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  Population & community ecology
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Topics relating to population & community ecology include competition, considered here, and KEYSTONE PREDATOR, and POPULATION BIOLOGY dealt with in other sections.

This section on competition includes a sub-section on AGONISM below.

Research study 1

schematic showing prey allocation between 2 potentially competing sea-star species Pisaster ochraceus and Leptasterias hexactisInterspecific ompetition is convincingly demonstrated in a study on the ochre star Pisaster ochraceus and the six-armed Leptasterias hexactis in areas of San Juan Island and Olympic Peninsula, Washington where both species co-occur.  Both species feed mainly during summer and there is broad overlap in diet (see illustration on Left). However, note the specialisation in diet selection by each species.  The smaller-sized Leptasterias tends to favour smaller-sized graph showing effect on size of sea star Leptasterias hexactis of presence or absence of competitor sea star Pisaster ochraceusprey, while the reverse is true for the larger-sized Pisaster.  What happens to size of Leptasterias if Pisaster is absent?  This is simply but elegantly tested by selecting 3 island-reefs on which both species live and removing all large Pisaster from one, adding most of them to another, and leaving the third untouched as a control.

Measurements of size of Leptasterias over the next 15mo show that mean live mass of Leptasterias increases by 86% on the first reef, decreases by 40% on the second reef, and stays more or less the same on the third reef (see graph above graph showing inverse relationship in size between 2 potentially competing sea stars Pisaster ochraceus and Leptasterias hexactisRight).  The results suggest that once equilibrium is reached there should be an inverse correlation between biomass-density of the 2 species in habitats with similar prey availability.  This is shown in a comparison of 13 different habitats in the graph on lower Right. Where Pisaster is rare, biomass-density of Leptasterias is high, and where Pisaster is abundant, biomass-density of Leptasterias is low.  Menge 1972 Ecology 53: 635; data on prey eaten from Menge 1974 Ecology 55: 84.

NOTE  1091 SCUBA observations on the 2 species feeding on 17 invertebrates disclose a 71% overlap.  Only the 6 most common prey species are shown in the illustration

Research study 2

histograms showing size-frequency distributions of 2 sea-star species Pisaster ochraceus and Leptasterias hexactis in San Juan Island, WashingtonThe foregoing Research Study on Pisaster and Leptasterias appears to demonstrate the principle of competitive exclusion, that “complete competitors cannot coexist”. However, other studies at Point Caution, San Juan Island, Washington show that the 2 species may coexist in the same general habitat, despite considerable overlap in prey sizes eaten (see size-frequency distributions in histograms). for example, 64% of Leptasterias overlap in diet with small Pisaster, and over 90% of Leptasterias overlap in diet with all size ranges of Pisaster

That the 2 sea-star species are, in fact, able to coexist in this area likely relates to small-scale resource partitioning including: 1) microhabitat separation, especially at low tide, with Leptasterias staying farther back both in crevices and under rocks, and 2) diet separation in which Leptasterias eats small- to medium-sized motile prey, small Pisaster eats small sessile prey, and large Pisaster eats larger more motile prey.  Thus, the food resource is divided along a combination of resource dimensions and not along a single resource “axis”.  In areas where Pisaster is absent or in low numbers, Leptasterias widens its diet to include larger prey.  The nature of the interaction, based on laboratory observation of the 2 species, appears to be of the interference type.  Pisaster is aggressive towards the smaller species, pinching it with its pedicellariae and generally causing it to go off its food.  Menge 1972 Ecology 53: 635; Lubchenco & Menge 1974 Ecol Monogr 44: 189.

NOTE the data are combined here for 2 different years

Research study 3

A follow-up study on the island-reef transplants of Pisaster ochraceus in San Juan Island, Washington described in Research Study 1 above shows that within 2yr the mean live masses of Leptasterias hexactis are approaching new equilibria in the experimental areas.   The author concludes that interspecific competition with Pisaster ochraceus results in smaller-sized Leptasterias.  Release from this competition leads rapidly to a new, larger equilibrium size of Leptasterias.  These data reinforce the idea that in this interaction, Pisaster is the superior competitor.  It eats more of commonly sought-after prey, eats them more quickly, and is aggressively dominant to its smaller competitor.  Menge 1974 Ecology 55: 84.

Research study 4

photograph of sea star Pisaster giganteus courtesy NOAA, Channel Islands National Marine Sanctuary, CaliforniaIn areas around San Diego, California sea stars Pisaster giganteus are often found feeding in apparent competition with the carnivorous/scavenging whelk Kelletia kelletii.  Interestingly, although the sea star is known to feed on the whelk in the fieldphotograph of snail Kelletia kelletii courtesy Gerald and Buff Corsi, California Academy of Sciences, tests of 150 Kelletia paired with P. giganteus in the laboratory reveal no avoidance behaviour by the whelk.  Although the author does not describe the relationship as competititon, it seems that if the 2 species are exploiting a common food resource that is in short supply, it is likely to be a competitive relationship.  Rosenthal 1971 Fish Bull 69: 669. Photograph of P. giganteus courtesy NOAA, Channel Islands National Marine Sanctuary, California; photograph of K. kelletii courtesy Gerald & Buff Corsi and California Academy of Sciences.

photograph of snail Kelletia kelletii competing with sea star Pisaster giganteus for a mussel courtesy Rosenthal 1971 Fish Bull 69: 669.NOTE  during 2.5yr of field study the author cites 65 observations of this kind of “convergent” feeding in the 2 species, but adds that Kelletia may also be found in association with leather stars Dermasterias imbricata and pink stars Pisaster brevispinus.   

