
The rise in the deer population is probably boosted by the excellent conditions offered by the Coto del Rey, an area located north of El Rocío and composed of Mediterranean forest. As Francisco Carro, responsible for monitoring deer populations at ICTS Doñana, explains: ‘In this autumn's censuses, this estate has recorded a significant increase in the abundance of deer. This is probably because the Coto del Rey has no livestock, so there is more pasture and the deer have less competition’. In particular, this is where the second highest abundance of all the annual autumn trails has been detected (25.5 contacts/km).
Furthermore, the species increases in all transects, especially in the Doñana Biological Reserve (5.16 contacts/km in 2023 vs. 4.32 in 2022); Puntal (0.4 vs. 0.26); Algaida-Los Sotos (3.35 vs. 2.13); and Marismillas (2 vs. 1.16). These values reflect both the general increase of the species and its preferences for the use of the different farms, as the animals can move between them.
Census and population control
Ungulates (wild boar and deer) and lagomorphs (rabbits and hares) in the Doñana Natural Area have been counted every year since 2005 from a specially adapted vehicle that travels through all the existing habitats. These counts are carried out in March and September, the latter coinciding with the rutting season.
Adapted vehicle for the census of animals in Doñana
Compared to other areas on the peninsula, deer population densities in Doñana are moderate. ‘Although there are no predators or hunting operations in Doñana, the environment is less productive, as the grazing areas represent only a fraction of the Park and the resources they offer are shared with livestock and other wild ungulate species. In addition, the absence of predators that can regulate the population increases the risk of the population growing beyond the carrying capacity of the vegetation, especially in years of drought, which can degrade ecosystems,’ says Santamaría. It is therefore very important to monitor population size and its impact on vegetation to assess whether it needs to be regulated.
International research project on the effects on vegetation
The Spatial Ecology Group is developing the international research project RESILGRAZE, which analyses how the relationship between vegetation and large herbivores functions in high nature value pastoral systems in the European Atlantic region. These systems include cattle (horse and cow), coexisting with several species of wild herbivores (deer, fallow deer, roe deer). The aim is to find tools for early prediction of possible mismatches between vegetation production and its consumption by the herbivore community. This project has the participation of the Universities of A Coruña, Groningen (The Netherlands) and Edinburgh (Scotland).
When the usual cycles of drought and wetter years occur, processes of overgrazing are triggered, reducing food production in subsequent years. These processes can be moderate and local, thanks to the resilience of these ecosystems and the regulation processes of herbivore populations; or they can reach thresholds where vegetation is severely and 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 how much food there is for deer (and other herbivores) using satellite imagery and GPS collars and to analyse how deer move to find and exploit the areas with the most and best grass. Finally, the impact they have on vegetation composition and productivity is also measured.
The work already carried out shows, for example, that the availability of different types of vegetation in Doñana helps to increase resilience in years when it rains too little or even when it rains too much. Francisco Carro, who collaborates in the project, explains that ‘if the farms have different types of vegetation, the impact of these extreme years on herbivore populations is less’. Moreover, on farms with high densities, the effect of livestock on vegetation is much more severe than that of deer and fallow deer; however, when there are moderate densities, the presence of livestock favours wild ungulates, by selecting better quality grass types for them.
Achieving an appropriate balance between the different herbivores is therefore essential for the management of the Park. Especially for the tree vegetation, which is having severe problems in establishing and growing young individuals, caused by the lowering of the aquifer and the excess of herbivores. ‘To help the regeneration of the noble trees and shrubs, Park staff are having to protect the young plants with fencing. This method may work temporarily, but it is not a sustainable solution. Our contribution is to try to determine what is the impact of both types of herbivores, wild and domestic, and what densities are compatible with the regeneration of vegetation. And how to adapt these numbers to the changes that are already occurring in the climate’, says Luis Santamaría.