Tides and Predation in Intertidal Species Distribution

Rocky Intertidal Ecology I. Population Structure (review) II. Settlement & Recruitment Bipartite life cycle of benthic marine organisms with pelagic larvae review Larvae survive, grow, develop, disperse III. Zonation Pelagic Environment IV. Experiments that changed Ecology settlement reproduction Benthic Environment Adult Juvenile survive, grow, mature review Bipartite life cycle of benthic marine organisms with pelagic larvae review Closed Populations Production Supply Little or no exchange among populations Production Definitions: Recruitment a) settlement - the permanent transition from the pelagic environment to the benthic environment (from the planktonic dispersive stage to the sessile/sedentary benthic stage) b) recruitment - the addition of individuals to a population • could be same as settlement, BUT we rarely observe settlement (it is brief, unpredictable, involves small propagules, often occurs at night)… Supply Open Populations Production Supply Significant exchange among populations Supply Production (Keough and Downes 1982) i) subjective, arbitrary as to when and what is measured (e.g., daily, weekly, monthly, annually) ii) incorporates settlement and post-settlement mortality and movement iii) therefore, this estimate and its relationship to settlement depends on: - sampling frequency - patterns of post-settlement mortality (density-independent and density-dependent) • operationally, a recruit is what we record as a new individual in a population 1 Recruitment In the beginning… when ecologists first wandered into the rocky intertidal… t1 100 80 No. of indiv.s mortality rate = 0.4 (40%) t2 mortality rate = 0.6 (60%) 60 40 I. Pattern: species distributed in discrete zones relative to elevation and tidal height t3 20 0 settlement time • How does sampling frequency influence relationship between settlement and recruitment ? • What if rate of mortality varies among sites? • How does this influence your conclusion about settlement? Rocky Intertidal Zonation mussels gooseneck barnacles anemones, tunicates, sponges coralline algae Pisaster 2 i) zonation exhibited across a very short distance in intertidal - implies very strong determinants of distribution and abundance - great opportunity to examine causes of zonation ii) Almost all of the early ecological studies on zonation concentrated on processes affecting individuals after settlement ( postsettlement : i.e., juveniles and adults in benthic habitat) Reasons: i) those were the life stages for which patterns were described (zonation) ii) logistically easier to work on benthic stages than larvae iii) pool of larvae thought to be limitless (thus, what happened after settlement would be more important) II) Tides as a determinant of zonation patterns in intertidal II) Tides as a determinant of zonation patterns in intertidal i) intertidal zone is an ecotone -- the interface of two distinct habitats: terrestrial & marine (higher more terrestrial tolerant; lower more marine tolerant) ii) what could cause such sharp boundaries in species distributions? • seemed clear that such boundaries had to be the result of physical factors (rapid shift in environmental conditions and successful species) iii) what was the most likely physical factor that could cause zonation in intertidal areas? -- Tides! MOON SUN Tide Level Diurnal (twice daily) tides X Topography of ocean basin modifies tidal current so that the 2 high tides (and 2 low tides) per day are not same height. • this is referred to as mixed semi-diurnal tides gravitational pull & centrifugal force X 12:00 Noon X X 9:00 PM X X X 12:00 Midnight mean sea level X 0 ft mark at MLLW (mean lower low water) = mean lowest tides over the year at that location 6:00 AM 3 • when moon is on the same side as sun (new moon), or opposite sun (full moon), tides are strong (spring tide) • when moon is 90o to sun and moon, tides are weak (neap tide) • time of a tide lags ~ 50 minutes each day because moon rotates 12o around the earth each day night day 1 day freetidetables.com II) Tides as a determinant of zonation patterns in intertidal a) Tides are sinusoidal -- might lead you to believe that the height in intertidal relationship between immersion time (time underwater) and tidal height decreases smoothly (and linearly or gently curvilinear) with increasing tidal height: high An early example exploring effects of immersion (tides) on zonation… b) Example: Doty 1946 (Ecology 27:315-328) i) Pattern: sampled distributions of species of macroalgae along CA and Oregon coasts and found: Six distinct zones high crustose coralline reds (cc) Gigartina (G) Endocladia (En) elevation in intertidal Egregia (Eg) MLLW fleshy reds (fr) Laminaria – kelp (L) low low Immersion (= time submerged) Mean no. hr Does this correspond intuitively to sharp breaks in zonation patterns? iv) Test: Calculated the actual immersion times over the tidal range: ii) General hypothesis: tidal fluctuation and differential tolerance to immersion causes zonation of species in intertidal iii) Specific hypothesis: species zones will correlate with discrete zones of immersion ( critical tide levels ) iv) Test: Calculated the actual immersion times over the tidal range: 24 height in intertidal 20 high Mean no. hr of immersion 16 12 8 4 0 low low Immersion (= time submerged) Mean no. hr v) Results: dramatic non-linear steps in immersion… L fr Eg En G CC high height in intertidal v) Results: dramatic non-linear steps in immersion that correlated with algal zonation! vi) Conclusion: physical factors - immersion - control species distribution and determine zonation of species in intertidal 4 c) Questions / observations: