Wednesday, March 28, 2007


In 1969 and 1970, Richard Levins introduced the term “metapopulation” in his work on the biological control of pests (Hanski and Gilpin, 1997; Levins, 1969). He used models of migration, extinction, and local fluctuation to study the population processes of pests in a heterogenous environment (Levins, 1969). Levin’s work marked the beginning of contemporary metapopulation biology.

The literal meaning of the word metapopulation is a ‘population of populations’. Hanski and Gilpin (1997) define a metapopulation as a set of local populations within a larger area, where migration from one local population to other habitat patches are possible. These groups of local populations usually occur in suitable, discrete (i.e. separate and discontinuous) habitat patches that are scattered in a landscape. This spatial arrangement allows the populations to interact via the dispersal of individuals across a matrix of unsuitable habitat (Baguette and Schtickzelle, 2003). This ‘ensemble’ of populations results in dynamic interactions between local populations through migration (Marquet, 2002). These interactions are explained and interpreted by metapopulation modelling and theory.

Image 1. The Glanville fritillary butterfly
(Melitaea cinxia)

The aim of this forthcoming presentation will be to introduce metapopulation theory within the context of butterfly metapopulations. Butterflies will be used for the simple reason that their populations are often structured in space in a manner that is broadly consistent with the metapopulation concept. As a result the concept will be more clearly illustrated. This case study approach will furthermore highlight the relevance of the metapopulation concept to wildlife conservation and current environmental issues.


Baguette M,Schtickzelle N (2003) Local population dynamics are important to the conservation of metapopulations in highly fragmented landscapes. Journal of applied ecology 40: 404-412.

Hanski IA, Gilpin ME (1997)Metapopulation Biology- Ecology,Genetics, and Evolution. Academic press, San Diego. ISBN 0 12 323446 8

Levins,R (1969) Some demographic and genetic consequences of environmental heterogeneity. Entomological Society of America 15:237-240

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Recently I read an interesting article in the Science and Technology section of the Sunday Times. The article was on genetically modified insects, specifically mosquitoes in order to control disease.

We are all aware of Malaria, but not many of us know how it “works” or what it actually is. Malaria is caused by a parasite (representatives of the genus Plasmodium) that infects the blood and is transmitted to potential hosts through the saliva of the mosquito. According to the article, Malaria is second only to HIV and AIDS when it comes to how many people it kills each year. This is estimated to be around 2.7 million lives annually.

What scientists have done is to genetically engineer a mosquito that is resistant to the parasite itself, which can drastically impact the efficiency of transmission. The proposal is to release these mosquitoes into the wild where malaria is prevalent and where the natural biodiversity exists which includes the genetic component of the wild mosquitoes. In wild mosquitoes, the actual infection with malaria parasites does impact that mosquito’s reproductive potential, however previous studies with genetically modified mosquitoes proved that the wild forms were still fitter. Basically what this means is that even though genetically modified mosquitoes could have been introduced into the wild population earlier, they would have been out-competed by the wild forms which were genetically predisposed to survive better in the environment. Now unfortunately, the scientists seem to have unlocked this barrier and have now produced a genetically modified form that is fitter than the wild form!

So you may ask what this has to do with biodiversity then? Well, since a component of biodiversity is genetic, the introduction of a genetically modified mosquito would eventually cause the local extinction of the wild gene pool. What is more alarming is the apparent lack of forethought in the scientists who have not even mentioned the concern over the effects of co-evolution in parasitism, which can occur at a faster rate in the parasite than in the host. The Red Queen Hypothesis explains that parasites and their hosts are in a continuous battle or evolutionary “arms race” and that each has to keep up to remain within the folds of a dynamic equilibrium. If the mosquitoes that are introduced have a genetically predisposed higher resistance to the actual parasite, it is almost certain that this would fuel a reciprocal response in the parasite’s evolution in order to survive. If this response is to produce more aggressive forms of Plasmodium spp., the result could be even worse for infected people…

Something to think about.

David Vaughan
Senior aquarist, Quarantine
Two Oceans Aquarium
Cape Town, South Africa
+27 21 418 38 23

Article in Sunday Times, 25 March 2007, Science and Technology, page 33.

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Air pollution affects biodiversity on a great scale. The atmosphere, lithosphere, and hydrosphere are negatively affected by pollution [1]. Air pollution affects lower life forms more than higher life forms. Plants are generally more affected than animals on land, but not in fresh water. A decline in most species due to pollution is evident except for a minority that increase. I will be focussing on plants and how they are affected by air pollution.

