Buffelgrass Invasion Research Articles

Pollination biology of Gossypium turneri: Autonomous selfing provides reproductive assurance in an endangered wild cotton from Sonora, Mexico

AbstractThe wild cotton, Gossypium turneri, is a rare and endemic species from the state of Sonora, Mexico, threatened by habitat destruction, tourism development, and buffelgrass invasion in the municipality of Guaymas. As a consequence, G. turneri is now classified as endangered under Mexican legislation. This study aimed to document several aspects of the reproductive biology of this wild cotton to suggest conservation plans. The flowers of G. turneri remain open for only 1 day. Throughout the day, the flowers are visited by a diverse group of insects, including bees and butterflies. Pollination treatments revealed that this species is self‐compatible, capable of autonomous self‐pollination, and showed mixed evidence of pollen limitation. Although the flowers exhibit herkogamy, curved styles were observed that make the stigma come into contact with anthers. These results indicate that autonomous self‐pollination is caused by style curvature. Fruit set values and seed production per fruit in open pollinated control flowers were low, suggesting that this species produces a small number of seeds. These results provide new insights into this rare species, indicating that the species has a mechanism that provides reproductive assurance under low pollinator visitation, allowing for its persistence. The collection of seeds from the remaining populations for germplasm banks and the maintenance of live plants in botanical gardens are proposed for the conservation of this rare species.

Root microbiome diversity and structure of the Sonoran desert buffelgrass (Pennisetum ciliare L.).

Buffelgrass (Pennisetum ciliare) is an invasive plant introduced into Mexico's Sonoran desert for cattle grazing and has converted large areas of native thorn scrub. One of the invasion mechanisms buffelgrass uses to invade is allelopathy, which consists of the production and secretion of allelochemicals that exert adverse effects on other plants' growth. The plant microbiome also plays a vital role in establishing invasive plants and host growth and development. However, little is known about the buffelgrass root-associated bacteria and the effects of allelochemicals on the microbiome. We used 16S rRNA gene amplicon sequencing to obtain the microbiome of buffelgrass and compare it between samples treated with root exacknudates and aqueous leachates as allelochemical exposure and samples without allelopathic exposure in two different periods. The Shannon diversity values were between H' = 5.1811-5.5709, with 2,164 reported bacterial Amplicon Sequence Variants (ASVs). A total of 24 phyla were found in the buffelgrass microbiome, predominantly Actinobacteria, Proteobacteria, and Acidobacteria. At the genus level, 30 different genera comprised the buffelgrass core microbiome. Our results show that buffelgrass recruits microorganisms capable of thriving under allelochemical conditions and may be able to metabolize them (e.g., Planctomicrobium, Aurantimonas, and Tellurimicrobium). We also found that the community composition of the microbiome changes depending on the developmental state of buffelgrass (p = 0.0366; ANOSIM). These findings provide new insights into the role of the microbiome in the establishment of invasive plant species and offer potential targets for developing strategies to control buffelgrass invasion.

Mitigating buffelgrass invasion through simulated targeted grazing: understanding restoration potential in a variable precipitation regime

The potential for targeted grazing for restoration through the removal of invasive species represents an effective tool for practitioners. This study examines the impact of targeted grazing across highly variable precipitation on buffelgrass, a widely introduced African grass that often escapes cultivation. We simulated targeted grazing, overgrazing, and no grazing by clipping at two intensities and not clipping in a third treatment. We did this in plots with buffelgrass invasion and documented buffelgrass size and reproduction, as well as native plant community diversity. Additionally, as a further proxy for grazing, we tracked root allocation by buffelgrass in mowed and unmowed plots for 25 months while documenting large variability in precipitation. We found that targeted grazing reduced the negative impact of buffelgrass on the native community; on the other hand, overgrazing reduced the native community diversity. Precipitation interacted with simulated grazing such that buffelgrass subjected to grazing was less tolerant to drought and future simulated herbivory after rains returned. Long‐term targeted grazing may lead to reductions in buffelgrass that allow the restoration of the native plant community, which benefits the biodiversity of flora and fauna, and supports multiple uses of rangelands.

