As Professor Crawley rightly observes in 'Flora of Berkshire' (2005) - which is an impressively detailed account of the flora of a county in S.E.England, "..extinction is an hypothesis rather than a fact. You can not prove a negative, and it is always possible that intensive search might turn up a species that was thought to be extinct". In this massive tome (running to almost 1400 pages), Crawley states that 38 native species have been lost in Berkshire since 1800; the positive note is that no native species has been lost in the twenty years prior to the publication of the book. I do not know if that positive note has been maintained over the following 12 years - the problem with printed books is that up-dates tend to take decades to appear (the previous 'Flora of Berkshire' had been in 1968, with the very first in 1897). As for Buckinghamshire, the first was in 1926, whilst a check-list came in 2005 but no sign of a new flora, so likely to be more than a century after the first before it appears.
Crawley continues, covering the conservation of rare and declining species. He felt that some, might be relatively safe in National Nature Reserves, Wildlife Trust properties or well-managed Sites of Special Scientific Interest, but many are not. The loss of sites at which uncommon species used to grow has, in his words, "been truly appalling". Being a plant ecologist and having studied plants up-close for decades, he exhibits an exceptional level of understanding of the habitats where each species dwells. He has identified the following contributing factors, as having all taken their toll on native plant life: urban expansion, improvement of agricultural land, pond-filling, ploughing, drainage, herbicide application on arable fields and intensively managed pastures, intentional fertiliser application, unintentional eutrophication (air pollution, dog walking etc.), acid rain, water pollution, river bank engineering, boat traffic, land-fill, scrub encroachment after grazing animals have been removed, invasive alien species, road construction, 'tidying up' of urban waste ground, application of herbicides to footpaths, railways and roadside verges.
The Professor considered we should act immediately (and this was 13 years ago) to reverse the decline of the rare plants he listed (221 species. Beyond that it would be a worthy long-term objective to re-introduce all the native species which have been lost - this would take a huge amount of work, dedication and commitment.
Pertinently, he felt that there was a risk that fears about the consequences of climate change would deter conservationists from making the necessary effort i.e. fatalists, giving up, depressed and lacking motivation due to the prospect of plants being 'doomed to find themselves growing in the "wrong places", all too soon! His response was that this type of ecological defeatism needs to be countered with enthusiastic optimism. Conservation must take place now, while there are things still worth saving, an approach I wholeheartedly agree with.
We are so fortunate in the UK, having more field botanists, a few professional, a majority amateurs (many of whom are of professional standard) whose expertise has enabled us to understand our flora better than anywhere on earth - indeed we have some of the best in the world. Nevertheless, despite this exceptional grasp, still there is a fight on to care better for our environment and the plant species that attempt to live there. If one returns to India, to the NW Himalaya, which is my region of expertise and I know its flora better than anyone ever has, I am alarmed that for much of it, a reliable basic up-to-date check-list does not exist, let alone information on altitudinal and geographic range, or ecology. Most of the lists and partial floras produced in India (whether printed or on-line) are riddled with misidentifications, errors, omissions and incorrect information). A majority of the species claimed to be 'Critically Endangered' are no such things, actually being widespread, some even common and abundant, nobody actually knows which species are genuinely rare, which should be considered a national scandal but international bodies blindly accept the submissions by so-called botanists - as for CITES, those species submitted for the Indian Himalaya, like the IUCN, most are fraudulent. The 'Camp Methodology' is fundamentally flawed. The Botanical Survey is not "fit for purpose". Why have both Pakistan (with a fraction of the resources) and China more or less completed on-line digital floras, yet India has not completed one tenth (and to a much poorer standard), still has not even a basic check-list. Too many Indian botanists seldom venture into the field, relying instead on copying entries from Hooker's 'Flora of British India' (completed in volumes, in the 19th century). If British botanists copies entries from floras compiled in the late 19th century or early 20th, they would be laughed at - though they might use such volumes as a basis for assessing the changes which were taking place.