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Vol. 12 No. 3 - July 2006

Keystone Species: The Concept, their Ecological

Significance and Determining their Keystone Status

1R.S. Tripathi and 2P. Law

Keystone Species and their Ecological Significance

The concept of keystone species was first introduced by Paine in 1966 while establishing the importance of predation in maintenance of the prey species diversity in the rocky inter-tidal zone of pacific coast in North America. Paine stated that 'keystone species' were the species whose activity and abundance determine the integrity of the community and its unaltered persistence through time i.e. its stability. According to De Levo and Levin (1997), keystone species are individual species or groups of species whose removal from the ecosystem may result in dramatic changes in the structure and functioning of that ecosystem.

The keystone species play a central and critical role in maintenance of community structure and ecosystem functioning. If an ecosystem can be returned to a state in which the keystone species flourish, then all the other species, which depend on them, will also flourish. The importance of biodiversity in environmental management beside socioeconomic development and well being of human society, has led to the development of various techniques for conservation of floristic and ecological diversity. Some simple ways of managing the natural systems should be evolved so as to retain and conserve the identity of a landscape or region for a better tomorrow. One of the simplest ways of doing so is by identifying species, which play the key role of holding together the entire biological community or ecosystem. These species are known as 'keystone species' in ecological term.

The central core of keystone concept is that only a few species have uniquely important effect on the community or ecosystem by virtue of their uniquely important traits and attributes. Only those species can be considered as keystone species that had a significant effect on 'time window' of other species. For example, changes in climate may differentially affect the growth rate of emergent species in a forest, which in turn could affect other species. In most of the cases, it is indeed groups of species rather than individual species that assume importance and these species groups could be referred to as the 'keystone groups' or 'functional groups'. Keystone species or 'keystone species groups' play a vital role in maintaining ecosystem and regulating the biodiversity. Loss of vital function, and changes within the ecosystem or community would follow if such species groups are removed from the system. These species are 'responsible' for the existence of an ecosystem of certain type and create possibilities for the development of other types of communities. Biodiversity within an area can be characterized by measures of species richness, species diversity, taxic diversity, and functional diversity, each highlighting different perspectives. Functional diversity refers to the varieties of functions carried out by different species and groups of species known as functional groups. According to Smirnova (1998), there is a correlation between structural and taxonomic diversity. The maximum taxonomic diversity could be expected in a climax landscape, which develops due to the structural diversity of population mosaic produced by all key species of the biota and the spatial and temporal heterogeneity of these mosaics. The population dynamics of keystone species define the pattern of succession of vegetation. Turnover cycles of matter and energy flows in an ecosystem are dominated by the life activity of keystone species, and these activities determine the major shifts in ecosystem structure at the spatial and temporal scales. Population mosaics of keystone species have largest spatial-temporal dimensions, and population mosaics of subordinate species are thereby determined by the keystone species. Keystone species are responsible for the existence of the ecosystem and maintenance of its species diversity. So the biodiversity in any ecosystem can be manipulated by perturbations in such uniquely important species.

In recent years the overly expansive usage of the 'keystone species' concept has led to redefinition of the term. According to current interpretation, keystone species are only those species, which have a large disproportionate effect due to their greater biomass and/or abundance in the communities in which they occur. Moreover, those species, which drive ecosystem processes or energy flows, are generally referred to as 'key species', but only a few of them are 'keystone species'. It needs to be emphasized that the term 'keystone species' should be applied to those species whose role in nature includes the potential to affect the abundance of other competitively dominant species. A major research challenge for ecologists is to predict which species in the community would become keystone species.

Keystone species differ from one ecosystem to the other in time and space. The structural organization and function of ecosystem will alter when keystone species disappear for some reason or when a new and stronger keystone species comes up. It needs to be underlined that keystone species are only those species whose populations either support or essentially alter the main vegetation pattern of the ecosystem. Under such comprehension in a forest ecosystem only trees can be considered as the keystone species.

Trees are the key species that drive the system in many tropical and temperate forest ecosystems, and out of these key species, only few species or groups of species are keystone species, which play a crucial role in the maintenance of ecosystem stability through their keystone roles and functions.

In forest ecosystem it is rather difficult to ascertain the effect of species upon each other, and in many cases dominant species seem to be driving the whole community. Amongst the dominant species again there is a wide degree of differences in relation to their contribution toward the community make-up. One of the possible ways of characterizing keystone species in the forest ecosystem is through the competitiveness of the species along the successional gradient and focusing on their role, which supports or contributes towards maintaining an existing type of vegetation.