Effects of buffelgrass removal and nitrogen addition on soil microbial communities during an extreme drought in the Sonoran Desert

Buffelgrass (Cenchrus ciliaris), an invasive perennial grass in arid regions of the United States, has drastically modified natural ecosystems. Understanding the aboveground–belowground links between buffelgrass invasion and soil microbial communities will be critical for developing a comprehensive understanding of arid ecosystems and for deploying successful control strategies. In a randomized block‐field experiment located in Tucson, AZ, U.S.A., we investigated the effects of buffelgrass removal via hand pulling and nitrogen addition (and their interaction) on soil microbial communities during an extreme drought (summer 2020). We found that experimental treatments did not significantly impact bacterial and archaeal community diversity and composition, while plant removal affected fungal community diversity and composition. In addition, the removal treatment increased the proportion of putative chitinolytic bacteria (genus Lysobacter) and decreased the proportion of putative fungal endophytes (genus Darksidea). Buffelgrass manual removal appeared to favor fungal endophyte death around and inside of leftover intact roots, which may result in an increase of chitinolytic bacteria thriving on the degradation of fungal cell walls. Overall, our results suggest that hand removal—which is a common type of buffelgrass control—can alter soil fungal communities and the proportion of certain microbial functional groups. As these changes could subsequently affect native plants, more research is needed to develop a comprehensive understanding of the effect of buffelgrass control efforts on target plants as well as surrounding ecosystem dynamics.

Homeowner perceptions and responses to buffelgrass invasion risk in the Tucson, Arizona Wildland-Urban Interface

In this study, we aimed to analyze homeowners' level of awareness and perceived risk about buffelgrass invasion in the Tucson, Arizona Wildland-Urban Interface (WUI), as well as the factors influencing their participation in buffelgrass control and fire risk mitigation efforts. Data for the study were generated through the administration of an online survey among 117 members of Home Owner Associations (HOAs) in the Tucson WUI. The results showed that the overwhelming majority of respondents were aware of buffelgrass, but their knowledge about buffelgrass control mechanisms appeared to be limited. Respondents also more frequently expressed concern about the risks posed by buffelgrass invasion to general targets, such as the Sonoran Desert ecosystem, native plants and wildlife than risks to their private property and neighborhoods. The results also showed that the level of involvement in HOAs, and leadership in HOAs had significant positive effects on homeowners' participation in buffelgrass control efforts. Homeowners' duration of residence also had a significant negative effect on participation in buffelgrass control efforts, suggesting that newcomers may be more involved than long-term residents. Similarly, the number of months respondents spent in Tucson per year had a negative effect on the number of hours spent on buffelgrass control efforts. Respondents’ perceived risk about buffelgrass invasion also had a positive effect on the hours spent on buffelgrass control as well as their level of involvement in fire risk mitigation efforts. These results highlight the importance of local institutions and community heterogeneity in social responses to threats in WUI communities. Policies aimed at building the resilience of WUI communities need to account for their complexity as coupled social-ecological systems.

Collective Action and Invasive Species Governance in Southern Arizona

Invasive plants can have significant negative effects on human and ecological communities, including reduced productivity and biodiversity and increased fire risk. Effective mitigation of invasive species likely requires action by heterogeneous actors who span jurisdictions, sectors, and levels of governance. While there has been significant research to develop targeted mitigation techniques that slow or halt the spread of specific invasive plants, there has been relatively little complementary work to develop knowledge about the implementation of these management techniques through effective governance systems. To address this gap, we interviewed and conducted archival research on land managers involved in the mitigation of buffelgrass (Pennisetum ciliare, syn: Cenchrus ciliarus) invasion in southern Arizona to investigate how existing and emerging governance arrangements encourage or undermine individual and collective action to manage invasive plants. Our results show that a key challenge of managing invasive species is identifying the mechanisms that will allow heterogeneous actors to overcome internal barriers to coordination with others and enable collective action. These internal barriers are multifaceted, involving laws and policies, cultural traditions and mandates, the availability of monetary and human resources, and information on causes and consequences of species invasion and effective approaches to mitigation. Approaches to solving these problems must include improved knowledge of internal institutional structures and the opportunities and barriers they present to collective action, the preferences of heterogeneous actors when presented with information about future ecosystem conditions absent coordination, the factors that prevent individuals within different organizations from following through on commitments to participate in collective action institutions, and how each of these conditions affects the availability and persistence of resources for mitigation. Together, improved knowledge of the relationships between these factors may provide new approaches to proactive management of emerging resource management challenges, from invasive species to emerging diseases.

