encyclopedia of ecology || rocky intertidal zone

Ecosystems | Rocky Intertidal Zone 3107

Rocky Intertidal ZoneP S Petraitis and J A D Fisher, University of Pennsylvania, Philadelphia, PA, USA

S Dudgeon, California State University, Northridge, CA, USA

ª 2008 Elsevier B.V. All rights reserved.

Introduction

Physical Aspects of the Shore

Attached Organisms

Mobile Organisms

Zonation

Figure 1 Closeup of predatory snails, mussels, barnacles, and

brown algae in Maine, USA. Photo by P. S. Petraitis.

Rocky Intertidal Shores as an Important System in

Development of Ecology

Unresolved Problems and Future Directions

Further Reading

Introduction

The British ecologist A. J. Southward described the

intertidal zone as ‘‘the region of the shore between

the highest level washed by the waves and the lowest

level uncovered by the tide,’’ and thus communities on

rocky intertidal shores are primarily defined by the

tides and the presence of hard surfaces. The types of

organisms, the number of species, and the distribution

and abundance of individual species found in a parti-

cular rocky intertidal community also depend on the

physical aspects of the shore, the supply of resources,

food and larvae from overlying water, the biological

interactions among the species present, and the regio-

nal pool of species. Although rocky intertidal shores

cover only a small fraction of the Earth’s surface, they

contain a large diversity of organisms – ranging from

highly productive microalgae to transient vertebrate pre-

dators (Figure 1).

Physical Aspects of the Shore

Tides

Tides are caused by the gravitational effects of the Moonand Sun, which ideally produce a cycle of two high tidesand two low tides per day. However, the amplitude andfrequency of the tides are altered by the phases of theMoon, the Earth’s orbit and declination, latitude, and theconfigurations of the shoreline and the seafloor. The tidalrange tends to be smaller toward the equator and can varyfrom several meters in high latitudes to less than tens ofcentimeters near the equator. Configuration of the coastand the ocean basin can cause harmonic resonances andcreate tides that vary dramatically in amplitude and fre-quency. In extreme cases, the reinforcing and cancelingeffects can produce a single high and low tide per day oralmost no change over the course of a day.

The timing of low tides can have a profound effect byexposing organisms to extreme conditions. For example,the lowest tides in the Gulf of Maine, USA tend to occurnear dusk or dawn, and so organisms are rarely exposed tomid-day sun in the summer but are often exposed tobelow freezing temperatures on winter mornings. In con-trast, the lowest summer tides in southeastern Australiaoccur mid-day and expose organisms to extraordinarilyhigh temperatures.

Characteristics of the Shore

Any firm stable surface in the intertidal zone has thepotential to support the organisms that commonly occurin rocky intertidal communities, and at low tide, intertidalhabitats can range from dry rock to filled tide pools. Rocksurfaces can vary from very hard to relatively soft rocksuch as from granite to sandstone and can range fromsmooth platforms to irregular fields of stone cobbles andboulders. Topography, inclination, color, and texture ofthe rock affect rate of drying and surface temperature,which can limit the distribution and abundance of species.

Figure 2 Extensive brown algal beds in Maine, USA. Photo by

P. S. Petraitis.

3108 Ecosystems | Rocky Intertidal Zone

Man-made surfaces such as rock jetties and wooden pierpilings and biogenic surfaces such as mangrove roots canalso support communities that are indistinguishable fromthe communities found on nearby rocky shores.

Tide pools can be very different than the surroundingshore because of thermal variability, changes in salinityfrom evaporation and runoff, and changes in pH, nutri-ents, and oxygen levels caused by algae. Pools oftensupport residents such as sea urchins, snails, and fishthat would otherwise be restricted to subtidal areas.

