Cinzia Cantacessi

Department of Veterinary Medicine, University of Cambridge, Madingley Road CB3 0ES, Cambridge, United Kingdom 

Throughout the last decades, the world has witnessed an unprecedented biodiversity crisis, spurred by a myriad of factors including, but not limited to, climate change, urbanisation, disruption of natural ecosystems by human activities and a steady decline of animal and plant species. Such loss of biodiversity represents an existential threat to our planet, as it exacerbates food insecurity in developing countries and increases the risk of ‘spillover’ of pathogenic organisms from animals to humans and vice versa.

To mark the 2023 International Day for Biological Diversity, we would like to draw attention to an often neglected group of organisms – that of parasites.

Parasitology has long highlighted the key roles of parasites in influencing communities of (vertebrate and invertebrate) hosts. In their 2002 review article [doi.org/10.1017/S0031182002001476], Robert Poulin and Fatima Jorge comprehensively summarise evidence that metazoan parasites (= worms) profoundly shape the survival, growth, behaviour of intertidal invertebrates spanning rocky shores and soft-sediment flats.

For instance, changes in the abundances of selected host species as a consequence of parasitism may have downstream effects on the prevalence and distribution of other species, whilst parasite-mediated changes in host behaviour may translate into modifications of habitats that, in turn, may prove favourable for some species and deleterious for others. Thus, achieving a better understanding of parasite biodiversity is a key area of research that is often underestimated. 

Importantly, in a more recent article [doi.org/10.1017/S003118201800118X], Poulin and Jorge assembled a large dataset of geographical coordinates where ~5000 helminth species were first described, and applied statistical tools to assess the consistency of such reports across time and space – in other words, how much parasite biodiversity is yet to be discovered? The results were not encouraging, with the authors reporting obvious biases in helminth species discovery towards some geographical areas over others and inconsistencies in the number of new descriptions over time.

In order to fill this important gap, Poulin and Jorge propose setting up teams of taxonomists covering multiple higher taxa in order to take advantage of a limited number of host specimens, and/or banks of preserved parasite specimens awaiting descriptions by qualified taxonomists. Collections of parasites available in natural history collections represent a tremendous resource for such investigations – indeed, in their recent article [doi.org/10.1017/S0031182023000458], Sebastian Botero-Canola and Scott Gardner provide unprecedented insights into communities of helminth parasites infecting mammals in each ecoregion of the Nearctic. The authors report a positive correlation between parasite diversity and annual mean temperature, and between the latter and seasonal precipitation and parasite richness in carnivores; conversely, they report a negative correlation between seasonal precipitations and parasite diversity.

From these articles, as well as several others, it is clear that substantial gaps still exist in our understanding of parasite biodiversity, and in our current appreciation of how much of this diversity (i) has already been lost and (ii) is yet to be discovered. Filling these gaps is nonetheless extremely important, as parasite communities have co-lived and co-evolved with their hosts over millions of years; since parasite biodiversity is key to maintaining vertebrate community structure, its loss may result in significant alterations of this fine balance with unintended, far-reaching consequences for all living beings.