Vocabulary:

• Linkage density: = average number of links per species; measures the complexity of interactions across a food web
• Connectance: the proportion of possible links between species that are realized, calculated as the number of links divided by the maximum number of possible links
• Apparent competition: prey species share a predator, but do not compete for resources.
• Top-down control: predator populations limit the abundance of prey species, often resulting in a trophic cascade.
• Trophic cascade: occurs when top predator populations change, causing reciprocal changes in the populations of lower trophic levels. Often results in drastic changes to ecosystem structure and functioning.
• Bottom-up control: the productivity and abundance of populations at any given trophic level are limited by the productivity and abundance of populations below them.
• Basal Species: species with no prey in a food web and commonly shown at the bottom. These are usually autotrophs or detritivores.
• Intermediate Species: non-top-level consumers. Includes any organism which is connected to a higher and lower trophic level.
• Apex Predator: carnivore at the top of a food chain with no natural predators. Commonly keystone species.
• Trophic position: shows where certain species fit in relation to all other species in that trophic level.
  • Calculated from the fraction of food they consume from other trophic positions [1]

Outline of Notes:

Food Webs:

• Food webs are a way of visualizing relationships between organisms based on their trophic level.
  • An organism’s trophic level is determined by its feeding behavior:
    • Top/Apex predators occupy the highest trophic level
    • Intermediate species occupy the trophic levels between top predators and basal species:
      • Mesopredators sit just below top predators
      • Herbivores sit above autotrophs
    • Basal species occupy the lowest trophic level
      • Usually autotrophs or detritivores
• Arrows are used to indicate the type of interaction occurring between organisms:
  • Up arrows (↑): Indicate the flow of energy from food item to consumer
  • Down arrows (↓): Indicate regulatory effects from one population to another
    • These can be positive or negative, and indirect or direct

• • Example: Predators have a direct negative effect on herbivores, and an indirect positive effect on plants. Predators directly reduce herbivore populations by consuming them, thus reducing grazing on plant populations, allowing plant populations to increase.

• 

  What can food webs tell us about a species?

  • Feeding strategies
  • Generalist vs specialist
  • Competition
  • Possible trophic cascades
• In the example to the right:
  • P = Top predator
  • C = Consumer
  • H = Herbivore
  • A = Autotroph

Quantifying Food Web Dynamics

• S = number of species
• L = number of links
• S² = maximum number of links
• C = Connectance = L/S²
  • Connectance: the proportion of possible links between species that are realized
• LD = linkage density = L/S
  • Linkage density: average number of links per species. Measures the complexity of interactions across a food web.
• Chain length: average the number of connections between an apex predator and a producer
  • Ex: Chain length for P1 = 3:
    • 
• Mean chain length (ChLen) = the average length of every possible chain on the food web.

1. Calculate the length of every possible chain

2. Divide the total length by the number of total chains

• • Example: 12 + 8 + 3 = 23 (total length of every possible chain)  / 11 (total number of chains) = 2.09 mean chain length
• Compartmentalization:
  • Food webs can be compartmentalized by component species. Each compartment is indicated by a different color, and each species and compartment is connected by only a single species interaction (see figure 3).

Species Interactions:

Food webs can illustrate different species interactions.

• Species interactions can be direct or diffuse:
  • Direct relationship: a one-to-one relationship between two species.
    • Ex: A specific pollinator may only visit a specific flower, and that flower may only be visited by that one pollinator species.
  • Diffuse relationship: one species may interact with several other species.
    • Ex: A pollinator, like a bee, may visit several different types of flowers
    • Ex: A flower may be visited by many different species of pollinators
• Species interactions can also be direct or indirect:
  • Direct interactions: occur when species interact through direct contact.
    • Ex: predation
  • Indirect interactions: species interact through changes in another species
    • Ex: an increase in lion populations may cause an increase in grass abundance by decreasing antelope populations.
    • Apparent competition: an increase in one prey species causes an increase in predator populations, increasing predation on other prey species. The resulting effects appear to be a competitive interaction, even though the two species do not interact and do not compete for resources.
      • Ex: An increase in the population of antelopes (H1) increases the populations of lions (P). An increase in lion populations increases predation pressure on both antelopes (H1) and zebras (H2), ultimately decreasing zebra populations (H2).
        • The increase in antelope abundance (H1) corresponding to the decrease in zebra abundance (H2) appears to be a competitive interaction, even though the two species do not directly interact.

