A new approach for ecosystem-scale manipulations of bird abundance and species richness

Blog written by Chelsea L. Wood. Read the full paper here.

For generations, ecologists have relied on manipulative experiments to explore the dynamics of ecological communities. Some of the most influential studies in the ecology canon are experimental manipulations – think Bob Paine’s experimental exclusion of the keystone predator Pisaster ochraeceous, Stephen Carpenter’s whole-lake manipulation of nutrients, Dan Simberloff and EO Wilson’s island biogeography experiments in the Florida Keys, or Gene Likens’ forest-clearance experiments at Hubbard Brook. Correlational and comparative studies have their place for detecting and exploring the generality of patterns, but experimental manipulations are needed to understand the causal relationships that underlie ecological patterns.

But despite the value of manipulative experiments, they can be exceedingly difficult to execute, particularly when the focal community contains large-bodied, vagile species. For example, several studies point to the ecological importance of birds as predators, nutrient importers, hosts, seed dispersers, pollinators, and scavengers. But progress in understanding the ecological role of birds at the ecosystem level has been hampered by the difficulty of performing experimental manipulations of bird abundance across large spatial extents.

Our paper, recently published in Ecology and Evolution, presents a new method for experimentally increasing the abundance and richness of birds at the scale of entire aquatic ecosystems, with minimal cost, risk to wildlife, and need for maintenance. This approach involves the use of attractants that encourage birds to use a particular site, instead of deterrents that discourage birds from using that site or physically preventing their access to the site (approaches whose efficacy often attenuates over time). Our approach was effective at increasing the abundance and species richness of water‐associated birds at central California ponds.

Figure 1. Map of study sites in the East Bay region of central California. Eight experimental ponds were located in Joseph D. Grant County Park (circles) and eight were located on San Felipe Ranch (triangles). Of these, eight were randomly assigned to the bird attractant treatment (blue) and eight were randomly assigned to the control treatment (red)

We worked at 16 small ponds located on two adjoining properties in the East Bay area of central California (Figure 1). To attract birds to attractant‐treatment sites, we added perching habitat, nesting habitat, two mallard duck decoys (one male, one female), and one floating platform to each pond (Figure 2). We then assessed bird abundance by monitoring ponds with trail cameras. We compared the change in bird species richness and abundance from before the manipulation (i.e., 2014) to two years after manipulation (i.e., 2017) in control versus attractant treatments (a before–after–control–impact or BACI design).

Figure 2. Attractant manipulations installed at Glorious Pond, Joseph D. Grant County Park. bb = bird nesting boxes, fp = floating platforms, yellow arrows indicate added perching habitat

We found that our bird-attractant treatments augmented both bird species diversity and bird abundance. Bird species richness declined over time in both treatments, probably due to the effects of a prolonged drought that affected California during the time period of our experiment, but the decline in bird richness was less pronounced in the attractant compared to control treatments. Total bird abundance (across all species) increased in the attractant treatment while it declined in the control treatment. The bird species in which the attractant treatment had the most positive influence on abundance were American Robins, Black Phoebes, California Quail, Western Kingbirds, unidentified passerines, raptors, and waterbirds; together, these taxa accounted for 83% of total bird detections.

Our results suggest that simple, inexpensive modifications to existing pond habitat can produce a substantial change in bird abundance and richness – providing a way forward for field experiments that effectively quantify the ecological role of birds. It is especially notable that the manipulations were effective two years after their implementation; this allows for experiments with long temporal durations, a key feature for assessing ecological processes that occur on long time scales.

The manipulations we implemented were inexpensive, easily maintained, and unobtrusive. We estimate that our attractant treatments cost approximately US$103 per pond ($60 for wood duck box, $25 for generic bird box, $2 for fence posts to mount bird boxes, $6 for duck decoys, $10 for materials to construct floating platform), and required fewer than two person‐hours to install. In addition to their low cost, our manipulations were durable and easily maintained: despite the presence of large mammals (e.g., deer, pigs, coyote, cows) that might trample or otherwise compromise attractants, we observed no negative wildlife interactions. Manipulations required very minimal maintenance; we checked on ponds once per year and spent ~15 person‐minutes per pond per year re‐positioning floating platforms or duck decoys, supplementing shoreline perching habitat, or (for only one pond over the two‐year experiment) re‐mounting a fallen bird box. Importantly, the manipulations were unobtrusive and inconspicuous. This low visibility minimizes the chance that the treatments will be noticed by human visitors, reducing the likelihood of vandalism, theft, and objections by neighbors, park users, landowners, or land managers concerned about the aesthetic value of ponds. In fact, one of the land managers we worked with was enthusiastic about these manipulations, which she hoped would contribute to the conservation value of wetlands under her stewardship. The low cost, ease of maintenance, inconspicuousness, and conservation benefits of our approach allowed us to maximize the size and number of manipulated ponds, increasing statistical power and biological realism.

There are numerous potential applications of our approach to manipulating bird abundance and richness. We plan to use this method to perform a large‐scale, long‐term bird manipulation experiment in central California ponds. Our aim is to quantify the effect of increases in local bird abundance and richness on the composition of pond communities, and particularly on the transmission of parasites within ponds. Birds play a variety of roles in these pond ecosystems: as dispersers of parasites, predators of hosts, and hosts for vectors and the pathogens they transmit. Manipulative experiments are therefore necessary to disentangle the potential effects of change in bird biodiversity on disease processes and to discover the net effect of bird biodiversity loss on the prevalence of disease in ponds. Our method of bird augmentation might also be useful for scientists working on other questions about the ecological roles of birds, or in other ecosystems. Most bird manipulation experiments to date have investigated the role of birds as predators using bird deterrence, and bird exclusion is a suitable approach for assessing the impacts of bird predation on community composition at small spatial scales. However, because our approach can be deployed across larger spatial scales than traditional caged or netted bird exclosures, it can also be used to investigate processes that occur at large spatial scales: for example, nutrient export/import, seed dispersal, and scavenging/decomposition. Our approach could also be easily adapted to augment birds across large plots in other relatively open ecosystems—for example, grasslands, meadows, open woodlands, tundra, marshes, wetlands, dunes, and beaches.

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