Gary N. Ervin

Herbivory: caterpillar saliva beats plant defences.

Blood-feeding arthropods secrete special salivary proteins that suppress the defensive reaction they induce in their hosts. This is in contrast to herbivores, which are thought to be helpless victims of plant defences elicited by their oral secretions. On the basis of the finding that caterpillar regurgitant can reduce the amount of toxic nicotine released by the tobacco plant Nicotiana tabacum, we investigate here whether specific salivary components from the caterpillar Helicoverpa zea might be responsible for this suppression. We find that the enzyme glucose oxidase counteracts the production of nicotine induced by the caterpillar feeding on the plant.

An ecological perspective of allelochemical interference in land–water interface communities

Allelochemical interactions among aquatic macrophytes and between macrophytes and attached microbial assemblages (epiphyton) influence a number of ecological processes. The ecological importance of these interactions, however, is poorly understood; we hypothesize that paucity has resulted, in part, from (1) a narrow focus on exploration for herbicidal plant products from aquatic macrophytes, (2) the difficulties in distinguishing resource competition from allelopathic interference, and (3) a predominance of approaching aquatic allelopathy from a terrestrial perspective. Based upon recent thorough investigations of allelopathy among aquatic vascular plants, chemical compounds that influence competitive interactions among littoral organisms are amphiphilic compounds that tend to remain near the producing organism (e.g., polyphenolic compounds and volatile fatty acids). Production of these compounds may be influenced by relative availability of nutrients (particularly phosphorus and nitrogen), inorganic carbon, and light. Macrophyte strategies of clonal reproduction, in an effort to persist in these highly productive and competitive habitats, have contributed to reduced reliance upon sexual reproduction that is correlated with allelopathic autotoxicity among several dominant wetland plant species. Although few studies document the importance of allelochemical interactions in the wetland and littoral zones of aquatic ecosystems, abundant evidence supports the potential for significant effects on competition and community structure; effects of altered nutrient ratios and availability on plant chemical composition; and resultant effects on trophic interactions, particularly suppression of herbivory, competitive attached algae and cyanobacteria, and heterotrophic utilization of organic matter by bacteria and fungi.

EVALUATING NON-NATIVE SPECIES AND WETLAND INDICATOR STATUS AS COMPONENTS OF WETLANDS FLORISTIC ASSESSMENT

We evaluated a potential index for quantifying wetland floristic quality, based on the Floristic Quality Assessment Index (FQAI) developed and tested in other regions of the United States. Principal reasons for this study were 1) FQAI is based on plant species’ coefficients of conservatism, which are unavailable for most of the world and 2) FQAI value calculation mathematically neglects exotic (non-native) species. The index that we evaluated, termed the Floristic Assessment Quotient for Wetlands (FAQWet), incorporated components of total species richness, wetland affinity of species present, and the contribution of native versus exotic species to wetland vegetation quality. Thus, this index incorporated factors demonstrated to be influenced by the degree of human activity on the landscape while including both native and non-native plant species, and it used information that is readily available for most plant species encountered in wetlands of the United States. Adequacy of this index at representing perceived human influence on wetlands was evaluated by sampling vascular plant assemblages in 53 wetlands across northern Mississippi, USA. Correlation of FAQWet scores with semi-quantitative disturbance indicators suggested that FAQWet Index calculations weighted by proportion and frequency of native species performed best overall and were comparable with the currently used FQAI, although all indices displayed low correlation with indices of human activity. Analyses of individual components of the disturbance index, along with information on hydrogeomorphology and hydrologic alteration, helped account for some unexplained variation in the relationship between floristic quality and disturbance. For example, there were more exotic species and a lower overall degree of wetland affinity in vegetation of depressional wetlands than in lake fringe and riverine wetlands. The most important benefits of the FAQWet index in these studies were a lack of correlation with total plant species richness (unlike the FQAI), a stronger correlation with non-native species richness than was the case with the FQAI, and the ease of obtaining wetland indicator status and nativity status for FAQWet calculations. Results also highlighted the potential danger of ignoring exotic species in floristic assessments because of the relatively strong correlations of non-native species richness with human activity, hydrologic impairment, and floristic index scores.

