Theoretical Ecology

Gregorius, Hans-Rolf; Kosman, Evsey

doi: 10.1007/s12080-017-0363-ypmid: N/A

Recently, the notion of diversity, which is directed towards (effective) numbers of types (states of a trait such as species and genotypes), is increasingly used as an umbrella term akin to “variation”, thus including classical metrics of dispersion among others. This is probably due to the growing interest in functional aspects of variation which involve variable differences between types. Though the traditional notion of diversity does not cover these aspects, it shows up in many interpretations. To overcome this ambiguity, the traditional notion of diversity is extended in this paper to include variable differences with emphasis on their general significance as structuring features. For this purpose, structure is conceived to be captured by the representation of types via variable differences and abundances. Structural diversity then results from application of traditional measures of diversity to the relative structural representations of types in addition to their relative abundances. Since diversity as effective number of types alone provides no information about their mutual distinctness and the range covered by them, connections to measures of dispersion are indispensable. This is considered via two approaches that rely on dispersion characteristics and one approach that allows for an assessment of structural diversity for controlled levels of type distinctness. Effects of structure on dispersion and diversity are analyzed. The use of the approaches for discovering rarely considered characteristics of phylogenetic structure is demonstrated.

Gregorius, Hans-Rolf; Kosman, Evsey

doi: 10.1007/s12080-018-0392-1pmid: N/A

The article “Structural type diversity: measuring structuredness of communities by type diversity”, written by H.-R. Gregorius and E. Kosman, was originally published electronically on the publisher’s internet portal (currently SpringerLink) on 15 January 2018 with open access.

Roos, André

doi: 10.1007/s12080-018-0374-3pmid: 30931015

Ecological theory about the dynamics of interacting populations is mainly based on unstructured models that account for species abundances only. In turn, these models constitute the basis for our understanding of the functioning of ecological communities and ecosystems and their responses to environmental change, natural disturbances and human impacts. Structured models that take into account differences between individuals in age, stage or size have been shown to sometimes make predictions that run counter to the predictions of unstructured analogues. It is however unclear which biological mechanisms that are accounted for in the structured models give rise to these contrasting predictions. Focusing on two particular rules-of-thumb that generally hold in unstructured consumer-resource models, one relating to the relationship between mortality and equilibrium density of the consumer and the other relating to the stability of the equilibrium, I investigate the necessary conditions under which accounting for juvenile-adult stage structure can lead to qualitatively different model predictions. In particular, juvenile-adult stage structure is shown to overturn the two rules-of-thumb in case the model also accounts for the energetic requirements for basic metabolic maintenance. Given the fundamental nature of both juvenile-adult stage structure as well as metabolic maintenance requirements, these results call into question the generality of the predictions derived from unstructured models.

Phillips, Jessica; Peacock, Stephanie; Bateman, Andrew; Bartlett, Mackenzie; Lewis, Mark; Krkošek, Martin

doi: 10.1007/s12080-018-0375-2pmid: 30931016

A tension between cooperation and conflict characterizes the behavioral dynamics of many social species. The foraging benefits of group living include increased efficiency and reduced need for vigilance, but social foraging can also encourage theft of captured prey from conspecifics. The payoffs of stealing prey from others (scrounging) versus capturing prey (producing) may depend not only on the frequency of each foraging strategy in the group but also on an individual’s ability to steal. By observing the foraging behavior of juvenile coho salmon (Oncorhynchus kisutch), we found that, within a group, relatively smaller coho acted primarily as producers and took longer to handle prey, and were therefore more likely to be targeted by scroungers than relatively larger coho. Further, our observations suggest that the frequency of scrounging may be higher when groups contained individuals of different sizes. Based on these observations, we developed a model of phenotype-limited producer-scrounger dynamics, in which rates of stealing were structured by the relative size of producers and scroungers within the foraging group. Model simulations show that when the success of stealing is positively related to body size, relatively large predators should tend to be scroungers while smaller predators should be producers. Contrary to previous models, we also found that, under certain conditions, producer and scrounger strategies could coexist for both large and small phenotypes. Large scroungers tended to receive the highest payoff, suggesting that producer-scrounger dynamics may result in an uneven distribution of benefits among group members that—under the right conditions—could entrench social positions of dominance.

Ong, Theresa; Vandermeer, John

doi: 10.1007/s12080-018-0376-1pmid: N/A

Critical transitions whereby small changes in conditions can cause large and irreversible changes in ecosystem states are a cause of increasing concern in ecology. Here, we focus on the irreversibility of these transitions, formally known as hysteresis. We explore how simple correlations between parameters in Lotka-Volterra predator-prey equations result in a variety of complicated hysteretic patterns. These patterns include “unattainable” stable states that once lost may never be recovered. We suspect these patterns to be common in natural systems, where interactions between diverse assemblages are unavoidable. Thus, understanding underlying hysteretic structures may be necessary for rescuing lost ecosystem states and avoiding future losses.

