Food Web Theory in Conservation Management

Henrietta Ansell

Abstract

Food web theory is an effective theory when applied to conservation management of predators. Food web theory is a dynamic map of predator prey interactions, defined by the flow of energy between different trophic levels. It can be used by exploring interruptions in trophic levels to identify factors of decline in a population. Food webs can also be used to investigate indirect causes of decline through a bottom up approach and recognize relationships between species in an ecosystem. Food web theory is particularly applicable when applied to top predator affects, result of habitat loss and apex predator influence on invasive and non-invasive species. Ultimately, food web theory helps us understand interactions of species within ecosystems, which allows us to make predictions that aid environmental management.

Introduction

Ecological theory uses existing data, observation and new ideas to create cohesive theoretical framework to both explain existing systems and make new predictions (Marquet, et al., 2014). Theory allows us to make predictions about environments and apply management plans effectively. Food web theory is particularly useful in this scenario. Food webs are maps that distinguish energy flows throughout species in an ecosystem (see fig 1) (Thompson et al., 2012). Each stage is defined as a “trophic level”. Food web theory describes a framework to understand each species role within an ecosystem. Originally, food webs were considered a static system that become vulnerable when key parts fluctuate or are removed (de Ruiter et al., 2005). A new approach to food webs suggests that they are dynamic, describing changing relationships of trophic levels and species as open and flexible (de Ruiter et al., 2005). By investigating food web theories within conservation management, we can identify why species populations are fluctuating.  In my discussion I will be investigating top predator decline, habitat loss creating bottom up effects and finally how apex predators can aid the coexistence of native and non-native species. These three examples have shown to be effective when applied to conservation management.

Fig 1: Basic food web structure (Hui, 2012). Primary producers provide energy to primary consumers, which is passed to secondary consumers and on to tertiary consumers. Food webs can be more or less complex as well as, more fluid and dynamic when applied to real life ecosystems. 

Top Predators

Top predators have an important role in ecosystem composition. According to food web theory, a change in top predator populations will affect lower trophic level abundance (Vander Zanden et al., 2006). By investigating ecosystem food webs, we can identify potential causes for species decline within different trophic levels. For instance, if top predator, such as a coyote, is in decline, an opportunity for mesopredators (smaller predators, such as racoons and opossums) to increase in population, arises. Thus, bird life is heavily predated by the increase in mesopredator abundance (fig 2) (Crooks & Soulé, 1999). This change in species abundance shows how dynamic food webs are, as the system adjusts to the absence of a top predator. In order to manage bird populations, food web theory identifies that conservationists should focus on restoration of top predators in order to control mesopredator populations. By using food web theory, key problems are identified and outline appropriate management plans to resurrect the issue (Vander Zanden et al., 2006). By using theory, we can create more effective and economical decisions to restore and conserve ecological composition.

Fig 2: Top predator population abundance on bird population. (Ansell, 2019). Circle size represents population abundance. 

Habitat loss

Habitat loss is the number one cause of biodiversity loss globally (Fletcher et al., 2018). Top predators in particular are vulnerable to habitat degradation (Pimm, 1991). Overflow effects of habitat loss are also prevalent due to the interruption of food webs that interlink with surrounding areas. When stream boundaries are cleared of vegetation, it decreases the opportunity for organic matter to interact with stream food web systems (Lake et al., 2007). Organic matter is an important food source for shredder insects that inhabit these areas. As a result, Insects go elsewhere for nutrition or starve. Consequently, predatory fish that rely on insects as a food source are unable to feed (Lake et al., 2007) (fig 3). Therefore, predatory fish decline is indirectly caused by habitat loss in surrounding areas. In this case, food web theory is used to identify the issue as habitat loss. These findings are an important bottom-up process to consider and apply to agricultural practices. When forestry is cleared for cattle, a follow-on effect on surrounding streams is created. By utilising food web theory as a bottom up approach, conservationist can understand why predatory fish are declining.

Fig 3: Vegetation, Scraper Insects And Predatory Fish (W.V.D.E.P, 2019).

Apex Predators Aiding the Co-existence of Native and Non-Native Species

Non-native species continue to disperse infiltrating new habitats and creating novel ecosystems. The effect on invasive species on native ecosystems has been extensively debated (Wallach et al., 2015). Non-indigenous species can cause disruption by intruding native food webs. However, new species increase biodiversity locally and can potentially form positive relationships with natives (Wallach et al., 2015). Eradication techniques can be costly, detrimental to biodiversity and ineffective. By using food web theory, we can establish new techniques to deal with the negative effects of invasive species. Apex predators are large bodied predators the sit at the highest tropic level (Wallach et al., 2015). Constant patterns show that apex predators can structure communities by limiting populations of both native and non-native species (Wallach et al., 2015). For example, when apex predators’ control mesopredators, species on lower trophic levels can thrive. In the absence of apex predators such as wolves, both native deer and non-native wild horses reach high populations within North America (Wallach et al., 2015). Subsequently leading to over grazing and biodiversity loss. However, by introducing an apex predator or multiple, reductions of both species will result in the increase of vegetation (Wallach et al., 2015). In Using food web theory, conservationists and ecologists can investigate further into the idea of managing apex predators, and what kind of effects this will have on native and non-native species.

New Zealand Context

New Zealand struggles with the impact invasive predators have on endemic species and as a result, pest control is a major part of conservation management (Zavaleta et al., 2001). Stoats (Mustela erminea) predate on both rodents and native bird life. In one attempt to poison exotic predators an unexpected result occurred; rodents were effectively eradicated, but stoat populations persisted (Zavaleta et al., 2001). Consequently, stoat predation focused on native fauna (Zavaleta et al., 2001). In addition to this, ridding only stoat populations will result in a population boom for rodents, which will also lead to an increase in predation on native species. This example highlights how important it is to incorporate food web theory in conservation management. Eradicating one trophic level can have detrimental effects on others. In this case, it is incredibly important to exterminate all mammalian predators to avoid over-predation.

Conclusion

The use of food web theory in predator management will allow effective predictions to be carried out. By assessing changing trophic levels in ecosystems, we can understand declines and increases in populations. Predator population fluctuation can have cascading effects on lower trophic levels. By using a bottom up approach, we are also able to find why predators are declining. Additionally, we can use food web theory to manipulate populations at different trophic levels and increase abundance in others. To conclude, ecological theory has a practical place within environmental management and the recovery of communities.

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