In the American Southwest, grasslands and shrublands coexist as alternative stable states under a similar climatic regime. Over the next century, climate models predict a ~10–20% decrease in winter precipitation with a projected shift in the monsoon causing the bulk of the precipitation to fall later in the year when temperatures are cooling. Aridity is expected to increase due to prolonged droughts and higher temperatures. Current research has shown that the dominant grass (black grama) is disproportionately affected by drought and that shrubs are metabolically active at cooler temperatures, they benefit from late season rains and are expected to become the climax community. Our study area encompasses both stable states providing a natural laboratory to investigate how changes in plant assembly will affect the small mammal community. Using long-term data sets on net primary production, precipitation, and small mammals generated by the Sevilleta long term ecological research center (1989 to present), we will be able to analyze the differences in community composition and density of the small mammal populations in each stable state. In addition, our research will determine if the foraging guild of the small mammals and/or changes in plant assembly directly or indirectly (lagged) affect community composition.
Results/Conclusions
A Bray Curtis ordination in shrubland versus grassland showed differences in small mammal community composition based on foraging guild. Herbivores differed in the two landscapes (tracking vegetation), the omnivore and carnivore communities did not, implying that they utilize the two landscapes similarly. Changes in 29% of mice species explain 90% of these differences. The results from community composition metrics indicate no directional change in the small mammal communities, they are just reordering from year to year. Community stability is constant across decades because it is looking at an aggregate of all species, a long time span will not capture annual or biannual population booms and busts. Densities change from year to year and site to site so you will see more variation in stability over shorter time periods. The shrubland was more specious due to physical heterogeneity allowing more niche partitioning. Our data show that there is more synchrony in the grassland due to a larger number of herbivores which are closely tied to vegetation. We created a multigroup structural equation model (MGSEM) to determine a best fit hierarchical network of linear relationships that explain the differences in the small mammal communities in relation to plant community assembly.