Parasites occupy a nuanced position in ecosystems, performing essential ecological tasks while also presenting risks to their hosts. Research from the University of Washington shows that fish parasite abundance declined as waters warmed from 1880 to 2019. This trend matters because many parasites help move energy through food webs and aid in regulating wildlife populations. The study, published in the Proceedings of the National Academy of Sciences, represents the largest global dataset on wildlife parasite abundance to date. It indicates that parasites are especially sensitive to a changing climate, a finding with wide ecological implications.
“Many people expect climate change to trigger parasite outbreaks as the world warms,” one scientist noted, underscoring a nuanced picture: parasites depend on hosts and can be highly sensitive to environmental shifts. Parasites that require three or more host species during their life cycles, including a large portion of fish-associated parasites, showed pronounced declines when historical fish samples were analyzed. On average, they decreased by about 11 percent per decade. In fact, nine out of ten parasite species that disappeared entirely around 1980 depended on three or more hosts.
Worrying for ecosystems
Results suggest that single-host parasites remained relatively steady, while those with multiple hosts collapsed more dramatically. When declines affect species valued by humans, conservation actions typically follow. Although parasites can evoke fear or disgust, the outcome is troubling for ecosystems, as it signals disruptions in energy transfer within food webs and potential impacts on top predators.
The study highlights that parasites with complex life cycles play a key role in diverting energy through food webs and supporting the largest predators in the ecosystem. This work contributes to a broader plan aimed at protecting parasite diversity where it matters for ecological balance.
Another finding involves a new approach to reconstruct past parasite populations. In this method, mammals and birds are preserved using skin or feathers, while fish and amphibians are preserved in liquid to retain the parasites present at death. Researchers examined fish samples held in natural history collections and identified a range of multicellular parasites, including crustaceans, and striking tapeworms with hook-covered heads. The team counted a total of 17,259 parasites from 85 species in 699 fish specimens.
The warmth of the sea, a culprit
Three potential causes were considered for the observed parasite declines: the abundance of host species, pollution levels, and sea surface temperature. The strongest explanatory factor was ocean warming. In the study area, sea surface temperature rose by about one degree between 1950 and 2019, aligning with the observed reductions in parasite counts.
One investigator described the lifecycle of host-dependent parasites as a delicate machine, where multiple steps must align for successful development. Any disruption along this chain can lead to outages in parasite production. The researchers note a notable reduction in parasite abundance at Puget Sound and express concern that similar patterns may be spreading beyond this region.
As the ecosystem story unfolds, it remains possible that similar trends occur in other well-studied systems. The researchers advocate attention to parasite dynamics across diverse habitats, suggesting that these patterns could reflect broader planetary changes rather than isolated events.
To advance this line of inquiry, a new emphasis on museum collections and specimen preservation is helping scientists link past and present parasite communities. The approach encourages ecologists to identify focal ecosystems, locate the right specimens, and assess whether these trends are unique to a single region or more widely distributed. The implications extend to how human activities intersect with parasite diversity and ecosystem services depending on them.
Overall, the findings point to the possibility that parasitic species are under pressure, with potential consequences for human well-being as fewer parasite-derived ecological services are available. The work cited here adds to a growing body of evidence on how climate-driven changes ripple through food webs and affect conservation outcomes. For more information, see the study cited in the Proceedings of the National Academy of Sciences. (Citation: PNAS, 2023).