EXERGY ANALYSIS OF A COMPLEX, MULTI-SPECIES EVOLUTIONARY HISTORY IN AN ISOLATED ECOLOGICAL NICHE

This study extends an older two-species interaction model proposed by the same Authors a few years ago to describe three possible interaction modes (namely cooperative, parasitic and competitive) between different species. As in the previous model, we consider here a set of species occupying the same ecological niche but introduce a more complex interaction among the species. In the absence of predators, species N1 (the “prey”) would thrive in the assigned niche and -if undisturbed for a long period of time- reach its carrying capacity. The other species (three in this exercise) occupy the very same niche and display different interaction modes with the first one and among each other: N2 and N3 prey on N1, N3 preys also on N2 while N4 competes directly for N1's food resources. We assume that the resources available within the niche can be quantified by their exergy content. Since the preys are also “resources” for the predators, they are assigned an exergy value proportional to the average calorific nutritional value of their meat. The present study is more than a theoretical exercise in exergy resource allocation: it is intended to offer a contribution to a recovery plan prompted by a recent series of articles and news originating from the New Zealand Environmental Agency that described the situation of a critically endangered species, the Kākāpō (pronounced “ka·kuh·pow”), or Strigops habroptilus, a large, flightless, nocturnal parrot found only in New Zealand. It is the world's only flightless parrot, has a heavy build and nocturnal habits, is herbivorous with a low basal metabolic rate and a reported lifespan of up to 100 years. Originally preyed on by hawks and eagles, the Kākāpō developed a defensive strategy that allowed its survival over significant geological times. For several reasons the bird was exposed to new alien predators brought into its niche by humans in the last 1000 years or so and -unable to adapt- was on the verge of extinction until a conservation program was launched in the 2000. This is our species N1. To simplify the treatment, we consider only two of Kākāpō's predators: stoats (N2) and rats (N3), both of which actively raid the nests and kill chicks. The competitor species (N4) is the possum, who feeds on the same two- or three plant types as the Kākāpō. After having quantified the resources, an evolution equation for each species is derived, and the behaviour of the 4 coupled equations is studied under a set of simplifying assumptions derived by the published field data. The resulting time evolution curves confirm that without strongly proactive conservation measures the Kākāpō would go extinct within few years and can only survive in environmental niches from which its “alien” predators have been eliminated. The model also provides an indication for the limit specific carrying capacity (individuals/km2) and describes how this limit depends on the parameters of the model.