Ecological Concept of Pest Management – II

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.