Ecological Stoichiometry and Density Responses of Plant-Arthropod Communities on Cormorant Nesting Islands

<div><p></p><p>Seabirds deposit large amounts of nutrient rich guano on their nesting islands. The increased nutrient availability strongly affects plants and consumers. Consumer response differs among taxonomic groups, but mechanisms causing these differences are poorly understood. Ecological stoichiometry might provide tools to understand these mechanisms. ES suggests that nutrient rich taxa are more likely to be nutrient limited than nutrient poorer taxa and are more favored under nutrient enrichment. Here, we quantified differences in the elemental composition of soil, plants, and consumers between islands with and without nesting cormorant colonies and tested predictions made based on ES by relating the elemental composition and the eventual mismatch between consumer and resource stoichiometry to observed density differences among the island categories. We found that nesting cormorants radically changed the soil nutrient content and thereby indirectly plant nutrient content and resource quality to herbivores. In contrast, consumers showed only small differences in their elemental composition among the island categories. While we cannot evaluate the cause of the apparent homeostasis of invertebrates without additional data, we can conclude that from the perspective of the next trophic level, there is no difference in diet quality (in terms of N and P content) between island categories. Thus, bottom-up effects seemed mainly be mediated via changes in resource quantity not quality. Despite a large potential trophic mismatch we were unable to observe any relation between the invertebrate stoichiometry and their density response to nesting cormorant colonies. We conclude that in our system stoichiometry is not a useful predictor of arthropod responses to variation in resource nutrient content. Furthermore, we found no strong evidence that resource quality was a prime determinant of invertebrate densities. Other factors like resource quantity, habitat structure and species interactions might be more important or masked stoichiometric effects.</p></div

Mean (±SE) body N:C (A), P:C (B), and N:P (C) mass ratios for terrestrial herbivores (H), terrestrial detritivores (D), adult chironomids (C) and terrestrial predators (P).

<p>Mean (±SE) body N:C (A), P:C (B), and N:P (C) mass ratios for terrestrial herbivores (H), terrestrial detritivores (D), adult chironomids (C) and terrestrial predators (P).</p

<b>Table 3.</b> Results of linear mixed effects model (lme) testing for differences in elemental ratios (mean ± SE) of brackish invertebrates between reference islands and cormorant islands (abandoned and active cormorant islands with low and high nest density).

<p>Shown are the number of samples (n), number of islands (islands), F- and p- values from ANOVA for lme, the slope for wave exposure, and mean ± SE over all islands. Significant effects in bold (p<0.05).</p

Relationship between soil and plant nitrogen (%N) (A) and phosphorus (%P) (B) on active, abandoned and reference islands.

<p>The horizontal range corresponds to the range of the soil N and P. The grey diagonal line represents the 1∶1 relation. See <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0061772#pone.0061772.s003" target="_blank">Table S3</a> for ANOVA tables.</p

Relationship between consumer and resource N:C (A and B), P:C (C and D), and N:P (E and F) for terrestrial arthropods (A, C, E) and brackish invertebrates (C, D, F).

<p>Solid lines indicate herbivores, dashed lines detritivores, and dotted lines predators. The horizontal range corresponds to the data range of the resource. The grey diagonal line represents the 1∶1 relation. <u>Resource: Plants</u>: Col: Collembola, Iso: Isopoda, Lep: Lepidoptera larvae, Cer: Cercopidea, Cur: Curculionidae, Chr: Chrysomelidae, Aph: Aphids; <u>Resource: adult Chironomidae:</u> Ara: Araneidae, Lin: Linyphiidae, Tet: Tetragnathidae, Lyc: Lycosidae; <u>Resource: Collembola:</u> Car: Carabidae; <u>Resource: Epiphytes</u>: The: <i>Theodoxus fluviatilis,</i> Chi: Chironomidae larvae, Id: <i>Idotea</i> spp, Gam: <i>Gammarus</i> spp, Ia: <i>Jaera albifrons; </i><u>Resource<i>: Fucus:</i></u> Id_f: <i>Idotea</i> spp.</p

Results of linear mixed effects model (lme) testing for differences in elemental ratios of terrestrial arthropods and insects between reference islands and cormorant islands (abandoned and active).

<p>Shown are the number of samples (n), number of islands (islands), F- and p- values from ANOVA for lme. Significant (p>0.05) differences are bold in the table.</p

Results of linear regressions (lm) and generalized linear models (glm) testing for a linear relationship between plant quality (leaf N:C and P:C-content) and plant quantity (aboveground plant biomass (g/62.5 cm²) and arthropod densities.

<p>Shown are the direction of effect positive (+) and negative (−). Only significant results are shown (p>0.05).</p

Relationship between the elemental mass ratios (N:C and P:C) of invertebrate taxa(A and C), the elemental mismatch between consumers and their resources (B and D) and the effect size of terrestrial arthropods (density) between reference and active cormorant islands (E1_terr) and reference and cormorant islands (E2_terr) and of brackish water invertebrates (biomass) between reference and active cormorant islands with high nest density (E1_brack).

<p>Relationship between the elemental mass ratios (N:C and P:C) of invertebrate taxa(A and C), the elemental mismatch between consumers and their resources (B and D) and the effect size of terrestrial arthropods (density) between reference and active cormorant islands (E1_terr) and reference and cormorant islands (E2_terr) and of brackish water invertebrates (biomass) between reference and active cormorant islands with high nest density (E1_brack).</p

ANOVA table for analysis of arthropod densities as a function of island category (reference island (RF), abandoned cormorant island (AB), active cormorant island (AC) and cormorant island (CO) including both abandoned and active islands).

*<p>For Chironomidae E2 = E1.</p><p>Shown are df, error df, F- and p- values, mean (±SE) individual numbers per island and effect size (E1 and E2). Significant (p>0.05) differences are bold in the table, marginal significant differences (p = 0.051–0.099) are cursive and bold.</p

Soil (A) nitrogen (NH₄⁺ and NO₃⁻) (mg/100 g dry soil) and (B) phosphorus (mg/kg dry soil) content and (C–E) elemental ratios (mean ± SE) of herbs and grasses on reference islands (RF) (non-cormorant islands), abandoned cormorant islands (AB) and active cormorant islands with low (COL) and high (COH) nest density.

<p>Different letters indicate significant differences in linear mixed effect model (A and B) and post-hoc test (C–E).</p