Ecosystems and Ecology

Transcription

1 Ecosystems and Ecology Author: Prof Koos Bothma Licensed under a Creative Commons Attribution license. SOME FEATURES OF ECOSYSTEMS Because ecosystems vary from being simple to complex there can be no list of essential features. What can be concluded is that complex ecosystems are difficult or impossible to replace once they have been destroyed. Moreover, although specific ecosystems may be relatively stable, new ecosystems may arise when the basic components of ecosystems are altered naturally or by human intervention. Novel ecosystems Novel ecosystems are also known as emerging ecosystems and they result when the biotic components occur in combinations and relative abundances that have not yet occurred previously within any given biome. These ecosystems are the result of deliberate or inadvertent human action. As more and more parts of the Earth become transformed by human actions, these novel ecosystems increase in importance. Novel ecosystems either arise from the degradation of natural ecosystems or from the abandonment of ecosystems that had been managed intensively. It is not yet clear if novel ecosystems are persistent and what values they may have. Nevertheless, it is likely that it may be extremely difficult or costly to return novel ecosystems to the previous ecosystem state from which they developed, although this does not disallow the development of appropriate management objectives or approaches. Novel ecosystems can originate in several ways: Human impact on natural ecosystems may have resulted in the loss of essential organic elements or the introduction of new ones, urban degraded landscapes around natural ecosystems may create barriers to essential dispersal processes, and direct removal of organic components (e.g. soils, vegetation and animals) can create changes in the abiotic components of the environment or decrease the genetic part of the organic component to the extent that regeneration becomes impossible. In harsh environments the total constraints wil be greatest in the abiotic components while in more benign environments the constraints will be mainly in the biotic components. The global transport of alien organisms is a factor in the development of novel ecosystems. Not only are new biota being spread into existing ecosystems but new parasites and diseases enter and affect them. Novel ecosystems are therefore appearing globally at ever more rapid rates. The illegal trade in wildlife products is currently 1 P a g e

2 spreading viruses globally into previously unaffected human populations and ecosystems (see Smith et al below). Ecosystems can cross two types of threshold: biotic thresholds that are created by dispersal barriers and result in unusual combinations of organisms and functional groups, and abiotic thresholds that are caused by factors such as deforestation, soil erosion, overgrazing or climate change. Invasive organisms may also create major changes in existing ecosystem while even the removal of target invaders could allow other invaders with new impacts to enter an ecosystem and create more alteration. New biotic assemblages affect key interactions and processes in ecosystems. Because they result from human intervention it becomes vital to conserve less impacted or largely natural ecosystems. It is equally unwise to develop costly programmes to attempt to repair natural ecosystems once they have crossed a critical threshold of change. The only possible alternative in the latter case would be to explore and use the benefits of the resultant novel ecosystem. The rub lies in the way in which humans interact with their natural environment. Genomics Genomics is a progression from molecular studies of relatively simple model systems to complex natural systems. The relatively recent ability to sequence genomes from key plant types is revealing the molecular drives of community composition and ecosystem processes. A genome is the haploid set of chromosomes which contains the complete set of genetic material but does not reflect the genetic diversity of a species. The word is a blend of the words gene and chromosome (gen-ome). In Greek the word means I become or I am born. Interaction between trophic levels has been shown to alter the composition of a diverse community of fungi, insects, arachnids and avian predators. It is also possible that indirect genetic interactions among relatively few species such as a tree, browser, mutualist and/or pathogen and their environment may structure the composition and abundance of a larger community of organisms and define ecosystem processes, such as the cycling of nutrients. Heritability in a community through the tendency of related individuals within a species to support similar communities and ecosystem processes link ecological interaction in an ecosystem with genomics. Inter alia such genetic knowledge now extends the understanding of the function of condensed tannins and lignin in natural forest ecosystems. Genomic perspectives can further aid in understanding community structure and ecosystem processes by identifying the specific genes, alleles and Earth surface mechanisms upon which the heritability of communities and ecosystem phenotypes are based. A genomics approach to ecosystem processes is important because these processes represent the combined effects of the interactions among multiple species, environmental variation and complex feedback mechanisms. Consequently, quantifying the 2 P a g e

