Gary R. Huxel

Weak trophic interactions and the balance of nature

Ecological models show that complexity usually destabilizes food webs,, predicting that food webs should not amass the large numbers of interacting species that are in fact found in nature. Here, using nonlinear models, we study the influence of interaction strength (likelihood of consumption of one species by another) on food-web dynamics away from equilibrium. Consistent with previous suggestions,, our results show that weak to intermediate strength links are important in promoting community persistence and stability. Weak links act to dampen oscillations between consumers and resources. This tends to maintain population densities further away from zero, decreasing the statistical chance that a population will become extinct (lower population densities are more prone to such chances). Data on interaction strengths in natural food webs indicate that food-web interaction strengths are indeed characterized by many weak interactions and a few strong interactions.

When is a trophic cascade a trophic cascade

Trophic cascades are the time-honored focal point of food-web dynamics. They are the best loved example of indirect effects in undergraduate ecology textbooks and they represent a potentially useful application of theory. Researchers have found them from the Arctic to the tropics. But, can we agree on what they are? Here, we seek to clarify the terminology of trophic cascades and call for a consensus on how to quantify cascading effects in the future.

Rapid displacement of native species by invasive species: effects of hybridization

Abstract The introduction of non-native populations can lead to the competitive exclusion (displacement) of native populations. This has been hypothesized to be further exacerbated by the potential of hybridization, which can dilute or genetically assimilate the native genotype leaving no “pure” natives. With relatively moderate to high rates of immigration, the loss of the native species can be rapid with or without hybridization. Using single-locus, two-allele models, I find that species replacement can occur very rapidly and the time to displacement decreases rapidly with increasing immigration and selection differential. Immigration and selection act in two different ways: increasing immigration results in displacement by overwhelming the native; whereas increasing the selection differential in favor of the invader leads to displacement via genetic assimilation. The implications of these results are the need for more empirical studies on the immigration patterns of invasive species and their potential for interbreeding with natives.

Food Web Stability: The Influence of Trophic Flows across Habitats

In nature, fluxes across habitats often bring both nutrient and energetic resources into areas of low productivity from areas of higher productivity. These inputs can alter consumption rates of consumer and predator species in the recipient food webs, thereby influencing food web stability. Starting from a well‐studied tritrophic food chain model, we investigated the impact of allochthonous inputs on the stability of a simple food web model. We considered the effects of allochthonous inputs on stability of the model using four sets of biologically plausible parameters that represent different dynamical outcomes. We found that low levels of allochthonous inputs stabilize food web dynamics when species preferentially feed on the autochthonous sources, while either increasing the input level or changing the feeding preference to favor allochthonous inputs, or both, led to a decoupling of the food chain that could result in the loss of one or all species. We argue that allochthonous inputs are important sources of productivity in many food webs and their influence needs to be studied further. This is especially important in the various systems, such as caves, headwater streams, and some small marine islands, in which more energy enters the food web from allochthonous inputs than from autochthonous inputs.

Microcosms as Models for Generating and Testing Community Theory

Microcosms as biological models have played a central role in the development of contemporary ecological though. From Gause`s (1934) pioneering analysis of competitive exclusion to Huffaker`s (1958) explicit examination of a spatial resource and Tilman`s (1977,1982) elegant exploration of competitive mechanisms, laboratory analyses have provided essential insight into real-world ecology. Conversely, phenomena encountered in the field have been systematically examined in the laboratory, extending and refining our understanding of the dynamical range of both mechanism and process. Extension of these model-driven insights to the field has been integral in the development of important themes within the collective gestalt of academic ecology. This interchange between laboratory and field studies is typified by recent laboratory models of competition, predation, community, assembly, and landscape assembly. 74 refs., 3 figs.

The construction and assembly of an ecological landscape

1. An ecological landscape consisting of discrete interconnected patches was constructed in the laboratory. Each patch in the landscape was a 1-litre aquatic microecosystem containing producers and consumers. 2. Species invaded and spread throughout the landscape in a specific sequence following prescribed invasion pathways. 3. Species distribution among landscape patches was heterogeneous and converged to one of several alternative states despite identical initial conditions. 4. Differences in structure which developed among patches were the result of the assembly processes which occurred in each patch and among interconnected patches

ALTERNATIVE PREY AND THE DYNAMICS OF INTRAGUILD PREDATION: THEORETICAL PERSPECTIVES

A rich body of theoretical literature now exists focused on the three-species module of intraguild predation (IGP), in which a top predator both attacks and competes with an intermediate predator. Simple models of intraguild predation are often unstable, either because one consumer is excluded, or because sustained oscillations emerge from long feedback loops. Yet, many natural IGP systems robustly persist. Standard models of intraguild predation simplify natural systems in crucial ways that could influence persistence; in particular, many empirical IGP systems are embedded in communities with alternative prey species. We briefly review the key conclusions of standard three-species IGP theory, and then present results of theoretical explorations of how alternative prey can influence the persistence and stability of a focal intraguild predation interaction.