The amount of wave surge affects the types of organ-isms found on the shore and their distribution. Wavesurge and breaking waves tend to expand the extent ofthe intertidal zone and distribution of species by continu-ally wetting the shore and allowing species to extendfarther up the shore. Wave surge can also cause mobileanimals to seek refuge and can limit the distribution ofslow moving species, and the force of breaking waves candamage and sweep away organisms. Sand and debris suchas logs swept up by the waves can scour organisms off thesurface. In areas of low wave surge, sedimentation of sandand silt may bury organisms or clog gills and other filter-feeding structures.

Attached Organisms

Unlike terrestrial habitats, which depend largely on localplant material to support resident animal populations,rocky intertidal assemblages are supported not only byalgal primary production but also by secondary produc-tion from suspension feeders, such as barnacles andmussels, which link the ocean’s productivity to the shore.

Algae

The term ‘algae’ refers to an extraordinarily diverse andheterogeneous group comprising about seven majorlineages, or roughly 41% of the kingdom-level branchesin the Eukarya domain. Most lineages consist of unicellularmicroalgae, but the multicellular macroalgae that dominatemany rocky shores worldwide occur in only three groups(Rhodophyta, Chlorophyta, and Phaeophyta) (Figure 2).

Microalgae are ubiquitous and although inconspicu-ous, they are important members of rocky intertidalcommunities. For example, diatoms are the primaryfood source of many grazing gastropods and form bio-films, which facilitate settlement of invertebrate larvaeand stabilize meiofaunal assemblages.

Benthic macroalgae (i.e., seaweeds) dominate manyrocky shores, especially the low- and mid-intertidalzones of temperate regions, and many exhibit morpholo-gies adaptive for life on wave swept shores. The idealizedbody plan of a seaweed consists of a holdfast, a stipe, andone or more blades. The holdfast usually attaches the alga

either by thin encrusting layers of cells tightly appressedto the rock surface or by a massive, thick proliferation oftissue that often produce mucilaginous ‘glues’ to adherethe tissue to the rock. The stipes are analogous to plantstems and display remarkable material properties thatenable seaweeds to withstand the tremendous hydrody-namic forces imposed by breaking waves. The blade is theprincipal structure for the exchange of gases and nutri-ents, and the capture of light for photosynthesis. Bladesalso contain reproductive tissue, either within a vegeta-tive blade, or in sporophylls (i.e., special blades forreproduction). Some larger brown seaweeds, such asfucoids and kelps, have gas-filled floats called pneumato-cysts that buoy the blade so that it remains closer to thesurface where light intensity is greater.

The diversity and complexity of the life cycles of mostseaweeds contributes to their great abundance on rockyshores. The life cycle of most seaweeds consists of analternation of separate gametophyte and sporophytegenerations. The two generations can either look thesame (i.e., isomorphic) or different (heteromorphic). Insome species, the heteromorphic generations are so differ-ent that they were originally described as different species.Heteromorphic life histories are hypothesized to representan adaptation to grazing pressure, and heteromorphic gen-erations clearly show tradeoffs with respect to competitiveability, resistance to disturbance and longevity associatedwith upright foliose and flat encrusting morphologies.

Sessile Invertebrates

Adults of many invertebrate species are attached perma-nently to the rock or other organisms (epibiota). Theseinclude members of the phyla Porifera (sponges), Cnidaria(hydroids and sea anemones), Annelida (tube-buildingpolychates), Arthropoda (barnacles), Mollusca (mussels


and clams), Bryozoa (moss animals), and Chordata (tuni-cates). Suspension feeding – either by pumping waterthrough a sieve structure or trapping particles carried oninduced or external currents – is a common feature ofsessile animals and serves to transfer inputs of energy andnutrients produced in the water column into the intertidalzone via the ingestion of plankton. Additionally, by feedingon locally derived detritus, suspension feeders capture someof the nutrients that are produced by neighboringinhabitants.

Sessile intertidal animals are often physically or che-mically defended against predation and display plasticphenotypes in response to changing environmental con-ditions because they are fixed in place and cannot move toavoid predators. For example, the presence of the preda-tory gastropod Acanthina angelica induces change in theshell shape of its barnacle prey Chthamalus anisopoma, andthe barnacle forms a curved shell making it more difficultfor the predator to attack.

