Biogeographic range shifts in dolphins and porpoises
Biogeographic range shifts in dolphins and porpoises refer to the changes in the distribution of these marine mammals in response to environmental changes, particularly those driven by climate change. Dolphins and porpoises inhabit various aquatic ecosystems worldwide, with most species found in marine environments and a few in freshwater. As global temperatures rise, many species are likely to shift their ranges toward cooler waters, impacting species diversity and interactions. For instance, in regions like the British Isles, warmer waters have led to an increase in warm-water dolphin species while reducing the presence of cold-water species. These shifts also affect their food sources, as changes in water temperature can disrupt food webs, leading to scarcity and altering competitive dynamics among species.
Additionally, climate change poses threats such as habitat fragmentation, which can affect genetic diversity and population stability, particularly for freshwater species like river dolphins. Ocean acidification further complicates the situation by impacting prey availability and disturbing sound transmission in water, crucial for echolocation in dolphins and porpoises. Overall, the biogeographic range shifts of these mammals highlight the broad ecological implications of climate change, affecting not only their survival but also the health of marine ecosystems.
Subject Terms
Biogeographic range shifts in dolphins and porpoises
Definition
Dolphins and porpoises are aquatic, carnivorous mammals found worldwide from polar to tropical waters. Most species are marine, with a few having freshwater populations, but only the Indian River dolphin (Platanista gangetica), La Plata River dolphin (Pontoporia blainvillei), and the Yangtze River dolphin (Lipotes vexillifer) are strictly relegated to freshwater habitats.
The term dolphin can be used to refer to the Odontoceti (toothed whales), but here it will be used to refer to the dolphin family Delphinidae (approximately thirty-six species), the four families of river dolphins (one species apiece), and the porpoise family Phocoenidae (six species). Taxonomic uncertainty makes the exact number of species an open question.
Dolphins and porpoises face a variety of threats, including targeted hunting, entanglement in fishing gear, pollution, disease, habitat degradation, water diversion, acoustic disturbance, and competition with fisheries. The 2021 IUCN Red Lists five species and six subspecies of the forty-one dolphin species as endangered, and extensive surveys suggest that the Yangtze River dolphin is almost extinct.
Significance for Climate Change
Global warming is expected to induce biogeographic range shifts in dolphins and porpoises, as water temperature is often a good predictor of the presence of these species. For example, off the coast of Scotland, an increase in the number of warm-water species since the 1980s was coincident with an increase in water temperatures over the same period, while the abundance of cold-water species diminished. Around the British Isles in general, the white-beaked dolphin (Lagenorhynchus albirostris) prefers colder waters, and the short-beaked common dolphin (Delphinus delphis) prefers warmer waters. If these waters warm further, the short-beaked dolphin could expand its range at the expense of the white-beaked dolphin.
On global scales, the diversity of whales (including dolphins and porpoises) in deep water is maximum at sea surface temperatures of 21° Celsius, which occur commonly at midlatitudes. When applied to warming scenarios for the next few decades from the Intergovernmental Panel on Climate Change, these results suggest that biogeographic ranges would shift so that the diversity of whales in the tropics would decline while the diversity in higher latitudes would increase.
Climate change could induce range shifts in dolphins and porpoises by affecting species lower in the food web and thereby making preferred food items less abundant. This could also change competitive relationships between species. For instance, the 1982-1983 El Niño-Southern Oscillation event caused a reduction in squid off of California and coincided with the displacement of short-finned pilot whales (Globicephala macrorhynchus) by Risso’s dolphins (Grampus griseus), which are able to dive more deeply to obtain scarce prey.
Climate change might also fragment dolphin and porpoise populations, which has implications for genetic diversity and generally reduces population persistence. For instance, water diversions for shipping and irrigation have already fragmented the populations of the three freshwater river dolphins. If precipitation patterns change, so too could the demand for water withdrawals and impoundments, meaning that the ranges of these species could become further fragmented. Climate change could also fragment marine populations. For example, warming could enhance the degree to which the warm-water current flowing through the Bay of Biscay divides northern and southern populations of the harbor porpoise (Phocoena phocoena).
Ocean acidification, a result of dissolving into marine waters, could affect the food supply of dolphins and porpoises. Dolphins and porpoises are often the top predators in food webs based on organisms that have (CaCO3) body structures, which are susceptible to dissolution in acidified waters. For example, the pantropical spotted dolphin (Stenella attenuata) consumes pteropods, which are known to be sensitive to acidification. Moreover, many species of fish that are prey to dolphins and porpoises are dependent on coral reef habitats, which are susceptible to warming-induced bleaching and decalcification from acidification.
Another consequence of ocean acidification is that acidified water absorbs sound less effectively, making the environment “noisier.” Dolphins and porpoises depend upon echolocation to locate prey, discern their environment, and locate one another. Many species already suffer from acoustic disturbances caused by boat traffic, seismic exploration, and military exercises, which can propagate tens of kilometers underwater, and such disturbances are associated with behavioral changes, strandings, internal injury, and death. Deep water and waters at high latitudes are expected to be especially affected since the acidification reaction is temperature-dependent.
Global warming could exacerbate the threats of disease and pollution. Warmer temperatures and shifting currents are expected to extend the range of disease-causing agents. For example, warmer temperatures are associated with outbreaks of the bacterium Vibrio known to infect dolphins. Climate change might also influence the frequency and severity of harmful algal blooms.
Bibliography
Burns, William C. G., and Alexander Gillespie, eds. The Future of Cetaceans in a Changing World. Ardsley, N.Y.: Transnational, 2003.
Culik, Boris M. Review of Small Cetaceans: Distribution, Behavior, Migration, and Threats. Bonn, Germany: United Nations Environment Programme/Convention on Migratory Species Secretariat, 2004.
Reeves, Randall R., et al., comps. Dolphins, Whales, and Porpoises: 2002-2010 Conservation Action Plan for the World’s Cetaceans. Gland, Switzerland: International Union for Conservation of Nature, 2003.
Soulé, Michael E., Elliot A. Norse, and Larry B. Crowder. Marine Conservation Biology: The Science of Maintaining the Sea’s Biodiversity. Washington, D.C.: Island Press, 2005.
Temple, Andrew H., Ute Langner, and Michael L. Berumen. "Management and Research Efforts Are Failing Dolphins, Porpoises, and Other Toothed Whales." Scientific Reports, vol. 14, no. 7077, 25 Mar. 2024, doi.org/10.1038/s41598-024-57811-7. Accessed 20 Dec. 2024.