If you’ve ever been lucky enough to come across a pond that wasn’t there before, you’ll know why I say seasonal ponds are magical. All over the world, in every biogeographic region, there are ponds that come and go. Here, on the US’s eastern seaboard, our seasonal ponds fill up in fall and winter as rain and snow collects in small depressions. The ponds literally squirm with life in spring. But as spring turns to summer, evaporation and percolation whisk the water away and the ponds disappear. The ponds are variously known as vernal ponds, temporary pools, ephemeral waters, karst sinkholes, and seasonal wetlands, to list just a few of their names.
Because of their unusual water cycle, vernal ponds are unique and essential habitat for many endangered, rare, and endemic species. We are also discovering that the presence of a vernal pond has dramatic consequences for the ecology of the surrounding landscape. From our mid-Atlantic ponds, upwards of 1000s upon 1000s of salamanders, frogs, and toads emerge from a given pond and scatter across the nearby landscape. These animals eat, and in turn are eaten, making up an essential part of the local web of life. Indeed, in the region surrounding a vernal pond, the total mass of amphibians that were spawned in the pond outweighs all the birds, mammals, and other vertebrates in the area. This shifting of biological mass from pond to land, via a great migration of amphibians, is a pretty unusual biological phenomenon. But it just one part of seasonal ponds' magic.
The extreme fluctuations of wet and dry have led to some very bizarre adaptations in ponds’ species. To understand why we have to understand a bit about their water cycle. Seasonal pools are typical small, only a few to 50 meters wide, and they are shallow. If they were much deeper they wouldn’t loose all their water annually and hence they wouldn’t be seasonal. The water level’s seasonality drives one of the essential features of seasonal ponds - they are (mostly) devoid of fish. We’ll get to the bizarre killifish exceptions in a moment, but this lack of fish is what makes seasonal ponds so attractive to salamanders, toads, and frogs. There is far less predation of amphibian eggs in seasonal ponds than in year-round ponds. In addition to the annual cycles of flooding and drying out, seasonal ponds are hydrologically isolated from other waters. In other words, they don’t have streams flowing into or out of them. The hydrological isolation means that the critters that live in them evolve in relative isolation - leading to high levels of endemism.
Plants that live in vernal ponds are unusual. Most land plants can’t survive having their roots inundated with water, while most water plants can’t stay drying out. Only a small proportion of plants can survive both extremes. One of the problems with standing water for plants is that the roots can’t exchange gases with the atmosphere. They can’t get carbon dioxide to photosynthesize and they can’t release oxygen. Thus some vernal pond plants have developed roots which extend above the water, such as the knees of the Bald Cypress, to allow the free exchange of gases. Cattails evolved to have more pore space in their stems to allow for more gas exchange. To cope with the drier parts of the year, some ephemeral pond plants use the relatively rare photosynthetic CAM mechanism. CAM plants open their stomata, to collect carbon dioxide for photosynthesis, at night, rather than during the day. They thus loose far less water through their open stomata than daytime-carbon-collecting-plants because night time is cooler. But the great majority of seasonal pond plants use the a more common strategy of germinating, maturing, and producing seed in one season. Then if they die when the water level changes, the species will continue via their seeds. Some plants even manage to have several life cycles in one favorable season. These are, of course, annual and ephemeral plants whose seeds can survive dessication then germinate when the waters return.
The communities of vernal pond plants can be very stunning. Sloping out of the pond there are different moisture regimes which form distinctive vegetation zones. Thus when in flower, you get the appearance of concentric rings of color as seen here and (scroll down the page) here. Finally, I can’t talk about vernal pond plants without mentioning that high bush blueberries often grow near their edges.
But it is the animal communities of seasonal ponds that excite most people. Let’s start with one of the strangest: a family of fish that live in seasonal ponds. These are the so called killifish of which there are 1,270 species and they live in scattered ponds all over the world. The fish have eggs which can survive a period of complete dryness using a process called diapause - a pause in development. When the waters do return, the eggs hatch and the fish mature rapidly to lay their eggs before the waters dry out again, killing the adults. Some killifish mature in as little as 3 weeks. There are even some adult killifish which can survive a few weeks out of water. And as a group, killifish have shown remarkable resistance to various kinds of pollution, surviving industrial chemical pollutants up to 8,000 times higher than the previously believed lethal dose. Tough critters.
Fairy shrimp are far more common inhabitants of seasonal ponds. They are a crustacean that can be up to 2 inches long, even rarely reaching 5 inches, and they swim “upside down” with their 11 pairs of legs upright. They paddle their legs to filter out food and to propel themselves. Because of this unusual method of getting around, fairy shrimp are quite slow and don’t thrive where there are lots of predators. But they thrive in seasonal ponds. Indeed their presence is used as an indicator, in the mid-Atlantic, that a water hole is indeed a vernal pond. These crustaceans survive the dry season by laying eggs that are capable of cryptobiosis. That is, they can survive drying out by reducing or stopping all metabolic processes. When water returns, the eggs hatch in about 30 hours and a fairy shrimp is born. They typically take about 50 days to mature. The females will lay 1-6 broods of both quickly hatching eggs and those with thicker shells that can survive many years of dryness in the pools sediments.