Plants constantly take up atmospheric gases i.e. air everyday to sustain their biological processes. Vegetation growing under optimum conditions is most susceptible to air pollution [2]. As air pollution is for the most part man-made, we are the main source of this phenomenon. Pollution can be derived from two kinds of sources namely, stationary and multiple point sources. Stationary point sources include backyard fires (on a small scale) and the burning of a thousand tons of coal each day in coal-fired electrical power plants (on a large scale). Multiple point sources are usually mobile and include automobiles and other vehicles. The vehicles are the most important source of atmospheric pollutants as they release carbon monoxide. This is followed by industrials sources which release sulphur oxides, steam and electric power plants, space heating and lastly refuse burning. Agricultural chemicals also form part of air pollution [3].

The uptake of pollutants depends on the concentration gradient between the ambient air and the absorptive sites within the leaf. It also depends on the conductance of the stomata. The toxic effect of a pollutant may thus be almost directly related to the functioning of the stomata. Stomata openings are related to the physiological activity of the plant in that they regulate gas exchange; correlation exists between the extent of air pollution effects and the degree of opening of the stomata [2]. Pollutant flow may be restricted by the physical structures of leaves or scavenging by competing chemical reactions. However, as conditions change the ambient dose to which plants are exposed does not necessarily reflect actual cellular exposure. The initial flux of gases to the surface is controlled by boundary layer resistance i.e. the amount of gas able to contact the surface. This includes epidermal characteristics and air movement across the leaf.

At slower wind speeds (less that 2m/s), boundary layer thickness decreases as wind speed increases. Thus more pollutant enters the leaf when air is in motion. Pubescence is also important in that leaf hairs provide major areas of impact. Cuticle wax is also important in limiting uptake even if the cuticle is thin. Stomatal resistance is the most critical. Resistance is determined by stomatal number, size, anatomical characteristics for example the degree to which stomata is sunken and the size of the stomatal apperature [4].

The effect of pollution on the plants is usually visible in one form or the other. Pollution injury can be classed as acute, chronic (chlorotic) or hidden. In acute conditions intercostal leaf areas first take on a water-soaked appearance. The leaves then become dry and bleached to an ivory colour in some species while in others they become brown to brownish red. In the case of chronic injury the leaves become yellow and bleach until most of the chlorophyll and carotenoids are destroyed. This is caused by absorption of gas, insufficient to cause acute injury or absorption of sub lethal amounts over an extended time [3].

As the pollutants are taken up a “damage process” is followed. The epidermis is the first target as air pollution passes through the stomata and acts on this opening. The intercellular spaces are next affected as the pollutants dissolves in the surface water of the leaf cells changing the pH of the cells. In the second step the walls of the mesophyll cells are affected. As the walls contain cellulose, the cell membranes are most likely affected, notably their protein components.

As the pollutants react within the plant it is not necessarily in its original form. The pollutants pass into solution and form free radicals with electric charges. These radicals are more reactive and toxic. In the third step the cell organelles are affected for example, the chloroplast and mitochondria. In the case of the chloroplast the inner thylakoid membrane is the most sensitive. The enzymes of thylakoid and protein components of membranes are most likely to be targets. The precise protein will vary with the pollutant. Enzymes essential to carbon dioxide fixation is especially sensitive. In the mitochondria respiration, carbohydrate and lipid metabolism is adversely affected by air pollution. Changes in the ultra structure of the organelles are the first symptom of injury. The symptoms vary with the pollutant [4]. Some particular processes of sexual reproduction in plants are known to be very sensitive to toxic gases [5]. This therefore causes long-term changes to population ecology.

From the above information it is obvious to see that air pollution has severe adverse effects on the ultra structure and biological processes of plants. As plants form the bases of all food chains and also supplies us with oxygen, we should value and treasure them. Many of our forest ecosystems will be destroyed or at least be disturbed, resulting in considerable changes in plant communities and losses of plant resources and ecosystems. We should therefore increase our awareness of pollution in general and see what we are able to do to decrease pollution levels.

Air pollution also changes the distribution of many plants species and plant communities. It reduces biodiversity and does not respect boundaries set by conservation areas and nature reserves. Air pollution therefore contributes to the decline of biodiversity on a global scale. This global impact is also evident with climatic changes i.e. increase in temperature caused by gases polluting the atmosphere [6].

Something needs to be done to reduce pollution at the source. This can be done by reducing energy demands, conserving energy, switching of fuel and having technical pollution controls. The sixth major extinction is being triggered by humans’ inconsideration for our planet. Deforestation and fossil-fuel combustion have caused an increase in carbon dioxide by 30% in the past three centuries. We have already caused the extinction of 5-20% of the species in many groups of organisms. How much more disaster are we going to cause and what will it take to bring about a reformation? Air pollution is only one factor that influences biodiversity but controlling it can make the world of difference.