Assessing ecological uncertainty and simulation model sensitivity to evaluate an invasive plant species’ potential impacts to the landscape

Ecological forecasts of the extent and impacts of invasive species can inform conservation management decisions. Such forecasts are hampered by ecological uncertainties associated with non-analog conditions resulting from the introduction of an invader to an ecosystem. We developed a state-and-transition simulation model tied to a fire behavior model to simulate the spread of buffelgrass (Cenchrus ciliaris) in Saguaro National Park, AZ, USA over a 30-year period. The simulation models forecast the potential extent and impact of a buffelgrass invasion including size and frequency of fire events and displacement of saguaro cacti and other native species. Using simulation models allowed us to evaluate how model uncertainties affected forecasted landscape outcomes. We compared scenarios covering a range of parameter uncertainties including model initialization (landscape susceptibility to invasion) and expert-identified ecological uncertainties (buffelgrass patch infill rates and precipitation). Our simulations showed substantial differences in the amount of buffelgrass on the landscape and the size and frequency of fires for dry years with slow patch infill scenarios compared to wet years with fast patch infill scenarios. We identified uncertainty in buffelgrass patch infill rates as a key area for research to improve forecasts. Our approach could be used to investigate novel processes in other invaded systems.

Buffel grass (Cenchrus ciliaris) eradication in arid central Australia enhances native plant diversity and increases seed resources for granivores

AbstractQuestionsBuffel grass (Cenchrus ciliaris L.) is a drought‐adapted invasive plant that has become a serious environmental weed in many arid and semi‐arid systems. This paper examined whether eradication of buffel grass infestations: (i) increases frequency, richness and diversity of native vegetation and seed bank pools; and (ii) improves availability of seed resources for granivores.LocationAlluvial ironwood/corkwood woodlands in arid central Australia.MethodsWe assessed differences in floristic and seed bank composition between buffel‐infested sites and sites where buffel grass had been eradicated ca. 12 years previously. Plant species frequency data from nested‐quadrat sampling were amalgamated into plant functional groups to examine their relationship to buffel treatment. A seed flotation method was employed to: (i) assess seed bank composition of functional groups at eradicated vs. infested sites; and (ii) test whether the mass of seeds available for granivores was higher at buffel‐free sites.ResultsBuffel‐eradicated sites supported richer and more diverse vegetation and seed banks across all functional groups except perennial grasses. The effect was strongest for perennial forbs, annual/short‐lived forbs, and annual/short‐lived grasses. The overall mass of seeds of non‐buffel grass species was ca. 10‐fold higher at buffel‐removed sites. Numbers of seeds of species in the 0.001–0.009 9 g and 0.000 1–0.000 99 g size classes, both of which contain species with seeds consumed by granivores, were ca. 20‐ and 14‐fold higher respectively at buffel‐removed sites.ConclusionsBuffel grass removal at localised scales provides islands of habitat with improved opportunities for native plant re‐establishment and abundant foraging resources for granivores. Future research must disentangle the relative importance of dispersal vs. residual seed banks for community restoration after buffel grass invasion. Extended delays in eradication could allow seed bank reserves to deteriorate to a state that no longer permits regeneration.

Buffelgrass invasion and glyphosate effects on desert soil microbiome communities

Buffelgrass (Cenchus ciliaris) is a drought-tolerant invasive grass in the Americas and Australia that significantly impacts native plant communities and ecosystems. Despite the clear need to develop a comprehensive understanding of how buffelgrass is able to invade and rapidly establish in arid ecosystems, there is still a lack of knowledge as to if and how this weed might change the soil microbiome in a way that affects its dominance in the presence of management. We investigated the effect of buffelgrass on soil microbial communities in areas that have either been exposed to or not exposed to glyphosate in Saguaro National Park, Arizona USA. We found that buffelgrass roots in invaded areas are surrounded by a distinct soil community that includes a greater number of nitrifiers than in uninvaded soil. We also observed increases in arbuscular mycorrhizal fungi, ectomycorrhizal fungi and methanotrophs with buffelgrass invasion compared to uninvaded soil. Finally, we found no evidence of glyphosate effects on the soil microbiome. Overall, our study results suggest that buffelgrass can escape the limitation of nutrient availability in arid ecosystems by directly or indirectly modifying the soil microbiome. The competitive dominance of buffelgrass in arid systems might be indirectly enhanced by nitrifiers and fungal symbionts, which are often involved in rapid biomass accumulation. This work highlights the importance of soil microbiome considerations in weed science research.

