Large Species Shifts Triggered by Small Forces

DOI: 10.1086/422204
Background

Understanding the mechanisms behind shifts in community composition will help in explaining often sudden and contrast shifts for threatened ecosystems such as coral reefs. The rate of a community shifts occurs is typically varied through time, but ends in a rapid and stark shift in community composition. One explanation put forth to explain rapid shifts in community composition stochastic disturbance events. However, an alternative explanation is that systems may shift between alternative attractors. As a review, an attractor can be considered a set of numerical values that a dynamical systems tends to gravitate towards. So an alternative attractor would be a different set of numerical values that would have a slight gravitational pull on this system, but I would imagine that this effect is weak under ‘normal’ circumstances. Evidence for alternative attractors is usually explained by acknowledged feedback mechanisms between organisms and their environment. Interestingly, there are a few instances where feedbacks have yet to be described. This paper provides a model that considers a complex web of interactions in multi-species communities as a source of alternative attractors. Specifically, this paper seeks to link alternative attractors in community composition to response in environmental change.

Modeling 

Researchers use a modified Lokta-Volterra model that allows for the exploration of external forces, from environmental conditions, on the carrying capacity of each species. Environmental noise added to induce variability using a recurrence equation. A type II functional response was modeled for predator-prey relationships, with the growth of predators proportional to their consumption.

150 communities randomly drawn species pools were constructed with 100 simulations selecting randomly drawn initial conditions of all species. At 4,000 days researchers checked to if the system had arrived at a point of attraction, and rather that same point was reached with other systems.

Results

Researchers observed that the probability of alternative attractors was strongly dependent on the way interspecific competition terms were selected. Typically, adding a second trophic level typically induced complex cycling or chaotic attractors.  Concluding remarks suggest that alternative attractors a fairly common for multiple species models. Influence of the environment on response variables was generally related to the amplitude of fluctuation from the environmental parameter.