Eunice Pinn

Ecology is the study of the interrelationships existing between organisms and their environment. Living things do not exist in isolation. Individuals interact with others of their own species, other species and the physical and chemical environment that surrounds them. Their activities can affect their surroundings and can also influence the activities of other organisms. Similarly, different factors within the environment can affect the activities of organisms.

A species is a group of interbreeding individuals reproductively isolated from other such groups. All the individuals of a given species in an area are termed a population . Different species that tend to occur together in a particular geographic area constitute an ecological community . The community, or group of communities, and their surrounding physical and chemical surrounding environment are termed an ecosystem . Ecosystems, however, may be considered on a variety of scales. For example, at the largest scale, the entire planet may be considered as a single ecosystem, with its various terrestrial, freshwater and marine communities. At the other end of the scale, a rock pool may be considered an ecosystem. Both examples have the biological and abiotic (non-biological) components required for the ecosystem definition.

Populations, communities and ecosystems are all regulated by various factors. The major controlling factors for communities and ecosystems are energy, the physical environment and the interactions between species. Populations and communities are controlled by tolerance levels to abiotic factors such as light, salinity, temperature and nutrient availability. Biological regulation of populations also occurs through the interactions of different species and individuals with one another, particularly competition and predation.

Energy

Ecosystem function relies on energy and its transfer. In most cases, the source of the energy is usually the sun. In recent years, however, ecosystems have been discovered in the deep sea that derive their energy from chemicals rather than light.

In the majority of cases, light energy is captured from the sun by autotrophic organisms (also known as producers), and converted to sugars through a process known as photosynthesis . The main producers in the marine environment are seaweeds and phytoplankton . Seaweeds are members of the algal kingdom rather than being plants. Phytoplankton are microscopic algae. They are single celled, but can often form chains. Seaweeds are particularly important in coastal areas, but in the open ocean phytoplankton are the most important. The main producers in the terrestrial environment are vascular plants, particularly trees and grasses (see Dorset's Coastal and Marine Habitats ). These have a much more complex anatomy as they require adaptations to a terrestrial environment. Adaptations include an internal transport system for water and nutrients, strengthening tissue and an epidermis that allows photosynthesis and respiration to occur whilst protecting the plant against excessive water loss.

The producers grow and reproduce, providing the food for the heterotrophic component of the system, the consumers. Heterotrophic organisms are defined as all forms of life that obtain their energy from the consumption of producers, or the consumption of those organisms that have consumed producers, or through the absorption of dissolved organic matter in the environment. The arrangement of autotrophs and heterotrophs is called the trophic structure , in which each layer is called a trophic level .

As energy is transferred from one trophic level to the next, much is lost through heat and metabolic use by the organisms. The amount lost is variable, but substantial, with estimates ranging from 80% to 95% of the energy in that level. The system, thus, becomes self-limiting. This can be visualised as a trophic or energy pyramid (Figure 1). The producers or autotrophs are at the bottom, the second level includes the primary consumers, i.e. the herbivores that consume the plant material. The third level contains the secondary consumers, i.e. the carnivores that consume the herbivores. The next level contains the tertiary consumers, i.e. the carnivores that consume other carnivores. This continues until there is no energy left to support another layer. All levels above the first contain omnivores (organisms that consume both plant and animal material). The last element of trophic structure is the decomposers . These are organisms (mainly bacteria) that break down the complex organic molecules of dead organisms, and release reusable simple molecules that can be used by autotrophs and heterotrophs. In Figure 1, the size of each layer relates to the amount of biomass contained within that layer.

Figure 1: Trophic pyramid

In recent years, communities have been found in the marine environment that do not derive their energy from the sun. These communities were probably the most exciting and significant discovery in marine ecology since the 1970s. In 1977, near hot-water geysers on the sea floor animals were observed in extremely abundant communities in an otherwise barren sea floor. It has since been discovered that these hydrothermal vent communities rely on hydrogen sulphide for their energy requirements. Sulphur bacteria use the hydrogen sulphide in a process known as chemosynthesis to build complex organic molecules and grow. This process is very similar to photosynthesis but does not require light (Figure 2). These bacteria are then eaten by heterotrophs or live symbiotically with larger animals. Symbiosis is defined as a mutually beneficial relationship between two organisms. In the hydrothermal vent communities, symbiotic relationships occur between the sulphur bacteria, which provide their host with a food source, and many of the dominant species in the community which provide the bacteria with protection and, through their blood, a ready supply of hydrogen sulphide.

