Abiotic Factors

Abiotic Factors Shaping Life

Abiotic factors are the non-living, physical and chemical components of the environment — light, temperature, water, soil, and altitude among them — that constrain and shape the distribution, physiology and behaviour of organisms.

Light

Light drives photosynthesis and also acts as a signal that organisms use to time their activities.

• Phototaxis is a locomotory response in which a motile organism moves its whole body either towards light (positive phototaxis, as in many phytoplankton and insects attracted to a lamp) or away from light (negative phototaxis, as in earthworms and many soil organisms that burrow away from illuminated surfaces).
• Photokinesis, by contrast, is not a directed movement towards or away from a source — it is a change in the speed of random movement in response to light intensity. An organism showing photokinesis moves faster or slower depending on how much light there is, without orienting itself towards or away from the source.
• Biological rhythms: many physiological and behavioural activities repeat in regular cycles keyed to environmental light cycles. A circadian rhythm is one that repeats on an approximately 24-hour cycle — sleep-wake cycles, hormone release, and body-temperature fluctuation in animals are classic circadian rhythms, persisting even in constant laboratory conditions because they are driven by an internal "biological clock" that is normally reset (entrained) daily by the light-dark cycle.
• Photoperiodism is the response of an organism to the relative duration of light and darkness in a 24-hour cycle (day length). Photoperiod controls seasonal reproduction and migration in many animals and flowering time in many plants — animals use lengthening or shortening day length as a reliable seasonal cue, more reliable than temperature, to time breeding so that offspring appear when food is plentiful.
• Bioluminescence is the biochemical production and emission of light by a living organism, typically through an enzyme-catalysed (luciferase-luciferin) reaction. Fireflies use bioluminescent flashes for mate attraction and signalling; many deep-sea fish and other marine organisms use bioluminescence for communication, camouflage (counter-illumination) or luring prey in the permanently dark ocean depths.
• Ultraviolet (UV) radiation, an invisible high-energy component of sunlight, can damage DNA and proteins in exposed tissues. Excess UV exposure is linked to skin cancers and cataracts in humans and can suppress immune function; many organisms possess pigments (such as melanin) or behavioural strategies (nocturnal activity, seeking shade) that reduce UV damage.

Temperature

Temperature governs the rate of virtually every biochemical reaction inside an organism, which is why it is one of the most powerful abiotic factors of all.

• Van't Hoff's rule states that the rate of a biochemical/physiological reaction roughly doubles (or more) for every 10°C rise in temperature, within the tolerable range — this is why cold-blooded (ectothermic) animals become sluggish in cold weather and hyperactive in warm weather, and why metabolic rate in most organisms is strongly temperature-dependent.
• Every species has a minimum effective temperature (below which its metabolism effectively stops), a maximum effective temperature (above which proteins denature and function fails), and an optimum effective temperature at which its physiological processes and growth are most efficient. This tolerance range defines much of a species' geographic distribution.
• Cyclomorphosis is a striking example of temperature (and associated seasonal factors) shaping body form over the course of a year, seen classically in the freshwater crustacean Daphnia (water flea). Across the seasons, Daphnia individuals show cyclical changes in body shape — for instance, developing a taller helmet-like head and longer tail spine in warmer months (when predation pressure from fish that hunt by sight is often higher and the changed shape may reduce predation or aid suspension in less viscous warm water) and reverting to a rounder, more compact form in cooler months. Because the same genetic population changes shape with the seasons rather than through natural selection acting across generations, cyclomorphosis is regarded as a direct, reversible response to seasonally changing abiotic conditions (see figure: cyclomorphosis in Daphnia).

Soil

Soil supplies anchorage, water and mineral nutrients to plants, and its texture, structure, pH, mineral content and moisture-holding capacity strongly influence which plants — and, through them, which animals — can live in a given area. Soil-dwelling organisms (earthworms, burrowing insects, soil microbes) are themselves shaped by soil texture and aeration, and in turn contribute to soil fertility through decomposition of organic matter.

Water and Osmotic Adaptations

Water availability and its salt concentration relative to an organism's body fluids create a constant osmotic challenge.

• Stenohaline animals can tolerate only a narrow range of external salinity (most strictly marine or strictly freshwater species), while euryhaline animals can tolerate a wide range of salinity (such as salmon, which move between sea water and fresh water during their life cycle, or many estuarine species). …