**1. Introduction**

It could be thought, initially, that this chapter will be one more that deals with the impact that certain invasive species cause on the biota of a certain area, region, or country. Not exactly, though too. It is necessary to recognize that this matter - much approached and treated at different levels - still leaves ample room for suggestions and debates. In the text that follows, several examples of intervention in favor of a specific bird species are shown, to increase its populations and "remove" it from a classification "critically endangered" or remarkably close to it.

The measures implemented by different public administrations and/or conservation entities, to try to recover a specific species and avoid its classification as endangered, near endangered, etc., have provoked, and still do, a series of reactions both target species in question, such as related species or prey species; even on those that we would never have believed could be affected by the recovery plans of the former.

Admittedly, some of these reactions were not easy to predict, others were not. In some cases, political or personal "honor" criteria have prevailed, in an apparent maelstrom or race, to have more individuals than another "competing" region or nation.

Also, shortcuts have sometimes been sought. Some original, others of questionable ethics, with reasonable doubts to be protected by scientific criteria.

At times, there has been a sin of precipitation and rapid search for striking results, rather than the application of biological knowledge, and of considering a probable long-term projection.

"A wild population is defined as a set of individuals of a species that inhabits a certain area". "Density is the number of specimens per spatial unit (surface or volume). It is often more useful than the absolute size of the population since density determines and conditions fundamental aspects such as competition for resources" [1].

"Traditionally, indicator species have been considered those that, by being present in a certain system, indicate that said ecosystem is healthy, from the physical, chemical, or biological point of view (or, by the on the contrary, that it is deteriorated, as occurs with the species of aquatic invertebrates that indicate contamination). They are usually species that are easy to detect and "monitor" so that the demographic changes of their populations can be detected in time and interpreted in terms of other variables of conservation interest that are more difficult to measure" [2].

Dedicating all efforts to the recovery of a species, you can avoid the symptoms that warn us of its risk situation, but not end with its origin or, what is worse, camouflage the situation of other less striking species, but in a similar situation. "No species serves to indicate anything about its ecosystem of origin when it is kept in captivity, or conditions of probation, away from enemies and provided with food" [2].

Recovery involves much more than just increasing population numbers. Density can and should have a limit.

These premises constitute the basis on which this chapter will work: The growth in the number of specimens/surface unit of a specific species, which has been "helped" in different ways, is affecting other populations in such a way that it has displaced or eradicated them from specific areas, endangering their existence.

Not all cases of increase in population density in birds are due to human intervention, through the execution of direct intervention measures, to intentionally favor a certain endangered species. However, behind most animal overpopulations - not just birds - are human actions that, consciously or unconsciously, have caused them.

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*The Limit to the Density of Species (A Reflection on Human Intervention in Conservation...*

The examples discussed below refer to situations that occurred in Spain. We know of multiple similar actions in other countries. The reader will be able to relate and apply each case to its environment but will agree with us that a limit to the

Traditionally, raptors have been considered "harmful" in a simplistic dichotomous classification, in contrast to "beneficial birds." The diurnal raptors have been especially "persecuted", for predating on rabbits (*Oryctolagus cuniculus),* red-legged partridges (*Alectoris rufa*), and other species, which man uses for his consumption. Birds of prey were considered protected species, in Spain, from the year 1973. However, their habitats were not protected until several years later. It could be the paradox that an imperial eagle (*Aquila adalberti*) would fly over the entire Spanish territory without risk of being shot, but, incoherently, it would starve because its habitat had been destroyed, it did not have preys, or it could not rest safely or

The custom of poisoning carrion and eggs did not cease with the law. Raptors continued to perish due to the "silent" effect of strychnine, a situation that also affected corvids. The secondary poisoning of scavenger birds occurred by ingesting the corpses of mammals considered vermin, such as foxes, which had been poisoned, or even wild boars, which had eaten the deadly bait intended for foxes. There was another important decrease in the populations of griffon vultures due to the use of the tractor in agriculture, because it marked the end of the use of animals to plow and harvest agricultural fields: animals that were a basic contribu-

Traditionally, the corpses of domestic cattle were abandoned in the field, in the same place where they died (in the case of extensive livestock) or transferred to a

In the case of the herds in the extensive regime, the location of the corpses depended completely on chance, so the ethological patterns of the search flights of

The European Union, with the emergence of the bovine disease, called spongiform encephalopathy, which is transmissible to humans, issued regulations that prohibited these practices, making it mandatory to hire incineration services for carcasses. Suddenly, the most important source of food for scavengers was eliminated, which from then on only had carcasses from hunting. In some localities, points of deposit of carcasses of cattle were authorized, in which the animals were

During that time, vultures attacked disadvantaged animals, such as during parturition, tearing of genitalia and killing the calf or lamb [5]. Situations that, until then, had been exceptional, but continues occurring on more frequent occasions today (**Figure 1**). As a contrast [3–5], as follows: "A sheep of my property died in childbirth, I dragged it to a clearing where the vultures could see it better, they fell on it and almost ate it completely but they did not touch the corpse of the dead lamb as well and cause the sheep to die. It is the case of a dead animal" [4] and "A cow disappeared from a farm in Portezuelo, a town very close to Acehúche. It was found almost eaten by vultures and suddenly we saw something moves on the corpse; it was a calf that had not been touched and left ahead. In this case they were able to

reproduce, because there are no trees on which to build the nest.

point away from the facilities if they were housed animals.

eviscerated (being buried or incinerated these carcasses).

attack the mother even before she was dead, but we are not sure" [4].

*DOI: http://dx.doi.org/10.5772/intechopen.97436*

density of the species is necessary.

**2.1 The griffon vulture (***Gyps fulvus***)**

**2. The case of …**

tion to their subsistence.

the vultures did not suffer alterations.

*The Limit to the Density of Species (A Reflection on Human Intervention in Conservation... DOI: http://dx.doi.org/10.5772/intechopen.97436*

The examples discussed below refer to situations that occurred in Spain. We know of multiple similar actions in other countries. The reader will be able to relate and apply each case to its environment but will agree with us that a limit to the density of the species is necessary.