i) Should upper and lower limits both be set by physical factors? ! Where do intertidal species come from? ! Why would physical factors determine lower limit? ii) critical tidal levels were less good at explaining animal distributions; particularly mobile animals – does this cause rejection or modification of the general hypothesis? iii) Alternative hypotheses? Upper limits Lower limits a) radiation (UV) a) submersion b) desiccation b) lack of settlement - larval supply c) thermal stress c) biotic interactions d) freshwater e) lack of settlement - larval supply f) food availability g) biotic interactions c) Questions / observations: iv) How best to distinguish among these alternative hypotheses? v) What processes or mechanisms are responsible for each alternative? III) Connell and the experimental revolution a) Most ecological field studies prior to study we are to examine were purely descriptive (like Doty s) Joe Connell PhD in Millport, Scotland Ecology 1961; Ecol. Monographs 1961 III) Connell and the experimental revolution b) System: two species of barnacles and a carnivorous gastropod in the rocky intertidal: i) (Semi)Balanus balanoides - a large barnacle Cited over 1000 times! ii) Chthamalus stellatus – a small barnacle iii) Thais (Nucella) lapillus – predatory snail Chthamalus stellatus Semibalanus (Balanus) balanoides Nucella lapillus c) Pattern: disjunct vertical distribution of adults of the two barnacle species in intertidal zone 1) adult distributions more restricted than recruit distribution: - at upper limits for both species - at lower limits for Chthamalus 2) distribution of Cthamalus and Balanus overlap extensively as recruits but little as adults 3) Thais only found in adult distribution of Balanus 5 Chthamalus Balanus d) Hypotheses (and circumstantial evidence) 1) Upper limits a) set by settlement distribution? - inconsistent with observation that settlement occurs higher than adult distribution b) species interactions? (untested by Connell) Why untested? i) Chthamalus - few organisms above it (including predators) ii) Balanus - no evidence from other work that Chthamalus negatively affects Balanus d) Hypotheses (and circumstantial evidence) d) Hypotheses (and circumstantial evidence) 2) Lower limits 1) Upper limits a) Balanus c) Heat stress / desiccation - data and observations were consistent with this hypothesis (mechanism untested) i) settlement did occur above adult distribution for both species (rejects settlement hypothesis) i) set by settlement distribution? - perhaps, because lower limits of both recruits and adults are similar ii) set by predation by Thais? ii) much greater mortality for juveniles above adult distribution - Connell tested by experiments with cages - paired replicates at different tidal heights within and below range of Balanus d) Hypotheses (and circumstantial evidence) d) Hypotheses (and circumstantial evidence) 2-a-ii) Lower limit of Balanus set by predation? 2-a-ii) Lower limit of Semibalanus set by predation? i) Experimental design: C tidal height a hth lu ma Ba lan s U C C us C U Caged plots to exclude predators ii) Prediction: ep Xr l i ca te s! U C U U C Caged plots to exclude predators Un-caged plots with predators Un-caged plots with predators iii) Results: Balanus Density (no./m0.25) low high low tidal height high tidal height a) predation was more intense at lower limits, but… b) Balanus were not completely excluded at any tidal height c) lower limit not entirely set by predation 3) other Biological Interactions? e.g., competition with algae (untested) 4) Physical factors? — immersion— untested — why? 6 d) Hypotheses (and circumstantial evidence) 2) Lower limits d) Hypotheses (and circumstantial evidence) 2) Lower limits b) Chthamalus i) Physical processes — immersion b) Chthamalus iii) Predation/Competition — tested both at once (allows for test of multiple causality and interaction among factors) - placed rocks with Chthamalus juveniles in tidepools (of same tidal ht) - compared survivorship to those not in tidepools - no major difference in survivorship - attached rocks with juvenile Chthamalus and Balanus at several tidal heights - conclusion: immersion not important - removed Balanus from ½ of each rock ii) Settlement distribution — unlikely - half of rocks at each tidal height were enclosed in cages (to exclude predators) - settlement distribution much lower than adult distribution - indicates post-settlement mortality d) Hypotheses (and circumstantial evidence) 2-b-iii) Lower limits of Chthamalus set by predation and/or competition? d) Hypotheses (and circumstantial evidence) 2-b-iii) Lower limits of Chthamalus set by predation and/or competition? ii) Predictions: Balanus i) Experimental design: only predation important Thais Chtham. percent cover - remove Balanus from ½ of each rock - half of rocks at each tidal height were enclosed in cages (to exclude predators) d) Hypotheses (and circumstantial evidence) 2-b-iii) Lower limits of Chthamalus set by predation and/or competition? Balanus ii) Results: Chtham. percent cover +T +T -T Thais -T + Balanus -T +T only competition important -T +T both C and P important (additive effects) +T -T – Balanus + -T -T +T -T +T + C and P interact (non-additive effects) -T +T – + Balanus -T +T +T – + – Chthamalus Width of bar represents strength of importance - Balanus - Balanus limits Chth distribution and abundance (competition) - Thais normally does not control Chth by itself - Balanus and Thais jointly affect Chth distribution and abundance - Thais eats Balanus, enhancing Chth distribution and abundance 7 III) Connell and the experimental revolution 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 8