Permanent pools and ponds differ from lakes in that all of the water in a pool is in the photic zone, i.e., the water is shallow enough for sunlight to reach the pool bottom: lakes are deep and have areas too deep for sunlight to penetrate. Many smaller pools and ponds in Wales have been lost since the mid 20th Century, primarily through drainage and habitat succession, the latter as a consequence of under management or neglect. The remaining pools and ponds are critical for maintaining the aquatic fauna and flora and are important breeding grounds for small fish, amphibian and invertebrate populations, pools on lowland heaths and bogs are particularly important for dragonflies and damselflies.