Mobile Organisms

Mobile invertebrates and vertebrates that are found onrocky intertidal shores are typically divided into twocategories based on the amount of time spent betweentidemarks. Resident species remain in the intertidal zonethroughout most of their life and face a large range oflocal physical conditions that they mitigate by a variety ofbehavioral and physiological adaptations. Many residentsfind shelter during low tides, either between rocks, underalgae, or in tide pools, while other species attach toexposed rock surfaces just ahead of the incoming tide.Transient species are those that spend only a small partof their life cycles in the intertidal zone (e.g., as juveniles)or are those that enter and leave the intertidal zone duringlow or high tide.

Figure 3 Rocky shore in Central California, USA with elephant

seals on the beach. Photo by S. Dudgeon.

Invertebrates

Large, mobile invertebrate consumers are ecologically themost intensively studied guild on rocky shores andinclude species from Turbellaria (flatworms), Crustacea(e.g., crabs, shrimp, amphipods, and isopods), Annelida(e.g., polychaetes), Gastropoda (e.g., snails, nudibranchs,and chitons), and Echinodermata (sea urchins, brittlestars, and sea stars). Herbivores range from grazers ofdiatom films to browsers of macroalgae, and predatorsexploit a variety of methods (crushing, stinging, drilling,and partial consumption) to overcome the defenses oftheir prey.

Small mobile metazoans (roughly 0.1–1 mm and col-lectively termed meiofauna) thrive on and among thealgae, animals, and the trapped sediments on rocky shores.Meiofauna include consumers from many invertebrate

phyla, that – due to their small sizes, extremely highabundances, and high turnover rates – are an importantguild of consumers whose effects have largely beenneglected in comparison to studies of larger invertebrates.

Vertebrates

Vertebrates tend to be transient species that use theintertidal zone to feed or hide and include fish and marinemammals that enter at high tide and birds and terrestrialmammals that enter at low tide (Figure 3). For instance,marine iguanas (Amblyrhynchus cristatus) of the GalapagosIslands, Ecuador forage extensively on intertidal algae onlava reefs during low tides. The major exceptions areresident intertidal fishes, which are often cryptic andless than 10 cm in length. Resident and transient fishesinclude hundreds of species from dozens of families,though members of the families Blenniidae, Gobiidae,and Labridae are the most common.

Birds and mammals, characterized by high endother-mic metabolic rates and large body sizes, have significantimpacts on intertidal communities even at low densities.Birds include locally nesting and migratory species andcan remove millions of invertebrates during a season. Inaddition, birds in some communities provide major inputsof nutrients via guano and prey remains. More than twodozen terrestrial mammals, mostly carnivores, rodents,and artiodactyls, have been reported as consumers orscavengers of rocky intertidal organisms on every conti-nent except Antarctica. Most recorded prey species aremollusks, crabs, or fish. Probably one of the most unusualcases is a population of feral rabbits on a small island offthe coast of South Africa that forage on seaweeds in theintertidal zone. Given the mobility of vertebrates, theirimpact on rocky intertidal shores has been difficult toassess and intertidal activity is often discovered by finding


exclusively intertidal animals or algae in the gut contentsof otherwise pelagic or terrestrial species.

Little is known about the effects of harvesting byhumans in the rocky intertidal zone. Results from a fewlarge-scale studies in Australia, Chile, and South Africa,however, have demonstrated that harvesting has had sig-nificant effects on intertidal assemblages.

Zonation

Patterns

Rocky intertidal shores often display a vertical zonationof fauna and flora associated with the strong environmen-tal gradient produced by the rise and fall of the tides. Forexample, most moderately exposed rocky shores of thenorthern hemisphere have kelps at the littoral sublittoralinterface, followed by rhodophyte algae dominating thelow intertidal zone, by fucoid algae, mussels, and barna-cles dominating the mid-intertidal zone, and bycyanobacteria, lichens, and a variety of small tufted,encrusting, or filamentous ephemeral seaweeds occurringin the high intertidal zone. While species from manyphyla may be found together, often a single species orgroup is so common; vertical zones are named accordingto the dominant group (e.g. the intertidal balanoid zonenamed after barnacles in the family Balanidae).

