title used in an account of west-coast marine invertebrates entitled A Snail's Odyssey
  Habitat & community ecology
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Interspecific competition

  Considered in this section is interspecific competition, while topics of INTRASPECIFIC COMPETITION and OTHER HAZARDS OF INTERTIDAL LIFE are dealt with in other sections.
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Research study 1

photograph of barnacles Semibalanus cariosus and Balanus glandula competing for space on a rockLong-term observation of barnacle interactions in the San Juan Islands and Waadah Island, Washington shows that, in the absence of predators, Semibalanus cariosus dominates when competing for space with Balanus glandula and Chthamalus dalli, while B. glandula dominates when competing for space with C. dalli.  The size difference between these last 2 species is substantial and exclusion involves overgrowth, uplifting, and squeezing to death of Chthamalus by the larger and superior Balanus Dayton 1971 Ecol Monogr 41: 351.


In this photo a single Semibalanus cariosus appears
to be holding its own against several Balanus glandula 1X

photograph of barnacles Balanus glandula and Chthamalus dalli competing for space
One older generation and several younger generation Balanus glandula are surrounded by encroaching Chthamalus dalli (grey colour) 2X
photograph of barnacle Balanus glandula undercutting a Chthamalus dalli
Balanus glandula undercuts an encroaching Chthamalus dalli while, in turn, a smaller C. dalli is being undercut and/or crushed by conspecifics 3X
photograph of barnacles Balanus glandula and Chthamalus dalli competing for space
Two Balanus glandula overgrow 3 smaller Chthamalus dalli in a competition for space 3X
Research study 2

While small body size may be disadvantageous for Chthamalus in one-on-one interactions with larger species of barnacles, smallness does have some graph showing percentage cover of barnacles Balanus glandula and Chthamalus dalli with and without herbivores being presentecological advantages.  These are: 1) settling and surviving in small-sized protective intertidal crevices, 2) reaching reproductive maturity faster and having quicker generation cycle, and 3) being less attractive to predators.  A common belief on the west coast and elsewhere is that chthamaloid species are excluded from all but the highest levels of the intertidal zone because of competition with balanoids.  Thus, the smaller chthamaloid species is thought to occupy a spatial refuge created by the physiological intolerance of balanoids for high intertidal life.  While this pattern is, indeed, true for many shores of the world where balanoids and chthamaloids coexist, studies at Edmonds, Washington over many years show a different pattern, one in which upper limits of Balanus glandula may, in fact, exceed those of Chthamalus dalli and persist over time.  In other temperate-zone shore communities characterised by intense predation and physical disturbance, chthamaloids may actually achieve greatest numbers lower in the intertidal zone than other barnacle types. 

Shown in the lower graph is a circumstance when motile herbivores (e.g., limpets) are present in an area where C. dalli is the dominant space occupier. When herbivores are absent, however, B. glandula is dominant (see upper graph). In summary, the author cautions against generalising about a single influence, e.g., competition, as being a dominant ecological force in setting such patterns, especially at the risk of ignoring other possible influences, such as predation and disturbance.  Paine 1981 Paleobiology 7: 553.



NOTE  this Research Study is a response to an earlier publication by researchers at The Johns Hopkins University, Maryland and Scripps Institution of Oceanography, La Jolla, California who presented an overview of chthamaloid and balanoid evolution and their present-day distributions in several areas of the world, including the west coast of North America, and concluded that through competitive exclusion balanoids have caused the ecological restriction and decline of chthamaloids.  In particular, they point to the presence in balanoids of a tubiferous wall structure, which is stronger and faster growing than that of chthamaloids, and which led in their opinion to the elimination of chthamaloid barnacles from mid and low levels on the shore where they were at a competitive disadvantage.  Stanley & Newman 1980 Paleobiology 6: 173.

Research study 3

In response to the idea presented in the above Research Study that predation may have been more important than competitive exclusion in restricting chthamaloids from all but the higher intertidal regions, the researchers at La Jolla, California and Baltimore, Maryland point out 4 main facts in a follow-photograph showing barnacles at upper limit of distribution on the shoreup publication: 1) while chthamaloids have dwindled during the Cenozoic Era, balanoids have undergone a remarkable adaptive radiation, 2) balanoids are competitively superior to chthamaloids in battles for space, 3) balanoids grow comparatively quickly, and 4) peak diversity of living balanoids coincides precisely with an evolutionary absence of chthamaloids from the low intertidal/subtidal regions.  The authors counter the idea presented in Research Study 2 above that predation may have been a strong factor governing chthamaloid evolution by noting an absence of evidence that predation has “intensified markedly in the marine realm during the last 40 myr or so”.  On the strength of these and other arguments the authors stick by their idea that competitive exclusion has been the principal factor leading to the decline of chthamaloids below the upper intertidal zone.  Newman & Stanley 1981 Paleobiology 7: 561.

