An important component of biodiversity at a given time is mostly represented by juveniles
Immature specimens represented 59% of all the captured specimens on average, ranging between 39% - 76% per plot. This result is similar to those obtained in other studies (Soukainen et al, 2020, Malumbres-Olarte et al, 2020a, Malumbres-Olarte et al, 2019; Cardoso et al 2004; Russell-Smith & Stork 1995, Silva 1996), which ranged from 40% to 70%, and offers a first insight on the relevance of juvenile stages in spider inventories and the conclusions derived from them.
Overall, the number of species estimated from juveniles by metabarcoding was lower than that of adults. This could suggest that, at the time of the samplings, there were fewer species in the juvenile stage than in the adult stage. However, this finding could well be an artifact related to the sampling methods, as immature spiders are smaller than adults and may be more difficult to detect by direct sampling techniques. The combination of both life stages yielded a total of 491 different species, 35% higher than the richness we obtained considering only adults in the whole sampling. The degree to which immature stages contributed to diversity, however, was not constant across all our plots. In almost half of the individual samplings, the total number of species including juveniles more than doubled the richness obtained with adults. These results indicate that a large part of the diversity may be ignored by spider bioinventories that are performed exclusively on adults. Interestingly, the contribution of juveniles was higher in southern parks than in northern ones. Although knowing the reasons behind this pattern would require further study, we suspect the difference may be related to a phenological delay between both latitudes.
Although most families recovered a similar species richness in adult and in juvenile stages, some had important differences (Fig. 3). In the case of araneids, the fact that the number of captured juveniles was almost twice as high as the number of adults may have made the number of species captured as immatures greater because it increased the chances of sampling additional species for this stage. The additional diversity found only as juvenile spiders may also be related to the phenology of this family. Larger orb-weavers (araneids) mature in autumn in temperate zones, while smaller species tend to mature earlier (Levi, 1973). This observation fully matches our findings, as most of the araneids that were found exclusively as juveniles are large species of orb-weavers, such as Argiope lobata Pallas, 1772, Larinioides patagiatus (Clerck, 1757) or several Araneus species.
Also interesting is the case of linyphiids sheet-weavers, whose adult species richness was higher than juvenile richness in the northern parks (as much as twice as high in PP), but lower in the southern parks. Linyphiids are known to be much more diverse in temperate regions than in the subtropics and tropics (Cardoso, Pekár, Jocqué & Coddington, 2011). We indeed found more total species in northern parks, which have a more continental climate, than in southern parks, with a significantly drier and warmer climate. The differences in adult and juvenile richness between north and south, however, may indicate the existence of different predominant phenologies within linyphiids at both latitudes.
Although our data had a large taxonomic scope (order level), there are still some caveats to our analyses. All the MOTUs that could not be assigned to any nominal species could provide important additional information. Nevertheless, it is expected that completeness of reference databases will increase in coming years and the accuracy of taxonomic assignments in metabarcoding studies will improve.