''We are caus­ing mag­netic noise''

Down to Earth - - SCIENCE -

Sev­eral hu­man-made fields, which are stronger than the Earth's mag­netic field, are ham­per­ing nav­i­ga­tion abil­i­ties of an­i­mals, says NATHAN F PUTMAN , a post-doc­toral re­searcher at the depart­ment of fish­eries and wildlife, Ore­gon State Univer­sity, US. Edited ex­cerpts from an in­ter­view Why is the abil­ity to nav­i­gate im­por­tant for an­i­mals? Dif­fer­ent parts of an an­i­mal's range are best suited for dif­fer­ent ac­tiv­i­ties. Some lo­ca­tions might serve as good shel­ter, oth­ers as good feed­ing grounds, oth­ers as places where it is safest to re­pro­duce or rear the young. An­i­mals need to ef­fi­ciently tran­sit be­tween th­ese dif­fer­ent lo­ca­tions, whether a few me­tres apart or across con­ti­nents. How do an­i­mals en­sure they nav­i­gate cor­rectly? A but­ter­fly can­not look from Maine and see that it is mak­ing progress to­ward Mex­ico, nor can a sal­mon see from the Aleu­tian Is­lands that it is get­ting closer to­ward Van­cou­ver. What th­ese an­i­mals need is an ori­en­ta­tion sys­tem that in­forms them whether they are head­ing in the cor­rect di­rec­tion. The earth's mag­netic field pro­vides such in­for­ma­tion. They can de­ter­mine where they are on the globe, based on what the mag­netic field feels like. This is pos­si­ble be­cause there are pre­dictable gra­di­ents in the field's in­ten­sity (stronger to­ward the poles and weaker to­wards the equa­tor) and the an­gle that field lines in­ter­sect the sur­face of the earth (that is, field lines are per­pen­dic­u­lar to the sur­face at the mag­netic poles and grad­u­ally be­come par­al­lel as one moves to the mag­netic equa­tor). Th­ese two gra­di­ents form a bi­co­or­di­nate grid over many parts of the earth that al­lows an­i­mals to as­sess their lo­ca­tion.

Given that the earth's mag­netic field is quite weak (rel­a­tive to some hu­man-made fields), for an­i­mals to use such sub­tle mag­netic fields for nav­i­ga­tion re­quires an ex­quis­ite sen­si­tiv­ity. Un­for­tu­nately, this may put the an­i­mals in con­flict with some of the things that hu­mans do. How is this hap­pen­ing? Hu­mans are do­ing all sorts of things that in­crease the mag­netic "noise" in our en­vi­ron­ment. Much of this is be­ing done while we still have a very rudi­men­tary idea of how an­i­mals use mag­netic cues. Whether off­shore wind farms or wave en­ergy de­vices will dis­rupt the mi­gra­tion of marine an­i­mals is dif­fi­cult to know, but th­ese projects are mov­ing for­ward re­gard­less. What do we do to avoid the prob­lem? It would be wise for us to learn more about how an­i­mals use the earth's mag­netic field to nav­i­gate as we go for­ward mod­i­fy­ing what ap­pears to be an im­por­tant en­vi­ron­men­tal fea­ture for many species.

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