Table of Contents
Water as an Abiotic Factor
Water is one of the most important abiotic factors because it affects almost every physical and chemical process in organisms and ecosystems. In ecology, water availability and distribution largely determine where organisms can live, how they are built, and how they behave.
This chapter focuses on how water in the environment affects organisms, not on the internal properties of water as a chemical substance (covered elsewhere).
Forms and Availability of Water in Habitats
In nature, water appears in different physical states and forms that shape habitats:
- Liquid water
- Lakes, rivers, oceans, groundwater, soil water, water films on plant surfaces.
- Basis of aquatic ecosystems and crucial for land ecosystems via soil moisture.
- Solid water (ice, snow, permafrost)
- Ice cover on lakes and seas, glaciers, snowpack, permanently frozen ground.
- Limits liquid water availability; influences temperature and length of growing seasons.
- Water vapor
- Water in the air; influences humidity, cloud formation, and precipitation.
- Affects evaporative water loss of organisms (transpiration in plants, evaporation from animal surfaces).
The amount of water that is freely usable for organisms is often much less than total water present:
- In soils, much water is bound so tightly to particles that roots cannot absorb it.
- In salt water, total water is high but high salinity limits direct use for many organisms.
- In ice and permafrost, water is present but not available in liquid form.
Water as a Limiting Factor
According to the concept of the tolerance range, each species has:
- A minimum water availability it needs to survive.
- An optimum range where growth and reproduction are best.
- A maximum beyond which too much water becomes harmful.
Water can limit life in two opposite ways:
Water Shortage (Drought Stress)
Occurs when:
- Precipitation is low (deserts, semi-deserts, arid steppes).
- Soils are sandy or rocky and drain quickly.
- Winds and high temperatures increase evaporation and transpiration.
- Water is present but frozen (cold deserts, high mountains, polar regions).
Consequences:
- Cells lose water, turgor (pressure in plant cells) declines.
- Metabolic processes slow or stop.
- Enzymes and membranes can be damaged.
- For many organisms, long-term water shortage means death, unless they have adaptations.
Water Excess (Waterlogging, Flooding)
Occurs when:
- Soils stay saturated (swamps, marshes, rice paddies).
- Floods or tidal inundations are frequent.
- Drainage is poor due to clay soils or permafrost layers.
Consequences:
- Oxygen diffusion in waterlogged soil is very slow; roots can suffer from lack of oxygen.
- Toxic substances can accumulate in anoxic (oxygen-poor) conditions.
- Many terrestrial plants and soil animals cannot tolerate long-term waterlogging.
Thus, both too little and too much water can limit species distribution and abundance.
Ecological Gradients Related to Water
Water creates characteristic ecological gradients that structure the biosphere:
Humidity and Precipitation Gradients
From very dry to very wet habitats:
- Hyperarid deserts (almost no plant cover)
- Semi-arid regions (succulents, drought-tolerant shrubs, grasses)
- Temperate regions (forests, grasslands)
- Humid tropics (rainforests, cloud forests, wetlands)
Each step along this gradient is characterized by typical life forms and adaptations to water conditions.
Soil Moisture Gradients
Even within a small area (e.g., a meadow or forest), soil moisture can vary:
- Dry slopes vs. moist depressions
- Well-drained sandy soils vs. water-retentive clay soils
- Areas near streams vs. far away
Along this gradient, plant communities change from:
- Xerophilic species (tolerate dryness)
- Through mesophilic species (moderate moisture)
- To hygrophilic species (require very moist conditions)
Aquatic vs. Terrestrial Habitats
Water availability divides the living world into two major habitat types:
- Aquatic habitats
- Standing waters: ponds, lakes, wetlands.
- Flowing waters: streams, rivers.
- Marine habitats: coastal zones, open ocean, deep sea, estuaries.
- Terrestrial habitats
- Soils, rock surfaces, vegetation surfaces.
At the transition zones (shorelines, tidal zones, wet meadows), organisms often face rapidly changing water conditions (periodic flooding/drying).
Water Potential and Movement in the Environment
Water moves continually in the environment:
- As precipitation and runoff.
- By infiltration and percolation into soils.
- By evaporation from water surfaces and transpiration from plants.
