Ecological dynamics

An ecosystem is never the constant. It is influenced by surrounding ecosystems and by factors which are not constant. The reaction of an ecosystem to these changes are called dynamics. There are predictable forms of dynamics, such as tidal dynamics, seasonal dynamics and more or less coincidental dynamics such as the weather situation. Man also influences dynamics, for example by increasing the supply in nutrients.

Tidal dynamics

The tide provides many changes in an ecosystem in the course of a day. Twice a day, the mud-flats flood and twice a day they lay dry. The most obvious change is the water level, however numerous other factors change with this fluctuation. The oxygen level and the temperature of the tidal bottom is continually changing through the tides. In the summer, the bottom can warm up immensely during low tide, and consequently cool off during flood. It is just the opposite in the winter. The tides transport nutrients and offer seals a temporary rest area. The currents which are created under influence of the tides move bottom particles, thereby continually changing the form of the bottom. In this way, the tidal zone is very dynamic. However, not many organisms are able to handle all of these changes.

Seasonal dynamics

The tides determine the daily rhythm in an ecosystem, and likewise the season determines the yearly rhythm. The changes in the year, for example, are the length of time the sun shines daily and the intensity of the sun and thereby the temperature. There are more storms in general in the autumn than in the summer. This also leads to other changes in the ecosystem. The growth of phytoplankton is determined by seasonal dynamics. It is dependent upon three factors: light, temperature and nutrients. There is an over-abundance of nutrients in the wintertime. Nevertheless, very few phytoplankton grow due to the lack of light. In the springtime, the amount of light increases. Diatoms are the first algae to profit from this. After a while, the supply of silicon is used up and the diatoms are no longer able to grow. In the meantime, it has grown lighter and the sea is warming up, at which time other algae profit. In the autumn, the phytoplankton decreases its production again when the amount of light decreases.

Succession

Succession is the accumulated change of vegetation over time. Other species keep appearing, whereby the one vegetation gradually changes into another one. The first plant species found in an area are called pioneer species. The pioneer communities are characterized by the appearance of only a few species. However, there are many individual plants per species present. These pioneer plants have adapted to extreme environmental situations.

The process of waters turning into land is a form of succession. This process takes place in wet areas such as ditches, pools of water and meres. Beacuse plant remnants end up in the water, the bottom rises locally. Other plants which were unable to grow here can now flourish. The ditch or pool eventually dries up in this manner.

Succession on the salt marsh

An example of succession is the growth of a salt marsh. In undisturbed situations, a new sandbank should be able to be created somewhere due to the dynamics of the tides. When this sandbank grows large enough, eelgrass will grow. The eelgrass slows down the current, allowing more bottom particles to settle. The sandbank continues to grow. The water level above the bank decreases. Salicorn can germinate if the bank is regularly exposed. Seaweed needs more seawater than salicorn. Since the sandbank is rising, the competing position for salicorn in relation to seaweed is continually improving; seaweed makes room for glasswort. Glasswort also catches bottom particles. The bank becomes even dryer and therefore becomes more suitable for sea meadow grass. And this succession of species continues to carry on. The environmental situations grow less and less extreme and the dynamics are fewer. As a standard rule, nature strives to maintain the dynamics at as low a level as possible; for example, by lying high enough so the tides won't reach it. Since the situations are remaining more constant, the species composition is also changing more slowly. When a salt marsh only floods a few times per year, a relatively stable vegetation forms containing salt-tolerant plants, such as sea lavender, sea wormwood, sea purslane, sea aster and salt sandspurry.

Succession in the dunes

Natural succession also occurs in the dunes. The series begins with sea rocket and sand couch, both of which can germinate on pile of sand. The plants catch sand and thereby form the beginning of a dune. Marram grass follows as soon as fresh water is found in the dunes. Lyme grass can also be found at this stage. More and more sand is deposited and sea holly and sand evening primrose appear in sheltered areas. And the succession continues in this manner. When the dunes are older, leaching and decalcifying occurs, and interesting differences between the north and south slope of the dune appear. On the northern slope, heath species will establish themselves. Especially plant communities with crowberry and common polypode are typical. Heather (Calluna vulgaris) is found more inland on northern slopes.