Developing an expert elicited simulation model to evaluate invasive species and fire management alternatives

AbstractInvasive species can alter ecosystem properties and cause state shifts in landscapes. Resource managers charged with maintaining landscapes require tools to understand implications of alternative actions (or inactions) on landscape structure and function. Simulation models can serve as a virtual laboratory to explore these alternatives and their potential impacts on a landscape. To be useful, however, managers need to participate in model development to ensure that model structure can evaluate the response of key resources to plausible actions. Here, we detail development of a state‐and‐transition simulation model (STSM) to evaluate buffelgrass (Cenchrus ciliaris L. syn Pennisetum ciliare (L.) Link) in Saguaro National Park (SNP), Arizona, USA, through collaboration between managers and researchers. We integrate expert knowledge and research to create and parameterize a stochastic, spatially explicit STSM to evaluate specific management objectives. We also develop a dynamic link between the STSM and a fire behavior model to allow exploration of potential novel processes introduced to the ecosystem by buffelgrass invasion. Our projections show that buffelgrass can be expected to increase on the landscape and that the integration of fire into the model accelerates the projected rate of invasion and increases degradation of resources of management concern. We highlight the benefit of engaging end users in the modeling process so that the model is targeted to evaluate management objectives, in this case retention of saguaro cacti (Carnegiea gigantea (Engelm.) Britton & Rose) on the landscape. Being able to integrate an external model that can help address the unique characteristics of a problem such as the introduction of fire into the SNP desert ecosystem increases the ability of simulations to address complex ecological and management questions.

Documenting a plant invasion: The influence of land use on buffelgrass invasion along roadsides in Sonora, Mexico

Buffelgrass is an African grass commonly established in pastures in arid and semiarid regions as forage for livestock. However, once established in pastures, this grass spreads and invades adjacent habitats. We evaluated buffelgrass cover along invaded roadsides and adjacent lands in the state of Sonora, Mexico to explore how land use affect invasion. Additionally, we evaluated the abundance and influence of buffelgrass in five invaded plant communities. We recorded buffelgrass in 63.6% of the sampled sites along roadsides and in 17.5% of adjacent lands in the state of Sonora. Land use has a significant effect on adjacent roadside buffelgrass cover, probably through propagule pressure. Buffelgrass abundance ranged from 1633 to 10,399 adult plants per hectare and from 0 to 1811 seeds/m2 in invaded plant communities. Our analysis of five invaded sites revealed a significant effect of buffelgrass invasion through a reduction on species diversity. Overall, our data from five sites suggest that buffelgrass invasion in rangelands is having an impact of native plant communities in the state of Sonora.

Germination requirements and the influence of buffelgrass invasion on a population of Mammillaria grahamii in the Sonoran Desert

Buffelgrass invasion in the Sonoran Desert affects the regeneration of plant species and the diversity of plant communities. To study the impact of buffelgrass (Cenchrus ciliaris) invasion on the cactus Mammillaria grahamii, we compare density, population structure and association with nurse plants between an undisturbed natural site and an adjacent buffelgrass invaded site, in the Sonoran desert, Mexico. As germination and establishment are important phases determining species survival, we also studied germination requirements of M. grahamii seeds under controlled conditions. Five 5 × 20 m plots were used to compare plant cover, density, population structure and association in each site. A total of 128 individuals in 500 m2 were found in the natural area; 33 were in open areas and 95 under trees. About 71% of individuals under canopies were associated with two nurse species. In the invaded area 7 individuals were found under trees. M. grahamii seeds required light for germination, water potentials higher than - 0.6 MPa and temperatures above 25 °C. Seeds do not germinate after one minute exposure ≥100 °C. Results suggest that buffelgrass invasion affect cacti populations by changing the structure of a desert scrub, burning native plants, decreasing nurse plants, tree cover and changing microclimate.