For organisms, an important concept is water potential:
- It describes the tendency of water to move from one place to another.
- Water moves from regions with higher water potential (more freely available water) to lower water potential (more bound water, higher solute concentration, or more negative pressure).
Ecologically:
- Water flows from wet to dry zones in soils.
- Saltier environments draw water out of less salty cells or tissues (osmotic effects).
- Transpiring leaves with low water potential “pull” water from soil through roots and stems.
Adaptations to Water Conditions in Plants
Plants are bound to one place; they cannot move to seek water. Therefore they show especially clear adaptations to the water regime of their habitat.
Hydrophytes – Plants of Very Wet and Aquatic Habitats
Hydrophytes live in or on water, or in permanently water-saturated soils.
Typical ecological conditions:
- Water is abundant; desiccation is rarely a problem.
- Gas exchange (especially oxygen) is often difficult, especially below the water surface or in waterlogged soils.
- Light is reduced with depth and turbidity.
Characteristic features (ecological perspective):
- Reduced support tissue: water provides buoyancy; stems and leaves can be soft and flexible.
- Often large, thin leaves in submerged plants, increasing surface area for gas and nutrient exchange.
- Air-filled spaces (aerenchyma) for gas transport within the plant, crucial in low-oxygen conditions.
- Weakly developed or specialized roots; water and dissolved nutrients are taken up over large areas of the plant surface.
Habitat types:
- Floating plants on water surfaces.
- Submerged plants in open water.
- Plants of marshes, fens, and bogs with roots in saturated soils.
Hygrophytes – Plants of Moist Habitats
Hygrophytes grow in consistently moist but not permanently flooded sites (e.g., moist forests, stream banks, shaded ravines).
Ecological situation:
- Water is rarely limiting.
- Air humidity is high; transpiration losses are relatively low.
Ecological characteristics:
- Often thin, large leaves with big surface areas.
- Frequently many stomata and sometimes stomata on elevated structures; little need to restrict transpiration.
- Reduced protective layers (cuticle) compared to plants in dry habitats.
Mesophytes – Plants of Moderately Moist Habitats
Mesophytes occupy habitats with intermediate, variable water availability.
Ecological situation:
- Periods with sufficient water alternate with drier phases.
- Many cultivated plants and typical forest and meadow plants belong here.
Ecological characteristics:
- Balanced leaf structure between water-saving and gas exchange efficiency.
- Ability to react physiologically to short-term drought (closing stomata, wilting as a protective reaction).
Xerophytes – Plants of Dry Habitats
Xerophytes inhabit environments where water is scarce or only available for short periods (deserts, dry steppes, dry rocky slopes, Mediterranean climates, cold deserts).
Ecological challenges:
- High evaporation due to heat, wind, and strong sunlight.
- Precipitation is rare, irregular, and often low.
- Soils may hold little water or drain quickly.
- In cold deserts, water is frozen for much of the year.
Water-saving and water-storage strategies include:
- Reduction of water loss
- Thick waxy cuticle, sometimes additional protective layers (hairy leaf surfaces, sunken stomata).
- Reduced leaf surface (small, needle-like, rolled, or even replaced by spines).
- Photosynthesis shifted to stems in some plants (green stems).
- Stomata opening mainly at cooler, more humid times (e.g., at night in certain species).
- Water storage
- Succulence: water-storing stems or leaves, sometimes also roots.
- Large vacuoles and special storage tissues.
- Special life cycle strategies
- Short-lived annual plants that germinate quickly after rare rainfalls, grow, flower, and form seeds in a very short time.
- Seeds that survive long dry periods in a resting state.
- Deciduous behavior in dry seasons (losing leaves to reduce transpiration).
From an ecological viewpoint, the diversity of xerophytic strategies allows different species to occupy slightly different niches within dry habitats.
Water and Animals: Ecological Aspects
Animals can move and therefore respond more flexibly to changing water conditions, but they still face ecological constraints related to water.
Aquatic Animals
Living permanently in water (freshwater or marine) or moving between water and land (amphibious lifestyles) involves:
- Osmotic challenges
- Freshwater animals: their body fluids are usually more concentrated than the surrounding water; they tend to gain water and lose salts.
