1. INTRODUCTION
Ecosystem engineers alter the availability of resources for other
species by causing physical state changes in biotic or abiotic materials
(Jones, Lawton & Shachak, 1994; Wright, Jones, & Flecker, 2002; Buse
et al., 2008). Given the important role they play in local environments,
the literature surrounding ecosystem engineers is historically focused
on how their actions affect other species (Jones et al., 1994;Robles
& Martin, 2013;Tarbill,
Manley, & White, 2015;Wiebe,
2017), but little research has been published on what external factors
influence the engineers themselves (seeMikusinski,
2006;Jusino,
Lindner, Banik, & Walters, 2015). Importantly, little has been done
to investigate how ecosystem engineers choose breeding and young rearing
grounds
(Nilsson,
Johnsson, & Tjernberg, 1991; Garmendia, Cárcamo, & Schwendtner,
2006). Understanding these driving factors is essential to understanding
the ecology of not only the ecosystem engineers themselves, but the
organisms that rely on them for their own breeding and nesting grounds
as well.
The modifications made by ecosystem engineers have far-reaching
consequences and directly impact not only ecological associations, but
also the behavior of animals within an ecosystem. For example, animal
movement and community composition may be altered by the actions of
local ecosystem engineers (Lill & Marquis, 2003; Bangert &
Slobodchikoff, 2004). In this way, ecosystem engineers can indirectly
influence local trophic levels through multi-level environmental
modifications, such as altering local invertebrate diversity and
abundance, which in turn may increase foraging opportunities for other
vertebrates (Lill & Marquis, 2003; Bangert & Slobodchikoff, 2004), or
by providing more suitable species specific habitat for nesting
(Showalter & Whitmore, 2002)
Although insects themselves can act as ecosystem engineers (Bell &
Whitmore, 1997; Lill & Marquis, 2003; Bangert & Slobodchikoff, 2004),
they can also act as crucial resources for other ecosystem engineers at
higher trophic levels
(Hess
& James, 1998;Pechacek
& Kristin, 2004). For example, declines in insect richness and
abundance have been reported with parallel declines in a number of
insectivorous ecosystem engineers, such as woodpeckers
(Lister &
Garcia, 2018,Møller,
2019,Karr,
1976;Benton,
Bryant, Cole, & Crick, 2002;Rioux
Paquette, Pelletier, Garant & Bélisle, 2014;Narango,
Tallamy, & Marra, 2017;Bowler,
Heldbjerg, Fox, Jong, & Böhning‐Gaese, 2019). Therefore, ecosystem
engineering activities may be better understood by looking at the
distribution and abundance of their food resources.
Woodpeckers are avian ecosystem engineers that have a large proportion
of insects in their diet
(Jones
et al., 1994;Tarbill
et al., 2015), and control the location, construction, and
availability of nesting cavities, a limiting resource for secondary
cavity nesting birds (SCB; i.e. species that require a cavity to nest in
but cannot create the cavity themselves). Woodpeckers are primary
excavators of nesting cavities, often creating multiple cavities within
their home range each year to avoid predation, external parasite
buildup, and cavity wood degradation (Loye & Carroll 1998; Husak &
Husak, 2002; Wiebe, 2017). Once abandoned, these cavities are used by a
variety of secondary cavity nesting species
(Martin
& Eadie, 1999,Pakkala,
Tiainen, Piha, & Kouki, 2019). Woodpeckers select nesting sites based
on characteristics that protect their eggs and nestlings from predation,
tending to nest high in moderately to heavily decayed trees with wide
diameters at breast height (DBH), and with limited vegetation covering
the cavity entrance (vegetation cover, Mannan, Meslow, & Wight, 1980;
Li & Martin, 1991; Loye & Carroll, 1998; Newlon, 2005; Jusino et al.,
2016). Additionally, the shape of woodpecker cavities functions to
exclude nest predators by having small entrance holes and deep depths
(Sedgwick & Knopf, 1990; Li and Martin, 1991; Martin, Aitken, & Wiebe,
2004; Rhodes, O’donnell, & Jamieson, 2009). Given the nest construction
preferences of woodpeckers, the cavities they leave behind are often
superior nesting spaces when compared to naturally occurring cavities,
both of which are used by SCB (Martin & Li, 1992; Maziarz, Broughton,
& Wesolowski, 2017).
