The Spanish ibex is a unique species, exclusive of the Iberian Peninsula, which was at the brink of extinction by the late nineteenth and early twentieth centuries. In fact, two subspecies out of four disappeared, merely relegated to a few scattered groups at ‘Sierra de Gredos’ on the Central System Mountain Range. Nowadays, fortunately, its population is widely spread and the Spanish ibex has become the quintessential herbivore and an emblem of ‘Sierra de Guadarrama’ National Park, thanks to a Reintroduction Plan that reached its highest peak when 67 individuals were released at ‘Hueco de San Blas’ (La Pedriza) in the nineties. That population has increased up until 5.400 individuals in 2017.
The Spanish ibex, as the main herbivore of the mountainous and rocky areas of the Peninsula, plays a fundamental role in ecosystems, and it has been so for millennia. They are the so called ecosystem’s engineers as they maintain essential processes (e.g. nutrients cycle, seed dispersal), enhance biodiversity (they generate vegetation patches with diverse flora and fauna) and mobilise the energy from the lowest trophic levels (plants, mosses and fungi) to higher trophic levels (predators, scavengers, decomposers). Hence, a system without herbivores is an impoverished one, dysfunctional and incomplete. As plants, herbivores and predators have been coexisting for millions of years, and these interactions are the motor for the biodiversity that we know and enjoy, then, why should we worry about goats and plants if they have been coexisting forever? The main reason why is the alteration of that equilibrium, due to an excess of these herbivores or the lack of them, both equally detrimental.
With few individuals, landscapes become alike, losing herbaceous’ habitat biodiversity related to flora and fauna (butterflies, reptiles, orchids) and the roles developed by them for the ecosystem as plants pollinators, seed dispersal vectors, organic matter decomposers, fertilizers, etc. Moreover, structural biodiversity (vegetation with different shapes and sizes) decreases and forest fuel increases and homogenises and, thus, fire risk and propagation intensify.
On the contrary, overpopulation put unsustainable pressure on vegetation, degrading it and compromising its regeneration and that of the accompanying organisms (pollinators, seed dispersal vectors, mycorrhizas, etc.). Moreover, in the worst case scenario, vegetation coverage is intensely reduced (including lichens and rock mosses) aggravating erosive processes and losing the soil that supports the whole ecosystem created throughout the centuries.
As goats do not consume every species equally, the more appealing ones get really damaged. At ‘Sierra de Guadarrama’ National Park, we have been able to observe how certain species (hollies, yews, rowans, snowy mespilus, maples, oaks, whitebeams, etc.) stop growing naturally, both in shape and height, and remain as squat trees and restricted to those areas where goats -their mouth, hoofs or horns- do not reach. Constant and excessive pressure, on the goats’ part, causes the consumption of every sprout with leaves and flowers, preventing them from producing the fruits and seeds they need to regenerate and perpetuate. Thus, numerous species conservation status- some of which are protected- is threatened, as well as their habitats. In fact, the Habitats Directive of the European Union establishes the need to preserve these species and habitats of Community Importance, some of which are well represented in the National Park as well as being part of the reason why the protected area was declared (rock vegetation, peatland and subalpine pastures, juniper shrubs, pyrenean broom shrubs, pyrenean oak and birch woodland, etc.).
Hence, reliable, objective and quantifiable indexes are needed in order to evaluate the vegetation status related to herbivore’s populations, allowing the comparison, in both space and time, between herbivores’ pressure and its effect on flora and vegetation. In consequence, easily obtained and sensitive to herbivore’s pressure indexes have been found, such as goat alteration indexes (consumption, stamping or stripping the bark and rubbing the horns), abundance and regenerated vegetation’s structure (if there are young individuals that could turn into reproductive individuals in the near future), damages to threatened or rare species, adequate shape and growth, changes in biodiversity levels, etc. Moreover, indicator species have been proposed to conduct more detailed and direct monitoring related to the goats’use of the National Park´s territory and habitats in order to detect more sensitive or damaged areas and guarantee their conservation or restoration.
For the last few years, 180 sample plots have been established to learn about the conservation state of ligneous vegetation and its evolution in time. In 2019, approximately one third of the ligneous species presented unsustainable herbivory´s values in some plots, specially in rocky areas with numerous protected species. La Pedriza, Loma Pandasco, Hueco de San Blas, Cuerda Larga, Peñalara and Cabezas de Hierro summits are an example of areas suffering from intensive pressure where ligneous vegetation –primarily threatened species- and its regeneration are affected. There is no doubt that only through systematic and regular monitoring the effect of Spanish ibex on flora and landscapes can be evaluated, as well as the efficiency of possible actions in order to adapt population levels to more sustainable values, allowing ecological processes, biodiversity and geological landscape conservation.