Systematic reduction of natural enemies and competition across variable precipitation approximates buffelgrass invasiveness (Cenchrus ciliaris) in its native range

Invasive grasses cause devastating losses to biodiversity and ecosystem function directly and indirectly by altering ecosystem processes. Escape from natural enemies, plant-plant competition, and variable resource availability provide frameworks for understanding invasion. However, we lack a clear understanding of how natural stressors interact in their native range to regulate invasiveness. In this study, we reduced diverse guilds of natural enemies and plant competitors of the highly invasive buffelgrass across a precipitation gradient throughout major climatic shifts in Laikipia, Kenya. To do this, we used a long-term ungulate exclosure experiment design across a precipitation gradient with nested treatments that (1) reduced plant competition through clipping, (2) reduced insects through systemic insecticide, and (3) reduced fungal associates through fungicide application. Additionally, we measured the interaction of ungulates on two stem-boring insect species feeding on buffelgrass. Finally, we measured a multiyear smut fungus outbreak. Our findings suggest that buffelgrass exhibits invasive qualities when released from a diverse group of natural stressors in its native range. We show natural enemies interact with precipitation to alter buffelgrass productivity patterns. In addition, interspecific plant competition decreased the basal area of buffelgrass, suggesting that biotic resistance mediates buffelgrass dominance in the home range. Surprisingly, systemic insecticides and fungicides did not impact buffelgrass production or reproduction, perhaps because other guilds filled the niche space in these highly diverse systems. For example, in the absence of ungulates, we showed an increase in host-specific stem-galling insects, where these insects compensated for reduced ungulate use. Finally, we documented a smut outbreak in 2020 and 2021, corresponding to highly variable precipitation patterns caused by a shifting Indian Ocean Dipole. In conclusion, we observed how reducing natural enemies and competitors and certain interactions increased properties related to buffelgrass invasiveness.

Extreme drought impacts have been underestimated in grasslands and shrublands globally

Climate change is increasing the frequency and severity of short-term (~1 y) drought events-the most common duration of drought-globally. Yet the impact of this intensification of drought on ecosystem functioning remains poorly resolved. This is due in part to the widely disparate approaches ecologists have employed to study drought, variation in the severity and duration of drought studied, and differences among ecosystems in vegetation, edaphic and climatic attributes that can mediate drought impacts. To overcome these problems and better identify the factors that modulate drought responses, we used a coordinated distributed experiment to quantify the impact of short-term drought on grassland and shrubland ecosystems. With a standardized approach, we imposed ~a single year of drought at 100 sites on six continents. Here we show that loss of a foundational ecosystem function-aboveground net primary production (ANPP)-was 60% greater at sites that experienced statistically extreme drought (1-in-100-y event) vs. those sites where drought was nominal (historically more common) in magnitude (35% vs. 21%, respectively). This reduction in a key carbon cycle process with a single year of extreme drought greatly exceeds previously reported losses for grasslands and shrublands. Our global experiment also revealed high variability in drought response but that relative reductions in ANPP were greater in drier ecosystems and those with fewer plant species. Overall, our results demonstrate with unprecedented rigor that the global impacts of projected increases in drought severity have been significantly underestimated and that drier and less diverse sites are likely to be most vulnerable to extreme drought.

Evidence of plant-soil feedback in South Texas grasslands associated with invasive Guinea grass

Plant-soil feedback (PSF) processes play an integral role in structuring plant communities. In native grasslands, PSF has a largely negative or stabilizing effect on plant growth contributing to species coexistence and succession, but perturbations to a system can alter PSF, leading to long-term changes. Through changes to soil abiotic and biotic properties, invasion by non-native plants has a strong impact on belowground processes with broad shifts in historical PSFs. Guinea grass, Megathyrsus maximus, an emerging invasive in South Texas, can efficiently exclude native plants in part due to its fast growth rate and high biomass accumulation, but its impacts on belowground processes are unknown. Here, we provide a first look at PSF processes in South Texas savannas currently undergoing invasion by Guinea grass. In this pilot study, we addressed the question of how the presence of the invasive M. maximus may alter PSF compared to uninvaded grasslands. Under greenhouse conditions, we assessed germination and growth of Guinea grass and the seed bank in soil collected from grasslands invaded and uninvaded by Guinea grass. We found that Guinea grass grown in soil from invaded grasslands grew taller and accumulated higher biomass than in soil from uninvaded grasslands. Plants grown from the seed bank were more species rich and abundant in soil from uninvaded grasslands but had higher biomass in soil from invaded grasslands. In South Texas savannas, we found evidence to support shifts in the direction of PSF processes in the presence of Guinea grass with positive feedback processes appearing to reinforce invasion and negative feedback processes possibly contributing to species coexistence in uninvaded grasslands. Future work is needed to determine the mechanisms behind the observed shifts in PSF and further explore the role PSF has in Guinea grass invasion.