Combinations of various physical factors acting upondifferent inhabitants in intertidal zones that vary in theirexposure to waves can lead to complex patterns of dis-tribution and abundance along shorelines in a particularregion. Nevertheless, some general patterns are evident ata regional scale. Geographically, vertical zonation pat-terns are most pronounced on temperate rocky shoreswhere species diversity is high and tidal amplitudes tendto be greatest. On rocky shores in the tropics, biotic zonesare compressed into narrow vertical bands because ofsmall tidal amplitudes. In polar regions, annual ice scourand low species diversity tend to obscure any conspicuousvertical zonation.

Causes

It is often stated that the upper limits of organisms are setby physical factors, whereas the lower limits are set bybiological interactions but there are many exceptions tothis rule. The specific causes of the zonation seen on mostrocky shorelines vary with geographic location, but zona-tion results primarily from behavior of larvae and adults,tolerance to physiological stress, the effects of consumers,and the interplay between production and the presence ofneighbors.

Adult movements and larval behavior during settlementfrom the plankton onto rocky shores have major effects onthe distribution of animals. For example, studies of barnacles

have shown that vertical zonation of larvae in the watercolumn contributes to corresponding vertical zonations ofboth larval settlement and adults on the shore, a patternpreviously ascribed solely to interspecific competition. Forseaweeds, behavior is a relatively unimportant cause of theirzonation since adult seaweeds are sessile and settling sporesare mostly passively transported.

Marine organisms living higher on the shore arefaced with more frequent and extreme physiologicalchallenges than their lower shore counterparts, andthe upper limits of intertidal distributions for mostspecies are set by cellular dehydration. Dehydrationcan occur either from freezing during winter or simplydesiccation associated with long emersion times. Hightemperatures and wind, which accelerate the rate ofwater loss from tissues, exacerbate the effects ofdesiccation.

Primary and secondary production by sessile organismscan be limited at higher tidal elevations because nutrientsand other resources can be acquired only when immersed.Respiration rates of seaweeds and invertebrates are tem-perature dependent and thus can be greater when anorganism is exposed at low tide. For seaweeds, prolongedexposure to dehydration also reduces photosynthesis.

The reduced productivity associated with increasedexposure at higher tidal elevations modifies intra- andinterspecific interactions. For instance, competitionbetween seaweeds, which may be intense lower on theshore, is reduced at higher tidal elevations and enablescoexistence. Competition among intertidal seaweeds ishierarchical with lower shore species dominating thoseof the higher shore. Thus, fucoid species of the midintertidal zone are outcompeted for space in the lowzone by foliose red seaweeds that pre-empt spacewith an encrusting perennial holdfast. There is also acompetitive hierarchy among mid intertidal zone fucoidswith those typically occurring lower on the shore compe-titively dominant to those higher up. This is mostapparent on European rocky shores where the diversityof intertidal fucoids is greatest.

Grazing rates tend to be greater lower on the shore,although there are cases of herbivory by insects settingthe upper limits of ephemeral green algae. Grazing bysea urchins at the interface with the sub-littoral zone canlimit the lower distributions of macroalgae, but there islittle evidence for grazing on perennial seaweeds settingthe lower limits of those taxa within the intertidalzone. Grazing of perennial seaweeds is most intense atthe sporeling stage soon after settlement. Grazing bygastropods and small crustaceans certainly contributesto losses of biomass of established individuals, but doesnot affect distributions within the intertidal zone. Incontrast, the grazing of established ephemeral speciesboth on emergent rock and tidepools is intense duringspring and summer in many regions eventually


eliminating those algae from their respective habitats.There are also many examples of consumers using sea-weeds as habitat as well as food.

