FUNDING AND FIELD ASSISTANCE

Funding for this study was provided by D.O.D. Threatened & Endangered Species funds from Eglin Air Force Base, a National Science Foundation DDIG, and grants from Sigma Xi, Florida Ornithological Society, WPI International, PEO International and the Virginia Tech Graduate Research Development Program.

Many thanks to the RCW Research Team, Jackson Guard personnel at Eglin AFB and seasonal field crews for their valuable assistance and support.

INTRODUCTIONINTRODUCTIONDirect and indirect species interactions within ecological communities may play a strong role in influencing or maintaining community structure. Complex community interactions pose a major challenge to predicting ecosystem responses to environmental change because predictive frameworks require identification of mechanisms by which community interactions arise. Cavity-nesting communities are well suited for mechanistic studies of species interactions because 1) cavity-nesters interact through the creation of and competition for cavity nest-sites and 2) cavity availability is relatively easy to measure and manipulate. In this study, we use a cavity ‘nest-web’ and path analysis as a predictive framework for identifying potential indirect species interactions between the federally endangered Red-cockaded Woodpecker and other cavity-nesting birds in northwest Florida. We then use experimental manipulation of cavity availability to test predictions about potential species interactions.

CONCLUSIONSCONCLUSIONS

PART 1: NEST WEBS AND PATH MODELINGPART 1: NEST WEBS AND PATH MODELINGMETHODS: From 2002-2005, we monitored abundance and nests of cavity-nesters on 36 48ha plots at Eglin Air Force Base (Fig. 1). We constructed a nest web using nest cavities for which we could identify the excavator (Fig. 2). We then developed a path diagram to depict an indirect relationship identified in the nest web (Fig. 3). Observational units in the model were research plots from 2003-2005 (n=92), excluding plots which had experimental manipulations from a concurrent study on large cavity availability (n=16, see Part 2). We developed our path model using AMOS v.5.0 and assessed model fit with chi-square analysis (significant at P > 0.05) and root mean square error approximation (RMSEA, significant at P < 0.05).

EXPERIMENTAL RESULTSEXPERIMENTAL RESULTS

The application of metal restrictor plates caused a decline in large SCN nests found in Red-cockaded Woodpecker cavities and a switch to the use of snags by the Northern Flicker. Large pine snags apparently buffered the impact of metal restrictor plates. The use of snags for nesting requires either the ability to find existing cavities in snags or excavate new cavities. The Northern Flicker, capable of cavity-excavation, had greater flexibility than SCNs in switching to the use of available snags when Red-cockaded Woodpecker cavities were restricted and this flexibility was reflected in our experimental results. These results indicate that Red-cockaded Woodpecker cavity management can indirectly impact large SCNs.

CAVITY EXCAVATION AND ENLARGEMENT AS MECHANISMS CAVITY EXCAVATION AND ENLARGEMENT AS MECHANISMS FOR INDIRECT INTERACTIONS IN AN AVIAN COMMUNITYFOR INDIRECT INTERACTIONS IN AN AVIAN COMMUNITY

Lori A. Blanc, Department of Biology, Virginia Tech University, Blacksburg, VA 24061, E-mail: lblanc@vt.eduJeffrey R. Walters, Department of Biology, Virginia Tech University, Blacksburg, VA 24061, E-mail: jrwalt@vt.edu

Figure 3. Path diagram depicting the indirect relationship between the RCW cavities, pine snags, Northern Flicker and large SCNs, including the southeastern American Kestrel and Eastern Screech-Owl. Arrows reflect hypotheses about causal relationships between variables; associated numbers indicate the path coefficient (i.e. direct effect of independent variable on adjacent dependent variable). The model explained 35% of variation (R2) in large SCN nests and had a strong fit to observed data. When modified to include only snags suitable for nesting (dbh > 20cm), model fit improved (X2 = 5.0, P=0.41; RMSEA = 0.008). In addition, the path between pine snags and Northern Flicker became significant (P=0.001).

Figure 4. Metal restrictor plates -- a cavity management technique commonly used for management of the federally endangered Red-cockaded Woodpecker. We used restrictor plates to experimentally reduce the number of enlarged Red-cockaded Woodpecker cavities and prevent Red-cockaded Woodpecker cavity enlargement.

Study SiteStudy Site

Figure 1. The study was conducted at Eglin Air Force Base in the Florida panhandle.

Figure 2. A nest web diagram depicting cavity creation and use within the cavity-nesting bird community at Eglin Air Force Base. N indicates the number of nests found for a species; E indicates the number of nest cavities excavated by that species. Links between the Secondary Cavity Nester (SCN) level and the Primary Excavator (PCE) level represent the proportion of SCN nests found cavities excavated by the indicated PCE. Links between the PCE and Tree level show the proportion of PCE nests found in that tree resource.

Bars reflect mean change in relative abundance from 2003 to 2005, including StdErr.

