Ecosystems and Ecology
Author : Prof Koos Bothma
Licensed under a Creative Commons Attribution license .
It took millions of years for the Earth to build up a fragile and balanced diversity of plants, animals and micro-organisms. Some organisms have lived for millennia in a little changed state while others are relatively new arrivals. Those that have perished will never return, neither will those thousands that are currently disappearing in large part because of human influences such as habitat destruction; the introduction of invasive, exotic species; and the overutilization of the Earth’s renewable natural resources.
The consequences can be profound and do not preclude the extinction of humans. An example is the fragmentation and loss of habitat for plant pollinators. These organisms are vital in sustaining viable plant populations and should they disappear due to the injudicious use of pesticides or the fragmentation of their habitat then it will affect the entire food web of an ecosystem and could cause the total collapse of ecosystem functioning.
Booby colony, Lambert's Bay, South Africa - January 1998
Since the Swedish botanist Carolus Linnaeus in the 1700s published his series of books titled the
Systema Naturae as a system of classifying all living organisms, some 1,5 to 1,75 million species and subspecies have been identified and named scientifically. The most recent third edition of the reference book Mammal species of the world: a taxonomic and geographic reference by Wilson and Reeder (2005) show the addition of 171 new mammal species from 1982 to 1992 and 260 from 1993 to 2005. There are
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Ecosystems and Ecology now 5416 known species of mammals and many more still undoubtedly remain to be named and described. This latest edition is based on 9373 scientific publications. Estimates of the total number of species of organism on Earth range from 5 to 100 million. Whatever the total, some 50 per cent of them are insects and many of these insects are also herbivorous. Beetles are some of the most various and pervasive creatures on Earth.
Some 99 per cent of all the known species that ever lived on the Earth has already become extinct in five mass periods of extinction. The world is now experiencing a sixth and human-induced extinction of living organisms. The early organisms were mainly single-celled microbes but some 540 million years ago animal life became more complex and increased rapidly in diversity and abundance. Some 250 million years ago more than 90 per cent of the then known marine animals became extinct, while the dinosaurs disappeared some 65 million years ago. Recovery after every mass extinction took millions of years.
Migrating humans killed most of the megafauna in Australia and the Americas within a few thousand years of arriving there. Such loss of biodiversity will affect ecosystems, animal and human health drastically.
White rinoceroses in open Acacia, open woodland, Lewa Wildlife Conservancy, Kenya - January 1993
The term biodiversity is simply a contracted form of the term biological diversity. It encompasses the total variety of life on Earth in all its many manifestations. A longer definition is that biodiversity is the variability among all living organisms from all sources in the terrestrial, marine and other aquatic ecosystems and the ecological complexes of which they are part. This definition includes diversity within species, between species and between ecosystems. The term biodiversity can be applied to any unit of land surface be it large or small. It can also be linked to specific time frames; such as known geological epochs, periods, eras or eons; or to a specific time frame of the present time.
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Ecosystems and Ecology
The world Convention on Biological Diversity in 2002 compiled a list of 31 indicators to be used to report on the progress on the stated objective to achieve a significant reduction in the rate of biodiversity loss by
2010. By 2010, most of these indicators of the state of biodiversity did show declines over 2002 but there were no significant recent reductions in the rate of decline whereas indicators of pressure on biodiversity had increased. Moreover, the rate of loss did not appear to be slowing.
The indicators of biodiversity loss operate at the genetic, population, species and ecosystem level.
Declines were apparent in the population trends of vertebrates; habitat specialist birds and shorebirds worldwide; and in the extent of forests, mangroves, seagrass beds and coral reefs. The two indicators of freshwater quality and trophic integrity in marine ecosystems were stable and showed marginal improvements respectively. The overall biodiversity loss at the species level through extinction had accelerated and the rate of change had become negative.
There were increasing trends in the majority of the indicators of pressure on biodiversity with increased pressures being shown in the aggregate human consumption of the ecological assets of the Earth, the deposition of reactive nitrogen, the number of alien species in Europe, the proportion of fish stocks being overharvested and the impact of climate change on European bird populations. There were no indicators that showed a significant reduction in the rate of increase of the pressure on biodiversity. Data on the global fragmentation of habitats are not available but this process is suspected to be increasing in forests while the larger river systems are still being fragmented by the construction of dams and reservoirs.
