Editing Marine mammal (section)

===Keystone species===
{{Further|Keystone species}}
Sea otters are a classic example of a keystone species; their presence affects the ecosystem more profoundly than their size and numbers would suggest. They keep the population of certain [[benthic]] (sea floor) herbivores, particularly [[sea urchin]]s, in check. Sea urchins graze on the lower stems of [[kelp]], causing the kelp to drift away and die. Loss of the habitat and nutrients provided by kelp forests leads to profound [[Cascade effect (ecology)|cascade effects]] on the marine ecosystem. North Pacific areas that do not have sea otters often turn into [[urchin barren]]s, with abundant sea urchins and no kelp forest.<ref>{{Cite journal|title = Killer Whale Predation on Sea Otters Linking Oceanic and Nearshore Ecosystems|journal = Science|year=1998|issn = 0036-8075|pmid = 9774274|pages = 473–476|volume = 282|issue = 5388|doi = 10.1126/science.282.5388.473|first1 = J. A.|last1 = Estes|first2 = M. T.|last2 = Tinker|first3 = T. M.|last3 = Williams|first4 = D. F.|last4 = Doak|bibcode = 1998Sci...282..473E| s2cid=8925215 }}</ref> Reintroduction of sea otters to British Columbia has led to a dramatic improvement in the health of coastal ecosystems,<ref name=dfo>{{cite web|url=http://www.dfo-mpo.gc.ca/species-especes/profiles-profils/seaOtter-loutredemer-eng.html|title=Aquatic Species at Risk&nbsp;– Species Profile&nbsp;– Sea Otter|publisher=Fisheries and Oceans Canada|access-date=29 November 2007}}</ref> and similar changes have been observed as sea otter populations recovered in the Aleutian and Commander Islands and the [[Big Sur]] coast of California.<ref name = vanblaricom/> However, some kelp forest [[ecosystem]]s in California have also thrived without sea otters, with sea urchin populations apparently controlled by other factors.<ref name = vanblaricom/> The role of sea otters in maintaining kelp forests has been observed to be more important in areas of open coast than in more protected bays and [[estuaries]].<ref name=vanblaricom/>

{{Multiple image|align=left|image1=Antarctic Fur Seal Pups play amid Tussock Grass (5723988869).jpg|alt1=Two furry, dark-brown seal pups in the sand, sitting next to some tall, green grass|width2=150|image2=Blanchon-idlm2006.jpg|alt2=A white seal pup on the snowy ground with large black eyes and nose|footer=[[Antarctic fur seal]] pups (left) vs. Arctic [[harp seal]] pup (right)}}
An apex predator affects prey population dynamics and defense tactics (such as camouflage).<ref>Lepak, Jesse M.; Kraft, Clifford E., Weidel, Brian C. (March 2006). [http://www2.dnr.cornell.edu/cek7/Publications/Lepak_et_al_2006.pdf "Rapid food web recovery in response to removal of an introduced apex predator"] (PDF).  ''Canadian Journal of Fisheries and Aquatic Sciences'' '''63''' (3): 569–575. {{ISSN|0706-652X}}. {{open access}}</ref> The polar bear is the apex predator within its range.<ref name=apex1/> Several animal species, particularly [[Arctic fox]]es (''Vulpes lagopus'') and [[glaucous gull]]s (''Larus hyperboreus''), routinely scavenge polar bear kills.<ref name="behavior"/> The relationship between ringed seals and polar bears is so close that the abundance of ringed seals in some areas appears to regulate the density of polar bears, while polar bear predation in turn regulates density and reproductive success of ringed seals.<ref>{{cite book |last1=Amstrup |first1=Steven C. |last2=Marcot |first2=Bruce G. |last3=Douglas |first3=David C. |year=2007 |title=Forecasting the range-wide status of polar bears at selected times in the 21st Century |location=Reston, Virginia |publisher=U.S. Geological Survey |url=http://www.plexusowls.com/PDFs/forecasting_polar_bears_amstrup_etal_lowres.pdf}}</ref> The [[evolutionary pressure]] of polar bear predation on seals probably accounts for some significant differences between Arctic and [[Antarctica|Antarctic]] seals. Compared to the Antarctic, where there is no major surface predator, Arctic seals use more breathing holes per individual, appear more restless when hauled out on the ice, and rarely defecate on the ice.<ref name="behavior"/> The fur of Arctic pups is white, presumably to provide [[camouflage]] from predators, whereas Antarctic pups all have dark fur.<ref name="behavior">{{cite book |last1=Stirling |first1=Ian|first2=Dan |last2=Guravich|year=1988 |title=Polar Bears |location=Ann Arbor, MI|publisher=University of Michigan Press |pages=27–28|url={{Google books|plainurl=yes|id=ViOiGWPQRjIC|page=27}}|isbn=978-0-472-10100-9|oclc=757032303}}</ref>

