Benthic invertebrates

Predator populations

Each species is sensitive to changes in intra- and inter-specific interactions which will influence the development of the benthic communities and the stability and persistence of benthic communities is influenced by biological interactions. The interactions between infaunal species may be in competition for space (Woodin, 1976) or as the result of survival, migration and recruitment patterns. For example, surface active Nereis diversicolor had a negative effect upon Corophium spp. (Jensen & Andrew, 1993). Species which are commensal, parasitic, symbiotic or epizoic depend on the presence of other species as hosts or partners. Environmental changes removing the latter will cause the species reliant on them to disappear.

Community composition will be sensitive to synchrony (or otherwise) between the population dynamics of predators and the different prey species and species reproducing during times of minimal predator activity could significantly reduce the effects of predation. For example, as shown by Eagle (1975) and Tyler & Banner (1979) where larval settlements of predators of Abra alba were consumed by the dominant epifauna. Settlement activity and timing is also important in determining community type as some species such as Pygospio, Protodrilus and Ophelia are attracted by certain sediments on which they settle but they have different times of settlement.

The community composition is sensitive to changes in food availability. For example, Buchanan and Moore (1986) found that a decline in quantities of organic matter changed the infauna of a deposit feeding community which is essentially food limited. This was by largely reducing the Ophelina acuminata population which had previously out competed the other species and dominated the benthic population. In turn, this removed the competitive pressure from the other species and produced a period of instability as several species became dominant.

Decreased grazing by macrofauna can affect intertidal sediment stability with the resultant increase in diatoms correlated with an increase in sediment shear strength (Underwood & Paterson, 1993). For example, the feeding activities of C. volutator may reduce sediment stability of intertidal mudflats by decreasing shear strength. The latter is the result of lowered mucopolysaccharide production, caused by the removal of the microflora by Corophium, and increased water content, due to bioturbation (Gerdol & Hughes, 1994).

Wading bird predators of intertidal sand and mudflats regulate the macrofaunal community and factors that increase the predation may affect this regulation. This may be in a direct way or indirectly by altering the behaviour of certain species e.g. the crawling behaviour of Corophium in response to feeding activity of sandpipers (Boates et al, 1995). In addition, predators are sensitive to other predators’ behaviour, for example, Thrush et al (1994) discussed the negative predation interaction between shorebirds and eagle rays on the infaunal community.

Size-dependant predation is important for the study of the dynamics of size-structured populations such as those on intertidal sand and mudflats (Gibson et al, 1995). In general, density dependent mortality can arise from patchy distributions or predator-behaviour which influences the overlap in time or space between predator and prey. For example, the brown shrimp Crangon crangon is a major predator of juvenile plaice and flounder (Pihl, 1990) and predation rates are inversely related to the size of the fish. Predator-prey interactions could be enhanced also by high densities in small nursery areas, simply due to an increased encounter rate and possibly increased habitat overlap (Modin & Pihl, 1996).

Plaice and flounder have developed different temporal and spatial strategies to avoid predation during settlement (Modin & Pihl, 1996). Thus, an early arrival of plaice on the nursery grounds ahead of the predatory shrimp gives time for the predation risk to be reduced by growth. Flounder settle in the high-risk period during or subsequent to the immigration of shrimp but survivors occupy less predator-hazardous habitats.

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