Recruitment is the measure of the number of young individuals (e.g., fish and coral larvae, algae propagules) entering the adult population, in other words, it is the supply of new individuals to a population. Recruitment can play a critical role in the resilience of coral populations through the number of individuals and different species that repopulate a reef. Its importance for community dynamics and coral populations varies by species, habitat and reef location. The rates, scales, and spatial structure of dispersal among populations drive population replenishment, and therefore have significant implications for population dynamics, reserve orientation, and resiliency of a system. For dispersing larvae, the number of new recruits entering a population is primarily related to five factors:
- physical oceanographic processes (e.g., upwelling vs. downwelling)
- the abundance of larvae in the water column
- larval behavior (e.g., vertical migration in the water column to catch currents going in different directions)
- the availability of suitable substrate for settlement
- the ecological factors that affect survivorship after settlement (e.g., competition, predation, food supply)
All of these processes affecting the magnitude of recruitment into a system can influence the spatial patterns of coral reef species communities and assemblages. For coral bleaching, larval recruitment is a particularly critical component of the recovery process. Reefs that have been severely damaged are reliant on the arrival of larvae from corals that have survived the bleaching event elsewhere and their successful settlement, survival and growth.
To better understand the resilience potential of a site, managers will need to ask: “What are natural rates of recruitment at this site?” Seeking answers to the following questions pertaining to recruitment rates will provide managers with a better understanding of recruitment at their sites and will facilitate implementation of successful management strategies:
1. What physical oceanographic conditions characterize the site?
Large scale physical oceanographic processes, such as ocean currents, upwelling, eddies, and El Niño events, can cause considerable mixing and long-distance transport of pelagic larvae. These large-scale processes, in turn, affect recruitment patterns at smaller, site-level scales. Currents and areas of upwelling will have a direct effect on the extent of larval transport to distant locations and the flux of larvae over particular sites, and thus overall patterns of recruitment. At the smaller, site-level scale, other physical processes can factor into larval dispersal and recruitment patterns such as micro-currents, light, areas of flow constriction, salinity, depth, surface orientation, and sedimentation. These mechanisms can facilitate larval retention near source populations. To better understand and connect these large scale processes to local areas, managers should examine the oceanographic current complex within the area. For example, areas of high upwelling and high flow currents would be expected to result in high larval recruitment because of the great flux of water over the community. Information on surface ocean currents and tides provide managers with the general movement patterns and expected larval distribution. Managers can also seek help and support from local oceanographers if information is not easily available. Furthermore, managers can also perform recruitment studies and experiments at their sites to identify the settlement and recruitment patterns. One method to quantify the density of coral recruits and to get a better idea of differential recruitment within an area is to place settlement tiles (onto which coral larvae attach and can be examined) throughout the site and compare the settlement between sites and against oceanographic patterns. This can provide general trends of the availability of larvae to the system.
2. Where are the sources of larvae for the site?
The production, settlement, and survival of larvae are dependent on the availability of source areas7. The source of larvae to an area can be an external location; or the source can be locally derived, if larval production and settlement occur within the natal site. These self-recruiting systems are not dependent on outside sources of larvae for replenishment. The pattern of larval exchange, and the degree to which larvae originate from outside populations, helps to explain connectivity. A large amount of self-seeding leads to low connectivity, while high rates of larval exchange with other populations generate high connectivity1.
Historically, the consensus had been that populations of reef fishes were demographically open over large spatial scales, with recruits to a population originating from adults elsewhere. This implied that larvae settling on a reef were not derived from eggs spawned on that reef, but from eggs spawned somewhere else (external sources). However, the paradigm has shifted to consider many reef fish populations as “closed” populations that are self-recruiting2,3,4,5.
