Witman Jon D.

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Witman
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Jon D.
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  • Dataset
    Percent algal substrate grazed by sea urchins in trophic cascade experiments in Gal?pagos Islands during 2012 (GMR Trophic Cascades project)
    (Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2021-04-01) Witman, Jon D. ; Smith, Franz
    Percent algal substrate grazed by sea urchins in trophic cascade experiments in Gal?pagos Islands during 2012. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/635717
  • Article
    Consumer mobility predicts impacts of herbivory across an environmental stress gradient
    (Ecological Society of America, 2019-10-12) Lamb, Robert W. ; Smith, Franz ; Witman, Jon D.
    Environmental stress impedes predation and herbivory by limiting the ability of animals to search for and consume prey. We tested the contingency of this relationship on consumer traits and specifically hypothesized that herbivore mobility relative to the return time of limiting environmental stress would predict consumer effects. We examined how wave‐induced water motion affects marine communities via herbivory by highly mobile (fish) vs. slow‐moving (pencil urchin) consumers at two wave‐sheltered and two wave‐exposed rocky subtidal locations in the Galapagos Islands. The exposed locations experienced 99th percentile flow speeds that were 2–5 times greater than sheltered locations, with mean flow speeds >33 cm/s vs. <16 cm/s, 2–7 times higher standing macroalgal cover and 2–3 times lower cover of crustose coralline algae than the sheltered locations. As predicted by the environmental stress hypothesis (ESH), there was a negative relationship between mean flow speed and urchin abundance and herbivory rates on Ulva spp. algal feeding assays. In contrast, the biomass of surgeonfishes (Acanthuridae) and parrotfishes (Labridae: Scarinae) was positively correlated with mean flow speed. Ulva assays were consumed at equal rates by fish at exposed and sheltered locations, indicating continued herbivory even when flow speeds surpassed maximum reported swimming speeds at a rate of 1–2 times per minute. Modeled variation in fish species richness revealed minimal effects of diversity on herbivory rates at flow speeds <40 cm/s, when all species were capable of foraging, and above 120 cm/s, when no species could forage, while increasing diversity maximized herbivory rates at flow speeds of 40–120 cm/s. Two‐month herbivore exclusion experiments during warm and cool seasons revealed that macroalgal biomass was positively correlated with flow speed. Fish limited macroalgal development by 65–91% at one exposed location but not the second and by 70% at the two sheltered locations. In contrast, pencil urchins did not affect algal communities at either exposed location, but reduced macroalgae by 87% relative to controls at both sheltered locations. We propose an extension of the ESH that is contingent upon mobility to explain species‐specific changes in feeding rates and consumer effects on benthic communities across environmental gradients.
  • Article
    Markov chain analysis of succession in a rocky subtidal community
    (University of Chicago Press, 2004-07-01) Hill, M. Forrest ; Witman, Jon D. ; Caswell, Hal
    We present a Markov chain model of succession in a rocky subtidal community based on a long-term (1986–1994) study of subtidal invertebrates (14 species) at Ammen Rock Pinnacle in the Gulf of Maine. The model describes successional processes (disturbance, colonization, species persistence, and replacement), the equilibrium (stationary) community, and the rate of convergence. We described successional dynamics by species turnover rates, recurrence times, and the entropy of the transition matrix. We used perturbation analysis to quantify the response of diversity to successional rates and species removals. The equilibrium community was dominated by an encrusting sponge (Hymedesmia) and a bryozoan (Crisia eburnea). The equilibrium structure explained 98% of the variance in observed species frequencies. Dominant species have low probabilities of disturbance and high rates of colonization and persistence. On average, species turn over every 3.4 years. Recurrence times varied among species (7–268 years); rare species had the longest recurrence times. The community converged to equilibrium quickly (9.5 years), as measured by Dobrushin’s coefficient of ergodicity. The largest changes in evenness would result from removal of the dominant sponge Hymedesmia. Subdominant species appear to increase evenness by slowing the dominance of Hymedesmia. Comparison of the subtidal community with intertidal and coral reef communities revealed that disturbance rates are an order of magnitude higher in coral reef than in rocky intertidal and subtidal communities. Colonization rates and turnover times, however, are lowest and longest in coral reefs, highest and shortest in intertidal communities, and intermediate in subtidal communities.
  • Dataset
    Residence times of consumers in subtidal areas on Isla Baltra, Gal?pagos during 2012 (GMR Trophic Cascades project)
    (Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2021-04-01) Witman, Jon D. ; Smith, Franz
    Residence times of consumer species during a trophic cascade experiment conducted at 8- 10 m depth at Isla Baltra, Galápagos Islands from July 13 to July 19, 2012. Data represent counts of consumer species photographed with GoPro cameras at 1 second intervals during daylight hours in two adjacent areas of the bottom, named Baltra crest and Baltra gully. The consumers were photographed in the vicinity of treatments where sea urchins Lytechinus semituberculatus were enclosed in fences on substrates of consumable benthic algae. Data represent raw data of entrance and exit times from image analysis and the calculated difference. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/630458
  • Dataset
    Sea urchin (Eucidaris) quadrat counts at twelve sites in the Galápagos Islands in 2016-2017
    (Biological and Chemical Oceanography Data Management Office (BCO-DMO). Contact: bco-dmo-data@whoi.edu, 2022-04-26) Witman, Jon D.
    Sea urchin (E. galapagensis) densities showed high variability across twelve study sites in the Galápagos Islands in 2016-2017. Sea urchin densities ranged from a low of 0.17 individuals per 0.25 square meter quadrat at the Pinzon site to a maximum of 7.7 individuals per quadrat at the Champion site in January 2016. For a complete list of measurements, refer to the full dataset description in the supplemental file 'Dataset_description.pdf'. The most current version of this dataset is available at: https://www.bco-dmo.org/dataset/872905