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<br />1-2.
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<br />53. 1 thank J. I. BuL S. Al. Cha.•nbers, D. Jablonski, and C. Al. Pease for helpful
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<br />Pubuc Health Service grant GM27120.
<br />Recruitment Dynamics in Complex Life Cycles
<br />JONATHAN ROUGHGARDEN, STEVEN GAINES, HUGH POSSINGHAM
<br />Organisms living in the marine rocky intertidal zone
<br />compete for space. This, together with predation, physical
<br />disruption, and differing species tolerances to physiologi-
<br />cal stress, explains the structure of the ecological commu-
<br />nities at some sites. At other sites the supply of larvae is
<br />limiting, and events in the offshore waters, such as wind.
<br />driven upwelling, explain the composition of intertidal
<br />rnmmunities. Whether the community ecology at a site is
<br />governed by adult-adult interactions within the site, or by
<br />limitations to the supply of larvae reaching the site, is
<br />determined by the regional pattern of circulation in the
<br />coastal waters. Models combining larval circulation with
<br />adult interactions can potentially forecast population
<br />fluctuations. These findings illustrate how processes in
<br />different ecological habitats are coupled.
<br />marine biologist, Gunnar Thorson, observed that a majority of the
<br />marine invertebrate species whose adult phase lives on rocks or
<br />burrowed in mud have a two-phase life cycle (3). The conspicuous
<br />adult phases of barnacles, starfish, snails, dams, worms, and so forth,
<br />arc usually preceded by nearly invisible larval phases that live and
<br />feed in the coastal waters for a fed- days to a few months, depending
<br />on the species. Most fish also have a two-phase life cycle. The
<br />dynamics of a two-phase species can, in principle, be affected at
<br />either phase. But Thorson further noted that two-phase species have
<br />large fluctuations in abundance when compared to otherwise similar
<br />one-phase species. Thus, the larval phase, and not the adult phase,
<br />was implicated as the point at which fluctuations affect marine
<br />population dynamics. Because most coastal marine populations have
<br />rN Transport
<br />Offshore water column
<br />H UMANnT HAS LONG BEEN PERPLEXED BY ERRATIC FLUC-
<br />tuitions in the abundance of commercially exploited ma-
<br />rine populations, such as sardines, herring, squid, lobsters,
<br />and crabs. One of the first models of theoretical ecology was
<br />proposed by Vito Volterra to explain such fluctuations as oscilla-
<br />tions resulting from a nonlinear predator-prey interaction (1).
<br />Although Volterra's model is still of mathemarical interest, fluctua-
<br />tions in marine populations are not regular enough to be considered
<br />oscillations (2) and their cause has retrained mvsterious.
<br />Most biologists assume that marine population fluctuations are
<br />somehow caused by events in the offshore waters. The great Danish
<br />1. Roughgarden is a professor in the Deparmnrnt of Biological Sciences. Stanford
<br />Universm, Stanford, CA 94305. and at the Hopkins Marne Station. Oeeanview
<br />Bouk%mni Pacific G-,c, CA 93950. S. Gaines is an aSS<* M Professor in the Graduate
<br />Program in Ecology and Evolutiaean• Biology, Brown Unnesin•, Pro6denoe Rl
<br />02912. H. Poss-Olam is a posrdocioral rewards assooae in the Department of
<br />Biological Sciences, Sanford Onn•asrry.
<br />ibsstlr
<br />Transport
<br />-"V%
<br />Transport
<br />
<br />Ba„r,es L"rT.. c.s,=.:s._-1 earnrww
<br />larva 1 Reelty inlartirJwl hnhlttrt=' 1 (inra
<br />Transport f Transport
<br />Fig. 1. Schematic of interacdons between species in a community of the
<br />rockv intertidal zone Physical contact between adult animals attached to the
<br />rocks Inds to hicrarchieal competition for space As shown with a line
<br />tcrmmating in a dot, an individual of Balarsus glandula to crgrows or crushes
<br />an individual of Cheitamalss dalli. Also, mormlit• from abiotxe mechanisms,
<br />and from predarion by the starfish Pisasrer oehraeem, affects B. glandula more
<br />than C. dalli. Both species release a larva to the water column that may
<br />evenrualh mum to settle on vaant space, as illustrated by the arrow
<br />coupling each life arie to offshore transport mcchanisms.
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<br />SCIENCE, VOL. 24.I
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