How Does Water Loss Affect Biodiversity?

Water loss degrades biodiversity at every scale of life, from soil microbes to migratory birds, and across every type of ecosystem, from rivers and wetlands to forests and peatlands. When water disappears from a landscape, the effects are not limited to the obvious: fish dying in shrinking rivers or frogs losing their breeding ponds. Drought and water withdrawal reshape food webs, scramble the timing between species that depend on each other, and can push entire biomes toward irreversible collapse. The breadth of damage is what makes this topic so difficult to summarize in a single sentence, and so important to understand across its many dimensions.

Freshwater Fish Face Steep Losses

Rivers are among the ecosystems most directly harmed by water loss, and freshwater fish are the clearest casualties. When river discharge drops, whether from climate change, irrigation withdrawals, or dam operations, the available habitat shrinks and fragments. Fish that depend on particular flow conditions, water temperatures, or river connectivity lose access to spawning grounds and feeding areas. A global modeling study projected that in rivers where discharge declines, up to three-quarters of local fish species could be headed toward extinction by 2070. The median losses were far lower, but even the middle-range projections showed meaningful declines in fish richness across many river basins worldwide.¹

These aren’t purely theoretical concerns. In rivers where discharge has already dropped significantly, researchers have documented exactly the kinds of species losses the models predict. The relationship between river flow volume and the number of fish species a river can support is well established, and it runs in one direction: less water, fewer species. What makes this particularly worrying is that freshwater fish account for a disproportionately large share of global vertebrate diversity relative to the tiny fraction of the planet’s surface that freshwater covers.

Wetland Drying and Amphibians

Amphibians are arguably the group of animals most tightly bound to water availability, since most species need standing water to breed. When ponds and wetlands dry up or hold water for shorter periods, breeding habitat vanishes. During California’s extreme drought from 2012 to 2015, roughly four out of five temporary ponds and more than a quarter of permanent ponds dried out. Juvenile amphibian occupancy plummeted across species: bullfrog site occupancy fell by half, Pacific chorus frogs dropped from being present at over 90% of sites to around 74%, and California newt occupancy fell from 75% to 53%.²

A similar pattern has played out in Yellowstone National Park, where warming temperatures have caused a fourfold increase in permanently dry ponds compared to conditions measured roughly sixteen years earlier. Among the ponds that still hold water, fewer support amphibians and each pond supports fewer species than before.³ Looking ahead, modeling of Pacific Northwest wetlands suggests that between 12% and 23% of wetlands currently used for amphibian breeding could become unsuitable by the 2080s, with some individual watersheds losing up to 80% of suitable breeding habitat.⁴

There is an interesting wrinkle in the California data, though. The same drought that hammered native amphibians also knocked back invasive fish, whose pond occupancy dropped from about 20% to 8%. After the drought broke, native amphibian populations rebounded. So in some cases, drought can temporarily level the playing field between native species and their invasive competitors, even as it causes real short-term harm.

How Trees Die When Water Runs Out

Forests depend on water moving through tree trunks the way blood moves through arteries. When drought becomes severe, air bubbles form inside the water-conducting tissue and block flow, a process researchers call hydraulic failure. Once enough of that internal plumbing is blocked, branches and then entire canopies die. In eucalypt forests during extreme drought, trees that had lost all their foliage showed near-total blockage of their water transport system, while healthier trees with intact canopies had far less damage.⁵

This mechanism operates across tree species and continents. Research on piñon pine die-offs in the American Southwest found substantial hydraulic failure in both roots and branches that mapped onto the landscape patterns of canopy death.⁶ In European Norway spruce, the collapse of the water-transport system during severe drought happened with startling speed: trees that appeared safe were pushed past the point of no return in a very short window.⁷ The speed matters because it means drought does not always kill forests gradually. A forest that looks fine in June can be full of dead and dying trees by September.

When mature trees die, the ripple effects on biodiversity are enormous. Trees are habitat for insects, birds, epiphytes, and fungi. They shade streams, stabilize soil, and regulate local humidity. Losing them changes the character of an entire ecosystem, often favoring grasses and shrubs that support far fewer associated species.

The Invisible Casualties Underground

Soil is full of life that most people never think about. Bacteria, fungi, and protists in soil break down organic matter, cycle nutrients, and help plants absorb what they need. Drought disrupts all of this. When soil dries out, microbial community structure shifts, functions degrade, and the interactions between microbes and plant roots weaken.⁸

The damage is especially pronounced in the zone immediately around plant roots, where the exchange between microbes and plants is most active. Research has shown that drought sharply reduces the diversity of bacteria, fungi, and protists in that root zone, simplifies the networks of interactions among microbial species, and lowers the abundance of genes responsible for breaking down carbon and cycling nitrogen.⁹ In plain terms, drought doesn’t just kill the microbes that are there. It dismantles the collaborative networks they rely on, making the soil less capable of supporting plant growth even after moisture returns. That reduced soil function feeds back into aboveground biodiversity loss by weakening the plants that animals depend on.