The plants on the southern slope have a difficult time. The temperature fluctuates immensely. On sunny days, the temperature can reach 50°C. Very dry periods can also occur. The plants which attempt to grow here must be well adapted to this extreme environment. The vegetation consists of mosses and lichen, with scattered clumps of grey-hair grass. The heath dog violet is also found on this side of the dunes. When rabbits are found in the dunes, many herbal plants are also found, such as bedstraw and dune milkwort. From the difference in the growth between the two slopes, it appears that the strength of the dynamics influences the eventual climax situation.

Sudden changes in the ecosystem of the North Sea

The NIOZ has been collecting various data sets since 1970, including the breeding success of eiders on Vlieland, the biomass of the fauna and the amount of algae in the Marsdiep. Up till now, scientists have only used the data mainly for studying the effects of eutrophication (due to phosphate and nitrate) in the Wadden Sea and North Sea.

However, certain correlations became apparent when the various data sets were compared to one another. For example, the various changes in the breeding success of the eiders occurred very suddenly and not gradually, which would be the most logical expectation if indeed the gradual increase in eutrophication was the cause of the changes.

What exactly happened during the sudden changes? The algae biomass doubled between 1976 and 1978, followed by the biomass in benthic animals in 1980. And in 1978, the breeding success of the eiders on Vlieland increased explosively from 7 chicks in 1977 to more than 1000 chicks a year later in a more or less stable colony.

Furthermore, additional studies at the end of the 1970s showed that major changes had not only occurred in the wadden ecosystem. The same remarkable changes were found when studying data sets on the North Sea. Different species of sharks and crabs disappeared in 1978 while in that same year the composition of the zooplankton changed and the bird populations a few years later.

Initially, one thought it was caused by a freshwater bubble from Iceland that had reached the North Sea via the Atlantic Ocean in 1977 (the great salinity anomaly). Actually, it was not really freshwater but the saline concentration was lower than normal. This amount of slightly fresher water ‘departed’ in 1968 from a marine area north of Iceland for the east coast of North America. It started crossing the Atlantic Ocean in 1970 and reached the North Sea in 1977. The anomaly continued via the Norwegian coast and the Barentsz Sea along the east coast of Greenland, reaching Iceland again in 1981. This explanation does not seem to be the only correct one since in the meantime it is known that majors changes not only took place in the North Sea in 1977, but in the Mediterranean Sea and the west coast of North America as well. And the saline anomaly was never in those places.

What is so remarkable is that such major changes occurred more often as the past century progressed. It seems to show a kind of regularity. Important years are 1965, 1977/1978 and 1990, although major increases in concentrations of benthic life, fish and birds also occurred in the North Sea around 1935 and 1955. Such regularity often turns one to look at an astronomical explanation. Perhaps the appearance of sun spots is the cause. Sun spots have been occurring more intensively since the end of the war.

Research institutes in Plymouth (Great Britain) and Helgoland (Germany) have data sets over a long period of time. The sudden large scale changes in life in and on the sea in 1977/1978 were also found at these institutes. Major changes took place in Wadden and North Sea life in 1990. The breeding success of the eiders disappeared within a couple of years. Mussels and plaice were having hard times. The herring population greatly decreased once again, after having recovered to a relatively good level following the tremendous fall in 1977. In other words, the entire North Sea ecosystem was reacting, but to what?

Many more series of data are necessary than are now available in order to produce a good hypothesis. The present hypothesis is that a change is occurring in the climate. Which change that could be is unclear. Meteorological institutes in the Netherlands and Germany are being consulted to answer this question. This change could be causing possible alterations in ocean currents. Even the composition of the water could possibly be changing slightly. Nitrogen levels in particular could be fluctuating. An altered biology can mean more nitrogen ending up in the bottom so that less remains in the water. That affects the biomass in sea and the phytoplankton. Which means less amounts of other algae, fewer zooplankton, less fish and eventually fewer or different birds. In this way a new balance is created, which can change again very suddenly after a period of time. The breeding success of eiders is a good example.

Source: de Vleet, Ecomare