Lyn C. Branch

EFFECTS OF LANDSCAPE STRUCTURE IN FLORIDA SCRUB: A POPULATION PERSPECTIVE

Understanding how patchy landscapes affect the distribution and abundance of species is a central issue in applied ecology. We examined the effects of landscape structure on the distribution of the Florida scrub lizard (Sceloporus woodi) in naturally fragmented habitat. Scrub lizards were absent from 89 of 132 patches, and their distribution was explained in multifactor models by two landscape variables: patch size and patch isolation. Mark–recapture studies of S. woodi and experimental displacement trials indicate that this species exhibits low vagility within a patch and that movement through dense habitat, which often characterizes the landscape matrix, is restricted. Poor dispersal ability relative to the degree of isolation of patches in this landscape may account for large gaps in its distribution. To examine the applicability of the scrub lizard model to other species, we applied the model to a sympatric lizard, the six-lined racerunner (Cnemidophorus sexlineatus), which is similar in body size, fo...

The influence of landscape, patch, and within-patch factors on species presence and abundance: a review of focal patch studies

Understanding the influence of large and small-scale heterogeneity on species distribution and abundance is one of the major foci of landscape ecology research in fragmented environments. Although a large number of studies have addressed this issue individually, little effort has been made to synthesize the vast amount of literature published in the last decade. We reviewed 122 focal patch studies on 954 species published between 1998 and 2009 to determine the probability of species responding significantly to landscape, patch, and within-patch variables. We assessed the influence of taxonomic, life history, and methodological variables on probability of response to these 3 levels. Species in diverse taxa responded at high rates to factors at all three levels, suggesting that a multi-level approach is often necessary for understanding species response in patchy systems. Mammals responded at particularly high rates to landscape variables and therefore may benefit more than other taxa from landscape-level conservation efforts in fragmented environments. The probability of detecting a species response to landscape context, patch, and within-patch factors was influenced by a variety of methodological aspects of the studies such as type of landscape metric used, type of response variable, and sample size. Study design issues rarely are discussed by authors as reasons why a particular study did not find an effect of a variable, but should be given more consideration in future studies.

Effects of natural habitat fragmentation on an endemic scrub lizard (Sceloporus woodi): an historical perspective based on a mitochondrial DNA gene genealogy

The Florida scrub lizard, Sceloporus woodi, is endemic to scrub habitat patches along the central portion of the Florida peninsula and xeric coastal regions. Scrub ecosystems are the patchily distributed remnants of previously widespread habitats formed during the Pleiocene and early Pleistocene. Scrub lizards appear to have limited dispersal capabilities due to high habitat specificity and low mobility. To assess the population structure and phylogeography of S. woodi, 135 samples were collected from 16 patches on five major ridges in Florida, USA. Analysis of 273 bp of mitochondrial DNA (mtDNA) cytochrome b reveals a very strong geographic distribution of genetic diversity. Haplotype frequencies are significantly different in 63 of 66 comparisons between patches. With one exception, samples from the five major ridges are characterized by fixed differences in haplotype distribution and deep evolutionary separations (3–10%). Fixed genetic differences were also observed between northern and southern segments of several ridges. Analysis of molecular variance (amova) shows an estimated 10.4% total genetic variation within patches, 17.5% among patches (within ridges), and 72.1% among ridges. This strong population structure among patches within ridges indicates that the distribution of S. woodi is tightly linked to sandy scrub habitat and that the discontinuous distribution of scrub habitats significantly inhibits dispersal and gene flow. Phylogeographic analyses indicate a pattern of dispersal down the Florida peninsula during the late Pliocene–early Pleistocene, followed by habitat fragmentation and vicariant isolation events. Therefore, the deep genetic structuring among scrub lizard populations on separate ridges is attributed to ancient isolation events induced by a shift from dry (xeric) to wet (mesic) conditions on the Florida peninsula. These findings indicate that some scrub lizard populations have persisted in isolation for time frames in excess of 1 Myr, providing a case history on the genetic consequences of habitat fragmentation.

The relative influence of habitat loss and fragmentation: Do tropical mammals meet the temperate paradigm?

