Not just the usual suspects: Insect herbivore populations and communities are associated with multiple plant nutrients

Abstract

Abstract. The relationship between plant nutrient content and insect herbivore populations and community structure has long interested ecologists. Insect herbivores require multiple nutrients, but ecologists have focused mostly on nitrogen (an estimate of plant protein content), and more recently phosphorus (P); other nutrients have received little attention. Here we document nutrient variation in grass and forb samples from grassland habitats in central Nebraska using an elemental approach; in total we measured foliar concentrations of 12 elements (N and P, plus S, B, Ca, Mg, Na, K, Zn, Fe, Mn, and Cu). We detected significant variability among sites for N, P, Mg, Na, K, and Cu. We next used a model selection approach to explore how this nutritional variation and plant biomass correlate with grasshopper densities (collectively and at the feeding-guild level), and principal component analysis to explore nutrient correlations with grasshopper community species composition. When all grasshoppers were pooled, densities varied among sites, but only P was associated with abundance of the elements shown to vary between sites. Different responses occurred at the feeding-guild level. For grass specialists, densities were associated with N, plus P, Mg, and Na. For forb specialists, N and P were often associated with density, but associations with Na and K were also observed. Finally, mixed-feeder abundance was strongly associated with biomass, and to a lesser extent P, Mg, Na, and Cu. At the community level, B, Ca, Zn, and Cu, plus biomass, explained .30% of species composition variation. Our results confirm the positive association of N and P with insect herbivore populations, while suggesting a potential role for Mg, Na, and K. They also demonstrate the importance of exploring effects at the feeding-guild level. We hope our data motivate ecologists to think beyond N and P when considering plant nutrient effects on insect herbivores, and make a call for studies to examine functional responses of insect herbivores to dietary manipulation of Mg, Na, and K. Finally, our results demonstrate correlations between variation in nutrients and species assemblages, but factors not linked to plant nutrient quality or biomass likely explain most of the observed variation.

Dietary phosphate affects food selection, post-ingestive P fate, and performance of a polyphagous herbivore

“…This contention is supported by experimental tests of plant nutrient limitation (Bishop et al, 2010;Elser et al, 2007) and correlations between host plant P content and grasshopper abundance (Joern et al, 2012). Indeed, P levels in terrestrial foliage range widely.…”

“…Comparisons of the P content (%P, as percentage of dry mass and generally dominant in determining C:P ratio) of the plants and herbivores of freshwater and terrestrial systems suggest that P limitation should be as common in terrestrial ecosystems as it is in aquatic systems (Boswell et al, 2008;Elser et al, 2000a;Joern et al, 2012;Lemoine et al, 2014). This contention is supported by experimental tests of plant nutrient limitation (Bishop et al, 2010;Elser et al, 2007) and correlations between host plant P content and grasshopper abundance (Joern et al, 2012).…”

“…It is also well recognized that the nutritional landscape and requirements of animals are multi-dimensional, with striking examples of the importance of particular nutrients (e.g. changes in sodium availability influence reproduction, growth rate and abundance of insects; Joern et al, 2012;Kaspari et al, 2014;Smedley and Eisner, 1996). Nevertheless, we still have a relatively incomplete picture of the relationships between nutrient concentrations and herbivore performance, providing a major impediment to predictive ecology.…”

“…The mean global foliar %P is 0.12%; a few plants have P concentrations as high as 1.00%, but most leaves (87%) have values between 0.05% and 0.45% (Elser et al, 2000a). Joern et al (2012) compared elemental compositions of grasses and forbs collected from central Nebraskan grasslands and found a P range from about 1500 to 3600 ppm (0.15-0.36%P). Boswell et al (2008) showed that Schistocerca americana grasshoppers maintained a body P content of about 1% throughout development, which is at least threefold higher than the values for host plants reported by Joern et al (2012).…”

“…Joern et al (2012) compared elemental compositions of grasses and forbs collected from central Nebraskan grasslands and found a P range from about 1500 to 3600 ppm (0.15-0.36%P). Boswell et al (2008) showed that Schistocerca americana grasshoppers maintained a body P content of about 1% throughout development, which is at least threefold higher than the values for host plants reported by Joern et al (2012). Surprisingly, as yet there are few studies of terrestrial herbivores that have measured the effect of dietary P level below 0.45%P and across the ecologically observed range on consumer growth and survival.…”