Biological and environmental contrasts between aquatic and terrestrial systems have hindered analyses of community and ecosystem structure across Earth's diverse habitats. Ecological stoichiometry provides an integrative approach for such analyses, as all organisms are composed of the same major elements (C, N, P) whose balance affects production, nutrient cycling, and food-web dynamics. Here we show both similarities and differences in the C:N:P ratios of primary producers (autotrophs) and invertebrate primary consumers (herbivores) across habitats. Terrestrial food webs are built on an extremely nutrient-poor autotroph base with C:P and C:N ratios higher than in lake particulate matter, although the N:P ratios are nearly identical. Terrestrial herbivores (insects) and their freshwater counterparts (zooplankton) are nutrient-rich and indistinguishable in C:N:P stoichiometry. In both lakes and terrestrial systems, herbivores should have low growth efficiencies (10-30%) when consuming autotrophs with typical carbon-to-nutrient ratios. These stoichiometric constraints on herbivore growth appear to be qualitatively similar and widespread in both environments.

Traditionally, herbivorous insects are thought to exhibit enhanced performance and outbreak dynamics on water‐stressed host plants due to induced changes in plant physiology. Recent experimental studies, however, provide mixed support for this historical view. To test the plant‐stress hypothesis (PSH), we employed two methods (the traditional vote‐counting approach and meta‐analysis) to assess published studies that investigated insect responses to experimentally induced water‐deficit in plants. For insects, we examined how water deficit affects survivorship, fecundity, density, relative growth rate, and oviposition preference. Responses were analyzed by major feeding guild (sap‐feeding insects and chewing insects) and for the subguilds of sap‐feeders (phloem, mesophyll, and xylem feeders) and chewing insects (free‐living chewers, borers, leaf miners, and gall‐formers). Both vote counting and meta‐analysis found strong negative effects of water stress on the performance of sap‐feeding insects at large and on members of the phloem‐ and mesophyll‐feeding subguilds in particular. Both analytical techniques demonstrated a nonsignificant response for chewing insects at large due to the offsetting effects of water stress on the different subguilds. For example, our analyses found consistent positive responses for borers, negative responses for gall‐formers, and inconsistent responses for free‐living species and leaf miners. Overall, our analyses strongly challenge the historical view that herbivorous insects exhibit elevated performance and outbreak dynamics on water‐stressed plants. Rather, there is widespread evidence that many phytophagous insects, especially sap‐feeders, are adversely affected by continuous water stress. Despite enhanced foliar nitrogen during times of plant stress, concurrent reductions in turgor and water content interfere with an herbivore's ability to access or utilize nitrogen. To explain the discrepancy between the observed outbreaks of phytophagous insects on water‐stressed plants in nature and the negative effects detected in many experimental studies where plants are continuously stressed, we propose a “pulsed stress hypothesis” whereby bouts of stress and the recovery of turgor allow sap‐ feeders to benefit from stress‐induced increases in plant nitrogen. Our finding that phloem‐ feeding insects respond positively on intermittently stressed plants but exhibit poor performance on continuously stressed ones is consistent with this hypothesis and suggests that the phenology of water stress as it mediates nitrogen availability may hold the key to understanding how water stress affects the population dynamics of insect herbivores.

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Dispersal is considered a vital life history characteristic for insects exploiting temporary habitats, and life history theorists have often hypothesized an inverse relationship between dispersal capability and habitat persistence. Most often, this hypothesis has been tested using interspecific comparisons of dispersal capability and qualitative estimates of habitat persistence. Consequently, most assessments have failed to control for possible phylogenetic nonindependence and they also lack quantitative rigor. We capitalized on existing intraspecific variation in the dispersal capability of Prokelisia planthoppers to examine the relationship between habitat persistence and dispersal, thereby minimizing possible phylogenetic effects. Two congeneric species (Prokelisia marginata and P. dolus) occur in the intertidal marshes of North America, where they feed exclusively on cordgrasses (Spartina). Because these planthoppers exhibit wing dimorphism, flight—capable adults (macropters with fully developed wings) are easily differentiated from flightless adults (brachypters with reduced wings). Thus, dispersal capability can be readily estimated by the percentage of macropters in a population. At a regional spatial scale, we found a highly significant negative relationship between dispersal capability (percent macroptery) and habitat persistence. In this system, habitat persistence is influenced by a combination of marsh elevation, winter severity, and tidal range, which interact to determine the ability of planthoppers to endure through winter in their primary habitat for development. P. marginata develops primarily in low—marsh habitats during summer, habitats that can be subjected to pronounced winter disturbance due to ice scouring and/or extensive tidal inundation. Levels of winter disturbance of the low marsh are extreme along the Atlantic coast, intermediate along the Pacific, and low along the Gulf. Both the failure of P. marginata populations to remain through winter in this habitat, and the dispersal ability of these populations (92%, 29%, and 17% macroptery, respectively), are correlated with levels of disturbance. Thus, in regions where winter disturbance is high, levels of dispersal are correspondingly high to allow for recolonization of extirpated habitats from overwintering sites on the high marsh. Unlike P. marginata, P. dolus develops primarily in high—marsh habitats, which are much less disturbed on all coasts during winter. Consequently, this species remains year—round in its primary habitat for development, and most populations exhibit relatively low levels of macroptery (<10%). When raised under common garden conditions, many more macropters of both species were produced from Atlantic compared to Gulf populations. Thus the proportion of macropters produced from the populations used in this experiment paralleled the incidence of macroptery measured in the field, providing evidence that the geographic variation in dispersal capability in both species has in part a genetic basis. The results of this stu...

Phytophagous insects have a much higher nitrogen and phosphorus content than their host plants, an elemental mismatch that places inherent constraints on meeting nutritional requirements. Although nitrogen limitation is well documented in insect herbivores, phosphorus limitation is poorly studied. Using factorial experiments in the laboratory and field, in which levels of soil nitrogen and phosphorus were manipulated, we studied the relative consequences of macronutrient limitation for two herbivores, namely the phloem-feeding planthoppers Prokelisia dolus and P. marginata. These planthoppers inhabit the salt marshes of North America where large stands of their Spartina host plant are found. Notably, these congeners differ in their dispersal abilities; P. marginata is dispersive whereas P. dolus is sedentary. Both nitrogen and phosphorus subsidies enhanced the nitrogen and phosphorus content of Spartina. When P. dolus and P. marginata were raised on plants with an enriched nitrogen signature, they exhibited greater survival, grew to a larger size, developed more rapidly, and achieved higher densities than on nitrogen-deficient plants. However, P. marginata experienced greater fitness penalties than P. dolus on nitrogen-deficient plants. Phosphorus limitation and associated fitness penalties were not as severe as nitrogen limitation for P. marginata, and were not detected in P. dolus. The tempered response of P. dolus to N- and P-deficient Spartina is probably due to its greater investment in feeding musculature and hence ability to compensate for nutrient deficiencies with increased ingestion. To cope with deteriorating plant quality, P. dolus employs compensatory feeding, whereas P. marginata disperses to higher quality Spartina. When its option of dispersal is eliminated and P. marginata is confined on nutrient-deficient plants, its performance is drastically reduced compared with P. dolus. This research highlights the importance of interfacing herbivore life-history strategies with ecological stoichiometry in order to interpret the consequences of macronutrient limitation on herbivore performance and population dynamics.