3 genetic covariance among species is important in understanding the evolution of communities and ecosystems. A merger of genomics with community and ecosystem ecology may ultimately provide the key to understanding how complex natural ecosystems are being formed and maintained. Food-web matrices and feedback mechanisms The more mature an ecosystem becomes, the more it is composed of interrelated food-web matrices and feedback mechanisms. Food-webs are pathways for the cycling of energy through an ecosystem. Solar radiation is the source of the energy and photosynthesis is a first step in primary production in an ecosystem. The energy is thereafter transferred to different trophic levels with the larger herbivores followed by the larger carnivores (if they are still present) as the higher trophic levels. Only 10 to 20 per cent of all the solar energy that reaches an ecosystem is transferred to a next higher trophic level within it, which means that the number of trophic levels in any ecosystem can never be large. Moreover, some of the energy is lost through respiration. Eventually all organic matter is broken down again by microorganisms to be recycled through the ecosystem. Reptiles are abundant in the Kgalagadi Transfrontier Park of the south-western Kalahari ecosystem where snakes may prey in lizards The terrestrial ecosystems produce two-thirds of the primary production in the world, and the oceanic ones only a third. The tropical rain forests produce most primary production and are also the most intricate ecosystems. As latitudes increase further from the equator, so primary does production decreases. Extreme desert, rock, sand and ice, for example, have a net primary productivity that is 0.2 per cent of that of a tropical rainforest while the savannas have a net primary productivity that is 36.1 per cent of that of a tropical rainforest despite being the second highest net primary producers in the world. The production efficiency of energy also varies according to the taxonomic class of the organisms that are involved. Micro-organisms that are short-lived, have a small size and a rapid population turnover rate 3 P a g e

4 have an extremely high energy production efficiency while in vertebrates it is generally around 10 per cent. Energy efficiency is the percentage of assimilated energy that is incorporated into new biomass. Once energy has been transferred randomly into heat it can no longer be used as energy by organisms, except momentarily to maintain body temperature. The proportion of nutrients in living biomass also increases from the poles to the equator. Consequently they vary greatly between various biomes and ecosystems. Decomposition processes in the cold regions are slow and nutrients are trapped for long periods in dead organic matter. In contrast, decomposition of organic matter happens rapidly in tropical areas such as the rainforests where most of the nutrients are present in living organic matter. Food-web matrices link various trophic levels in mature ecosystems. A food-web consists of various linked food chains. A food chain consists of pathways of energy transfer from basal species to the large carnivores which form the apex of a food chain. Each trophic link contains a variable number of types of organism. The Knysna Estuary in South Africa, for example, contains four trophic levels involving a total of 15 types of organism of which only one is an apex predator. The number of trophic levels and the mean number of types of organism per trophic level in the savannas is still unknown. Only at extremely low levels of primary productivity does there seem to be an upper limit on the number of trophic levels. However, species diversity is significantly highest in the most productive regions which allow each consumer to feed on a limited number of organism types at a lower trophic level. Intricate food-web matrices with long food chains typically experience such severe population fluctuations that the extinction of the top or apex predators is more likely there than in those with simpler food chains. In general too, carnivores are more likely to be opportunistic consumers that will attack any potential prey item of the right size at a given time irrespective of its position in the food chain or food-web. The black-backed jackal Canis mesomelas is an abundant hunter-scavenger in the Kgalagadi Transfrontier Park of the south-western Kalahari ecosystem as here at Bedinkt windmill in May It often follows hunting lions Panthera leo and leopards Panthera pardus to scavenge what is left of their prey but this exposes it to a predation risk too 4 P a g e

