title used in an account of west-coast marine invertebrates entitled A Snail's Odyssey
title used in an account of west-coast marine invertebrates entitled A Snail's Odyssey
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Post-recruitment effects on community structure

  Reproductive events include post-recruitment effects on community structure considered in this section, and
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Research study 1

graph showing that settlement of barnacles Balanus glandula is proportional to the fraction of vacant space availablegraph showing predation by sea stars on newly settled barnacles Balanus glandulaAlthough it seems obvious that the rate of settlement of barnacles Balanus glandula onto an area of rock surface will be proportional to the fraction of vacant space available (see graph on Right), how variations in settlement rate later affect the kinetics of the intertidal community assemblage is not so obvious.  In fact, studies in Monterey Bay, California suggest that settlement variability appears to play as important a role in structuring the community as post-settlement processes such as predation and competition. In areas where barnacle settlement is low, population density varies from year-to-year, but usually in direct accordance with yearly differences in setlement.  In such locations mortality, as from sea-star predation, is independent of the area occupied by barnacles.  In areas where barnacle settlement is high, however, population density varies greatly within years, but only slightly between years regardless of yearly differences in settlement.  In these densely occupied locations mortality becomes cover-dependent owing to increased predation by sea stars Pisaster ochraceus. The sea stars are more attracted to the areas with greater density of barnacles and, when they get there, their feeding efficiency goes up because they can eat clusters of barnacles rather than single individuals (see graph upper Left).  This is evidenced in the graph by the fact that all points indicating less than 0.8 survivorship are associated with discrete patches of basal plates characteristic of Pisaster predation.  Additionally, where excessive crowding leads to the formation of hummocks, mortality goes up through removal of the hummocks by waves and log-battering.  The importance of competition for space as a determinant of community structure as, for photograph of hummock of barnacles Balanus glandula partly torn awayexample, between species of Balanus and Chthamalus, may also apply mainly to locations with high settlement rates.  Gaines & Roughgarden 1985 Proc Natl Acad Sci 82: 3707.

NOTE  from their population models the authors credit these within-year oscillations in abundance, each with a period of 30wk, to time-lags based on growth.  The authors describe the cycling process as commencing with heavy settlement onto spaces typically cleared by Pisaster, followed by rapid growth, and then a recurrence of sea-star predation. 

NOTE  think of an area “swamped” by potential settlers each year, where most or all of the available space is regularly filled


Hummock of barnacles Balanus glandula created by
competition for space. Part of the hummock has
been removed, possibly by sea stars Pisaster
or by floating debris in the waves 1X

Research study 2

An obvious feature of barnacle assemblages Semibalanus cariosus in areas of the San Juan Islands, Washington during the early 1980s was the presence of a cohort of older, larger (>20mm diameter) individuals.  By studying photographic records dating back a decade, researchers could trace this cohort to an exceptionally good year for survival of Semibalanus in 1970.  This followed an unusual spell of sub-zero temperatures coincidental with low spring tides that led to massive mortality of predatory whelks Nucella spp. in 1969-70. This mortality enabled more of the spring 1970 settlement of Semibalanus to reach size refuge (10-14mm diameter) before the whelk population recovered 1-2yr later.  The single mortality event structured S. cariosus populations for more than a decadeSebens & Lewis 1985 J Exp Mar Biol Ecol 87: 55.

Thatched barnacles Semibalanus
isolated on the shore 1X

histograms showing the importance to community structure of a dominant 1970 cohort of barnacles Semibalanus cariosus in future generations The cohort in question settled in 1970 and could be followed for the next dozen years or so
Research study 2.1

The role that recruitment plays in population dynamics of barnacles in south-central Alaska is investigated for Semibalanus cariosus1 by a researcher at the Institute of Marine Science, Fairbanks.  The question addressed is classic ecology, an attempt to determine the relationship between recruitment success and later adult population density, but has the additional interest of being conducted close to the geographic northern limit2 (Kachemak Bay) of a species.  Results over a 3yr study period show considerable yearly and between-site variability in recruitment3 success, part of the variability owing to different levels of predation.  However, in only one year is initial larval settlement linearly photograph of barnacle Semibalanus cariosus with predatory snail Nucella ostrinacorrelated with subsequent adult density; in the other years recruitment greatly exceeds the capacity of the habitat to accommodate them.  This leads to intense intra-specific competition for space.  Interestingly, even during a low recruitment year there are ample recruits to maintain “normal” population density.  The author suggests that even at its northern limit of distribution recruitment of S. cariosus appears not to be the primary process limiting population numbers. Carroll 1996 J Exp Mar Biol Ecol 199: 285.

