Dynamic Responses in a Plant-Insect System to Fertilization by Cormorant Feces

Theoretical arguments suggest that increased plant productivity may not only increase consumer densities but also their fluctuations. While increased consumer densities are commonly observed in fertilization experiments, experiments are seldom performed at a spatial and temporal scale where effects on population fluctuations may be observed. In this study we used a natural gradient in soil fertility caused by cormorant nesting. Cormorants feed on fish but defecate on their nesting islands. On these islands we studied soil nutrient availability, plant nutrient content and the density of Galerucella beetles, main herbivores feeding on Lythrum salicaria. In a common garden experiment, we followed larval development on fertilized plants and estimated larval stoichiometry. Soil nutrient availability varied among islands, and several cormorant islands had very high N and P soil content. Plant nutrient content, however, did not vary among islands, and there was no correlation between soil and plant nutrient contents. Beetle densities increased with plant nutrient content in the field study. However, there was either no effect on temporal fluctuations in beetle density or that temporal fluctuations decreased (at high P). In the common garden experiment, we found limited responses in either larval survival or pupal weights to fertilization. A possible mechanism for the limited effect of fertilization on density fluctuations may be that the distribution of L. salicaria on nesting islands was restricted to sites with a lower N and P content, presumably because high N loads are toxic

Development.time_Pupal.mass_Audusseau_et_al

Data on pupal time (day) and pupal mass (mg) according to species (A. urticae, A. io, P. c-album), family (or clutch), and plant fertilization treatment

Larvae_Pupae_body_content_Audusseau_et_al

Data on body content of nitrogen and phosporus (% dry mass) according to species (Aglais urticae, Aglais io, Polygonia c-album), plant fertilization treatment, and stage (Larva L or Pupa P)

survival_Audusseau_et_al

Number of individuals that failed to reach emergence and number of individuals that reached adulthood according to species (A. urticae, A. io, P. c-album), family (or clutch), and plant fertilization treatment

Plant_nutrient_content_Audusseau_et_al

Leaf content of nitrogen and phosphorus (% dry mass) of plants in the four fertilization treatments and plants collected in nature. Time corresponds to the number of day from the beginning of the experiment

Analysis of deviance table (type II Wald Chi-square test and type III Wald Chi-square test when a model with an interaction term has been selected).

<p><i>Note</i>: Fertilizer, sex, and start date were considered as fixed effects of the models. Family or clutch were considered as a random effect of the mixed models (see text for more details). For size models, Akaike Information Criterion (AIC) values were calculated using maximum likelihood (ML) but other parameters were from REML estimation.</p><p>Values represent the effect of fertilizer, sex, their interaction, and start date on the deviation of development time to pupation (DT) and pupal mass from the control treatment for <i>Aglais urticae</i>, <i>Polygonia c-album</i>, and <i>Aglais io</i>.</p

Leaf N:P ratios of plants in the different fertilization treatments and plants sampled in nature.

<p>Sample sizes were as follows: plants in the control (<i>N</i> = 9), +P (<i>N</i> = 11), +N (<i>N</i> = 11), +N+P (<i>N</i> = 11), and plants sampled in nature (<i>N</i> = 36). For the analysis of leaf N:P ratio according to plant fertilization treatment, all comparisons were significant (Tukey’s tests; <i>p</i> < 0.01) except plants in the control and +P treatments that showed similar N:P ratios (Tukey’s test; <i>p</i> = 0.47). The results from the Wilcoxon pairwise comparisons showed that the N:P ratio of plants sampled in nature were significantly different from plants in the control, +P, and +N treatments (<i>p</i> < 0.05) and overlapped with the N:P ratio of plants in the +N+P treatment (<i>p</i> = 0.17). The horizontal line corresponds to the median of leaf N:P ratios. The bottom and top of the box correspond to the 25th and 75th percentiles, respectively.</p

Deviation of development time from the control (% ± se) (a) and deviation of pupal mass from the control (% ± se) (b), for <i>Aglais urticae</i>, <i>Polygonia c-album</i>, and <i>Aglais io</i> larvae according to plant fertilization treatment.

<p>Deviation of development time from the control (% ± se) (a) and deviation of pupal mass from the control (% ± se) (b), for <i>Aglais urticae</i>, <i>Polygonia c-album</i>, and <i>Aglais io</i> larvae according to plant fertilization treatment.</p