Commentary on How Habitat Choice Dictates Aquatic Beetles’ Richness, Abundance and Composition

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Commentary on How Habitat Choice Dictates Aquatic Beetles’ Richness, Abundance and Composition

Overview

         Binckley & Resetarits (2005) investigate how abundance and distribution patterns of organisms can be a result of two distinct mechanisms. The first and most dominant paradigm is the random dispersal which is succeeded by non-random and site-specific mortality. The second paradigm in communal ecology is habitat selection which incorporates individual redistribution across habitat patches according to perceived as opposed to realized fitness. The researchers conducted two experiments to quantify how non-lethal availability of predatory fish impacts the species richness and mean abundance of aquatic beetles. They hypothesized that habitat selection behaviour would be a key driver in species diversity and distribution. Furthermore, Binckley and Resetarits (2005) predicted that according to earlier research by Wellborn et al. (1996), the distribution of predatory fish would be a major determinant of the distribution and abundance of numerous aquatic taxa at the landscape-level.

            In the first experiment, they crossed non-lethal presence or absence of a single Enneacanthus obesus with supplemental habitat complexity. Considering that habitat complexity is known to reduce predation, the researchers examined beetle response to a familiar predator, complex habitats preference and if or not the said preferences were influenced by the fishes. Twenty-four experimental ponds were established and ordered in six linear blocks each made up of four pools in an area surrounded by pine and hardwood forest. Later, the ponds were covered with a screen of tight-fitting fiberglass to stop premature infiltration of insects. The ponds were then filled with tap water and two days later zooplanktons and phytoplankton were added. Then the four treatments were assigned to the pools randomly. Later on, fishes were put below screens and screens submerged under water. Habitat complexity was then added on top of the lids. In effect, this allowed chemical communication between the beetles and predators while preventing any physical interactions. Later, beetles were collected out of the pools and preserved in ethanol. Using MANOVA, effects of blocks, as well as treatments on the mean abundance of beetles and species richness, were examined.

            In the second experiment, the same methodology as in the first was used. Twelve experimental ponds over four blocks were established and three randomly selected treatments assigned to ponds within blocks. The three were two Enneacanthus gloriosus and two Aphredoderus sayanus as well as fishless controls. Every week beetles were collected for an overall three weeks. Like in the first experiment, MANOVA was used for analysis with non-orthogonal multivariate and univariate contrasts examining treatment differences.

            The findings indicated that mean species abundance and richness reduced in the non-lethal presence of fishes. Moreover, the magnitude of effects increased in the second experiment. Notably, the complexity of the habitat had nil effect as well as no interaction with either presence or absence of fishes in the first experiment. Response to A.sayanus had no big difference with controls in the second experiment. Researchers, however, are still investigating the response of beetles and tree frogs to A. Sayanus (Binckley & Resetarits 2003).

            Furthermore, the experimental approach used by the researchers showed that presence of tiny predatory fishes notably reduced beetle mean abundance and species richness solely through habitat selection behaviour. Additionally, aquatic beetles clearly preferred fishless ponds with their species abundance and richness similar to those of previously done surveys. Having curiously noted the reduced abundance of a few beetle species in ponds that had fishes in the second experiment, the authors suspected it to be the consequence of density and not predator identity.

            Overall, the experiments played a crucial role in substantiating suggestions that habitat selection is a basic process that drives the abundance and distribution of aquatic life forms. Additionally, results obtained affirm that for species that can detect and hence avoid fishes, decreasing local mortality goes with increasing predator numbers. Crucially, however, the authors note that modern ecologists lack a deep understanding of processes that operate at different scales relate to impact patterns of diversity and distribution. They, therefore, go on to explain that such interactions are generated by habitat selection depending on specific habitat characteristics.

Critique

            To test their hypothesis, Binckley & Resetarits (2005) performed two rather distinct experiments. In the first, beetle response was tested to a known predator only. Though it is clear that habitat complexity decreases predation, it would have been wise to widen the scope of the first experiment to include not-so-obvious predators. Such an experiment would have yielded even more tantalizing details regarding species abundance and richness. Moreover, the location of the ponds in a forested field might have brought in some situational and environmental bias in the experiment. Perhaps, to counter such a possibility, the researchers would have considered replicating their experiments in different environmental setups to cater for the various kinds of niches that beetles are known to inhabit. Another key weak-point of the first experiment is the addition of 0.4kg of dried leaf litter together with 1.01 of zooplankton as well as phytoplankton. The researchers here never pointed out the reason for the varying amounts of the two and whether any more or any less of the same would have resulted in different outcomes of the experiment. For instance, maybe the amount placed would have affected the fish’s level of predatory behaviour towards the beetles. Furthermore, collection and preservation of beetles after seventeen days seems rather too soon to be used as a predictor for activities that happen over very long periods in the natural environment.

            I am also concerned at the pedestrian way the second experiment was presented. From the outset, one may make a mistake of taking it as just another small experiment, yet it is in fact perhaps the most detailed. Its inclusion of two distinct fish species and a fishless control means that a more detailed description of the step by step procedures would have left little chance for filling gaps among readers. A curious reader might have wished to know how the two species differ significantly as to both be included in the same experiment. Moreover, one may wish to know whether any of their characteristics would have required researchers to use specialized handling. Even more distressingly for me is the fact that in the description of the two experiments, no clear link between them has been made. As such, the description of the second experiment appears like a continuation of the first which is not the case. In effect, the specific goals of each of the two experiments should have been made clear. Hypotheses and predictions were also overlooked and as such, the reader is forced to deduce them from the available literature. That lack of the two crucial statements negates the true nature of any scientific inquiry. Additionally, the results section is also rather short and short of a deeper explanation.