NOTE1   there are 2 other species Balanus glandula and B. balanoides included in the study but, as both occur in far fewer numbers than S. cariosus and as their recruits could not, in any case, be differentiated in the field, they are not included here

NOTE2   ice scour north of 60o is apparently the most important determinant of northern distributions of barnacles in Alaska, but of course the extent of this influence may change depending upon changing patterns of global warming.  The study site at Kachemak Bay, Alaska is at 59o35’N

NOTE3   the study uses both natural rock surfaces and plastic settlement plates, each type covered with predator-excluding mesh walls.  These walls exclude larger predators such as whelks and sea stars, but not smaller ones such as limpets and nemerteans

Adult barnacles Semibalanus cariosus with a predatory snail Nucella
and several barnacle recruits of different species 2X

Research study 3

photograph of Monterey Bay to show differences in seawater temperature on either side of the BayWhat effect does variation in recruitment intensity of an integral successional stage in a rocky-shore community, such as a barnacle, have on later community structure?  This familiar ecological question is addressed in Monterey Bay, California by manipulating densities of Balanus glandula and Chthamalus spp. in experimental plots at 2 locationsThe 2 locations are on either side of Monterey Bay, and have markedly different seawater temperatures and substratum types (see photograph on Left).  Note that in July 2000 water temperatures at Sand Hill Bluff on the north side of the Bay are considerably warmer than at Hopkins Marine Station on the south side.  The authors’ research hypotheses are that variation in recruit density of barnacles will influence community composition through effects on competition for space or on facilitation.  The first comes into play at high levels of recruitment and/or when space is limiting; the second, if space is not limiting.  In the latter case, increased recruit density may actually lead to general increases in species abundances.  In one of several experiments at the 2 sites the authors manipulate densities of Chthamalus in spring in replicate 8 x 10cm plots by first clearing the plots, applying crushed barnacles to enhance settlement of cyprids, then later culling recruits to densities of 200 per plot (high density) or 50 per plot (low density).  The results are complex and will not be presented here but, overall, the authors find no evidence that recruitment differences influence community composition through competition for space; rather, facilitation seems to be the more important process. The authors conclude that while differences in intensity of recruitment produce measurable changes in community composition, these changes may be short-lived.  Forde & Raimondi 2004 J Exp Mar Biol Ecol 301: 1.

NOTE in each location the upper intertidal area hosts only a few dominant motile organisms (limpets, 2 spp; snail predators, 2 spp) and sessile organisms (barnacles, 2 spp; algae, 2-3spp)

NOTE  the presence of an early coloniser speeds up, or facilitates, establishment of a later coloniser

Research study 4

graph showing relationship between size of cyprid larva and size of later juvenile in the barnacle Balanus glandulaIt is well documented that the health and vitality of a marine invertebrate larva will affect its growth and survival after settlement.  This is confirmed for barnacles Balanus glandula in a combined laboratory and field study at the Oregon Institute of Marine Biology, Coos Bay. The researchers manipulate factors such as temperature and food ration1 during larval culture to produce cyprids of different sizes.  Larger cyprids produce juveniles of larger size than smaller cyprids (see graph on Right). When transferred to the field the larger cyprids2 grow faster as juveniles and often survive better than smaller juveniles.  By translocating3 naturally settled barnacles into the laboratory within 2d of settlement, the authors additionally find, not surprisingly, that initial juvenile size is a good predictor of juvenile growth performance later on. The authors note that the effects of larval nutrition may persist for some time in juveniles.  Emlet & Sadro 2006 Integr Comp Biol 46: 334.

NOTE1  larvae and juveniles are fed diatoms Skeletonema costatum

NOTE2  these larger cyprids have more lipids and protein than smaller ones. Smaller juveniles are created by feeding larvae on reduced rations during the last naupliar stage

NOTE3  larvae are allowed to settle on 8 x 8cm slate plates drilled with arrays of small conical pits to induce settlement in the field