title for learn-about section on whelks & relatives in A SNAIL'S ODYSSEY
  Habitat & community ecology
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Demography & habitat complexity

  Topics in this diverse section include demography, considered here, and SHELL MORPHOLOGY & HABITAT, SHORE-LEVEL SIZE GRADIENTS, and AGENTS OF COMMUNITY STRUCTURE considered in other sections.
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Research study 1

graph showing changes in relative density of whelks Nucella spp. in San Juan Island, Washington over a 6-yr periodA 5yr study of whelks Nucella ostrina and N. lamellosa in San Juan Island, Washington reveals the following general demographic features: 1) N. ostrina reaches maturity in 1yr and survives for 2yr or less, while N. lamellosa matures after 4yr and breeds for several more years, 2) population numbers of both species vary from year to year, corresponding “roughly” to changes in food supply 3) recruitment varies irregularly, with patterns being different in adjacent areas, 4) mortality is higher in N. ostrina than N. lamellosa, perhaps because of harsher physical conditions in its upper-shore habitat, and 5) general demographic patterns of the 2 species respond similarly to events in their common environment. Note in the graph depicting population numbers of the 2 species over the study period, that both populations drop to about half their 1969 sizes, then eventually rise again.  Spight 1982 J Exp Mar Biol Ecol 57: 195.

NOTE the author's original graph shows data for 2 locations, but these are combined here into single lines for visual clarity

NOTE a detailed overview of life-history patterns of the whelks N. ostrina and N. lamellosa can be found in Spight 1979 p. 135 In, Reproductive ecology of marine invertebrates (Stancyk, ed.) Univ S Carolina Press, Columbia, SC

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Research study 2

graph showing survival of populations of whelks Nucella over 18-mo period at Bodega Bay, CaliforniaMany marine invertebrates exist in patches, usually related to features of habitat, access to mates, or availability of prey.  A question that may be asked in relation to populations of any organism is “what is the smallest patch size that will persist over time, that is, not lead to extinction?”.  The question has direct relevance to conservation of extinction-prone species in nature reserves and captive breeding populations, and can be tested with whelks Nucella ostrina because they lack a free-living larval stage.  In order to answer this question, researchers at the Bodega Marine Laboratory, California set out 7 sets of different-sized plates1 suspended 30cm above a sandflat in Bodega Harbor.  The snails don’t normally cross sand so the experimental populations are reasonably isolated.  The plates are graded in size starting from 1m2 and geometrically decreasing in size in 6 steps to 1/64 m2.  Each plate has the same density2 of whelks at the start, namely, 128 individuals . m-2 Thus, initial population size is proportional to plate area, or "patch" area.  The plates are set out for 9mo from August to establish a solid cover of barnacles Balanus glandula, and then the snails are added.  After a further 18mo period the researchers find that no degree of subdivision of habitat, or "patch" size, leads to extinctions on all replicate3 plates (see graph).  The snails appear to have sufficient food and they reproduce on all plates.  By the end of 18mo, the populations are effectively entering their third generation.  The main mortality of the whelks is from birds and crabs Metacarcinus magister.  The authors conclude that there is no evidence for a distinct threshold size for population persistence of N. ostrina under the conditions imposed. The approach is unique for marine ecosystems and certainly justifies further applications. Quinn et al. 1989 Conserv Biol 3: 242.

NOTE1  the plates are Formica plastic on a wood base, representing a "patch". The researchers note some evidence of movement between plates, possibly by individuals being transported in floating seaweeds, or perhaps current transport of juveniles?