Upper limit of barnacle distribution on a rocky shore
in Barkley Sound, British Columbia: small-sized
Balanus glandula
intermixed with Chthamalus dalli 0.5X

Research study 4

phtoograph of a mussel Mytilus californianus with barnacles, mostly Balanus glandula, growing on itIn interactions between sea mussels and barnacles on rocky west-coast shores, the mussel is the dominant competitor for space and the barnacle the inferior one.  However, the outcome of such competitive exclusion may not be elimination of the inferior species if the dominant species somehow modifies the environment such that it promotes the existence of the former.  In fact, on the west coast, mussels Mytilus californianus often host dense populations of barnacles including Semibalanus cariosus and Balanus glandula, with the latter predominating, suggesting that rather than being reduced through competitive exclusion, their population numbers are in fact enhanced because of the extra colonisable surface being made available.  This hypothesis is tested at 3 locations in Oregon and one in Washington by assessing extent of barnacle cover on mussels in areas with 100% cover of mussels and 0% cover of barnacles on the rock surface.  Results show that average area of live barnacles on mussels is 128% of the underlying primary rock space.  The authors conclude that on average the presence of the dominant competitor actually enhances population density of the inferior competitor.  On the basis of their results, the authors propose adding a third type of interspecific competition to the usually accepted 2 used in rocky-intertidal ecology paradigm (interference competition and preemptive competition).  The proposed third one is termed substrate displacement, where secondary habitat suitable for the inferior competitor increases with increasing colonisation by a dominant competitor, thus enhancing living conditions for the former.  Lee & Ambrose 1989 Oikos 56: 424.

NOTE  area of a mussel valve is estimated as the area of an ellipse

NOTE  mussels lie obliquely to the surface and also may be layered, thus leading to greater than 100% theoretical space available to be colonised by barnacles

Mussel Mytilus californianus with growth of barnacles. The barnacles are
mostly Balanus glandula with a few darker-coloured Chthamalus dalli 1.2X

Research study 5

photograph of mixed growth of brown algae Pelvetiopsis and Fucus, and barnacles Balanus glandulaSuccessional events following experimental clearing of plots in the intertidal community on the central Oregon coast follows the pattern: barnacles Chthamalus dalli are the first to colonise, but are competitively displaced by Balanus glandula. Once Balanus is established, the macroalgae Pelvetiopsis limitata, Fucus distichus, and Endocladia muricata colonise.  Rates of colonisation in experimental plots that graph showing effect on barnacles Chthamalus dalli of removal of barnacles Balanus glandulaare near to one another vary greatly, apparently as a result of variation in the timing of Balanus recruitment. 

The fine-scale processes of succession are investigated by independent removals of graph showing effect of removal of barnacles Chthamalus dalli on abundance of Balanus glandulabarnacles.  When Balanus is removed, Chthamalus thrives (see graph above Right); however, removal of Chthamalus has no effect on recruitment of Balanus (graph on lower Right). Interestingly, algae only colonise after barnacles become established, with the facilitation effect being stronger for Balanus than for Chthamalus

graph showing effect on algal cover of presence of barnacles Balanus glandulaWhen Balanus is removed, algal density decreases significantly in comparison with undisturbed control plots (see graph on Left). The presence of barnacle tests on the substratum interferes with foraging by limpets; hence, permitting survival of more algal spores.  The author notes that 2 successional models are involved: Chthamalus-Balanus being an example of the tolerance model and barnacle-algae being an example of the facilitation model.  Consumers (limpets) affect the succession in 2 ways.  First, by bulldozing Balanus larvae as they attempt to settle, the limpets delay the establishment of Balanus and the competitive exclusion of Chthamalus.  Second, the limpets delay the establishment of macroalgae.  The study is a nice reiteration of what is known already on processes of succession in seashore communities, and adds valuable insight into the role played by consumers in modifying the rate of succession.  Farrell 1991 Ecol Monogr 61: 95; see also Farrell 1989 J Exp Mar Biol Ecol 128: 57.