Regulatory Effects on Food Webs:

• Top-down control: predator populations limit the abundance of prey species, often resulting in a trophic cascade.
  • Trophic cascade: occurs when top predator populations change, causing reciprocal changes in the populations of lower trophic levels. Often results in drastic changes to ecosystem structure and functioning.
• Bottom-up control: the productivity and abundance of populations at any given trophic level are limited by the productivity and abundance of populations below them.

 Blog-style Summary:

Food chains can be a very effective tool for wildlife conservation and management. Food chains typically show the main energy flow from primary producers (autotrophs), through consumers, to the apex predator. Food chains can be used to see what the effects may be of top-down or bottom-up control. Top-down control involves controlling predator populations in an effort to control the abundance of intermediate species, or prey populations; this type of control often results in a trophic cascade. Trophic cascades occur when top predators are added or removed, influencing the abundance of every organism that comprises the lower trophic levels of an ecosystem. Because food chains are incredibly simplified, they are best applied when looking at one specific species, such as wolves.

For example, if wolf populations are increased in an area, deer populations will decrease, allowing grass populations to increase as a result. Bottom-up control is the opposite approach to top-down; instead of changing the abundance of a top predator, the abundance and productivity of producers is modified. In our wolf example, if we increase the number of primary producers (grass), the amount of deer will increase, thus increasing the abundance of wolves in the ecosystem.

Food webs provide a more in-depth and accurate version of ecosystem interactions, by incorporating several individual food chains. While both webs and chains represent the flow of energy through an ecosystem, they serve different purposes. Most species consume multiple food sources and utilize a different food resource when their primary source is unavailable. In these instances, food webs are better as they include these alternate food sources. By visualizing the many species that interact and compete for available resources, food webs also enable ecologists to investigate and quantify relationships between species and across trophic levels.

For example, Trophic cascades are events that are initiated by either removing a top predator or introducing a top predator into an area. One of the best-known trophic cascades in the United States was documented in Yellowstone National Park. Following the removal of wolves in the 1920s, elk populations in the area were released from predation pressure and quickly grew. This growth in the elk population increased grazing pressure and caused plant populations in the area to decrease. In 1995 wolves were reintroduced back into Yellowstone National Park. Wolves began to prey on elk, causing a reduction in the elk population. This reduction in the elk population allowed the plant populations to recover and increase in abundance (Ripple et al. 2012).

Several types of interspecies interactions exist in a food web beyond predator and prey relationships. Competition, mutualism, predation, and parasitism all exist within food webs. When most people think of food webs they oversimplify it and assume that energy flows in a single direction toward the apex predator but in reality, it’s much more complicated. The direction and flow of nutrients in a system goes up through intermediate and apex predators, sideways between intermediates, and even cycles through decomposers to contribute to bottom-up effects. (Learn more about the detritus cycle in Chapter 11: Ecosystem Ecology.)

        Food webs are an important tool for understanding the complex nature of relationships between every organism in an ecosystem. Understanding the complexity of these trophic interactions serves a multitude of purposes. When an invasive species is introduced into an ecosystem, the magnitude of the effects can be determined by how it changes the interactions in the food web. When we as a scientific community can understand these changes and anticipate future implications to the ecosystem, we can create a management plan that is tailored to the specific problem at hand.

Spotlight on NC:

Impacts of Feral Hogs in North Carolina

Background

Invasive feral swine have lived in North Carolina for centuries. Originally brought over from Europe for food and hunting, feral hogs today have become a nuisance causing crop destruction, forest destruction, and wetland damage [1]. This is largely caused by the generalist nature of the pigs. They have minimal predators to control their populations and will eat just about anything [1]. The species has become a nuisance for farmers by eating crops and trampling land. Their omnivorous diet allows them to wreak damage on a multitude of environments, eating things from insects to crops to forest understory [2]. Their rapid introduction has prevented the adaptation of native organisms to their behavior and diet [1]. Their invasive ways and generalist diet cause large impacts on the surrounding ecosystems’ food webs.

Food Web Applications

In North Carolina, feral hogs have spotty populations on the coast and in the Great Smoky Mountains. They are especially detrimental to mountain ecosystems since many sensitive plant and animal species reside there. [3]. They transform the environment through their foraging, turning up the soil as they search for plants and insects to eat. They can also cover large areas in a short amount of time and do not discriminate between natural and agricultural lands. While young hogs can be predated upon by coyotes, bobcats, and foxes, adults have no natural predators. This is due to a combination of their large adult size and rapid reproduction; any natural predators they may have are not able to keep the population in check [1]. This means that there are no top-down controls to their population. Since they are omnivorous generalists, their extremely broad diet prevents them from feeling the effects of
bottom-up controls as well.
 

