I think, your line of thought is right, if you underline "Natural evolution". If you include our human actions in this nature - we are also part of nature - then you are right, there is nothing to do for us, why should we stem the tide?

If you think, human actions cause or accellerate change in natural evolution, the approach is different. The biodiversity, with the exception of big natural disasters like vulcanism or the impaction of a meteor increases naturally. Many human actions lead to decrease of biodiversity. If you think, humans are a big natural disaster, you have nothing to do.

For the case of naturally rare species, it does not immediately follow that they are destined to be extinct. They have been like this for millions of years, if left alone (by human) they should probably able to persist for many million more years.

For endangered and threatened species, however, their endangerment is almost always related to human activities. Some people destroy them, so, some people have to try to save them from being destroyed.

For the philosophy and moral background of why we save species I suggest reading on history of conservation biology. It should be available in any conservation biology textbook.

There are no absolute optima in the world, but only relative optima, that is optima which are relative to an interest, goal, puropse, value. So, could you please specifiy your interest ..., in order to make your question answerable?

Dear Kirchhoff,

My question is general not specific. Studies are suggesting to restore the population and restoring species simply because they could locate only few individuals of that species in natural habitat. For me this less number of individual may be due to the process of evolution, we need not have to supplement the population in natural habitat if there is no species specific human disturbance is there. On the other hand when we are going for restoration of that particular species naturally what determines the number of seedling to be planted?? it could be 10, 100, 1000s ... all make a different impact on ecosystem... may be this introduction may change the structure, composition of the forest

Dear Sreejith Ashtamoorthy, 

I'm not sure if I get your problem right. Your goal is to restore the population of a specific (tree?) species (in a forest in Kerala?) which is rare (what is a scientific property) and which is regarded as endangered (what isn't a scientific property but a goal)? If you plant individuals of this rare (tree?) species you will, of course, change the ecosystem. But, what's the problem with that? Do you have a second goal, e.g. to maintain a forest that is as free as possible from human influence? If so, you should establish a population of the endangered speices which is not too much above the minimum viable population size. However, according to metapopulation theory and concepts of spreading the (extinction) risk, this might best be done by establishing a set of interlinked local populations. ...

Very interesting.  There were many who believed this was the case with the California condor yet millions of dollars have been spent to save the species.  But your actual question is framed along the lines of "carrying capacity," in which there are a plethora of models and methods to help you determine maximum sustainable yield.  I recommend searching in natural resource management literature for detailed methodology.

Dear Sreejith Ashtamoorthy,

Very interesting question! However, I strongly doubt, that there is a more general answer than "it depends on the species and the specific environment".

Perhaps it is a better idea to concentrate on ecosystem processes, for example: how are the seeds of the trees dispersed and are the species involved in this dispersial present? If your forest is near a river - are there still natural / semi-natural flooding conditions? Etc.

If you have an idea ( I hesitate to say "know"), why a species is rare and/or threatened, you might get an idea, what to do.

kind regards

Irene

Hello Dr. Sreejith Ashtamoorthy

Seem to be your question touching broad subject areas such as selective pressure and natural selection. 

According to your field, as far as I realized you relying on subject areas which is species reintroduction or population dynamic 

There is a little hint from Dr. Andy Quayle (Senior Lecturer, Sparsholt Campus 2013)

 Critical habitats elements - Current population (of certain species) = available space for introduce population (available Critical habitats elements)

 As an simple example;

If 100 adult macaws needs 50 tree holes for nesting (tree holes are Critical habitats element for macaws); In a rainforest contained 150 tree holes and 200 macaws, so habitat available for another 100 adults 

This question is also touching on the area of management philosophies. Of course carrying capacity is something to be investigated, but it is not a given that any particular ecosystem can have all species within it at carrying capacity. The only way for this to be true is if the organismic model of communities (Clements, 1936) is 100% true: that there is an optimal community structure that all things inevitably end up at, or strive to be at.

Instead, you need to consider the natural disturbance properties of an ecosystem (types, rates, severities), and understand these properties over a significant period of time to account for a wide array of forcing agents and responses. From here you will arrive at looking at the Natural Range of Variation , or Historic Range of Variation of a system. This is much more in line with the individual model of community structure (Gleason). You will find that the species of concern has a range associated with its abundance.

here is where management philosophies and values enter the picture. You (or the land manager, or society) must decide what is desired for a particular system. All outcomes are not possible at once, you must choose a place/point within the NRV/HRV, and from there you can decide if Species X or Y needs to have more or less presence in the system.

You must know the carrying capacity.

The species have no number limit to live in area if there were no destruction of it but if only one individuale will participate in this destuction the carrying capacity will be off.

A glass which contains max 30 ml and we wiil put over 30 ml, the exed water will be  lost or like a bus with max passenger of 60 body and it will contains 70 body : it will be destroyed !!!

The principle of carrying capacity is ok but the question is how we practically assess the carrying capacity of individual species in field??