Drought is a long period with much less precipitation than usual, and it can change an entire ecosystem. When soil, streams, lakes, and plants lose water, organisms must spend more energy finding the resources they need to survive. Plants often respond first by closing leaf pores, slowing growth, or wilting, which reduces the food and shelter available to animals.
Because every part of an ecosystem is connected, drought can cause effects that spread through many species and habitats.
The main driver of drought stress is water shortage in soil and surface water. Low soil moisture reduces photosynthesis and primary production, so less energy enters the food web. As rivers and ponds shrink, water warms, oxygen levels can fall, and fish and amphibians become stressed.
Dry vegetation also becomes easier to ignite, which increases wildfire risk and can rapidly transform habitats.
Understanding How Drought Affects Ecosystems
Soil does not dry evenly. Water near the surface disappears first through evaporation, while deeper water may remain available to long roots. The amount held in the root zone matters more to many plants than a brief rain shower.
Clay soils can hold a lot of water but may grip it so tightly that roots cannot pull it out. Sandy soils release water more easily, yet they drain fast. As drying continues, plants must pull harder to move water from soil into roots.
Eventually, the water column inside their tissues can break. This is called hydraulic failure. It can damage water carrying cells and may kill a plant even before every leaf turns brown.
Plant responses create tradeoffs. Closing stomata slows water loss, but it also restricts the carbon dioxide needed to make sugars. With less sugar, a plant has less energy for roots, new leaves, seeds, chemical defenses, and repair.
Some species drop leaves early to conserve water. Others grow deeper roots or keep small waxy leaves. These traits help explain why one species survives a dry season while another declines.
Seedlings are especially vulnerable because their roots reach only shallow soil. A drought can therefore change which plants dominate an area for years after rainfall returns.
Food webs feel this loss through both quantity and timing. Fewer flowers can mean less nectar for insects. Fewer insects can leave birds with too little food during nesting season.
Herbivores may eat bark, roots, or unusual plants when grasses fail, putting further pressure on the remaining vegetation. Animals can move toward water, causing crowding around springs or small ponds. Crowding raises competition and can make disease spread more likely.
Predators may follow prey into these limited areas. In this way, drought does not affect every organism in the same way. It reorganizes where organisms live, what they eat, and when they reproduce.
Water quality changes matter as much as water quantity. A smaller stream has less volume to dilute salts, fertilizer runoff, sewage, and waste from animals. Slower water can become warmer during the day.
Warm water holds less dissolved oxygen, while bacteria use oxygen as they break down dead material. Fish may gather near cooler inflows, and species that need fast, cold water can disappear first. In dry landscapes, bare soil is easier for wind to carry away.
When rain finally arrives, hard dry ground may absorb little water. Fast runoff can cause erosion, flash flooding, and muddy streams. This shows why drought can produce damage during a later storm as well as during the dry period.
When studying drought, follow the chain from weather to soil, plants, animals, and water bodies. Compare conditions with the normal pattern for that place and season, since a rainfall amount that is ordinary in one region may be severe drought in another. Notice delays in the system.
A tree may look healthy while using stored water, yet show damage months later. Recovery can be delayed too, especially after seed loss, erosion, or fire. Human choices affect the outcome.
Heavy groundwater pumping, removing streamside vegetation, and overgrazing can intensify drought stress. Water conservation, healthy soils, native plant cover, and protected wetlands can help ecosystems retain water and recover more steadily.
Key Facts
- Drought occurs when precipitation is far below normal for weeks, months, or years.
- Low soil moisture limits plant growth and reduces primary production.
- Photosynthesis equation: 6CO2 + 6H2O + light energy = C6H12O6 + 6O2.
- Transpiration is water vapor loss from plant leaves, and it often decreases during drought as stomata close.
- Water level drop in streams and lakes can reduce habitat space and lower dissolved oxygen for aquatic life.
- Dry fuel plus heat plus oxygen increases wildfire risk: fuel + oxygen + heat = fire.
Vocabulary
- Drought
- A drought is an extended period of unusually low water availability caused by below-normal precipitation and other drying conditions.
- Soil moisture
- Soil moisture is the water held in soil that plants can absorb through their roots.
- Primary production
- Primary production is the creation of plant biomass through photosynthesis, forming the energy base of most food webs.
- Transpiration
- Transpiration is the movement of water from plant roots to leaves and then into the air as water vapor.
- Food web
- A food web is a network of feeding relationships that shows how energy and matter move through an ecosystem.
Common Mistakes to Avoid
- Thinking drought only affects deserts. This is wrong because forests, grasslands, wetlands, farms, rivers, and lakes can all experience drought stress.
- Assuming plants die only because they are hot. Heat can make drought worse, but plants are mainly stressed because they cannot take up enough water for photosynthesis, cooling, and cell support.
- Ignoring water quality when water levels drop. Lower water levels can make water warmer and more polluted, which can reduce dissolved oxygen and harm aquatic organisms.
- Treating wildfire as separate from drought. Drought dries grasses, leaves, and branches, creating fuel that can burn more easily when an ignition source is present.
Practice Questions
- 1 A lake is normally 4.0 meters deep in summer, but during drought it drops to 2.8 meters. By how many meters did the lake level decrease, and what percent of the normal depth was lost?
- 2 A grassland usually produces 900 grams of plant biomass per square meter in a growing season. During drought, primary production falls by 35%. How many grams per square meter are produced during the drought?
- 3 Explain how a decrease in soil moisture can lead to fewer predators in an ecosystem, even if the predators do not eat plants directly.