The invasiveness of Guinea grass (Megathyrsus maximus) is characterized by habitat and differing herbivore assemblages in its native and invaded range

A significant challenge of global change is the human-mediated movement of pasture grasses and their subsequent impact on ecosystem processes when they become invasive. We must understand invasive grass ecology and their natural enemies in native and introduced ranges to mitigate these impacts. Guinea grass (Megathyrsus maximus) is a pantropically introduced pasture grass that escapes intended areas and invades native ecosystems – threatening biodiversity and ecosystem function. The success of invasive plants has often been attributed to ecological release from stressors, including natural enemies and resource availability. Our objective was to assess Guinea grass functional traits across three different habitat types in native and invaded ranges by documenting ungulate and arthropod abundance, diversity, and feeding guilds. Guinea grass functional traits were assessed in three habitat types: grassland, riparian, and woody thickets around nitrogen-fixing Prosopis glandulosa in its introduced range in Texas, USA, and Senegalia mellifera in its native range in Kenya. We characterized Guinea grass functional traits by measuring plant height, cover, biomass, root-to-shoot ratios, and reproductive traits. We then examined the phytophagous arthropod and ungulate abundance and feeding guild diversity across the three habitat types. We hypothesized that functional trait expression related to invasiveness would be associated with Guinea grass in its introduced range. Also, we hypothesized that the abundance and diversity of phytophagous arthropods and ungulates would be lower in the invaded range. Finally, we hypothesized that Guinea grass functional traits would differ between the three habitat types, given the habitat types’ innate differences in resource availability. We found that Guinea grass was 2.5 times taller and 3.3 times more productive and covered 2.5 times more area in its invaded versus native ranges. Introduced Guinea grass had higher reproduction rates with 2.5 times more reproductive tillers, while habitat type drove vegetative reproduction with 15 times more stoloniferous establishment in wooded and riparian sites than grasslands. Texan ungulate communities were less species-rich, less functionally diverse, and less abundant than the Kenyan ungulate community. The phytophagous arthropod diversity on plants was twice as high on Kenyan Guinea grass than on Texan Guinea grass. Total arthropod family richness was nearly double, with 15 families represented in Kenya and 8 in Texas. These results suggest that Guinea grass has escaped a rich assemblage of arthropods and ungulates and likely explains some of its spread in introduced ranges. This study demonstrates how the invasive success of Guinea grass can be understood in terms of its competitive ability and interaction with natural enemies in the introduced and native ranges and may inform future biological control.

Pseudacteon notocaudatus and Pseudacteon obtusitus (Diptera: Phoridae), two new species of fire ant parasitoids from South America

Ongoing studies in South America of phorid flies of the genus Pseudacteon Coquillett 1907 have revealed two further new species in this genus that are described here: P. obtusitus and P. notocaudatus. Both species are parasitoids of Solenopsis (F.) fire ants.

Myrmecomorba nylanderiae gen. et sp. nov., a microsporidian parasite of the tawny crazy ant Nylanderia fulva

A new microsporidian genus and species, Myrmecomorba nylanderiae, is described from North American populations of the tawny crazy ant, Nylanderia fulva. This new species was found to be heterosporous producing several types of binucleate spores in both larval and adult stages and an abortive octosporoblastic sporogony in adult ants. While microsporidia are widespread arthropod parasites, this description represents only the fifth species described from an ant host. Molecular analysis indicated that this new taxon is phylogenetically closely allied to the microsporidian family Caudosporidae, a group known to parasitize aquatic black fly larvae. We report the presence of 3 spore types (Type 1 DK, Type 2 DK, and octospores) with infections found in all stages of host development and reproductive castes. This report documents the first pathogen infecting N. fulva, an invasive ant of considerable economic and ecological consequence.

Geographic distribution suggests that Solenopsis invicta is the host of predilection for Solenopsis invicta virus 1

Solenopsis invicta virus 1 (SINV-1) was found regularly and prevalently in S. invicta. In sampled locations where S. invicta and S. geminata are sympatric (specifically, Gainesville, FL and Travis, TX), SINV-1 was detected in S. geminata. Conversely, in areas in which S. geminata and S. invicta are allopatric, SINV-1 was not detected in S. geminata; these locations included north Australia (n=12), southern Mexico (n=107), Hawaii (n=48), Taiwan (n=12), and the Johnston Atoll (n=6). A similar relationship was observed for S. richteri. In areas in which S. invicta and S. richteri were sympatric, SINV-1 was detected in the S. richteri population, but in areas in which S. invicta and S. richteri were allopatric, SINV-1 was not detected. These occurrences suggest that S. invicta is the host of predilection, or preferred host for SINV-1, and that the congenerics, S. geminata and S. richteri serve as either accidental, reservoir, or transfer hosts. The minus genome strand of SINV-1 was detected in S. geminata and S. richteri indicating that these species may serve as functional hosts capable of supporting SINV-1 replication. SINV-1 was not detected in S. xyloni regardless of its proximity to S. invicta. These results suggest that SINV-1 may be an example of pathogen spillover or pollution.

Indirect competition facilitates widespread displacement of one naturalized parasitoid of imported fire ants by another

Species abundances in natural systems are usually close to some equilibrium, making mechanisms that maintain or prevent species coexistence difficult to discern. Biological control projects provide an opportunity to observe systems transition between equilibriums as a result of the influence of the newly introduced species. In the southeastern United States and Texas, species of phorid fly parasitoids are being sequentially introduced as control agents for imported fire ants. The first two species introduced, Pseudacteon tricuspis and P. curvatus, partition the host niche based upon body size and co-exist broadly in their native range in Argentina, indicating they would form a co-existing and complementary suite of parasitoids in North America. This study examines the interaction between these parasitoids at multiple temporal and spatial scales. Surprisingly, data at all scales reveal that as P. curvatus establishes at a site it competitively displaces P. tricuspis. However, the speed of this reduction appears to differ between ecoregions, suggesting that the rate of displacement depends on environment. At the site where P. curvatus has been established the longest, this population interaction approaches complete displacement. Tests of potential mechanisms causing this displacement reveal that direct competition for host workers alters the operational sex ratio of the P. tricuspis population, but the strength of this effect is insufficient to explain the displacement. Experiments reveal the operation of a strong, indirect effect whereby locally common species preempt reproductive opportunities from rarer species by inducing host behavioral defenses. Finally, a re-examination of published data from their native range reveals that a previously overlooked negative relationship between the densities of these two species also exists there, suggesting that the same processes as those reported here also operate in South America.