Only three indicators address the benefits to humans from the maintenance of biodiversity. There were declines in the population trends of vertebrates that were being utilized and the aggregate species extinction risk has increased. The mammals, birds and amphibians that are being used for food and medicine showed declining populations as did that of the birds that are being traded internationally, mostly as pets. Already in 1991 it was estimated that some 2 to 5 million birds were being traded internationally every year. In 2012 BirdLife International estimated that nearly 4000 species of bird are being traded worldwide of which 1461 are on the restricted trade list of CITES and some 20 are being threatened with extinction in the wild. The latter includes the African grey parrot Psittacus erithacus . The indicators for which there were no reliable data included the number of domesticated animal breeds that are at risk of extinction, although 9 per cent are already extinct, the languages that are spoken by fewer than 1000 people (22 per cent of the currently known 6900 languages) and the more than 100 million poor people who live in remote regions within threatened ecosystems where they at least partially depend on the benefits of biodiversity.
There are sufficient data to show that the target of the level of biodiversity conservation in the world that was set for 2010 had not been met. All the global pressure indicators showed increasing trends although there were some system-specific exceptions with decreasing pressure trends for particular populations, taxa and habitats. Nevertheless, with political will and adequate resources, global biodiversity loss can be
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Ecosystems and Ecology reduced or reversed. However, this will require sustained investments in coherent global diversity monitoring and the development and application of indicators that are essential in tracking and improving the effectiveness of the biodiversity responses.
In assessing land management in fragmented habitats the importance of matrix habitats in maintaining biodiversity has to be emphasized. Resource management that maintains or improves the suitability of matrix habitats for organisms is fundamental to biodiversity conservation. Matrix habitats are linked, suitable habitat patches that are essentially surrounded by unsuitable habitat for a given cohort of organisms. Isolated conservation areas will only become effective in biodiversity conservation when there are suitable and ecologically effective links between them.
The importance of maintaining matrix habitats in fragmented habitats has already been emphasized in
Section 7.2 above. To do so requires knowledge of the ecological processes that influence the distribution of species at different scales. To date, most land management decisions have been based on the abundance of single species or functional groups. It has been suggested that future land management programmes be based on the following basic questions concerning biodiversity: which groups of organism are relevant, how does their domain of scale relate to the land area to be managed, what processes are likely to be important as determination of species distribution at land management scales, and how will the proposed land management activities interact with these processes?
Humans commonly perceive landscapes at kilometre-wide scales in which many biological processes and environmental features contribute to biodiversity. Functional heterogeneity determines species distribution and land management has to focus on groups of species that react to similar kinds and scales of heterogeneity while identifying the sources of that heterogeneity. This is done by dividing biodiversity into taxonomic, trophic, and functional or body size groups. Ultimately the measured biodiversity depends on the number and types of group being considered.
After distilling the view that is obtained of biodiversity to such groups or species there remains a variety of perspectives and scales. The common scale-independent characteristics of species can be used to generate scale-specific key land management questions. The three characteristics that determine species distribution across all scales are habitat requirements, dispersal capabilities, and the size and location of the geographic range. The potential of occupying a habitat depends on the physiological limitations of a species and the presence and effect of competitors, predators and pathogens. The habitat therefore represents the smallest unit of land management. The next and larger unit of land management is the landscape which involves the distribution of species which in turn are based on the abundance and spatial distribution of suitable habitats and interacting species. The habitat matrix approach allows species to disperse among suitable habitat patches provided that such dispersal is not prevented by barriers between the landscapes. The largest land management unit is the geographic scale which involves infrequent or long-range dispersals, regional population dynamics, meta-population management
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Ecosystems and Ecology and speciation. In this context the concept of transfrontier parks becomes important (see Section 8.4). For example, African elephants can only exist without becoming agents of land degradation when they can move about over vast areas in a natural cycle of abundance that may span several centuries.