Biodiversity impacts of an invasive grass: ant community responses to Cenchrus ciliaris in arid Australia

Buffel grass (Cenchrus ciliaris) is a highly invasive species that thrives in semi-arid environments and has the capacity to transform native vegetation outside its native range. However, there is limited information on the effects of buffel grass invasion on native fauna. We used an experimental approach to investigate the impact of buffel grass on the native ant fauna near Alice Springs in semi-arid central Australia. A series of plots where buffel grass was removed and native vegetation had regenerated (B−), paired with adjacent control plots heavily invaded by buffel grass (B+), were used to assess the impact of buffel grass on ant diversity and composition, and on rates of seed dispersal by ants. Differences in ant diversity were also compared between two microhabitat types: bare ground and under cover, to examine the extent to which any impacts were a simple function of change in vegetation cover. Ant abundance and richness were approximately 50 % higher in B− compared with B+ plots, and higher abundance was especially pronounced for the very thermophilic Hot-Climate Specialists. Ant species composition varied significantly between plot types. B− plots supported more species and individuals in both bare and covered microhabitats, which suggests that the differences in ant diversity was not simply through changes in vegetation cover. Rates of seed removal by ants were marginally higher in B− plots. Our findings indicate that buffel grass has a major impact on a dominant faunal group of arid Australia, and possibly reduces the delivery of an important ecosystem service. In addition, our study demonstrates the potential for ecosystem recovery following effective buffel grass management.

Buffel grass and climate change: a framework for projecting invasive species distributions when data are scarce

Invasive species pose a substantial risk to native biodiversity. As distributions of invasive species shift in response to changes in climate so will management priorities and investment. To develop cost-effective invasive species management strategies into the future it is necessary to understand how species distributions are likely to change over time and space. For most species however, few data are available on their current distributions, let alone projected future distributions. We demonstrate the benefits of Bayesian Networks (BNs) for projecting distributions of invasive species under various climate futures, when empirical data are lacking. Using the introduced pasture species, buffel grass (Cenchrus ciliaris) in Australia as an example, we employ a framework by which expert knowledge and available empirical data are used to build a BN. The framework models the susceptibility and suitability of the Australian continent to buffel grass colonization using three invasion requirements; the introduction of plant propagules to a site, the establishment of new plants at a site, and the persistence of established, reproducing populations. Our results highlight the potential for buffel grass management to become increasingly important in the southern part of the continent, whereas in the north conditions are projected to become less suitable. With respect to biodiversity impacts, our modelling suggests that the risk of buffel grass invasion within Australia’s National Reserve System is likely to increase with climate change as a result of the high number of reserves located in the central and southern portion of the continent. In situations where data are limited, we find BNs to be a flexible and inexpensive tool for incorporating existing process-understanding alongside bioclimatic and edaphic variables for projecting future distributions of species invasions.

Habitat use and behaviour of birds in areas invaded by buffel grass (Cenchrus ciliaris L.) and in restored habitat

Context Buffel grass (Cenchrus ciliaris L.) is an introduced pasture grass that has spread over large areas of semiarid Australia with potentially significant negative consequences for biodiversity. Previous studies suggest that the response of bird communities to the changes in habitat associated with buffel grass invasion is complex. Aims This study compares the behaviour of birds between sub-sites with predominantly native vegetation where buffel grass had been removed and control sub-sites with buffel grass, which were left unmanaged, at two locations in central Australia. Our objective was to evaluate whether removing buffel grass at small scales had measurable benefits for local bird species and to better understand how the presence of buffel grass affects bird behaviour. Methods We assessed differences in microhabitat use and behaviour of all species combined and when separated into above ground, flexible and ground foraging guilds. We also tested for direct correlations between different cover types and the microhabitat use and behaviour of birds within guilds, with some further analyses of three common species individually. Key results Management of buffel grass was associated with changes in the microhabitat use and behaviour of birds when all species were combined and for all foraging guilds. Buffel grass cover was negatively correlated with the proportional time birds spent on the ground and with proportional time spent foraging. However, of the three species examined individually only the behaviour of white-winged trillers (Lalage tricolor) was consistently correlated with ground vegetation cover. Our observations also suggest birds spent less time overall at sites with high buffel grass. Conclusions Managing buffel grass in small areas creates islands of habitat that provide important opportunities for foraging at a localised scale. Buffel grass and other ground cover affects the behaviour of birds collectively, but individual species responses vary. Implications Management of buffel grass in small areas with high conservation value should be considered. Our study will help to inform land managers of the benefits of controlling buffel grass on a localised scale.