Figure 2: Comparison of photosynthesis and chemosynthesis.

The Physical Environment

The place where organisms are found is referred to as their habitat . Some examples of marine habitats include rocky shores, sandy beaches, coral reefs, estuaries, and the open ocean. Examples of terrestrial habitats are given in Dorset's Coastal and Marine Habitats. Habitats are characterised by their abiotic features. Consequently, the physical and chemical environment dictates which individuals can live there. It is an individual's tolerance to abiotic factors such as temperature, water movement, salinity, nutrient availability, etc., that limits where they can survive. Each habitat can be divided into microhabitats . For example, consider a rocky shore: this contains a variety of microhabitats including the rock itself, rock pools, sediment on the shore, and the underside of boulders. The seaweed will also provide places for organisms to live and is therefore considered to be a microhabitat. In general, the more complex the habitat, the more microhabitats it will contain. A good example of this is coral reefs, which have an extremely complex structure and therefore thousands of microhabitats.

The rocky shore provides an excellent example of how different abiotic factors influence where organisms live. The distribution of species on the shore from the high tide to the low tide mark is determined by factors such as length of air exposure during low tide, ability to withstand the force of the waves, rock type, and rapid changes in salinity and temperature. Rocky shores are dominated by the tide, which produces a gradient of environmental conditions. At the highest part of the shore, conditions are almost terrestrial with wetting mainly from wave splash and only rarely submerged. The lowest part of the shore is rarely uncovered, except on the lowest spring tides and only for brief periods. In between, individuals experience a range of exposure and submersion depending on their position on the shore. This range of environmental conditions results in species zonation because no organism is equally well suited to all levels on the shore.

Different levels on the shore are occupied by different assemblages of species, each individual having its main abundance within a particular zone where conditions are most favourable for it. Above and below this zone, numbers will decrease or be absent as environmental conditions are less suitable.

The niche is defined broadly as the role of an organism in its community. A niche is made up of every environmental and biological variable affecting the species. For any given variable, e.g. temperature, the range within which the organism can exist and reproduce is defined as part of its niche. The different tolerances to all the different variables combine to form the fundamental niche . The fundamental niche is the range of abiotic conditions which a species can theoretically occupy, whilst the realised niche is that part of the full range that is left to a species after biotic interactions such as competition and predation. Ecological niches can be broad or narrow and species at the extremes of this are defined as generalists or specialists. Generalists can tolerate a wide range of conditions whilst specialists have very restricted range of tolerances.

Interactions between species

The most important interactions occurring between individuals are competition and predation. Both of these are biological regulators of populations. Their importance also varies in different communities.

Competition is defined as the interaction between individuals for a limited resource . Competition can be intraspecific (between individuals of the same species) or interspecific (between different species). In a competitive interaction, either the competitors manage to share the resource, or one excludes the other. Where the resource is shared, the fitness of the competitors is reduced, i.e. their growth and reproduction are limited as energy is utilised in the competitive interaction. Where two species are competing for the same limited resource, the dominant competitor will out-compete the inferior competition. This can lead to one species being excluded. The competitive exclusion principle states that no two species with exactly the same requirements can coexist in the same place at the same time, i.e. species with identical niches cannot exist together.

The amount of competition is linked to the amount of niche overlap . The more similar the requirements of two individuals, the more competition there will be between them. As populations increase, so competition for resources increases. This increased competition absorbs energy that would otherwise be used for growth and reproduction. Consequently, populations can be limited by competition.

Predation is defined as the consumption of one species by another. This definition includes the traditional carnivore, but also includes grazers (herbivores) and seed eaters. Carnivores and herbivores vary considerably in their ability to regulate prey populations. Sometimes, the predator may have little effect on the prey population and its removal would have little discernible effect on prey population numbers. In other cases, the predator may be the most important factor regulating prey population numbers. If the predator is removed, the prey population can explode. Occasionally, this can have dramatic consequences for the rest of the community. Keystone species are predators whose removal causes great changes in the presence and abundance of many species in the community, most of which are not the prey of the predator.

