Frequency-dependent fitness and reproductive dynamics contribute to habitat segregation in sympatric Jewelflowers

Coexistence results from a complex suite of past and
contemporary processes including biogeographic history, adaptation,
ecological interactions, and reproductive dynamics. Here we explore
drivers of local micro-parapatry in which two closely related and
reproductively isolated Streptanthus species (Jewelflower,
Brassicaceae) inhabit continuous or adjacent habitat patches and occur
within seed dispersal range, yet rarely overlap in fine-scale
distribution. We find some evidence for abiotic niche partitioning
and local adaptation, however differential survival across habitats
cannot fully explain the scarcity of coexistence. Competition may
also reduce the fitness of individuals migrating into occupied
habitats, yet its effects are insufficient to drive competitive
exclusion. Experimental migrants suffered reduced seed production and
seed viability at sites occupied by heterospecifics, and we infer that
heterospecific pollen transfer by shared pollinators contributes to
wasted gametes when the two congeners come into contact. A minority
disadvantage may reduce effective colonization of patches already
occupied by heterospecifics, even when habitat patches are
environmentally suitable. Differential adaptation and resource
competition have often been evoked as primary drivers of habitat
segregation in plants, yet negative reproductive interactions –
including reproductive interference and decreased fecundity among
low-frequency migrants – may also contribute to non-overlapping
distributions of related species along local tension zones.
Methods
All data (.csv files) and code (.R) files used for analyses.  Code files conduct tests and create figures presented in the article.