Testing the resource use overlap hypothesis in invasive species control

Background/Question/Methods

Theories of niche partitioning and competitive exclusion suggest that where species utilize resources differently in space and time more species can be supported. We apply these theories to test whether restoration of native species with niches that overlap with an invasive species will be more effective in controlling the invasive than restoration using native species with complementary resource use.  Bothriochloa ischaemum (KR bluestem) is a C4, perennial grass, introduced into grasslands of Texas and the Midwest to improve degraded rangelands.  The species is highly invasive and its introduction has resulted in widespread homogenization of these ecosystems.  Through a target neighbor greenhouse competition experiment focused at the seedling establishment phase, we aimed to assess the suppressive abilities of five native herbaceous species and correlate this information with the species overlap in resource use in space and time with KR Bluestem.  The study focused on the following native species:  Schizachyrium scoparium, Bouteloua curtipendula, Sorghastrum nutans, Bothriochloa laguroides, Lupinus texensis.  KR Bluestem and all native species were grown as plugs in seedling trays for three months.  Well-established seedlings were transferred simultaneously to construct neighborhoods and then grown in a greenhouse in full sunlight thereafter.  Treatments consisted of three individuals of all six species grown in monoculture plus three of each native species grown with KR Bluestem as the target.  Five pots per treatment were harvested at five dates over the course of one year.  The following response variables were measured:  root depth, plant height, leaf area, leaf mass, above and below ground biomass (per pot and per individual where possible) and soil available NPK.   

Results/Conclusions

For all measures of fitness, KR Bluestem was unaffected by competition with the native species.  In contrast, fitness measures for native species were either neutrally or negatively affected by growing in competition with KR Bluestem.  We found no significant differences in soil resource use among the various treatments.  Our results suggest that overlap in resource use does not determine the effectiveness of native species to suppress a non-indigenous, invasive species at the seedling stage under controlled greenhouse conditions.