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Population Invasion - A Threat to Ecosystem
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Objectives:

 

  • To study the basic concept of pest population invasion and its impacts on the ecosystem.
  • To understand the concept, "spread of pest population" using mathematical simulator.
     

 

Theory:

 

Invasion ecology has grown to become an important multi-disciplinary subfield of ecology with growing links to many other disciplines. This covers all the aspects relating to the introduction of organisms to an ecosystem, their ability to establish in the ecosystem, invading the target region, and the interactions with the resident organisms in the new ecosystem location. When a population is surrounded by favorable habitat, it will spread outward until it occupies the entire available habitat in the ecosystem. Such an appearance of population in a new area is known Population invasion. Invasive species is simply defined as non-indigenous species that threatens the diversity or abundance of native species. They disrupt a particular habitat or wild land area by loss of natural controls through their dominant colonization. The examples include Asian long horned beetle, freshwater mussel, sea lamprey and rusty crayfish (Figure 1), which explain the concept of population invasion . The rate at which the population spreads depends on the rate of population growth and percentage of the population that disperses at a fixed time period.

 

                   Figure 1:   a)  Freshwater mussel                            b)  Sea lamprey                              c)  Rusty crayfish

 

Image source:en.wikipedia.org/wiki/

 

 

Pests are likely to get introduced into an area through transportation and suitable environmental conditions and they start to exploit the resources and spread at a very fast pace. They cause the displacement of the native species of that region due to the competition for the natural resources and food. Sometimes, the displacement of this native species can also be due to the aggressive nature of the pest population making them to abandon their shelters. It also causes the explosion of the natural resources, for eg: the Asian long-horned beetle that originated from China has been known to cause damage to the maple wood trees of North America.  Generally, when an invasive species is introduced into a new area through some transportation media, it starts to feed on a variety of species being one of its traits i.e., opportunistic feeding. It also breeds at a very fast pace being able to get multiplied in huge numbers in a short generation time. It is also difficult to identify their existence as in the case of the larvae of the Asian long-horned beetle.

 

Case Studies on Pest Invasion:

 

Invasion of Asian long-horned beetle

 

Considering the specific case of the Asian long-horned beetle (Figure 2), the concept of pest population invasion can be explained in a better way. The Asian long-horned beetle commonly referred to as “Starry Sky” or “Sky Beetle”, originated mainly from china and the Far East regions. It was known to have spread to North America through the wood packing material transported from China in the 1980’s.

 

Figure 2: Asian long-horned beetle

 

Image source: en.wikipedia.org/wiki/

 

This pest is a serious threat to many species of deciduous hardwood trees. It is known to bore deep into a tree’s heartwood, where it feeds on the tree’s nutrients eventually leading to the death of the tree due to its tunneling (Figure 3). They are known to make 35 to 90 individual depressions into the host tree’s bark and lay an egg in each of the pits. The eggs hatch in 10 – 15 days and the white caterpillar-like larvae tunnels into the tree’s phloem and cambium layers beneath the tree bark. After several weeks, the larvae tunnel deeper in the tree’s heartwood where they mature into pupae. The pupae hatch into adults inside the tree over the winter months. It was found out that period of transition from the egg to the larval stage is a prolonged one, during which the organism remains virtually invisible. During this period, it is difficult to detect it and it is easily and inadvertently transported by human activity, especially when using the wood packing materials. Once delivered to a potential colonization site in the temperate zone of North America, a ready supply of host arboreal species is virtually guaranteed. It is often very difficult to find the beetle's hidden passage except for the traces left out like egg deposit borings of emergent adults.

 

 

 Figure 3: Asian long-horned beetle tree tunneling

 

 Image source: en.wikipedia.org/wiki/

 

 

In the United States, Asian long horned beetle has no known natural predator, for any stage of its life cycle. It has been identified that only limited biotic and abiotic factors within the temperate zone of North America to limit the potential. The beetle attacks many different types of hardwood trees, including maple (Norway, sugar, silver, and red), birch, horse chestnut, poplar, willow, elm, ash and black locust. Unlike most cerambycids (long horned beetles) inhabiting the temperate zone, the Asian long horned beetle feeds on both healthy and weakened trees as opposed to the recently dead or dying wood. In addition, unlike most wood-feeding insects inhabiting a living host, the Asian long horned beetle is polyphagous (feeding on many kinds of food) as opposed to monophagous (feeding on one kind of food), feeding on a wide variety of host species. Females oviposit at different locations from exposed roots to the trunks except the smallest branches of the host trees allowing a high concentration of infestation of a single host organism. Asian long horned beetle larvae consumes healthy bark and xylum, making tunnels into the healthy tree which start horizontally and turn upwards to a length of about 10 cm. In great numbers this destruction of the healthy heartwood of the tree first reduces the strength of the host organism and with repeated or continued infestation can lead to its death.

