Top-down effects of small mammals on the invasive plant Halogeton glomeratus post-fire

Background/Question/Methods

Fire and plant invasions are reshaping deserts and often interact by changing fire frequency, species diversity, timing of resource acquisitioning, and soil chemistry. One particularly important invasive plant species in deserts of North America is Halogeton glomeratus (halogeton). The USDA has classified halogeton as a noxious weed because of its toxicity to sheep and cattle and its ability to leach salts into soil.  Its toxicity comes from high concentrations of oxalates in the plant tissue which allows for easy establishment because very few herbivores will consume it.  Halogeton establishes rapidly after fire, is a prolific seed producer with small, easily dispersed seeds.  After fire, halogeton has abundant nutrient resources available and water from summer monsoon events allowing for abundant growth to flower and fruit quickly.  Our hypotheses are 1) fire will provide a bottom-up nutrient pulse for the plant community aiding in the establishment and increased growth of halogeton; 2) halogeton will establish quickly in fire disturbed plots because of its toxicity towards herbivores.  Our study is a full factorial design focusing on pre- and post-fire effects with the inclusion and exclusion of small mammals on vegetation. The study is located in Rush Valley, UT within the Great Basin.

Results/Conclusions

From the study we found that halogeton established abundantly in the burned as well as in the unburned treatments. Surprisingly, halogenton establishment was extremely low where small mammals were present. The reason for our surprise is that halogeton is toxic to cattle, sheep, and we assumed all mammals to be affected.  The implication of this is that small mammals can have a direct, top-down influence on vegetative succession of burned landscapes.  With the exclusion of small mammals halogeton was able to establish and grow prolifically on burned Wyoming sagebrush sites because of the available nutrients, water, and lack of competition. Our data suggest a strong top-down effect occurring on a noxious weed that cancels the bottom-up effect that fire produces on an invasive plant species.    This finding brings new questions and ramifications for community interaction between fire, invasives, and the role of small mammals. One persistent question is why there is high variability in weed establishment interannually. Our data suggest that some of the variability in establishment and persistence of halogeton is controlled by small mammal abundance rather than simply the more conventional bottom-up resources.