Ecosystem Trophic Structure, Part 1: Top Carnivores

Having established some basic concepts and had an introductory look at the prehistory of megafauna and megafaunal loss (and the culprits) in the last post, the plan moving forward is to explore the salient impacts this megafaunal perdition could entail, first via the investigation of the roles megafauna play in the environment on a topic by topic basis.

Perhaps the most conceptually accessible effect that megafauna have is on ecosystem trophic structure and this is the aspect we will be investigating over the next couple of posts.

Top Carnivores and the Collapse of Trophic Cascades 

Top carnivores (carnivores situated at the top of a food chain) as a subsection of megafauna, have a substantial impact on the trophic structure of ecosystems, regulating the abundance and behaviour of smaller herbivores and mesopredators (mid-trophic level predators that both prey and are preyed upon - mostly omnivorous so not reliant on any one food source – making them superpredators for smaller prey if left unchecked) and creating inhospitable landscapes that herbivores try to avoid (often referred to rather forbiddingly as ‘landscapes of fear’).

A well-known and well-researched example highlighting this megafaunal effect is encapsulated in Ripple & Brescha 2011, a study of the effect of 1995/96 reintroduction of grey wolves to Yellowstone National Park following their extirpation (localised extinction) more than 70 years before.

The wolves’ extirpation caused the collapse of a three level (predator = wolves, prey = elk, plants) trophic cascade, opening the door for elk (no longer under significant predation pressure) to significantly alter habitat, soils and woody plants in the park. Over the 70-year absence of the wolves, herbivory (essentially plant-eating) of the booming elk population inhibited the recruitment (addition of new organisms to a species population) of plants such as aspen, cottonwood and willow. Given the rare opportunity to study a naturally occurring experiment outlining the effects of large predators on an ecosystem, Ripple & Brescha did the following over the period following the reintroduction of wolves into the park. 

Method

• Carried out aspen/cottonwood recruitment surveys (taking into account other factors that might affect tree recruitment)
• Summarised trends in wolf, elk, bison, beaver populations
• Summarised temporal trends in willow stem cross-sectional ring area growth (higher areas indicating more willow growth and less browsing suppression)
• Synthesised data from other literature concerning vegetation changes in YNP from 1996 to 2010 (year before publication of their article)

Results

Summarised in the figure below. With the reintroduction of wolves in 1995/96, elk populations decreased; consequentially the percentage of aspen leaders browsed decreased whilst aspen height, willow ring area and cottonwood, beaver and bison populations all rose.

The overall changes since wolf reintroduction were as would be predicted by ecological theory - put simply; more wolves, fewer elk (with altered behaviour), and more plant biomass.

Examining some consequential ‘butterfly’ effects instigated by wolf reintroduction in more detail:

• Under threat of predation by wolves (back to landscape of fear idea) YNP elks exhibited altered habitat use, movements, group sizes and vigilance – all likely contributing to reestablishment of the historical wolves-elk-browsing plants cascade and a return to recoupling of behavioural and density effects of wolves on elk with fire disturbance expediating further plant recruitment. Furthermore, new and continuing recruitment of such woody browse species (species with parts to be browsed upon (eaten)) has potential rippling effects on abiotic processes (such as decomposition of nitrogen into plant accessible forms) and biotic functions. Decreases in elk herbivory, resulting in decreased coyote numbers and increased cover and forage could lead to increases in small herbivore populations, in turn significantly affecting the prey base for mid-size predators such as foxes and badgers.
• Resurgent willows providing a more structurally complex habitat for songbirds, leading to an increase in songbird population richness. The resurgence of willows, being a major component of beaver diet in YNP was also at least partially responsible for the increase in beaver populations, the implications of which are huge. Production of dams and ponds by beavers decreases streambank erosion / increases sediment retention, raising wetland water tables, modifying nutrient cycling, all of which play an important role in plant and animal diversity in riverside ecosystem. Recovering riparian (relating to wetlands adjacent to rivers / streams) vegetation can work in tandem to help stabilise eroding channels and add complexity to river/pond shapes.
• Wolf-killed carcasses could benefit scavengers like ravens, magpies, eagles.
• Less browsing on berry-bushes by elk -> higher berry production -> more food for birds and bears -> increased shrub establishment through seed dispersal after consumption, transport and defecation of berries by birds and bears (linking to another idea to be looked at in its own right later).

Conclusions

The reintroduction of wolves seems to have initiated a restructuring of YNP’s ecosystems, re-establishing a historical trophic cascade. Beyond direct implications for wolf management as a conservation tool in ecosystem restoration (a positive step in itself), this study evinces the crucial role that large predators can play in establishing and maintaining resilient, robust wildland ecosystems. As Ripple and Brescha put it ‘Predation and predation risk associated with large predators appear to represent powerful ecological forces capable of affecting the interactions of numerous animals and plants, as well as the structure and function of ecosystems’.