title for learn-about section of A SNAIL'S ODYSSEY
  Population & community ecology
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Community succession

  This part of population & community ecology deals with community succession, while topics of MUSSEL-BED DIVERSITY, INTRASPECIFIC COMPETITION, INTERSPECIFIC COMPETITION, and EXTENT OF GENETIC DIFFERENTIATION are considered in other sections
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Research study 1
 

photograph showing patch recolonisation in a mussel bed on a wave-exposed shore in British Columbiadrawings showing patch recolonisation in a mussel bed
Patches are formed within the Mytilus californianus zone by wave-shear during winter storms and general log-battering.  The ecology of such patches has attracted much interest, especially with regard to the successional events leading to their recolonisation.  Initially, the patches are colonised by diatoms, then by ephemeral algae such as Ulva spp. and Porphyra spp., then by perennial algae including Halosaccion glandiforme, Mastocarpus papillata, and Endocladia muricata.  These successional stages are followed by acorn barnacles Balanus glandula and Semibalanus cariosus, then goose barnacles Pollicipes polymerus and sometimes bay mussels Mytilus trossulus, and then by the mussel Mytilus californianus.  This last species requires secondary space, that is, substrates such as algae, barnacles, and byssus threads, for larval settlement, but later becomes competitively dominant over all other sessile species.  These events take about 5yr depending upon degree of wave exposure.  Dayton 1971 Ecol Monogr 41: 351; Paine & Levin 1981 Ecol Monogr 51: 145; Wooton 2002 Ecol Res 17: 249.

NOTE the primary space is the rock surface

Patch in a mussel bed in a state of being colonised by acorn barnacles Balanus glandula. The algae
in the background are sea palms Postelsia palmaeformis. In the foreground is a mixed growth of
mussels Mytilus californianus, goose barnacles Pollicipes polymerus, and acorn barnacles B. glandula

 
Research study 2
 

photograph of mussels Mytilus californianus "flowing" into a patch in the mussel bed being colonised by acorn barnacles Balanus glandulaThe mechanism driving these successional events is generally thought to be competition with larger, later-colonising species assuming competitive dominance over smaller early succession species, until finally sea mussels dominate.  Sea mussels Mytillus californianus exhibit “brute force” competitive dominance: small barnacles are smothered, larger barnacles such as Semibalanus cariosus are overgrown and abraded, and goose-neck barnacles Pollicipes polymerus are slowly crushed to death.  But what role do predators play in this?  Both Mytilus californianus and Semibalanus cariosus have refuge in size from predation by whelks Nucella spp., and thus could potentially monopolise all space were it not for predation by sea stars Pisaster ochraceus from lower positions on the shore, and creation of new colonisable spaces by log damage.  Studies on the Olympic Peninsula, WA also show that by consuming the early successional stages, predators such as whelks and birds, and herbivores such as chitons and limpets, actually accelerate succession.  Suchanek 1981 Oecologia 50: 143; Paine 1984 Ecology 65: 133.

Mussels Mytilus californianus "flowing onto" a patch in the bed
through shear weight of numbers. Any newly settled spat of acorn
barnacles Balanus glandula being covered by the mussels will be killed

  Photographs of some of the major "players" in mussel-bed succession:
 
photograph showing competition for space between mussels Mytilus californianus and thatched barnacles Semibalanus cariosus, with predatory whelks Nucella canaliculata in plentiful numbers
Mussels M. californianus monopolise all primary space & the only thatched barnacles S. cariosus surviving are ones growing on the mussels themselves. Whelks Nucella canaliculata prey on newly settled barnacles and mussels, but not on adults, which havae reached size-refuge 0.3X

Some of the "players" in the competitive struggle for primary rock space in a mussel bed are sea mussels Mytilus californianus, red algae Endocladia muricata, acorn barnacles Balanus glandula, and limpets Lottia digitalis 0.25X

In extreme wave-exposed sites large sea palms Postelsia palmaeformis compete with mussels M. californianus for primary space. This particular plant is attached to several mussels and may tear them free in a storm. Note the severe abrasion of Postelsia's holdfast by the mussels
 
