Predation

Competition

Recruitment and lifecycles

The main predators in intertidal and subtidal areas are birds, fish and epifaunal crustacea such as crabs and shrimps (Meire et al, 1994). These aspects of mediating relationships have been detailed above.

The faunistic variation in these physically controlled environments reflects the species tolerance and sensitivity to those conditions. Competition between organisms occurs in response to a limitation of resources - the abundance of reproductive mates (intra-specific competition) and food and space (inter- and intra-specific competition). Competition for space and food is unlikely to be a limiting feature in the high energy sedimentary environments (sandbanks). This is because the populations are small, due to the harsh conditions, and many organisms swim and feed in the water column at high tide and only shelter temporarily in the sediment at low tide (Peterson, 1991). Densities are kept low by the disturbance of sediment in high energy areas and so there is probably no limitation of space (Peterson, 1991).

In many marine, sedimentary communities, deposit and detritus feeders compete for food and suspension feeders compete for space (Levinton, 1979). Thus the large populations inhabiting intertidal mudflats and, to a lesser extent intertidal sandflats, will have inter- and intra-specific competition for food. Because of this, resource partitioning may occur among certain deposit feeders to avoid competition as shown for the gastropod Hydrobia and the amphipod Corophium which ingest different size food particles (Fenchel, 1972). Inter-specific competition may be relatively low in intertidal mud and sandflats because of the restricted community diversity.

Most macrofauna are iteroparous in that they breed several times per lifetime. The fecundity is closely linked to the limited food supply with temperature changes an important controlling factor. Many polychaete worms including Nephtys spp. and spionids release eggs and sperm into the water where, after fertilisation, the larvae enter the plankton for a short time before settling to the substratum (Rasmussen, 1973). The passive movement of these stages again reinforces the importance of understanding the hydrographic regime to interpret the factors influencing the community structure.

The presence of high densities of adult invertebrates may inhibit the recruitment of potential colonising stages from the water (Olafsson et al, 1994). This may account for juveniles occupying less favourable parts of the intertidal areas, for example juvenile Arenicola and Nephtys settle at areas outside the optimal distribution for the adults. However, many juveniles and adults are mobile and can enter the water column and relocate themselves. Larvae from species such as Nephtys settle in low energy areas and then migrate to the more favourable areas favoured by the adults (Peterson, 1991). Recruitment is then linked with the hydrographic regime which allows the dispersal and eventual settlement of metamorphosing larvae. This then allows for the ‘hydrographic concentration’ of new recruits to a population.

Although some sediment dwellers have a benthic and brooding mode of reproduction (e.g. amphipods and oligochaetes), most are planktonic spawners (Rasmussen, 1973) and the settlement of Nephtys caeca did not take place in the intertidal zone, suggesting sublittoral larval recruitment. The number of larger individuals increased markedly with decreasing level on the shore. Nephtys caeca is polytelic (which is discrete, iteroparous) and on European coasts breeds in its second and subsequent years (Olive et al, 1981). The spawning is highly synchronised, and an elevation of the water temperature could be the triggering factor for gamete release (Olive, 1978). Nephtys caeca has a diverse population structure which allows a better recovery from a poor recruitment. Some species show spatial variation in their life cycles, for example, different populations of Corophium volutator display one or two generations per year depending on their location.

Severe exposure such as that occurring on subtidal mobile sandbanks restricts diversity, by eliminating sedentary forms, especially bivalve molluscs, and encouraging the numerical dominance of agile swimmers such as haustoriid amphipods and isopods. These species have a short life span (r strategists) and the fauna is characterised by its flexibility. The population dynamics of the fauna in exposed habitats may be based on long term breeding success, e.g. 6-7 years for tellinids with a cohort produced which may then dominate the population (Pearson & Barnett 1987). The opportunist pollution-tolerant polychaete Capitella capitata (which is also an r strategist) has both benthic and planktonic larvae and breeds throughout the year, this means it is able to colonise impacted or stressed areas very quickly.

Subtidal mobile sandbanks are usually dependent on an input of colonising organisms and have few species with benthic reproduction, thus any disruption to the delivering currents will cause changes. In addition, some sandbanks are likely to be sinks of materials as centres of gyres. The community of these areas in most cases will not contain rare species given the dispersal mechanisms of the species and the nature of the areas. The larvae of many benthic species e.g. Ophelia bicornis, Protodrilus spp., Pygospio elegans and Phoronis spp. can differentiate between substratum types and settle upon the preferred grade of sediment. The larvae of the reef forming polychaete Sabellaria spinulosa seeks contact with the tubes of adults and will settle in these areas before commencing metamorphosis, hence some of the biological components influencing other biological components.

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