The keystone species of the temperate zone are trees, which create and support forest communities, as well as pathogens and herbivores that destroy forest communities and create possibilities for the development of other types of communities. The desired level of solar energy, water and nutrient are the most essential resources for the existence of organisms in a given community. Thus the term keystone species can be applied to those species, which directly or indirectly affect energy flows, and hydrological and nutrient cycling and recycling, Based on this viewpoint, trees can be considered as keystone species of forest ecosystem; they are responsible for the existing vegetation playing a role of habitat modifier by manipulating nutrient status, water availability, and light gradient of the system. Trees also suppress light demanding species and help the shade-tolerant species to successfully colonize the area. Thus the canopy trees can be considered as keystone species in forest ecosystem.

The current level of conceptual understanding of the effects of biodiversity on ecosystem processes is so primitive that at this stage it is possible to recognize the linkages at the level of functional groups only. In any ecosystem there are diverse types of functions performed by different species or species groups. However, no two species or individuals are identical. It may be noted that species diversity within the functional groups or genetic diversity within the species has important ecosystem consequences.

Although certain species have much greater influence than others on an ecosystem structure, not all ecosystems include the same species that exert such pervasive influence on them. In fact most ecosystems are somewhat sensitive to the loss of a few species, though some losses have greater impact on the system than others. Nevertheless, identification of such species, which would function as keystone species in an ecosystem can help in the conservation of that ecosystem. The fact that some species matter more than the others, becomes especially clear in the case of 'keystone species' or 'ecosystem engineers' or 'organisms with high importance value for the community'. These terms may differ in usage, but all refer to those species whose loss or removal results in disproportionately greater impact on the community when compared to the loss of other species. Members of the functional groups maintain and determine the resilience of the ecosystem by spreading a wide range of ecological niches exploited by the component species.

The contribution of individual species toward ecosystem development varies in time and space, and accordingly, not all species are equally important when we look at the community stability and functioning. The community function may be maintained by a species or summed effects of a few species. Some species undoubtedly play more significant role than others in ecosystem function. The varieties of functions that a species can perform are limited and consequently, an increase in species richness also increases functional diversity, producing an increase in ecological stability.

Within a community it is not possible to substitute species for one another, rather there are a good number of combinations of species that can produce similar ecological roles. There is no intrinsically unique level at which biotic diversity affects ecosystem processes. Based upon their ecological roles and the specific ecological niches that they exploit, species can be divided into 'functional groups'. A functional group refers to a group of species, which perform ecologically similar roles in ecosystem processes.

Biodiversity versus ecosystem stability and functioning has been a matter of academic concern for sometime now. It is only recently that attempts have been made to clearly delimit the role of biodiversity in ecosystem function, and the role of individual species in community stability and its functioning. The relationship between biodiversity and functioning of ecosystems is far more complex and only fractionally understood. To determine the ecological importance of biodiversity one must focus attention on the aspects of biodiversity that control resilience, i.e. the ability of the system to maintain its characteristic pattern and rates of processes in response to the variability inherent in its climate regime. A species may regulate biogeochemical cycles, alter disturbance regimes, or modify physical environments, and thereby it could be assumed that biodiversity has, a direct link with ecosystem stability.

A community is an entity composed of functionally interdependent species. The internal structure of a community is made up of important functional groups of species. Each of these species occupies what we call as ecological niche that depicts the ecological conditions in which the species occurs and determines its functional position within the community. When several species have similar niches, and also similar function they form together a functional group. A few or several of such species have a critical functional role that has a direct or indirect effect on the stability and survival of the community. Such s species o or t the g groups o of s species a are certainly the 'keystone species'. The loss of such species would result in widespread changes in the community structure and function, and may often lead to species loss or elimination.

Determining the Keystone Status of Species

All species are important for the existence of an ecosystem and for the maintenance of its various functions, but as mentioned earlier, all are not equally important. The identification of species and groups of species, which play key role in maintaining the ecosystem stability and resilience by influencing the structure and function of an ecosystem is a stupendous task, and very few attempts have been made in this direction.

Communities viewed in terms of functional groupings in general prove to be much more stable than when viewed in terms of species composition. In defining ecosystem or plant communities it is difficult to separate the effects of human and natural disturbance. The dynamic view of communities defines the complexity of characteristics of the community by emphasizing on its spatial heterogeneity and a non-linear causation. The community and ecosystem can be understood and managed better when the species are grouped to the degree possible into classes that possess similar characteristics and behaviours. This can be possible through a detailed study of community structure, which will also provide a better understanding of species distribution and their status in natural ecosystems. Here, it may be mentioned that the grouping of plants in different life forms by Raunkiaer was one of the most widely accepted functional groups classifications.

