Environmental gradients and herbivore feeding preferences in coastal salt marshes

Herbivory patterns along the intertidal gradient of Juncus roemerianus salt marshes

Grasshopper herbivory can vary substantially among locations within a salt marsh or among marshes, but its variability along the marsh intertidal gradient (extending from the shoreline to the upland fringing forest) is not well reported. Previous papers have shown that grasshopper herbivory may affect nutrient processes in salt marsh ecosystems, but how such effects are tied up to the intensity of herbivory and how they vary spatially is poorly known. To help address these gaps, we evaluated whether grasshopper herbivory intensity and herbivore abundance together with other plant characteristics (such as total leaf length, plant live and dead biomass, plant nutrient content and plant nutrient standing stocks) varied along the intertidal gradient of two black needlerush marshes in the Northern Gulf of Mexico. Our results show that in one marsh grazing intensity decreased from the shoreline to the forest tree line, but in the other there was similar grazing intensity across the entire intertidal gradient. None of the measured plant characteristics followed the differences in herbivory found along the intertidal gradient and between salt marshes. We also found that, in the salt marsh with decreasing herbivory towards the upland edge, the combination of herbivory, plant nutrient content and plant nutrient standing stocks suggest two different functional zones along the intertidal gradient, one of nutrient availability and recycling near the shoreline and another one of nutrient inmobilization near the upland fringing forest. In concert, the results suggest that grasshopper herbivory intensity may vary along the intertidal gradient in some marshes, but not in others. In turn, spatial differences in herbivory along the intertidal gradient, if they occur, may influence nutrient processes, such as recycling and storage, leading to associated spatial differences in nutrient dynamics in the salt marsh.

Herbivore impacts on marsh production depend upon a compensatory continuum mediated by salinity stress

Plant communities are disturbed by several stressors and they are expected to be further impacted by increasing anthropogenic stress. The consequences of these stressors will depend, in part, upon the ability of plants to compensate for herbivory. Previous studies found that herbivore impacts on plants can vary from negative to positive because of environmental control of plant compensatory responses, a.k.a. the Compensatory Continuum Hypothesis. While these influential studies enhanced our appreciation of the dynamic nature of plant-herbivore interactions, they largely focused on the impact of resource limitation. This bias limits our ability to predict how other environmental factors will shape the impact of herbivory. We examined the role of salinity stress on herbivory of salt marsh cordgrass, Spartina foliosa, by an herbivore previously hypothesized to influence the success of restoration projects (the scale insect, Haliaspis spartinae). Using a combination of field and mesocosm manipulations of scales and salinity, we measured how these factors affected Spartina growth and timing of senescence. In mesocosm studies, Spartina overcompensated for herbivory by growing taller shoots at low salinities but the impact of scales on plants switched from positive to neutral with increasing salinity stress. In field studies of intermediate salinities, scales reduced Spartina growth and increased the rate of senescence. Experimental salinity additions at this field site returned the impact of scales to neutral. Because salinity decreased scale densities, the switch in impact of scales on Spartina with increasing salinity was not simply a linear function of scale abundance. Thus, the impact of scales on primary production depended strongly upon environmental context because intermediate salinity stress prevented plant compensatory responses to herbivory. Understanding this context-dependency will be required if we are going to successfully predict the success of restoration efforts and the ecological consequences of anthropogenic disturbances.

Preference and performance in plant-herbivore interactions across latitude--a study in U.S. Atlantic salt marshes

High-latitude plants are often more palatable to herbivores than low-latitude conspecifics. Does increased plant palatability lead to better herbivore performance? Our field and laboratory work investigated (A) whether high-latitude plants have traits indicating that they should be higher-quality foods for herbivores; (B) whether geographic differences in plant quality are more important than local adaptation of herbivores. We studied 3 plant species and 6 invertebrate herbivores in U.S. Atlantic Coast. Past studies had shown high-latitude individuals of these plants are more palatable than low-latitude conspecifics. We documented plant traits and herbivore performance (body size) in the field across latitude. We collected individuals from different latitudes for factorial (plant region x herbivore region) laboratory experiments, examining how herbivore performance was affected by plant region, herbivore region, and their interaction (i.e., local adaptation). Field surveys suggested high-latitude plants were likely of higher quality to herbivores. Leaf nitrogen content in all plant species increased toward high latitudes, consistent with lower leaf C/N and higher leaf chlorophyll content at high latitudes. Furthermore, leaf toughness decreased toward higher latitudes in 1 species. The body size of 4 herbivore species increased with latitude, consistent with high-latitude leaves being of higher quality, while 2 grasshopper species showed the opposite pattern, likely due to life-history constraints. In the laboratory, high-latitude plants supported better performance in 4 herbivore species (marginal in the 5th). The geographic region where herbivores were collected affected herbivore performance in all 6 species; however, the pattern was mixed, indicating a lack of local adaptation by herbivores to plants from their own geographic region. Our results suggest that more-palatable plants at high latitudes support better herbivore growth. Given that geographic origin of either plants or herbivores can affect herbivore performance, the nature of plant-herbivore interactions is likely to change if climate change “reshuffles” plant and herbivore populations across latitude.

