Background/Question/Methods
Two major biotic factors known to reduce plant performance and abundance are plant competition and insect herbivory. Although both factors have been demonstrated to severely limit the individual performance and population dynamics of some plants, the combined effects of competition and insect herbivory on regeneration probability, especially on an introduced plant species, are less known. We experimentally evaluated the combined effects of competition exerted by neighbouring prairie vegetation and herbivory imposed by native insects on seedling establishment, growth and survival of the relatively sparse, introduced Cirsium vulgare (bull thistle), compared to seedling response of the more common, co-occurring native congener, C. altissimum (tall thistle). Based on the biotic resistance hypothesis, we predicted that the interspecific competition and herbivory reduce regeneration of the sparse introduced thistle more than that of the more common native thistle. The field experiment was conducted in a western tallgrass prairie in Nebraska. We manipulated vegetation cover to alter the strength of interspecific competition and sprayed with insecticide to reduce the magnitude of insect herbivory on both C. vulgare and C. altissimum. We evaluated the herbivore reduction treatment, and compared seedling emergence, survival, and growth of two thistle species over two consecutive years.
Results/Conclusions
The results show that seedling emergence success differed between the two species and the two competition levels. Further, both competition and herbivory affected seedling survival and growth of both C. vulgare and C. altissimum. However, the relative effect of the two interactions were different for the two thistle species. Survival of C. vulgare seedlings was more strongly limited than C. altissimum by both herbivory, exerted by native insects, and by competition imposed by established vegetation. In contrast, C. altissimum had relatively high survival, and it was much less affected by reduction in interspecific competition and in insect herbivory. Additionally, seedling growth of C. altissimum was only limited by competition. We conclude that both insect herbivory by native insects and interspecific competition strongly limit the regeneration of the introduced C. vulgare, and do so more for C. vulgare than for the native C. altissimum, providing significant biotic resistance to invasive regeneration in western tallgrass prairie.