Induced defenses in herbivores and plants differentially modulate a trophic cascade

Inducible defenses are dynamic traits that modulate the strength of both plant–herbivore and herbivore–carnivore interactions. Surprisingly few studies have considered the relative contributions of induced plant and herbivore defenses to the overall balance of bottom-up and top-down control. Here we compare trophic cascade strengths using replicated two-level and three-level plankton communities in which we systematically varied the presence or absence of induced defenses at the plant and/or herbivore levels. Our results show that a trophic cascade, i.e., significantly higher plant biomass in three-level than in two-level food chains, occurred whenever herbivores were undefended against carnivores. Trophic cascades did not occur when herbivores exhibited an induced defense. This pattern was obtained irrespective of the presence or absence of induced defenses at the plant level. We thus found that herbivore defenses, not plant defenses, had an overriding effect on cascade strength. We discuss these results in relation to variation in cascade strengths in natural communities.

Inducible colony formation within the Scenedesmaceae: adaptive responses to infochemicals from two different herbivore taxa

We studied the occurrence of colony formation within 40 different strains of Scenedesmaceae (Chlorococcales, Chlorophyta) in response to grazing-released infochemicals from the herbivorous zooplankters Brachionus calyciflorus Pallas (Rotifera) and Daphnia magna Strauss (Cladocera). With the exception of two strains, all strains showed similar responses to both B. calyciflorus and D. magna infochemicals, either no response or inducible colony formation. Colony size was found to increase with B. calyciflorus infochemical concentration and could be described by a sigmoid function. The increase in colony size was more pronounced in the Scenedesmus species tested than in Desmodesmus species, which was probably due to higher threshold infochemical concentrations for colony induction in Desmodesmus. Therefore, the adaptivity of colony formation to the herbivory threat only holds above the threshold concentration for colony induction and as long as maximum colony size has not been attained. Taking this into account, our results suggest that inducible colony formation is a common adaptive response of many Scenedesmaceae to the threat of herbivory. [KEYWORDS: Brachionus calyciflorus; Daphnia magna; Desmodesmus; herbivory; inducible defences; infochemicals ; Scenedesmus]

Effect of macrophyte community composition and nutrient enrichment on plant biomass and algal blooms.

Submerged freshwater macrophytes decline with increasing eutrophication. This has consequences for ecosystem processes in shallow lakes and ponds as macrophytes can reduce algal blooms under eutrophic conditions. We hypothesize that the productivity of submerged vegetation, biomass change under eutrophication and the suppression of algal blooms may be affected by macrophyte community composition. To test our hypothesis, we established three macrophyte community types in 36 fishless experimental ponds: one dominated by the oligotrophic species Chara globularis, one dominated by the eutrophic species Potamogeton pectinatus and a diverse vegetation which became co-dominated by Elodea nuttallii and C. globularis, and we fertilized half of the ponds. The macrophyte communities produced different amounts of biomass and they responded differently to fertilization. The community dominated by Potamogeton produced the lowest overall biomass, but was not affected by nutrient addition. The communities dominated by Chara and co-dominated by Elodea and Chara produced more than four-fold the amount of biomass produced in Potamogeton communities under oligotrophic conditions, but were strongly negatively affected by nutrient addition. Phytoplankton abundance did not differ significantly among the plant community types, but showed large variation within community types. There was a significant negative relationship between spring macrophyte biomass and the probability of summer algal blooms. The occurrence of algal blooms coincided with low daphnid densities and high pH (>10). We conclude that the macrophyte community composition, characterized by the dominant species, strongly affected the amount of biomass production as well as the short-term response of the vegetation to nutrient enrichment. Macrophyte community composition had no direct effect on algal blooms, but can affect the occurrence of algal blooms indirectly as these occurred only in ponds with low (<100 g/m2 DW) spring macrophyte biomass. Keywords: Aquatic plant; Chara; Clear water; Ecosystem function; Elodea; Eutrophication; Phytoplankton; Potamogeton; Productivity;Turbidity

Heterogeneity in food-web interactions of fish in the Mwanza Gulf, Lake Victoria : a quantitative stable isotope study

Stable isotope analyses and derived population-level metrics were used to quantitatively analyse spatial and seasonal heterogeneity in the fish trophic dynamics in relation to environmental variables in Mwanza Gulf, Lake Victoria (Tanzania). The fish community in Lake Victoria, including the top predator Nile perch, is generally omnivorous with a heavy reliance on invertebrates. This is in contrast to findings based on stomach content analyses of Nile perch, which showed a stronger reliance on fish. We tested two hypotheses: (1) during the rainy seasons multiple carbon sources influence the food-web structure inside the Gulf, leading to increased carbon ranges and trophic diversity. (2) During dry periods, the food-web structure mainly relies on pelagic primary production, reducing carbon ranges and trophic diversity. Carbon sources indeed varied seasonally and spatially, affecting the fish community at the highest trophic levels. With the onset of rains, carbon sources became spatially highly differentiated with enriched δ13C values of fish in shallow water inside the Gulf and depleted δ13C values in open waters. Metrics associated with niche size correlated significantly with seasonally varying environmental variables, while δ13C ranges correlated with spatially varying environmental variables

Inducible colony formation within the Scenedesmaceae: adaptive responses to infochemicals from two different herbivore taxa