NOTE  3 theoretical models of succession are recognised: 1) tolerance model: where early successional stages have little or no effect on later stages; 2) facilitation model: where early successional stages speed up establishment of later stages; and 3) inhibition model: where early successional stages slow up establishment of later stages.  See Connell & Slatyer 1977 Am Nat 111: 1119.

Research study 6

Beds of the competitively dominant barnacles Semibalanus cariosus in San Juan Island, Washington are often characterised by the presence of much bare space on the rock surfaces.  The large adults have reached a size refuge from predation by whelks Nucella spp., but newly settled Semibalanus on these bare patches of rock surface are eaten by the whelks and bulldozed away by limpets.  Does the presence of the large barnacles modulate in some way the action of the whelk consumers in maintaining the bare space?  This is tested on a rocky shoreline in San Juan Island, Washington using photograph of several barnacles Semibalanus cariosus with limpetsnatural bare patches as well as patches cleared in nearby Semibalanus beds.  Results show that similar ecological interactions are taking place as in the past to maintain patches without adult Semibalanus, but the whelks and limpets apparently have no effect on maintenance of bare patches within nearby beds of large barnacles.  Berlow & Navarrete 1997 J Exp Mar Biol Ecol 214: 195.

NOTE  the site used in this study is the same as that used for pioneering studies a quarter of a century ago, and the idea of repeating the original work is a good one. For the original see Dayton 1971 Ecol Monogr 41: 351 at LEARNABOUT MUSSELS: POPULATION & COMMUNITY ECOLOGY: COMMUNITY SUCCESSION

NOTE  it is not possible to do justice to this large and complex study in the space available in the ODYSSEY, other than to draw the reader’s attention to it


Several isolated Semibalanus cariosus among
a scattering of Balanus glandula and limpets 0.7X

Research study 7

map showing distribution of 6 west-coast barnacle species used in research studyIn ecology, character displacement is an outcome of competition in which 2 species living in the same area have evolved differences in morphology or other characters that lessen competition for food resources.  In areas of non-overlap the 2 species may be hard to tell apart, but in areas of overlap, the populations diverge in one character or another and are more easily distinguished.  In other words, they have “displaced” one another in one or more characters, presumably with the competitively dominant species doing the displacing. Where the species’ distributions do not overlap, the differences are minimised or lost.  This interesting concept is examined in a study of 2 west-coast barnacle communities at Los Angeles, California and Bamfield, British Columbia (see accompanying map), using various shell dimensions (height, volume, and basal area) and schematic showing character displacement in populations of potentially competing west-coast barnacle specieslength of the feeding cirri as the potentially “displaceable” characters.  As shown on the map, each community consists of 4 species, with 2 species, Balanus glandula and Pollicipes polymerus, being common to both.  At Bamfield the 3 acorn barnacles show at least some overlap in vertical distributions on the shore (comparable data for the Los Angeles community are not presented by the authors) .

The analytical techniques used are complex, but the results in their simplest form reveal no significant effect on body size, but there is a remarkably even displacement in ramus (cirrus) length exhibited in both barnacle communities, despite their large geographical separation and different species composition (see figure above Right). No one species overlaps another in length of feeding appendage.  Note in the figure that mean lengths of rami are evenly displaced in the Bamfield data, but not in the Los Angeles data.  Moreover, both B. glandula and P. polymerus have significantly shorter feeding appendages at Los Angeles than at Bamfield. Based on other evidence, the authors suggest that competition causes the difference, but whether it relates to competition for food or space, or both, is not known.  The authors make a compelling argument for feeding as being the operative competitive force, but it is hard to imagine that direct competitive interactions for food between the much larger (and more discretely clustered) Polymerus and the smaller B. glandula could exist.  That goose barnacles generally feed out of contact with acorn barnacles, and on different food types and sizes of particles, are not points raised by the authors.  The study is interesting and certainly provocative. The authors remark that the “real” value of their work may be in stimulating further research into competition and character displacement in barnacles.  Marchinko et al. 2004 Ecology Letters 7: 114. 

NOTE  the rationale for character displacement is the competitive exclusion principle, where in a stable community no 2 species with identical niche requirements can coexist

NOTE  other characters are ecological, behavioral, and physiological, all assumed to be genetically based. The authors provide data on several morphological features, but only the set dealing with ramus (cirrus) lengths is shown here