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Harvard Forest Symposium Abstract 2006

  • Title: Prey availability directly affects physiology and nutrient allocation in ten carnivorous plant species
  • Primary Author: Elizabeth Farnsworth (New England Wild Flower Society)
  • Additional Authors: Aaron Ellison (Harvard University)
  • Abstract:

    We examined the effects of prey availability on the photosynthetic rate, fluorescence, growth, architecture, and foliar nutrient content of ten species of carnivorous pitcher plants in the genus Sarracenia. Two plants of each Sarracenia species were assigned to one of six feeding levels ranging from 0g per feeding to 0.25g of finely ground wasps (for small species); 0g-0.5g (for species of intermediate sizes); and 0g-1.0g (for large species); plants were fed once weekly for 7 weeks. We tested the hypothesis that enhanced nutrient levels provided by controlled feedings of insect prey would result in increased photosynthetic rates of one or more leaves, increase relative biomass allocation to phyllodes (leaves not involved in prey capture) rather than predatory pitchers, increase overall plant biomass, and reduce stress-related fluorescence. Previous field studies showed that leaf traits of carnivorous Sarracenia purpurea and Darlingtonia californica differed substantially from the predictions of "universal" scaling relationships developed primarily for autotrophic species; photosynthetic rates are much lower than expected given observed levels of foliar nitrogen, phosphorus, and leaf mass per unit area. We expanded our sampling to nine additional species to determine if this phenomenon is characteristic of carnivorous species generally. We also sought to determine interspecific differences in these effects, hypothesizing that the species that utilize enzymatic pathways rather than pitcher inquilines to digest prey would show smaller-magnitude effects because they must invest additional nitrogen and phosphorus into enzymes.


    Twelve 6-year-old plants of each species (N=120 plants) were grown in a Sphagnum medium in 4-inch pots in the Harvard University greenhouse, and watered at the base daily. All plants were measured nondestructively (numbers of pitchers and phyllodes, length to hood, hood length, hood width, basal diameter, midstem diameter, mouth diameter) on 6 June 2005. Individual plants were randomly assigned to one of six feeding levels (two plants per level) such that similar starting size distributions were equivalent among treatments. Maximal photosynthetic assimilation rates (Amax) were measured on all plants during the week of June 13, 2005. The two species that naturally host a food web of inquilines (S. purpurea and S. rosea) were inoculated in June with a standardized volume and mixture of field-collected bacteria and protozoa; the other species were left intact. The first and second pitchers produced in each plant were fed once/week with a fixed dose of finely-ground hymenoptera moistened in 5ml of distilled water. Doses were scaled to species size to avoid "overdosing" and injuring leaves and to reflect ranges of prey capture observed in the wild. Feeding levels ranged from 0g (control plants) to 0.25g for the small species, S. rosea, S. purpurea, S. psittacina, S. alabamensis, S. jonesii, and S. rubra; 0g to 0.5g for intermediate-sized species, S. minor and S. alata; and 0g to 1.0g for the largest species, S. flava and S. leucophylla. Amax and chlorophyll fluoresence (Fo, Fv, and Fm) were measured on the first and second pitchers produced by all plants, phyllodes (as available), and fully expanded youngest leaves of all plants (as available), using a LiCor 6400, after seven weeks of feeding on the week of July 31. Phyllodes and pitchers were recounted at this time. Plants were subsequently harvested, aboveground and belowground biomass separated, and the areas of the first, second, and youngest leaves measured using a LiCor 3100 leaf area meter. All tissues were dried for ~1 wk at 70 deg C and dry biomass determined. Samples were ground and sent to the University of Vermont Agricultural and Environmental Testing Laboratory for determination of C, N, and mineral content (P, Ca, Mg, K, Na).


    Maximal photosynthetic rates and foliar total chlorophyll content of the youngest mature leaves were positively and significantly correlated with feeding level, and these relationships were consistent among species. Overall photosynthetic rates were low, comparable to the two species previously tested (S. purpurea and D. californica), indicating that vital metabolic rates are similar among many species of carnivorous plants. Variable to maximal fluorescence ratios of the younger leaves (Fv/Fm), which are positively correlated with the efficiency of Photosystem II, were positively and significantly correlated with feeding level. The oldest, first-produced pitchers did not show significant responses to feeding level in terms of photosynthetic rates, fluorescence, area, or biomass. Whole-plant biomass was unaffected by feeding level, but plants fed higher doses of insects produced a significantly higher proportion of phyllodes than control or low-dose plants. Higher nutrient availability was correlated with lower foliar N:P ratios, and better-fed plants shifted from being P-limited to being N-limited. Species utilizing digestive enzymes did not differ consistently from those using inquilines to process prey.


    These findings indicate that prey availability enhances photosynthetic rates and photosystem efficiency in many pitcher plant species, but these increases mostly appear in younger leaves. This accords with stable isotope data (Butler et al. 2005), which demonstrate that nutrients captured by older pitchers are rapidly translocated to younger leaves (we await data on foliar N, P, and K from this experiment to test whether this mechanism holds). As predicted by models of trade-offs in allocation between photosynthetic versus predatory leaves, better-fed plants produced phyllodes in preference to the pitchers that contribute less efficiently to growth. The departure of multiple species from so-called "universal" scaling relationships may reflect a general "cost of carnivory." This is the first multi-species, controlled feeding experiment using realistic prey to measure these physiological parameters directly and thus elucidate mechanisms of nutrient allocation and allometry in carnivorous plants.

  • Research Category: Physiological Ecology, Population Dynamics, and Species Interactions