The effects of predation and competition are also interlinked within the ecosystem. The effect of a predator on one species will influence its ability to compete with other species. Predator mediated coexistence occurs when the preferred prey of a predator has its dominant competitive ability reduced. This normally dominant species is no longer able to out-compete its rivals. Without the presence of the predator, this species would normally exclude its rivals from the community.

Biodiversity

Life in the sea varies from very simple, single celled, microscopic organisms such as diatoms to complex, vertebrate organisms such as fish, dolphins and whales. More simple and fragile life forms can exist in the marine environment than in terrestrial environments because water provides them with support, flotation, transport and protection. Consequently, very simple reproductive processes are enabled and the need for structural complications such as skeletons and protective coverings is minimised. At the other end of the scale is the Blue Whale, the largest organism ever to have existed on the planet.

Biodiversity , or biological diversity, simply means the ‘variety of life'. It can be viewed at many different levels from the variety within a species (genetic), between species (taxonomic) and of ecosystem (ecological). For example, wild Atlantic Salmon, after several years at sea, return to breed in the very river where they were born. The salmon from each river in the UK are all the same species (Salmo salar) but they are genetically different: genetic diversity . On a global basis, there are six different species of salmon: taxonomic diversity . The rivers all over the world to which the salmon return are all slightly different. They each have their own assemblages of species and communities: ecological diversity .

Some communities have extremely high species diversity, e.g. coral reefs. There are two contrasting theories as to why this occurs. The equilibrium theory suggests that the species diversity of a community is usually in a state of equilibrium and that high diversity is due to a high number of habitats and/or niches maintained by various feedback mechanisms (niche diversification) and the stable nature of the physical environment. The other theory, nonequilibrium or intermediate disturbance hypothesis, suggests that communities and species are rarely in a state of equilibrium and high species diversity is maintained through continual and gradual environmental change and periodic disturbance. This promotes a changing species composition through the presence of many species that are not highly specialised. In this model, where communities experience little disturbance, competitive interactions can continue, and exclusions occur. This results in communities of low diversity. Where disturbance is frequent, species are eliminated if they cannot attain maturity and reproduce before the next disturbance. When disturbances are intermediate in occurrence diversity increases. Slower-growing and -reproducing species are given a chance to establish themselves. However, competitive exclusions are not given a chance to occur before the next disturbance event.

Measurement of Biodiversity

Biodiversity is measured in a wide variety of ways depending upon the level of diversity being considered. This includes:

•  a -diversity is concerned with the species diversity within a habitat or community. This is the most common level at which biodiversity is assessed.

•  b -diversity corresponds with the variation in diversity from one habitat (community) to another. It is also concerned with the extent and rate of change of species richness and distribution along a gradient.

•  g -diversity is the species richness of a range of habitats in a geographic area

Since a -diversity is the most commonly measured level, it is the only one that will be dealt with here. The simplest measure of biodiversity at the species level is species richness , i.e. the number of species present (denoted S). This however, takes no account of the number of individuals present belonging to each species. The relative abundance of each species is termed evenness . In a community with high evenness, most of the species present will have similar abundances, i.e. no single species dominates the community. Communities can differ in both these attributes. A highly diverse community would have a high species richness and a high species evenness. A community with an identical number of species but which is dominated by a single species would generally be considered to be less diverse, despite having the same number of species.

Of the many diversity and evenness indices, the Shannon-Wiener Index (denoted H') and Simpson's Index (denoted D') are the most widely used. The Shannon-Wiener Index is sensitive to changes in the number and abundance of rare species, whilst Simpson's Index is more sensitive to changes in the abundance of dominant species.

The proportion of each species in the sample is estimated ( p i ), logged and then multiplied by themselves. The resulting values are then summed for all species in the sample. As a check, S p i should equal 1. The original calculation used logs to base 2, but any base can be used. Natural logs are the most common base used today. Shannon-Wiever Index varies from zero to a maximum equivalent to the log of species richness ( Ln S ). See table 1 for an example calculation of data collected from Kimmeridge Bay.

Dead men's fingers.

