Guano manure is an effective fertilizer and gunpowder ingredient due to its high levels of phosphorus and nitrogen and also its lack of odor. Superphosphate made from guano is used for aerial topdressing. Soil that is deficient in organic matter can be made more productive by addition of this manure.
It is estimated that there is only enough phosphorus from current resources to last about 30 years. Currently vast volumes of phosphorus are needed to produce fertilizer, as it is an essential plant macronutrient. Guano is rich in phosphorus and is an effective phosphorus fertilizer.
Guano has been harvested over several centuries along the coast of Peru, where islands and rocky shores have been sheltered from humans and predators. The Guanay Cormorant has historically been the most important producer of guano; its guano is richer in nitrogen than guano from other seabirds. Other important guano producing species off the coast of Peru are the Peruvian Pelican and the Peruvian Booby.
The high concentration of nitrates also made guano an important strategic commodity. The War of the Pacific (1879 to 1883) between the Peru-Bolivia alliance and Chile was primarily based upon Bolivia's attempt to tax Chilean guano harvesters and over control of a part of the Atacama desert that lies between the 23rd and 26th parallels on the Pacific coast. The discovery during the 1840s of the use of guano as a fertilizer and saltpeter as a key ingredient in explosives made the area strategically valuable.
In this context the US passed the Guano Islands Act in 1856 giving citizens discovering a source of guano the right to take possession of unclaimed land and entitlement to exclusive rights to the deposits. However, the guano could only be removed for the use by citizens of the United States. This enabled US citizens to take possession of unoccupied islands containing guano.
By the end of the 19th Century, the importance of guano declined with the rise of artificial fertiliser, although guano is still used by organic gardeners and farmers.
One of the major innovators in guano harvesting was Benjamin Drake Van Wissen, an Australian engineer who at the beginning of the 20th Century managed mining operations of large guano deposit located on Nauru, designing an efficient harvesting machine for the guano that maximized the extraction of phosphates.
The ideal type of guano is found in exceptionally dry climates, as rainwater drains the guano of nitrates. Guano is harvested on various islands in the Pacific Ocean (for example, the Chincha Islands and Nauru) and in other oceans (for example, Juan de Nova Island and Christmas Island). These islands have been home to mass seabird colonies for many centuries, and the guano has collected to a depth of many metres. In the 19th century, Peru was famous for its supply of guano.
Bat guano is usually mined in caves and is associated with a corresponding loss of troglobytic biota and diminishing of biodiversity. Guano deposits support a great variety of cave-adapted invertebrate species, which rely on bat feces as their sole nutrient input. In addition to the biological component, deep guano deposits contain local paleoclimatic records in strata that have built up over thousand of years, which are unrecoverable once disturbed.
The greatest damage caused by mining to caves with extant guano deposits is to the bat colonies themselves. Bats are highly vulnerable to regular disturbance to their roosts. Some species, such as Phyllonycteris aphylla, have low fat reserves, and will starve to death when regularly disturbed and put into a panic state during their resting period. Many species will drop pups when in panic, with subsequent death, leading to a steady reduction in population. Research in Jamaica has shown that mining for bat guano is directly related to the loss of bat species, associated invertebrates and fungi, and is the greatest threat to bat caves on the island.