Last Saturday, snapshots of a spring peeper (see A Peeper’s Problem) were used to springboard a discussion regarding habitat fragmentation and the conservation of species that exhibit behavioral characteristics not exclusively bound to a single ecological community type. The general idea was that saving a forest from commercial harvest, or conserving a wetland, is an essential step towards preserving biodiversity; however just as important to conservation efforts is the protection of wildlife corridors and other thoroughfares used by flora and fauna. In that post, the spring peeper was forwarded as an exemplar of a species whose natural history requires spatial dispersion between differing communities; those communities used for reproduction (wetlands) and those used during non-reproductive adulthood (forests). To further delineate the “metacommunity” concept, the current post aims to look at this idea through the bulging eyes of a different, though not wholly dissimilar, species – the squirrel frog.
As fate would have it, on the very same stormy night that the peeper was victimized by the paparazzi, another frog also happened into the viewfinder – as though he knew that a herpetologically-laden week of community ecology discussions at Ecographica was eminent…
Meet Hyla squirella, the squirrel frog:
The squirrel frog is common throughout the Southeastern United States and like Pseudacris crucifer, Hyla squirella is a terrestrial tree frog that undertakes journeys from “water-to-wood and back again” during its life cycle. These travels are bound to reproduction and early development in water, with maturation and adulthood driving them to arboreal existence in the uplands.
Metamorphosis from tadpole to froglet is the starter pistil for transitioning to the trees, with seasonality - specifically the rainy season - as one of the big signals for movement water-ward. Thus, H. squirella represents a biological link between two ecologically distinct communities; a wetland with depressional geomorphological features that are awash in aquatically adapted plants, invertebrates and fish with fluctuating levels of hydrology and nutrients, VERSUS a forest system with epiphytic plants, wood boring insects and a generally more arid microclimate.
A set of distinct ecological communities that are biologically entangled through the spatial dispersion of commonly hosted, interacting species is one way of defining the term “metacommunity.” Essentially, a metacommunity is an order of ecological organization above the community level. So, moving backwards through the hierarchy, a metacommunity is a set of distinct communities, a community is a set of distinct populations and a population is a set of individuals. And, just as individuals interact and associate with each other under rules established by population dynamics, communities can interact with each other in a landscape through processes that can be described in terms of a metacommunity dynamic.
In traversing ecologically unique community structures, the niche of the squirrel frog presuppose many risks inherit to a world of pavement and progress, but Hyla squirella enters the battle well equipped and is armed with the decision making tools afforded by natural selection.
For example, during ovipositioning the squirrel frog deposits its eggs in elongate, slender strings with each egg lined-up, one after the other, like dominos made of pearls; these strings are laid in waters that have been positively selected for their ability to supply young tadpoles with the resources required for growth and development. For the herbivorous Hyla tadpoles, this means that an abundance of algae, plants and inorganics can be found attached to submersed logs, rocks and other structures. The tads are suspension feeders; this means that they scour the surface of substrates for nutrients, akin to tiny vacuum cleaners, leaving no surface un-sampled. Although food acquisition is vital to the tadpoles ultimate success, another key decision also enters into the equations of the mother frog’s evolutionarily provided calculator – predator avoidance.
In considering the processes under which metacommunal species weigh the risk of death against the benefits of nutrient availability (growth) within a given habitat, C.A. Binckley (Old Dominion University) and W.J. Resetarits (University of Southampton) examined the squirrel frog’s preferences in natal ponds. They constructed 54 experimental ponds in which they controlled nutrient availability and the occurrence of fish that prey on hylid eggs. By comparing the total eggs deposited at each artificial pond, they were able to correlate the pond preference of mother frogs for expressed nutrient availability and predatory risk. Their study demonstrated that within a variable landscape, metacommunal species display habitat choosing behaviors that are in accordance with optimization theory and predicted foraging behavior. In other words, the research showed that the squirrel frogs exhibited a pond choosing behavior which can be affectively viewed as the frog weighing the risk of death against the opportunity for growth; with the frog trying to achieve the lowest possible “mortality /growth” value.
In a similar fashion, it’s a safe bet that similar “decisions” are undertaken by H. squirella when choosing upland habitats as an adult… The decision making toolset programmed into the genes of the squirrel frog not only provides the ability to survive and excel within a variety of community types, it also is the instrument through which communities as distinctive as wetlands and upland forests are linked. Alteration of one habitat, one species, or even one gene, can have reverberations in far ranging ecosystems; this is precisely why conservation of those connections is of the utmost importance.
Binckley, C., & Resetarits, W. (2008). Oviposition behavior partitions aquatic landscapes along predation and nutrient gradients Behavioral Ecology, 19 (3), 552-557 DOI: 10.1093/beheco/arm164
Winter that wasn't
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