Woodpecker resources can be defined both in terms of food (mainly wood
burrowing insects, largely in the order Coleoptera) and in the number of
trees suitable for excavation (Bonnot, Millspaugh, & Rumble,
2009 ;Rota,
Rumble, Lehman, Kesler, & Millspaugh, 2015). These resources have
been shown to be directly linked to woodpecker nest site location and
home range sizes (e.g. the area used by a bird in its daily movements)
(Worton, 1989; Powell, 2000;Wiktander,
Olsson, & Nilsson, 2001;Pasinelli,
2007). For example, the Black-backed woodpecker (Picoides
arcticus ) selects nesting sites based on infestations of the mountain
pine beetles (Dendroctonus ponderosae ) (Rota et al., 2015), and
the Three-toed woodpecker’s (Picoides dorsalis ) home range size
is negatively correlated with the number of trees with suitable DBH for
cavity excavation (Pechacek & d’Oleire-Oltmanns, 2004). However, no
studies to date have looked at the impact of food resources on both the
nest site location and home range sizes of woodpeckers, which in turn
directly impacts neighboring SCB.
The Golden-fronted woodpecker (GFWO, Melanerpes aurifrons ), is a
poorly studied, medium sized bird, whose range extends from Central
America to Texas (Wetmore, 1948; Sauer, Link, Failon, Pardieck, &
Ziolkowski, 2013; Schroeder, Boal, & Glasscock, 2013). GFWO numbers are
in decline across their Texas distribution, and are considered a species
of concern in the Texas Wildlife Action Plan (Bender, 2007). As with
other woodpecker species, GFWO act as ecosystem engineers, providing
nesting cavities for SCB throughout their range (Husak & Maxwell,
1998). Determining the factors that influence the nest site location and
construction of cavities is crucial to not only understand the
conservation needs of GFWO, but also for the conservation and basic
ecology of SCB that may rely on the cavities created by GFWO.
To investigate relationships between the GFWO and local SCB nesting
successes, we conducted an observational study on GFWO nesting success
(≥ 1 fledgling) in relation to nesting site locations, home range sizes,
local insect biomass, and cavity construction, along with the nesting
success of the four most common SCB in our study area, the Black-crested
Titmouse (BCTI; Baeolophus atricristatus ), Ash-throated
Flycatcher (ATFL; Myiarchus cinerascens ), Brown-crested
Flycatcher (BCFL; Myiarchus tyrannulus ), and Bewick’s Wren (BEWR;Thryomanes bewickii ) in the southern Texas Tamaulipan Brushlands
(Baumgardt, Morrison, Brennan, Pierce, & Campbell, 2019).
The objectives of our study were to determine 1) the role of insect
availability in nest site location and home range size of GFWO, 2) the
role of nest metrics (e.g. DBH, vegetation cover) in the nesting success
of GFWO and the four species of SCB, and 3) if SCB cavity selection and
nesting success differed between abandoned woodpecker cavities and
natural cavities. We predicted 1) insect abundance would be greater at
GFWO occupied sites versus GFWO unoccupied sites and that home range
size would be negatively correlated with the availability of insect
orders commonly eaten by birds, 2) the same cavity metrics would
influence nest success in both GFWO and SCB species and 3) that SCB
would tend to nest in, and have higher nest success in abandoned
woodpecker cavities compared to natural cavities, and that abandoned
woodpecker cavities would share characteristics making them more
suitable for nesting birds, compared to natural cavities.