Do Priority Effects Benefit Invasive Plants
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Biol Invasions (2012) 14:2617–2624 DOI 10.1007/s10530-012-0257-2 ORIGINAL PAPER Do priority effects benefit invasive plants more than native plants? An experiment with six grassland species Timothy L. Dickson • Jennifer L. Hopwood • Brian J. Wilsey Received: 7 August 2011 / Accepted: 28 May 2012 / Published online: 13 June 2012 Ó Springer Science+Business Media B.V. 2012 Abstract Invasive, non-native plant species often species would decrease plant biodiversity. To test this outcompete native species and reduce biodiversity. hypothesis, we grew three pairs of invasive and native Invasive plants frequently begin growth before native plant species from three different functional groups/ plants, yet few studies have examined whether inva- plant families (C3 grasses/Poaceae, non-leguminous sives win in competition partly by colonizing disturbed forbs/Asteraceae, and legumes/Fabaceae). We seeded sites more quickly or by beginning growth earlier in the each of the species 3 weeks before seeding the other season than native plants (i.e. due to priority effects). five species into large pots in a greenhouse. Consistent We hypothesized that invasive plant species would with our hypothesis, we found much stronger priority benefit more from priority effects than would compa- effects with invasive than native species. Each invasive rable native species and that earlier growth of invasive species formed a near-monocultures when seeded first (97.5 % of total biomass, on average) whereas native species did not similarly dominate (29.8 % of total Electronic supplementary material The online version of biomass, on average). Similarly, Simpson’s species this article (doi:10.1007/s10530-012-0257-2) contains diversity was 81 % higher when the initially sown supplementary material, which is available to authorized users. species was native rather than invasive. The literature T. L. Dickson Á J. L. Hopwood Á B. J. Wilsey suggests that invasive species in the field often begin Department of Ecology, Evolution, and Organismal growth earlier in the spring than native species and that Biology, Iowa State University, Ames, IA 50011, USA climate change may increasingly allow invasives to e-mail: [email protected] begin growth before native species, indicating invasive J. L. Hopwood priority effects may become increasingly common. e-mail: [email protected] B. J. Wilsey Keywords Dominance Á Earlier growth Á e-mail: [email protected] Exotic species Á Non-native species Á Plant invasion Á Priority effect Present Address: T. L. Dickson (&) W.K. Kellogg Biological Station, Michigan State University, Hickory Corners, MI 49060, USA Introduction Present Address: Faster growth, greater fecundity, and other mecha- J. L. Hopwood The Xerces Society for Invertebrate Conservation, 628 nisms are proposed to explain the superior competi- NE Broadway Suite 200, Portland, OR 97232, USA tive ability of invasive plant species (Pysˇek and 123 2618 T. L. Dickson et al. Richardson 2007). Priority effects may also alter of no studies that have examined the effects of earlier competition between invasive and native plants. growth of invasive species on plant diversity, even Although priority effects can strongly affect plant though some pairwise studies have examined the competition and the eventual composition of plant effects of earlier invasive species growth on single communities (Ko¨rner et al. 2008), only a few studies native species (Abraham et al. 2009; Grman and have examined the role of priority effects in compe- Suding 2010; Stevens and Fehmi 2011). Priority tition between invasive and native plants (Seabloom effects are highly relevant to the composition and et al. 2003; Abraham et al. 2009; Grman and Suding dynamics of plant communities on disturbed soil, and 2010; Stevens and Fehmi 2011), and we are unaware knowledge of invasive priority effects can be applied of any studies that have examined how invasive to restoration plans (Suding et al. 2004; Young et al. priority effects alter plant diversity. 2005). Although few studies have examined differences in We hypothesized that invasive plant species would priority effects between invasive and native plant benefit more from a priority effect than would species, many studies have dealt with the general topic comparable native species and that earlier growth of of priority effects (Collinge and Ray 2009 and invasive species would decrease plant biodiversity. To references therein). Priority effects occur when the test this hypothesis we compared three native species arrival or earlier growth of one or more species leads to with invasive species in the same functional groups a different community structure than would form if all and plant families as the native species. We created species began growth simultaneously (Shulman et al. separate treatments where each species began growth 1983). These different communities may lead to 3 weeks before the others. We also created a seventh alternative stable states (Suding et al. 