The value of the index ranges from 0 to 1. At zero, all the individuals belong to the same species and at one they belong to different species. See Table 1 for an example calculation of data collected from Kimmeridge Bay.

Table 1:Calculation of Shannon-Wiener and Simpson's diversity indices.

Factors Affecting Biodiversity at the Local Scale

A variety of factors can influence biodiversity at the local scale beyond those biological interactions already discussed. These include habitat complexity and changes to the habitat.

Habitat complexity has been shown to be very important in the terrestrial environment with increased complexity leading to increased diversity. Investigations on the relationship between habitat complexity and biodiversity in the marine environment have only begun relatively recently. One example of this is the influence of piddocks on intertidal biodiversity on soft rock shores, including Lyme Regis (Pinn et al ., 2002).

The relative softness of chalk and clay, and their friable nature, is favourable to the creation of a variety of microhabitats. Erosion, either physically or biologically generated, creates crevices, clefts and ledges, which provide a variety of microhabitats suitable for different forms of life. Piddocks are one of the most characteristic faunal groups inhabiting soft rock shores. These bivalve molluscs are similar in dimensions to the edible mussel. Unlike most molluscs, however, piddocks live in a burrow, which they bore in the surface of soft rocks. Piddocks tended to aggregate together in patches ranging from around 20 square metres to over 1500 square metres. Where abundant, their borings can severely compromise the structural stability of the substratum, resulting in increased rates of erosion.

Piddock burrows are approximately conical in shape, with the opening being narrower than the base of the burrow. The shape of the burrow, however, was found to be modified by population density. In areas of high population density, the burrows become either J shaped or very convoluted. At Lyme Regis, 50% or more of the burrows present were found to be no longer occupied by piddocks. These empty burrows added considerably to the small-scale complexity of the shore profile and provided refuge, from predation or desiccation stress, for other marine organisms.

At Lyme Regis, a comparison was made of the species richness in three microhabitat types: open rock, Piddock burrows, and piddock-like. The presence of Piddocks significantly increases the number of species observed (Figure 3). Hence the burrowing activity of piddocks enhanced local biodiversity with, on average, about twice as many species living on areas of rock with burrows than on adjacent areas of rock without burrows.

Figure 3: Comparison of the species richness in three microhabitat types at Lyme Regis.

The species found living in old Piddock burrows included sponges (e.g. Breadcrumb Sponge Halichondria panicea), periwinkles ( Littorina littorea ), Dog Whelks (Nucella lapillus ), crustaceans (e.g. the broad clawed Porcelian Crab Porcellana platycheles and the Hairy Crab Pilumnus hirtellus ) and polychaetes (e.g. Peacock Worm Sabella pavonina and the Greenleaf Worm Eulalia viridis ). In addition, a comparison of the occupants of three different size groups of old Piddock burrows (2-6mm, 8-12mm and >14mm burrow opening) was made. Burrow occupancy by non-Piddock species ranged from 0-10% in small burrows, 10-45% in medium sized burrows and 5-30% in large burrows. Small burrows regularly had the lowest diversity of occupants and medium sized burrows the greatest.

The additional habitat complexity and associated species diversity generated by the burrowing activity of Piddocks is relevant to assessing and understanding the factors regulating biodiversity along these soft rock shores. Soft rock shores such as chalk are rare in Europe and areas of soft rock along the south coast of England are of particular conservation interest. The Dorset coastline contains some of the best examples of soft rock shores in the UK, comprising a significant proportion of the UNESCO World Heritage Site.

At the other end of the scale, biodiversity can be reduced by factors such as habitat destruction, fragmentation and degradation. Habitat destruction, such as filling and drainage of salt marshes and human-induced erosion, is one of the major sources of biodiversity loss or reduction because it removes the habitats upon which species depend. Habitat fragmentation , where previously continuous habitats are broken into smaller isolated parcels, is also an important loss of biodiversity. As large tracts of habitat are broken up, foragers and predators no longer have a continuous range of habitat over which to feed. In addition, planktonic colonisation becomes more difficult. A fragment may still contain a species, but not enough individuals to maintain the population in the long term. However, fragments have longer edges, which may create new habitats for some species. Habitat degradation is probably the most important potential source of biodiversity loss. For example, nutrient increases can lead to phytoplankton population increases which lead to loss of light to the seabed. This can have dramatic negative consequences for sea grass beds and the species that rely on them. Degraded habitats are typically species-poor environments.