Stream Invertebrates and the Loss of Sensitive Species

The small creatures that live in streams and rivers, insects, crustaceans, worms, and snails, are often the first to respond when flow declines. As streams dry, the mix of species shifts away from pollution-sensitive and moisture-dependent taxa toward hardier generalists and drying specialists.¹⁰ This is more than an academic distinction. Many of those sensitive species are the ones ecologists use as indicators of stream health, so their disappearance means both a direct loss of biodiversity and a loss of our ability to monitor what’s happening.

The type of stream habitat matters too. Riffle habitats, the shallow, fast-flowing stretches where water tumbles over rocks, are hit harder by flow reduction than deeper pools. Communities in riffles show steeper declines in the number of species and in the abundance of organisms that can’t tolerate drying.¹¹ Since riffles are where much of the oxygen exchange and organic matter processing in a stream occurs, losing their inhabitants has consequences that cascade through the rest of the aquatic food web.

Migratory Birds Lose Their Stepping Stones

Waterbirds migrating across continents depend on chains of wetlands and lakes as refueling stops. In arid regions, these stopover sites are often endorheic, meaning they have no outlet to the sea and are sustained entirely by the balance between inflows and evaporation. That balance is shifting. In the Great Basin of the western United States, shrinking freshwater and declining viability of perennial saline lake systems are fracturing a critical link in the Pacific Flyway, one of North America’s major migratory corridors. Millions of waterbirds depend on this region during breeding, migration, and wintering, and the loss of reliable water there threatens the entire flyway.¹²

This isn’t unique to North America. The drying of endorheic water systems worldwide is creating new bottlenecks along continental flyways, squeezing migratory waterbirds into fewer and more crowded stopover sites.¹³ Crowding at remaining sites increases disease transmission, competition for food, and vulnerability to localized disturbance. A chain is only as strong as its weakest link, and for migratory birds, a single missing wetland in the middle of a desert crossing can mean the difference between completing a journey and dying along the way.

Food Webs Unravel from the Top Down

Water loss doesn’t just remove individual species. It restructures the relationships among all the species that remain. Experimental work on freshwater food webs has shown that drought triggers substantial losses of species and the feeding links between them, with predators and larger-bodied organisms being especially vulnerable.¹⁴¹⁵ Losing predators matters because they regulate the populations of species below them. Without predators, prey populations can spike, overgraze their own food sources, and destabilize the whole system.

There is some resilience built into these networks, though. Research has found that surviving consumers in drought-impacted food webs tend to broaden their diets, feeding on a wider range of resources than they did before. These new feeding links partially re-balance the food web’s overall structure, preserving some of its organizational properties even as species are lost.¹⁶ But “preserving topology” is not the same as preserving function. A food web that has lost its top predators and rearranged its remaining links is a fundamentally different ecosystem, even if the wiring diagram looks superficially similar.

Drought Disrupts Pollination

Flowering plants need pollinators, and pollinators need nectar and pollen. Drought squeezes both sides of this relationship. Drought-stressed plants produce fewer flowers and less nectar per flower, which means there’s less food available for bees, butterflies, and other pollinators.¹⁷ Greenhouse experiments and multi-year field observations have confirmed that drought reduces both pollen and nectar production.¹⁸

The downstream effects can be severe. In experiments simulating drought conditions on zucchini, a crop that depends entirely on insect pollination, severe drought reduced the caloric value of nectar offered to pollinators by about 95% per hectare. Even moderate rainfall reduction cut caloric value by roughly a third.¹⁹ When pollinators can’t find enough food, they visit fewer flowers, which means fewer seeds are produced, which in turn means fewer plants in the next generation. For wild plant communities, this creates a feedback loop where drought reduces not just the current year’s reproduction but the future population of both plants and pollinators.

Timing Mismatches Between Species

Many ecological relationships depend on precise timing. A plant needs to flower when its pollinators are active. A bird needs to arrive at its breeding ground when insects are available to feed its chicks. Water loss, and the warmer conditions that often accompany it, can throw off these schedules in ways that harm both partners.

In subalpine wildflowers, earlier snowmelt and drier summers compress the window between flowering and seed production. Some species rush through their reproductive cycle so quickly that the animals that depend on their flowers, fruits, or seeds, from pollinators to seed-eating birds, find resources available for a shorter period or at an unexpected time.²⁰ In Japanese deciduous forests, a spring ephemeral herb that depends on bumblebee queens for pollination illustrates the problem sharply: when snow melts early, the plant flowers before the bees have emerged from hibernation. The result is greater pollen limitation and reduced seed production.²¹ These timing mismatches are subtle. Nobody walks through a forest and sees them happening. But they erode reproductive success year after year and can eventually drive population declines in species that look perfectly healthy in the moment.

