Nonnative plant shifts functional groups of arthropods following drought Authors: Adam B. Mitchell & Andrea R. Litt The final publication is available at Springer via http://dx.doi.org/10.1007/s10530-016-1072-y in Biological Invasions. Mitchell, Adam B. , and Andrea R. Litt. "Nonnative plant shifts functional groups of arthropods following drought." Biological Invasions 18, no. 5 (May 2016): 1351-1361. DOI: 10.1007/ s10530-016-1072-y. Made available through Montana State University’s ScholarWorks scholarworks.montana.edu Nonnative plant shifts functional groups of arthropods following drought Adam B. Mitchell . Andrea R. Litt Abstract Nonnative plants alter the composition of native plant communities, with concomitant effects on arthropods. However, plant invasions may not be the only disturbance affecting native communities, and multiple disturbances can have compounding effects. We assessed the effects of invasion and drought on plant and arthropod communities by comparing grasslands dominated by nonnative Old World blue-stem grasses (OWBs, Dichanthium annulatum) to grasslands dominated by native plants during a period of decreasing drought severity (2011–2013). Native plant communities had more species of plants and arthropods (/ m2) than areas dominated by OWBs during extreme drought, but richness was comparable as drought severity decreased. Abundance of arthropods was greater in native plant communities than in OWB communities during extreme drought, but OWB communities had more arthropods during moderate and non-drought conditions. We observed a shift in the arthropod community from one dominated by detriti-vores to one dominated by herbivores following plant invasion; the magnitude of this shift increased as drought severity decreased. Both plant communities were dominated by nonnative arthropods. A nonnative leafhopper (Balclutha rubrostriata) and native mites (Mochlozetidae) dominated OWB communities as drought severity decreased, and OWBs may serve as refugia for both taxa. Nonnative woodlice (Armadil-lidium vulgare) dominated native plant communities during extreme and non-drought conditions and abundance of this species may be associated with an increase in plant litter and available nutrients. Given the importance of arthropods for ecosystem services, incorporating arthropod data into conservation studies may demonstrate how changes in arthropod diversity alter ecosystem function where nonnative plants are dominant. Introduction Nonnative plants often reduce richness of native plant communities and change vegetation structure (Levine et al. 2003; Gaertner et al. 2009; Hejda et al. 2009; Vila` et al. 2011). These changes have concomitant effects on other organisms at different trophic levels, espe-cially arthropods (Bezemer et al. 2014; Litt et al. 2014; van Hengstum et al. 2014). However, the response of arthropods to changes in plant composition or struc- multiple environmental stressors may provide insights ture resulting from nonnative plants may differ by taxa for conservation (Paine et al. 1998). or functional group (Litt et al. 2014). We developed a field-based study to compare plant Plant-feeding arthropods are most likely to be and arthropod communities between landscapes dom- affected directly by changes in plant composition inated by nonnative grasses (Old World bluestem (Bezemer et al. 2014; Litt et al. 2014). Many plant- grasses or OWBs, Bothriochloa and Dichanthium feeding arthropods require specific plant species for spp.) and landscapes dominated by native plants. food or reproduction sites (Bernays and Graham 1988; OWBs alter the composition of native plant and Tallamy 2004; Burghardt et al. 2010; Forister et al. arthropod communities (Hickman et al. 2006; Woodin 2015) and novel plants may not be recognized as et al. 2010; Cord 2011) and we sought to build on habitat by native arthropod species. Herbivorous previous work by focusing specifically on changes in arthropods may not utilize nonnative plants due to the composition of arthropod communities at the level physical and chemical defenses (Graves and Shapiro of functional group and species. Investigating changes 2003; Burghardt et al. 2010). Plant-feeding arthropods in arthropods from a functional group perspective generally are less abundant and diverse where nonna- could demonstrate how ecological properties change tive plants are dominant (Litt et al. 2014; van following disturbance events such as plant invasion. Hengstum et al. 2014, but see Bezemer et al. 2014). We predicted that OWB-dominated landscapes would Although some arthropods do not feed on living have fewer plant species, as well as lower species vegetation, plant invasions still may have indirect richness and abundance of herbivorous arthropods, but effects on these