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Cross-ecosystem bottlenecks alter reciprocal subsidies within meta-ecosystems


Klemmer, Amanda; Galatowitsch, Mark; McIntosh, Angus (2020), Cross-ecosystem bottlenecks alter reciprocal subsidies within meta-ecosystems, Dryad, Dataset,


Reciprocal subsidies link ecosystems into meta-ecosystems, but energy transfer to organisms that do not cross boundaries may create sinks, reducing reciprocal subsidy transfer. We investigated how the type of subsidy and top predator presence influenced reciprocal flows of energy, by manipulating the addition of terrestrial leaf and terrestrial insect subsidies to experimental freshwater pond mesocosms with and without predatory fish.  Over 18 months, fortnightly addition of subsidies (terrestrial beetle larvae) to top-predators was crossed with monthly addition of subsidies (willow leaves) to primary consumers in mesocosms with and without top predators (upland bullies) in a 2 x 2 x 2 factorial design in four replicate blocks. Terrestrial insect subsidies increased reciprocal flows, measured as the emergence of aquatic insects out of mesocosms, but leaf subsidies dampened those effects. However, the presence of fish and snails, consumers with no terrestrial life stage, usurped and retained the energy within in the aquatic ecosystem, creating a cross-ecosystem bottleneck to energy flow. Thus, changes in species composition within donor or recipient ecosystems within a meta-ecosystems can alter reciprocal subsidies through cross-ecosystem bottlenecks.


Experimental set-up

The 18-month pond mesocosm experiment took place at the University of Canterbury’s Cass Mountain Research Station, South Island, New Zealand. In January 2013, thirty-two 1,100-litre cattle tanks (mesocosms) were filled with ground water, 1 cm of gravel substrate, common pond macrophytes from the area (submergent Myriophyllum and emergent Carex), and two 10-cm diameter ceramic pots positioned on their side to provide fish habitat. Experimental tanks were inoculated with five litres of filtered pond water, 380 ml aliquots of concentrated phytoplankton and zooplankton, and 300 ml filtered fine particulate organic matter. To ensure representation of various trophic levels and feeding groups, a range of benthic invertebrates from local ponds and lakes were added to tanks in natural densities based on exploratory surveys (predatory invertebrates: 10 Procordulia dragonflies and 10 Xanthocnemis damselflies; primary consumers: ~200 Potamopygrus snails, ~100 Chironominae midges and 50 Triplectides caddisflies). In addition to those taxa, two 0.3 m2 sweeps of benthos with a 1-mm mesh D-net from local ponds (one each from a permanent and temporary pond) were added to the tanks to increase diversity of rare taxa to mimic naturally occurring food webs. Tanks were allowed to be naturally colonized by terrestrial dispersal of adult stages of invertebrates from January to May 2013 before treatments were initiated; colonization continued throughout experiment.

The experiment had a fully crossed 2 x 2 x 2 factorial design, with the presence/absence of subsidies to primary consumers (leaves) and the presence/absence of subsidies to top-predators (beetle larvae) crossed with the presence/absence of top-predators (fish). Each treatment was replicated four times and randomized within four spatial blocks of tanks. Ten adult upland bullies (Gobiomorphus breviceps) (totalling 26.6 +/- 0.4 g wet mass – low end of natural densities), a common native predatory fish in New Zealand freshwater ecosystems, were added in naturally occurring sex ratios (~1 male to ~9 females) as the top predator to “fish” treatment tanks on 14 May 2013. These adult fish reproduced in December 2013, and young-of-the-year (YOY) upland bullies were present from this point onward.

Four grams of air-dried riparian willow leaves (Salix fragilis) were added as resources to primary consumers (“leaf subsidy”) every four weeks, beginning 22 May 2013. Every two weeks, beginning 22 May 2013, two grams (wet weight) of live terrestrial beetle larvae (Tenebrio sp.) were added as resources to predatory fish (“insect subsidy”). Beetle larvae were used so we could easily control subsidy additions.

Emergence sampling

To measure the effects of terrestrial subsidies on aquatic emergence, we sampled adult emerging aquatic invertebrates from the tanks during the spring and summer of 2013-14.  Emergence traps were first deployed in late spring (15 November 2013), 178 days after the experiment began, during the period where emergence was expected to increase towards the summer peak. Adult invertebrates were collected from traps every two to four weeks, for a total of seven sampling dates (days 203, 232, 247, 264, 274, 298, and 322 of the experiment). Floating emergence traps, loosely tethered over the centre of the tanks (covering 0.08 msurface area), were constructed with 1-mm mesh and a collection jar containing 50% ethanol to preserve invertebrates. Large emerging invertebrates, such as Odonata, were sometimes found attached to mesh on the inside of the trap rather than in the collection jar, so traps were carefully inspected for all emerged invertebrates during each sampling occasion. Invertebrates were preserved in 70% ethanol until later processing in the laboratory.

Adult aquatic invertebrates collected from the traps were identified to family or order, and then photographed using a Leica DFC450 microscope camera and body length measured in Adobe Acrobat Pro.  Lengths were converted to dry mass using length-weight regressions. Adult invertebrates were categorized as predatory invertebrates or primary consumers based on their aquatic larval stages.

Snail to non-snail ratio sampling

Five mesh sampling baskets (0.04 m2) containing cobble and fabric cut to mimic leaves were placed in the tank as sampling devices for invertebrates > 0.5 mm in length. At the conclusion of the experiment, on 3 November 2014, one basket was removed and all invertebrates were preserved in ethanol for later identification. Tanks were then destructively sampled to measure fish and large invertebrate biomass. This involved a combination of basket (invertebrates of body length < 5.0 mm) and whole-tank sampling (for invertebrates > 5.0 mm) for final invertebrate biomass estimates in mg/m2.

All invertebrates > 0.5 mm were identified to lowest taxonomic unit using Winterbourn et al. 2006. Invertebrate length was measured as described above. Dry weights of invertebrates were calculated using length-weight regressions. The ratio of snail : insect primary consumer biomass for tanks was calculated with snail taxa as: Mollusca Gyraulus sp., other Planorbidae, Physa sp., Potamopyrgus sp., and Sphaeriidae. All other primary consumers had complex life cycles and were included in the insect category.


Usage Notes

Metadata files with column descriptions for Klemmer et al_Dryweight_emergence.csv and Klemmer et al_Dryweight_snail_to_insect.csv


Brian Mason Scientific & Technical Trust

The Miss E. L. Hellaby Indigenous Grasslands Research Trust

New Zealand International Doctoral Research Scholarship

National Institute of Food and Agriculture, Award: ME0-22101

Brian Mason Scientific & Technical Trust

The Miss E. L. Hellaby Indigenous Grasslands Research Trust

New Zealand International Doctoral Research Scholarship