Research study 3
 

graph showing effect on growth of seaweed Odonthalia flocculosa in the presence and absence of sea musses Mytilus californianusIn addition to their physical effects on seaweeds seen in the Research Studies above, mussels also interact with seaweeds chemically.  Like most marine invertebrates the principal excretory product of mussels is ammonia which, in enclosed systems such as tidepools, may represent a potential nitrogen source for growth of macroalgae.  That this chemical interchange is possible is shown by experiments in Oregon where growth of red algae Odonthalia floccosa contained with sea mussels Mytilus californianus over 24d in laboratory mesocosms is about 40% greater than growth in the absence of mussels (see photograph of red alga Odonthalia flocculosagraph on Right; asterisks indicate significant differences between means).  Odonthalia is commonly found growing on shells of sea mussels.  The author of the study notes that ammonia augmentation of this sort may be important in high-intertidal pools that are isolated from oceanic nitrogen inputs for extensive periods.  Bracken 2004 J Phycol 40: 1032.

NOTE  lit. “middle world or universe”, referring to experimental containers in which a portion of an ecosystem, in this case a tidepool, is isolated or simulated, allowing cause-and-effect manipulations to be done on one or more elements under controlled laboratory conditions.  Sizes of “cosms” are relative, but for aquatic systems such as ponds or tidepools, would generally range from jam-jars to large ice-cream buckets in size.  In the Oregon study, each "mesocosm" is 2.5 liters in volume

 

 

The red alga Odonthalia flocculosa grows on rocks and sometimes attached to
mussels throughout the intertidal region, and is commonly found in tidepools 1X

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What alternative explanation(s) might there be for these results, something the author of the study has considered? Check out the possibilities below, then CLICK HERE for explanations.

The enhanced growth of Odonthalia may be an artifact of laboratory conditions, and field tests are required for confirmation. 

Other chemicals essential for growth of algae are excreted by mussels. 

Carbon dioxide is required for growth of plants and is released as a by-product of metabolism by mussels. 

Other invertebrates growing on the mussel shells add to the ammonia available to the algae. 

 
Research study 4
 

graph showing interrelationships of cover of intertidal kelp Hedophyllum with occurrence of El Nino event in 1997 with sea-star presence or absenceIn Tatoosh Island, Washington mussel beds Mytilus californianus (sometimes with M. trossulus) exist in stable state with a mid-intertidal canopy-forming dominant kelp Hedophyllum sessile, the mussels being somewhat higher in the intertidal zone than the kelp.  Sea stars Pisaster ochraceus feed on smaller-sized mussels, but larger ones reach size-refuge and this continues to reinforce the stability of the community.  Major interruptions in the steady-state condition occur from time-to-time, most notably with severe winter storms or incursions of warmer waters related to El Niño events, but recovery usually follows.  Note in the graph the effect that a strong 1997 El Niño event has on percentage cover of Hedophyllum but, by about 2000, the alga appears to be in strong recovery.  At that time researchers from the University of Washington remove sea stars Pisaster ochraceus from one of several test sites, allowing greater survival of mussels.  Hedophyllum continues to dominate for a time but, by about 2001, presumably as a result of space competition with the burgeoning mussel population, population numbers of the alga crash and the alga is replaced by mussel beds.  This alternate community state persists for several years, seemingly stable even in the absence of a major consumer. Paine & Trimble 2004 Ecol Lett 7: 441.

 
Research study 5
 

Beds of sea mussels Mytilus californianus are thought to represent a climax community. But do the beds change much over time in degree of cover, thickness, composition, and so on?  This is examined by researchers at the University of California, Los Angeles for beds of Mytilus californianus located at 21 sites along the California coast, including several in the Channel Islands.  The researchers compare historical data from the 1970s and 1980s with measurements made in 2002.  Results show that mussel cover and biomass at the southern sites have declined by 40 and 51%, respectively, during that time, but with little change at central and northern California sites.  The causes of the declines are unknown, but may include such factors as increased seawater temperature, people traffic, and pollution. Smith et al. 2006 Mar Biol 149: 537.

NOTE  the historical data are taken from several research theses and federal-government-sponsored baseline studies conducted in the California Bight region

 
Research study 6
 

photograph of experimenntal array of mussels and sensors to determine wave forces within mussel patches in the intertidal zonePertinent to the above study on patch colonisation in Mytilus-californianus beds are results from research at Hopkins Marine Station, Pacific Grove, California showing that wave forces are significantly reduced within patches.  Thus, settling larvae and spores may be buffered from full exposure to shearing forces of waves.  The author creates experimental open areas of different sizes within a “bed” of glued-down mussel-replicas on a 60 x 60cm aluminum plate, within which are attached a test object of 1cm diameter (see accompanying photograph). The experimental “patch” array is attached to a vertical rock face low in the surf zone.  Transducers fixed both within and without the cleared area measure wave forces, the difference between the reference and treatment transducers providing an approximate of actual force.  One set of results, as an example, indicate that a 1cm-diameter test object within a 5cm-diameter patch experiences up to 60% reduction in wave forces as compared with an unsheltered object.  The mitigating effect drops off as patch size increases and is not evident in patches greater than 30cm diameter.  The author notes that data from only a single location on the shore should be extrapolated to other locations and other wave conditions only with caution.  O’Donnell 2008 Mar Ecol Progr Ser 362: 157.