species (Wolkovich 2010; Lenda et al. higher richness and abundance of detritivorous arthro- 2013). Changes in litter composition, soil moisture, pods, than native plant-dominated landscapes. mineralization, and soil pH following plant invasion In 2011, a severe drought occurred throughout the can have adverse effects on the composition and introduced distribution of OWBs (NDMC-UNL abundance of litter-and-soil-dwelling arthropods 2014). The drought persisted for several years in the (Kappes et al. 2007; Wolkovich et al. 2009; Alerding southern portion of this range and provided us with the and Hunter 2013). Conversely, where nonnative plants added opportunity to explore relationships between are dominant, the abundance and availability of plant plant invasion and drought severity on plant and litter may increase, which may allow litter-associated arthropod communities. OWBs are drought tolerant arthropods to increase in abundance (Litt et al. 2014). and more productive than native grasses under dry Plant invasion may be only one of many stressors conditions (White and Dewald 1996; Bhat et al. 2011; influencing communities of native plants and arthro- Porensky et al. 2014), which may exacerbate effects of pods; concurrent stressors can interact to have novel drought-induced stress on local plant and arthropod effects (Vitousek et al. 1996; Paine et al. 1998). For communities. We predicted that species richness for example, nonnative plants may better tolerate drought both plants and arthropods would decrease in the conditions, further reducing the ability of native plants presence of drought for both plant communities, but to compete (Everard et al. 2010; Crous et al. 2012). the magnitude of the decrease would be greater in Drought also may reduce abundance or richness of OWB-dominated areas than in areas dominated by arthropod communities through changes in availabil- native plants. ity or quality of habitat or food (Kindvall 1995; Scheirs and De Bruyn 2005; McCluney and Sabo 2012; Buchholz et al. 2013). Litter-feeding arthropods Methods may shift feeding preference to moist litter and predaceous arthropods may reduce hunting activity Study area (i.e., searching for and attacking prey) that is energet- ically expensive when water resources are limited We conducted our research at the Welder Wildlife (Sabo et al. 2008; McCluney and Sabo 2009). Few Refuge (N 28.12155, W 97.442808), a 3157-ha refuge studies have examined the combined effects of plant located 12 km northeast of Sinton, Texas. We sampled invasion and drought on arthropod communities, and within a plant community at the southernmost border understanding how native communities respond to of the refuge, which was classified historically as a mesquite-buffalograss community (Box 1961). How- perennial species that appeared dead were counted due ever, this plant community now is dominated by to uncertain dormancy responses to drought. We Kleberg bluestem (Dichanthium annulatum), a repre- grouped plant species into specific cover classes that sentative species of OWBs; our work occurred in the included grasses, forbs (herbaceous broad-leafed southern portion of the introduced range of Kleberg plants), and woody plants. We also recorded bare bluestem. We also sampled within a plant community ground and litter (dead vegetation not attached to soil) that was dominated by native grasses, but without as cover classes. Finally, we measured the height of the OWBs, which was 1.3 km from the OWB monocul- tallest plant of each species for each quadrat, and ture. Prior to the study, the native plant community averaged among species within each quadrat. We used was relatively undisturbed, and the OWB-dominated species richness and canopy cover of plants as measures plant community had been mowed annually to serve as of community richness and composition, and plant a fire break (T. Blankenship, personal comm.). density and height as measures of vegetation structure. We established 10, 6 9 9-m plots within the study area, five in the OWB-dominated community and five Soil sampling in the native plant community. We established plot size based on a concurrent study (Mitchell 2014). We Prior to each field season, we collected 1 L of soil selected plots at random; plots in the OWB-dominated from each quadrat up to a depth of 15 cm to determine community were at least 1.5 m apart and plots in the soil chemistry. Soil samples were analyzed by Texas native plant community were at least 32 m apart. We Plant and Soil Labs (Edinburg, TX) to quantify soil measured plant, soil, and arthropod characteristics on pH, organic matter (% O.M.), and available nutrients 2 two, 1-m quadrats per plot, every month from June– (NO3 and P2O5). We used these soil characteristics to August during 2011 through