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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 |
