We're going to start this week with an introduction to ecosystems and the services and benefits that it provide to society. A common definition for an ecosystem is a biological community of interacting organisms and their physical environment. One of the most important aspects of this definition is the term interactions. These interactions between different species and between species and the physical environment can lead to what some people call emergent properties; characteristics that you wouldn't predict as a function of the individual characteristics of the species and also characteristics of the ecosystem that make it resilient to stress and change. A good example of this is tropical forests, where through moisture recycling, in other words, transpiration from the forests goes back into the atmosphere and then is recycled as rainfall. One can actually have a rainfall environment that's much moister than one would expect from the moisture that's available to the region, but transport by winds from outside the region. So, for instance, in the Amazon, it's estimated that about 30% of the rainfall that occurs in the Amazon is recycled moisture that through this process of transpiration, releasing water back into the atmosphere and then falling is rainfall. Another example of how ecosystems work together to maintain a suitable environment is through nutrient cycling. Again in tropical forests, most soils in tropical forests are what we call heavily leached, that means, they are very low in nutrients because there's been essentially washed out into groundwater by the heavy large amounts of rainfall that filtered through the soils. So, tropical forest systems get around this by keeping all the nutrients very tightly trapped in vegetation both living vegetation and the dead vegetation and soils and then reuse that very very rapidly without letting it easily get back into the soils and be leached out. So, this type nutrient cycle really just makes the nutrients easily available and maintains the nutrient levels needed for these tropical forests to grow and produce for their primary productivity. If we look at ecosystems more generally, we can think of them at multiple different geographic scales. At the biggest scale, we can think of the earth as an ecosystem all on its own and that's sort of the fundamental idea behind James Lovelock's Gaia hypothesis where he defines the earth as a almost like a living organism that where all the interactions act to have feedbacks and regulatory processes that keep the earth within the system within certain habitable limits. We can go down to almost like the, not quite the macroscopic scale, but very very small scales. So, for instance, think of a rock pool on the coast. That rock pool it could be considered an ecosystem in itself. It's got organisms in their physical environment of water, so you've got various marine organisms all interacting along with plant organisms in the rock pool as a very small ecosystem. We can also think of ecosystems in terms of the physical settings within which they occur. The three main categories today would be terrestrial ecosystems, so those that are based on physical land surface. Freshwater ecosystems, those occurring in freshwater systems such as rivers and lakes and marine ecosystems. Also water-based, but in this case in saline water, salty water conditions. We also get ecosystems that can span these different physical environments. So, two examples of that might be, would be wetlands where you have mixed freshwater and terrestrial environment and as you have the seasonal variation in rainfall, the wetlands expand and contract and the species have to cope with both being able to live in both aquatic or waterborne and dryland environments. Along coastlines you have this on beaches and rocky coasts, you have a situation where the tide comes in and out and these ecosystems also have to deal with periods of exposure to air and periods of being underwater. So, almost like an amphibian type of ecosystem. Going from the global down was to find a scale. So, next sort of step down is what's called a biome. Biome is a very large community of plants and animals that occupies a large and distinct region and the important thing about biomes is their limits are often defined by climatic conditions, both temperature and rainfall. There are a number of biomes that occurred at the global scale and six of those occur in Africa. If we look at Africa starting at the equator and particularly in the Western Equatorial regions, we have tropical evergreen forests. Almost like as a bull's eye in the middle of Africa, and then radiating out from that we have different zones with different biomes occur as one gets to essentially drier and warmer conditions. So, after the tropical evergreen forests we are going to a biome called tropical deciduous forest. Then that sort of transitions into tropical woodlands which is a more scattered sort of forest grassland environment, tropical grasslands and then finally, into a desert biome and then right at the tip, the north and the south of the continent in the Mediterranean climates, we have temperate shrub lands. In Southern Africa, that's the famous biome, and in the north, it's the Mediterranean shrub land ecosystems. But within these biomes, there's a lot of variability and we talk about eco-regions as being distinct areas with similar vegetation patterns that occur within a particular biome. For instance, the WWF has defined 119 different eco-regions in Africa. So, an example of that might be in a tropical forest biome. In low-lying areas, you might get forest systems that are dominated by palm trees that are quite resilient to periodic flooding and then in more elevated areas you might get more traditional woody tropical forest trees and there's a very different eco-regions. When we thinking about ecosystems today, it's really important to remember that people are an integral part of these ecosystems and these days tend to talk about social-ecological systems and this is where we have society and people in society and the structures that society creates being critical, regulating and modulating functions in the natural ecosystems. People could be seen as almost like a keystone species within these ecosystems. They're really important because they interact with virtually all other species and three good or bad practice can have very strong negative and positive consequences on the functioning and the structure of these ecosystems.
