No abstract

Summary1. Globalization and climate change trigger species invasions and range shifts, which reshuffle communities at an exceptional rate and expose plant migrants to unfamiliar herbivores. Dominant hypotheses to predict plant success are based on evolutionary novelty: either herbivores are maladapted to consume novel plants (enemy release hypothesis), or novel plants are maladapted to deter herbivores (biotic resistance hypothesis). Since novelty can work both ways, it fails to consistently predict when herbivores will consume novel over non-novel plants. Surprisingly, the value of using plant traits to predict herbivore consumption of novel plants remains largely unexplored. We hypothesized that (i) plant traits explain generalist herbivore consumption rates of novel and non-novel plants, and (ii) any effect of novelty will be grounded in consistent trait differences between native and novel plants. Lastly, we expected to find (iii) differences in plant traits and plant consumption rates across latitude. 2. To test these hypotheses, we measured the consumption rate of plant species for a tropical and a temperate generalist herbivore in controlled feeding trials by offering them a large variety of 40 plant species from different geographical origins. Therefore, whether a plant was novel depended on the herbivore used, allowing us to disentangle plant identity from plant novelty. We also measured plant chemical traits and determined whether traits, geographical origin or novelty best explained herbivore consumption rates. 3. Both generalist herbivores consumed more of plants with a high nitrogen-to-phenolic compounds ratio, irrespective of the plant's novelty to the herbivore. A pattern of increasing plant's nitrogen-to-phenolics ratio with latitude could explain why both the tropical and temperate herbivore consumed more of plants from temperate regions. Plant novelty and its geographical origin no longer explained consumption rates once differences in nitrogen-to-phenolic compounds ratio were taken into account. 4. We show that differences in plant traits along a latitudinal cline determine herbivore consumption rates, irrespective of whether plants are novel or familiar. Therefore, we propose that integrating evolutionary novelty theory with plant traits and biogeography will increase our understanding of the consequences of plant species migration beyond biogeographical barriers.

Global warming is expected to strengthen herbivore-plant interactions leading to enhanced top-down control of plants. However, latitudinal gradients in plant quality as food for herbivores suggest lower palatability at higher temperatures, but the underlying mechanisms are still unclear. If plant palatability would decline with temperature rise, then this may question the expectation that warming leads to enhanced top-down control. Therefore, experiments that directly test plant palatability and the traits underlying palatability along a temperature gradient are needed. Here we experimentally tested the impact of temperature on aquatic plant growth, plant chemical traits (including stoichiometry) and plant palatability. We cultured three aquatic plant species at three temperatures (15, 20, and 25°C), measured growth parameters, determined chemical traits and performed feeding trial assays using the generalist consumer Lymnaea stagnalis (pond snail). We found that rising temperature significantly increased the growth of all three aquatic plants. Plant nitrogen (N) and phosphorus (P) content significantly decreased, and carbon (C):N and C:P stoichiometry increased as temperature increased, for both Potamogeton lucens and Vallisneria spiralis, but not for Elodea nuttallii. By performing the palatability test, we found that rising temperatures significantly decreased plant palatability in P. lucens, which could be explained by changes in the underlying chemical plant traits. In contrast, the palatability of E. nuttallii and V. spiralis was not affected by temperature. Overall, P. lucens and V. spiralis were always more palatable than E. nuttallii. We conclude that warming generally stimulates aquatic plant growth, whereas the effects on chemical plant traits and plant palatability are species-specific. These results suggest that the outcome of the impact of temperature rise on macrophyte stoichiometry and palatability from single-species studies may not be broadly applicable. In contrast, the plant species tested consistently differed in palatability, regardless of temperature, suggesting that palatability may be more strongly linked to species identity than to intraspecific variation in plant stoichiometry.

Invasive plant species are among the major threats to freshwater biodiversity. Few experimental studies have investigated whether native plant diversity can provide biotic resistance to invaders in freshwater ecosystems. At small spatial scales, invasion resistance may increase with plant species richness due to a better use of available resources, leaving less available for a potential invader (Complementarity effect) and/or the greater probability to have a highly competitive (or productive) native species in the community (Selection effect). In submerged aquatic plant communities, we tested the following hypotheses: (1) invader establishment success is greatest in the absence of a native plant community; (2) lower in plant communities with greater native species richness, due to complementary and/or selection effects; and (3) invader establishment success would be lowest in rooted plant communities, based on the limiting similarity theory as the invader is a rooted submerged species. In a greenhouse experiment, we established mesocosms planted with 0 (bare sediment), 1, 2, and 4 submerged plant species native to NW Europe and subjected these to the South African invader Lagarosiphon major (Ridl.) Moss. We used two rooted (Myriophyllum spicatum L., Potamogeton perfoliatus L.) and two non-rooted native species (Ceratophyllum demersum L., Utricularia vulgaris L.) representing two distinct functional groups considering their nutrient acquisition strategy which follows from their growth form, with, respectively, the sediment and water column as their main nutrient source. We found that the presence of native vegetation overall decreased the establishment success of an alien aquatic plant species. The strength of this observed biotic resistance increased with increasing species richness of the native community. Mainly due to a selection effect, the native biomass of mixed communities overyielded, and this further lowered the establishment success of the invader in our experiment. The strongest biotic resistance was caused by the two native plant species that were of the same functional group, i.e., functionally most similar to the invader. These results support the prediction of Elton’s biotic resistance hypothesis in aquatic ecosystems and indicate that both species richness and functional group identity can play an important role in decreasing establishment success of alien plant species.

Non-native species introductions are widespread and can affect ecosystem functioning by altering the structure of food webs. Invading plants often modify habitat structure, which may affect the suitability of vegetation as refuge and could thus impact predator-prey dynamics. Yet little is known about how the replacement of native by non-native vegetation affects predator-prey dynamics. We hypothesize that plant refuge provisioning depends on (1) the plant’s native status, (2) plant structural complexity and morphology, (3) predator identity, and (4) prey identity, as well as that (5) structurally similar living and artificial plants provide similar refuge. We used aquatic communities as a model system and compared the refuge provided by plants to macroinvertebrates (Daphnia pulex, Gammarus pulex and damselfly larvae) in three short-term laboratory predation experiments. Plant refuge provisioning differed between plant species, but was generally similar for native (Myriophyllum spicatum, Ceratophyllum demersum, Potamogeton perfoliatus) and non-native plants (Vallisneria spiralis, Myriophyllum heterophyllum, Cabomba caroliniana). However, plant refuge provisioning to macroinvertebrate prey depended primarily on predator (mirror carp: Cyprinus carpio carpio and dragonfly larvae: Anax imperator) and prey identity, while the effects of plant structural complexity were only minor. Contrary to living plants, artificial plant analogues did improve prey survival, particularly with increasing structural complexity and shoot density. As such, plant rigidity, which was high for artificial plants and one of the living plant species evaluated in this study (Ceratophyllum demersum), may interact with structural complexity to play a key role in refuge provisioning to specific prey (Gammarus pulex). Our results demonstrate that replacement of native by structurally similar non-native vegetation is unlikely to greatly affect predator-prey dynamics. We propose that modification of predator-prey interactions through plant invasions only occurs when invading plants radically differ in growth form, density and rigidity compared to native plants.