The relative influence of habitat loss vs. habitat fragmentation per se (the breaking apart of habitat) on species distribution and abundance is a topic of debate. Although some theoretical studies predict a strong negative effect of fragmentation, consensus from empirical studies is that habitat fragmentation has weak effects compared with habitat loss and that these effects are as likely to be positive as negative. However, few empirical investigations of this issue have been conducted on tropical or wide-ranging species that may be strongly influenced by changes in patch size and edge that occur with increasing fragmentation. We tested the relative influence of habitat loss and fragmentation by examining occupancy of forest patches by 20 mid- and large-sized Neotropical mammal species in a fragmented landscape of northern Guatemala. We related patch occupancy of mammals to measures of habitat loss and fragmentation and compared the influence of these two factors while controlling for patch-level variables. Species responded strongly to both fragmentation and loss, and response to fragmentation generally was negative. Our findings support previous assumptions that conservation of large mammals in the tropics will require conservation strategies that go beyond prevention of habitat loss to also consider forest cohesion or other aspects of landscape configuration.

Enhancing the area–isolation paradigm: habitat heterogeneity and metapopulation dynamics of a rare wetland mammal

Conservation of species in fragmented landscapes often is guided by spatially realistic metapopulation theory. However, convincing cases of metapopulation dynamics are uncommon, especially for vertebrates. Moreover, there is concern that the patch area and isolation paradigm for metapopulations is an oversimplification for heterogeneous landscapes. We tested predictions from metapopulation theory for a rare wetland mammal (round-tailed muskrat, Neofiber alleni) and asked whether it was necessary to use a habitat-informed version of the area-isolation paradigm that included patch quality and matrix heterogeneity. In each of two years, we surveyed 457 isolated wetlands in central Florida, USA, for presence-absence of Neofiber and evaluated logistic regression models of patch occupancy, extinction, and colonization. We documented metapopulation dynamics in which patch occupancy was constant between years (26% of patches occupied) due to balanced local extinctions (n = 45) and recolonizations (n = 46). Neofiber was both habitat and dispersal limited. Local extinctions were related negatively to patch area, patch quality (cover of maidencane grass, Panicum hemitomon), and distance to nearest roadside ditch. Patch colonization depended on patch area, patch quality, and spatial connectivity to potential source wetlands. Despite the importance of patch quality, Neofiber did not exhibit a habitat-tracking metapopulation on an annual time scale. Cost-distance modeling suggested effective distances that included high costs for moving through forested matrix habitats generally were better than Euclidean distances for predicting patch colonization and occupancy. Two dominant land uses were tied to turnover dynamics: cattle grazing decreased habitat quality of wetlands, and presence of pine (Pinus spp.) plantations decreased functional connectivity. The simple area-isolation paradigm was not adequate for characterizing spatial dynamics of the Neofiber metapopulation. Nevertheless, we contend that the metapopulation approach remains a useful conservation framework for many species if landscape heterogeneity is embraced and explicit effects of land-use practices on turnover processes are considered.

Habitat Patch Size Affects Demographics of the Florida Scrub Lizard (Sceloporus woodi)

Abstract Habitat destruction and fragmentation are often cited as primary causes for declines in reptile and amphibian populations. Smaller habitat patches constrain population size resulting in increased extinction risk caused by stochastic processes. However, patch size also may affect populations in a deterministic manner. We used mark and recapture techniques to examine effects of patch size on demographics of the Florida Scrub Lizard (Sceloporus woodi). We examined populations in eight scrub habitat patches that differed in size but were similar with respect to other habitat characteristics. Abundance, survivorship, and recruitment were positively associated with patch size for both sexes. Individual growth rate increased with patch size for males but was negatively associated with patch size for females. Although our data indicate that patch size strongly affects demographics of scrub lizards, mechanisms responsible for these patterns are unknown. We discuss conservation implications of area-sensitive demography for local and regional populations. In particular, we suggest caution when constructing metapopulation models that assume homogenous survival and fecundity rates for all patches regardless of patch size.