A trophic cascade triggers collapse of a salt-marsh ecosystem with intensive recreational fishing

Overexploitation of predators has been linked to the collapse of a growing number of shallow-water marine ecosystems. However, salt-marsh ecosystems are often viewed and managed as systems controlled by physical processes, despite recent evidence for herbivore-driven die-off of marsh vegetation. Here we use field observations, experiments, and historical records at 14 sites to examine whether the recently reported die-off of northwestern Atlantic salt marshes is associated with the cascading effects of predator dynamics and intensive recreational fishing activity. We found that the localized depletion of top predators at sites accessible to recreational anglers has triggered the proliferation of herbivorous crabs, which in turn results in runaway consumption of marsh vegetation. This suggests that overfishing may be a general mechanism underlying the consumer-driven die-off of salt marshes spreading throughout the western Atlantic. Our findings support the emerging realization that consumers play a dominant role in regulating marine plant communities and can lead to ecosystem collapse when their impacts are amplified by human activities, including recreational fishing.

Consequences of omnivory for trophic interactions on a salt marsh shrub

Although omnivory is common in nature, its impact on trophic interactions is variable. Predicting the food web consequences of omnivory is complicated because omnivores can simultaneously produce conflicting direct and indirect effects on the same species or trophic level. We conducted field and laboratory experiments testing the top-down impacts of an omnivorous salt marsh crab, Armases cinereum, on the shrub Iva frutescens and its herbivorous and predatory arthropod fauna. Armases is a "true omnivore," consuming both Iva and arthropods living on Iva. We hypothesized that Armases would benefit Iva through a top-down trophic cascade, and that this benefit would be stronger than the direct negative effect of Armases on Iva. A field experiment on Sapelo Island, Georgia (USA), supported this hypothesis. Although Armases suppressed predators (spiders), it also suppressed herbivores (aphids), and benefited Iva, increasing leaf number, and reducing the proportion of dead shoots. A one-month laboratory experiment, focusing on the most common species in the food web, also supported this hypothesis. Armases strongly suppressed aphids and consumed fewer Iva leaves if aphids were available as an alternate diet. Armases gained more body mass if they could feed on aphids as well as on Iva. Although Armases had a negative effect on Iva when aphids were not present, Armases benefited Iva if aphids were present, because Armases controlled aphid populations, releasing Iva from herbivory. Although Armases is an omnivore, it produced strong top-down forces and a trophic cascade because it fed preferentially on herbivores rather than plants when both were available. At the same time, the ability of Armases to subsist on a plant diet allows it to persist in the food web when animal food is not available. Because omnivores feed on multiple trophic levels, their effects on food webs may differ from those predicted by standard trophic models that assume that each species feeds only on a single trophic level. To better understand the complexity of real food webs, the variable feeding habits and feeding preferences of different omnivorous species must be taken into consideration.

Herbivore responses to nutrient enrichment and landscape heterogeneity in a mangrove ecosystem

Complex gradients in forest structure across the landscape of offshore mangrove islands in Belize are associated with nutrient deficiency and flooding. While nutrient availability can affect many ecological processes, here we investigate how N and P enrichment interact with forest structure in three distinct zones (fringe, transition, dwarf) to alter patterns of herbivory as a function of folivory, loss of yield, and tissue mining. The effects of nutrient addition and zone varied by functional feeding group or specific herbivore. Folivory ranged from 0 to 0.4% leaf area damaged per month, but rates did not vary by either nutrient enrichment or zone. Leaf lifetime damage ranged from 3 to 10% of the total leaf area and was caused primarily by the omnivorous tree crab Aratus pisonii. We detected two distinct spatial scales of response by A. pisonii that were unrelated to nutrient treatment, i.e., most feeding damage occurred in the fringe zone and crabs fed primarily on the oldest leaves in the canopy. Loss of yield caused by the bud moth Ecdytolopha sp. varied by zone but not by nutrient treatment. A periderm-mining Marmara sp. responded positively to nutrient enrichment and closely mirrored the growth response by Rhizophora mangle across the tree height gradient. In contrast, a leaf-mining Marmara sp. was controlled by parasitoids and predators that killed >89% of its larvae. Thus, nutrient availability altered patterns of herbivory of some but not all mangrove herbivores. These findings support the hypothesis that landscape heterogeneity of the biotic and abiotic environment has species-specific effects on community structure and trophic interactions. Predicting how herbivores respond to nutrient over-enrichment in mangrove ecosystems also requires an assessment of habitat heterogeneity coupled with feeding strategies and species-specific behavior measured on multiple scales of response.