Spatial pattern and severity of fire in areas with and without buffel grass (Cenchrus ciliaris) and effects on native vegetation in central Australia

AbstractThe spread of buffel grass (Cenchrus ciliaris) in semi‐arid Australia in recent decades has substantially increased ground cover and fuel loads, particularly in open woodland vegetation communities. The resulting alteration of fire regimes may be the most significant impact of buffel invasion on ecological communities in these areas. Broad scale management of buffel grass is currently not an option in Australia but it is becoming increasingly relevant to assess the benefits of restoring areas of native vegetation where preventing buffel grass invasion is no‐longer possible. We managed buffel grass in a series of experimental plots from 2008–2012. In June and August 2011, two unplanned fires burnt through the plots providing a unique opportunity to compare the outcome of wildfire, including the spatial pattern of fire, and the effect on ground vegetation and on a long‐lived, perennial overstorey species, in replicated managed and unmanaged plots. The area of ground that remained unburnt was much greater in managed plots (with predominantly native vegetation) than unmanaged (predominantly buffel grass) plots and where the managed plots did burn the fire was more patchy. This had direct implications for the richness of ground layer plant taxa following fire and the extent to which overstorey trees were exposed to fire. Fire increased pre‐existing differences in the number of taxa in the ground level vegetation, an effect that persisted for the duration of our study, suggesting that fire accelerates direct negative competitive effects between buffel grass and native grasses and forbs. Hakea divaricata (fork‐leafed corkwood) trees in unmanaged buffel grass sites suffered higher burn intensities, and their long‐term viability at this location is likely to be threatened if fires fuelled by buffel grass continue. Our results demonstrate clear benefits of removing fire‐enhancing invasive plants from areas of high conservation value.

Community and Ecosystem Effects of Buffelgrass (Pennisetum ciliare) and Nitrogen Deposition in the Sonoran Desert

AbstractBuffelgrass is a non-indigenous, invasive, C4 grass that was introduced throughout much of southern Texas, the Southwestern United States, and northern and central Mexico to improve degraded rangelands. The successful introduction and spread of buffelgrass follows a trajectory similar to that of other invasive C4 grasses in arid and semiarid ecosystems. In the Plains of Sonora of the Sonoran Desert (Mexico) buffelgrass is favored by widespread removal of native vegetation and seeding, but, why, following initial introduction, the species persists remains unclear. In this study, we addressed two concerns associated with buffelgrass invasion in the Plains of Sonora. We hypothesized that under arid rangeland conditions, buffelgrass outcompetes native herbaceous species (1) through rapid acquisition of limiting nutrients (here assumed to be nitrogen) and (2) under conditions with high nitrogen input. In summer 2002, a 2 by 2 factorial experiment was established with buffelgrass removal and nitrogen addition in both intact desert and converted buffelgrass grassland habitats. In winter 2003, we found that, regardless of habitat type, buffelgrass removal had a positive effect on abundance, biomass, and richness of native herbaceous species while addition of nitrogen, as urea (at 50 kg N ha−1yr−1 or 9.18 lbs N ac−1yr−1), and disturbance resulted in reduction in abundance and biomass. Nitrogen addition did not negatively alter buffelgrass cover. Nitrogen addition had the expected result of increasing initial, peak and total NO3 and NH4 mineralization with the exception of NO3 measures in intact desert. Removal of buffelgrass did not result in significant increases in soil NO3 or NH4 with the exception of peak NH4 in intact desert. Results of this study support observations that native herbaceous species are displaced by buffelgrass invasion and that nitrogen pollution will likely favor buffelgrass over the native herbaceous species in this ecosystem.

