Image by Calvin Teo, on wikimedia
It is well documented that baby sea turtles rely on the direction of starlight and moonlight reflected off the water's surface to help them find the ocean when they emerged from their nests. Turtles in urbanised areas go in the wrong direction, heading towards the brighter buildings and street lamps.
Artificial light can mess up coral sex and disrupt other reef activities (from Light Pollution Offers New Global Measure Of Coral Reef Health ScienceDaily 24 Nov 08). Lab studies show that light can disrupt coral reproduction, which is timed to moonlight. Light at levels that would seem insignificant to humans can be incredibly significant to marine organisms and even terrestrial organisms.
Artificial light tends to benefit predators, which is why many organisms rely on darkness to maximize their odds of survival. In Florida, lighting was seen to disrupt foraging behaviour of dune mice which naturally avoided foraging during the full moon. One beach mouse subspecies, the Pallid beach mouse, has already become extinct. (Lights From Beachfront Development Harm Endangered Beach Mice ScienceDaily 30 Nov 04)
Light can also disrupt migration patterns of birds, possibly leading them astray from food and safe passage. Most bird migration takes place at night. (Birds Migrate Together At Night In Dispersed Flocks, New Study Indicates ScienceDaily 7 Jul 08)
Light also causes bird death directly. Each year, it is estimated that millions of birds collide with communication towers. A study found that a simple alteration of the lighting scheme on these towers may reduce bird mortality by as much as 71%. (Change Of Lighting Could Drastically Reduce Bird Death By Collision With Communication Towers ScienceDaily 20 Mar 09)
Even in daylight, our urban structures create artificial polarised light that confuse wildlife (Light pollution forms 'eco-traps' Mark Kinver, BBC News 16 Jan 09)
Water is the primary source of horizontal polarised light in the natural world, and many animals - including birds, insects and reptiles - had highly developed polarisation vision. This particular form of light played a key role in the animals' lifecycle, such as finding breeding and feeding sites.
Artificial sources of polarised light: any kind of shiny, black object - oil, solar cells, asphalt - causes problems. The closer they are to wetlands, the bigger the problem.