No difference between the change in Northern Flicker (NOFL) abundance in experimental and control plots (U = 70.5, P = 0.42).

Decrease in American Kestrel (AMKE) abundance in experimental plots, but not significantly more than control (U = 71, P = 0.40).

Marginally significant increase in Red-cockaded Woodpecker (RCW) abundance in experimental plots (U = 54.5, P = 0.08).

Bars reflect mean change in the number of Northern Flicker (NOFL) nests found from 2003 to 2005, including StdErr. Asterisk indicates significant difference (P<0.05) between control and experimental plots.

Decrease in number of NOFL nests found in RCW cavities in experimental plots, although not significantly more than in controls

(U = 78, P = 0.11).

Significant increase in NOFL nests found in snags in experimental plots (U = 48.5, P = 0.01), indicating a switch to the use of snags.

Bars reflect the mean change in the number of large SCN nests found from 2003 to 2005, including standard error. Asterisk indicates significant difference (P<0.05) between control and experimental plots.

Number of large SCN nests found in Red-cockaded Woodpecker (RCW) cavities decreased significantly more in experimental plots than in controls (U = 89.5, P = 0.01).

No change in number of large SCN nests found in snags (U = 64.5, P = 0.38).

In summary, we identified an indirect interaction between the Red-cockaded Woodpecker and large SCNs, mediated by the Northern Flicker and used path analysis to show how cavity excavation and enlargement can drive these relationships. Then, through experimental manipulation, we a) confirmed the role of these 2 mechanisms in creating habitat for large SCNs and b) demonstrated that metal restrictor plates can disrupt this indirect relationship by affecting Northern Flicker cavity-enlargement behavior. Recent studies indicate that indirect interactions based on modification of traits (e.g., behavior), such as that shown here, may be highly important and widespread phenomena within ecological communities.

This study also provides an empirical example of how, in ecological communities where complex species interactions occur, single-species management can have indirect impacts on non-target species. In some cases, indirect effects may conflict with ecosystem management goals and subsequently, have broad implications for conservation management. Indeed, it has been proposed that biodiversity conservation include protection of critical interspecific interactions. Finally, this research demonstrates how a study of basic ecological principles can inform conservation management and concommitally, how the application of a management technique can aid in the study of basic ecological principles. Our findings highlight the need to develop a better understanding of how mechanisms underlying species interactions can influence community structure and subsequently cause unexpected community responses to environmental change.

METHODS: We randomly selected 8 plots for experimental manipulation along with 8 control plots. Plots within both groups contained a comparable range of enlarged Red-cockaded Woodpecker cavities and snag availability. Following the 2003 field season, we installed metal restrictor plates on all inactive and enlarged Red-cockaded Woodpecker cavities within experimental plots (n=135 cavities, Fig. 4). Metal restrictor plates are commonly used to repair and prevent enlargement of Red-cockaded Woodpecker cavities by other woodpecker species. Experimental manipulation was complete in January 2005. The experimental portion of this study consists of 2003 (pre-treatment) and 2005 (post-treatment) data. To control for annual variation, we used the amount of change in abundance and number of nests found in each plot between 2003 and 2005 for analysis. For each plot we subtracted pre-treatment from post-treatment values and used the difference for analysis. We used a Mann Whitney U test to detect significant differences in the amount of change between treatment and control plots.

PART 2: EXPERIMENTAL STUDYPART 2: EXPERIMENTAL STUDY

135 cavities restricted (8 treatment plots / 8 control)

EXPERIMENTAL MANIPULATION

RESULTSRESULTS

Model fit: X2 = 5.8, P=0.33; RMSEA = 0.04

A concurrent experimental study enabled us to test relationships shown in our path model. We experimentally blocked 2 pathways within the model, including a) that between enlarged Red-cockaded Woodpecker cavities and Northern Flicker presence and b) that between enlarged Red-cockaded Woodpecker cavities and large SCNs. Experimental manipulations were designed to prevent the cavity enlargement behavior of the Northern Flicker and remove enlarged Red-cockaded Woodpecker cavities from the research plots.

The federally endangered Red-cockaded Woodpecker, endemic to fire-maintained southern pine forests, is the only species in North America that excavates cavities in living pine. Sap wells excavated around the cavity help prevent nestling predation by rat snakes, which can climb the bark of living pine. Other woodpecker species, such as the Northern Flicker and Pileated Woodpecker, often enlarge these cavities, subsequently creating nesting habitat for large, secondary-cavity nesters (e.g., Wood Ducks, Eastern Screech-Owls, American Kestrels, Fox Squirrels). Over 27 vertebrate species are known to used Red-cockaded Woodpecker cavities.

A dead tree, referred to as a ‘snag’. Dead wood is critical for cavity-excavation by most woodpecker species. There are many species that cannot excavate, but require cavities for nesting. These ‘secondary cavity nesters’ (SCN) rely on woodpeckers to excavate cavities.

Photo Credit: Kevin Rose

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