Light

Light is an essential abiotic resource whose ecological effect depends on three separate properties: its quality (wavelength or colour), its intensity (the actual energy delivered, measured in units like gram-calories), and its duration (day length, which itself varies seasonally). Light governs growth, pigmentation, migration timing and reproductive cycling in many species — birds' gonads, for instance, become more active as day length increases. High-intensity or high-frequency light such as UV or X-rays can even induce gene mutations. Because light is essential for vision, organisms adapted to permanently dark habitats, such as cave-dwellers, often show poorly developed eyes or have lost them altogether. Diapause (a suspended developmental phase) in some animals is also triggered by changing light conditions. Three specific light-driven behaviours are recognised: phototaxis is directed movement of a whole organism toward light (as in moths, positive phototaxis) or away from it (as in Euglena, Volvox and earthworms, negative phototaxis); phototropism is directed growth or orientation toward light (as in a sunflower turning to face the sun, positive phototropism) or away from it (as in plant roots growing away …