[1] McNeely, J.A., Gadgil, M., Lévêque, C., Padoch, C. & Redford, K. (1995). Human influences on biodiversity. In: Global biodiversity assessment, V.H. Heywood (ed.), Cambridge University Press, Cambridge, pp. 711—821. 0-521-56481-6 ISBN

[2] Stern, A.C., Wohlers, H.C., Boubel, R.W., Lowry, W.P. (1973). Fundamentals of Air Pollution. Academic Press, New York.

[3] Kozlowski, T.T., Mudd, J.B. (1975). Responses of Plants to Air Pollution. Academic Press Inc., New York.

[4] Anderson, F.K., Threshow, M. (1991). Plant stress from Air Pollution. John Wiley and sons, New York.

[5] Scholz, F., Gregorious, H.R., Rudin, D. (1987). Genetic Effects of Air Pollution in Forest Tree Populations. Springer-Verlag, New York.

[6] Leemans, R. (1996) Biodiversity and global change. In: Biodiversity, a biology of numbers and difference, K.J. Gaston (ed.), Blackwell Science, Oxford, pp. 367—387. 0-86542-804-2 ISBN



Mangrove ecosystems are essentially tropical to subtropical ecosystems, structurally dominated by trees and shrubs, some herbaceous plants and vines with associated biota. Mangroves predominantly occur along coastal areas and inhabit the fringes of estuaries (Nybakken 2005). Mangroves, seagrasses and coral reef tropical ecosystems are discrete ecosystems frequently occurring in close proximity to one another and interact with one another through the exchange of energy in the form of dissolved organic matter and faunal migration (Kitheka 1997).

Factors needed for the development of mangroves

Mangroves occur on soft, muddy, dark substrata that are frequently waterlogged, creating an anoxic environment due to reduced interstitial circulation and high bacterial activity (Nybakken 2005). These ecosystems occur in coastal areas or estuaries which are well protected from wave action. The latter explain why mangroves develop most extensively in regions behind coral reefs (Hogarth 1999). Reduced wave action allows for the settling out of fine silts and sediments which associated organic matter suspended in river inflows. In addition reduced wave action is required for the settling and establishment of new seedlings (Nybakken 2005). Mangroves are essentially facultative halophytes and have unique adaptations to cope with high salinities. Mangroves are terrestrial flowering plants that have reinvaded salt water and hence cannot survive in water of too high salinity. Being and estuarine ecosystem, mangroves experience continuous fluctuations in salinity with tidal action (Hogarth 1999). The distribution of mangrove forests is dictated by the relative sea surface temperature. They are mainly distributed within the winter position of the 20 °C isotherm (Nybakken 2005). However, these ecosystems may occur further south or north where currents bring warm water to the east coast of continents. Due to their sensitivity to freezing, mangroves do not extend into temperate habitats (Nybakken 2005). Mangroves require tidal action for their survival as the latter inundates roots with oxygenated salt water and replenishes nutrients. Tidal action prevents soil salinities from reaching lethal levels especially in areas with high rates of evapotranspiration (Nybakken 2005). The duration of tidal flooding dictates the degree of sedimentation in mangrove ecosystems. However, if root systems (aerial roots) are submerged for too long, mangroves can literally downed due to a lack of oxygen (Hogarth 1999).
Mangroves have adaptations that allows then to outcompete their terrestrial counterparts.

Mangroves are facultative halophytes, inhabiting stressful anoxic, saline environments because of their inability to compete with terrestrial freshwater angiosperms (Nybakken 2005). These ecosystems thrive in these seemingly stressful environments due to the acquisition of physiological, morphological and reproductive adaptations that allow them to cope with anoxia and osmotic problems.Many species of mangrove plants such as Bruguiera sp actively excrete salt via the roots, whereas other accumulate salt in older leaves which they later shed (for example Xylocarpus sp) (Hogarth 1999). Most mangroves have succulent sclerophylous leaves containing specialized water storing tissue. Alternatively as seen in Avicennia sp and Sonneratia sp, the leaves can contain salt exuding glands on the ventral and dorsal surfaces (Hogarth 1999). To reduce the osmotic gradients for the outward diffusion of water from the plant tissue, some mangrove species store amino acids in their internal fluid (Nybakken 2005). Rhizophora is less successful in preventing salt uptake, and as a consequence the internal salt concentrations may reach as high as 3 ‰, more than 100 times that of terrestrial plants (Branch and Branch 1981). Mangroves have a range of xeromorphic features in order to cope with the osmotic loss of water. These features include a thick leaf epidermal tissue layer, covered by a waxy cuticle. The leaves often contain fine hairs on the ventral surfaces and stomata are sunken (Hogarth 1999).
As adaptations to anoxic conditions most mangrove plants are shallow rooted with horizontal cable roots extending just below the mud surface. In Avicennea marina, vertical above ground pencil roots extend from the cable roots (Branch and Branch 1981). These pencil roots or pneumatophores contain lenticels which function in aeration of the roots (Nybakken 2005). Similarly Bruguera gymnorrhiza and Xylocarpus sp have knee roots and blade roots respectively, that branch from its cable roots and functions in gas exchange (Branch and Branch 1981). Rhizophora sp lacks cable roots but are shallowly anchored by a system of prop roots. In addition stem tendrils which extend from the braches or stem functions in gas exchange (Nybakken 2005). Sonneratia alba has above ground pneumatophores similar to that of Avicennia sp, however these are not pencil-like but can be more than 10 cm in diameter and are associated with fungal hyphae aiding in aeration and nutrient acquisition (Hogarth 1999). In most mangroves the section of the pneumatophores (or above ground root) penetrating the soil is often adapted with specialized aerenchyma tissue. Aerenchyma has a regular arrangement of air spaces in its interior called lacunae lending both floatation and aeration throughout the plant (Hogarth 1999).