Depending on the focus of study some may find the floristic composition most important, while others may find the persistence of certain processes and functions (such as biomass production, nutrient cycling, evapotranspiration, and energy flows) more important in the maintenance of the community. However, advocating the integration of both community dynamics and structural organization of the ecosystem with its functional processes appears to be more reasonable. Functional grouping of the components of vegetation can be done adopting various criteria such as phytosociological associations, life form, overall morphology position in the canopy, phenorhythmics or phenology, biomass and nutrient partitioning, structure of organs such as leaves or roots and the physiological characteristics. In many ecosystems dominant species seem to drive the system and play a critical role. These species could be considered as keystone species. Studies of their ecology, spatial distribution and their relationship with other components of ecosystem are important from functional perspective.

The question as to how species diversity is maintained in natural communities, and which species or group of species plays more important role in influencing ecosystem functions, has been agitating the minds of many ecologists during the past two decades. Several investigations were carried out to answer this and related questions in the tropical and subtropical forests in various parts of the world. Sustainable forest management requires a knowledge of not only the species composition but also of the functional relations due to which these floristic components are existing in the forest ecosystem. There are many ways that help to quantify the functions of different species and their drastic effects when they are eliminated from their natural habitat or ecosystem. Species such as 'indicators', 'ecosystem engineers' etc. were identified by different researchers to see the effects of species or group of species on certain ecological processes.

The keystone group includes those species or group of species, which greatly influence the presence or absence of other species. In the animal kingdom quite a few of these species have been identified and listed from different ecosystems, however, only a few examples are available from forest community. Some species, which contain high levels of nitrogen, phosphorus and potassium in their leaf tissue in spite of their growth in a highly infertile soil serve as keystone species in terms of nutrient conservation in some protected forest ecosystems of Meghalaya.

Generally, under the climatic conditions of India, the outcome of succession is the dominance of a plant community by a few tall long-lived tree species with a few understorey species surviving within their shade. The intermediate disturbance hypothesis recognized the keystone role of climax species in suppressing community diversity due to gradual reduction of species number as succession proceeds. The other extreme is the elimination or exclusion of longlived, potentially dominant trees and shrubs due to suppression of their seedlings by certain fast growing weedy herbs and shrubs, which serve as keystone species in this kind of situation. It has been shown by several researchers that in high light and productive environment fast growing herbs and shrubs can quickly suppress the slower growing tree seedlings. There is also some evidence to suggest that the elimination of keystone weeds can lead to the invasion by tree species. In the successional forests of north-eastern hill region of India a number of species have been categorized as keystone species. Species like Alnus nepalensis can play a very important role in nutrient cycling through nitrogen fixation, while many bamboo species (Dendrocalamus hamiltonii, Bambusa tulda and Bambusa khasiana) play a keystone role in conservation of nutrients like nitrogen, phosphorus and potassium in jhum fallows in northeast India. Similarly, the invasion of introduced nitrogen fixing shrub Myrica faya in Hawaiian Island has been shown to produce large-scale effects on nitrogen cycling by greatly increasing the amount of this essential plant nutrient in soil.

Some keystone plant species have been identified based on the phenological attributes and their 'mobile links'. These keystone mutualists are typically plants that provide criterion support to large complexity of 'mobile links' (mobile links are animal pollinators and dispersers on which plants depend for seed production and propagation). During periods of resources scarcity, certain plant parts/products dubbed 'keystone resources', assume major importance as mainstay of primary consumers. In many tropical forests there seem to be a very few plants that regularly produce edible reproductive structure (fruits, seeds, flowers) during the period of minimum fruit availability. Such plants can certainly be referred to as the keystone species since they sustain the myriads of primary consumers including pollinators and dispersers. In view of the above considerations, the identification of keystone species and study of their population dynamics in forest ecosystem are important for biodiversity manipulation and management as well as for the maintenance of the forest.

During the course of ecological studies on the sacred groves of Meghalaya, the senior author and his collaborators at NEHU, Shillong have identified keystone species and keystone groups based on their phytosociological characteristics such as IVI, position in the canopy, growth form/life form, important association with other species (e.g. epiphytic growth), phenological associations linked with phenological behaviour of plants (such as 'mobile links', food resources availability for primary consumers), successional characteristics such as light demanding or shade-tolerant nature of species, biomass and nutrient allocation pattern in different plant parts, shoot/root ratio of seedlings, and their role in nutrient cycling (litter quality, nutrient release, and nitrogen fixation). The pioneering researches made in this direction by the Ecology Group at the North-Eastern Hill University, Shillong need to be further intensified.

1INSA Senior Scientist, National Botanical Research Institute, Rana Pratap Marg, Lucknow-226 001, India (formerly Professor of Botany, North-Eastern Hill University, Shillong, India)

2Department of Botany, Lady Keane College, Shillong, India

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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