An effective method for ecosystem-scale manipulation of bird abundance and species richness.

Pieter T. J. Johnson,Margaret Summerside,Chelsea L. Wood

doi:10.1002/ece3.5509

Pieter T. J. Johnson, Margaret Summerside + Show 1 more

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https://doi.org/10.1002/ece3.5509

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Abstract

Manipulation experiments are a cornerstone of ecological research, but can be logistically challenging to execute—particularly when they are intended to isolate the ecological role of large, vagile species, like birds. Despite indirect evidence that birds are influential in many ecosystems, large‐scale, multi‐year bird manipulation experiments are rare. When these studies are conducted, they are typically realized with caged or netted exclosures, an approach that can be expensive, risky for wildlife, and difficult to maintain. In cases where caged exclosures are not appropriate, alternate approaches are needed to allow rigorous empirical studies on the ecological role of birds. Here, we present and validate a method for experimentally increasing the abundance and richness of birds at the scale of entire aquatic ecosystems. Unlike bird exclusion, this approach is experimentally tractable, appealing to land managers, and possible to deploy over large spatial scales. We tested the efficacy of our approach for increasing bird abundance and species richness at 16 central California ponds. Based on bird visitation data obtained by summer camera trapping, our approach significantly increased bird species richness and abundance at manipulated ponds compared to control ponds. Attractant treatments mitigated the negative effects of a major drought on bird species richness and generated a near doubling of bird abundance in the presence of attractants. Treatments had no effect on most mammal species, with the exception of ground squirrels, which increased in abundance in the presence of attractants. These results suggest that attractants are effective in increasing bird abundance and richness. We encourage researchers to consider this approach for experimentally isolating the ecological role of birds in aquatic and open terrestrial ecosystems, especially in cases where cost or logistical constraints preclude the use of caged or netted exclosures.

Highlights

Manipulation experiments have driven tremendous progress in ecol‐ ogy
With a combination of enhanced nesting, roosting, and perching habitat, we demonstrate the efficacy of this approach in increasing the abundance and species richness of water‐associated birds at cen‐ tral California ponds, while leaving the abundance of most other
The bird attractant treatments we implemented were effective in augmenting the abundance and species richness of birds at eight central California ponds, even two years after the establishment of treatments

Summary

Introduction

Manipulation experiments have driven tremendous progress in ecol‐ ogy. Correlational and comparative work can point to potential eco‐ logical roles for particular taxa, but experimental manipulations are needed to understand causal relationships (Lubchenco & Real, 1991). Birds can function as key nutrient importers in some habitats, including lakes (Manny, Johnson, & Wetzel, 1994), islands (Croll, Maron, Estes, Danner, & Byrd, 2005; Stapp, Polis, & Sanchez Pinero, 1999; Young, McCauley, Dunbar, & Dirzo, 2010), urban forests (Fujita & Koike, 2007), and intertidal ecosystems (Bosman, Toit, Hockey, & Branch, 1986). Beyond their direct effects, birds function as a transport system for other or‐ ganisms, both free‐living and parasitic. Other ecological roles for birds include seed dispersal, polli‐ nation, scavenging, and pest control (Sekercioglu, 2006)

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