Land managers should therefore examine how and why species distribution, richness or composition changes with increasing distances between land units of the same size. This variation is a function of organism attributes, such as dispersal ability and niche breadth, and site characteristics, such as the rate of change in environmental variables in space and time. The niche of an organism is its role in a specific environment; such as a producer, primary or secondary consumer; its feeding behaviour or other speciesspecific attributes. Moreover, different processes and variables will explain biodiversity depending on the group of species, the scale and the type of landscape that are involved.
Within a landscape unit, management scales are lar ger than habitat or species’ range levels but they are smaller on a patch-matrix or mosaic of suitable habitats and for movement or dispersal between such habitats. Alternatively environmental heterogeneity can be viewed on a gradient approach. In most landscape units, both environmental gradients and the effects of habitat mosaics on dispersal will be important although they may be scale dependent within the same unit. A further factor of importance is the influence of human behaviour through its interaction with biological patterns.
On the geographical scale, regions of endemism become important because they contain much of the biodiversity in a given region. A fundamental land management guideline is to maintain productive ecosystems that capture energy and support food-webs. There must also be a coordinated regional management approach and an understanding of historical geographical processes such as glaciation and deforestation. Such a consideration of historical events is necessary when interpreting the responses of species groups to land management strategies but extrapolation to environmentally similar areas may not be appropriate. The identification and allocation of management resources to land management units where regional losses have not yet occurred should also be a priority.
Biodiversity is consequently being managed at several spatial and temporal scales. Consequently land management decisions interact across various scales. The land management framework that is being applied will depend on which groups of organism are being considered, how their scales of land area relate, what the likely important processes are and how the land management activities will interact with these processes. Land managers are therefore required to make decisions that are based on many organisms and concepts. Consequently interdisciplinary and inter-institutional interaction and cooperation
(One Health approach) become vital.
The biomes and their associated multitude of ecosystems in Africa vary between literature sources.
Generally, however, the following terrestrial biomes are recognized in Africa: deserts, forests, fynbos
(macchia), grasslands, the Nama-Karoo, savannas, the succulent Karoo and thickets. The tops of the
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Ecosystems and Ecology high mountains are sometimes recognized as an additional biome. Except for the tropical forests and high mountain peaks these biomes all occur in southern Africa. The vegetation of southern Africa is discussed in detail by Cowling, Richardson and Pierce (2004) in the references.
Tankwa Karoo, Tankwa Karoo National Park, South Africa - October 1988
Considerable biodiversity benefits can be obtained by regenerating grasslands to facilitate introducing grazing animals to regions where they once occurred but have since disappeared. In doing so, livestock management should mimic the movements of wild herbivores to avoid trampling and patch overutilization.
Reforestation is another approach that will increase biodiversity and improves the use and cycling of nutrients and energy.
Biodiversity conservation can be improved greatly by protecting existing plant communities and restoring degraded landscapes. The current and real threat that a third of the world ’s flora already faces extinction requires rapid, collaborative programmes that apply various models that have been shown to produce positive results. There are many aspects that are relevant to plant ecology and survival but the three essential biological elements appear to be seed size, canopy height and the leaf area of essential plants in plant communities. Environmental elements of major importance are phosphorus and nitrogen availability, fire frequency, flood duration, annual temperature and annual rainfall.
Larger areas usually contain more plant species than smaller ones. Moreover, the temperate and tropical floras are most species-rich while those of deserts, sub-arctic regions and the oceanic floors are relatively poor in species. The greater the area of habitat that is protected the more species it will contain. For every doubling of area size there is an approximate 20 per cent increase in the number of species that will be present. Nevertheless, some rare species are habitat-specific and only require small units of suitable habitat. They will consequently only be likely to be encountered when large units of land on the landscape scale are explored. The reverse happens when natural landscapes are destroyed through
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Ecosystems and Ecology logging, ploughing, draining wetlands and overgrazing rangelands. Every time that a natural area is reduced in size by a half, the number of plant species present will be reduced by 20 per cent.
Habitats with intermediate levels of plant biomass appear to have the largest number of plant species.
Species richness is low at a low plant biomass because of high levels of stress or disturbance and only a few types of plant can tolerate environmental extremes. However, the vast majority of freshwater wetlands occur in areas with a low plant biomass making some largely infertile sites significant to the conservation of plant diversity. This species-biomass ratio is valid in some other ecosystems and is generally known as the humped-back model.