O’Dwyer, James

doi: 10.1007/s12080-018-0377-0pmid: N/A

Competition in ecology is often modeled in terms of direct, negative effects of one individual on another. An example is logistic growth, modeling the effects of intraspecific competition, while the Lotka-Volterra equations for competition extend this to systems of multiple species, with varying strengths of intra- and interspecific competition. These equations are a classic and well-used staple of quantitative ecology, providing a framework to understand species interactions, species coexistence, and community assembly. They can be derived from an assumption of random mixing of organisms, and an outcome of each interaction that removes one or more individuals. However, this framing is somewhat unsatisfactory, and ecologists may prefer to think of phenomenological equations for competition as deriving from competition for a set of resources required for growth, which in turn may undergo their own complex dynamics. While it is intuitive that these frameworks are connected, and the connection is well-understood near to equilibria, here, we ask the question: when can consumer dynamics alone become an exact description of a full system of consumers and resources? We identify that consumer-resource systems with this property must have some kind of redundancy in the original description, or equivalently there is one or more conservation laws for quantities that do not change with time. Such systems are known in mathematics as integrable systems. We suggest that integrability in consumer-resource dynamics can only arise in cases where each species in an assemblage requires a distinct and unique combination of resources, and even in these cases, it is not clear that the resulting dynamics will lead to Lotka-Volterra competition.

Kath, Nadja; Boit, Alice; Guill, Christian; Gaedke, Ursula

doi: 10.1007/s12080-018-0378-zpmid: N/A

Allometric trophic network (ATN) models offer high flexibility and scalability while minimizing the number of parameters and have been successfully applied to investigate complex food web dynamics and their influence on food web diversity and stability. However, the realism of ATN model energetics has never been assessed in detail, despite their critical influence on dynamic biomass and production patterns. Here, we compare the energetics of the currently established original ATN model, considering only biomass-dependent basal respiration, to an extended ATN model version, considering both basal and assimilation-dependent activity respiration. The latter is crucial in particular for unicellular and invertebrate organisms which dominate the metabolism of pelagic and soil food webs. Based on metabolic scaling laws, we show that the extended ATN version reflects the energy transfer through a chain of four trophic levels of unicellular and invertebrate organisms more realistically than the original ATN version. Depending on the strength of top-down control, the original ATN model yields trophic transfer efficiencies up to 71% at either the third or the fourth trophic level, which considerably exceeds any realistic values. In contrast, the extended ATN version yields realistic trophic transfer efficiencies ≤ 30% at all trophic levels, in accordance with both physiological considerations and empirical evidence from pelagic systems. Our results imply that accounting for activity respiration is essential for consistently implementing the metabolic theory of ecology in ATN models and for improving their quantitative predictions, which makes them more powerful tools for investigating the dynamics of complex natural communities.

Donohue, John; Piiroinen, Petri

doi: 10.1007/s12080-018-0380-5pmid: N/A

The concept of density-dependent population growth is fundamental to our understanding of how populations persist. While it is generally agreed that negative density dependence must occur at high densities, the direction of density dependence may be negative (pure negative density dependence) or positive (demographic Allee effect) at low densities. In this article, we present a technique to link the direction of density dependence to generic ecological factors. This technique involves exploiting the presence of a particular bifurcation, known as a saddle-node-transcritical interaction. We first provide a method to detect this bifurcation in a given model and then demonstrate its ecological relevance using several existing mechanistic models. With a mathematical framework in place, we are able to identify scenarios in which neither a weak Allee effect nor pure negative density dependence are possible. More generally, we find conditions on parameter values that are necessary for transitions between pure negative density dependence and demographic Allee effects to occur.

Kubo, Yuki; Lee, Joung-Hun; Fujiwara, Takahiro; Septiana, Ratih; Iwasa, Yoh

doi: 10.1007/s12080-018-0381-4pmid: N/A

Partnership programs have gained importance in forestry management. In Indonesia, profit sharing and agroforestry are examples of partnership programs between forest managers and local communities. In this paper, we analyze potential conflicts among participants in these programs. First, we derive a recursive formula to determine the future value of a compartment of plantation to the society, which includes both the forest owner and the local community. While trees are young, the land is also used for agriculture, which is an agroforestry program. When there is a high rate of future discounting and a high rate of natural disturbances, the society may find it profitable to continue the agricultural use of the land. Second, we calculate the profit for the forest owner and the local community separately. To prevent illegal logging, the owner shares a fraction of the profit obtained by selling logs with the local people, which is a profit-sharing program. Illegal logging greatly reduces the profit for the forest owner, especially when trees are tall. Illegal logging of old cohorts is harmful to the local people as well. In contrast, illegal logging of young cohorts provides profit to the local people because they will be hired to replant young trees. Our analysis shows an “overlooking period” in which a conflict of interest exists between the forest owner and the local community. We indicate that the overlooking period can be mitigated by coordination of the shared profit and the wage for the workers.

Pfab, Ferdinand; Stacconi, Marco; Anfora, Gianfranco; Grassi, Alberto; Walton, Vaughn; Pugliese, Andrea

doi: 10.1007/s12080-018-0382-3pmid: N/A

We present a model for the population dynamics of the invasive fruit fly Drosophila suzukii and its pupal parasitoid Trichopria drosophilae. Seasonality of the environment is captured through a system of delay differential equations with variable delays. The model is used to explore optimal timing for releasing parasitoids in biological control programs. According to the results, releasing parasitoids is most effective between late spring and early summer when the host population begins to increase. A single parasitoid release event can be more efficient than multiple releases over a prolonged period, but multiple releases are more robust to suboptimal timing choices. The findings can be useful for optimizing parasitoid release and should be transferable for similar systems. More generally, the model is an example for stage-structured resource-consumer dynamics in a varying environment.