Wesner, JS, EJ Billman, and MC Belk. 2012. Multiple predators indirectly alter community assembly across ecological boundaries. Ecology 93: 1674-1682. (see Publications for a .pdf).
Choosing where to lay eggs is critical to an organism’s fitness. Make the wrong choice, and your offspring are toast. It’s no surprise, then, that variation in habitat quality can often lead to variation in colonization, which is a key process during community assembly. Quality is defined in various ways, but one of the most obvious is the presence of predators, and organisms will often avoid colonizing habitats with predators when given the choice. But two features may complicate this choice, 1) predator diversity, and 2) the location of predator-free habitats in the landscape.
We measured insect colonization of aquatic mesocosms that contained predatory trout, dragonflies, or both. In addition, we varied the position of predator-free controls; some were adjacent to mesocosms with predators, while others were isolated. Based on previous research, we expected colonization in the adjacent controls to be intermediate between colonization in the predator treatments and isolated controls. We also expected colonization in the treatment with two predator species to be lower than colonization in treatments with single species.
Surprisingly, rather than showing intermediate preferences, we found that egg-laying insects (mostly chironomids) did not distinguish between treatments with single predator species, multiple predator species, or adjacent controls. They avoided all of these in favor of isolated predator-free mesocosms. This occurred despite the fact that adjacent and isolated predator-free controls were identical. Thus, colonization did not vary solely as a result of intrinsic habitat quality, but also as a result of the quality of adjacent habitats. This means that habitats that might appear identical to us based on intrinsic sampling may be perceived differently by organisms that colonize them. If these response scale-up, our results have consequences for traditional habitat models, as well as the design of conservation reserves.
Wesner, JS. 2012. Emerging aquatic insects as predators in terrestrial systems across a gradient of stream temperature in North and South America. Freshwater Biology 57: 2465-2474.
For most aquatic insects, emergence to adulthood means two things: sex and death. As a result, emergence is often synchronous (sex requires other individuals, of course), resulting in mating swarms. These swarms also make good meals for terrestrial predators, subsidizing their diet. More food can feed more predators, so streams that produce more emerging insects also tend to support more birds, spiders, lizards, etc. From a bottom-up perspective, the importance of this subsidy is clear. However, not all insects are limited to sex and death. Some, like dragonflies, feed as adults, meaning they’re both prey (bottom-up) and consumers (top-down) in terrestrial food webs, with potentially important consequences.
Dragonflies are also more diverse and abundant in warm climates. In contrast, stoneflies, some of which are predators in the stream but never as adults, are more abundant in cold climates. This means that predation by adult aquatic insects may vary along a temperature gradient. In this paper, I asked how a latitudinal changes in average stream water temperature might affect both the magnitude and trophic structure (proportion of predators) of insects emerging from streams. I used published benthic insect datasets (i.e. larval stream datasets) as a proxy for emergence, because emergence collections are relatively rare. Collections ranged from Alaska to Brazil, spanning average mean stream temperature from 4 to 25 degrees Celsius.
The trophic structure, but not the magnitude, of potential emerging insects varied across the temperature gradient. Warm streams had proportionally more adult predators than cold streams (range: 0-12% by abundance, and 0-86% by biomass), but not necessarily more overall prey. Thus, the “bottom-up” potential of emerging subsidies was consistent, while the “top-down” potential varied widely. I argue that this top-down potential should be considered more widely in studies of aquatic-terrestrial linkages, particularly in warm climates.
Wesner, JS and MC Belk. 2012. Habitat relationships among biodiversity indicators and co-occurring species in a freshwater fish community. Animal Conservation 15, 445-456.
What if conservation of a single species simultaneously benefited other species in the ecosystem? That is the allure of “indicator species” or “surrogate species”, particularly those that seem to occur in places of high species diversity. Unfortunately, an association between a given species’ presence and high diversity does not necessarily mean that conservation of that species will improve diversity. We demonstrated this in the latest (October) issue of Animal Conservation.
We collected fishes from a bunch of sites in the Bear River Drainage, which straddles the UT/WY border before flowing into the Great Salt Lake. The drainage is home to northern leatherside chub (NLC), a species of conservation concern. NLC is a working man’s minnow, all grays and browns, clear fins, the kind of fish that is too small to even be considered a trash fish, and so nondescript that it wasn’t described until 2004. But it knows where to live. Streams with NLC tend to have higher fish diversity than streams without NLC, a pattern that was also true for three other common fishes during our surveys.
Given those associations, we might conclude that restoring the habitat of any of those species would benefit most other species in the drainage. We would be wrong. Instead, the species we examined showed mostly idiosyncratic habitat relationships. In other words, the main habitat associated with one species was neutrally, or sometimes even negatively, associated with other species. Had we only focused on the species-diversity associations, and not examined the underlying habitat associations, we would have falsely supported using one or more of those species as “surrogates” for other fish.