Another plant diversity model states that the more different habitats there are in a region the greater will be the number of species. On the landscape level this implies that the more kinds of gradient there are, the greater the range of environmental conditions and the higher the plant diversity. The larger the number of extreme environments that are involved in the gradients the higher the total plant diversity is expected to be.
It is also known that some plants occur more commonly in some regions than others although this pattern is not universal and the mechanisms involved are still unknown. However, one generalization that is emerging is that a majority of the diversity within any one vegetation type is represented by rare plant species with a low frequency of occurrence. Since such rare plants are often not identified by observers it is further likely that any frequency of occurrence rates that are found will be underestimates.
It is well known that a few plant species will dominate in any particular habitat. Most of the remaining plant species occur in a matrix of dominant plant species because weaker competitors for the available environmental resources restrict the less abundant ones. The larger canopy-forming plants form a matrix of biomass in woodlands and forests while grasses are the potential dominant plants in herbaceous communities.
A final model involves a non-equilibrium approach when recurring disturbances remove biomass from the community and a suite of plant species exploit such gaps. Disturbances can be single events such a fire, or physical events such as an animal burrow or erosion and pollution. Biodiversity in this sense is then a product of the rates of disturbance and recovery. When any of or both these rates are altered then biodiversity also changes. However, the loss of plant species does not only follow disturbances. In prairie grasslands, for example, the loss of species-rich indigenous grasses follows long-term low-level nitrogen deposition. Such nitrogen deposition seems to be ecosystem-specific and requires the maintenance of mature and fully functional ecosystems.
The need to protect natural plant communities and restore them in degraded landscapes remains paramount. The broad principles which have been described above will allow it to be done properly and scientifically. The task that remains is to refine them and then to adapt them to local circumstances.
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Ecosystems and Ecology
In the south-western Kalahari ecosystem that was described in Section 5, large trees are scattered through sparse vegetation and mainly open dune landscape. These trees are focal points for animal activity because they supply birds and small rodents with nest sites, shade and food. Faeces, nest material and other detritus increase the sodium phosphate and potassium levels that are available in the soils to other plants that grow beneath the trees by 2.5 times in comparison with the surrounding shrubland. Such soils are conducive to the germination of plants with fleshy fruits which in turn are beneficial to avian frugivores. In savannas generally there is a minimum density of trees that is required per unit of land surface because the most palatable grasses only germinate in the shade of large trees.
Overabundant woody plants are, however, detrimental because the grass cover becomes depleted by competition for space and growth resources.
Elephant damage to a tree, Satara, Kruger National Park, South Africa - December 1982
In Australian farmlands, biodiversity is greatly enhanced by the presence of trees. With the current predicted decline by two-thirds in the number of trees there by 2100, the consequences will be a predicted concomitant bat and bird decline of up to 50 per cent. The ultimate consequences will be a reduction in economically important ecosystem benefits such as the provision of shade for livestock, wildlife and pest control. Many other species also depend on the presence of trees (or shade) and the removal of trees is likely to cause fundamental changes in biodiversity. Eventually some Australian
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Ecosystems and Ecology woodlands may suffer a regime shift and change to treeless grasslands much as how the Serengeti
Plains of East Africa were created.
Early humans have changed the pristine ecosystems of Australia immensely by causing the extinction of numerous types of megafauna and by changing fertile woodlands into monotonous shrubland with a depaupered biodiversity. The extinctions included the loss of a large flightless herbivorous bird
Genyormis newton whose diet and incubation partially depended on trees before woodland mosaics with an abundant grass layer disappeared through the use of fire for various reasons. Many of the extinct species of the megafauna were browsers and their overhunting and extinction in Australia may have triggered the expansion of shrublands.
There is no doubt that humans play a pivotal role in both the extinction and conservation of biodiversity on
Earth. As much as 45 000 years or more ago, early human migrants had already started the extinction of a rich Australian megafauna, and that of the Americas followed when the first humans arrived there from north-east Asia over the Beringia land bridge some 15 500 years ago.
