Andrea R. Litt

Effects of invasive plants on arthropods.

Non-native plants have invaded nearly all ecosystems and represent a major component of global ecological change. Plant invasions frequently change the composition and structure of vegetation communities, which can alter animal communities and ecosystem processes. We reviewed 87 articles published in the peer-reviewed literature to evaluate responses of arthropod communities and functional groups to non-native invasive plants. Total abundance of arthropods decreased in 62% of studies and increased in 15%. Taxonomic richness decreased in 48% of studies and increased in 13%. Herbivorous arthropods decreased in response to plant invasions in 48% of studies and increased in 17%, likely due to direct effects of decreased plant diversity. Predaceous arthropods decreased in response to invasive plants in 44% of studies, which may reflect indirect effects due to reductions in prey. Twenty-two percent of studies documented increases in predators, which may reflect changes in vegetation structure that improved mobility, survival, or web-building for these species. Detritivores increased in 67% of studies, likely in response to increased litter and decaying vegetation; no studies documented decreased abundance in this functional group. Although many researchers have examined effects of plant invasions on arthropods, sizeable information gaps remain, specifically regarding how invasive plants influence habitat and dietary requirements. Beyond this, the ability to predict changes in arthropod populations and communities associated with plant invasions could be improved by adopting a more functional and mechanistic approach. Understanding responses of arthropods to invasive plants will critically inform conservation of virtually all biodiversity and ecological processes because so many organisms depend on arthropods as prey or for their functional roles, including pollination, seed dispersal, and decomposition. Given their short generation times and ability to respond rapidly to ecological change, arthropods may be ideal targets for restoration and conservation activities.

Interactive Effects of Fire and Nonnative Plants on Small Mammals in Grasslands

ABSTRACT Invasions by nonnative plants have changed the structure of many terrestrial ecosystems and altered important ecological processes such as fire, the dominant driver in grassland ecosystems. Reestablishing fire has been proposed as a mechanism to restore dominance of native plants in grasslands invaded by nonnative plants, yet fire may function differently in these altered systems, potentially affecting animals in novel ways. To assess whether invasions by nonnative plants alter the effects of fire on animals, we performed a manipulative experiment in semi-desert grasslands of southeastern Arizona that have been invaded by a perennial, nonnative grass from Africa, Lehmann lovegrass (Eragrostis lehmanniana). We applied fire to 36 of 54 1-ha plots established along an invasion gradient where dominance of E. lehmanniana ranged from 0% to 91% of total live plant biomass. Over the 5-year period from 2000 to 2004, we used mark-recapture methods to assess how population and community attributes of small mammals varied along the gradient of nonnative grass and in response to fire. We quantified changes in presence of 17 species, abundance of 9 species, total abundance of all species combined, species richness, and species composition. Based on 11,226 individual mammals from 24 species, we found that effects of nonnative-grass dominance varied with habitat preferences of each species, resulting in composition of the small-mammal community changing predictably along the invasion gradient. As dominance of nonnative grass increased, presence and abundance of granivorous heteromyids and insectivores (e.g., Chaetodipus, Perognathus, Onychomys; pocket mice and grasshopper mice) decreased, whereas presence and abundance of omnivorous and herbivorous murids (e.g., Reithrodontomys, Sigmodon; harvest mice and cotton rats) increased. Species richness of the small-mammal community averaged 8.4 species per plot and was highest at intermediate levels of nonnative-grass dominance where vegetation heterogeneity was greatest. Abundance of all small mammals combined averaged 26.9 individuals per plot and did not vary appreciably with nonnative-grass dominance. During the 4- to 8-week period immediately after fire, abundance of 6 of the 9 most common species changed, with 5 species decreasing and 1 species increasing on burned plots relative to unburned plots. During this same time period, species richness of small mammals decreased by an average of 3 species (38%) and total abundance of all species combined decreased by an average of 16 individuals (61%) on burned plots relative to unburned plots. Effects of fire on vegetation biomass, on presence of 9 of 17 mammalian species, and on abundance of 4 of 9 mammalian species remained evident ≥2 years after fire. Effects of fire on most small-mammal species varied with the degree of nonnative-grass dominance, suggesting that fire functioned differently 0n areas invaded by nonnative plants relative to areas dominated by native plants. Specifically, effects of fire on presence of 12 of 14 species and abundance of 7 of 9 species varied along the gradient of E. lehmaniana. During this post-fire period, however, composition of the small-mammal community in areas dominated by nonnative grass transitioned towards composition of areas dominated by native grasses, suggesting that fires had some restorative effect on habitat for small mammals. The relative strength of this effect will likely depend in general on the structural and compositional contrasts between invaded and native plant communities. Despite the reported ineffectiveness of fire at reducing dominance of nonnative plants, restoring fire to grasslands invaded by nonnative plants can help maintain the mosaic of vegetation conditions necessary to support the diverse assemblage of animals that inhabit these fire-governed ecosystems.

