Factors shaping the abundance of two butterflies sharing resources and enemies across a biogeographic region

Aim: Intra-specific variation in species relative abundance is
shaped by a complex interplay of abiotic and biotic factors, making it
both necessary and challenging to assess their combined relative
importance in explaining variations across space and time. We used two
congeneric butterfly species for which extensive count data and a deep
understanding of their natural history are available to test three
hypotheses explaining intra-specific variation in their abundance: (H1)
seasonal dispersal behaviour driven by climate, (H2) resource
availability, and (H3) apparent competition mediated via shared
parasitoids. Taxon: Gonepteryx rhamni (Brimstone) and G. cleopatra
(Cleopatra). Location: NE Iberian Peninsula, where both species coexist,
and a nearby archipelago (Balearic Islands), where only Cleopatra occurs.
Methods: We analyzed spatial abundance variations for both species in the
mainland and island-mainland differences in the abundance of Cleopatra.
Abiotic and biotic factors, including temperature, host plant and
overwintering habitat availability, larval parasitism and density
dependence were tested to explain the observed variations. Results: H1 can
explain variation in butterfly abundance between mainland regions since in
warmer summers populations increased in cooler areas but decreased in
warmer areas. H2 explains the variation within mainland climate regions
with a strong positive relationship between resource availability and
abundance but is unlikely to explain the island-mainland variation in the
abundance of Cleopatra. H3 could neither explain biogeographical variation
in abundance because although richer parasitoid communities were found on
the mainland, larval mortality rates were similar or lower on the mainland
than in the islands. Main conclusions: Climate and resource availability
jointly account for variation in butterfly abundance across the mainland,
but neither these factors nor parasitism can explain island-mailand
differences. Both coexisting butterfly species and their larval
parasitoids may have undergone evolutionary processes, resulting in
spatial segregation that promotes the coexistence of the two butterfly
species on the mainland.