Species-level conservation too often still tends to focus on species that are globally or regionally threatened with extinction. Nevertheless, common and widespread species are also of significant conservation concern because most of the recently threatened or extinct species were previously regarded as safe. Many of the common and wide-spread species are currently showing massive population declines. This will have major ramifications for ecosystem functioning and the factors that underlie these declines are likely to intensify in the future.
The natural dynamics of these threats and changes are still being understood poorly. However, it is already clear that the majority of these species are in a state of decline as a direct or indirect consequence of human activities. Moreover, when compared with the geological record, the current rates of global decline and extinction of species are greater than what would be otherwise expected. What is unknown, however, is for how long human impacts have been determinants of the conservation status of species. Nevertheless, virtually all the continents have lost the majority of their large-bodied terrestrial animals broadly after humans first moved out of Africa some 65 000 years ago.
In addition to overexploitation, the major threat of extinction lies in habitat degradation and loss, the introduction of exotic species and in extinction cascades. Habitat degradation and loss often acts in combination with overexploitation. It has been estimated conservatively that at least 20 to 25 per cent of the number of individual breeding birds in the world has been lost because of changes in land cover alone. In contrast, many bird species such as cattle egrets Bubulcus ibis , helmeted guineafowls Numida meleagris and queleas Quelea spp. have benefited from agriculture and have increased in abundance.
The impact of changes in land use can be extremely rapid particularly where species are concentrated geographically.
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Ecosystems and Ecology
The American chestnut tree Castanea dentata has the best documented history of decline following the introduction of alien species. Once abundant across much of eastern North America as an extremely hardy hardwood competitor it nearly became extinct following the accidental introduction of the pathogens
Phytophthora cinnamoni which causes root rot and Cryphonectria parasitica which causes chestnut blight. The entire remnant population still suffers from the effects of these two pathogens. Moreover, other
American tree species are also under threat of extinction from introduced alien pathogens.
Evidence of extinction cascades are more rare. However, the black-footed ferret Mustela nigripes that once inhabited the entire Great Plains region of North America has declined to near extinction following
European colonization. This happened largely because its main prey species, the black-tailed prairie dog
Cynomys ludovicianus, was nearly eradicated as an agricultural pest. Many common or widespread species that have declined to extinction will have had species-specific parasites that will also have become extinct unless they adapted to new hosts. The deliberate control and eradication of some pests and parasites and their vectors have already resulted in the decline of previously common and widespread species such as tsetse fly Glossina spp. in Africa.
It is not necessary for a species to be abundant and widespread to have significant consequences for ecosystem functioning and structure. The scale of reduction is equally important as is shown by the consequences of declines in herbivores on nutrient cycling and the effect of the loss of apex predators on populations of large herbivores. Moreover, declines with similar effects are still happening. Evidence is growing that changes in land cover are likely to be accompanied by the continued loss of biodiversity with attendant repercussions for numerous ecosystem functions and services. This matter requires urgent conservation attention through interdisciplinary and inter-institutional research programmes.
The effects of human activities on the composition and diversity of biological communities have been reviewed extensively by Hooper et.al
. (2005) (see reference below). According to these authors, these human activities and effects are increasing species invasion and extinction rates at all scales from local to global. Such changes can potentially alter ecosystem properties and the goods and services that they provide that are vital to humans. The functioning of ecosystems and their properties depend to a great degree on biodiversity. Especially important facets are the fundamental characteristics and abundance in space and time of the organisms that are present in an ecosystem.
The effects on species are a response to climate, resource availability and any disturbances that influence the properties of ecosystem. These can all be modified by human actions. While there is now broad scientific consensus on many facets of the relationship between biodiversity and ecosystem functioning, the next management step will be the integration of the knowledge about the biotic and abiotic controls of ecosystem properties, the structure of ecological communities, and the forces that drive the extinction of indigenous species and invasion by alien ones. Further integration of this ecological knowledge should be established within the social and economic constraints of potential ecosystem
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Ecosystems and Ecology management programmes. The ultimate solution seems to lie in understanding the complexity of ecosystem management while taking strong steps to minimize global biodiversity loss. Clearly this again requires interpersonal and inter- institutional cooperation.
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