Increased duration of aquatic resource pulse alters community and ecosystem responses in a subarctic plant community

Abstract

Allochthonous resource movement across ecosystem boundaries creates episodic linkages between ecosystems. The sensitivity of the community to external resources of varying duration can alter the baseline upon which future pulses of allochthony can act. We explored the terrestrial ecosystem response to pulsed inputs of lake-derived resources with a manipulative experiment in a subarctic heathland where we assessed plant community and nutrient availability responses to additions of midge carcasses (Diptera: Chironomidae). Insect carcasses were added as either a one-time pulse or a 4-yr press to simulate differing durations of allochthony, which is common in the area. We found that midge pulses significantly elevated soil inorganic [N] in the first year (7× over background levels) but were significantly diminished (1.5×) by the second year after the initial pulse. The press treatment continued to elevate total soil inorganic [N] to 13× over background levels by the fourth year of midge additions, but then declined to 3.6× background in year 5 when experimental midge additions had ceased. In contrast to the soil inorganic N response, plant biomass was similar in pulse-addition and control plots over the course of the experiment. However, by the second year of the study plant biomass in press-addition plots were significantly higher than controls (>50%), and continued to increase over the 4 yr of the press treatment. Midge addition stimulated dominance of graminoids and thatch litter in plots that had previously been primarily heathland vegetation, a response that persisted 4 yr post-midge addition. Our findings suggest that soil and plant community responses to persistent insect carcass deposition (e.g., press) into heathland vegetation has the potential to carry forward in a way that modifies the baseline ecosystem conditions upon which additional allochthony may act.

Trophic transfer from aquatic to terrestrial ecosystems: a test of the biogeochemical niche hypothesis

“…These aquatic subsidies of various bio‐elements change nutrient availability in the terrestrial system (Gratton et al. ) and can thus trigger a change in elemental composition throughout the food web. This mechanism can be especially important in young terrestrial ecosystems with developing soils lacking critical nutrients.…”

“…, ) and can have a fertilization effect in the surrounding area (Gratton et al. ). The input of midges from lakes to surrounding terrestrial ecosystems may moreover provide an opportune scenario to test hypotheses of ecological stoichiometry, such as the biogeochemical niche hypothesis (Peñuelas et al.…”

“…These emergent aquatic insects naturally fertilize the nutrient‐poor terrestrial ecosystem, which is mostly nitrogen limited (Gratton et al. ), and set up ideal conditions to test the biogeochemical niche hypothesis at the whole food web level. We hypothesize that plants will show stronger stoichiometric differences among sites than arthropods, which we expect will be more similar to each other due to consumers’ higher homeostatic capacity (Fagan et al.…”

Reducing allochthonous resources in a subarctic grassland alters arthropod food webs via predator diet and density

“…Accumulated living and dead plant biomass at Mývatn—presumably aided by annual midge fertilization at this location (Gratton et al. )—represent a resource pool that could sustain detritus‐consuming arthropods even in the absence of midge inputs over the four years of this study. Available aboveground plant biomass was measured at our site in 2009, and an average of 9203 g/m 2 dw was harvested (R. Jackson, M. Raudenbush, and C. Gratton, unpublished manuscript ).…”

Tri‐trophic interactions: bridging species, communities and ecosystems

“…Likewise, research on subsidies adopting a tri‐trophic view has focused largely on how movement of resources between terrestrial and aquatic ecosystems boosts predator populations to strengthen trophic cascades (e.g. Nakano & Murakami ; Sabo & Power ; Spiller et al ; Gratton et al ), but effects of subsidies on other tri‐trophic phenomena have not been addressed (but see Knight et al ). Unstudied questions with respect to subsidies and meta‐community dynamics affecting landscape‐level TTIs include the following: how the degree of mobility varies among trophic levels; whether natural enemies affect herbivore movement, how this varies based upon herbivore traits (e.g.…”