EVSC20004 Lecture Notes - Lecture 8: Chthamalus, Biogeography, Starfish
18 May 2018
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LECTURE 8: ROCKY SHORES – ECOLOGY
• Competition and predation mediated by the environmental conditions in which they occur
Barnacles
• Competition for space between Semibalanus and Chthamalus
• At mean low water level, barnacles are generally rare
Joe Connell (1961) - zonation of 2 types of barnacles
• Larval settlements patterns explain some of the difference in adult distribution although
settlement overlaps in the upper intertidal
• Importance of competition: transplanted baby Chthamalus down the intertidal zone, showed
they were quickly overgrown by Semibalanus - outcompete in a shared environment
• Desiccation: in high intertidal, Semibalanus die due to desiccation - physical gradient
• Predation: removed predatory species and showed lower limit of Semibalanus was controlled
by predation
• Combination of physical factors, settlement preferences, competition and predation is
responsible for zonation patterns in studied rocky shore
• Experiment by Connell: led to generalisation that upper limit in intertidal zone is regulated by
physical factors, whereas lower limit is regulated by biological factors
o Obvious exceptions exist to this generalisation
Semibalanus
Chthamalus
• Dominates high intertidal zone
• Larval settlement large range, down low there is
no adults
• Outcompetes Chthamalus
• Cool adapted species
• Dominates mean high tide level
and below
• Larval settlement up high, no
adults
• Warm adapted species
• Effect of climate change on zonation patterns - re-organisation of competitive interactions,
may lead to changes in community assemblages
o Range shift: species ranges change geographic location
o Range contraction: range gets smaller
Cape Cod Barnacles, East Coast USA
• Classical bio-geographic barrier - Cape Cod, stops mixing of flora and fauna
• Colder water to north, warmer water to south
• Cannel built through Cod, both Semibalanus and Chthamalus living in the cannel
• Survival of Chthamalus in transplants beyond its northern limits - higher survival rate when no
competition - can survive beyond normal range
• South of Cape Cod: both compete, Chthamalus dominates high intertidal
o Semibalanus cannot outcompete in intertidal - too hot for them, desiccation
• North of Cape Cod: waters too cool for Chthamalus and Semibalanus occurs right to top of
high intertidal, northern limit of Chthamalus is determined by temp and competition
• Transplant Chthamalus north 80km, survive well and don’t have to compete
• As land and water temp rises, likely to be numerous range extension of intertidal species
and changes in competitive interactions among them
o Effect on food webs and assemblages
Document Summary
Lecture 8: rocky shores ecology: competition and predation mediated by the environmental conditions in which they occur. Barnacles: competition for space between semibalanus and chthamalus, at mean low water level, barnacles are generally rare. Joe connell (1961) - zonation of 2 types of barnacles. Larval settlements patterns explain some of the difference in adult distribution although settlement overlaps in the upper intertidal. Larval settlement large range, down low there is no adults: outcompetes chthamalus, cool adapted species. Chthamalus: dominates mean high tide level and below. Species interactions: all species interact with other species through competition, predation and mutualism, predators may modify the dominance achieved by competition & strongly affect species composition, keystone species: pisaster (sea star) in californian rocky intertidal. Large-scale disturbances may lead to unpredictable colonisation, leading to alternate stable states: partly explains why we observe different patches in intertidal zone with different domination species despite the predictable outcomes of the processes of competition.
