Research ArticleECOLOGY

Capacity to support predators scales with habitat size

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Science Advances  04 Jul 2018:
Vol. 4, no. 7, eaap7523
DOI: 10.1126/sciadv.aap7523
  • Fig. 1 Biomass distributions among predator and prey trophic levels in 29 grassland rivers varying in both habitat size measured by discharge (in cubic meters per second) and the abundance of prey resources [primary consumers (in grams per square meter)].

    (A and B) Individual predator sizes accounting for 50% of the cumulative predator biomass [P50; DM] in rivers with fish (black circles) and in rivers lacking fish (open triangles). (C) Availability of prey energy at the base of the food web at each river measured by the biomass per unit area of primary consumers (in grams of DM per square meter). (D and E) Combined biomass of predators per unit area of stream (in grams of DM per square meter). (F) Relationship between habitat size and biomass per unit area of predators plotted with primary consumer biomass set at its mean to illustrate the independent effect of habitat size. (G) Relationship between habitat size and biomass per unit area of predators plotted with habitat size set at its mean to illustrate the independent effect of primary consumer biomass. The data for (F) and (G) were generated using the equation Bpred = 0.92H0.25 Bprey0.83, where Bpred is predator biomass per unit area, H is habitat size, and Bprey is primary consumer biomass per unit area. P50 predator mass was unavailable for two fishless rivers where predatory invertebrates were group-weighed.

  • Fig. 2 Overall effect of habitat-size constraints on trophic structure.

    Habitat size (A), by constraining predator size (B), subsequently affects predator/prey biomass ratios (C and D) and capacity to support predator biomass (C) per unit of prey mass (D) based on our sampling in South Island, New Zealand rivers. For habitats sampled, a reduction in habitat size from 1.0 to 0.01 m3·s−1 [stream discharge (Q) associated with reduced width (W)] reduces predator size (for example, brown trout total length, from 248 to 51 mm) based on a back calculation of fish mass accounting for 50% of the cumulative biomass (P50) using Eq. 1 [P50 (in grams of DM) = 46.26H1.04]. Applying results for Eq. 5 (that is, Bpred = 0.92H0.25 Bprey0.83, where Bpred is predator biomass per unit area, H is habitat size, and Bprey is primary consumer biomass per unit area, with units indicated in the diagrams) and assuming similar resource availability across habitats [(D), 3.23 g of DM·per square meter], this reduction in predator size leads to a substantial reduction in the predator biomass (C) being supported per unit of prey biomass.

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/4/7/eaap7523/DC1

    Section S1. Drivers of secondary production

    Section S2. Intermediate predators

    Fig. S1. Variation in major drivers of secondary production.

    Fig. S2. Variation in stream water–specific conductivity in relation to discharge from 15 rivers in the Waimakariri and Rakaia river catchments, New Zealand.

    Fig. S3. Patterns in abundance of predatory invertebrates.

    Table S1. Location (New Zealand map grid) and habitat size (measured in terms of discharge and stream order) of sites sampled from the Waimakariri and Rakaia river catchments of the South Island, New Zealand to provide information on predator and prey abundance, together with the source of the data.

  • Supplementary Materials

  • This PDF file includes:
    • Section S1. Drivers of secondary production
    • Section S2. Intermediate predators
    • Fig. S1. Variation in major drivers of secondary production.
    • Fig. S2. Variation in stream water–specific conductivity in relation to discharge from 15 rivers in the Waimakariri and Rakaia river catchments, New Zealand.
    • Fig. S3. Patterns in abundance of predatory invertebrates.
    • Table S1. Location (New Zealand map grid) and habitat size (measured in terms of discharge and stream order) of sites sampled from the Waimakariri and Rakaia river catchments of the South Island, New Zealand to provide information on predator and prey abundance, together with the source of the data.

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