5 Soil food-webs are the basis of the transfer of solar energy between species in an ecosystem. Above the ground the energy moves from the primary producers (plants) to the primary consumers (herbivores) and then to the secondary or apex consumers (carnivores). At the end of the process the decomposers such as fungi and bacteria recycle the nutrients several more times before a specific chain of energy transfer ends. Trophic cascades are top-down interactions that describe the indirect effects of higher trophy level consumers on ecosystem functioning. They include herbivory and carnivory and some examples will be discussed later. Ecological communities Ecological communities range in geographic size from biomes to ecosystems, landscapes, bioregions, plant communities, animal and plant populations and habitats. The habitat is the place where the ecological requirements of a specific organism are met and the niche of an organism is its ecological role in an ecosystem. Ecological communities are relatively small assemblages of organisms that occur in a defined space at a given time and they define ecosystems in combination with the physical environment. However, because communities are not divorced from their specific abiotic environments, the distinction between ecosystems is more a matter of scale than nature. A community comprises all the organisms in a given area whether they are micro-organisms, plants or animals. The major problem with the study of communities is that only a portion of all the organisms that form part of them can readily be studied and counted. Consequently the most common or conspicuous organisms are often emphasized and some rarer but ecologically vital organisms may be overlooked. In describing a community, its biodiversity is important. However, simple taxonomic lists are inadequate because they ignore the numerical importance of each taxon. For example, most grassland communities contain ten common types of grass, at times called key grasses, which support the bulk of the larger herbivores. As in wildlife management generally, the objective of studying a community is important. In an extreme case, a scientist may only be interested in a specific community of rare birds or mammals. When plants are involved, the communities are often described in terms of biomass which defines the rate of primary production per species in a quantitative and qualitative way. Two indices of abundance and species richness are Simpson s Index and Shannon s Index. More details on these indices appear in the references below. Some communities have clear and sharp boundaries but usually they merge gradually. Vegetation communities are the basis of wildlife management because they differ in a characteristic way and contain different plant species compositions and vegetation structure which in turn affect their ability to sustain larger herbivores. They are usually characterised by the dominant plant types. The description of plant communities requires a variety of ordination techniques which treat communities to statistical analyses to define their structure and the abundance of specific species, and defines their boundaries. These results 5 P a g e

6 indicate the relationship between a community and its physical environment and describe the nature of the community. Community boundaries imply the existence of discrete units and distinct boundaries but this is not a fundamental question. What matters most is the level of organization within a community rather than when and where it occurs. The plant community level of organization defines the nature of the interaction of various organisms within the community, usually at a given place and time. The stability of a plant community measures its resistance to disturbance while its resilience measure its ability to return to a specific state after having been disturbed, and the speed with which it does so. Stable and resilient communities have the ability to persist for a long time. Communities have specific characteristics because they possess stabilizing and resilient properties. Both these properties are sensitive to severe changes in the physical environment. A community which is only stable within a narrow range of environmental conditions is dynamically fragile. Conversely, one which is stable within a wide range of environmental conditions is dynamically robust. Complex communities contain more pathways for energy transfer than simple ones. Such complexity has originally been linked to better stability. However, it is increasingly evident that increasing complexity eventually creates instability because the food-webs that occur in complex communities may contain nonessential random elements. Moreover, increased complexity may lead to increased stability in certain characteristics of a community and to decreased stability in others. There are also indications that the relationship between complexity and stability may depend on the relevant trophic level. Therefore, no single relationship between stability and complexity exists as a generalization. Edge effect Edge effect is the occurrence of greater species diversity and biological density in an ecotone (the zone of intergradation between two ecological communities) than in any of the adjacent ecological communities. In ecosystems it is the effect of juxtaposition of contrasting environments. For example, where land adjacent to a forest has been cleared to create an open forest boundary, sunlight and wind now penetrate the forest to a greater extent for a specific distance and alter it environmentally and biologically. In the Amazon rain forest it is already estimated that the penetrating edge effect exceeds that of cleared areas in surface area. This allows fires to penetrate the forests and is slowly transforming them. Edge effect also involves succession because of vegetation that is spreading outwards. The higher biodiversity that this creates is the result of increases in plant and animal diversity. Edge effects are major drivers of change in fragmented landscapes but are highly variable in space and time. Extreme weather events also drive edge effects, and where trees form part of a community high winds make its boundaries vulnerable. In contrast, edge effects are more productive in terms of animal production than closed stable plant communities. However, rare animal and plant populations with undisturbed habitat requirements can be affected negatively by the creation of edge effects. 6 P a g e