If you look at a wetland in April and August you’ll likely only notice similar things about the map location. Different birds will be standing in the shallows, different insects will be flying from the water and some plants will have completely disappeared from the site. If you only looked at one month for your species count, your report would be accurate but misleading. This is what I’m aiming to explain throughout this essay. Biodiversity is most often referred to as a list of species living in a particular area, but I feel it should be seen as a timetable. Seasonality affects what species are present, what they’re doing and who relies on them. It’s therefore important to know when species are present. If this gets out of sync, ecosystems can collapse despite all species being alive.
Understanding seasonal biodiversity
Seasonal biodiversity refers to the predictable changes in species composition, abundance and behaviour throughout the year. The study of these seasonal changes, such as leaf emergence, insect emergence, bird arrival and fish spawning, is known as phenology.
In temperate and polar regions seasonal changes are mainly driven by temperature fluctuations. In much of the tropics it is driven by rainfall and in floodplains by water level. All these factors mean that a season is just an altered environment and species have evolved to utilise or survive that specific time period.
There are several ways around seasonal periods. Some species migrate, such as birds and whales, and some become inactive, like hibernating bats or seeds lying dormant in the soil for months. Others adapt, like the snowshoe hare turning from brown to white. This means that what was a single ecosystem is now multiple ecosystems happening in the same place but across different times.
Examples
The Serengeti follows the rain
Approximately 1.3 million wildebeest migrate in a loop of several hundred kilometres through Tanzania and Kenya to find fresh grass. Zebra and gazelle follow. Hundreds of thousands of calves are born in a short period of time early on in the year, causing a large surplus of prey for predators. Most calves will survive due to the sheer number.
This migration helps to feed the landscape. Every year wildebeest drown whilst crossing the Mara River. Subalusky and colleagues estimated that around a thousand tonnes of dead biomass is added to the river. This means that the bones will take years to decompose, so despite having one dramatic season, the fish, scavengers and microbes will benefit. All the animals rely on the same timetable.
Forest floor rush
Temperate forests are most interesting early in spring. Wildflowers such as bloodroot and trillium will flower, seed and disappear within a few weeks. These wildflowers are racing against the trees. Once the canopy is full of leaves the forest floor is dark, meaning they can’t photosynthesise enough to survive. They therefore only get a short period of light during the year.
This affects the animals too. Early pollinators will come out to feed on the flowers. Several species rely on ants to disperse their seeds. Later in the year caterpillars hatch on fresh leaves. Returning songbirds will time their chicks to coincide with the peak in caterpillars. All of these links rely on the previous link. If one part of the chain fails, the whole thing breaks down.
Fish in the Amazon forest
Around Manaus the rivers in the Amazon fluctuate by approximately ten metres each year. During the high water months, massive sections of forest are underwater. Fish swim between the trees eating the fallen fruit and seeds. Tambaqui have powerful jaws to crack open hard nuts and act as seed dispersers whilst swimming through the flooded forest. This relationship was described in great detail in Michael Goulding’s book The Fishes and the Forest (1980).
I find this to be the most unusual example of the five. The fish act like a forest animal for part of the year and the tree relies on it to reproduce. However, once the water levels drop, the fish return to the channels and the forest returns to land. If dams cause a lack of flood pulse, this relationship will be affected.
Salmon feeding the forest
Pacific salmon spend the majority of their lives in the ocean but will return in a seasonal rush to their stream of birth. They will spawn and die in large numbers. The carcasses are taken into the trees by bears, eaten by eagles and wolves, and left to sink into the ground.
Studies carried out by James Helfield and Robert Naiman on Alaskan rivers found that plants near the spawning streams received roughly 22 to 24 percent of their leaf nitrogen from the salmon. The Sitka spruce growing by the streams were growing over three times faster compared to those growing by a stream without salmon. Trees that would take approximately 300 years to grow to 50 cm wide were growing to this size in around 86 years. The ocean is brought into the forest once a year and the forest thrives.
The winterless bird: the Arctic tern
Arctic terns were tracked using tiny light sensors by Carsten Egevang and colleagues (2010). They were found to fly from Greenland to the Antarctic and back, averaging around 70,900 km per year. As they fly between the two polar summers, they experience the most daylight of any animal on Earth.
This is a good example to use as it shows how one species can link far apart ecosystems. The tern’s survival relies on the conditions in the Arctic, Atlantic and Southern Ocean during different times of the year. It can’t be protected by a single reserve in one country.
