Rocky Intertidal Ecology -- part II The development of experimental ecology I. Intertidal Zonation, part II 1. Follow ups on Connell 2. Predation 3. Exceptions II. Horizontal Distribution 1. Variation in Relative Importance 2. Alternate Stable States Connell and the experimental revolution review Impacts: 1) Connell s rule : upper limits set by physical processes, lower limits set by species interactions 2) The dawn of appreciation and exploration of experimental field ecology The explosion of field experiments in the intertidal 1. David Wethey (1984, Biological Bulletin) a) System - same as Connell s (Semibalanus and Chthamalus) but in New England on east coast of US b) Pattern - same as Connell s (Semibalanus and Chthamalus) but focus on explaining upper limits of Semibalanus c) General hypotheses: i) Solar exposure sets upper limit of Semibalanus ii) Competition with Semibalanus sets lower limit of Chthamalus tidal height Chthamalus Semibalanus d) Specific hypotheses: Shading areas above upper limit of Semibalanus will 1) allow Semibalanus to extend higher 2) cause decrease in Chthamalus because of increase in Semibalanus e) Experimental Design: - control treatment (unmanipulated) to compare with manipulations - manipulations designed to test for artifacts of main manipulation Treatment Predators Shade Roof 1) Unmanipulated control + 3) 2-sided cage with opaque plastic roof + + + 4) 2-sided cage with screen roof + 5) 2-sided cage with clear plastic roof + + 6) Full cage with opaque plastic roof + + 7) Full cage with screen roof 8) Full cage with clear plastic roof + Many treatment comparisons test effects of shading, others predation, and others cage artifacts. Which comparisons test which effects? Wethey (1984) i) cage or roof material effects (i.e., artifacts) ii) effects of predators (above predator zone) iii) Semibalanus survived above rmal adult distribution in shade iv) Chthamalus died at higher rate in shade because of increased abundance of Semibalanus survivorship No Shade (1,3,4,6,7) Semi Chth Shade (2 & 5) g) Conclusions: i) Upper limit of Semibalanus set by solar exposure ii) er limit of Chthamalus set by competition with Semibalanus 2. Importance of predation in determining zonation Robert a) System / Pattern: i) Rocky intertidal in Pacific Northwest (Olympic Peninsula, Washington) ii) Mytilus californianus dominant in mid-intertidal - why t higher? (assumed desiccation) - why t lower? lower limit remarkably stable mussels can migrate, and they settle below adult distribution - so settlement may t be very important 1
a) System / Pattern (cont d): iii) Pisaster ochraceus main predator on mussels occurs mainly in lower intertidal - upper limit may be set by desiccation? b) General hypothesis: lower limit of Mytilus set by predation by Pisaster c) Specific hypothesis: in areas where Pisaster is removed, Mytilus distribution will expand lower into the intertidal Rocky Intertidal Zonation mussels gooseneck barnacles anemones, tunicates, sponges coraline algae Pisaster d) Test: i) removed Pisaster from lower intertidal at two sites ii) replicate control area at each site with removals problem with experimental design: replication of removal and control at each site -- Can you distinguish treatment vs. area effects? f) Conclusions: i) predation sets lower limit of mussels ii) supports general paradigm that biotic interactions set lower limits of distribution in intertidal e) Results: i) over several years, Mytilus distribution extended into lower intertidal zone ii) where Mytilus extended into lower intertidal zone, species diversity declined ather story g) Postscript: i) After experiment ended, Paine quit removing Pisaster, but cont d to sample sites: er limit of Mytilus high low removals time Tatoosh site Mukkaw site a) at one site, lower limit moved back up as Pisaster reinvaded b) at other site, it did t! g) Postscript: ii) Two important implications: a) Experimental design: site-site variability can mask experimental results replication at the scale of sites is needed b) Patterns: Distributions can be the result of temporary environmental conditions (in this case the reduction of Pisaster) - mussels do settle to lower intertidal, and if they can survive long eugh, they can grow and escape predation by their greater size - ather example: Southern California species that recruit to and remain in central California during episodic El Niños 2
The experimental revolution, cont. 3. Exceptions to the paradigm (of upper and lower limits) a) Upper limits determined by physical factors? Underwood 1980, Oecologia a) System: Grazing gastropods and foliose macroalgae in intertidal of Australia b) Pattern: Grazers occurs in zone above the alga they eat c) General hypotheses: 1. grazing determines upper limit of foliose algae 2. physical factors determine upper limit of algae 3. both grazing and physical factors 4. other factors - e.g., spores don t settle above upper limit of algae d) Specific hypotheses: 1. areas cleared and kept free of in mid-intertidal will become colonized by foliose algae 2. areas shaded will become colonized by foliose algae 3. only areas both cleared of and shaded will become colonized by algae mid lower foliose algae e) Test: 1. full cage (with roof) provides shade and excludes 2. roof only provides shade only 3. cage with roof ( fence ) only excludes 4. open is control shade roof only open full cage fence algae never occurred in open plots algae colonized the grazer exclusions ( fences ), but t the roof-only or the open plots in fenced areas: algal cover reached 100% but never lived long eugh to reproduce high cover due to continuous recolonization by new spores algae only grew and survived to reproduce in the shaded full cages (with roof) shade algal cover algal reproduction interaction Underwood 1980 f) Conclusions: upper limit of distribution set by biotic factor: grazing! upper limit of reproduction set by interaction between and physical stress 3. Exceptions to the paradigm (of upper and lower limits) er limits determined by biological factors? a) intertidal organisms are adapted to marine and terrestrial habitats b) most studies find that lower limit set by biotic interactions, but c) exceptions: - Littorina (snail) limited to very high intertidal and will die if submerged too long - two macroalgae, Silvetia and, die if submerged too long d) few studies have tested this!!!! mid lower foliose algae Silvetia compressa 3