Geographic patterns of genetic diversity from the native range of Cactoblastis cactorum (Berg) support the documented history of invasion and multiple introductions for invasive populations

Spread of the invasive cactus-feeding moth Cactoblastis cactorum has been well documented since its export from Argentina to Australia as a biocontrol agent, and records suggest that all non-native populations are derived from a single collection in the moth’s native range. The subsequent global spread of the moth has been complex, and previous research has suggested multiple introductions into North America. There exists the possibility of additional emigrations from the native range in nursery stock during the late twentieth century. Here, we present mitochondrial gene sequence data (COI) from South America (native range) and North America (invasive range) to test the hypothesis that the rapid invasive spread in North America is enhanced by unique genetic combinations from isolated portions of the native range. We found that haplotype richness in the native range of C. cactorum is high and that there was 90% lower richness in Florida than in Argentina. All Florida C. cactorum haplotypes are represented in a single, well-defined clade, which includes collections from the reported region of original export from Argentina. Thus, our data are consistent with the documented history suggesting a single exportation of C. cactorum from the eastern region of the native range. Additionally, the presence of geographic structure in three distinct haplotypes within the same clade across Florida supports the hypothesis of multiple introductions into Florida from a location outside the native range. Because the common haplotypes in Florida are also known to occur in the neighboring Caribbean Islands, the islands are a likely source for independent North American colonization events. Our data show that rapid and successful invasion within North America cannot be attributed to unique genetic combinations. This suggests that successful invasion of the southeastern US is more likely the product of a fortuitous introduction into favorable abiotic conditions and/or defense responses of specific Opuntia hosts, rapid adaptation, or a release from native enemies.

Abundance patterns of dragonflies along a wetland buffer

Local abundance of animals with aquatic and terrestrial life stages may be useful to determine criteria for protective buffers around wetlands. Maiden flights and daily commutes of adult Odonata (damselflies, dragonflies) occur between wetland breeding area and adjacent upland habitat used for foraging, maturation, and nocturnal roosting. We measured abundance of dragonflies adjacent to a wetland in Mississippi, USA to determine if abundance varied with distance from water. Sexually mature males and combined females/prereproductive adult males (females-immatures) were recorded 10– 160 m from the littoral edge of a 185 ha shallow reservoir. The number of dragonflies was dominated by Celithemis eponina throughout the study period. Mean abundance did not change with distance from water out to 160 m, both for all species combined and for each of three dominant species. In the assemblage, mature males outnumbered females-immatures in the 10–40 m distance, whereas the reverse occurred in the 130–160 m distance. At the species-level, there was a mixed response in the mature male: female-immature ratio, with little resemblance to the assemblage pattern. Results of this study suggest that wide buffer zones around wetlands may be essential to protect Odonata assemblages, especially females and sexually immature adults. Furthermore, odonate flight behavior may serve as a useful biocriterion to determine the width of ecologically significant wetland buffers.

Distribution of Adult Odonata Among Localized Wetlands in East-central Mississippi

Abstract We measured species richness and composition of adult Odonata and inferred habitat preferences among man-made wetland sites and surrounding tracts of natural bottomland forest. Cumulative species richness and composition were described by proportion coefficients and beta diversity indices. The three man-made sites provided open space resources, and more species were observed in each than in the floodplain forest. Twenty-nine of 42 species documented over a four-month period were observed in only one or two of the four wetlands studied. Large differences in species assemblages between the immediately adjacent ditch and marsh sites were the best evidence for high habitat affinity because distance and structural barriers to movement were absent. Such compositional asymmetry may reflect differential vegetative and reproductive suitability of the habitats. Results suggest that the open-canopy wetlands supported higher diversity of adult Odonata, and that distinct odonate assemblages were found among different habitat types in this flood-plain wetland complex.

SPATIO-TEMPORALLY VARIABLE EFFECTS OF A DOMINANT MACROPHYTE ON VASCULAR PLANT NEIGHBORS

Although considerable attention has been focused on competition among wetland plants, surprisingly few studies have considered the role of facilitation in structuring freshwater wetland plant assemblages. Positive interaction among plants has been investigated thoroughly in saline marshes, which display a definite, predictable pattern of species zonation, with facilitation playing an important role during colonization of bare or disturbed patches of marsh. The few studies that have investigated facilitation in freshwater marshes have implicated the processes of sediment oxygenation and sediment stabilization. Whereas both processes are known to be influenced by many freshwater hydrophyte species, they have not been considered widely as mechanisms of interspecific facilitation. In the tussock-forming rushJuncus effusus L., late-Spring collapse of culms away from the tussock center results in a morphology that increases light availability atop tussocks and appears to establish a unique spatio-temporal component to the effect ofJuncus on neighbors. In previous work, shading around the tussock periphery, influenced largely by this collapse of culms, was shown to suppress production and species richness of neighbors; however, various plant species have been observed to grow directly upon theJuncus tussocks in the void left by collapsed culms. To quantify this latter phenomenon, vascular plant abundance was evaluated on tussocks ofJuncus effusus in four Mississippi (USA) beaver wetlands to determine the likelihood and nature of positive interactions betweenJuncus and neighbors. Tussocks in three of these wetlands were examined for tussock microtopography and diameter, relative water depth atop the tussock, and colonization by other plant species. Species richness of colonizing plants correlated positively with tussock diameter in two of these wetlands, and no colonizers were present in the third. Tussock microtopography and elevation differed significantly among wetlands, but species richness was unrelated to relative water depth atop the tussocks. In the fourth beaver marsh, two sets of surveys were conducted to determine whether the species assemblages uponJuncus tussocks differed from those across the entire wetland. Plant assemblages onJuncus tussocks, and on elevated substrata in general, were significantly different from assemblages not associated with elevated surfaces. Thirteen species were associated exclusively with elevated rooting substrata, ten of which were exclusive toJuncus effusus tussocks. The present results suggest that small-scale spatial and temporal shifts occur in the effects ofJuncus on neighboring plants. Effects shift from primarily competitive shading interactions resulting from a dense shoot canopy to facilitative interactions atop tussock mounds after mature culms collapse. Facilitation likely is mediated through characteristics of the tussock mound, such as provision of a stable rooting substratum or oxygenation of sediments by theJuncus effusus root system.

