Geology topics

Phillip J. Zwank

Publications and source records attributed to Phillip J. Zwank.

5 recordsLinked to original sources

USGS1994

Herbivory on shoalgrass by wintering redheads in Texas

An estimated 80% of redheads ( Aythya americana ) winter on the Laguna Madre of south Texas and Mexico and feed almost exclusively on shoalgrass ( Halodule wrightii ) rhizomes. Shoalgrass abundance has decreased by 60% over the past 30 years, and because the effects of shoalgrass loss on wintering redheads are unknown, we initiated a study to define habitat selection criteria and document the effect of wintering redheads on shoalgrass in the lower Laguna Madre, Texas. Redheads consumed an average of 75% of shoalgrass rhizome biomass at collection sites each winter. When rhizome biomass was grazed to a mean biomass of ltoreq 0.18 g dry mass/core (approximately 10 g dry mass/ml), shoalgrass did not recover to its previous level the following growing season. Thirty-three percent of the sites (10) were grazed below 0.18 g dry mass/core during both years of the study, while 64% (19) were grazed below 0.18 g during 1 or the other of the 2 winters. Ramet number was positively correlated ( P < 0.001, r 2 = 0.54) with rhizome biomass; however, this relationship was influenced by grazing intensity. Heavy grazing reduced the amount of rhizome attached to each ramet compared with ungrazed ramets. Grazing had no effect on root biomass ( P = 0.388), rhizome moisture content ( P = 0.553), or soil magnesium, phosphorous, and potassium ( P = 0.102, 0.499, 0.162, respectively). Redhead presence increased ( P = 0.042) soil nitrogen levels. Foraging areas selected by redheads within the lower Laguna Madre had lower ( P = 0.026) salinities (24 ppt) than areas not selected (35 ppt). Redheads did not select foraging areas in relation to crude protein levels in rhizomes. Shoalgrass habitat in the Laguna Madre should be protected from further losses and enhanced where possible.

Texas↗

USGS1988

Habitat Suitability Index Models: Black-bellied whistling-duck (breeding)

A review and synthesis of existing information were used to develop a model for evaluating the quality of habitat for breeding black-bellied whistling-ducks. The model is scaled to produce an index between 0.0 (unsuitable habitat) to 1.0 (optimal habitat). Habitat suitability index models are designed to be used with Habitat Evaluation Procedures previously developed by the U.S. Fish and Wildlife Service. Guidelines for model application and techniques for measuring model variable are provided.

FWS/OBS↗

USGS1987

Habitat Suitability Index Models: Forster's tern (breeding) - Gulf and Atlantic coasts

The nesting range of Forster's terns hosts three allopatric breeding populations. The first and most important breeding area, in terms of the number of nes t i ng pairs, includes the western guIf coas t from the Louisiana-Mississippi border to northern Tamaulipas, Mexico (American Ornithologists' Union [AOUJ 1983). In addition, small numbers of Forster's terns have nested in Mobile County, Alabama (Imhof 1976). Although this species has not been recorded nesting in Mississippi (J. Jackson, Mississippi State University, Starkville; pers. comm.), it is observed in the coastal regions of that State every summer, and several thousand nest in adjacent Louisiana (Portnoy 1977; Clapp et ale 1983). The two largest colonies of Forster's terns documented in the literature were both in Louisiana: one of 2,750 pairs in Lake Borgne on the Louisiana-Mississippi border and one of 2,263 pairs in Calcasieu Lake (Portnoy 1977).

FWS/OBS↗

USGS1985

Habitat Suitability Index Models: Lesser snow goose (wintering)

The lesser snow goose may have the largest population of any goose in the world (Cooch 1958; Kerbes 1975; Ogilvie 1978). Its arctic breeding range has greatly expanded since the mid-1950's (Bellrose 1976). In the United States, it normally ranks behind only the Canada goose (Branta canadensis) in population size (Bellrose 1976; Owen 1980) and in harvest (Carney et al. 1981, 1982). Because up to 70% of the geese harvested annually by hunters in Texas and Louisiana are lesser snow geese (Carney et al , 1981), its importance as a waterfowl species of the Gulf of Mexico coast cannot be overemphasized. Surveys taken in the Central and Mississippi Flyways from 1955 to 1975 have shown an upward trend in the number of wintering snow geese (Bellrose 1976). On the wintering grounds snow geese tend to remain in large, very conspicuous flocks that please both hunters and non-consumptive viewers of wildlife.

FWS/OBS↗

USGS1983

Habitat Suitability Index Models: Mottled duck

The mottled duck is a mallard-like resident species of the Gulf of Mexico coast, from the marshes of Pearl River on the Lou i si ana-Ht ss t ss ipp i border to the Al varado Lagoon near Veracruz, Mexico (Bell rose 1976). The highest densities of nesting mottl ed ducks are found in brackish to fresh coastal marshes (H. Bateman, Louisiana Department of Wildl ife and Fisheries, Baton Rouge; pers , comm.). Mottled ducks also inhabit prairie land near prairie potholes in Texas and flooded rice fields in Texas (Engel ing 1950) and Louisiana (Linscombe 1972).

FWS/OBS↗