• demographic swamping;
  • establishment disadvantage;
  • fitness;
  • habitat differentiation;
  • hybridization;
  • introduced species;
  • Moraceae;
  • Morus alba;
  • Morus rubra;
  • small populations


  • 1
    The effects of hybridization on the abundance of parental taxa depends on their relative frequency, the viability of hybrid offspring and the degree of ecological differentiation among parental and hybrid taxa. Habitat overlap can facilitate competition for suitable sites and threaten the persistence of parental taxa, especially those in small populations.
  • 2
    Here we examine ecological differentiation between the endangered North American red mulberry (Morus rubra), introduced white mulberry (M. alba) and red × white hybrids in a reciprocal transplant experiment. Fitness of red, white and hybrid mulberry was estimated as survival and above-ground biomass of seedling and juvenile life stages, transplanted into open (white habitat) and shaded (red habitat) forest environments. In addition, all taxa, including reciprocal hybrids (R × W, W × R), were compared in a common garden in full sun.
  • 3
    In the reciprocal transplant study, red mulberry was consistently less fit than white and hybrid mulberry regardless of transplant habitat; F1 hybrids were as fit as white mulberry. In the common garden, red mulberry and hybrids with red mothers had lower fitness than white mulberry and hybrids with white mothers. Reciprocal hybrid crosses differed significantly with respect to survival and cumulative fitness, but not biomass.
  • 4
    Red mulberry is not ecologically differentiated from white or hybrid mulberry in the three transplant environments examined; rather, it is consistently the least fit taxon. Therefore, all else being equal, hybridization with white mulberry and the subsequent presence of hybrids will place red mulberry at a strong disadvantage during establishment.
  • 5
    The results highlight the potential effect of hybridization by introduced species on the abundance of native species, particularly at the northern extremes of its geographical range, where populations are small and native environments may be degraded or at the limits of ecological tolerance.