Buffelgrass (Pennisetum ciliare) Invades Lands Surrounding Cultivated Pastures in Sonora, Mexico

AbstractWe examined relationships between land disturbance and the extent and abundance of exotic buffelgrass (Pennisetum ciliare) at the interface of cultivated pastures and native desert lands in Sonora, Mexico. Plot and transect surveys of lands inside and outside pasture fences and general linear mixed models revealed complex relationships among buffelgrass, native vegetation, distance from pasture fences, and three categories of land disturbance (undisturbed, moderate, and severe). Results illustrate that buffelgrass invasion is extensive in lands surrounding pastures, and that buffelgrass abundance declines steeply with distance from pasture fences. The role of disturbance is weak but significant in its interaction with distance from the fence. Buffelgrass is more successful at colonizing severely disturbed lands than native vegetation, and its decline in abundance on severely disturbed lands is relatively more gradual than on other disturbance regimes, so landscapes where severely disturbed lands are interspersed with buffelgrass pastures could become centers of extensive buffelgrass invasion. In light of its potential to transform the Sonoran Desert, buffelgrass outside pastures warrants attention in a region-wide control scheme, as well as in future research, which ideally would involve remote sensing.

Propagule pressure, not fire or cattle grazing, promotes invasion of buffel grass Cenchrus ciliaris

Summary The invasion of exotic grasses into savannas is occurring globally. Despite threats to ecosystem processes and biodiversity, few studies have investigated factors that facilitate invasion success. The abundance of the exotic perennial Cenchrus ciliaris buffel grass was assessed across woodland clearings at 46 sites in Queensland, Australia. The influence of fire and grazing on the invasion success were also examined experimentally within 18 sampling stations (including cattle grazed and ungrazed treatment pairs) with unburnt, low‐intensity and high‐intensity fire treatments. Mixed‐effects logistic regressions suggest that the probability of buffel grass occurring in remnants was predominantly a function of the abundance of buffel grass in adjoining paddocks. The abundance of buffel grass in remnants was greater where the boundaries with clearings faced the prevailing wind adjacent to paddocks with high buffel grass abundance. There was a significant positive association between buffel grass and tree canopies at approximately half of the sites, and the strength of this association was positively related to the amount of coarse sand in the surface soil. This association was interpreted as nutrient enrichment from litter fall on sandy infertile soils being a requirement for invasion. The invasion of buffel grass at the experimental site accelerated with above‐average rainfall after a prolonged drought, and invasion continued even after imposing all combinations of fire and grazing. Cattle grazing modestly enhanced invasion, relative to the absence of grazing, but this difference was only significant without burning. The occurrence of fire in both the survey and experiment was not associated with enhanced buffel grass invasion. Invasion of buffel grass may be inevitable over large areas of savanna, especially when abundant rainfall follows periods of drought. However, invasion will be minimized in areas where buffel grass has not been planted, areas without cattle grazing and in areas with low soil fertility. Synthesis and applications. The invasion of buffel grass accelerates during abundant rainfall after drought and is enhanced by propagule pressure, dispersal trajectories and under trees, probably as a result of nutrient enrichment. Disturbance had little influence on invasion, and there is no support for fire‐promoted invasion as predicted by the grass–fire cycle theory.

What Drives the Conversion of Native Rangeland to Buffelgrass (Pennisetum ciliare) Pasture in Mexico’s Sonoran Desert?: The Social Dimensions of a Biological Invasion

The introduction of invasive exotic plants has many social dimensions. Although a diverse literature identifies some of the social drivers of exotic plant introduction and subsequent invasion, relatively little attention has been given to the motives of individuals involved. This research focuses on the extensive conversion of native rangeland to exotic buffelgrass (Pennisetum ciliare) pasture by ranchers in Mexico’s Sonoran Desert using data gathered through systematic interviews and ordinary least squares regression modeling to demonstrate how a few social variables determine the extent of buffelgrass introductions. Results show that land allocation to pasture is determined chiefly by ranch size, with significant roles also played by rotational grazing, buffelgrass seed harvest, and exposure to government research. Results are contextualized and explored in depth, illustrating how the extent of rangeland-to-pasture conversion in this part of the Sonoran Desert is determined by direct and indirect social factors. The study also highlights implications for buffelgrass invasion.