Agrostate: Ecological Concept of Pest Management

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)

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Bismark Bangali

BSC in Agrotechnology

Khulna University.

Agrostate