A couple of key ecological processes at play here, and they touch on resiliance of species.
Species survival in the long term has two major requirements - sufficient viable habitat (refugia) and viable corridors between said refugia. Reasoning is to do with maintaining genetically viable populations of a species. Usually a single site is insufficient to maintain genetic viability, and is why conservation bodies now look to landscape scale conservation (i.e. not just using island arks, but “rewildling” / restoring the greater landscape to be wildlife friendly and allowing ecological processes to take place relatively unhindered by humans). There are many published papers on this issue, but probably the two key references are Macarthur & Wilson 1967 - The Theory of Island Biogeography (think of refugia as islands in a landscape of human impacts) and a derived application of species usage of these islands through the development of Metapopulation Theory / Ecology (most succinctly presented by Hanksi, 1999, in his book on the subject, using fritillary butterflies’ distribution and occurrance in the Danish islands).
Nature reserves, either large or small, together with the David Attenborough style tv documentaries, give the impression to many not involved in wildlife conservation that all is well with the rest of life on Earth… whereas we are quite literally on the brink of our own extinction ('cos we can’t survive without the rest of life on Earth). Rachel Carson’s Silent Spring (1963) is just one of many “apocalyptic tales” that attempt to educate the many deniers / greedy types (see/read “An Inconvenient Truth” and “Earth In The Balance”, by Al Gore for some of the issues in the bigger picture). One final pointer about nature reserves - they are part of the conservation toolkit, but not the whole story. I would strongly recommend anyone involved in nature conservation read John Mackinnon’s 1992 paper “Avenues of Futility in Conservation” (pdf) which highlights the many dead ends, rabbit holes, failures and red herrings that conservation efforts get sidelined by, purportedly in an effort to educate and facilitate conservation - whereas conservation needs to lobby, to actively oppose the greed, and become “even more cunning”… scary reading, but essential to get conservation focussed on the straight and narrow… It is more relevant now than when it was written at the time of the Rio Convention (The UN’s “Convention on Biological Diversity”, which remains a big exercise in paper pushing by administrators).
I absolutely agree.
I would like to add that some peculiar kind or reserves such as microreserves can be very effective in protecting extremely rare taxa distributed in very small areas (e.g. very narrow endemics; very isolated peripheral populations).
On the other hand it can happen that many rare taxa fall outside the existing protected areas. It these cases it is hard to foster conservation actions because of the many economic interests on natural areas.
Here we have some subjects who claim that protected areas “do no work”. It is true that some protected areas could work better but in many cases it can happen when their management is purposedly wrong or their funding is maliciously lowered year by year.
I don’t think the kind of people who will use the presence of a reserve to justify reckless destruction elsewhere would be dissuaded from destroying the environment if there wasn’t a reserve. If they felt the need to put any kind of environmental spin on their actions, they would probably say there can’t be anything important in this area because no one has bothered protecting any of it.
When it comes to destruction as a result of property development, there seems to be huge ignorance, probably deliberate ignorance, over the idea of habitat. Developers will talk of mitigation in terms of hectares as though any hectare is as valuable as another for biodiversity. So they will justify destroying an ancient wood or wetland by saying an equal area will be set aside for wildlife elsewhere. Or worse, they will talk in terms of trees: Our development will only require the felling of 70 trees, and we are going to plant 300.
To add that protected areas cover >15% of the Earth’s land surface and >7% of the seas. Whilst effectiveness varies hugely (is it in the best place for wildlife conservation? was it under threat in the first place? is it well regulated, invested in, managed?) this is a very substantial chunk of our planet. https://www.protectedplanet.net/en
There’s a fair bit of work on the effectiveness of protected areas in different parts of the world on tackling land use change, on leakage to other areas and so on. One recent study here seems a bit gloomy - https://www.pnas.org/content/116/46/23209
At my old school there was a plan to cut down two 250-300 year old sycamores for a temporary construction road, but ‘don’t worry we’ll replant them when we’re done’. Since then a local arboretum has occasionally gone around putting ‘price tags’ on all the trees to raise awareness for the idea that ‘not all trees are the same’