NOTE  the author records 95 attacks by Pisaster on Kelletia, but never witnesses escape behaviour in the snail

A sea star Pisaster giganteus and a scavenging snail
Kelletia kelletii
feed on the same prey mussel



Research study 1

photograph of arm-wagging behaviour in agonistic bat stars Asterina miniata courtesy Wober 1975 Biol Bull 148: 483.graph showing the relationship between density of bat stars Asterina miniata and the frquency of agonistic encounters betwee pairs of individualsSometimes sea stars appear to interact agonistically, but the exact nature of what is going on between the “protagonists” is not well understood.  Perhaps as a consequence of their radial symmetry, e.g., diffuse nervous system, absence of a "brain", sea-star behaviour seems to be quite “primitive”.  Agonistic behaviours described for sea-star species include spacing-out behaviour in aquariums, perhaps mediated through pinching by pedicellariae, and “arm-wrestling”.  A researcher in Monterey Bay, California using SCUBA observes that if 2 bat stars Asterina miniata are placed within 1cm of one another they will engage agonistically.  The author describes a complex series of arm movements including extracting, lifting, holding, feinting, arching, dropping, pushing, and locking but, in the end, it is generally the most “overlapped” individual that withdraws.  Bouts last about 30min, but may be longer, especially if the 2 “combatants” are attempting to share the same food item.  Prior to actual contact, 2 individuals may arch their rays out of contact with one another and “face off” for several moments. Sexes of 19 interacting pairs of Asterina show no significant deviation from randomness; hence, the bouts appear not to be sexually related.  Not surprisingly, the incidence of pair-wise interaction increases with increasing density in the habitat (see graph). The author briefly discusses territoriality in Asterina populations, and this may be a subject worthy of further research, both in this species and in asteroids in general.  Wobber 1975 Biol Bull 148: 483.

NOTE  agonism is a behavioural term that encompasses all aspects of “aggressive” actions including threat, attack, and flight.  The term aggression emphasises the attack component

NOTE  Asterina is now known by the earlier genus name Patiria

Research study 2
  Similar complex agonistic interactions are described by the same researcher for sunflower stars Pycnopodia helianthoides in the Monterey Bay area, but with an additional behaviour, side-slipping (sliding of arms between, then over and onto the rays of the “opponent”).  Just as with bat-star bouts, the “loser” is generally the one that is more overlapped, especially if the overlapped part is the central disc.  On separation, one individual may pursue the other over distances of several meters.  The author admits to seeing no acts of cannibalism resulting from the interactions, but does discuss the possibility that the behaviour may be predatory rather than some form of intraspecific competition. Wobber 1975 Biol Bull 148: 483.

Below: two sunflower stars aggressively engage in Barkley Sound, British Columbia. The behaviour includes writhing, intertwining, fast crawling, and slipping of bodies past one another until they separate and crawl away 0.25X
photograph of 2 sunflower stars Pycnopodia helianthoides engaged in some sort of vigorous interaction photograph of two sunflower stars Pycnopodia helianthoides engaged in some sort of agonism
Research study 3

An interesting experiment in Tatoosh Island, Washington demonstrates spacing-out behaviour in ochre stars Pisaster ochraceus.  In the study, densities of sea stars are manipulated at 7 rock-bench sites so that 4 sites have greater, and 3 sites have lesser, densities than their original ones.  Within 48h graph showing how sea stars Pisaster ochraceus tend to return to original densities in several habitats when densities are adjusted to either greater than or lesser than their original densitiesafter the manipulation the sea-star abundances are nearly convergent on their original levels.  The actual mechanism of separation is not known, but the authors surmise that agonistic behaviour involving pinching by pedicellariae may be involved.  The results show that social interactions among P. ochraceus can alter local densities and, thus, rates of predation in the rocky intertidal region.  Palumbi & Freed 1988 Ecology 69: 1624.

photograph of a cluster of ochre stars Pisaster ochraceus at low tideNOTE  other data in the study show that the act of manipulation accounts for some, but not all, of the later movement of the sea stars. The graph at Day 0 indicates the relative changes in densities in the 7 areas resulting from manipulating the sea stars, 4 areas having their densities increased and 3 areas having their densities decreased. After 2d the sea stars have crawled either away from one another or towards one another, thus tending to restore their original densities

Ochre stars cluster together at low tide for
protection and to conserve water. Only when
the tide comes in do they space themselves out