Recent evidence, from a variety of fields, indicates that local retention of larvae in the natal habitat may be considerably more prevalent than previously thought, even in species with long larval durations. For example, studies in Kimbe Bay, Papua New Guinea, revealed high levels of self-recruitment (60%) in two reef fish species, clownfish (Amphiprion percula) and butterflyfish (Chaetodon vagabundus)6. This study, among others, suggests that the extent of larval dispersal between populations is lower than currently assumed, affecting connectivity among populations and having important implications for MPA design and resilience2,3. Most reef ecosystems are not exclusively self-recruiting or dependent on outside sources. Proportions of larvae originating from internal or external locations can vary widely within and between reef systems.
In terms of recovery from bleaching, it may be optimal to have a combination of both self-recruitment and external sources of recruitment. For example, if an exclusively self-recruiting coral community suffers mass mortality from a bleaching event, there is little prospect for recovery, since all of the sources of larvae were impacted by bleaching. Likewise, recovery of a coral community that depends solely on external sources is completely dependant on the arrival of suitable coral larvae that have survived the bleaching event elsewhere. A reef that both receives and provides larvae is more likely to be resilient in a bleaching event, because it has multiple options for recovery. Monitoring of recruits should be included in the site monitoring protocol to identify new individuals arriving to the location.
3. Is there suitable habitat for recruits?
Finding unoccupied space and suitable habitat on coral reefs is a competitive process for coral larvae. Substrate morphology is one important factor for coral larvae settlement and a possible determinant of coral community structure. Substrates, such as live coral, sediment, and fleshy macroalgae are unsuitable for coral recruit settlement. Scientists have discovered that the suitability of a surface for coral settlement is greatly determined by chemical or biological properties of the surface. The presence of algae can greatly reduce survivorship and settlement success of coral planulae. A recent study of the common Hawaiian reef-builiding coral Montipora capitata found a clear, negative relationship between density of early life history corals (1-3 polyps) and fleshy coral algal cover. Planulae that do settle in macro-algal dominated areas not only suffer from increased indirect, algal-induced mortality, but also experience lower recruitment success as algae are unlikely to serve as stable substratum for future colony growth12.The presence of chemical stimuli in crustose coralline algae (CCA) as well as in other substrates, such as dead coral have been shown to induce coral larvae settlement8. Recent studies have found that coral larvae appear to be able to recognize and respond to chemical signatures in CCA in the selection of settlement habitat location9. When evaluating the potential for coral larvae recruitment, managers should examine the availability of suitable habitat in the site, looking specifically for areas of CCA or patches of dead coral, which may provide adequate settlement substrate for new coral recruits. New recruitment of juvenile reef fish is also dependent on the availability of suitable habitat. For example, an 8-year study in Papua New Guinea demonstrates the interdependence of the relationship between juvenile fish and coral cover10. Over 8 years, a decline in coral cover was observed, followed by a dramatic decline in fish biodiversity and an overall phase-shift in reef fish community structure. Those species with a great dependence on living coral as juvenile recruitment sites revealed the greatest decline in abundance. This suggests habitat-limited recruitment and that obligate coral-dwelling species’ populations decrease with a decrease in healthy coral cover, compromising the long term resilience of the community.
4. What is the herbivory regime at the site?
Abundance and community structure patterns of herbivorous fish and invertebrate grazers can influence coral recruitment. The presence of grazing reef fish, such as parrotfish, reduce macroalgal cover and can facilitate enhanced coral recruitment11. In areas where there has been overfishing of herbivores, recovery of coral communities following a bleaching event has been compromised. Managers need to evaluate the types and extent of grazing occurring within the area. Management that is designed to enhance coral recruitment and resilience should include strategies that reduce algae cover through the maintenance of high levels of grazing reef fish and invertebrates.
1 Levin 2006
2 Swearer et al. 1999
3 Jones et al. 2005
4 Warner and Cowen 2002
5 Jones et al. 1999
6 Almany et al. 2007
7 Cowen et al. 2000
8 Norstrom et al. 2007
9 Harrington et al. 2004
10 Jones 2004
11 Mumby et al. 2007
12 Vermeij et al. 2009