Pretty damning paper on the current model of conservation in the UK, published in Global Ecology and Conservation
From the abstract:
We find that although 28% of UK land is reported by the UK government to be protected, only 11.4% of land area falls within protected areas designated primarily for nature conservation. Condition monitoring indicates that at most 43–51% of protected areas in the UK are currently in favourable condition, which suggests as little as 4.9% of UK land area may be effectively protected for nature. However, estimates of protected area coverage vary greatly depending on the types of protected areas considered ‘effectively protected’ as measured by management category and site condition. Taking the UK as an example of a country that has reportedly met the target, we suggest that global progress may have been overestimated, and that future targets and indicators need to focus on the quality as well as quantity of protected areas.
For example, the UK's National Parks don't meet the IUCN definition of a national park, because they're aiming to preserve historical landscapes rather than conserve biodiversity. This means that things like farming are subsidised, along with permitting intensification (such that the landscape isn't actually preserved anyway, but hey that's shifting baseline syndrome for you), with pretty disastrous consequences for nature.
Plus, of course, there's basically no funding or incentive to maintain the few areas where conservation is supposedly important, like SSSIs (which are mostly too small anyway). And there's the current scandal highlighting how many supposedly protected rivers and coastal areas are used by water companies as a dumping ground for sewage. Etcetera etcetera.
The UK is one of the richest countries in the world with a huge amount of expertise in doing this stuff. Which suggests that in most other countries the situation may be even worse, at least in places that don't give a crap.