Gulf story
As I live in Doha, I’m most interested in this example. Qatar lies along a major migration route, meaning that during the cooler months the coasts and wetlands are filled with migrating waders and flamingos. During the winter rains the depressions in the desert will temporarily turn green with annual plants. These will then turn into seeds for the rest of the year.
During the summer, approximately 90 km off the coast of Qatar, around the Al Shaheen oil field, there is one of the largest known aggregations of whale sharks. This was documented by Robinson and colleagues (2013). The sharks arrive in April, peak between May and August and are mostly gone by October. Densities of up to 100 sharks per square kilometre were recorded at peak.
They are there due to the timing. They are feeding on the eggs of the mackerel tuna, which spawn in the area during the hot months. Once the spawning season is over, they disperse throughout the region. This is one of the clearest examples I know of a food chain relying on a timetable. If only one survey was carried out during winter, there would be barely any of this. The most important spot in Qatar’s waters for the largest fish in the sea would appear empty.
The risk of out of sync timetables
Climate change poses a large risk to seasonal ecosystems. Different species will respond to different cues. Plants may respond to temperature, insects may respond to temperature but at a different rate, and migrants will respond to day length thousands of kilometres away from where they’re travelling to, meaning they can’t detect early springs. This can cause animals to be at the right place at the wrong time. This is known as phenological mismatch by ecologists.
The most famous example is from the Netherlands. Christiaan Both and his team studied nine populations of pied flycatchers, small birds which overwinter in West Africa and breed in Dutch forests. Over around two decades, the peak in caterpillars (the main food source for their chicks) had advanced by around 16 days. The flycatchers couldn’t keep up as they don’t leave Africa based on European weather. In areas where the caterpillars had advanced the most, the bird populations had dropped by around 90 percent. However, where the peak in food was late, the decline was minimal. The birds weren’t starving everywhere; they were just arriving too late.
This can be seen in many other places. Camille Parmesan and Gary Yohe carried out an extensive analysis of 172 species (2003) and found that spring events were advancing by an average of around 2.3 days per decade. This seems like a small change. However, different species are advancing at different rates. Monika Winder and Daniel Schindler showed that in Lake Washington the warming caused an advancement in the spring algae bloom, but the zooplankton that feed on them couldn’t keep up, causing a loss in connection. A later study led by Stephen Thackeray looked at over 800 time series and found that animals higher up the food chain tend to advance more slowly than those lower down. This is concerning, as often those advancing the slowest are waiting for their meal.
Visual mismatch
Snowshoe hares change colour based on day length, not snowfall. As the snow cover decreases, there are white hares on brown ground. Scott Mills and colleagues found that mismatched hares don’t survive as well (Mills et al., 2013). Survival rates decreased by up to seven percent per week of mismatch. In Kyoto, cherry blossom records go back around 1,200 years. In 2021 the trees fully blossomed on 26 March, the earliest date in the whole record. A thousand years of diaries makes for a very patient thermometer.
I don’t want to oversell this. The flycatcher example is a strong one, but it is one of the strongest. Not all mismatches will cause a drop in population. A review carried out by Samplonius and colleagues (2021) argued that evidence for real fitness costs from phenological mismatch is much weaker than the headline examples suggest. This is partly due to many studies not measuring the consequences, only the timing. Some species are flexible and some have natural adaptations to help them, such as a long breeding season or multiple food sources.
Overall, phenological mismatch is real, it is common and it can definitely cause declines. However, we don’t currently know how often it does. Long term monitoring is key.
Why this matters for protecting nature
There are three practical reasons.
Monitoring must be carried out with timing in mind. You may have a healthy looking site, but functions may be failing, such as a stopover that is no longer useful. Almost all of the studies I’ve mentioned require someone to record the dates over decades. This is easily underfunded and impossible to replicate.
Protection must be carried out throughout the whole journey. Terns, flamingos and whale sharks all cross borders, meaning a reserve in one country can be negated by a drained wetland in another. Natural flooding cycles, undammed rivers and connected habitats are just as important.
People can help collect data. Citizen science projects tracking first flowering, first frog calls or the arrival of migrants will give real phenology data.
Conclusion
Seasons are the clock life runs on. They decide when the forest floor will burst into life, when the river will feed the forest, when millions of animals will migrate over the grasslands and when the largest fish in the sea will appear off the coast of Qatar. An area isn’t just a collection of species; it is a show.
The quietest consequence of a changing climate can also be the most serious. Species that have evolved to interact with each other no longer do. The flycatcher and caterpillars are still there; they just aren’t matching up. To protect biodiversity, we must think about the calendar as much as the map.
By: Suriya Dev Saravanakumar
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