Mangroves are able to optimize the dispersal and survival of their seedlings by being viviparous. In this way the seed germinates and develops into a seedling while still attached to the parent plant. The seedling is only released once sufficient roots have developed. Once released into the water column, the seedling floats with prevailing currents to a new location where it is able to settle and set roots in shallow waters (Nybakken 2005).

Mangroves and Coral Reefs are never found in the immediate vicinity of one another

Mangroves and coral reefs are both tropical ecosystems that are frequently found in close proximity to one another and are interconnected via the exchange of energy in the form of organic matter and animal migration during spawning and feeding (Kitheka 1997). However, these two discrete ecosystems are never found in the immediate vicinity of one another due to the fact that they thrive in contrasting physical environments.
The interaction between mangroves and coral reefs are not clear cut. Coral reefs stabilize the seascape by dissipating wave action and over geological time create areas protected from wave action, favouring the development of mangroves, while mangroves (and seagrasses ) act as chemical and physical buffers to the influence of land runoff on coral reef ecosystems (Birkeland 1997). Mangroves have the capacity to filter land runoff, removing terrestrial particulate and dissolved organic matter, trap and bind sediment, essentially promoting downstream coral reef growth. However, sporadic events such as Hurricane Andrew that hit the Florida Peninsula in August 1992, is a reminder of why these two ecosystems occur some distance from each other (Hogarth 1999). Heavy precipitation and wave action flushed large quantities of accumulated material from mangrove and seagrass sinks into downstream coral reef ecosystems (Birkeland 1997). Coral reefs are extremely vulnerable to sedimentation, eutrophication in the form of dissolved organic matter and fluctuations in salinity. Fine sediments and silts cause clogging of the mouth parts of coral polyps and hence prevent respiration subsequently leading to smothering of these polyps. The inflow of fine silts increases the turbidity of reef water, essentially decreasing the amount of light reaching corals and so doing decreasing the photosynthetic ability of obligate mutualistic zooxanthallae (Birkeland 1997).

Associated with mangrove sediment and mud is large amounts of dissolved organic matter. Coral reef ecosystems thrive in oligotrophic waters (Koop et al. 2001). Eutrophication increases the growth of opportunistic fleshy macro algae which outcompete corals for space and prevent coral larval settlement (McCook 1999). Increased levels of dissolved organic matter also lead to sporadic algal bloom events which further increases the turbidity of the reef water. Increased levels of dissolved nitrogen and phosphorous tends to disrupt the mutualistic relationship between zooxanthallae and coral polyps and as a result reduce coral calcification (Ferrier-Pages et al. 2000). Coral reefs are stenohaline and are not able to tolerate the fluctuation in salinity which is brought about when brackish mangrove derived water enter highly saline coral reef water.

The success of mangrove ecosystems is attributable to their ability to exclude stronger terrestrial competitors from the seemingly stressful habitats in which they occur. Mangroves have unique physiological, morphological and reproductive adaptations which allow them to thrive in conditions in which their terrestrial counterparts would be unable to. Mangroves interact with other tropical coastal ecosystems (seagrasses and coral reefs) via the exchange of dissolved organic matter and provide a spawning ground for many reef fish and invertebrates. Due to the catastrophic effect that disturbed mangroves can have on coral reefs, these two ecosystems are never found in the immediate vicinity of one another.


Birkeland C (1997) Life and death of Coral Reefs. Chapman & Hall, New York, USA. ISBN 0412035413, pp 536

Branch G, Branch M (1981) The living shores of Southern Africa. Struik Publishers, Cape Town. ISBN 0869771159, pp 272

Ferrier-Pages C, Gattuso J, Dallot S, Jaubert J (2000) Effect of nutrient enrichment on growth and photosynthesis of the zooxanthallate coral Stylophora pistillata. Coral Reefs 19: 103-113

Hogarth P (1999) The Biology of Mangroves. Oxford University Press, New York. ISBN 0198502230, pp 228