The storm called Eloise swirled into being out in the watery wilds of the Southwest Indian Ocean in mid-January. It tracked westward, raged its way across Madagascar, then boiled up into an all-out tropical cyclone in the Mozambique Channel before making destructive landfall on the African mainland: the strongest tropical cyclone to strike Mozambique since 2019’s Cyclone Kenneth.

Sputtering its way to extinction over the southern African interior, Eloise unleashed fierce deluges – and among the places impacted was one of the oldest and most celebrated wildlife sanctuaries in the world.

Kruger National Park saw some gnarly flooding from Eloise and from followup rainfall on waterlogged soils. Swollen rivers such as the Sabie and Olifants caused closures of multiple bridges, entrances, and roads in the park. “Animals instinctively move to higher ground even before the storm arrives,” Kruger’s general manager of communications and marketing, Ike Phaahla, told Getaway Magazine in late January. “Nature normally takes care of itself, we have never had to rescue any animals before but we do a regular flyover to observe infrastructure and should there be a need to assist animals in distress we will do so.”

Kruger has experienced major floods numerous times, not least in 2000, when the southern part of the park was entirely cut off due to raging flows, and just a year ago, when floodwaters impacted northern Kruger in particular.

The floods that hit the Kruger National Park in 2000 were some of the most severe on record.

Tropical cyclones (also known as hurricanes or typhoons) and other tropical storms birthed in the Southwest Indian Ocean are responsible for a significant proportion of extreme precipitation events in northeastern South Africa, even though they provide only a fraction of the average annual rainfall. While about 11 weather disturbances in those seas each year intensify into tropical depressions, most storms entering the Mozambique Channel veer (or “recurve”) away from the southern African mainland; only a few make landfall. 

All of this is to say that tropical cyclones and overspilling waters are important, if only periodic, parts of Greater Kruger’s environmental fabric. Scouring away sediment and vegetation (including whole trees, in some cases), big, occasional floods – whether triggered by tropical cyclones or just garden-variety torrential rains – play a critical role sort of “resetting” the ecological stage in the region’s semi-arid rivers such as the Sabie. And research suggests the large woody debris piles that floods deposit in the Sabie are important for giving seedlings of riparian plants a protected and favourable foothold.

Both severe storms and floods are examples of ecological disturbances, which help shape the structure and diversity of ecosystems. In light of Kruger’s latest overflows, we thought we’d sketch out some of the basics of how floods – and tropical cyclones, for good measure – impact plants, animals, and ecological systems in general.

Rising Waters

Many ecosystems are partly defined by regular, seasonal flooding, whether it’s triggered by the rainy season of a tropical savanna climate or, at higher latitudes, by springtime snowmelt. Organisms in such systems are adapted to, and in many cases rely on, periods of high and low water, which also help determine the layout of different plant zones. Consider the lion-plied Okavango Delta, the Bangweulu Swamps, or the Upper Nile’s Sudd in Africa, the mighty Pantanal of South America’s Upper Paraguay basin or the bottomland várzea and igapó rainforests of the Amazon, the sheet-flow marshlands of the Florida Everglades, the billabongs of the northern Australian tropics – or, if you want a chillier example, the gigantic, seasonally flooded West Siberian Lowlands.

Like many ecosystems, Botswana's Okavango Delta, is partly defined by regular, seasonal flooding.

In the Terai-Duar lowlands of Nepal, Bhutan, and northeastern India, monsoon floods nourish towering riverine grasslands whose productivity, in turn, helps maintain a large and diverse community of ungulates, including the densest concentrations of that fantastic, well-armoured beast the greater one-horned rhinoceros.

Seasonal floods – and even those rarer, extreme floods that periodically impact watersheds – can improve fish spawning on low-gradient rivers, and inject a bunch of nutrients into the aquatic ecosystem from lands flushed by floodwaters. But the specific effects of floods depend on the scale and duration of the inundation, the physical characteristics of affected watercourses, and on many biological factors: For example, more young fish often die in floods than mature ones, and bottomland trees well-adapted to soggy conditions may do just fine swamped for a certain period of time but suffer if the high water is prolonged – and can be uprooted and washed away by fiercer floodwaters.