Genetic Isolation in Fragmenting Waterways

When stretches of river go dry, they don’t just kill the fish in those reaches. They create barriers that split populations apart. In Arctic Alaska, dry zones in rivers were significantly associated with increasing genetic differences among populations of Arctic grayling, even though these fish are strong swimmers capable of long-distance movement. The temporary barriers created by dry stretches were enough to isolate headwater populations and push them toward genetic divergence.²²

Similar patterns of low genetic diversity and strong population structure have been documented in threatened fish species across river basins where flow is variable and connectivity is intermittent.²³ Genetic diversity is a species’ insurance policy against future change. Populations with low diversity have fewer options for adapting to new diseases, new predators, or further environmental shifts. So water loss doesn’t just reduce the number of individuals in the short term. It narrows the genetic toolkit available for long-term survival.

When Drought Gives Invaders an Edge

Not all species lose equally during drought. Some invasive plants actually perform better than natives under dry conditions, thanks to greater physiological flexibility. Research in Mediterranean coastal dune ecosystems found that an invasive acacia species maintained high performance during the growing season and showed seasonal physiological plasticity that gave it a competitive advantage over native dune plants during dry periods.²⁴ In practical terms, drought can reshuffle the competitive balance in a community, clearing space for stress-tolerant invaders while weakening the natives that historically dominated.

As water becomes scarcer in more regions, this dynamic could accelerate invasions in ecosystems worldwide. Invasive species are already one of the leading threats to global biodiversity, and if drought conditions systematically favor them over native flora, the two pressures compound each other in ways that are difficult to reverse.

Large-Scale Tipping Points

Perhaps the most alarming dimension of water loss and biodiversity is the possibility of abrupt, irreversible shifts in entire biomes. The Amazon rainforest is the case that keeps ecologists up at night. Satellite-based analyses have provided direct evidence that the Amazon has been losing resilience since the early 2000s, meaning the forest is becoming less able to bounce back from disturbances like drought and fire.²⁵ If the forest crosses a tipping point, large portions could transition from closed canopy to open savanna-like vegetation, with staggering consequences for the millions of species that live there and for global carbon storage and climate regulation.

Repeated droughts are a key driver of this risk. Modeling work suggests that while the Amazon has some capacity to adapt to individual dry spells, a future climate characterized by permanent drought conditions could trigger the transition to open canopy, particularly in the southern Amazon where deforestation and drying trends overlap.²⁶ The Amazon generates a significant fraction of its own rainfall through moisture recycling. As forest is lost, less moisture is recycled, which dries the remaining forest further, which leads to more loss. It’s a self-reinforcing cycle that makes the tipping-point scenario a genuine near-term concern rather than a distant theoretical worry.

Peatlands as a Window into Loss and Recovery

Peatlands sit at the intersection of water and biodiversity in an instructive way. These waterlogged ecosystems depend entirely on high water tables to maintain the saturated, low-oxygen conditions that support their distinctive communities of specialized plants and animals. When peatlands are drained, whether for agriculture, peat extraction, or development, those specialist species disappear. Research on raised bogs in Europe found that the most ecologically valuable beetle species were present on intact bogs with healthy Sphagnum moss cover but absent or barely detectable at extraction sites even decades after peat mining had stopped. Restoring a suitable water regime was identified as the single most important factor for preserving these bog specialist communities.²⁷

The good news is that rewetting works, at least partially. Monitoring of rewetted montane peatlands has shown positive trends in plant, dragonfly, and butterfly diversity following restoration, though the recovery trajectories are nonlinear and remain incomplete after two decades.²⁸ Broader restoration programs in former peat extraction areas have shown that rewetting combined with targeted management, like re-establishing Sphagnum moss and managing water levels, can foster rapid vegetation recovery and create diverse wetland habitats that support a range of species.²⁹

Environmental Flows as a Conservation Tool

One of the most concrete things humans can do to protect aquatic biodiversity from water loss is to manage how much water stays in rivers. Environmental flows, sometimes called e-flows, are managed releases of water designed to maintain the ecological health of a river system. They are not about keeping rivers at pristine conditions. They are about ensuring enough water remains to support the species and processes the river depends on. Expanding the implementation of environmental flows has been identified as an important strategy for restoring the biodiversity and resilience of water-stressed rivers.³⁰

Determining the right amount of water is the hard part. One approach calculates the minimum flow needed to keep fish biodiversity within 10% of what it would be under natural conditions; in a case study, that came out to about 40% of the river’s average annual flow.³¹ That figure varies by river, by season, and by which species you’re trying to protect, but it illustrates the scale of the commitment involved. Protecting biodiversity means leaving a substantial share of river flow in the river, which puts environmental flows in direct tension with agricultural irrigation, municipal water supply, and hydropower generation. In water-scarce regions, those trade-offs are only going to intensify, making the political and economic dimensions of water management as important as the ecological ones.

What makes environmental flows particularly challenging is that they need to be maintained not just in average years but during droughts, which is precisely when the pressure to divert water elsewhere is greatest. A flow regime that works during a normal year but collapses during a dry year offers little protection to species whose survival depends on getting through the worst conditions. Getting this right requires not just scientific understanding but institutional commitment to treating rivers as infrastructure that provides irreplaceable ecological services, not just as plumbing for human use.