A permanent quadrat is a sampling area that is marked out and left in place, often with pegs or posts at its corners, so that exactly the same area can be sampled again and again. Its position is also recorded with a map, GPS or measurements from fixed landmarks.Permanent quadrats are used to study how a community changes over time, for example:
- succession on bare ground, a sand dune or an abandoned field
- recovery after a fire, flood or pollution incident
- the effects of management such as grazing, mowing or coppicing
- the effects of climate change over many yearsBecause the same area is measured each time, differences between surveys are due to real change rather than to sampling a different patch. This removes a major source of variation.Surveys are repeated at fixed intervals, such as monthly or yearly, and at the same time of year, so that seasonal changes do not hide long-term trends. The same measures of abundance and the same abiotic measurements should be used each time. Photographs from a fixed point are a quick extra record.Worked example: sheep are removed from a meadow containing a permanent quadrat. Grass cover is 80% at the start and 35% after three years, a fall of 80 − 35 = 45 percentage points. Over the same period bramble cover rises from 0% to 40%. This shows that grazing had been preventing woody plants from establishing.Limitations: markers can be moved or lost, and repeated trampling by recorders can damage the area being studied.
- succession on bare ground, a sand dune or an abandoned field
- recovery after a fire, flood or pollution incident
- the effects of management such as grazing, mowing or coppicing
- the effects of climate change over many yearsBecause the same area is measured each time, differences between surveys are due to real change rather than to sampling a different patch. This removes a major source of variation.Surveys are repeated at fixed intervals, such as monthly or yearly, and at the same time of year, so that seasonal changes do not hide long-term trends. The same measures of abundance and the same abiotic measurements should be used each time. Photographs from a fixed point are a quick extra record.Worked example: sheep are removed from a meadow containing a permanent quadrat. Grass cover is 80% at the start and 35% after three years, a fall of 80 − 35 = 45 percentage points. Over the same period bramble cover rises from 0% to 40%. This shows that grazing had been preventing woody plants from establishing.Limitations: markers can be moved or lost, and repeated trampling by recorders can damage the area being studied.