Ecological Concept of Pest Management – II (2)

Recolonization of crops by insect pests

 

The theory of island biogeography is also applicable to the recolonization of crops by insect pests after hibernation or aestivation.

 

Example

 

The incidence of sugar beet mild yellow virus declined rapidly with increasing distance from storage pits for beets in which the aphid vectors Myzus persicae and Rhophalosiphoninus staphyleae over wintered and contracted the virus. This indicates the important role that distance from the source place play in colonization.

 

Phases in Colonization:

 

During the colonization process, once the species equilibrium has been reached, a rapid shift from the dominance of r – strategies in the community to the dominance of k – strategies takes place. Thus four phases on colonization can be recognized.

 

• The nonreactive phase occurs during colonization before equilibrium is established and may often lead to overshooting the equilibrium number.
• Later, as competition increases, because of increasing population densities, and as more species of predators and parasitoids arrive, biotic interactions in the community assume a more important role. This interactive phase brings the number of species to equilibrium.
• Given longer periods of time, there is an assortative phase in the community, essentialially a reshuffling of k – strategies, resulting in the selection of those species that con coexist most efficiently. This may allow the species equilibrium number to increase slowly.
• Given even longer time beyond the assortative phase, one can conceive of an evolutionary phase in community development in which species are not only sorted but undergo genetic change as a result of selection during coexistence, so that species become mutually adapted for living together.

 

Factors Determining the Number of Arthropod Species in a Community:

 

Number of arthropod species in a community is correlated with following major factors:

 

1. Plant species diversity
2. Plant structural diversity
3. Distance of community from source of colonists
4. Length of contemporary time available for colonization
5. Evolutionary time available for coevolution between herbivores and their host plants.

 

 

Ecological/Community Succession:

 

Ecological succession, a fundamental concept in ecology, refers to more-or-less predictable and orderly changes in the composition or structure of an ecological community. Succession may be initiated either by formation of new, unoccupied habitat (e.g., a lava flow or a severe landslide) or by some form of disturbance (e.g. fire, severe windthrow, logging) of an existing community. The former case is often referred to as primary succession, the latter as secondary succession.

Establishment of K-selected species in Community:

 

§  As the age of the community increases, condition tend to change from severe to equable, and favors for the establishment of the K- selected species rather than r-selected species.

 

Driving force for community succession:

 

The main driving forces/ factor for the community succession are the following:

 

Force/Factor 1. Plant species that colonizes an area actually changes that area and thus make it more suitable for other colonists;

 

Force/ Factor 2. Different species of colonists arrive at different times.

 

 

Influence of Plant on the Development of Succeeding Community

 

 

The first factor can again be broken into four main influences that plants have on the development of the succeeding community.

 

 

1.            They create more shade and thus ameliorate the microclimate, making it possible for the shade tolerant species to colonize.

2.      They contribute organic matter, which changes soil texture and nutrient status;

3.            They produce chemicals ( secondary metabolic compounds) that may be toxic to other members of their own species or to other plant species; and

4.            They attract animals, including insects that change factors in the environment by burrowing in soil, leaving excrement, selectively eating plants, dispersing seeds, pollinating flowers and attracting their own predators and parasites.

 

The second factor: Different species of colonies arrive at different times:

 

§  Plant species with tiny, wind-dispersed seed arrive very early

§    Eventually enough large seeds arrive to outcompete the resident species and   succession results

§    Similar trends may be seen in the insect species that colonize an area, although insect size does not correlate so well with competitive ability.

 

 

Synoptic model:

 

The concepts of ecological succession, habitat stability, r- and k- selected and population dynamics have been nicely integrated into synoptic model by Southward (1975) and Southwood and Comins (1976).

 

 

As a very general conclusion of this chapter, agricultural ecosystem can be viewed in terms of two central concepts of ecology- island biogeographical theory and the succession of community.

(.......... end)

 Bismark Bangali

BSC  in Agrotechnology 

 

 

 

Khulna University.