UC San Diego UC San Diego Previously Published Works Title Complex responses to invasive grass litter by ground arthropods in a Mediterranean scrub ecosystem Permalink https://escholarship.org/uc/item/6n34h6hr Journal Oecologia, 161(4) ISSN 1432-1939 Authors Wolkovich, Elizabeth Mary Bolger, Douglas T. Holway, David A. Publication Date 2009-10-01 DOI 10.1007/s00442-009-1425-7 Peer reviewed eScholarship.org Powered by the California Digital Library University of California Oecologia (2009) 161:697–708 DOI 10.1007/s00442-009-1425-7 PLANT-ANIMAL INTERACTIONS - ORIGINAL PAPER Complex responses to invasive grass litter by ground arthropods in a Mediterranean scrub ecosystem Elizabeth Mary Wolkovich · Douglas T. Bolger · David A. Holway Received: 21 April 2009 / Accepted: 7 July 2009 / Published online: 8 August 2009 © The Author(s) 2009. This article is published with open access at Springerlink.com Abstract Plant invasions have tremendous potential to decline because of changes in soil temperature (F. mccooki) alter food webs by changing basal resources. Recent studies and habitat structure (P. vistana) associated with litter. We document how plant invasions may contribute to increased studied trends observationally and conducted a 3-year arthropod abundances in detritus-based food webs. An experiment in which we manipulated litter quantity. In con- obvious mechanism for this phenomenon—a bottom-up trast to other published studies, most detritus-based arthro- eVect resulting from elevated levels of detritus from the pod taxa declined in areas of high grass invasion, and, invasive plant litter—has not been explicitly studied. We within trophic levels, responses often varied idiosyncrati- examined the eVects of an annual grass invasion on ground cally. For the two most common taxa, a native ant arthropod assemblages in the coastal sage scrub (CSS) of (F. mccooki), and predatory mites in the Anystidae, we southern California. Bottom-up food web theory predicts experimentally linked declines in abundance to increased that the addition of detritus would increase generalist-feed- levels of invasive grass litter. Such declines, especially ing arthropods at all trophic levels; accordingly, we those exhibited by the most common ant taxa, could have expected increases in fungi, Collembola, and common pre- cascading eVects on the CSS ecosystem, where ants are dators such as mites and spiders. For the common ant taxa, numerically dominant and thus may have broad inXuences habitat alteration may also be important for predicting on food web and ecosystem properties. Our results high- responses. Thus we expected that Forelius mccooki and light that accurately predicting arthropod responses to inva- Pheidole vistana, the most common ant species, would sive plant litter requires careful consideration of the structural and food resources provided by detritus to each particular food web. Communicated by Nathan Sanders. Keywords Detritus · Food web · Coastal sage scrub · Bottom-up · Habitat structure Electronic supplementary material The online version of this article (doi:10.1007/s00442-009-1425-7) contains supplementary material, which is available to authorized users. Introduction E. M. Wolkovich (&) · D. T. Bolger Environmental Studies Program, Dartmouth College, Invasive species can transform the habitats they colonize by 6182 Steele Hall, Hanover, NH 03755, USA altering a suite of ecosystem properties (Mack et al. 2000). e-mail: [email protected] Invasion by a single species can initiate a cascade of E. M. Wolkovich changes in other trophic levels, drastically altering food Department of Biological Sciences, Dartmouth College, web composition and structure from the pre-invasion state 6044 Gilman, Hanover, NH 03755, USA (O’Dowd et al. 2003; Sakai et al. 2001). Plant invasions in particular, and the associated declines in numerically domi- D. A. Holway Division of Biological Sciences, University of California, nant native plant species, may alter communities via both San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0116, USA their living and dead (litter) plant matter. 