Figure 4 Grindstone Neck in Maine, USA with Mount Desert

Island in the background. This site was used by Menge and

Lubchenco in their groundbreaking work in the 1970s. Photo byP. S. Petraitis.

Rocky Intertidal Shores as an ImportantSystem in Development of Ecology

The rocky intertidal zone has been a stronghold for eco-logical research, and the success of intertidal experimentsstems in part from the fact that intertidal assemblages areoften comprised of the few species that are able to survivethe environmental variation associated with the cycling oftides. In addition, many resident intertidal species aresmall, common, and slow moving or fixed in one place.Thus rocky intertidal shores historically appeared as sim-ple, well-defined habitats in which easily observed andmanipulated local interactions control the dynamics ofthe assemblages. Such initial appearances, however, havebeen deceiving, and variation in recruitment of offspringfrom the plankton, a characteristic of many marine species,has stimulated an increased appreciation of the role ofoceanographic conditions.

Descriptive Studies: Research Prior to 1960

Descriptions of rocky shores and speculation about thecauses of vertical zonation go back more than 195 years.Before the 1960s, ecologists had published descriptions ofintertidal areas from more than a dozen large geographicalregions that spanned much of the globe and included bothsides of the North Pacific and North Atlantic; Greenland;the West Indies; South and Central America; the coastsof Africa; the Mediterranean; the Black Sea; Indian OceanIslands; Singapore; Pacific Islands, Australia, and Tazmania.These early accounts of the rocky intertidal remain a poten-tially valuable source for comparison to contemporarypatterns of species distributions due to local species extinc-tions and introductions.

The Rise of Experimental Studies: 1960–80

Direct experimental manipulation of intertidal organismsaccelerated in the 1960s with the groundbreaking work ofJ. H. Connell and R. T. Paine. Connell manipulated thepresence of two species of barnacles in Scotland by selec-tively removing individuals from small tiles fashionedfrom the sandstone rock from the shore. He showed thatthe lower limit of the high intertidal species Chthamalus

stellatus was set by competition with the mid zone speciesBalanus (now Semibalanus) balanoides and that the upperlimit of S. balanoides was set by physical factors. Paineremoved the predatory seastar Pisaster ochraceus from anarea of the intertidal shore in Washington and showedthat Pisaster was responsible for controlling mussels,

which are successful competitors for space and dominatethe intertidal shore in the absence of Pisaster. These earlyinvestigations provided a framework for the rapid growthof experimental studies that characterized the field inrecent decades (Figure 4).

In general, the observation and experimental manip-ulations of mobile consumers and their prey has oftenrevealed predation by mobile consumers as an importantfactor that contributes to the structure of rocky intertidalassemblages. Consumers have been repeatedly shown tobe prey species- and prey size-selective, while algal graz-ing consumers can inadvertently remove newly settledanimals and algae as well as their intended prey.

Supply-Side Ecology and External Drivers:1980–2005

Marine ecologists have known for a long time that successof many intertidal species depend on the supply of pro-pagules (larvae, zygotes, and spores) from the plankton,but it was not until the 1980s that experiments wereexecuted to assess how the supply of propagules influ-enced the patterns of distribution and abundance of adultsin benthic assemblages.

Propagule supply and early post-settlement mortalitymarkedly influence both the strength of interactions amongestablished individuals and overall patterns of distributionand abundance on rocky shores. Abundance of establishedindividuals is often directly proportional to the density ofsettlement and consequently, and strength of adult inter-actions depends on variation of settlement. In contrast, ifsettlement is high enough to consistently saturate the sys-tem, then local populations tend to be driven by stronginteractions among adults regardless of settlement varia-tion. In some cases, heavy early postsettlement mortality

Figure 5 The intertidal zone near Antofagasta in northern Chile,

a region with upwelling and abundant seaweeds. Photo by

P. S. Petraitis.


can lead to low densities of adults despite an abundance ofsettlers, and this has been shown for several seaweeds andmany invertebrate species. The causes of variation in pro-pagule supply can be classified into two broad categories –oceanographic transport or regional offshore production.Although invertebrate larvae and some macroalgal sporesare motile, their movements are most directly importantat small spatial scales near the substrate just prior to settle-ment. By and large, propagules of benthic species aretransported at the mercy of currents and other oceanictransport phenomena. For instance, coastal upwellingresults in a net offshore transport of propagules and leadsto a reduction in settlement along a shoreline. This com-monly occurs with invertebrate species that have longresidence times in the plankton. In contrast, seaweeds,which have very short planktonic stages, often dominateintertidal sites within regions characterized by seasonal orpermanent upwelling (Figure 5).

