There are several key causes of habitat loss:
Food and timber production
The intensification of agriculture has created a landscape where wildlife is largely excluded. The drive to produce more food has had a massive impact on the countryside around us. Fields have been made larger and easier to cultivate and harvest by removing hedgerows and ditches. River margins have been drained to create more productive land. Fertilisers have changed the natural soil chemistry allowing commercial crops to grow vigorously but have created soils no longer suitable for many native plants. New machinery has allowed areas once thought unproductive to be farmed. Pastures have been enriched to allow more livestock to graze, creating fields of short dense grass with only a very limited variety of species or spaces to attract insects and birds.
Along with food production, more land is used for the production of timber and other materials. While these are renewable natural resources, they are grown intensively in the same way as food crops. This creates a habitat with just a single species as other plants are excluded. In the National Park, dense stands of conifer have been planted close together, creating woodlands that have none of the light, deadwood, different aged trees or different species that make our native deciduous woodlands so diverse.
Development
Along with agriculture the building of roads and industrial, retail and housing development has covered areas once rich in wildlife with concrete, brick and tarmac. In many cases some of our most important wildlife areas were targeted for development such as flat, accessible river valleys or poor agricultural land. As cities, towns and villages have expanded, the gaps between areas of natural habitat have increased, isolating species in small pockets. This fragmentation effect prevents species moving from one area to another and so populations have become more at risk.
Pollution and alteration
Habitats can be lost even if not developed or farmed. Rivers are diverse habitats that contain a number of different conditions such as deep and shallow or fast and slow stretches of water. While the river may still exist, many rivers in the UK have been straightened, deepened or walled with flood defences which have destroyed the natural variety of conditions. Pollution from industry, agriculture and urban areas is often washed into rivers which affects the chemistry of the water. This can not only poison wildlife directly but can also make the river unsuitable for some species. Pollutants can reduce the oxygen content of the water and encourage the over abundance of algae, or toxins can build up in the sediment which will take years to wash out.