Solutions and Proposed Research

When coming up with a solution for the issues created by feral hogs, one’s first thought might be to go hunting. Hunting is often used as a form of population control and is sometimes encouraged for certain invasive species and organisms [1]. However, this is not entirely the case for the feral hog. In areas where the hogs are already established, this practice could be an effective start to controlling the population. In states where feral hogs are not an established animal, though, it has become a popular practice for hunters to bring hogs en masse from other states to hunt for sport. This introduces the hogs to areas where they had not previously resided and furthers their invasive nature to states that did not previously have a hog issue [4].

Some of the methods of hog population control that have proved to be successful include aerial gunning, corral trapping, and poisoning [2]. In the practice of aerial gunning, aircrafts track these feral hogs and shoot them from above. Corral trapping can have lethal or nonlethal connotations depending on the area. However, in most areas, the lethal route is considered the most ethical and effective method, since the hogs cause so much destruction. Poisoning is another method of control that entails setting up bait sites and laying out poisoned foods in large amounts for the hogs to eat [1].

While these control methods have been heavily researched and tested, the level of ecological disturbances that the feral pig causes calls for more research to be done. It is our suggestion that further research goes into methods of lethal population control, including finding more effective methods of poisoning. This could perhaps even be done by finding ways to make the crops they destroy unappealing or poisonous to them, but not to humans. We also suggest more funding and research goes towards creating policies that ensure hunters do not continue this “catch-and-release” method of sport hunting.

Conclusion

Feral swine can majorly disrupt North Carolina’s ecosystem due to their invasive, omnivorous nature. If they remain, the hogs could spread and invade further areas [4]. Removing the hogs from these ecosystems could allow the food webs to return to normalcy, creating a healthier ecosystem [2].

Featured Ecologists:

Christine Sprunger, PhD

Dr. Sprunger studies soil health and how different factors in agriculture management can manipulate it. She studies degraded systems and how to rehabilitate them, learning how nematodes can be soil health bioindicators and how climate change impacts the rhizosphere. She has done multiple studies with Kernza, a perennial grain that she is proving can be more profitable and keep soil healthier than annual grain crops. Dr. Sprunger has found that two harvests of Kernza a year increases nutrient cycling and availability, mineralizable carbon, and provides additional revenue. She also found that established perennial crops increase the amount of carbon available to microorganisms while stabilizing the soil food web. This is very important for the climate and the environment in general. Perennial crops decrease soil erosion dramatically and help rebuild topsoil through the microorganism communities they foster. This is what Dr. Sprunger wanted to do when she came into the field. She wanted to find solutions to rehabilitate soil because she was born in Haiti and she has seen the effects of degraded soil on a third-world country. Her research is showing that agricultural management practices have the ability to foster crop fields with higher productivity without losing profitability, a major concern of many farmers.

Earyn McGee, PhD Candidate 

McGee is broadly interested in research that takes place at the intersection of conservation biology and land management. She is studying the diets of three species of lizards in the Chiricahua Mountains to assess how the drying out of perennial streams can affect riparian lizard communities, which fall under the categories of both food web and ecosystem ecology. Her project involves a lot of fieldwork and “lizard lassoing,” as McGee calls it, and she uses both mark-recapture surveys and DNA sequencing of fecal matter as part of her methodologies. She is trying to determine how much the lizards rely on aquatic insects for sustenance, which could have heavy implications for wildlife inhabitants of arid environments that are being increasingly impacted by climate change. In addition to her herpetology work, McGee is also particularly interested in science communication and increasing the participation of underrepresented groups in STEM. She is a graduate student mentor for the Doris Duke Conservation Scholars Program at the University of Arizona, where she mentors undergraduate students from various backgrounds and introduces them to the intricacies of field research.

Student Contributors:

Note Outline: David Poling, Drake Sandman, Teddy Sarian, Raché Manning

Blog Style Summary: Sam Ellison, Steven Hart, Matthew Girouard, Roger Sandvik

Spotlight on NC: Elizabeth Mitchell, Cait Phipps, Jess Capes, Eva DeSantis

Featured Ecologists: Nicholas Terwilliger, Sasha Pereira