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454 RECRUITMENT STEVEN D. GAINES University of California, Santa Barbara The number of individuals of a species at any given place can increase in only two ways: new births and immigration . For most species on rocky shores, these two sources of population growth are one and the same. Since the young of most marine species are released into the sea as plankton , they are moved by ocean currents. If they survive the journey, their return to adult habitat at the shore is called recruitment. The recruits that replenish most populations on the shore are typically the offspring of adults from other sites, some from nearby, others from far away. STATIONARY ADULTS, TRAVELING YOUNG Most of the animals and plants found on rocky shores are either slow-moving or completely immobile. This trait makes them easy for scientists to study and easy for beachcombers to discover. Immobility, however, only characterizes the adults. As young, many are wanderers in a different habitat. Most marine species produce young that are microscopic and cast into the sea to drift on ocean currents. Some drift for a few minutes; others drift for months. Animals release larvae; seaweeds release spores. Although they are all quite small (typically much less than millimeter in length at release), they have a great diversity of shapes that often bear no resemblance whatsoever to their parents (Fig. ). Larvae that drift for more than a few hours typically develop through multiple stages that increase in size and may change greatly in shape. The energy needed for this growth comes most commonly from eating other plankton (in which case the larvae are called planktotrophic); but in some species the larvae receive enough nutritional stores from their mother, in the form of yolk, to complete development without eating anything else (such larvae are called lecithotrophic). THE PERILS OF THE PLANKTON Having the option of producing minute young who can feed on their own affords marine fish and invertebrates the opportunity to produce enormous numbers of offspring . Depending on the species, tens or even hundreds of thousands of larvae may be released in a single spawning event. The benefit of being able to make a minimal energetic investment in individual offspring is clear, but there are also inherent costs. If larvae must successfully obtain their own food to complete development, their fate depends on how much food they find and on surviving the challenges of life in the plankton long enough to complete development. These are nontrivial challenges. Larvae from intertidal mussels, for example, have been shown to vary greatly over time and space in their success at obtaining food. The consequences are acute, with large variation in the survival of larvae through development and large variation in their performance even after they recruit to adult habitat on the shore. More daunting still are the perils of developing while drifting in the plankton. Even if food is abundant, the risk of the larvae being eaten while drifting for weeks is quite high. Potential predators range from other small zooplankton all the way up to baleen whales. Moreover, if a larva is lucky enough to make it through the planktonic gauntlet, it has to complete its development within reach of habitat where it can thrive as an adult. Imagine the challenges for R LOST AT SEA Many patterns of coastal ocean circulation are known to alter the success of larvae in ultimately reaching appropriate adult habitat. Eddies, internal waves, tides, freshwaterdriven flows in estuaries, and several other circulation features have all been linked to variation in larval success through their transport of larvae either to or away from adult habitats. Perhaps the best studied of these connections , however, involves the prominent ocean process called upwelling. Coastal habitats with upwelling are some of the most productive ocean habitats on the planet. One of the key processes driving upwelling is winds. When winds blow along the coast, they can generate forces on surface waters of the ocean that are perpendicular to the coast because of the rotation of the earth and the ensuing Coriolis effect. As a result, surface waters are pushed away from the coast and replaced by deeper, colder, nutrient -rich water that is upwelled from below (Fig. ). The infusion of nutrients into surface waters provides the fuel for blooms of phytoplankton, which subsequently support large populations of higher trophic levels. From the perspective of larvae, the blooms in phytoplankton provide a rich source of food. The problem is that while the larvae are feeding...
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