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.

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Ecological Concept of Pest Management – II(1)

Colonization of Island/Theory of Island Biogeography:

 

• The theory of island biogeography explains the process of how colonization in island take place
• Annual crops may be regarded as islands that may have a life span of as little as one growing season and may be colonized by arthropods from surrounding area.

 

 

Species equilibrium in community

 

• Each island or each community has its own limit for species of arthropods, and once this has been reached, an equilibrium exists.
• The above equilibrium is not static, but dynamic, since new species may arrive and resident species become extinct. However, the number of species remains at equilibrium.
• Thus the equilibrium number clearly depends on the immigration rate of arthropods into the community and the rate of loss from the community by emigration and extinction.

 

 

Rate of Species immigration and extinction, their relationship and factors

 

• Naturally the rate at which new species become established is rapid at first, when many niches are vacant, but declines as the niches are filled.
• In the early stages of colonization most of the species that arrive are new to the island; the rate is rapid because the species are capable of rapid dispersal, while those arriving later are slow disperser. These factors combine to produce a concave immigration curve. The extinction curve is also concave, because the more the species, the more is the chance of becoming extinct, principally because of competition and predation as shown in the figure.

 

 

					P = Total number in the species pool on the mainland   Ŝ = equilibrium number of species at the intersection of the curves of the rates of immigration and the rates of extinction.   I = Initial rate of immigration

 

 

	Number of Species Present

 

Conditions setting the limits of the immigration curve:

 

• The total number of species on the mainland that are available as colonists. i.e., the species pool P.
• The initial rate of immigration I, which depends on P, and
• The distance of the island from the mainland

 

 

Influence of distance and size of islands on immigration and extinction

 

• Immigration to near island is rapid, because the chances of an individual leaving the mainland and reaching the island are high;
• Immigration to a distant island is slow, as the chances are greatly reduced.
• Thus we can develop a set of immigration curves that differ according to the distance of the island from the mainland.
• Extinction rates on islands also differ, not with the distance of the islands from the mainland, but with the size of the island. Naturally, as island size increases, the resource base for species increases, as both species diversity and structural complexity increase. Thus extinction rates are likely to be lower on larger islands and can be expressed by a series of curves as shown in figure.

 

 

			Ŝ = equilibrium number of species at the intersection of the curves of the rates of immigration and the rates of extinction.

 

 

 

	Number of Species Present

 

 

 

Types of Colonizer:

 

• Similar to the plants, the insects can be divided into good colonizers (r – strategists), i.e., those which have high dispersability and low competitive ability and good competitors (k – strategies) i.e., those which have low dispersiability and high competitive ability.

 

• Many pests of crops are  r – strategists. They are small and can be dispersed by wind and are often present in crops soon after germination of host plant (e.g., aphids, thrips)

 

• But the predators and parasitoids are k – strategists. They are slower in colonizing vacant sites. There is no selective pressure for them to colonize early, as they would encounter food shortage. This leads to pest outbreaks early in the season or after the insecticides application (because insecticides kill both pests and natural enemies)

(....continue)

 Bismark Bangali

BSC  in Agrotechnology 

 

 

 

Khulna University. 

Read more...