Regional offshore production influences the supplyof larvae to a coastal habitat in two ways. First, phyto-plankton production in nearby waters offshore affectsthe abundance of planktotrophic larvae that feed forseveral weeks in the plankton potentially leading togreater larval supply in areas with greater phytoplank-ton production. Second and in opposition, increasedproduction in offshore can generate increased resourcesand habitat for the associated pelagic community thatpreys upon larvae and thus leads to a reduced larvalsupply.

Unresolved Problems and FutureDirections

Marine ecologists have been remarkably successful inadvancing our knowledge of how strong local interactions

affect the composition of communities, yet it is not yetclear how the results of small-scale experiments can bescaled up into broad scale generalizations. This is oneof the major challenges of rocky intertidal ecologysince practical, everyday concerns of management, com-mercial harvesting, biodiversity, and restoration demandanswers on the scale of square kilometers of habitat,not square meters of experimental site. One currentapproach has been to use teams of researchers undertakeidentical small-scale experiments over a broad geogra-phical region (e.g., EuroRock in Great Britain andEurope) or over similar oceanographic conditions (e.g.,the ongoing studies of rocky shore in upwelling systemson the Pacific Rim by PISCO). Another approach hasbeen the integration of ‘real time’ physical, chemical,biological data from in situ and remote sensors (e.g.,satellites that can reveal near shore temperature andprimary productivity) with experimental studies on com-munity dynamics.

Neither approach solves the difficulties of workingwith large mobile consumers such as mammals, whoseimportance is under appreciated because of the difficul-ties inherent with studying mammals. Even the rat (Rattus

norvegicus) – the most widely recorded introduced inter-tidal mammal with the broadest documented intertidaldiet – likely remains underreported as a rocky intertidalconsumer from many coastal locations where it is knownto be established. It is likely that rocky intertidal organ-isms supply terrestrial consumers significant amounts ofenergy, yet there are few data on intertidal–terrestriallinkages and how intertidal shores serve as importantsubsidies for terrestrial habitats.

It is also unclear if detailed information from onearea can be informative about another area. For exam-ple, rocky intertidal shores on both sides of the AtlanticOcean look surprisingly alike with not only the samespecies of plants and animals present but also similaritiesin their abundances and distributions. The similarity isso striking that a good marine ecologist, knowing littlemore than the direction of the prevailing swells, can listthe 20 most common species on any 100 m stretch ofshoreline. The average beachcomber could not tell if heor she were in Brittany, Ireland, Nova Scotia or Maine.The causes of this similarity are not well understood.Rocky shores in Europe and North America may looksimilar because of strong biological interactions maintainspecies in balance or because of historical accident,and these opposing views are endpoints on a continuumbut represent one of the major intellectual debates inecology today.

Finally ecosystems are not static, and rocky intertidalsystems, which lie at a land–sea boundary, will be doublyaffected by climate change as both oceanic conditionssuch as storm frequency and surge extent, and terrestrialconditions, such as air temperatures, are altered. Such

Population Dynamics | r-Strategist/K-Strategists 3113

changes could affect local communities by altering thedisturbance dynamics and changing the geographic limitsof intertidal species.

See also: Saline and Soda Lakes; Salt Marshes.

Further Reading

Connell JH (1961) The influence of interspecific competition and otherfactors on the distribution of the barnacle Chthamalus stellatus.Ecology 42: 710–723.

