COS 82-2 - Do Invasive Plants Affect Diversity in Heterogeneous Grasslands? The Effect of Sericea Lespedeza (Lespedeza cuneata) Invasion on the Tallgrass Prairie.
Invasive species are suspected to be major contributors to biodiversity declines worldwide. However, invasive species effects are suggested to be scale-dependent, and hypothesized to paradoxically be positively related to biodiversity at large spatial scales. Recent meta-analyses and reviews also suggest that invasive species effects might not change with scale, and may be neutral across scales (i.e., grain and extent). Yet, experimental evidence supporting or rejecting the invasion paradox is lacking, particularly across heterogeneous grasslands. We manipulated eight large (333 – 809 ha) experimental landscapes with pyric herbivory – recoupling of fire and grazing – to test how an invasive legume (i.e., Lespedeza cuneata) affects tallgrass prairie plant communities at spatial grains ranging from 0.1 m2 to >3,000,000 m2. We collected plant species composition and canopy cover data from each of our experimental landscapes annually from July 1-31 during 2019-2021. To address how L. cuneata abundance and scale affects plant diversity and composition, we modelled three standard measures of plant diversity (i.e., Hill numbers 0-2) and functional group canopy cover (i.e., grasses and sedges, forbs, and shrubs) against L. cuneata cover using Linear Mixed-Models at each spatial grain. Significance for each model was declared at α = 0.05.
Results/Conclusions
The effects of L. cuneata invasion on both plant diversity and plant functional group abundances changed with spatial grain; most being significantly negative at small (0.1 m2) spatial grains and neutral or positive at our largest grain sizes ( >3,000,000 m2). Although the effects were nearly always statistically significant at small spatial grains, the variability of plant diversity across our experimental landscapes was only weakly explained (i.e., the R-square) by L. cuneata abundance across all grain sizes. Therefore, it is unclear whether any meaningful biological effect exists between L. cuneata and plant diversity across our study area at any scale. Processes like pyric herbivory are known to promote heterogeneity and biodiversity, and restrict invasive species like L. cuneata. It is possible that the influence of L. cuneata may be different across grasslands not managed for heterogeneity. Invasive species effects gleaned from small scale, highly manipulated, experimental studies do not reliably predict their effects across large heterogeneous landscapes. Therefore, management strategies based on results from small-scale studies of invasion are unlikely to increase or conserve biodiversity across large grasslands. Rather, processes that generate landscape heterogeneity, like pyric herbivory, are probably more important to promoting biodiversity across all scales.
