Territory Quality Transitions and Source–sink Dynamics in a Florida Scrub-Jay Population

Abstract

Source–sink theory provides an approach to identify habitat arrangements needed to sustain populations in spatially and temporally varying landscapes. Our objective was to investigate whether source–sink ideas could be applied to quantify how habitat arrangements influenced Florida Scrub‐Jay (Aphelocoma coerulescens) population dynamics, in order to enhance habitat restoration. From 1988 to 2001, we measured reproductive success, survival, immigration, emigration, pair bond fidelity, and the duration of delayed breeding by young. The arrangement of shrub height in each territory was used to classify habitat quality each year, according to the following categories: short (<120 cm tall), optimal (short plus 120–170 cm tall), tall mix (short or optimal and >170 cm), and tall (>170 cm). Annual demographic performance rates were calculated in each territory by summing the recruitment of potential breeders (the number of yearlings produced) and then subtracting the number of breeders that died. The mean demographic performance per breeding pair was −0.57, 0.33, −0.26, and −0.35, respectively, for short, optimal, tall mix, and tall territories. Optimal territories functioned as sources because recruitment exceeded mortality; hence, the optimal territories were net exporters to marginal habitat. Potential breeders were exported to sink territories by active dispersal and by “territory quality transitions,” whereby territories produced an excess of potential breeders while in optimal condition and then became marginal because of shifting territory boundaries and habitat conditions. Short, tall mix, and tall territories functioned as sinks because they were net importers, mortality exceeded recruitment, and because there were no density‐dependent reductions in their demographic performance. The population declined because there were too few optimal territories to offset declines in sinks, which usually had too much tall scrub even though most of the landscape had been burned recently. Successful habitat restoration requires greater emphases on improving habitat quality at the territory scale because this is the fundamental landscape unit related to demography. Source–sink ideas, complemented by territory quality transitions, provide a quantitative approach to directly relate habitat and demographic objectives. Corresponding Editor: D. J. Levey.

Adaptation to Marginal Habitats

“…Individuals in a sink habitat have on average lower reproductive value: their expected contribution to the future gene pool is smaller than that in a source, which renders natural selection in the sink less effective . The sink nature of marginal habitats has been directly demonstrated in a number of species, at various spatial scales (e.g., Blondel et al 2006, Boughton 1999, Breininger & Carter 2003, Manier & Arnold 2005, Stanton & Galen 1997, Sulkava et al 2007, Thingstad et al 2006. The lines show the local population growth (births minus deaths) as a function of local density for a high-quality core habitat (red) and a marginal habitat (blue); the asterisks indicate equilibria.…”

A rapid approach to modeling species–habitat relationships

“…These models were based on our knowledge of Florida scrubjay biology and published studies describing such relationships (e.g. Breininger 1992, Breininger and Carter 2003, Breininger and Oddy 2004, Breininger et al 1995, 1998, Woolfenden and Fitzpatrick 1984. Logistic regression models with Florida scrub-jay occupancy as the response variable were then fit for each hypothesis using SPSS 12.0.…”

“…To demonstrate the utility of the method for predicting habitat occupancy, we used model averaging (see Burnham and Anderson 2002) to predict Florida scrub-jay occupancy for a region of the study site which had an independent data set of Florida scrub-jay habitat occupancy. This area was part of a demographic study of Florida scrub-jays; all of individuals were banded within this area aPd territory boundaries were mapped every year since 1988 (Breininger and Carter 2003). We included models in the set such that the sum of their w1 values exceeded 0.9 (analogous to a 90 % confidence set of models, Burnham and Anderson, 2002).…”

“…We also found that level two of Height (I]) had a large positive effect on probability of occupancy (Table 4). Scrub height is important factor in determining habitat quality and our level two represents optimal conditions (Breininger and Carter 2003). In population centers Scrub-jays prefer and compete for breeding opportunities in optimal habitat Fitzpatrick 1984, Breininger et al 1995).…”

Parthenicus weemsi Henry (Hemiptera: Miridae): New Distribution Records and Seasonality of a Specialist Herbivore on Florida Rosemary, Ceratiola ericoides (Ericaceae)

“…Oak height is a critical limiting factor for Florida scrub-jays which have been documented to abandon territories where the oaks reached >3 m on the Lake Wales Ridge (Fitzpatrick et al 1991) and to suffer a net population loss in territories on Merritt Island on the Atlantic coast where patches of oak scrub in their territories were either taller than 1.7 meters or shorter than 1.2 meters (Breininger and Carter 2003). The lower limit of 5 years is based on the time required for re-sprouting oak stems to reach acorn-bearing height (Ostertag and Menges 1994), acorns being an important part of the scrub-jay's diet.…”