Longevity of species and community fluctuation

Reproductive modes and recruitment patterns

Stochastic factors in community development

For most CFT species there is little information under this heading, but for some of the more prominent ones studies have been made. Some of these have relatively long life spans ranging from 6 - 100 years.

The soft coral, Alcyonium digitatum, is a very prominent member of the CFT community. Observations of marked colonies have shown that colonies of 10-15 cm in height are between five and ten years old (Hartnoll, unpublished). The life span certainly exceeds 20 years - colonies have been followed for 28 years in marked plots (Lundälv, pers. comm.). The sea urchin, Echinus esculentus is the most prominent grazer in the CFT community, and a species which has suffered from commercial collecting (see section V.C). Specimens of 10 cm diameter are about 6 years old (Comely & Ansell, 1988). Gage (1992) determined an age of eight years for larger specimens, and suggested they may live to 16 or more years in Scottish waters. Nichols et al. (1985) give a life span of up to 12 years off Plymouth. The sea fan, Eunicella verrucosa is another species which has suffered from collecting. This is notoriously slow growing (5 cm height after 5 years, Keith Hiscock cited in Eno et al., 1996), colonies increasing in length by only about 10 mm per year at Lundy (Fowler & Pilley, 1992), and by 6 mm or less per year at Skomer where Bullimore (1986) suggested that the largest colonies may be over 100 years old. However, analysis of the detailed Skomer monitoring data (Gilbert, in prep.) suggests that smaller colonies grow faster, and that a colony of under 20 cm is probably no more than 5 years old. The various cup corals are described as slow growing and long lived on the basis of photographic monitoring (Fowler & Pilley, 1992). The starfish, Asterias rubens, is a common predator within many CFT biotopes. It has a life span of 7-8 years (Schafer, 1972). Some ascidians are long lived, 3-4 years in Boltenia echinata, 5-8 years in Ascidia mentula, and probably over 20 years in Pyura tesselata (Svane & Lundälv, 1981, 1982a, 1982b).

In contrast there are other CFT species which are short lived, essentially annual in fact. These include the ascidians Ciona intestinalis, Clavellina lepadiformis, Ascidiella aspersa and A. scabra (Costelloe et al., 1986; Dybern, 1965; Svane, 1983).

Information is restricted, but it is clear that a number of the more prominent members of the CFT community are relatively long lived, and fairly slow growing. It may be concluded that because of this communities which they dominate will be relatively stable with time, but that when they are damaged recovery to their original complexity may be slow. This recovery will be hindered by the fact that for a number of species recruitment was observed to be very spasmodic, particularly for species near the limits of their geographical range (Fowler & Pilley, 1992; Hiscock, 1998a). On the other hand communities dominated by the annual species will exhibit marked seasonal and year to year fluctuations - for example Ciona intestinalis (Costelloe et al., 1986). In general it appears that longevities, and community stability, increase with increasing depth, though hard data to support this are limited (Lundälv, 1985).

Detailed information on changes with time in circalittoral communities derive only from studies where fixed quadrats have been monitored (normally photographically) over a period of years. In Britain such studies have been carried out Lundy and the Scillies over the period from 1983 (see Fowler & Pilley, 1992 for summary), and at Skomer from 1982 (Bullimore, 1987). Various changes were detected, particularly in the abundance of species of cup corals.

• Thus in the Scilly Isles both Leptopsammia pruvoti and Caryophyllia smithii declined between 1984 and 1991.
• A number of the trends observed were similar at the Scilly, Lundy and Skomer sites, probably a response of southern species to climatic changes.
• Although various changes were observed, the general conclusions for studies at all three sites was that there was considerable stability both seasonally and from year to year, with conspicuous species represented by specimens of considerable age (Fowler & Pilley, 1992).

If this were generally true for CFT biotopes there would be encouraging implications for management strategies, and for the development of monitoring programmes to detect unusual levels of change. However, other studies discussed below show that in at least some CFT biotopes change on a seasonal and year-to-year basis are the norm rather than the exception.

The only other comprehensive north European studies are those on the Swedish west coast by Lundälv and his co-workers. Changes in abundance are described for a variety of ascidians and for the anthozoan Protanthea (for summary of work see Lundälv, 1985). Annual seasonal fluctuations were clear, and year-to-year variations also occurred, as did longer term trends: some changes were common to a series of sites, suggesting a common cause. The composition of the community at a site could change completely. One area was dominated by ascidians from 1970-81, but these were replaced by the tube worm Pomatoceros from 1982-93, with ascidians only beginning to reappear in the mid nineties (Lundälv, 1996). Lundälv’s work generally tend to confirm that stability of communities increases in more stable environments - those which are deeper and more sheltered.

