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Growth, Development, and Vertebrate and Invertebrate Herbivory of the Federally Endangered Spiranthes parksii Correll and Sympatric Congener Spiranthes cernua
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ABSTRACT Spiranthes parksii Correll, a terrestrial orchid protected under the Endangered Species Act, and its congener Spiranthes cernua (L.) Rich, were studied in the Post Oak Savanna Ecoregion of Central Texas in 2014 and 2015. The species are sympatric and each produces a single inflorescence in the fall with emergence of a basal rosette during flower senescence or early spring. Objectives of this study were to 1) assess variation in annual and seasonal growth 2) determine the impact of vertebrate and invertebrate herbivores on the rosette and flower phases, and 3) identify invertebrate herbivores that utilize S. parksii and S. cernua. To assess variation in annual growth patterns between years an analysis of precipitation, demographic (presence or absence), and growth data (leaf area and inflorescence height) was performed. From 2014 to 2015 there was a reduction in precipitation, plants present, plant height, and the number of flowering plants that survived to seed production. To determine the difference between vertebrate and invertebrate herbivores, a 2 x 3 factorial experiment was conducted. Plants were randomly assigned to one of five treatments: Control (accessible to Vertebrates and Invertebrates), Insecticide with no cage (Vertebrate Only), Cage with no insecticide (Invertebrate Only), Caged with insecticide (Cage+Insecticide; no vertebrate or invertebrate), and cage with mesh cover and no insecticide (Mesh; access by only small invertebrates). During the flower season, herbivory was visually estimated for plant stalk and inflorescence by 5 percent increments. For rosettes, herbivory was visually estimated for each leaf in 5 percent increments and averaged over the whole rosette. During the first flowering season, vertebrates consumed more reproductive tissue (46%) than invertebrates (3%), while in the second season there was no significant difference between the two at 19% and 2%, respectively. There was no significant difference in percent herbivory of rosettes by vertebrates or invertebrates at 9% and 14% in 2014 and 16% and 11% in 2015. Invertebrates that were observed consuming Spiranthes sp. inflorescences and rosettes were armyworms (Order Lepidoptera: Family Noctuidae), grasshoppers (Family Acrididae), and an unidentified member of the Actiinae subfamily. This experiment confirms that vertebrates have a direct effect on Spiranthes sp. fitness through removal of reproductive tissue and an indirect impact by consuming rosettes. In addition, it documents that invertebrate herbivores can have a similar effect on inflorescence and rosettes. This knowledge can be important in understanding the influence of plant-herbivore interactions on conservation and management plans for S. parksii.
Nally, Deseri Dawn (2016). Growth, Development, and Vertebrate and Invertebrate Herbivory of the Federally Endangered Spiranthes parksii Correll and Sympatric Congener Spiranthes cernua. Master's thesis, Texas A & M University. Available electronically from