It’s all a tricky balancing act, these flood-cycled ecosystems, exemplified by their half-land, half-liquid identity. In the Pantanal, aberrant flooding in the dry season can throw off the nesting routine of the jabiru stork; the jabiru gropes for fish and snails in the shallows and mud of drying-up ponds, and if the water level’s too high it struggles to feed its nestlings. The same goes for the wood stork, another shallow-water fisher, which in the Everglades can experience widespread nesting failure when adequate “dry-down” doesn’t occur.

In the seasonally flooded rainforest of the Peruvian Amazon, meanwhile, terrestrial mammals such as peccaries and armadillos retreat to natural river levees and other high ground as waters rise; during extreme floods, they may be confined to very limited acreage and experience enhanced competition and predation, or end up drowning if dry land’s unavailable.

Tropical Cyclones

What about tropical cyclones themselves? Like floods – which, as in Kruger, these tempests commonly provoke, from both downpours and coastal storm surges – they’re an important ecological disturbance. Their scope, though, is a bit more geographically limited: Fuelled by warm ocean waters, they form in particular nursery grounds, mainly between 10 and 30 degrees latitude, though after spinning westward longer-lived tropical cyclones often recurve to sputter out over colder waters or land. So they’re intense but inherent disturbances in many coastal parts of the tropics, subtropics, and lower temperate realms.

A satellite animation showing ten days in the life of tropical storm Eloise.

Given their regularity, furious power, and areal extent, it’s no surprise tropical cyclones exert a significant influence on the character of the coastal forests growing in their storm tracks. (The native forests of Guam are sometimes called “typhoon forests,” honouring their prime historical shaper.) The tempests open gaps in closed forest, allowing shade-intolerant plants a foothold; they also may flood the forest floor with nutrients, given the green litter they leave behind tends to be more nutrient-rich than the dead twigs and leaves trees shed as part of their normal cycles.

Hurricanes and typhoons may smash their way through existing mangrove swamps, but also deposit mounds and flats of sediment ideal for the establishment of new mangroves (even as some of that heaped-up sediment can smother the roots of standing trees).

The ecological effects of these storms aren’t limited to terra firma: With their surging waves, tropical cyclones clobber oyster beds and coral reefs, breaking apart their structure and smothering them in sand. But they also benefit coral communities by flushing out silt-choked lagoons and pulsing in cold waters to relieve heat-stressed reefs.

Tropical cyclones may blow seabirds far inland, strand marine mammals, squash alligators, and suffocate loads of fish by flooding waters with washed- or blown-in organic matter, the decomposition of which plummets dissolved oxygen levels. Many critters, however, appear to be mobile or resilient enough to endure these whopping whirlwinds without much trouble. A variety of wildlife, from bats and birds to fishes, seem to be able to detect shifts in barometric pressure and may thereby get a heads-up on approaching weather disturbances; changes in water level and salinity can also clue in aquatic creatures to a tropical cyclone and prompt them to move to one or another refuge.

gator crushed by three_2017_06_30.jpg
In 2017, Hurricane Matthew barrelled its way up the East Coast of the United States claiming the lives of more than 600 people, and – as it turns out – at least one American alligator. More here. Image: Bert Wyatt/USFWS via Savannah Coastal Refuges Complex/Facebook

Barometric changes were suspected as the probable signal for juvenile blacktip sharks to head for deeper water as Tropical Storm Gabrielle (2001) approached shore. In the Shark River Estuary of the coastal Everglades, Hurricane Irma (2017) seemed to prompt both common snook and young bull sharks to head downstream and, in some cases, out of the estuary entirely. Alligators being tracked with acoustic transmitters in the same system showed varying responses. Some moved from marshes and mangrove backwaters into river channels – perhaps, researchers speculated, following prey or carcasses flushed out by the storm. One gator that had been idling in upriver haunts abruptly headed significantly downstream, maybe tailing fish riding beefed-up freshwater flow from hurricane rains.

The extreme habitat modification (to put it mildly) that a tropical cyclone can engineer may significantly impact wildlife: locally depriving leaf- and fruit-eaters of food by stripping vegetation, say, or toppling the large, old trees used for nesting.

Timing also plays a role when it comes to cyclone effects on wildlife: Hurricane season in the North Atlantic overlaps with sea-turtle nesting and fall bird migration in North America, which sometimes means a washed-away or sand-drowned nesting beach or gaggles of avian travellers being swept far off-course.

