Even without external perturbations the climax does not necessarily remain unchanged forever. Observations in very old forest suggest that self destructructive biological changes may be occuring, which, in the individual, we would call aging. Thus, young trees may not be quite replacing the old ones as they die, or regenetation of nitrients may be lagging and the whole metabolism thus slowing down. There is little data at present, but we wonder if communities may not suffer gradual aging after reaching maturity, just as do individual organisms. Storms and disease, of course, could hasten the aging and death of a climax and the start of a new cycle of developmental stage. In fact, a cyclic climax may be a common phenomenon. The californis chaparral vegetation mentioned. This dwarf woodland almost seems to ‘’program itself’’ for periodic destruction by fire. As the community matures, latter and dead wood pile up faster than they can be decomposed during the long, dry summers. Antibiotic chemicals produced by the shrubs also accumulate in the soils and inhibit growth of ground cover. As the communitu becomes more and more combustible, fire sooner or later sweeps through the woodland. Detritus is removed, antibiotics neutralized, and the shrubs and trees killed back down to ground level. A successional development then repeats itself as the woody vegetation resprouts and grows to maturity again. In this way the aging communitu becomes youthful again for a while.

   So far we have emphasized the destabilizing effect of allogenic physical surges. But acte perturabations can also be stabilizing if they come in the form of regular pulses that can be utilized by adapted species as an extra energy subsidy. If fact, a rhythmic, short term perturbation imposed from without can maintain an ecosystem in some intermediate point in the developmental sequence, resulting in, so to speak, a compromise between youth and maturity. What we called ‘’fluctuating water level ecosystems’’. Estuaries, intertidal shores, rice paddies, and florida everglades are held in ahighly productive carly seral stage by daily or seasonal fluctuations in water levels to which the biota are strongly adapted and coupled in terms of life cycles. These pulse stabilized subclimaxes are very important components of the general landcape because the surplus net production that is a property of young systems passes into and helps nourish neighboring systems. This is one reason why ecologists are generally united in recommending that estuaries be preserved and utilized in their more or less natural state.