- Marine animals: surrounding water is often more saline; many must avoid losing too much water to the environment or gaining excess salt.
- Gas exchange and movement
- Oxygen availability varies with temperature, depth, and mixing; many animals show vertical migrations or adjust activity to oxygen levels.
- Body forms, buoyancy strategies, and locomotion are shaped by the density and viscosity of water.
The details of osmoregulation and excretion are treated in other chapters; ecologically, these processes determine which aquatic habitats a species can occupy (e.g., freshwater, brackish, marine, hypersaline lakes).
Terrestrial Animals
On land, preventing water loss and obtaining drinking water are key ecological challenges.
- Water loss
- Evaporation through skin and respiratory surfaces.
- Loss via excretion (urine, feces).
- Water gain
- Drinking water, water in food.
- Metabolic water produced during cellular respiration.
Ecological adaptations (without going into physiological details):
- Activity patterns: nocturnal or crepuscular activity to avoid midday heat.
- Use of microhabitats: burrows, leaf litter, shaded areas to reduce evaporation.
- Behavioral water conservation: avoiding unnecessary movements, social structures that affect microclimate (e.g., insect nests).
- Species in very dry regions (deserts) often obtain most water from food and metabolic processes, and some never drink free water.
Water and Microorganisms
Microorganisms (bacteria, archaea, fungi, protists) are especially sensitive to water activity in their environment—how much water is freely available, not just total water content.
Ecological points:
- Some microorganisms thrive only in very moist microhabitats (e.g., wet soil pores, biofilms, water films on leaves).
- Others tolerate or even require extremely dry or salty conditions (halophiles in salt lakes, microbes in dry crusts).
The presence and activity of microorganisms are crucial for decomposition, nutrient cycles, and soil formation, and thus water availability indirectly shapes entire ecosystems.
Water Regime and Ecosystem Structure
The water regime of an area (distribution of precipitation over time, evaporation rates, soil water storage) helps define its ecosystem type:
- Deserts and semi-deserts
- Low, irregular rainfall.
- Sparse, highly specialized vegetation and animal life.
- Grasslands and savannas
- Seasonal rainfall patterns with pronounced dry and wet seasons.
- Fire regimes often linked to seasonal dryness.
- Temperate forests
- More evenly distributed precipitation.
- Soil water storage and snowmelt play important roles.
- Wetlands (marshes, bogs, fens)
- Long-term water saturation; unique flora and fauna adapted to oxygen-poor conditions.
- Tropical rainforests
- High, almost year-round rainfall.
- Very high transpiration and evaporation rates; local water cycle strongly driven by vegetation.
Water does not act in isolation: it interacts with other abiotic factors such as temperature, light, and soil properties. For example:
- Warm, dry air combined with wind leads to high evaporation.
- Cold climates can be very dry ecologically, even when snow and ice are abundant.
Water as a Dynamic Environmental Factor
Water conditions in a habitat are rarely constant:
- Seasonal changes
- Wet and dry seasons, rainy and monsoon periods.
- Snowmelt in spring leading to floods.
- Short-term fluctuations
- Droughts, storms, local flooding.
- Changes in water level in streams and lakes.
Organisms respond ecologically by:
- Migration (e.g., animals moving to water sources, fish moving between river sections).
- Dormancy (e.g., seeds, spores, resting eggs, hibernation or estivation).
- Phenological changes (timing of flowering, breeding seasons) in relation to water availability.
Long-term shifts in precipitation patterns and water availability due to climate change can reorganize entire ecosystems and their species composition.
Summary: Core Ecological Roles of Water
- Water availability is a key abiotic factor that limits where organisms and ecosystems can exist.
- Too little or too much water can be harmful; each species has its own tolerance range regarding water.
- Water conditions create large-scale (biomes) and small-scale (microhabitats, soil gradients) patterns of life.
- Plants show especially clear structural and life-cycle adaptations to water, from hydrophytes to xerophytes.
- Animals and microorganisms also have ecological strategies to cope with water stress, both in aquatic and terrestrial environments.
- Changing water regimes over time lead to dynamic responses in populations and ecosystems and are central to understanding ecological change.