Trophic Cascades

Trophic Cascades

What are they and what do they have to do with Apex Predators?

 In general, a trophic cascade is said to occur when any change in the food web of an ecosystem has impacts that reach farther down that food web. A more scientific way of defining it which relates to Apex Predators is "a progression of direct and indirect effects of native predators on successively lower trophic levels" (Bescheta and Ripple 2009). These trophic cascades are important when discussing the management of apex predators because these large carnivores are at the top of their food web (by definition, apex predators have no natural predators). They often exert top-down control on the food chain in their ecosystem; through predation they control the abundance of prey populations. This is where the cascading effect begins, because the abundance of prey populations in turn will affect the abundance of their prey, all the way down to abundance of plants and nutrients in an ecosystem.

This diagram shows a simple version of a trophic cascade, only 3 levels are affected overall. In the system on the left, where the carnivore is present, there is an overall positive effect on the growth of plants. This is because the Carnivore has a negative effect on the Herbivore, which in turn has a negative effect on the plants, resulting in an overall positive effect on plants. However, in the system on the right, where the top predator has been eliminated, there is an overall negative effect on plant growth. (Graphic created by Caitlin Forster)

An actual example of a trophic cascade that very nearly follows the above flow chart occurred after the reintroduction of gray wolves into the greater Yellowstone area. In the early 1900's, the local apex predator, the gray wolf, was completely extirpated from the ecosystem. As a result, the elk populations, which had been freed from the top down control of wolf predation, exploded in number. Subsequent years showed severe overgrazing of elk on vegetation such as young willow and aspen trees as well as bunchgrasses (Bescheta and Ripple 2009). Researchers could pinpoint elk as the cause of overgrazing because fenced areas, which protected plants from elk, did not show overgrazing or reduced tree recruitment. However, when wolves were reintroduced into the system in the mid 1990's, tree recruitment increased and grazing patterns of elk were changed. This demonstrates that trophic cascades do actually happen in natural systems, and with far reaching effects.


Direct Effects:

Herbivory:
The loss of top level consumers quite often has an effect on herbivory, or plant predation, by altering the relative abundance of organisms that prey on plants. In the wolf example mentioned above, the loss of the top consumer resulted in larger ungulate populations and increased herbivory, but this is not necessarily the case. In some systems, herbivory can decrease with loss of apex predators and we see a boom in plant recruitment or a shift in plant populations (Estes et. al.).

Mesopredator Release:
When apex predators are removed from a system, lower level carnivores are freed from the control exerted on them by top predators either from direct predation or competition for the same resources. This effect has been coined "mesopredator release" and is more common than many people realize. When doing range analysis, scientists found that all ranges of almost all Apex Predators in North America have contracted, while the ranges of mesopredators have expanded (Prugh et. al.). Several factors contribute to improved mesopredator survivorship

1. Mesopredators require smaller habitats and are therefore less affected by habitat loss than Apex Predators.
2. There are higher incidences of Apex Predator/Human conflict and as fragmentation continues to worsen, so does large carnivore persecution
3. As fragmentation increases, the resources that are available to mesopredators increases as well. At first this may seem counterintuitive, but garbage, pet food and crops are considered resources to mesopredators.

It is important to note that mesopredators cannot simply bump up a trophic level and take on the role of an apex predator. Apex predators often exert behaviorally mediated control of trophic systems, such as when elk modify their foraging behavior away from riparian areas in the presence of wolves (Bescheta and Ripple), and mesopredators are not often equipped to fill these niches.


Indirect Effects:

Wildfire:
Trophic cascades also have far reaching indirect effects. Altering herbivory and the relative abundance of plants in a system also changes the fuel available for wildfires. In some fire adapted regions, this can reduce fire frequency while in other non-fire adapted regions it can increase. In either situation, the effects can be disastrous.

Soil quality and erosion:
Another consequence of altering vegetation in a habitat is the reduction of soil quality and the increase in erosion when native, well-adapted plants are eliminated from a system.

Invasive Species:
Invasive species "have left behind their natural predators and freed themselves from top down control" (Estes et al.). So when natural predators are eliminated from a system, which reduces the amount of top-down control that can be exerted, the system is left exposed to infiltration from non-native species.

Biodiversity:
When an apex predator is removed from a system that system fails to function at optimum levels, which reduces productivity, resource availability and habitat quality. Eventually this results in a less diverse system and biodiversity is reduced.

Top Down versus Bottom Up Control

Many studies on trophic cascades focus on the top down effects that apex predators have on their ecosystem, which as discussed above, are quite significant. However, another consideration that must be made is the relative strengths of these top down controls and how they compare to bottom-up control in different ecosystems. Bottom-up controls in ecosystems occur when production or nutrient availability in lower trophic levels affect productivity in higher trophic levels.  For example, in an arid desert ecosystem, is herbivory the limiting factor on plant growth, or is plant growth actually more limited by water availability? Instead of the above diagrams, that only show top down trophic interactions, this ecosystem might look something like this:

Synthesized Graphic by Caitlin Forster 

In this trophic diagram of a more arid ecosystem, bottom up controls have a much larger effect, shown by the relative arrow sizes. One might not expect the removal of the apex predator to have as large an effect as in the previous diagram, except that both carnivores and herbivores impact habitat quality, which functions as a bottom up control for plants. This shows that both top-down and bottom-up controls exist in trophic maps, and that which one dominates depends on a suite of conditions and how they are balanced in different ecosystems.