Abundance Abundance II. Horizontal patterns of distribution and abundance vesiculosus Semibalanus balaides Mytilus edulis Nucella lapillus 1. Variation in relative importance of ecological processes - Menge 1976, Ecology a) Background: we have focused on vertical zonation, but what about horizontal gradients? b) System: barnacles, mussels, algae, predatory snail in New England rocky intertidal c) Patterns: along a gradient from exposed to protected sites CHARACTERISTIC EXPOSED SHORE PROTECTED SHORE Dominated by Fucoid algae Free Space Rare (<10%) Common (40-90%) Predators/Grazers Uncommon (16-80/m 2 ) Common (108-450/m 2 ) Barnacle Cover d) General hypotheses: i) ii) Competition and predation are important in determining these patterns, but Importance of competition and predation differ in exposed and protected sites e) Specific hypotheses (experimental design): Complicated design using cages and cage controls to assess effects of: i) competition: barnacles, mussels, and algae ii) predation / grazing iii) exposure: importance and how it varied along gradient iv) all areas initially cleared Exposed Shores Protected Shores ( manipulation) Open (-) (-, -predators) (-, -predators, -mussels) (-Predators, -) ( manipulation) Algae () Open (-) (-, -predators) (-, -predators, -mussels) (-Predators, -) Algae () At exposed sites - same pattern for cages with and predator removals and removals (open areas) a) barnacles colonize then are out-competed by mussels ( additional effect of predators: see open areas) b) If mussels are removed then barnacles persist - pattern is similar to that in protected shores At protected sites - effects of predators a) mussel abundance is kept low by predators, allowing barnacles to colonize and persist in low numbers outside of cages b) without predators, barnacles are out-competed in cages by mussels c) barnacles persist in high numbers only if you remove mussels, predators, & algae d) if you remove only predators (including ) algae and barnacles colonize but get out-competed by mussels g) Conclusions: Different processes are important at exposed and protected sites: a) at exposed sites -- predation/grazing unimportant - competition is the primary organizing force in the system 1. predators are generally uncommon 2. mussels are competitively dominant (over algae and barnacles) b) at protected sites -- predation important 1. with predation, barnacles dominate if is removed 2. without predation mussels out-compete barnacles and algae 3. predation keeps competition from occurring with mussels (mussel abundance is kept low) c) Importance of predation varies with exposure; at exposed sites predators are uncommon, their feeding ability is reduced because they have to spend more time hanging on and t feeding h) More generally: General paradigm of community organization in rocky intertidal (Connell 1975, Menge and Sutherland 1976, Menge 1976, Lubchenco and Menge 1978, Underwood and Denley 1984) Importance to community organization Predation Competition Physical Processes Benign Severe Environmental harshness in habitats with relatively benign physical environments - predation structures communities with increasing environmental harshness - predation efficiency is decreased and competition becomes a major process structuring communities with even greater environmental harshness - importance of competition decreases and physical processes become most important local escapes from predation (in benign environments) or physical stress (in harsh environments) cause patchiness in the community 4
Percent Cover 2) Alternative stable states - Lubchenco 1978, Am. Naturalist a) Background: Why might sites exhibit different stable b) System: grazing snail and algae in New England rocky intertidal c 1 ) Patterns: spatial variation in community structure: a) Question: Why might sites exhibit different stable b) System: grazing snail and algae in New England rocky intertidal c 1 ) Patterns: spatial variation in community structure: Littorina littorea crispus vesiculosus Habitat Littorina / Tidepools common rare common low Tidepools intermediate intermediate intermediate high Tidepools rare common rare low Rock common rare common low Rock intermediate rare common intermediate Rock rare uncommon common high c 2 ) Patterns: spatial variation in species diversity varies as a function of grazer density and habitat type: Tidepools Rock (emergent) er d) Hypotheses: i) Littorina prefers to eat ii) out-competes other algae in tidepools (if Littorina) iii) Littorina can suppress competitive abilities of in tidepools iv) is competitively inferior on emergent rock surfaces e) Study Design: Why might sites exhibit different stable i) assessed food preferences of Littorina ii) manipulated density of Littorina in pools Lubchenco 1978 i) are favored food of Littorina (in pools and on rock) ii) patterns from pools (Littorina common) Littorina addition (rare before) Littorina removal (common before) Pools 1) can out-compete, but 2) high densities of Littorina can suppress effects of 3) intermediate densities of Littorina allow coexistence of most species 4) Littorines are a keystone species but maximum effect on diversity occurs at intermediate densities Rock 1) competitively inferior - but still favored prey 2) (mid) and (low) are superior competitors 3) Littorina s effect is to graze already uncommon species ( and other ephemerals) 4) Grazing on uncommon species speeds up competitive exclusion and acts to reduce species diversity Results Summary 1. in tidepools, intermediate densities of Littorina increase algal diversity (by eating the dominant competitor) 2. on emergent rock surfaces, Littorina reduces algal diversity (by eating the subordinate competitor) Tidepools Rock (emergent) er 5