Study Species: The endangered Golden-cheeked Warbler and Black-capped Vireo

 

The Golden-cheeked Warbler (GCWA) Dendroica chrysoparia is an endangered neotropical migrant songbird that breeds in the United States, primarily across 27 counties in Texas. Estimates of population sizes for this species based on banding data and surveys were 18,500 and 13,800 in 1962 and 1990, respectively (USFWS 1992). Studies have found that 11-34% of territorial males remain unpaired and that nest failures are common (Jette et al. 1998; Rappole et al. 2000). It has been suggested that availability of habitat on the wintering grounds might be a major factor limiting the population size of golden-cheeked warblers (Rappole et al. 1999; Rappole et al. 2003). The species is considered to have suffered a large reduction in breeding habitat (35%) and population size (25%), and loss of habitat in different portions of the breeding range has resulted in remaining habitat being highly fragmented (USFWS 1992).

 

 

 

 

 

 

 

The Golden-cheeked Warbler, and map showing current breeding range (GCWA images courtesy of Chris Murray).

 

THE BLACK-CAPPED VIREO

The Black-capped Vireo (BCVI) is a small endangered neotropical migrant songbird that breeds in northern Mexico and south-central Texas. It winters on the western coast of Mexico from the foothills of the Sierra Madre to Jalisco and Michoacan and probably farther south (Graber 1961) . BCVI breeds in “scrubby tree growth of the forest-grassland ecotone in the area of transition between the Austroriparian and Sonoran Biotas” (Graber 1961) . Plant species in the breeding habitats vary across the range, but they have a strong preference for early successional mixed oak-juniper habitats, with a high density of deciduous vegetation from 0-3m in height. BCVI habitat is characterized by 30-53% woody cover (USFWS 1991) . BCVI arrive in breeding habitat from late March to mid-April, and typically cluster in the habitat in groupings of 15-25 territories. Territory size is most often 2-4 acres in extent and nesting begins with the arrival of the female, which can extend until August (USFWS 1991) . Fire may play a very important role in habitats that undergo rapid succession and in renewing habitat that has overgrown. BCVI have been documented to occupy habitats very soon after a fire (Wink and Wright 1973) . It is estimated that over 50% of the breeding range of BCVI has been lost due to fragmentation, urbanization and agriculture (USFWS 1991) . This has resulted in an increase of habitat edges, which in turn has encouraged increased in numbers of the parasitic Brown-headed cowbird Molothrus ater. Nest parasitism by this species has become a major causative factor in reduced reproductive success and hence declining populations of BCVI (Barber and Martin 1997) .

 

 

 

 

 

 

 

 

 

Map showing current distribution of Black-capped Vireo (Map Source: Wilkins etal, 2006; images by Giri Athrey)

 

Estimating the Effective Population Size (Ne) based on temporal changes in allele frequencies.

“It is obvious that some sort of abstraction must always be made from crude enumerations to obtain the most significant population number from the standpoint of effects of inbreeding” (Wright 1931).

In theoretical population genetics it is possible to estimate how rapidly genetic variation is lost in a population of given size, but this is based on assumptions such as random mating, stable population size and non-overlapping generations (Kimura and Crow 1963; Leberg 2005). However several of these assumptions are not met in natural populations. To deal with the demographic realities of most natural populations, the rate of loss of genetic variation can also be estimated as the Effective population Size (Ne). Ne is defined as the number of individuals in an ideal population that would lose genetic variation at the same rate as the actual population (Crow and Denniston 1988; Kimura and Crow 1963). An ideal population is one that is temporally stable, and the effects of mutation, migration, mating system and selection are negligible, if not absent. The concept of effective population size (Ne) is of fundamental consideration in understanding random gene frequency drift in finite populations (Maruyama and Kimura 1980). The purpose of defining an effective population size is to investigate how genetic drift operates.

Considering how useful this information can be in understanding natural populations, it is surprising that only few studies have explicitly estimated this parameter. While dealing with small populations that have potentially suffered the consequences of stochastic events, random genetic drift determines how populations have changed over time. In such cases, the variance in allele frequencies (F) calculated based on allele frequencies from two time points can be a valuable estimator of genetic drift. Ne is related to the variance in allele frequencies, F as (in its most basic form, where T represents the number of generations):

This method, called the temporal method, has seen rapid development in the last decade with increasing computing power, but has also been criticized for its glaring inadequacies. However, efforts to estimate Ne have been notoriously difficult owing to violation of several assumptions that are inherent in many population models. Many existing estimators are considered inaccurate or biased as they violate one or several assumptions including, but not limited to, discrete generations, constant population size, and absence of migration.

