Is the world wormier than it used to be? We'll never know without natural history collections

Abstract

Many disease ecologists and conservation biologists believe that the world is wormier than it used to be—that is, that parasites are increasing in abundance through time. This argument is intuitively appealing. Ecologists typically see parasitic infections, through their association with disease, as a negative endpoint, and are accustomed to attributing negative outcomes to human interference in the environment, so it slots neatly into our worldview that habitat destruction, biodiversity loss and climate change should have the collateral consequence of causing outbreaks of parasites. But surprisingly, the hypothesis that parasites are increasing in abundance through time remains entirely untested for the vast majority of wildlife parasite species. Historical data on parasites are nearly impossible to find, which leaves no baseline against which to compare contemporary parasite burdens. If we want to know whether the world is wormier than it used to be, there is only one major research avenue that will lead to an answer: parasitological examination of specimens preserved in natural history collections. Recent advances demonstrate that, for many specimen types, it is possible to extract reliable data on parasite presence and abundance. There are millions of suitable specimens that exist in collections around the world. When paired with contemporaneous environmental data, these parasitological data could even point to potential drivers of change in parasite abundance, including climate, pollution or host density change. We explain how to use preserved specimens to address pressing questions in parasite ecology, give a few key examples of how collections‐based parasite ecology can resolve these questions, identify some pitfalls and workarounds, and suggest promising areas for research. Natural history specimens are ‘parasite time capsules’ that give ecologists the opportunity to test whether infectious disease is on the rise and to identify what forces might be driving these changes over time. This approach will facilitate major advances in a new sub‐discipline: the historical ecology of parasitism.

How to use natural history collections to resurrect information on historical parasite abundances

“…By the very nature of the time period that they most comprehensively cover (approximately the past century; Wood & Vanhove, 2022), natural history collections lend themselves to answering questions about anthropogenic change in parasite communities. One powerful design approach that can harness retrospective data such as these is before-after-control-impact (BACI).…”

“…There is a new frontier in helminthology: time (Harmon et al , 2019; Cook et al , 2020; Thompson et al , 2021; Wood & Vanhove, 2022). Many of the most contentious questions that concern the ecology of helminths could be resolved with data on helminth abundance and host use in the past few decades or centuries.…”

“…This handful represents just a tiny fraction of the total number of parasite species on Earth; the helminth endoparasites of vertebrates alone number between 100,000 and 350,000 species (Carlson et al , 2020b). Palaeo-parasitological and archaeo-parasitological techniques, like the quantification of helminth eggs in coprolites (Wood, 2018), can shed light on change in parasite burdens over deep time horizons (thousands to millions of years; Wood & Vanhove, 2022). But if helminthologists want a means of resurrecting information on many parasite species across the Anthropocene (i.e.…”

“…Simple parasitological dissections of specimens held in natural history collections can yield reliable information on the helminth and arthropod parasites of vertebrate hosts (Fiorenza et al , 2020). Because the world's museums hold billions of suitable specimens collected over more than a century (Constable et al , 2010; Holmes et al , 2016; Harmon et al , 2019), these potential parasitological datasets are broad in scope and finely resolved in taxonomic, temporal and spatial dimensions (Wood & Vanhove, 2022). Those museums that allow researchers access to specimens usually do so at no cost, meaning that the only expenses associated with this work are in labour, travel, shipping and consumable supplies – no field excursions, permits, or animal use approvals are needed, and no new vertebrates are sacrificed.…”

A reconstruction of parasite burden reveals one century of climate-associated parasite decline

“…For many other groups of organisms, ecologists rely on long-term data to infer current population status and trends and to project future threats: for example, some insects ( 9 ), amphibians ( 10 ), mammals ( 11 ), and birds ( 12 ) are on the decline, whereas invasive species ( 13 ) and synanthropic species ( 14 ) are on the rise. Given their substantial ecological influence, it would be useful to have similar long-term data for parasites ( 15 ), but almost no such data exist for parasites of nonhuman hosts ( 16 , 17 ). Those few datasets that do exist are plagued with issues of accessibility, completeness, temporal scope, and taxonomic coverage ( 17 , 18 ).…”