 

 Invasion of Sea lamprey                          

 

Petromyzon marinu, the sea lamprey, is present in the Great Lakes of North America. This is a North Atlantic river-running species living in the sea and spawning in the stream waters. But eventually it had established itself in the Lake Ontario and some other lakes in the New York State. Later the creature establishes naturally in the St Clair River, Lake Michigan, where they began to establish spawning runs in the streams flowing to these lakes. During this natural phenomenon, sea lamphery attacks lake trout (Salvelinus namaycush), which is a species of great commercial importance.  The sea lamprey is a combination of hunting predator and an ectoparasite which hangs to a fish and secretes an anticoagulant and some lytic fluids into its wounds, and rasps. They suck the flesh and a juice from the fish until the fish is dead (Figure 4). The population statistics of lake trout fishery since 1889 have been thoroughly analyzed for ten years after the lamprey invasion began to take effect, the numbers of lake trout taken in the American waters of Lake Huron and Lake Michigan fell from 8,600,000 pounds to only 26,000 pounds.

 

Figure 4: Sea lamprey wounds on a Chinook salmon, Garden River First Nation

 

 Image source: en.wikipedia.org/wiki/

 

 Invasion of Brontispa longissima

 

Brontispa longissima, otherwise known as the coconut hispine beetle (Figure 5), is one of the potential insect pests of coconut and other ornamental palm plant in the Southeast Asia. The pest is known to be endemic to Indonesia and Papua New Guinea and was recently introduced into the continental Southeast Asian countries.

 

Figure 5: Brontispa longissima


Both the larvae and adult beetle feed on the unopened leaf buds of the coconut palm, this invasion changes turns the leaves into brown and finally decreases the fruit production. Such defoliation will ultimately lead to the death of the invaded tree. Almost seventeen species of palm trees including nipa palm, oil palm and many ornamentals are attacked by this pest. The pest was found in the Mekong Delta of Vietnam and Maldives by late 1990's accidentally with a shipment of ornamentals. Later in 2004, the pest invaded to Myanmar followed by the Philippines in 2005. The increasing number of coconut industries in these countries makes the pest invasion a more critical catastrophic effect in these regions. One of the most promising biological control mechanisms for coconut hispine beetle is the larval parasitoid Asecodes hispinarum, a natural enemy to the pest. This parasitoid was successfully introduced into Samoa in the early 1980's to control this pest invasion in that area (Voegele et al. 1989).

 

Impacts of Invasive populations in the Ecosystem:

 

A successful invader in the ecosystem can in many cases inflict enormous damages to the ecosystem. The invaders alter fundamental physical features of ecosystem such as natural resources, nutrient cycling, and plant productivity and also to human health and productivity.

 

  • Disrupt the natural communities and other ecological processes. This will ultimately harm the native species in the ecosystem.
  • Species competition for the same natural resources such as food, water, space and shelter and thus disrupts the food chain. (Example: The brown tree snake in Guam).
  • Degrade the aquatic habitats and clog waterways (Example: Giant salvinia, in the south entirely cover the water bodies, thus prevents the sunlight from getting to the aquatic plant and phytoplankton and restricts the new oxygen from entering the water bodies.
  • Invasive species act as hosts for other damaging organisms (Example: Buckthorn is an invasive shrub that also carries oat rust that damages oat crops).
  • For humans, it act as a vector (carriers) for diseases (invasive snail carries schistosomiasis)
     

 

How to overcome the Population Invasion?

 

The best way to limit the impacts of invasive species is its prevention. Once an invasive species are identified it is so important to halt their intentional or unintentional spread in the ecosystem. Regular monitoring of the invasive species in conjunction with control methods is critical for overcoming the effects of the population invasion. Control of the pest with chemical agents such as pesticides, herbicides, or fungicides can effectively kill invasive species in the ecosystem, but later it become problematic to other organisms in the ecosystem including the humans and animals.  Also the prolonged use of chemicals will develop resistance mechanisms in the target species. Mechanical control is the method of changing the habitat condition to physically remove the invading species from the ecosystem. The habitat can be modified through the alteration of fire or flooding regimens, but it can be can be more expensive and labor intensive.  Biological control mechanism means introducing a natural enemy, either a predator or a parasite, generally from the invader’s native range is a critical method to control an invasive species that is established in a particular area of the ecosystem. This is the most eco friendly control mechanism for pest invasions with minimal expense, but on the other side, some control agents were not able to survive and some agents attacks the non-target native organisms of the ecosystem.

 

 

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