Coping with drought
Many organisms don’t mind that ponds dry out. In fact, a substantial diversity aquatic organisms can only thrive in ponds that regularly or at least occasionally dry out. In general, temporary pond species use different strategies to survive in temporary ponds. Many have drought resistant life stages in the form of encysted embryos (resting eggs) or orther cryptobiotic life stages such as larvae or adults encased in protective capsules of mucus. These allow populations to bridge the dry period in situ. Other organisms (e.g. amphibians, many water beetles and water bugs) have mobile adults that visit temporary ponds to lay eggs or deposit larvae but leave when the ponds dry out. These adults may survive in the terrestrial habitat matrix (in the case of amphibians) or move to more permanent aquatic habitats in the region (in the case of some water beetles and bugs). Most organisms with effective dormant life stages are crustaceans, rotifers and worms. They are passive dispersers that rely on vectors such as water, wind and adherence to mobile animals to disperse to new ponds. In turn, the insects and amphibians that thrive in temporary ponds are active dispersers that use different habitat cues to select in which pools they will reproduce. Wiggins and coworkers (1980) recognised that there are also survival strategies that capture a bit of both. Sometimes mobile organisms such as certain chironomids, odonates and beetles may colonize a temporary pond (either at the start or later during the inundation) but at least a portion of the eggs, larvae or adults have the ability to also persist in situ through the dry period. Another interesting mix of a mobile coloniser combined with a resistant life stage is found in Aedes mosquitoes. Here the adult female lays drought resistant eggs in dry pool basins before the water arrives.
To survive in a temporary pond, you have to be mobile, drought-tolerant or both!
The figure below shows the life cycle of a typical temporary pond organisms that uses resting eggs to bridge dry periods. The fairy shrimp Branchipodopsis wolfi from temporary ponds in Southern Africa. Resting eggs hatch from the sediment in the first 24 hours after inundation and this is stimulated by low conductivity and low temperatures. The larvae grow fast and when they are not limited too much by the high density of conspecifics they can reach maturity and start to produce their own resting eggs after just 7 days. Tom Pinceel and coworkers recently shed light on the complexity of this mechanism. Not all eggs hatch during each inundation, ensuring that at least a fraction remains dormant in the sediment. In case the pond would dry out before the fairy shrimp can reproduce, this prevents local extinction of the population
The curse of fish predation
Although permanent ponds can differ from permanent ponds in many aspects, the most crucial difference is that fish tend to be absent from temporary ponds and therefore temporary pond species generally don’t suffer much from fish predation. As a result, temporary pond zooplankton can be much larger than in permanent ponds with fish and insect predators can be quite diverse and abundant. Sometimes fish can end up in temporary ponds when they are transported there e.g. when ponds connect with rivers or permanent ponds. In Africa adventurous fish such as Clarias catfish can move overland to temporary ponds during rainy nights. Finally, some fish are uniquely adapted to the temporary pond environment and can survive the dry periods in slime coccoons (e.g. African lungfish) or as dormant embyros in the sediment (African killifish of the genus Nothobranchius). Temporary ponds house populations of typical boom-and-bust organisms (e.g. fairy shrimp) that profit from the abundance of resources at the start of an inundation, grow very fast and mature before the pond eventually dries out or before predators reach high densities. Despite the typical absence of fish, temporary ponds are certainly not enemy-free. Water beetles, and bugs typically thrive and so do several damselflies and dragonflies. We have even observed large water scorpions arriving in a rock pool on top of an isolated mountain by means of active flight.