Restoration Fire and Hurricanes in Longleaf Pine Sandhills

H istorical accounts characterize intact old-growth longleaf pine (Pinus palustris) forests as having a canopy of scattered large pines, clumps of younger pines, scattered hardwoods, and a low groundcover of shrubs and mixed forbs and grasses (Means and Grow, 1985; Myers, 1990; Frost, 1993; Schwartz, !994). The open forests of the southeastern United States were exposed to frequent, lowintensity fires caused by lightning strikes and set by Native Americans, which maintained this longleaf pine-dominated landscape. Hurricanes, which are a less frequent but consistent event in these systems, also modify the vegetation structure (Gordon and others, 1997). Thus, the openness of these forests is the direct result of storm events and fire, which together maintain hardwood species at low densities. Typically, mesic sandhills and flatwoods can sustain annual and biannual fires, whereas drier sandhills would burn every three to five years due to insufficient build-up of fine fuels (Robbins and Myers, 1992; Frost, 1993). Opencanopied longleaf pine forests support the highest plant species richness in North America (Walker and Peet, 1983; Hardin and White, 1989; Walker, 1993). This high diversity is a direct effect of fire, which reduces competition from trees and shrubs and exposes mineral soil for the establishment of seedlings of new species (Streng and others, 1993). Before the land was fragmented by roads and land clearing, lightning and human-induced fires were able to spread over vast areas because only waterways, wetland communities, recent burns, and weather conditions blocked their spread through the extensive grasslands below the longleaf pine canopy (Robbins and Myers, 1992; Frost, 1993). Fire coverage would have varied widely, from small areas, when fires were extinguished by the same storm that started the fire, to fires of thousands of acres (Stout and Marion, 1993). Many of the areas that remain in or have been planted in longleaf pine now suffer from an overabundance of hardwoods that have formed a high midstory. Hardwoods often resprout after logging or other operations and continue to dominate the midstory and canopy. Current management often does not include fire. As a result, most of the remaining 2 percent of the original 92.5 million acres once covered by longleaf pine communities is degraded. Establishment and growth of the pines, as well as habitat quality for many other species, have decreased.

Performance of Juvenile Cutthroat Trout Translocated as Embryos from Five Populations into a Common Habitat

AbstractThe distributions of most native trout species in western North America have been severely reduced, and conservation of many of these species will require translocation into vacant habitats following removal of nonnative species. A critical question managers have is “Does it matter which donor sources are used for these translocations?” We present a case study that addressed this question for a large native trout translocation project in Montana. We introduced embryos from five source populations of Westslope Cutthroat Trout Oncorhynchus clarkii lewisi to a large, fishless watershed in Montana following removal of nonnative fish with piscicides. Source populations providing embryos for translocations were three nearby ( 350 km from the translocation site), and a population in captivity for one generation comprised of i...

Effects of Increased Heteropogon contortus (Tanglehead) on Rangelands: The Tangled Issue of Native Invasive Species☆

ABSTRACT Heteropogon contortus recently and rapidly increased in dominance in grasslands where it once had been aminor component. Ecological effects of this increase are unknown, but landmanagers are concerned about the potential negative economic and ecological impacts. We examined compositional and structural characteristics of the vegetation community along a gradient of dominance of H. contortus to quantify changes, compare the effects to invasions by nonnative grasses, and provide insights about management. As H. contortus increased, grass richness decreased across the gradient by 6 species·m-2 (95% CI: 2–10) in summer and 10 species·m-2 (6–15) in winter. Cover of other native grasses decreased 8–10% in both seasons for every 10% increase in H. contortus. Presence of seven individual plant species and cover of five species decreased, but presence of five species and cover of one species increased with H. contortus. Canopy cover increased and soil nutrients were higher in dense H. contortus, potentially facilitating further ecological changes. We suggest that managing H. contortus and other species that become invasive within the ecosystem where they were once native likely requires reducing rather than wholly eliminating the species, which may differ from management strategies for nonnative species.

Nonnative plant shifts functional groups of arthropods following drought

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 bluestem 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 detritivores 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 (Armadillidium 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.

Plant and Small Vertebrate Composition and Diversity 36–39 Years After Root Plowing

Abstract Root plowing is a common management practice to reduce woody vegetation and increase herbaceous forage for livestock on rangelands. Our objective was to test the hypotheses that four decades after sites are root plowed they have 1) lower plant species diversity, less heterogeneity, greater percent canopy cover of exotic grasses; and 2) lower abundance and diversity of amphibians, reptiles, and small mammals, compared to sites that were not disturbed by root plowing. Pairs of 4-ha sites were selected for sampling: in each pair of sites, one was root plowed in 1965 and another was not disturbed by root plowing (untreated). We estimated canopy cover of woody and herbaceous vegetation during summer 2003 and canopy cover of herbaceous vegetation during spring 2004. We trapped small mammals and herpetofauna in pitfall traps during late spring and summer 2001–2004. Species diversity and richness of woody plants were less on root-plowed than on untreated sites; however, herbaceous plant and animal species did not differ greatly between treatments. Evenness of woody vegetation was less on root-plowed sites, in part because woody legumes were more abundant. Abundance of small mammals and herpetofauna varied with annual rainfall more than it varied with root plowing. Although structural differences existed between vegetation communities, secondary succession of vegetation reestablishing after root plowing appears to be leading to convergence in plant and small animal species composition with untreated sites.