NOTE  the replicas are cast from real but empty shells of Mytilus californianus in Silastic Rubber S, then the molds are fiilled with polyester resin.  The replica mussels are set in rings.  Removal of successive rings creates patch diameters of, in order, 5, 10, and 15cm, for comparisons

 
Research study 7
 

map showing study site in California for "mussel-trampling" studyhistogram comparing different levels of foot-traffic on mussel mass over a 1-yr periodAll “environmentally aware” visitors to the shore know not to trample on soft-bodied invertebrates and to be careful when turning rocks.  But what about walking over beds of sea mussels?  Their shells seem strong enough to support the weight of most people and, after all, they are often abundant enough to form a carpet such that a person’s mass is distributed over more than just a single mussel.  Now, imagine being on a rocky shore in British Columbia or Alaska, then transport yourself to a much more heavily frequented rocky shoreline at Monarch Beach in southern California (see map).  Here, researchers from California State University, Fullerton estimate overall annual use by humans (walkers, tidepoolers, collectors) during low-tide periods at a maximum of about 1 person per 10m of shoreline per 10min.  This may not seem like a crowd but, as we shall see, the effects on the sea-mussel community may not be insignificant.  To test effects of this amount of traffic, the researchers establish, and walk on, experimental 0.35m2 plots delineated in selected mussel beds.  Treatments are 0, 150, and 300 steps by an average-sized human (60-75kg) wearing soft-soled shoes, with average strides, over all parts of each plot, done once per month, for 1yr.  Results show significant decrease in number and biomass of mussels in both experimental plots as compared with control plots (see graph showing biomass changes).  The authors note that about 15% of the observed losses owe directly to mussels being crushed underfoot.  Overall, the researchers conclude that even such a modest amount of visitor foot-traffic can significantly reduce mussel density, biomass, and sizes.  Smith & Murray 2005 Mar Biol 147: 699.

NOTE  in addition, the authors include in their study the effect of bait-collectors removing 2 large mussels per month, but these data are not included here

NOTE  this equates to about 850 steps . m-2   

 
Research study 8
 

graph showing effect of mussel removal on coralline-alga abundanceMost research on succession in mussel communities suggests that a predictable series of successional events follows removal of the dominant species, in this case the sea mussel Mytilus californianus, but is this always true?  In fact, long-term removal1 of mussels in replicate plots in the mid-intertidal region over a 10-15yr  period on Tatoosh Island, Washington does alter the dynamics of the system in affecting temporal variabity of 3 subdominant species coralline algae Corallina vancouveriensis, acorn barnacles Semibalanus cariosus, and goose barnacles Pollicipes polymerus, but there is no predictability in which species becomes dominant even in closely sited plots.  In all cases, in plots with mussels experimentally removed, temporal variance relative to mean cover of each subdominant organism is lower than in control plots (see sample graph for Corallina).  The author suggests that in its typical domination to form monoculture-type communities, Mytilus interrupts the successional dynamics by amplifying environmental stochasticity2.  The author suggests that further investigation3 of this process is merited.  Wootton 2010 Ecology 91 (1): 42; see also Wooton 2013 Bull Mar Sci 89 (1): 337.

NOTE1  extent of removal depends on frequency of visits over the study period, but generally densities in the experimental plots were on average about 90% lower than in the control plots

NOTE2   lacking a predictable order or randomness, as opposed to deterministic

series of photos showing time-delay results of mussel removal from experimental plots on Tatoosh IslandNOTE3   this short account hardly does justice to the depth and complexity of the study.  It employs complex statistical analyses and is rich in jargon, and the reader is encouraged to read the paper carefully

Sample control plots (above) with
complementary experimental plots (below)
several months after clearing. Note the common presence of coralline algae and
goose barnacles in the experimental plots  

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