Many species of fairy shrimp exist - for the populations tend to be quite isolated from one another. Fairy shrimp lineages only mix with other population when carried by wind, on the feet of animals, or even sometimes by widespread flooding. But most fairy shrimps live their entire lives in one pond. There is one species of fairy shrimp that is believed to have been isolated from all other lineages for 23 million years. That’s pretty long in evolutionary terms, especially given their short life spans and hence many generations in 23 million years. For comparison, the last common ancestor between chimpanzees and humans was only about 6 or 7 millions years ago.
Another magical creature of seasonal ponds are seed shrimp, Heterocypris incongruens. This seed shrimp is tiny, typically less than a millimeter across. And like the fairy shrimp it is a crustacean, but the seed shrimp resembles a clam. The magical power of Heterocypris incongruens is that females of this species can lay viable eggs without fertilization by males, so called parthenogenesis. Research has found that parthenogenetic populations are better suited to survival in marginal locations than sexually reproduced populations. This is probably because at the extremes of the animal’s range it is difficult to find mates.
But probably the most beloved of seasonal pond creatures are amphibians. There are a number of fascinating ones. The eastern spadefoot is a toad, so named because they have a spur on their back feet which is typically 3 times as large as their toes. This spur is good for digging - and is used to burrow, butt first, into the ground. The spadefoot spends most of its life under ground in a kind of hibernation, emerging only to breed, in a seasonal pond, and occasionally to eat. Eastern spadefeet have one of the quickest maturation rates of any amphibian and can reach adulthood in as little as 2 weeks. But their path to adulthood is bizarre. They are polymorphic. This mean they have more than one form. Most spadefeet become ‘typical’ tadpoles with round bodies and a skinny tail that feed on vegetation and smaller critters. But some spadefeet become wholly carnivorous and develop a larger body, with a much bigger head, a slimmer body, and serrated mouth parts - all the better to eat you with my dear. The carnivores tend to grow faster than their siblings and will eat other spadefoot tadpoles - though they will preferentially feed on non-siblings. It is easy to understand how evolution favors cannibalism in such a resource limited ecosystem, but how do they know who their siblings are?s
Spotted salamanders show another remarkable adaptation to seasonal pool living. Small ponds like these are typically low in oxygen. Spotted salamanders have developed a symbiotic, or perhaps not so equal, relationship with green algae. We’ve know for over 125 that the greenish hew of spotted salamander eggs is caused by algae living within the egg capsule. It is believed the algae provides oxygen to the developing eggs of the salamander, and the eggs provide nutrients to the algae in the form of urine. But in 2010, scientists discovered that the algae was actually INSIDE of the embryo’s cells, not just adjacent to them. This is the first documented case of algae living inside the cells of a vertebrate. It is unexpected because animals typically aim to prevent foreign bodies entering their cells. So some unusual mechanism has enabled the salamander to turn off part of its immune system and allow the algae to grow inside the amphibian’s cells. The fact that this has been observed in a creature that can regenerate lost limbs has led some to conjecture that this may be linked to how regeneration works.
That’s a pretty long list of unusual adaptations: intracellular algae, parthenogenesis, diapause, cryptobiosis, polymorphy, root and stem adaptations, and CAM photosynthetic pathways, not to mention bizarre fairy ring like vegetation patterns.
And it makes me desperate to see and help protect these ecosystems. Probably the easiest way to find them is to follow the calls of amphibian parties in the early spring. On the US’ east coast, we can follow the call of the male wood frog whose loud call is reminiscent of a quacking duck, or that of the spring peeper whose calls sound like sleigh bells. Seeking vernal ponds in summer and autumn is a bit trickier but blackend, compressed leaf litter, grey soil, and watermarks on nearby tree trunks are all clues that you may have stumbled across a dried out seasonal pond. But if you decide to go on a seasonal pond hunt, consider cleaning your boots and disinfecting them with bleach before and after the visit. This is important to make sure we are not contributing to the transmission of amphibian diseases which are decimating amphibian populations throughout the Americas.
To help the critters that live in these ephemeral ponds, we can join one of the Big Night volunteer events, where people help migrating amphibians safely cross streets. While it may sound like it won’t help much - studies have shown that between 50 and 100% of amphibians attempting to cross roads are killed. In addition, the documentation that such volunteers collect have helped to convince local governments to install migration tunnels for amphibians. The Harris Center in New Hampshire provides a series of 5 videos for anyone interested in helping amphibians cross the road.
Seasonal ponds are sadly under great threat by continuing development, pollution, and climate change. They are a particular vulnerable ecosystem, because they are small and they are not always visible. Land use change is probably their greatest threat, including filling in and draining ponds as well as destruction and fragmentation of the surrounding habitat. Because of their small water volume, seasonal ponds are also very sensitive to pesticides, including mosquitos control practices, fertilizers, and atmospheric pollution. To make matters worse, the hydrological balance in seasonal ponds is delicate and we don’t know what impacts climate change will have on them. While the EU has seen fit to protect vernal ponds, efforts to protect them in the US are piecemeal with no federal regulation to protect them. If you are lucky enough to have a vernal pond in your area, I encourage you to build up your good karma and do what you can to help protect these magical places.
Also you could look to see if your local town will provide legal protection for a vernal pond on your property. My folks had the foresight to protect their pond and when we went to sell the property a prospective owner was denied their desire to fill in the pond.
Thanks for wring about this Pru. I love learning about these magical ponds. Hearing the peepers makes my heart sing with joy.