Abstract
The atmospheric boundary layer and lower free atmosphere, or aerosphere, is increasingly important for human transportation, communication, environmental monitoring, and energy production. The impacts of anthropogenic encroachment into aerial habitats are not well understood. Insectivorous birds and bats are inherently valuable components of biodiversity and play an integral role in aerial trophic dynamics. Many of these insectivores are experiencing range-wide population declines. As a first step toward gaging the potential impacts of these declines on the aerosphere’s trophic system, estimates of the biomass and energy consumed by aerial insectivores are needed. We developed a suite of energetics models for one of the largest and most common avian aerial insectivores in North America, the Purple Martin ( Progne subis ). The base model estimated that Purple Martins consumed 412 (± 104) billion insects*y-1 with a biomass of 115,860 (± 29,192) metric tonnes*y-1. During the breeding season Purple Martins consume 10.3 (+ 3.0) kg of prey biomass per km3 of aerial habitat, equal to about 36,000 individual insects*km-3. Based on these calculations, the cumulative seasonal consumption of insects*km-3 is greater in North America during the breeding season than during other phases of the annual cycle, however the maximum daily insect consumption*km-3 occurs during fall migration. This analysis provides the first range-wide quantitative estimate of the magnitude of the trophic impact of this large and common aerial insectivore. Future studies could use a similar modeling approach to estimate impacts of the entire guild of aerial insectivores at a variety of temporal and spatial scales. These analyses would inform our understanding of the impact of population declines among aerial insectivores on the aerosphere’s trophic dynamics.
Highlights
With increasing human use of the atmospheric boundary layer and lower free atmosphere for energy, communication, transportation, and remote sensing, our need to understand the aeroecology of animals whose life histories depend on this environment has increased [1]
The quality and maintenance of housing by people can strongly impact the reproductive output of birds and we suspect that the housing and protection of martins that are the subject of intensive study is likely to be better than average. Rather than use this model with an unrealistically high growth rate as the basis for all of our comparisons, we chose to compare among models with minimal annual population change (λ =1). We argue that these models are optimal for comparisons of energy dynamics because (1) the long-term average λ for Purple Martin populations must be close to 1 and (2) based on breeding bird surveys, this value is within 1% of the value for recent Purple Martin population dynamics, which indicate an average decline over the past few decades of about 1% per year range wide (λ =0.99)
We estimated that Purple Martins consume a minimum of 262 billion insects annually, which is likely to be a significant component of aerial trophic systems in many regions
Summary
With increasing human use of the atmospheric boundary layer and lower free atmosphere (aerosphere) for energy, communication, transportation, and remote sensing, our need to understand the aeroecology of animals whose life histories depend on this environment has increased [1]. Understanding the dominant ecological processes occurring in the aerosphere, including consumption of insects by vertebrate predators, bolsters our basic understanding of trophic interactions, which is important for effective conservation and management of aerial species and their habitats (e.g., [2,3]) These trophic relationships are of added interest owing to regional population declines in avian insectivores across broad spatial scales [4,5]. It is difficult to quantify the cumulative trophic impact of insectivorous birds because, at a minimum, it requires estimates of (1) total numbers of these birds, (2) energetic requirements of those birds, and (3) energy content of insect prey Despite this difficulty, a quantitative model of the magnitude of energy flowing through aerial trophic systems could provide a starting point for investigations of the potential ecological consequences of changes in the abundance of both predators and prey [8]. These types of estimates are relatively rare in the ecological literature [9]
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