Long-term effects of aeration and fire on invasion of exotic grasses in mixed-brush plant communities.

Abstract Invasion of exotic grasses into grasslands dominated by native plants changes fire cycles and reduces biodiversity. Brush management practices that create soil disturbance, such as aeration, may potentially result in invasion of exotic grasses and replacement of native vegetation. We tested the hypothesis that a long-term effect of aeration and prescribed burning is an increase in exotic grasses. The study was conducted at the Chaparral Wildlife Management Area in the western south Texas plains where four treatments were evaluated: aeration, warm-season burn, aeration followed by a warm-season burn, and no treatment (control). The experimental design was a randomized complete block with four replicates. We estimated percentage canopy cover of exotic grasses, native grasses, forbs, litter, bare ground, and woody and succulent plants in 2007. There was a multivariate main effect among treatments for the dependent variables absolute canopy cover of exotic grasses, native grasses, forbs, litter, and bare ground (Wilkss Lambda F15,179.84  =  2.78, P  =  0.001). Variables that contributed to the significant overall effect included litter (F3,69  =  4.32, P  =  0.008) and native grasses (F3,69  =  6.11, P  =  0.001). The multivariate main effect of treatment was significant (Wilkss Lambda F9,180.25  =  2.04, P  =  0.038) for the relative canopy cover of herbaceous species. Relative cover of exotic grasses was 31% higher (P  =  0.024) in control than in the prescribed burn treatment. Native grasses relative cover was 30% higher (P  =  0.003) in prescribed burn than in the control treatment. We did not detect differences among treatments in the percentage of total woody and succulent plants canopy cover (P  =  0.083). Under the environmental conditions at the time of the study, aeration and/or prescribed burning do not increase exotic grasses.

Improving geographically extensive acoustic survey designs for modeling species occurrence with imperfect detection and misidentification

Abstract Acoustic recording units (ARUs) enable geographically extensive surveys of sensitive and elusive species. However, a hidden cost of using ARU data for modeling species occupancy is that prohibitive amounts of human verification may be required to correct species identifications made from automated software. Bat acoustic studies exemplify this challenge because large volumes of echolocation calls could be recorded and automatically classified to species. The standard occupancy model requires aggregating verified recordings to construct confirmed detection/non‐detection datasets. The multistep data processing workflow is not necessarily transparent nor consistent among studies. We share a workflow diagramming strategy that could provide coherency among practitioners. A false‐positive occupancy model is explored that accounts for misclassification errors and enables potential reduction in the number of confirmed detections. Simulations informed by real data were used to evaluate how much confirmation effort could be reduced without sacrificing site occupancy and detection error estimator bias and precision. We found even under a 50% reduction in total confirmation effort, estimator properties were reasonable for our assumed survey design, species‐specific parameter values, and desired precision. For transferability, a fully documented r package, OCacoustic, for implementing a false‐positive occupancy model is provided. Practitioners can apply OCacoustic to optimize their own study design (required sample sizes, number of visits, and confirmation scenarios) for properly implementing a false‐positive occupancy model with bat or other wildlife acoustic data. Additionally, our work highlights the importance of clearly defining research objectives and data processing strategies at the outset to align the study design with desired statistical inferences.

Simulating cheatgrass (Bromus tectorum) invasion decreases access to food resources for small mammals in sagebrush steppe

Invasions by nonnative plants can alter the abundance of native animals, yet we know little about the mechanisms driving these changes. Shifts in vegetation characteristics resulting from nonnative plants can alter availability of food resources, predation risk, and foraging efficiency (both the access to and ability to find food), each providing a potential mechanism for documented changes in animal communities and populations in invaded systems. Cheatgrass (Bromus tectorum) is a nonnative grass that invades sagebrush steppe, resulting in declines in some small mammal populations. We examined whether changes in structural characteristics associated with cheatgrass invasion could alter foraging by small mammals, providing a potential mechanism for documented population declines. We quantified differences in vegetation structure between native and cheatgrass-invaded sagebrush steppe, then experimentally added artificial structure in native areas to simulate these differences. We placed grain at foraging stations and measured the amount removed by small mammals nightly. Adding litter at depths approximating invasion by cheatgrass reduced the average amount of grain removed in 2 of 3 study areas, but increasing stem density did not. Based on this experiment, the deeper litter created by cheatgrass invasion may increase costs to small mammals by decreasing foraging efficiency and access to existing food resources, which may explain population-level declines in small mammals documented in other studies. By isolating and identifying which structural attributes of cheatgrass invasion are most problematic for small mammals, land managers may be able to design treatments to efficiently mitigate impacts and restore invaded ecosystems.