As above stated, fencing is a commonly used tool in conservation biology 8,18. It is assumed that fencing has a positive effect over biodiversity, given that short-term response in plants is usually positive by avoiding herbivore grazing and trampling, human trampling, or collection 9,10. Even though, this assumption is usually poorly supported by empirical data 19. In fact to evaluate these long-term trends, even at short-term, is usually a neglected issue 18. In our study, for the three species analysed fencing has not always had positive consequences over the evaluated response variables at mid-term. In fact, we obtained contrasting results not only for species, but also for populations within a given species. Hence, medium or long-term effects of fences usually are related with complex both population and community dynamics (Newman et al. 2014), being not so straightforward, and supporting the idea that net results in mid or long term is species- and habitat-dependent 8.
One of the most influential factors in the net outcome of fencing is the change in vegetation cover in absence of herbivory, that may lead important changes in community structure and function (Newman et al. 2014). In our study, the best performance for fenced populations was obtained at intermediate vegetation cover values, while for patches that become more densely vegetated after fencing, target species populations tend to significantly decrease, in line with other research results 18. Herbivory alter vegetation cover and structure 21. Hence, in absence of herbivory interspecific competition tend to increase significantly 21,22, which may cause the negative consequences sometimes observed at mid or long term, especially for undercanopy species 18, conducting to a homogenization of plant community (Newman et al. 2014).
In general, altitude (as an arrange of different associated abiotic and biotic factors, Körner, 2007) is very influential in controlling an excessive increase in vegetation cover. In our cases, fenced high-mountain populations tended to perform better than in middle areas, as in high mountain areas environmental constrains limit cover and consequently the interspecific competence 24. As a rule of thumb, we could establish that fences will exert better results for target species on naturally open habitats.
Habitat or microhabitat disparity in a given species may provoke differences in fencing performance 11. For our target species, indicator variables (i.e. cover, density of focal species, biovolume, distance to the nearest neighbour, and fruit production) showed contrasting results for the populations within the same species and for the different indicator variables within the same population. Moreover, in most of the populations net outcome of the different features was not significantly different for fenced and unfenced species. This lack of a clear positive effect at mid-term over studied species do not justify the high costs associated with fencing installation and maintenance 13,14. Together with potential conflict with stakeholders that may entail fencing 8.
In some cases fencing could be even particularly detrimental for species 12. We found a quite detrimental effect for Solenanthus reverchonii, with a declining tendency in fenced patch. This fact relies on the very low fruit production, no recruitment and an increased interspecific competence (high vegetation cover and virtually no distance with the nearest neighbour). Interestingly, this species is exozoochorous (i.e. Dispersal of seeds by being carried on the surface of an animal) 25, thus the drastic change in habitat structure, sometimes becoming virtually impenetrable, may have restricted or even arrested seed dispersal 11. Afterwards, seeds could not find suitable open sites typically inhabited by the species 17,25. In this case fencing seemed to be positive at short term 17, but over the years the population trend has become clearly negative.
The present study is focused in three species with different biotypes and contrasted habitats. However, the low number of species do not allow us to generalize our results. Also, to have detailed and quantitative information about the grazing pressure, would be very helpful. This lack of information for the area is clearly a limitation of the present work. Another important issue is the complete absence of the data at the starting point. Differences appearing prior to the fence installation may be influential even after 17–35 years. In this sense, to compile detailed information about the habitat and target species at the starting point is crucial to evaluate the mid-term and long-term effects. Despite, these limitations, our study reveals that a previous evaluation (using the best available information), and a continuous monitoring is essential, not only at the short-term but also at mid- and long-term 20. Also, transformation of permanent fences in temporary ones could help to overcome partly the negative long-term effects of the total herbivory exclusion over community structure and threatened populations.
We have to rethink and re-evaluate all the existing fences, seeking for both ecological and economic viability. Furthermore, for establishing new fences an evaluation system must be implemented. In this regard, a plant functional type approach can be adopted 26, compiling all the existing experiences, the scant literature, grey literature, and technical reports. Also, bearing in mind that fencing is only an emergency solution, and comprehensive measures, such as herbivore control (both wild and domestic) are always preferable despite controversial. Ultimately, grazing problem is just one of the consequences of the termed tragedy of the commons27.