Important life history habitats for the conservation of marine species, difficulties in identifying appropriate reservation areas. By Nicholas ScottPosted: May 5, 2014
Protecting the unique diversity of marine species from overfishing, habitat degradation and other anthropogenic causes, is becoming more pronounced in the world. A commonly utilized technique for the conservation of endangered or vulnerable marine species (e.g. marine mammal protected areas) is to place certain essential habitats under protection (either closed or restricted). Marine protected areas are commonly referred to as (MPA). These MPA’s aim to protect vulnerable species and their essential life stages e.g. nursery areas and breeding grounds. The essential habitats are crucial to the species survival in many ways but are often difficult to identify. One of the many key areas for marine species is a nursery area; this is a particular location where larval survival and growth is much higher than other locations (Vasconcelos et al 2008). These locations play a crucial role in the population dynamics of many marine organisms, adding significant numbers of individuals to the adult population (Nagelkerken et al 2013). There are many other key habitats that vary greatly for each individual species but nursery areas are one of the more crucial habitats for the success of a population. If these essential habitats are overexploited or degraded the effects of this loss could be devastating to not only one species but also potentially many. The decision process for where a MPA is to be located is difficult; this is due to the large variety in marine organisms and their associated key habitats. When considering an area for reservation one must consider many different aspects of the life history of many different species.
- First I will be looking at the importance of nursery sites and the difficulties in identifying these areas.
- Next I will discuss the importance of settlement sites for reservation ecology.
- Lastly I will discuss the importance of connectivity (corridors between meta populations) in reservation ecology.
The identification of key life history stages/ areas that should be conserved to maintain depleting marine stocks is difficult. Conserving these important habitats and life stages is crucial to the survival of many vulnerable marine species. Improving our techniques for identifying these areas is an essential next step in the conservation process of marine species.
The importance of nursery sites is well documented in literature; Sundbald (2013) found that the size of a nursery area potentially influences the adult population. Nursery sites are defined as “a habitat that produces relatively more adult recruits per unit of area than other juvenile habitats used by a species“ (Vasconcelos et al 2008). These nursery areas are characterized by high densities of juveniles and increased survival rate, offering protection for juveniles during the vulnerable life stages (Vasconcelos et al 2011). These nursery areas however are not easily identified due to the high variability in marine species life cycles (Nagelkerken et al 2013). A process used by many species, (making the identification process difficult) is ontogenetic habitat shift (Mumby, P.J., 2006 & Nagelkerken et al 2013). Essentially the process is the movement to different habitats during different stages of their life cycle (Mumby, P.J., 2006). Nagelkerken et al (2013) suggests that not many species are restricted to just one habitat. Many use a mosaic of habitats on a daily basis. This makes identifying the key sites difficult due to requiring knowledge on all sites utilized by individual species. This is further complicated by the large variation in marine species and their associated habitats (species specific). Sessile organisms are much easier to work with compared to mobile organism, which can migrate between areas on a daily bases e.g. vertical migration or diel tidal shifts (Nagelkerken et al 2013). Some species show high site fidelity for nursery sites, Bonnet et al (2014) found that the juveniles of a sea krait species (sea snakes), are restricted to 2 small areas on 2 separate islands. The high site fidelity and low number of nursery areas would suggest that these areas are crucial for the maintenance of the sea krait populations (Bonnet et al 2014). This is due to the fact that only 2 sites exist meaning if one is lost a large drop in recruitment could occur placing stress on the entire population. A potential method for identifying nursery sites and their productivity is the use of stable isotope analysis within the otolith. Determination of the trace elements and stable isotope concentrations within the otolith, allows us to possibly determine the habitat location. This is due to site-specific concentrations of isotopes and trace elements that can be matched to the concentrations in the otolith. This technique could possibly allow us to determine areas which are supplying greater amounts of individuals to the adult populations. This could allow us to identify areas that are more productive than others, improving our knowledge for the selection of reservation areas (Vasconcelos et al 2011 & 2008, Gibb et al 2007). If this process works effectively this could make choosing areas for reservation much easier. Unfortunately this technique is very expensive and time-consuming limiting its usefulness.
Other key habitats for marine species that are potentially important in the maintenance of a functional population are settlement sites. These sites are essential for marine species that disperse their larvae into the water column. Dispersed larvae are transported in the water column by either active swimming or passive movement within the ocean currents. During this stage of their life cycle many metamorphose into a form that can settle onto a substrate for development into adults. Substrata that these dispersal larvae settle on are often species specific thus making them unique to that species and crucial for its life cycle. Stoner A.W. (2003) study on the Queen Conch (Strombus gigas) found that settlement sites were associated directly with specific sites, not specific habitats. This suggests that when determining the importance of a settlement site one cannot just look at the specific habitat it has chosen and expect the other individuals to settle there. But rather look for specific locations that may be utilized more frequently due to some unforeseen optimal variables. Nagelkerken (2013) suggests that these settlement sites could be potential bottlenecks for species recruitment. This could imply that these settlement areas are crucial for that marine species survival; Nagelkerken (2013) expresses the importance of identifying these areas in conservation of marine species. These areas are said to be difficult to identify mostly because larvae are very small but also because the settlement areas are often occupied for very short times (Stoner A.W. 2003). Stoner (2003) suggests that there are centres of settlement and/ or recruitment within large seemingly homogenous habitats. This supports evidence that settlement sites can be very specific for species and crucial for adult population maintenance. When trying to determine an area for reservation, it is important to take into account these areas of settlement and recruitment for they are crucial for the survival of the associated vulnerable life stages.
Another important habitat that must be considered when planning a MPA is the connectivity between meta-populations and life stage specific habitats. For many marine species going through their developmental stages, once they reach maturity they must migrate to the adult habitat (Vasconcelos et al 2011). Nagelkerken (2013) suggested that there are potentially species-specific pathways ontogenetic migrating organisms may follow, possibly due to favorable variables e.g. protection from predators. This also applies for connectivity between 2 subpopulations; Barnett (2011) suggests that N. cepedianus shows fine scale habitat partitioning within population due to high site fidelity by individuals. This could have important implications for the genetic fitness of the species. Gibb et al (2007) suggests that population segregation (e.g. site fidelity) makes subpopulation more vulnerable to fishing pressures and could have negative influences on the populations gene pool. This is due to segregated populations being able to be individually fished out, which may result in the decrease in recruitment for that sub population. This can result in an overall decrease in populations’ genetic diversity, potentially decreasing the fitness of the species with the potential on set of inbreeding depression. The identification of these connectivity sites, or corridors, is difficult due to high amounts of inhabitable areas that a mobile marine organism can occupy e.g. vertical and horizontal movement. Sampling is also made more difficult due to the fact it is underwater, adding additional constraints to the sampling process. Many species may have a huge range of corridors that may be utilized making it difficult, if not impossible, to determine the most important corridor. Focusing on the potential species-specific corridors and species that are vulnerable may help understand the importance of these corridors along with the implications it has for improving conservation efforts
It can be seen that it is extremely difficult to identify key life stages in marine species due to the high diversity seen in marine organisms. Research into these key life stages takes a long time to determine and is very expensive which puts constraints on the identification of key areas. These key areas appear to be crucial in the maintenance of a functional population and without them, the populations may collapse. In the future I believe we must focus on our identification techniques of these essential life stages so that we can improve the efficiency and productivity of all marine protected areas.
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