2004) and may control treatment where all six species began growth at contribute to stochasticity in community composition the same time. We conducted this greenhouse exper- (Trowbridge 2007). Priority effects are common when iment to specifically examine the role of priority one species begins growth before another, whether effects on biomass production in native and invasive growth is begun a few days (Ross and Harper 1972)or plants and to examine the role of priority effects on years earlier (Collinge and Ray 2009). community composition. Conducting the experiment There are several studies that suggest invasive in a greenhouse allowed us to control environmental species regularly begin growth before native species. characteristics and remove herbivory, permitting us to Wainwright et al. (2012) find that exotic forbs and focus on interactions between invasive and native annual grasses germinate more quickly than native species under nearly ideal growing conditions in the species in response to the rainy season in semi-arid absence of herbivory. Californian grasslands. Wilsey et al. (2011) find that 18 invasive herbaceous species begin growth three and a half weeks earlier in the spring, on average, than do Materials and methods 18 phylogenetically paired native species. McEwan et al. (2009) find that Lonicera maackii (Rupr.) Herder Experimental design (bush honeysuckle) begins growth 2–3 weeks earlier in the spring than native shrubs. Invasive plant species This experiment was conducted in Ames, Iowa in an also colonize disturbances and grow quickly in the Iowa State University greenhouse. We performed the absence of other individuals (Mack et al. 2000; experiment with replicated cylindrical pots 27.5 cm in Parendes and Jones 2000), both characteristics that diameter and 35 cm in height. Each pot was filled with could allow invasive plants to enjoy a priority effect homogenized Iowa topsoil to within 5 cm of the top of when compared to native plants. the pot. To remove the soil seedbank, the soil was If invasive plant species are able to begin growth steam sterilized shortly before it was added to the pots. earlier in the growing season, this may strongly Steam sterilization likely killed the soil microbial increase their fecundity and survival compared to community, and we were concerned that the lack of situations where invasives begin growth at the same soil microbes could prevent the germination or growth time as natives, and this earlier invasive growth may of some plant species. To replace the soil microbial also reduce plant diversity. Surprisingly, we are aware community, we collected an 8 cm deep soil sample 123 Do priority effects benefit invasive plants more than native plants? 2619 from Doolittle remnant prairie located 10 miles north sowed one of the six species into each treatment before of Ames, and we washed this soil sample through a the others. We then sowed the other five species sieve with 0.1 mm openings and collected 4 L of the 3 weeks after to be consistent with other priority effect resulting slurry. We then added 71 mL of this slurry to studies (Ko¨rner et al. 2008; Stevens and Fehmi 2011). each pot on 27 July 2008. It is likely that steam We also created a seventh control treatment where we sterilization and the addition of slurry created high sowed all six of the species at the same time. Thus, all nutrient availability that is representative of fallow species were eventually sown into each treatment, and agricultural fields (Gross et al. 1995). only the timing of sowing varied between treatments. We used three invasive plant species in this The initially sown species and the control treatment experiment, Dactylis glomerata L., Leucanthemum were sown on 25 July 2008, while the subsequently vulgare Lam., and Trifolium repens L. and three native sown species were sown on 16 August 2008. species, Elymus canadensis L., Ratibida pinnata These treatments were crossed with treatments that (Vent.) Barnhart, and Dalea purpurea Vent. All either used the same number of seeds of each species, or species except R pinnata were used in a study by used a pure-live seed approach that attempted to correct Wilsey et al. (2011) that showed invasive species were for germination so that the same number of germinating beginning growth earlier than native species, and individuals would be sown. Before we began the Wilsey et al. (2011) used a closely related species, experiment, we estimated seed germination rates by Ratibida columnifera (Nutt.) Woot. & Standl. The seeding each species individually into three pots in late plant species used in our experiment were from the May. Fifty seeds of each species were added to each pot functional groups C3 grass, non-leguminous forb, and and we recorded the number of seeds that germinated legume, respectively, and were from the families over a 40 day period. When adding the same number of Poaceae, Asteraceae, and Fabaceae, respectively. All seeds of each species during the experiment, we added six species were perennial. We chose the invasive- 41 seeds of each species per pot.