Invasive species impacts on ecosystem structure and function: A comparison of the Bay of Quinte, Canada, and Oneida Lake, USA, before and after zebra mussel invasion

As invasion rates of exotic species increase, an ecosystem level understanding of their impacts is imperative for predicting future spread and consequences. We have previously shown that network analyses are powerful tools for understanding the effects of exotic species perturbation on ecosystems. We now use the network analysis approach to compare how the same perturbation affects another ecosystem of similar trophic status. We compared food web characteristics of the Bay of Quinte, Lake Ontario (Canada), to previous research on Oneida Lake, New York (USA) before and after zebra mussel (Dreissena polymorpha) invasion. We used ecological network analysis (ENA) to rigorously quantify ecosystem function through an analysis of direct and indirect food web transfers. We used a social network analysis method, cohesion analysis (CA), to assess ecosystem structure by organizing food web members into subgroups of strongly interacting predators and prey. Together, ENA and CA allowed us to understand how food web structure and function respond simultaneously to perturbation. In general, zebra mussel effects on the Bay of Quinte, when compared to Oneida Lake, were similar in direction, but greater in magnitude. Both systems underwent functional changes involving focused flow through a small number of taxa and increased use of benthic sources of production; additionally, both systems structurally changed with subgroup membership changing considerably (33% in Oneida Lake) or being disrupted entirely (in the Bay of Quinte). However, the response of total ecosystem activity (as measured by carbon flow) differed between both systems, with increasing activity in the Bay of Quinte, and decreasing activity in Oneida Lake. Thus, these analyses revealed parallel effects of zebra mussel invasion in ecosystems of similar trophic status, yet they also suggested that important differences may exist. As exotic species continue to disrupt the structure and function of our native ecosystems, food web network analyses will be useful for understanding their far-reaching effects.     

Adaptations in a hierarchical food web of southeastern Lake Michigan

Two issues in ecological network theory are: (1) how to construct an ecological network model and (2) how do entire networks (as opposed to individual species) adapt to changing conditions? We present a novel method for constructing an ecological network model for the food web of southeastern Lake Michigan (USA) and we identify changes in key system properties that are large relative to their uncertainty as this ecological network adapts from one time point to a second time point in response to multiple perturbations. To construct our food web for southeastern Lake Michigan, we followed the list of seven recommendations outlined in Cohen et al. [Cohen, J.E., et al., 1993. Improving food webs. Ecology 74, 252–258] for improving food webs. We explored two inter-related extensions of hierarchical system theory with our food web; the first one was that subsystems react to perturbations independently in the short-term and the second one was that a system's properties change at a slower rate than its subsystems’ properties. We used Shannon's equations to provide quantitative versions of the basic food web properties: number of prey, number of predators, number of feeding links, and connectance (or density). We then compared these properties between the two time-periods by developing distributions of each property for each time period that took uncertainty about the property into account. We compared these distributions, and concluded that non-overlapping distributions indicated changes in these properties that were large relative to their uncertainty. Two subsystems were identified within our food web system structure (p < 0.001). One subsystem had more non-overlapping distributions in food web properties between Time 1 and Time 2 than the other subsystem. The overall system had all overlapping distributions in food web properties between Time 1 and Time 2. These results supported both extensions of hierarchical systems theory. Interestingly, the subsystem with more non-overlapping distributions in food web properties was the subsystem that contained primarily benthic taxa, contrary to expectations that the identified major perturbations (lower phosphorous inputs and invasive species) would more greatly affect the subsystem containing primarily pelagic taxa. Future food-web research should employ rigorous statistical analysis and incorporate uncertainty in food web properties for a better understanding of how ecological networks adapt.     

D-Pulegone: A feeding deterrent to deer mice (Peromyscus maniculatus) and prairie voles (Microtus ochrogaster)

Summary D-Pulegone, a monoterpene present in pennyroyålMentha sp. is repellent to a number of vertebrates, decreasing consumption of feed and granular particles. In the present study, several tests were used to determine the reliability of this substance as a feeding deterrent. D-Pulegone (10mM—1M) applied as a coating to apple pieces significantly (P < 0.001) decreased ingestion by deer mice,Peromyscus maniculatus, and prairie voles,Microtus ochrogaster. The deterrent effect of d-pulegone was not diminished following multiple exposures suggesting that this compound may be a promising candidate for use a a general vertebrate repellent. The inhibitory effect of 1M d-pulegone on apple ingestion in voles was abolished when contact was prevented by encasing the stimulus solution in plastic mesh capsules. We conclude that while d-pulegone has repellent properties following direct contact, volatile exposure alone may not be sufficient to elicit avoidance behavior.     

Absorption, deposition and distribution of dietary aluminium in immature rats: effects of dietary vitamin D3 and food-borne chelating agent

We report the levels of aluminium, calcium and potassium in selected tissues of growing rats administered dietary or subcutaneous aluminium, and also the effect of dietary aluminium in combination with cholecalciferol, or with lactose plus a dietary chelating agent. Dietary aluminium decreased the growth rate of normal rats and increased the deposition of aluminium in the tissues. Animals given lactose with a dietary chelator showed a 17 - 100% increase in brain, heart, and muscle aluminium concentration in comparison with those fed aluminium alone. Animals fed both aluminium with cholecalciferol also showed increased levels (12-39%) of aluminium, chiefly in muscle and heart in comparison with those fed aluminium alone. Aluminium deposition was correlated positively with Ca2+ and K+ levels among each of these tissues. We conclude that in normal growing rats aluminium deposition is increased in heart and muscle in the presence of vitamin D3 and in brain, heart and muscle in the presence of lactose and a dietary chelating agent.