Killer whales are apex predators throughout their global distribution, and can have a profound effect on the behavior and population of prey species. Their diet is very broad and they can feed on many vertebrates in the ocean including [[salmon]],<ref>{{cite book|first1=Lynne M.|last1=Barre|first2=J. B.|last2=Norberg|first3=Gary J.|last3=Wiles|year=2005|title=Conservation Plan for Southern Resident Killer Whales (''Orcinus orca'')|publisher=National Marine Fisheries Service (NMFS) Northwest Regional Office|location=Seattle|page=18|url=http://www.nwr.noaa.gov/Marine-Mammals/Whales-Dolphins-Porpoise/Killer-Whales/Conservation-Planning/upload/SRKW-propConsPlan.pdf|archive-url=https://web.archive.org/web/20080626121719/http://www.nwr.noaa.gov/Marine-Mammals/Whales-Dolphins-Porpoise/Killer-Whales/Conservation-Planning/upload/SRKW-propConsPlan.pdf|url-status=dead|archive-date=2008-06-26}}</ref> rays, sharks (even [[white shark]]s),<ref>{{cite journal|last1=Pyle|first1=Peter|last2=Schramm|first2=Mary Jane|last3=Keiper|first3=Carol|last4=Anderson|first4=Scot D.|year=1999|title=Predation on a white shark (''Carcharodon carcharias'') by a killer whale (''Orcinus orca'') and a possible case of competitive displacement|journal=Marine Mammal Science|volume=15|issue=2|pages=563–568|doi=10.1111/j.1748-7692.1999.tb00822.x|bibcode=1999MMamS..15..563P |url=http://www.prbo.org/cms/docs/marine/MMS.pdf|access-date=2016-08-02|archive-date=2012-03-22|archive-url=https://web.archive.org/web/20120322070431/http://www.prbo.org/cms/docs/marine/MMS.pdf|url-status=dead}}</ref><ref>{{cite journal|last=Visser|first=Ingrid N.|year=2005|title=First Observations of Feeding on Thresher (''Alopias vulpinus'') and Hammerhead (''Sphyrna zygaena'') Sharks by Killer Whales (''Orcinus orca'') Specialising on Elasmobranch Prey|journal=Aquatic Mammals|volume=31|issue=1|pages=83–88|doi=10.1578/AM.31.1.2005.83}}</ref> large baleen whales,<ref>{{cite journal|last1=Ford|first1=J. K. B.|last2=Reeves|first2=R. R.|year=2008|title=Fight or flight: antipredator strategies of baleen whales|journal=Mammal Review|volume=38|issue=1|pages=50–86|doi=10.1111/j.1365-2907.2008.00118.x|citeseerx=10.1.1.573.6671}}</ref> and nearly 20 species of pinniped.<ref>{{cite book|last1=Heimlich|first1=Sara|last2=Boran|first2=James|year=2001|title=Killer Whales|location=Stillwater, Minnesota|publisher=Voyageur Press|isbn=978-0-89658-545-4|oclc=46973039}}</ref> The predation of whale calves may be responsible for annual whale migrations to calving grounds in more tropical waters, where the population of killer whales is much lower than in polar waters. Prior to [[whaling]], it is thought that great whales were a major food source; however, after their sharp decline, killer whales have since expanded their diet, leading to the decline of smaller marine mammals.{{sfn|Perrin|2009|loc=[https://books.google.com/books?id=2rkHQpToi9sC&pg=PA360 p. 360]}} A decline in Aleutian Islands sea otter populations in the 1990s was controversially attributed by some scientists to killer whale predation, although with no direct evidence. The decline of sea otters followed a decline in [[harbor seal]] and [[Steller sea lion]] populations, the killer whale's preferred prey, which in turn may be substitutes for their original prey, now reduced by industrial whaling.<ref>{{cite journal|last1=Springer|first1=A. M.|title=Sequential megafaunal collapse in the North Pacific Ocean: An ongoing legacy of industrial whaling?|journal=Proceedings of the National Academy of Sciences|volume=100|pages=12223–12228|year=2003|doi=10.1073/pnas.1635156100|issue=21|bibcode=2003PNAS..10012223S|pmid=14526101|pmc=218740|doi-access=free}}</ref><ref>{{cite journal|last1=Demaster|first1=D|last2=Trites|first2=A|last3=Clapham|first3=P|last4=Mizroch|first4=S|last5=Wade|first5=P|last6=Small|first6=R|last7=Hoef|first7=J|title=The sequential megafaunal collapse hypothesis: Testing with existing data|journal=Progress in Oceanography|year=2006|doi=10.1016/j.pocean.2006.02.007|volume=68|issue=2–4|pages=329–342|bibcode=2006PrOce..68..329D}}</ref><ref>{{cite journal|last1=Estes|first1=J. A.|last2=Doak|first2=D. F.|last3=Springer|first3=A. M.|last4=Williams|first4=T. M.|title=Causes and consequences of marine mammal population declines in southwest Alaska: a food-web perspective|journal=Philosophical Transactions of the Royal Society B: Biological Sciences|volume=364|pages=1647–1658|year=2009|doi=10.1098/rstb.2008.0231|issue=1524|pmid=19451116|pmc=2685424}}</ref>