Another series of long term observations on fixed circalittoral sites was carried out on the east coast of the U.S.A. in Massachusetts (see Sebens, 1985a, for summary). Year to year variability occurred in the percentage cover of component species, but there was never a change in overall character. However the duration of study was six years, and it was seen in Lundälv’s work that sites could be stable for that duration, yet still subsequently undergo major changes.

Reproductive modes and recruitment patterns.

We have seen that the majority of CFT species are sessile so how do they get to new areas? The answer is that whereas the adults are indeed fixed in one place, the larval stages are generally highly mobile. The majority of CFT species (well over 90%) have planktonic larvae which float or swim in the water column, are carried by the currents, and dispersed to new locations. Thus the common soft coral Alcyonium digitatum has an actively swimming larva with a large store of energy-yielding yolk (Hartnoll, 1977). In captivity some larvae were still actively swimming fourteen weeks after hatching (Hartnoll, 1975). That duration is probably unnaturally long, but with a 1 knot drift (a fairly average rate) a floating larva can be carried 100 km in a mere two days.

Dispersal is one thing, but reaching a suitable habitat at the end is another matter. Obviously there must be a great loss of larvae which never reach the right place to settle. However, settlement is not a random process - larvae do have limited powers of swimming, and they show adaptive behaviour patterns to help find a place where they can survive (Crisp, 1974). A combination of responses to light and gravity, for example, can ensure that settlement occurs only on steep or downwardly facing surfaces. A preference to settle near to adults of the same species means that the environment must be favourable for survival. Clearly, despite the risks, pelagic larvae are an effective reproductive strategy. Most common CFT species produce such larvae, and experimental deployment of settlement plates shows rapid recruitment by a variety of species (Hextall, 1994).

Nevertheless there are some CFT species which lack pelagic larvae, and others whose pelagic larvae normally settle very quickly. In the soft coral, Parerythropodium coralloides, the eggs are brooded and the larvae crawl away from the parent to settle nearby (Hartnoll, 1975). The species tends to occur in large patches (successful settlement will be almost guaranteed), but not very commonly, and it is not known how it disperses over distances. The conspicuous plumose anemone, Metridium senile, is interesting in that it has pelagic larvae produced by sexual reproduction, and also buds off daughter anemones asexually from its base (Kaplan 1983). This might seem an ideal way to hedge ones reproductive bets, but few species have such flexibility. Other anemones reproduce asexually such as Gonactinia prolifera and Protanthea simplex. The jewel anemone Corynactis viridis also reproduces asexually to produce patches of a single colour. Swimming larvae which often settle within minutes of release occur in various ascidians (Olson, 1983) and bryozoans (Young & Chia, 1981).

The relevance of reproductive strategy to SAC management is that any species lacking a planktonic dispersal phase, or with otherwise constrained dispersal power, must be regarded as more vulnerable to locally adverse conditions. Once removed, it may not easily reappear, and management strategies should take account of this.

Stochastic factors in community development.

Previously ecologists have tended to emphasise the predictability of community composition in relation to the physico-chemical environment. More recently though, stochastic factors, centred upon the availability of larvae and the creation of vacant space, have come to the fore. This is the topical 'supply side' hypothesis (Underwood & Fairweather, 1989).

Figure - Alternative locally stable states of vertical rock wall community in Massachusetts.

One of the features of CFT communities is their fine-scale spatial variation - they tend to be very patchy. Whilst the infralittoral tends to be more predictable, circalittoral rock tends to be a mosaic of different species patches. The same is seen in some parts of the rocky intertidal, where it has been attributed to the effects of biological interactions and unpredictable recruitment (Hartnoll & Hawkins, 1985). Similarly in the subtidal, the different assemblages may represent 'alternate stable states' (Sutherland, 1974; Sebens, 1985a,b). In most CFT biotopes substratum space is very fully occupied, and the availability of space is a controlling resource for the settlement and growth of species. According to when free space is made available, and on which species are recruiting at that time, different assemblages of species may develop under the same physico-chemical conditions. Once established, often following a successional sequence (Hextall, 1994), these assemblages are stable for long periods (we have seen the long life span of many CFT species), and different assemblages may co-exist in close proximity. In Massachusetts Sebens (1985a) described four alternative locally stable states, dominated by the anemone Metridium senile, the soft coral Alcyonium siderium, the tunicate Aplidium pallidum and crustose corraline algae respectively. Once established each state is maintained by a different positive feedback loop (Figure 8).

The practical implications of this are that it makes the objective classification of communities, and the correlation of community composition with environmental variables, much more difficult - more discrete 'communities' will be described than in fact exist. If such areas are being monitored, community change may be considered an indication of environmental change, whilst it may be only part of a natural biological cycle involving a switch between stable states.

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