Effects of partitioning allochthonous and autochthonous resources on food web stability

The flux of energetic and nutrient resources across habitat boundaries can exert major impacts on the dynamics of the recipient food web. Competition for these resources can be a key factor structuring many ecological communities. Competition theory suggests that competing species should exhibit some partitioning to minimize competitive interactions. Species should partition both in situ (autochthonous) resources and (allochthonous) resources that enter the food web from outside sources. Allochthonous resources are important sources of energy and nutrients in many low productivity systems and can significantly influence community structure. The focus of this paper is on: (i) the influence of resource partitioning on food web stability, but concurrently we examine the compound effects of; (ii) the trophic level(s) that has access to allochthonous resources; (iii) the amount of allochthonous resource input; and (iv) the strength of the consumer–resource interactions. We start with a three trophic level food chain model (resource–consumer–predator) and separate the higher two trophic levels into two trophospecies. In the model, allochthonous resources are either one type available to both consumers and predators or two distinct types, one for consumers and one for predators. The feeding preferences of the consumer and predator trophospecies were varied so that they could either be generalists or specialists on allochthonous and/or autochthonous resources. The degree of specialization influenced system persistence by altering the structure and, therefore, the indirect effects of the food web. With regard to the trophic level(s) that has access to allochthonous resources, we found that a single allochthonous resource available to both consumers and predators is more unstable than two allochthonous resources. The results demonstrate that species populating food webs that experience low to moderate allochthonous resources are more persistent. The results also support the notion that strong links destabilize food web dynamics, but that weak to moderate strength links stabilize food web dynamics. These results are consistent with the idea that the particular structure, resource availability, and relative strength of links of food webs (such as degree of specialization) can influence the stability of communities. Given that allochthonous resources are important resources in many ecosystems, we argue that the influence of such resources on species’ and community persistence needs to be examined more thoroughly to provide a clearer understanding of food web dynamics.

Hierarchy underlies Patterns of Variability in Species Inhabiting Natural Microcosms

Relative variability of species has been shown to increase significantly with a decrease in their ecological range. Similarly, the distribution of collapse (e.g., extinctions, disturbances, population declines) magnitudes has also been shown to follow an inverse power-law form described by the 1/f ω curve. We hypothesized that the two, possibly general, patterns associated with ecological systems share a common underlying cause: the hierarchical structure of the system itself. To test the hypothesis we used a model system of 49 natural rock pools inhabited by 40 species of invertebrates. Three measures of species variability based on changes in abundance, distribution, and persistence in individual pools conform with the postulated negative exponential curves. Correspondingly, frequency distributions of changes of various magnitudes conform to the 1/f ω pattern. Examination of the contributions of species to the 1/f ω pattern revealed that species low in the system hierarchy (habitat specialists in this case) are responsible for the majority of small variation events (correlations between the ecological range and position on the 1/f ω curve range from 0.625 to 0.807 on the three measures of variability). This permits the conclusion that the two patterns are linked and constitute different expressions of the same hierarchical system structure.

Invasion Success and Community Resistance in Single and Multiple Species Invasion Models: Do the Models Support the Conclusions?

Eltons concept of community-level resistance to invasion has derived significant theoretical support from community assembly models in which species invade (colonize) singly at low densities. Several theoretical models have provided support to this concept and are frequently cited as providing evidence that invasion resistance occurs in nature. The underlying assumptions of these models however, are derived from island or island-like systems in which species invade infrequently at low abundances. We suggest that these island-like models cannot be generalized to systems in which species arrive in greater frequencies and densities. To investigate the effects of altering the basic assumptions of these original models, we utilized assembly algorithms similar to those used in previous studies, but allowed either two species to invade per time step or single species invasions at relatively high inoculation densities. In these models, invasion resistance only occurred when the invasion process was restricted to single species invading at low densities (as in previous models). When two species were allowed to invade per time step, invasion resistant states did not occur in any of 20 simulated communities, even after 10,000 invasion events. Relaxation of the assumption of invasion at low density also resulted in a lack of invasion resistance. These results may explain why the strict concept of complete invasion resistance appears only to operate in island and island-like systems.