NOTE2  natural densities in the study region range up to 400 . m-2

NOTE3  each of the 7 treatments has a minimum of 4 replicates, but 4 of the smaller-sized treatments have enough additional replicates for the combined area in each treatment to add up the area of one of the largest plates. Thus, there are 64 plates of 1/64 m2, 32 plates of 1/32m2, and so on

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Research study 3

photograph of whelk Nucella lima Gerald & Buff CorziWhat matters most to a whelk Nucella lima with respect to survival: microhabitat, size of shell, or colour of shell?  These factors are investigated by researchers from the University of Alaska for populations in Auke Bay, Alask, using multi-state models.  Some 2000 marked snails are released into 2 rocky-shore habitats 2km distant from one another and monitored over 3 successive summer periods.  As part of the input to the survival models, topographical features of each habitat such as boulders, flat areas, and crevices are itemised.  There are 6 capture-recapture sessions during the summers of 2006-2008.  Results show, perhaps unsurprisingly, that of the factors tested survival is most dependent upon microhabitat.  Interestingly, an indication of movement by snails into the microhabitat with highest survival over the period of study (boulders) suggests a possible adaptive habitat choice.  Confidence in the study is reduced by low recapture statistics, most notably after each winter season, but even during a single summer season.  For example, only an average of 28% of initial summer captures are recaptured in the same summer, and recoveries beyond 2 successive capture sessions are vanishingly small (<1%).  Chief predators at both sites seem to be birds, but the authors present no statistics on possible losses to predators over anyother causes, including losses by individuals just wandering away from the study sites.  Kovach & Tallmon 2010 Hydrobiologia 652: 49. Photograph courtesy Gerald & Buff Corzi.

NOTE  each habitat is 8m2, which on first thought seems rather small, but perhaps lending itself well for repeated capture-recapture sessions

NOTE  other factors in the model include season, shell length, shell colour (light, medium, dark), and survival time.  Possible tag loss is mentioned, but dismissed as minor

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Research study 4

To what extent does habitat complexity influence interactions between a whelk, its prey, and its predators.  This is examined by a group of researchers at UC Santa Barbara, California for the whelk Nucella emarginata, its prey mussels Mytilus californianus, and one of its chief predators, the sea star Pisaster ochraceus in both field and laboratory situations.  Habitat complexity is defined by depth of mussel bed.  If the bed is too thin the whelks may move away owing to risk of dislodgement by waves or increased exposure to predators; if too thick, the whelks risk having their movements impeded and/or becoming ensnared by byssus threads.  Results show that recapture of marked snails in the field is higher in more complex habitats, presumably because the snails remain longer owing to more interstitial hiding places.  In laboratory experiments in photograph of whelk Nucella emarginata feeding on mussels Mytilus californianus courtesy Biology Department, UC Fullertonwhich whelks are exposed to effluent from upstream sea stars, they are 1.5 times more likely to be found in thicker parts of the mussel substratum.  In 3-mussel-thick substratum, the whelks consume 2.5 times more mussels than ones in 1- or 6-mussel-thick substrata, and growth in the 3-mussel habitat is correspondingly greater.  The presence of sea-star effluent inhibits whelk growth in the lower-complexity treatments (1- and 3-layers), but not in the higher-complexity treatment (6-layers). The results generally support previous models that suggest that a predator’s feeding and growth rates will be highest at intermediate levels of habitat complexity owing to trade-offs between prey availability and predator access.  For example, the crowded conditions in the 6-layer habitat, although offering more refuge space from predators, may actually stifle the whelks’ movements and reduce their access to prey.  The authors summarise by noting that habitat complexity can have direct effects on the whelks’ distributions and interactions, and also mediate trade-offs among various outcomes.  Gosnell et al. 2012 Mar Biol 159: 2867.  Photograph courtesy Biology Department, California State University at Fullerton.

NOTE  in 42d laboratory experiments assessing effects of habitat complexity on feeding and growth rates of whelks, mussels are layered 1, 3, and 6 mussels deep.  The authors comment on their use of mussels both as the whelks’ food source and as the definer of habitat complexity - something that might have been avoided by using artificial mussels or perhaps natural but empty shells glued together

Whelk Nucella emarginata feeding on
mussels Mytilus californianus 1X
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