We studied the occurrence of colony formation within 40 different strains of Scenedesmaceae (Chlorococcales, Chlorophyta) in response to grazing-released infochemicals from the herbivorous zooplankters Brachionus calyciflorus Pallas (Rotifera) and Daphnia magna Strauss (Cladocera). With the exception of two strains, all strains showed similar responses to both B. calyciflorus and D. magna infochemicals, either no response or inducible colony formation. Colony size was found to increase with B. calyciflorus infochemical concentration and could be described by a sigmoid function. The increase in colony size was more pronounced in the Scenedesmus species tested than in Desmodesmus species, which was probably due to higher threshold infochemical concentrations for colony induction in Desmodesmus. Therefore, the adaptivity of colony formation to the herbivory threat only holds above the threshold concentration for colony induction and as long as maximum colony size has not been attained. Taking this into account, our results suggest that inducible colony formation is a common adaptive response of many Scenedesmaceae to the threat of herbivory. [KEYWORDS: Brachionus calyciflorus; Daphnia magna; Desmodesmus; herbivory; inducible defences; infochemicals ; Scenedesmus

A device to study the behavioral responses of zooplankton to food quality and quantity

In order to explore the behavioral mechanisms underlying aggregation of foragers on local resource patches, it is necessary to manipulate the location, quality and quantity of food patches. This requires careful control over the conditions in the foraging arena, which may be a challenging task in the case of aquatic resourceconsumer systems, like that of freshwater zooplankton feeding on suspended algal cells. We present an experimental tool designed to aid behavioral ecologists in exploring the consequences of resource characteristics for zooplankton aggregation behavior and movement decisions under conditions where the boundaries and characteristics (quantity and quality) of food patches can be standardized. The aggregation behavior of Daphnia magna and D. galeata x hyalina was tested in relation to i) the presence or absence of food or ii) food quality, where algae of high or low nutrient (phosphorus) content were offered in distinct patches. Individuals of both Daphnia species chose tubes containing food patches and D. galeata x hyalina also showed a preference towards food patches of high nutrient content. We discuss how the described equipment complements other behavioral approaches providing a useful tool to understand animal foraging decisions in environments with heterogeneous resource distributions.

Samlingsboplatser? : En diskussion om människors möten i norr 7000 f Kr - Kr f med särskild utgångspunkt i data från Ställverksboplatsen vid Nämforsen

This thesis deals with meetings between peoples during prehistoric times in the northern part of Norrland, Sweden. Particular attention is paid to the possible occurrence of more temporary meetings between people in larger groups at aggregation camps during the period ca 7000 – 0 BC. The study has had the aim of increasing our understanding of how peoples’ meetings and contact networks may have been framed. Thirteen sites that previous research has interpreted to be aggregation camps within our field of study have been analysed and interpreted. These are: Jokkmokk, Purkijaur, Nelkerim, Porsi, Lundfors, Norrfors, Överveda, Rappasundet, Hälla, Lillberget, Glösa, Sörånäset and Ställverksboplatsen (the Ställverket site). The Ställverket site at Näsåker (Nämforsen) has been the object of particular study. It has also been viewed in a broader context by analysis and interpretation of other ancient remains in the neighbouring area. I have argued that some interpretations arrived at in earlier research are problematical and that none of the thirteen sites can be said with certainty to have been an aggregation camp. Thus aggregation camps seem not usually to have been a part of the contact network in the area of study. Instead of using aggregation camps as meeting-places, the people involved seem, at certain times and places, to have maintained contact with each other by means of meetings at the base camps, notably the winter sites. These sites seem to have been rather sedentary and are positioned at fairly even distances from one another. I call this model the base camp model. Some grounds for applying the base camp model seem to exist at certain places in the inland region from the end of the Mesolithic era up to 0 BC. After that contact networks seem to change. In the coastal district it seems possible to apply it to some places from the transition between the Mesolithic – Neolithic Age up to about 2500 BC. Thereafter the picture is unclear. The study does also emphasise however that more in-depth studies are needed to strengthen the viability of the base camp model’s applicability, that there are still big gaps in the material and that much work still remains to be done in order to solve the problems of how aggregation camps can best be defined and how they can be identified archaeologically

Community stoichiometry in a changing world: combined effects of warming and eutrophication on phytoplankton dynamics

The current changes in our climate will likely have far reaching consequences for aquatic ecosystems. These changes in the climate, however, do not act alone and are often accompanied by additional stressors such as eutrophication. Both global warming and eutrophication have been shown to affect the timing and magnitude of phytoplankton blooms. Little is known about the combined effects of rising temperatures and eutrophication on the stoichiometry of entire phytoplankton communities. We exposed a natural phytoplankton spring community to different warming and phosphorus loading scenarios using a full-factorial design. Our results demonstrate that rising temperatures promote the growth rate of an entire phytoplankton community. Furthermore, both rising temperatures and phosphorus loading stimulated the maximum biomass built up by the phytoplankton community. Rising temperatures led to higher carbon:nutrient stoichiometry of the phytoplankton community under phosphorus limited conditions. Such a shift towards higher carbon:nutrient ratios, in combination with a higher biomass build-up, suggests a temperature-driven increase in nutrient use efficiency of the phytoplankton community. Importantly, higher carbon:nutrient stoichiometry of phytoplankton is generally of poorer nutritional value for zooplankton. Thus, although warming may result in higher phytoplankton biomass, this may be accompanied by a stoichiometric mismatch between phytoplankton and their grazers, with possible consequences for the entire aquatic food web. Read More: http://www.esajournals.org/doi/abs/10.1890/13-1251.1