The Effects of Trophic Cascades on the Biotic Interactions of Ecological Webs

All organisms have relationships to other organisms, whether it's a bird consuming a grasshopper or fungi receiving nutrients from decaying organic matter. Scientists use ecological webs to keep track of these relationships and depict cascading, direct and indirect biotic interactions between organisms in ecosystems. Cascading interactions are when feeding relationships affect abundance of populations which affects the next two trophic levels. Populations of producers, consumers, and decomposers are limited by their respective resources in a density-dependant fashion that cause these relationships between species to become fragile (HSS 1960).

Trophic cascades are also more likely to occur within higher trophic levels due to the fact that many herbivores and photosynthesizing organisms do not need to compete for food as much as carnivores (HSS 1960). Due to the multiple interactions and varying intensities of relationships that a species has, if a species population was significantly changed, it would cause a serious imbalance and lead to changes within the ecosystem.

Scientists observe and collect data in different ecosystems where trophic cascades are occurring to learn more about the fragility of these interactions and their effects on the rest of the ecosystem.

The objective of this essay is to explore and synthesize three different scientific journals that focus on trophic cascades and their effects on ecosystems. Three different experiments have shed light on the drastic effects that trophic cascades can cause on the ecological web and environment. The three trophic cascade scenarios that will be analyzed include a study on the introduction of arctic foxes into an ecosystem, a study on the effects of fish presence/absence in pond ecosystem webs, and a study on intraguild predations effects on a trophic cascade.

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Croll et al. in 2005 conducted a study on the many subarctic islands in Alaska about how the introduction of arctic foxes affected every trophic level and changed the community structure. Humans had introduced the foxes to the islands to increase fur production which eventually changed the abundance and composition of plant communities on the islands through the occurrence of a trophic cascade (Croll et al 2005). To complete this experiment, 12-32 grids were set up that were 30m by 30m and the species found within those grids were recorded. These grids were surveyed annually for three years in August along with an annual soil sample that was analyzed. The results unveiled the key ecosystem web relationships that changed in this metapopulation after the introduction of the arctic fox. The arctic fox consumes the native passerine bird and song sparrow which lead to the decrease of these populations. The decline in bird species reduced the amount of nutrient transport as well to the plant community. This decrease in population led to the increase of the prey that the birds would eat which included the mollusk and arachnid. The dipteran consumes the grass shrub and due to the decrease of the song sparrow, who preyed on the dipteran, the population increased leading to more grass shrubs being consumed. There were a lack of grass shrubs, bird species, dipterans, molluscs and arachnids overall in the islands that were fox infected. This is an example of a trophic cascade and how the introduction of a species can interrupt and disturb an ecological food web (Croll et al 2005). This metapopulation was perfect for observation due to the size, isolation, and the smaller amounts of dispersal on islands.

Knight et al. conducted a study on the effects of fish presence in ponds and the trophic cascades that occurred due to the presence of this key predator in the ecosystem web. There were 18 different ponds with and without fish at the University of Florida where fish, dragonflies, pollinators, and plant demographics were measured and monitored. The study found that fish consumes dragonfly larvae and so the ponds without fish had a higher dragonfly population (Knight et al 2005). Dragonflies consumes pollinators including bees, wasps, flies and other insects so with higher populations of dragonflies, the pollinators populations would decrease. Low population levels of pollinators then leads to lower population levels of plants within the ecosystem. With fish as a present top predator in the ponds, there was a lower population level of dragonflies, higher population of pollinators, and higher population of plants within the ecosystem. These two ecosystems change dramatically with the presence and absence of only one species (Knight et al 2005). Fish are an important link in the food web of the pond ecosystem and in the abundance of the plant populations of the pond which are crucial in determining pond health. The link between consumers and producers is very strong even though they are not directly connected through intraspecific relationships.

Finke and Denno conducted a study observing predator diversity and the effects of food web complexity on trophic cascades. This study was conducted through a field enclosure in a marsh ecosystem in New Jersey and focused on arthropods, spartina, and planthoppers. Planthopper densities were reduced by Tytthus predation when ended up increasing the population of spartina plants in the marsh ecosystem. Multiple species of spiders would eat the planthoppers along with other species of spiders. The decline in planthoppers lead to a lower amount of predation on the tillers. The planthopper density was not changed when there were predators and when there was not due to the lack of predator pressure from the intraguild predation in the arachnid class (predation on potential competitors). This indicates that the more predator diversity there is, the less intense a trophic cascade will be and the less likely it will be for a trophic cascade to occur (Finke, Denno 2004). Species richness and diversity are important in maintaining a healthy and balanced ecosystem.

These three studies prove that even one species can affect the entire ecosystem web greatly when there is an imbalance in the web equilibrium. Species evenness and richness is extremely important to every trophic level and the removal or placement of these species can cause great changes (Finke, Denno 2004). Human intervention and disruption within ecosystem webs have been increasing and have been occurring since the beginning of human existence (Finke, Denno 2004). There are many historical instances of human disturbance such as hunting the buffalos to almost extinction for fur, overfishing cod, and the passenger pigeon hunted to extinction for eating human crops. All organisms on different trophic levels are extremely interconnected in direct and indirect relationships within the same ecosystem (Croll et al 2005). Extinctions or extreme population changes cause a domino effect, which are trophic cascades. Laws and regulations on fishing and hunting are extremely important in preserving biological diversity and preserving ecosystem services such as disease and pest control. By following and understanding these laws and regulations, people can begin to monitor their actions more carefully. Humans should also begin reducing the amount of pesticides and herbicides that are used with harsh chemicals to preserve ecosystem services such as decomposition and pollination. It is important for humans to become educated on the strong bonds in ecological webs, the impacts that human activity can cause, and the ecosystem services that could collapse.

Citations

1. Croll, D. A. 2005. Introduced Predators Transform Subarctic Islands from Grassland to Tundra. Science, 307(5717), 1959-1961. doi:10.1126/science.1108485 
2. Finke, D. L., & Denno, R. F. 2004. Predator diversity dampens trophic cascades.Nature, 429(6990), 407-410. doi:10.1038/nature02554
3. Hairston, Smith, & Slobodkin. 1960. Community structure, population control, and competition. The American Naturalist, 421-425. Retrieved April 27, 2016.
4. Knight, T. M., Mccoy, M. W., Chase, J. M., Mccoy, K. A., & Holt, R. D. 2005. Trophic cascades across ecosystems. Nature, 437(7060), 880-883. doi:10.1038/nature03962