123 698 Oecologia (2009) 161:697–708 Numerous studies have shown that plant invasions gen- whether the detritus-based ground arthropod community erally increase basal resources through dramatic increases also exhibits a simple bottom-up response to invasion. in living plant biomass and litter (Liao et al. 2008). Bot- tom-up food web theory predicts this increase would in Hypotheses and predictions turn increase abundance of species in all trophic levels linked to these basal resources (Oksanen et al. 1981). In Plant invasions that increase a system’s litter produce two agreement with theory, recent studies have found that, notable changes aVecting arthropod communities: compared to uninvaded areas, invaded areas have similar increased basal resources and habitat alteration, including detrital food web composition with increased abundance microclimatic and structural changes. Such changes may be of arthropods at all trophic levels (Gratton and Denno particularly dramatic when—as in CSS—the invasion 2005; Kappes et al. 2007), and, for one system, stable iso- results in a dramatic change in the life form of the dominant tope studies indicate that a bottom-up eVect of detritus plant species and disturbance regime. We examined such contributes to these changes (Gratton and Denno 2006). In possible changes by conducting a 1-year observational contrast, grazing webs show reduced species and func- study followed by a 3-year manipulation of invasive grass tional group richness and gross declines in abundance litter. Bottom-up food web theory predicts that the addition across trophic levels (Ernst and Cappuccino 2005; Gratton of abundant detritus would increase generalist arthropod and Denno 2005), suggesting that grazers respond to the taxa at all trophic levels in CSS. We expected that grass change in food plants more than to the altered productivity litter would increase fungi because of its high C:N ratio of the system. (Wardle et al. 1999, 2004). We expected Collembola, con- However, several features of these studies limit their sidered generalist fungivores (Hunt et al. 1987), to track the generalization. First, observational studies (Ernst and increase in fungal resources and in turn subsidize predator Cappuccino 2005; Gratton and Denno 2005; Kappes et al. abundance: spiders (Araneae), harvestmen (Opiliones) and 2007) examine landscapes undergoing concurrent changes, Prostigmata mites (Dindal 1990; Wise 1993). In contrast, such as increased Wre frequency or N deposition, together ants are known to respond strongly to habitat alteration, with invasion (Didham et al. 2005, 2007). These methodo- especially changes to the thermal environment (Andersen logical limitations may obscure any clear link of invasion to 1997; Retana and Cerda 2000), as well as to food resources. food web changes, and especially to a bottom-up eVect of Furthermore, of the dominant arthropod taxa in this system, invasive plant litter. Second, studies have been conducted only ants are described suYciently well taxonomically and mainly in submersed systems—Xoodplains and salt ecologically (Menke and Holway 2006) to develop speciWc marshes (Gratton and Denno 2005, 2006; Kappes et al. predictions based upon the habitat alteration hypothesis. 2007)—thus their applicability to terrestrial systems is Thus, for the Wve most common native ant species, we unknown. based predictions on dietary preferences, morphology, and We studied the eVects of invasive plant litter in coastal environmental tolerances. We expected that Pheidole vis- sage scrub (CSS) ecosystems in southern California, a tana, a gracile species restricted to open arid and semi-arid semi-arid system extensively invaded by non-native annual habitats (Creighton 1950; Wheeler and Wheeler 1986), European grasses (Minnich and Dezzani 1998). This inva- would have diYculty moving through dense litter as a sion, similar to others in semi-arid systems, is associated result of its larger body and long legs. Forelius mccooki, a with an altered Wre regime (Keeley et al. 2005) and dra- species tolerant of high temperatures (Holway et al. 2002), matic landscape changes (Cione et al. 2002): uninvaded may decline in areas of high grass invasion as a result of CSS is shrub-dominated with extensive bare ground and decreased soil surface temperatures caused by the insulat- soil crusts (Mooney 1977), while invaded areas transform ing eVect of grass litter (Wolkovich et al. 2009). Because to a landscape of shrubs imbedded within a matrix of non- Pheidole clementensis and Solenopsis xyloni are known to native annual grasses (Cione et al. 2002). The invasion of harvest non-native seeds (Gregg 1969), we predicted they annual grasses, a plant functional type distinct from the would increase in response to the addition of non-native dominant shrubs, produces a striking change in basal grass litter. Unlike P. clementensis, S. xyloni has a much resources: invaded CSS habitat is four times more produc- wider geographical range and occurs in a wide variety of tive, has 50 times more litter and 30% higher concentra- habitats (Creighton 1950; Ward 2005). For this reason we tions of soil nitrogen compared to uninvaded habitat expected that this species would exhibit more Xexible (Wolkovich et al. 2009). Thus, the invasion has created the responses to leaf litter additions. Lastly, Solenopsis molesta potential for signiWcant bottom-up eVects. In contrast to is a klepto-parasite