Why is Biodiversity Important?

There are many ethical and moral reasons why biodiversity should be maintained. Everyone would agree that a marine world comprised only of two species, a seaweed and a worm, would be very boring. There are, however, some very practical reasons why the maintenance of biodiversity is so important.

Due to the interlinked nature of communities and the fact that many species play crucial roles within their community (e.g. recycling nutrients, or regulating prey populations), maintenance of diversity is vital if ecosystem function is to be retained. If a particular species were lost from the ecosystem, its role may be so important that the ecosystem collapses as it can no longer function properly.

Genetic diversity within a species will increase its ability to exist long term. For example, a single genetically identical population could be wiped out by disease, however, elsewhere this species may continue to exist if its genetic variability gives it some resistance to the disease. So, although a population is lost, the species is not. If every individual within the species was genetically identical, then the disease could spread throughout the entire species and make it extinct. So why is this important to us? One obvious example is food production through fishing and fish farming.

Marine biodiversity may also be a very important source of drugs and medicines for man. In recent years, some very important discoveries have been made. Chemicals obtained from sponges have been found to have antitumor properties, and a steroid from the Dogfish Shark ( Squalus acanthias ) helps on the treatment of fungal infections dangerous to AIDS and cancer patients. More than 1700 compounds with biomedical properties have been reported from marine sources in the last 10 years.

UK Biodiversity Action Plan

The ‘Convention on Biological Diversity' was one of the outcomes of the Earth Summit held in Rio in 1992. Signatories to the Convention were obliged to develop national strategies for the protection and sustainable use of biodiversity. The UK was a signatory and as such developed the UK Biodiversity Action Plan which was published in 1994 (www.JNCC.gov.uk ). The plan combined new and existing conservation initiatives with an emphasis on a partnership approach (www.ukbap.org.uk ). A steering group was set up to process four main areas identified in the plan:

•  key species and habitats

•  access to biodiversity databases

•  public awareness and involvement

•  monitoring systems

Priority species and habitats have been identified, for which action plans have been developed (www.english-nature.org.uk ). Species Action Plans (SAPs) provide information on the threats facing the species and opportunities for maintaining and enhancing their populations. Species Statements may also be produced which provide an overview of the status of the species and set out policies that can be developed to conserve them. Examples of Species Action Plans included the native oyster and the Pink Seafan. Priority Habitats Action Plans (HAPs) set out detailed actions to safeguard and enhance these habitats. Examples of Habitat Action Plans include saline lagoons, littoral and sub-littoral chalk, coastal shingle and lowland and upland heathlands. Following the development of the national biodiversity plans, local biodiversity plans have also been developed which identify local priorities and determine the contribution to the national Species and Habitat Action Plan targets. One example of a local action plan is the Dorset Biodiversity Strategy , which was launched in May 2003.

The Dorset Biodiversity Strategy aims to:

•  ‘ensure that national targets for species and habitats, as specified in the UK BAP, are translated into effective action at the local level

•  identify targets for species and habitats appropriate to the local area, and reflecting the values of people locally

•  develop effective local partnerships to ensure that programmes for biodiversity conservation are maintained in the long term

•  raise awareness of the need for biodiversity conservation in the local context

•  ensure that opportunities for conservation and enhancement of the whole biodiversity resource are fully considered

•  provide a basis for monitoring progress in biodiversity conservation, at both local and national level'

Marine and coastal priority species identified within the Dorset Biodiversity Strategy (www.wildlifetrust.org.uk/dorset/) include the Little Tern (Sterna albifrons), Pink Seafan (Eunicella verrucosa), Lulworth Skipper (Thymelicus aceteon), Scaly Cricket (Mogoplistes squamiger) and the Harbour Porpoise. Priority coastal and marine habitats include coastal vegetated shingle and coastal sand dunes, coastal salt marsh, littoral and sublittoral chalk, Maerl beds (Phymatolithon calcareum and Lithothamnion coralloides), saline lagoons and seagrass beds (Zostera marina).