• Caching rodents disproportionately disperse seed beneath invasive grass

      Sommers, Pacifica; Chesson, Peter; Univ Arizona, Ecol & Evolutionary Biol (WILEY-BLACKWELL, 2017-02-07)
      Seed dispersal by caching rodents is a context-dependent mutualism in many systems. Plants benefit when seed remaining in shallow caches germinates before being eaten, often gaining protection from beetles and a favorable microsite in the process. Caching in highly unfavorable microsites, conversely, could undermine the dispersal benefit for the plant. Plant invasions could disrupt dispersal benefits of seed caching by attracting rodents to the protection of a dense invasive canopy which inhibits the establishment of native seedlings beneath it. To determine whether rodents disproportionately cache seed under the dense canopy of an invasive grass in southeastern Arizona, we used nontoxic fluorescent powder and ultraviolet light to locate caches of seed offered to rodents in the field. We fitted a general habitat-use model, which showed that disproportionate use of plant cover by caching rodents (principally Chaetodipus spp.) increased with moonlight. Across all moon phases, when rodents cached under plants, they cached under the invasive grass disproportionately to its relative cover. A greenhouse experiment showed that proximity to the invasive grass reduced the growth and survival of seedlings of a common native tree (Parkinsonia microphylla) whose seeds are dispersed by caching rodents. Biased dispersal of native seed to the base of an invasive grass could magnify the competitive effect of this grass on native plants, further reducing their recruitment and magnifying the effect of the invasion.
    • The exotic invasive candle bush Senna alata (L.) Roxb. in Baja California Peninsula, México, a new threat for relictual oasis

      Navarro, José; Rodríguez-Estrella, Ricardo; Univ Arizona, Sch Nat Resources & Environm (REGIONAL EURO-ASIAN BIOLOGICAL INVASIONS CENTRE-REABIC, 2019-12-11)
      We recorded the invasive candle bush (Senna alata) for the first time in oases of southern Baja California peninsula in October 2013 and February 2015. Candle bush is likely an escape from residential plantings in this region. Data on density, plant cover, number of pods per plant, and seeds per pod are presented. In the oasis of Santiago, candle bush plant density was low and contained mainly mature plants, which made up 90% of the total population. In the oasis of San Jose del Cabo, population density was higher, but individuals were mostly shorter than 10 centimeters in height, and 60% of the population consisted of seedlings and young plants. Mature plants were seen mainly in full bloom and early fruiting stages. Candle Bush is invading very dry climates in oasis systems, this is the first documented record of the species invading this kind of wetlands and even starting to invade tropical deciduous forest around arroyos. The introduction pathways and time since the introduction of candle bush into the peninsula are unknown, but no records existed before 2013 when we found the population. We strongly recommend eradicating this invasive species from Baja California oases now, while it is in the early stages of invasion.
    • Provenance of invaders has scale-dependent impacts in a changing wetland ecosystem

      Amatangelo, Kathryn L.; Stevens, Lee; Wilcox, Douglas A.; Jackson, Stephen T.; Sax, Dov F.; Amatangelo, Kathryn L.; Stevens, Lee; Wilcox, Douglas A.; Jackson, Stephen T.; Sax, Dov F.; et al. (PENSOFT PUBL, 2019-11-19)
      Exotic species are associated with a variety of impacts on biodiversity, but it is unclear whether impacts of exotic specks differ from those of native species with similar growth forms or native species invading disturbed sites. We compared presence and abundance of native and exotic invaders with changes in wetland plant species diversity over a 28-year period by re-surveying 22 ponds to identify factors correlated with observed changes. We also compared communities found within dense patches of native and exotic emergent species with similar habits. Within patches, we found no categorical diversity differences between areas dominated by native or exotic emergent species. At the pond scale, the cover of the exotic grass Phragmites australis best predicted change in diversity and evenness over time, likely owing to its significant increase in coverage over the study period. These changes in diversity and evenness were strongest in younger, less successionally-advanced ponds. Changes associated with cover of P. australis in these ponds were not consistent with expected diversity decreases, but instead with a dampening of diversity gains, such that the least-invaded ponds increased in diversity the most over the study period. There were more mixed effects on evenness, ranging from a reduction in evenness gains to actual losses of evenness in the ponds with highest invader cover. In this wetland complex, the habit, origin and invasiveness of species contribute to diversity responses in a scale- and context-dependent fashion. Future efforts to preserve diversity should focus on preventing the arrival and spread of invaders that have the potential to cover large areas at high densities, regardless of their origin. Future studies should also investigate more thoroughly how changes in diversity associated with species invasions are impacted by other ongoing ecosystem changes.