The Human Factor

As with other ecological disturbances, human activity has altered the impacts of tropical cyclones and flooding. For one thing, we can influence their patterns and magnitude: We’ve certainly altered the hydrological regimes of rivers around the world and disrupted natural flood cycles. Anthropogenic climate change as well as human land use may be contributing to patterns of more intense flooding alternating with more intense droughts in both the Amazon Basin and the Pantanal.

Many scientists believe climate change is also influencing the frequency and intensity of tropical cyclones thanks to warming ocean waters and a wetter atmosphere. Research into hurricane impacts on rainforest in Puerto Rico showed trees fairly resilient in the face of mid-grade tropical cyclones may be extensively damaged by stronger ones, which may become more common. Fieldwork in Guam’s “typhoon forests” indicate another way human beings may indirectly degrade trees’ ability to withstand tropical-cyclone damage: A 2013 study suggested the greater stem breakage seen among a widespread, storm-adapted Guam cycad during Typhoon Chaba in 2004 compared to the more powerful Typhoon Paka in 1997 may have been due to infestations of two exotic insects. “A span of less than one decade allowed two alien invasions to eliminate the incipient resilience of a native tree species to tropical cyclone damage,” the researchers concluded.

When we’re talking about the ecological impacts of disturbances, the greater landscape context matters. In the past, a tropical cyclone may have given a rare foothold for pioneering plant species when it opened up canopy gaps and blowdowns in otherwise extensive closed forest. Nowadays, the same location – thanks to logging and land-clearing for agriculture and development – may have fewer big blocks of primary forest and an abundance of secondary growth. If a cyclone flattens remnant mature trees in these areas - “converting” them to the early-stage vegetation now dominant across the landscape – species that depend on forest interiors or old-growth are left with little shelter to retreat to.

Wildlife taking refuge from floods are, in many parts of the world, forced into human-dominated landscapes. Floodwaters may also, of course, bring aquatic critters into the same developed areas (see: the widespread crocodile warnings in Queensland from Cyclone Owen’s flooding in 2018).

A major example of how human activities and development can throw a wrench in age-old wildlife responses to flooding – and the patterns of flooding themselves – comes from those rhino-roamed Terai-Duar grasslands of Kaziranga National Park. Kaziranga, which lies along the Brahmaputra floodplain, has seen extreme floods more years than not recently, including last July, when as much as 95 percent of the park lay underwater.

As we’ve already mentioned, monsoon floods are an intrinsic part of Kaziranga’s ecology, maintaining and nutrifying the grasslands and also helping control invasive plants such as the water hyacinth. “It’s a riverine ecosystem, not a solid landmass-based ecosystem,” Kaziranga’s director, P Sivakumar, told The Indian Express during last year’s inundations. “The system won’t survive without water.”

Historically, many of Kaziranga’s animals have moved southward into the Karbi Anglong (Karbi Hills) during major floods in the alluvial lowlands. The floodplain and the hills are thus an integrated landscape. Nowadays, however, a busy highway and other development separate the national park from the Karbi Hills, forcing wildlife to either make a tricky crossing to reach high ground or stay behind amid floodwaters.

Recent decades have seen significant numbers of animals killed on the roadway, targeted by poachers, and drowned. Elephants, rhinos, and other big (and potentially troublesome) creatures fleeing floodwaters enter villages. During 2019 flooding, a Kaziranga tigress apparently trying to navigate her way to the Karbi Hills was found holed up inside a Bagori shop. Human alterations to the Brahmaputra watershed may also be increasing the frequency and scale of floods, exacerbating the issue.

Mitigation strategies include direct efforts to rescue flood-threatened animals, traffic controls and crossings on the highway, and bolstered land conservation in the Karbi Hills. Dozens of artificial highlands have also been built in Kaziranga to provide more refuge within its boundaries from floodwaters, though officials stress this is only a limited solution. “Ecologically, this is not a great intervention,” Kaziranga National Park Research Officer Rabin Sharma told Mongabay in 2019. “Highlands are required because animal corridors are blocked.”

Incidentally, more recent precipitation unrelated to Eloise – by the standards of weather systems, now long gone – continues to impact South Africa, including Kruger, which is seeing strong-flowing rivers (and some wet lionesses, too): 

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