Pets and feral animals have been shown to cause great amounts of destruction to native wildlife, but since very little research has been done in that field, things remain unclear. Most of the cases where the extreme case of extinction occurred were on islands, but this does not indicate that the mainland is safe from this. A lack of visible evidence, either scientific or not, does not indicate that such animals are not killing wildlife regularly, or that the consequences of having them around are negligible. There are several signs that some areas may be more vulnerable to the effects of invasive animals than others.

Introduction

For those of you that haven’t read my earlier article on the issues with keeping free ranging pets and having stray carnivores, I would recommend going through it first although it is not an absolute necessity to understand the concepts explained here. Also, some evolutionary concepts are dealt with so for a better understand consider going through my articles on evolution as well. Note that this is a clarification to those who are not convinced of the issues brought to light in the older article. I will also explain the issues associated with the introduction of non-carnivorous pets in a previously undisturbed environment. The driving factor behind my writing this article was conversations with people not familiar with the scientific way of thought that made me realise that unless the complete issues are spelled out, the ideas will not be understood. Additionally, another snake was nearly killed recently by an adopted feral cat (refer to my older article for the details of the snake killed). It must also be noted here that pets not only include cats and dogs but also livestock such a cows, goats, sheep and buffaloes.

The consequences of the introduction of animals into an area previously unexposed to it can be very subtle for the lay mind to comprehend since the interactions between humans and the environment are virtually ignored. Inter-connectedness is the quintessence of ecology without which none of its theories can be justified. Similar are the consequences of introducing alien plant species (alien here refers to those species from often from other continents, but not necessarily always so) or with disease. Lantana camara was introduced in India a couple of hundred years earlier and today it has replaced the understory in most of the peninsular dry deciduous forests and occupied a great portion of the arid scrub jungles. It, however, cannot be easily removed (as is with the case of free ranging pets and strays) as now several other animals and plants have developed certain crucial interactions with Lantana. Removal of Lantana has been largely unsuccessful for various reasons, which is atleast in this case advantageous but it may not be the case always.

What I wish to make clear through the course of this essay is that human intervention in the functioning of the Earth’s environment can no longer be ignored since human presence has grown disproportionately in the last few thousand years. This means that the repercussions of even growing food for even a tenth of the current population are so great that it could drive several species of native wildlife extinct.

Isolation in its various forms

What in essence happens when a ecosystem, previously unexposed to a certain kind of animal or plant, is suddenly introduced with an alien species? Tribal communities living in isolation were exposed to disease such as common cold and other such diseases by visiting people from modern society which spread rapidly (examples of which are seen in the Jarawas and explained in Jared Diamond’s Guns, Germs and Steel). But note that this is a closed environment. Such tribals typically live outside cities with limited interaction to the outside world. The consequences of the introduction of an alien species is typically the same in several cases. Unprepared and susceptible, at the mercy of the new species, most are affected, usually adversely. This must is to be kept in mind albeit it should also be adverted to the fact that this doesn’t occur in all cases.

In talking about carnivorous pets in my last article, I got too carried away by them and failed to touch on the consequences of herbivorous “pets” on undisturbed ecosystems, as was pointed out in one of the comments. What should one expect with free ranging herbivores such as cows and other livestock in such a scenario? Numbers of individuals introduced here are insignificant since the “lethal dose” of number of these animals will in any case be attained over a short period of time. Herds of goats and cows feed over large tracts of grass covered areas which is usually unable to regenerate due to their continuous onslaught. The effects are naturally amplified (given the additional absence of carnivores) in the case of smaller herbivores such as rabbits and hares which breed rapidly and clear large stretches of grassland. Uncontrolled grazing results in the denudation of natural forest in its various forms and hence native herbivores are the first hit. Since the native fauna and flora maintained an equilibrium between plant consumption and animal population, this is now disrupted.

So when do the effects of introduced species get felt? I have already suggested that extinctions due to introduced species has a high prevalence in the case of islands. What about islands is so peculiar? I will return to these questions a little later in the essay. A stable ecosystem is one where the population of each species is regulated or maintained in spite of predation, herbivory and other such interactions. That is to say that if a certain number of (for example) a bird species are killed each year, that species continues to survive because over time new individuals replace those lost. The predators continually switch over to alternate food sources and hence the pressure on the prey reduces (from the constant high pressure offered by specialist predators, which are typically rare in the wild). This is usually seen if the ecosystem is spread over a large area with a sufficiently large population of prey that is distributed through the area, allowing movement of populations across the area. If the predator species kill too many individuals of that bird species, the predators will have nothing to feed on and hence will die out with the bird population, but this extreme is quite rare. This is usually avoided and such an equilibrium is in all cases maintained by varying numbers of predator and prey. The same is the case with plant species and herbivores. Such plasticity exists in all ecosystems.

Islands are continually changing in their species composition. Rather than having an equilibrium between populations of species, there is a balance between extinctions (or to be more precise extirpations) and immigrations. A limited availability of resources allows only a small number of individuals to reside on it (excesses are not trimmed off and eventually drive the species extinct). The number of species also is considerably lower than on the corresponding mainland. Combined, the two mentioned factors allow a very low number of species, represented by a very small population inhabiting the island. Hence, local extinctions are frequent and seemingly causeless, although minor changes of any aspect are the cause. The same is the case with fragmented forests and “islands” on the mainland. Once extinction occurs, there is a greater likelihood of “immigrating” species to establish themselves. This fragile state of island biodiversity prevents the continuity of life once an invasive species is introduced since minor changes can disrupt the species composition. In addition, islands are more likely to have species that although might have evolved from those of the mainland, are considerably different and hence endemic. The extirpation (or extinction) of a species implies that the species that comes in its place could be altogether different, hence endemics and in most cases even other species lost on an island are lost forever. Island fauna is also tame since exposures to predators are typically lacking due to the absence of such a selection pressure.

