Biodiversity & Conservation

Species diversity is influenced by habitat stability and sediment type. The apparent harshness of the coarse sand/gravel beach environment belies the rich rewards for an organism capable of withstanding the rigours. There is an almost complete lack of competing species, a relative absence of predators (although the predatory isopod Eurydice pulchra may frequent the biotope as part of the surf plankton and prey upon Pectenogammarus planicrurus (Naylor, 1972), and an abundant supply of food in the form of macroalgae and other organic debris (Bell, 1995).

A critical relationship exists between the size of interstitial spaces in the substratum through which the species can pass and the size of Pectenogammarus planicrurus (Morgan, 1970). Selection experiments carried out with particles retained by 6.35, 3.35, 2.06 and 1.4 mm sieves revealed a clear preference for the 3.35 mm grade of particles. Further experiments showed that the passage of Pectenogammarus planicrurus was impaired by particles of the grade 2.06 mm, below which mortalities occurred. Following further work, Bell (1992) concluded that particle size was more important in determining the nature of the population of Pectenogammarus planicrurus than any other aspect of shore location, such as shore level (within tidal limits) and depth within the substratum. Bell (1992) consequently stated substratum preferences to be an important factor driving the population dynamics of the species, causing size-dependent migration between different grades of gravel. Short distance migration was suggested to be one reason why Pectenogammarus planicrurus enters the surf plankton (Morgan, 1968; Fincham, 1970; Bell, 1992).

Episodes of substratum instability would be expected particularly during the winter owing to storm generated wave action. In the calmer conditions that follow storm events, considerable quantities of macroalgal detritus are deposited on the gravel beach. Other macrofaunal species not especially found in the biotope may appear and exploit the food resource, these include archianellids of the genus Protodrilus, idoteid isopods and a variety of gammaridean amphipods, e.g. Atylus swammerdami and Gammarus salinus (Bell, 1995).

Where coarse sand and gravel beaches originate from relict glacial deposits, the particles are smooth and of a similar size, whilst very coarse particle beaches derived from the erosion of cliffs are more mixed and angular in shape. Coarse particle beaches are also likely to be found in wave exposed conditions where they are continually disturbed by plunging breakers. The beach is inherently unstable and the moving particles crush and grind against each other so that the surfaces are devoid of life other than microscopic species. The gaps between the particles are large so that on the ebbing tide water drains freely away because the capillary forces which hold on to the water are weak. It is in this interstitial space that the amphipod Pectenogammarus planicrurus lives. An important process along many coasts is the along-shore displacement of sediment or 'longshore drift'. As a coastal protection measure, barriers to longshore drift, 'groynes', are placed at right angles to the shore in the lee of which subsidiary currents are created which create a degree of substrate sorting and macroalgal debris deposited. Pectenogammarus planicrurus may be particularly abundant behind such obstructions to the current.

Very coarse sandy beaches are extremely unstable places, consequently macrophyte species do not become established owing to the lack of a stable substratum. In most situations, diatoms are the primary producers of the depositing shore, and are confined to the illuminated sediment surface layers. The phytoplankton of the sea also become a temporary part of the shore ecosystem when the tide is in and primary producers from other environments appear on the shore. These are invariably macroalgae that have become detached from rocky substrata and have been washed up, eventually they decompose on the beach and contribute to the energy budget of the shore system. Consequently most productivity on the depositing sandy shore may be categorised as secondary, derived from detritus and allochthonous organic matter, which is utilized by the fauna.

The amphipod, Pectenogammarus planicrurus, is the only species that is a permanent resident of the biotope and restricted to breeding within it. Other species, Eurydice pulchra and Gammarus salinus, may also be recorded in the biotope but are not faithful to it (see MarLIN reviews). On the west coast of Wales, Pectenogammarus planicrurus is an iteroparous breeder, with a semi-annual life history pattern (i.e. young produced early in the year mature and reproduce within the same year) and multiple overlapping generations (Bell, 1992). Females reach sexual maturity when body length is between 3-6 mm, and fecundity is related to female body size, ranging from 2 eggs per brood in small females to 14 per brood in large females. Mean brood size is 6.74 eggs (Bell & Fish, 1996). During the summer, the population turnover of Pectenogammarus planicrurus is high; a female might typically live for four weeks whilst those that survive and over-winter may live as long as eight months (Bell, 1992). For populations to persist in the harsh gravel/coarse sand beach, the species requires a high reproductive output at the population level, thus small brood sizes (brood sizes are typically larger in other species of intertidal amphipod) are compensated for by an early maturation and optimisation of reproductive trade offs, such as those between fecundity, egg size and parental female survival (Bell & Fish, 1996).

Beaches are dynamic environments, even when they are neither gaining nor losing sediment they are subject to short-term changes in response to wave regimes and weather conditions. Beach profiles show alteration as beach-face sands are re-cycled and decline as the component sand grains are reduced in calibre by attrition and weathering. As a consequence of the dynamic nature of the habitat the faunal component of the biotope is very sparse and low in species richness. Therefore, the community might be considered 'mature' only a few days or weeks after the last storm event, as the mobile species displaced from the biotope and those from adjacent area colonize the substratum via the surf plankton.

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