16
2 Genetic Resources: Status, Development, Losses, and Conservation
2.2.1
Review and Critical Analysis of the Methodologies of Recording of
Losses
Because of the significant differences between plant biodiversity in general and
agrobiodiversity (see Chap. 2.3), the decline in general plant biodiversity and its
determinants will be discussed very briefly, before the reduction of agrobiodiversity and its underlying causes are analyzed in more detail.
2.2.1.1
Genetic Reduction in Biodiversity and its Determinants
The physical decline and extinction of plant species, which causes genetic losses
as well as the transformation of plant species over evolutionary time, is a natural
process. On the one hand, the history of extinction in vascular plants shows some
major catastrophes, which had a significant influence on the structure and
composition of the vegetation and the extinction of species (Knoll, 1984). On the
other hand, the process of plant extinction is characterized by competitive
displacement by plants more able to adapt and the gradual adaptation of
environmental changes (Groombridge, 1992).
The primary causes for extinction of the plant species in recent history are the
human-induced habitat modification or fragmentation and habitat destruction (see
Fig. 2.2). Furthermore, other human activities may have either a positive or
negative impact on the existing biodiversity. The import of some new species into
a habitat may enrich biodiversity, if the exotic species finds a niche, or it may
reduce biodiversity if it has comparative advantages over traditional species.
In addition to these deterministic processes, stochastic processes affect the
popUlation dynamics of species as well. According to Schaffer (1987), stochastic
processes are determined by (1) demographic uncertainty as a result of random
events in the reproduction and survival of individuals; (2) genetic uncertainty as
random changes in genetic make-up; (3) environmental uncertainty caused by
unpredictable changes in weather, pests, and nutrition supply; and (4) natural
catastrophes, e.g., fires, floods, and droughts. The impact of these stochastic
processes on the level of biodiversity in a specific habitat does not always imply a
decline in biodiversity. The four underlying factors may as well maintain or even
improve the biodiversity. For instance, forest fires improve the regenerative
ability of the sequoia trees in California or enable the germination of certain plant
species which have been suppressed by other dominating plant species.
In addition to the direct impact of human intervention, the interaction of human
intervention with the stochastic processes is causing an over-proportional loss of
species. This process of isolation and fragmentation of populations leads to an
accelerated and over-proportional extinction of species (Schaffer, 1987).
Difficulties in recording extinction rates of plants are caused by the lack of
documentation of existing species at a given time in the past. It is difficult to
unequivocally demonstrate that extinction has occurred, because many species
may continue to persist for years in the seeds before germinating (Jenkins, 1992).
2 Genetic Resources: Status, Development, Losses, and Conservation
2.2.1
Review and Critical Analysis of the Methodologies of Recording of
Losses
Because of the significant differences between plant biodiversity in general and
agrobiodiversity (see Chap. 2.3), the decline in general plant biodiversity and its
determinants will be discussed very briefly, before the reduction of agrobiodiversity and its underlying causes are analyzed in more detail.
2.2.1.1
Genetic Reduction in Biodiversity and its Determinants
The physical decline and extinction of plant species, which causes genetic losses
as well as the transformation of plant species over evolutionary time, is a natural
process. On the one hand, the history of extinction in vascular plants shows some
major catastrophes, which had a significant influence on the structure and
composition of the vegetation and the extinction of species (Knoll, 1984). On the
other hand, the process of plant extinction is characterized by competitive
displacement by plants more able to adapt and the gradual adaptation of
environmental changes (Groombridge, 1992).
The primary causes for extinction of the plant species in recent history are the
human-induced habitat modification or fragmentation and habitat destruction (see
Fig. 2.2). Furthermore, other human activities may have either a positive or
negative impact on the existing biodiversity. The import of some new species into
a habitat may enrich biodiversity, if the exotic species finds a niche, or it may
reduce biodiversity if it has comparative advantages over traditional species.
In addition to these deterministic processes, stochastic processes affect the
popUlation dynamics of species as well. According to Schaffer (1987), stochastic
processes are determined by (1) demographic uncertainty as a result of random
events in the reproduction and survival of individuals; (2) genetic uncertainty as
random changes in genetic make-up; (3) environmental uncertainty caused by
unpredictable changes in weather, pests, and nutrition supply; and (4) natural
catastrophes, e.g., fires, floods, and droughts. The impact of these stochastic
processes on the level of biodiversity in a specific habitat does not always imply a
decline in biodiversity. The four underlying factors may as well maintain or even
improve the biodiversity. For instance, forest fires improve the regenerative
ability of the sequoia trees in California or enable the germination of certain plant
species which have been suppressed by other dominating plant species.
In addition to the direct impact of human intervention, the interaction of human
intervention with the stochastic processes is causing an over-proportional loss of
species. This process of isolation and fragmentation of populations leads to an
accelerated and over-proportional extinction of species (Schaffer, 1987).
Difficulties in recording extinction rates of plants are caused by the lack of
documentation of existing species at a given time in the past. It is difficult to
unequivocally demonstrate that extinction has occurred, because many species
may continue to persist for years in the seeds before germinating (Jenkins, 1992).