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.