Isolation also can be created by the construction of man-made barriers, especially in the case of plants and herpetofauna and other fauna with limited mobility. Roads, canals and other such structures limits further their mobility. Other more subtle selective pressures applied could also play a role in this.

Some form of isolation that is seen in cases where species have been introduced, plays an important role. Islands are isolated since they are less easily accessible to reptiles, amphibians and most plants from almost anywhere else. Plants, as already suggested cannot bear the brunt of the advance of herbivores unlike animals which tend to, over time, develop behavioural adaptations. There are however more complex forms of isolation, even on the mainland, explained below.

Consequences of invasion

Island fauna, as mentioned above, is tame and hence introduced predators (such as cats and dogs) can approach them without having to expend energy in stalking and other forms of subtlety. In the case of flora, islands are as susceptible to denudation as the mainland as plants (unlike animals) are not as dynamic and cannot survive the onslaught of a rapidly growing population of herbivores. Forest fragmentation ensues in part due to overgrazing by livestock and the removal of forest cover to meet food requirements with agriculture.

As a result of forest fragmentation, an original population of a species specific to that forest type, becomes isolated in these pockets. In most cases, likely including that of birds, the individuals from each region cannot or do not migrate to others due to territorial boundaries. The consequence is small unstable populations of species that are extremely susceptible to extirpation for a variety of reasons. Inbreeding causes loss of genetic diversity and consequently deleterious mutations accumulate causing lower likelihood of survival; loss of genetic variation also increases the risk of extirpation by zoonotic disease, which has been shown to have a greater prevalence in the fauna inhabiting fragmented forests even otherwise, such as in the case of Lion-tailed Macaques (Macaca silenus) and Civets in the Western Ghats (and additionally other mammal species); and predation itself can very likely wipe out the small population, the risks of which are greatly increased by introduced (typically animal) species which are either better adapted to hunting, or through competition for limited food resources.

Cows and other livestock are generally kept in large groups such that the yield of milk will be sufficient to not only satisfy individual and family needs but for its use as a commercialized source of livelihood. The cattle has to be provided not just dry feed but need fresh fodder for which a good source is grass in scrub jungles and meadows, and several shrubs in the same and other types of forest. These are readily used. As sensible as this may sound, large numbers of cattle are detrimental to the forest ecosystem. Selective feeding on plants, that is subsidised from predation by human intervention can result in the formation of an imbalanced ecosystem: native herbivores lose out on vital food resources and hence drop in numbers as a result of which predators are affected. This leads to a more permanent loss of forest cover in part since less faeces in the soil implies less fertility and hence native plant species don’t survive and disperse. The paucity of plants adversely affects bees, other pollinators on which birds and some animals feed, and affects native herbivore populations which again impacts the predators. This cycle repeats itself several times to leave either small fragments of forest or none at all and as a result not one but several species of biota is impacted.

Disease of animals, known as zoonotic disease, are on the rise as a result of close interaction of wildlife and humans. Diseases of urban and man-made origin are now prevalent in several feral and domestic animals. With the high incidence of these animals entering native forests, these diseases very likely will be transmitted to wildlife and being unequipped to deal with such, a large number of individuals will be lost. Additionally, forest fragmentation by human activities reduces wildlife to even smaller groups of individuals that face a greater likelihood of contracting zoonotic diseases from nearby human settlements. Smaller populations that have limited interaction with each other are more susceptible to disease. Diseases of livestock such as rinderpest are known to affect not only wild bovine species but reportedly affect cervines (i.e. Deer) and other related wildlife.

Species, selectively impacted by disease or predation, can cause additionally complications. Disease of carnivores resulting in their extermination causes a boom in herbivore populations which as a result of overgrazing leads to loss of habitat. A cascade of extinctions are likely to follow: this loss of food sources for herbivores can be detrimental to other herbivores which would have to resort to migration to survive and would otherwise perish. Any predatorial species dependent on these animals would also suffer. A similar result can occur by the loss of herbivores to disease or predation: with less fertile faeces, lowered fertility reduces the dispersal of trees or selectively propagates certain species which may or may not be beneficial to the ecosystem; the predators due to a lack of prey die out.

Solutions

Possible ways feral animals can be controlled are few: culling and sterilisation are possibly the most effective and easily implemented, although these are quite inhumane. Rescue agencies in cities catch and tend to such animals and usually give the animals out as pets later. This could be a more sensible and humane method although excesses have to be dealt with rapidly. Sterilization might be in the longer run more effective. Keeping domestic and adopted cats indoors at all times is the possibly the only way their killing of wildlife can be prevented. Dealing with cattle in India is further complicated by the fact that cows are considered sacred. I shall not advocate any measure to deal with cattle for this reason (although probably I will write about this issue later).

It now is apparent that these issues have to be better studied and areas that face greater risks have to be identified. Measures taken will have to be specific to those areas and have to be implemented rapidly without fail.