I am using empirical data coupled with simulation models to a) evaluate the performance of various available estimators of Ne based on empirical data, b) understand the role of assumptions and their violation as compared between empirical data and simulated allelic data, and c) explore the error involved in estimates of Ne when there are known assumption violations. By obtaining DNA samples from 100 year old museum specimens and DNA from contemporary populations from the same locations, I am looking at how habitat changes have influenced drift in populations of the endangered Golden-cheeked warbler and Black-capped Vireo. Summarized below is a table showing the locations and time points from which genetic samples will be used for temporal comparison.

 

Demographic, social and habitat characteristics influencing fine scale dispersal in the Black-capped Vireo

While habitat loss and fragmentation are regarded as the biggest threat to biodiversity today, species that are habitat sensitive – requiring specific habitat conditions - have been disproportionately affected. This is a significant concern for the Black-capped Vireo (BCVI). When habitat is limited, populations are sustained by a balance between dispersal and extinction among habitat patches. This dispersal is critical in maintaining a net positive rate of population growth.

Dispersal is one of the most important life history characteristics of any species, yet the least understood. For habitat specialist species, dispersal is extremely important as it determines whether new suitable habitats will be colonized, or whether species will successfully relocate following a disturbance event (Dieckmann et al. 1999; Hanski 1998). Another important concern, as yet unaddressed by previous studies on this species has been in understanding how breeding biology, habitat connectivity and fragmentation influence dispersal. Populations exhibit spatial structure due to social or demographic factors that impose limitations on individual movements: Natal dispersal, spatial segregation of kinship groups and sex-biased philopatry all together create social barriers that restrict gene flow, which in turn generate fine-scale structuring in populations (Chesser 1991). Promiscuity or extra-pair mating is important because the expectation for songbirds is social monogamy. This has a direct bearing on explaining why individuals disperse and in determining the genetic variation within populations. Closely related is variance in reproductive success – which when high can make populations more vulnerable to random events (Storz et al. 2001). Since dispersal is a result of breeding behaviors in most songbirds, teasing out the nexus of dispersal-breeding biology is of central importance in population studies. Habitat connectivity has long been considered important for movement between major refuges, but its importance to local, small spatial scale natal or breeding dispersal is not well understood (Andren 1994).

Current information on BCVI populations and dispersal are extrapolated from bird-banding studies, but there is often a huge disparity between data from these ‘observational’ methods and estimates based on genetic techniques (Koenig et al. 1996) . There is emerging evidence from dispersal studies from various taxa that observational studies may provide biased estimates if individuals (especially juveniles) disperse away from site of birth before being marked or banded, indicating that observational data of behavior and movement are not accurate indicators of a) survival, b) site of birth, c) frequency and extent of movement, d) relatedness between individuals (Waser et al. 2006) . This makes it necessary to implement approaches that use a consensus between observational and genetic data. Additionally, ongoing research in our lab on BCVI indicate that remnant populations are undergoing genetic differentiation (Barr et al. In. Prep.) . Isolation of some populations of this species and disruption of connectivity (both in terms of habitat and gene flow) clearly emphasizes the need for genetic information on dispersal. Understanding genetic, social and habitat features that influence patterns of movement of BCVI over a spatial scale is the major objective of this study. By using a combination of parentage analysis, parentage reconstruction, along with GIS data for habitats, I am trying to obtain a fine scale resolution on dispersal patterns in this species. A majority of the samples for this study is being collected at Kerr Wildlife Management Area.

 

Modeling population dynamics of Black-Capped Vireos

Another part of my current project is to build a matrix projection model of the Black-capped Vireo that incorporates dispersal data based on our empirical studies and a fire-dependent habitat decay model. The species depends on early succesional habitats of mixed oak-juniper in its breeding range. Periodic natural fire is known to have maintained such a habitat by reversion to early succesion every few years. However, since human occupation of most of central and west Texas, natural fire regimes have been suppressed and has led to the habitat becoming overgrown, and hence unsuitable for Black-capped habitat. In this situation, (hypothetical) a decaying habitat may appeal less to Black-capped Vireos and, in the absence of other suitable habitat the species may exist as a metapopulation with local extinctions, followed by recolonization of fire-regenerated habitats. On a spatial scale, this may appear as a number of populations going extinct in aged habitats, while 'blinking' on in habitats that recently experienced fire. Literature reveals that Black-capped Vireos move into habitats as early as two years post burn. This interesting dynamic has repurcussions for understanding the basis of dispersal, population growth and ultimately the future of the species and the role of managment in sustaining the species.