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Harvard Forest Data Archive
Decomposition Dynamics in the Sarracenia Purpurea Microecosystem at Harvard Forest 2010Related Publications
- hf169-01: changes in species richness and trophic diversity (preview)
- hf169-02: decomposition of ant carcasses (preview)
- Lead: Benjamin Baiser, Aaron Ellison
- Investigators: Roxanne Ardeshiri
- Contact: Aaron Ellison
- Start date: 2010
- End date: 2010
- Status: completed
- Location: Harvard Forest Greenhouse, Tom Swamp Tract (Harvard Forest)
- Latitude: +42.42 to +42.55
- Longitude: -72.29 to -72.10
- Elevation: 160 to 330 meter
- Taxa: Camponotus pennsylvanicus, Habrotrocha rosa, Metriocnemus knabi, Sarracenia purpurea, Wyeomyia smithii
- Release date: 2011
- EML file: knb-lter-hfr.169.8
- DOI: digital object identifier
- Related links:
- Nitrogen Cycling Dynamics in Sarracenia Purpurea at Harvard Forest 2004-2005
- Organic and Inorganic Nitrogen Uptake by Sarracenia Purpurea at Harvard Forest and Fort Albany ON 2007
- Prey Capture by Carnivorous Plants Worldwide 1923-2007
- Sarracenia Purpurea Prey Capture at Harvard Forest 2008
- Allochthonous Nutrients in the Sarracenia Microecosystem at Harvard Forest 2005-2007
- Study type: short-term measurement
- Research topic: physiological ecology, population dynamics and species interactions
- LTER core area: populations
- Keywords: biodiversity, decomposition, food webs, species richness, trophic structure
Ecological communities show great variation in species richness, composition and food web structure across similar and diverse ecosystems. Knowledge of how this biodiversity relates to ecosystem functioning is important for understanding the maintenance of diversity and the potential effects of species losses or gains on ecosystems. While research often focuses on how variation in species richness influences ecosystem processes, assessing species richness in a food web context can provide further insight into the relationship between diversity and ecosystem functioning and provide potential mechanisms underpinning this relationship. Here, we assessed how species richness and trophic diversity affect decomposition rates in a complete aquatic food web: the five trophic level web that occurs within water-filled leaves of the northern pitcher plant, Sarracenia purpurea. We identified a trophic cascade in which top-predators - larvae of the pitcher-plant mosquito - indirectly increased bacterial decomposition by preying on bactivorous protozoa. Our data also revealed a facultative relationship in which larvae of the pitcher-plant midge increased bacterial decomposition by shredding detritus. These important interactions occur only in food webs with high trophic diversity, which in turn only occurs in food webs with high species richness. We show that species richness and trophic diversity underlie strong linkages between food web structure and dynamics that influence ecosystem functioning. The importance of trophic diversity and species interactions in determining how biodiversity relates to ecosystem functioning suggests that simply focusing on species richness does not give a complete picture as to how ecosystems may change with the loss or gain of species.
The Sarracenia food web
Sarracenia purpurea is a long-lived perennial carnivorous plant with tubular leaves that open during the growing season and fill with 5 to 50 ml of rainwater. Once open, the leaves capture invertebrate prey that serves as the base of a fully-functional "brown" food web containing bacteria, protozoa, insect larval stages and other aquatic invertebrates. The Sarracenia food web decomposes captured prey and releases essential nutrients to the plant.
We collected pitcher-plant protozoa, copepods, mites, and dipteran larvae from pitcher plants growing in the Tom Swamp Bog, just north of Harvard Pond in central Massachusetts (42d 30m N, 72d 11m W). Sampling was approved by the Harvard Forest research committee. Protozoa were cultured with filtered pitcher fluid (see Food web assembly, below) in 50-ml screw-capped jars. The rotifer Habrotrocha rosa Donner was cultured with tap water and baker’s yeast in 50-ml screw-capped jars. Macro-invertebrates were held in 50-ml screw-capped jars with protozoa and bacteria as food sources. We rinsed each macro-invertebrate with filtered pitcher fluid before placing them into food webs to remove any "hitch-hiking" protozoa.