Denny MW (1988) Biology and Mechanics of the Wave-SweptEnvironment. Princeton, NJ: Princeton University Press.

Graham LE and Wilcox LW (2000) Algae. Upper Saddle River, NJ:Prentice-Hall.

Horn MH, Martin KLM, and Chotkowski MA (eds.) (1999) IntertidalFishes: Life in Two Worlds. San Diego, CA: Academic Press.

Koehl MAR and Rosenfeld AW (2006) Wave-Swept Shore: The Rigorsof Life on a Rocky Coast. Berkeley, CA: University of CaliforniaPress.

Levinton JS (2001) Marine Biology. New York: Oxford University Press.

Lewis JR (1964) The Ecology of Rocky Shores. London: EnglishUniversities Press.

Little C and Kitching JA (1996) The Biology of Rocky Shores. New York:Oxford University Press.

Moore PG and Seed R (eds.) (1986) The Ecology of Rocky Coasts.New York: Columbia University Press.

Ricketts EF, Calvin J, and Hedgpeth JW (1992) Between Pacific Tides,5th edn., revised by Phillips DW. Stanford, CA: Stanford UniversityPress.

Southward AJ (1958) The zonation of plants and animals on rocky seashores. Biological Reviews of the Cambridge Philosophical Society33: 137–177.

Stephenson TA and Stephenson A (1972) Life between Tidemarks onRocky Shores. San Fransisco, CA: W. H. Freeman.

Underwood AJ (1979) The ecology of intertidal gastropods. Advances inMarine Biology 16: 111–210.

Underwood AJ and Chapman MG (eds.) (1996) Coastal Marine Ecologyof Temperate Australia. Sydney: University of New South WalesPress.

Underwood AJ and Keough MJ (2001) Supply side ecology: The natureand consequences of variations in recruitment of intertidalorganisms. In: Bertness MD, Gaines SD, and Hay ME (eds.) MarineCommunity Ecology, pp. 183–200. Sunderland, MA: SinauerAssociates.

r-Strategist/K-StrategistsJ M Jeschke, University of Helsinki, Helsinki, Finland

W Gabriel, Ludwig-Maximilians-University Munich, Planegg-Martinsried, Germany

H Kokko, University of Helsinki, Helsinki, Finland

ª 2008 Elsevier B.V. All rights reserved.

Introduction

Historical Development of the r/K Concept

Problems of the r/K Concept

Aspects of the r/K Concept Used Today

Summary

Further Reading

Introduction

The concept of r-strategists and K-strategists lies at the

interface between ecology and evolution. It was devel-

oped in the 1960s and 1970s mainly by three US-

American scientists Robert H. MacArthur (1930–72),

Edward O. Wilson (1929–), and Eric R. Pianka (1939–).

The concept was especially important in the 1970s. One

short paper by Pianka from 1970 titled ‘On r- and

K-selection’ has been cited more than 1200 times accord-

ing to the ISI Web of Science. Although the concept as a

whole is not seen as accurate anymore today, parts of it

still are.In this article, we outline the historical development of

the r/K concept, followed by its problems as seen today.

We then describe its aspects that are still in use, namely

the observation that life histories show patterns within

and among species and the idea that selection regimes

vary with population density.

Historical Development of the r/KConcept

The r/K concept is based on the idea that environments

differing in population abundance and fluctuation should

select for different phenotypes. In a paper published

in 1950, Theodosius Dobzhansky compared evolution in

the tropics to evolution in temperate environments.

The tropics are more stable and populated by different

species than temperate environments, so ‘‘interrelation-

ships between competing and symbiotic species become

the paramount adaptive problem’’ (p. 220). On the other

hand, ‘‘Physically harsh environments, such as arctic tun-

dras or high alpine zones of mountain ranges, are

inhabited by few species of organisms. The success of

these species in colonizing such environments is due

simply to the ability to withstand low temperatures or to

develop and reproduce during the short growing season’’

(p. 220).

encyclopedia of ecology || rocky intertidal zone

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