Temporal effects on jasmonate induction of anti-herbivore defense in Physalis angulata: seasonal and ontogenetic gradients

Patterns of defensive enzyme (polyphenol oxidase (PPO)) induction were examined in Physalis angulata L. (cutleaf ground cherry) across seasonal and ontogenetic time gradients, in response to foliar jasmonic acid (JA) application. These studies showed strongest JA induction of PPO activity in young plants and during the month of July (mid-summer). In older plants, JA treatment was correlated with a decrease in PPO activity, and no significant effects of JA treatment were observed beyond September. Developmental patterns of induction corresponded with published studies of defense induction in other species, indicating active defense of young tissues and potential shifts in defensive strategy as plants transition from growth to reproduction. Peak induction during summer corresponds with periods of high solar irradiance and herbivorous insect populations, both of which are stresses against which PPO has been demonstrated effective.

Host plant defense signaling in response to a coevolved herbivore combats introduced herbivore attack

Defense-free space resulting from coevolutionarily naïve host plants recently has been implicated as a factor facilitating invasion success of some insect species. Host plants, however, may not be entirely defenseless against novel herbivore threats. Volatile chemical-mediated defense signaling, which allows plants to mount specific, rapid, and intense responses, may play a role in systems experiencing novel threats. Here we investigate defense responses of host plants to a native and exotic herbivore and show that (1) host plants defend more effectively against the coevolved herbivore, (2) plants can be induced to defend against a newly-associated herbivore when in proximity to plants actively defending against the coevolved species, and (3) these defenses affect larval performance. These findings highlight the importance of coevolved herbivore-specific defenses and suggest that naïveté or defense limitations can be overcome via defense signaling. Determining how these findings apply across various host–herbivore systems is critical to understand mechanisms of successful herbivore invasion.

Ecotype Variability and Edaphic Characteristics for Cogongrass (Imperata cylindrica) Populations in Mississippi

Abstract Cogongrass is a highly invasive, perennial grass that is found on all continents, except Antarctica. It continues to spread at an alarming rate in the southeastern United States. Cogongrass has been reported from a wide array of habitats; however, soils from areas where cogongrass grows have never been characterized. Live cogongrass plants, herbarium specimens, and soil samples were collected from 53 cogongrass populations from across the 10 physiographic regions and land use areas in Mississippi. Cogongrass leaf and inflorescence morphology varied among sites, and plants were found in soils varying widely in texture (ranging from 28 to 86% sand, 3 to 48% silt, and 6 to 43% clay), organic matter content (ranging from 0.9 to 5.0%), pH (ranging from 4.4 to 8.0), and nutrient status: 6 to 190 kg ha−1 (15 to 470 lb A−1) of phosphorus (P), 46 to 734 kg ha−1 of potassium (K), 150 to 7,620 kg ha−1 of calcium (Ca), 26 to 1,090 kg ha−1 of magnesium (Mg), 1 to 190 kg ha−1 of zinc (Zn), 145 to 800 kg ha−1 of estimated sulfur (S) based on organic matter, and 57 to 300 kg ha−1 of sodium (Na). These soil parameters were highly variable among cogongrass populations, even within physiographic regions or land use areas, and encompassed much of the soil physiochemical diversity within the state. Soil characteristics were significantly correlated with leaf length (Ca, K, Mg, P, Zn, and percentage of sand and silt), leaf width (Ca, P, Mg, and percentage of sand and silt), the leaf length-to-width ratio (K and P), inflorescence length (Na, P, and pH), inflorescence width (S, organic matter, and pH), and the inflorescence length-to-width ratio (S and organic matter). These data indicate that cogongrass is able to establish, emerge, grow, and reproduce on a wide array of soils in Mississippi. This ability provides cogongrass an advantage over other plant species that are more limited in the soil types that support their growth. Nomenclature: Cogongrass, Imperata cylindrica (L.) Beauv. IMPCY