A 5-yr 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

Many 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 Cancer 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