We assessed decomposition of ant prey in Sarracenia food webs of varying species richness and trophic diversity (TD). We assembled 10 species richness treatments (0-9 consumer species with the 0 level including bacteria only) with seven replicates each for a total of 70 food webs. We constructed interaction matrices and calculated TD for every combination of species at each species richness level (see Calculating trophic diversity (TD), below). We divided the range of TD equally into seven levels from low, where species have similar trophic niches, to high, where species have unique trophic niches, for each species richness treatment and randomly selected food webs with each of these values in order to determine the community composition for the seven replicates in each species richness treatment. We assembled each food web into an individual leaf on a separate plant (i.e. 70 plants). We selected the newest leaf on each plant for food web assembly and flushed each leaf with distilled-deionized water prior to assembly.
Food web assembly
We collected pitcher-plant fluid from the field and filtered out all particulates and protozoa greater than 1.2 um using a syringe filter (Arcodisc 32, 1.2 um mesh). The remaining fluid contained only bacteria and was mixed to ensure homogeneous bacterial assemblages in all pitchers. We cultured pitcher-plant yeast in petri dishes containing 15 g of agar and 1 g of yeast extract in 1 L of tap water and re-suspended yeast in the homogenized pitcher fluid. We inoculated each pitcher with 5 ml of pitcher fluid and 0.1 ml of protozoan cultures for the webs that contained protozoa. After one week, we added an additional 5 ml of pitcher fluid, 0.1 ml of rotifers, and required macro-invertebrate species to food webs. This total volume of pitcher fluid (10 ml) and the abundances of macro-invertebrates were based on averaged empirical data from pitcher plant food webs where the samples were collected. We plugged pitchers with cotton and marked the 10-mL line on each pitcher to control for evaporation. We checked water levels every two days and refilled pitchers to the 10-ml line with autoclaved well water.
Measuring ant decomposition
We collected carpenter ant workers (Camponotus pennsylvanicus) from a single nest to use as prey items for the experiment. We froze and then weighed each ant (+/- 1 ug) and randomly assigned one to each pitcher. We placed ants into pitchers on the same day that macro-invertebrates were added to the food web and removed the decomposed ants and ant parts two weeks later. We air dried the decomposed ants for two days and then re-weighed them (+/- 1 ug). We measured decomposition as percent ant mass lost over the two-week time period. To ensure that mass loss was not due to submersion in fluid and/or desiccation, we followed the same methods, but submerged 10 carpenter ants in glass jars filled with 10 ml of distilled-deionized water and placed them next to plants in the greenhouse as a control.
Measuring final TD, species richness, and species abundances
At the same time that we harvested the decomposed ants, we censused all the macro-invertebrates in each pitcher. Abundance of protozoa and rotifers was estimated as the average of five counts of 0.05-ml sub-samples. The protozoa Cyclidium and Peranema sporadically contaminated pitchers and unidentified microflagellates were found in nearly every pitcher. One pitcher contained two amphipods and another contained an unknown ciliate. The amphipods and unknown ciliate were not included in the sampling effect path model analysis. Final TD and species richness were calculated using the final community composition of each web including contaminant species. Final TD and species richness were used in all analyses.
This dataset is released to the public under Creative Commons license CC BY (Attribution). Please keep the designated contact person informed of any plans to use the dataset. Consultation or collaboration with the original investigators is strongly encouraged. Publications and data products that make use of the dataset must include proper acknowledgement.
Baiser B, Ellison A. 2011. Decomposition Dynamics in the Sarracenia Purpurea Microecosystem at Harvard Forest 2010. Harvard Forest Data Archive: HF169.
hf169-01: changes in species richness and trophic diversity
- plant: identifier: 1-70
- initial.s: species richness at the beginning of the experiment. Integer: 0 (bacteria only) through 9 (all species present).
- final.s: species richness at the end of the experiment. Integer: 1 – 9.
- initial.td: trophic diversity at the beginning of the experiment (unit: dimensionless / missing value: NA)
- final.td: trophic diversity at the end of the experiment (unit: dimensionless / missing value: NA)
hf169-02: decomposition of ant carcasses
- plant: identifier; 1 – 70
- mass.i: initial mass (unit: milligram / missing value: NA)
- mass.f: final mass (unit: milligram / missing value: NA)