Passive sampling effects and landscape location alter associations between species traits and response to fragmentation

As tropical reserves become smaller and more isolated, the ability of species to utilize fragmented landscapes will be a key determinant of species survival. Although several ecological and life history traits commonly are associated with vulnerability to fragmentation, the combination of traits that are most highly influential and the effectiveness of those traits in predicting vulnerability across distinct landscapes, remains poorly understood. We studied use of forest fragments by 25 mid- and large-sized neotropical mammals in Guatemala to determine how seven species traits influence vulnerability to fragmentation. We measured vulnerability in two ways: one measure that did not remove passive sampling effects (proportion of fragments occupied), and one that did (difference in occupancy rates within continuous and fragmented sites). We also examined the influence of species traits on patch occupancy rates of the same set of mammals on two landscapes in Mexico. When not accounting for passive sampling effects, body size, home range size, and vulnerability to hunting influenced how species responded to fragmentation. However, after controlling for passive sampling effects, only vulnerability to hunting strongly influenced sensitivity to fragmentation. Species that were heavily hunted were much less common in forest patches than in continuous forest sites of the same sampling size. The cross-landscape comparison revealed both similarities and differences in the species traits that influenced patch occupancy patterns on each landscape. Given the ubiquity of hunting in tropical environments, our findings indicate that management efforts in fragmented landscapes that do not account for hunting pressure may be ineffective in conserving heavily hunted tropical species. Our study also indicates that species traits may be useful in predicting relative patch occupancy rates and/or vulnerability to fragmentation across distinct landscapes, but that caution must be used as certain traits can become more or less influential on different landscapes, even when considering the same set of species.

Identification of Structural and Spatial Features that Influence Storm-Related Dune Erosion along a Barrier-Island Ecosystem in the Gulf of Mexico

Abstract Observations of hurricane impacts on coastal communities and predicted increases in tropical storm activity have spurred an interest in protection and restoration of coastal dunes. Dunes are valued for their role in the protection of infrastructure and aesthetic beauty. Past research on dune erosion has focused primarily on laboratory research or mathematical models. We directly assessed impacts of Hurricanes Ivan (2004) and Dennis (2005) on coastal dunes along Santa Rosa Island, a barrier island in the Gulf of Mexico. We recorded dune area for foredunes and secondary dunes, examined the spatial context of each dune, and recorded structural features of each dune before and after each hurricane. Both hurricanes severely impacted foredunes, and removal of foredune protection by Ivan led to severe impacts on secondary dunes from Dennis. Structural features of the dune, such as height and width, influenced resistance against storm-related erosion, but spatial context, such as location with respect to shoreline, also was important. Coastal dunes on this barrier island may be shifting to a state where their formation, recovery, and restoration are influenced heavily by tropical storm activity. The future success of coastal restoration efforts for maintenance of infrastructure or habitat protection should be cognizant of the importance of spatial context in addition to the structural and vegetation characteristics of dunes.

ASSOCIATIONS BETWEEN PATCH AREA AND VITAL RATES: CONSEQUENCES FOR LOCAL AND REGIONAL POPULATIONS

The negative association between patch area and extinction risk is one of the most ubiquitous observations in ecology. This association is often attributed to the disproportionate effects of stochastic fluctuations on small populations, but it also may be explained by deterministic relationships between patch area and vital rates (i.e., fecundity and survival). We explored the consequences of including deterministic relationships between patch area and vital rates in metapopulation models. Field studies have revealed that fecundity and monthly survivorship were positively associated with patch size for the Florida scrub lizard (Sceloporus woodi). Using simulation techniques, we compared metapopulation models of artificial landscapes that included vital rates for this species (scaled for patch area) to models that used averaged estimates of vital rates for all patches. These two types of metapopulation models produced dramatically different results with respect to the proportion of occupied patches and the extinction probability of the regional population. Including deterministic relationships between patch area and vital rates often resulted in either all patches being occupied or all patches going extinct, depending on the size of the local patches. Few artificial landscapes resulted in a dynamic colonization–extinction equilibrium characteristic of a “classic metapopulation.” In contrast, model simulations with demographic parameters equal for all patch sizes produced a more linear decline in extinction probability with patch size. Our results highlight considerations for application of metapopulation theory in conservation.

Predictive equations for biomass and fuel characteristics of Argentine shrubs.

Predictive equations for estimating shrub biomass in semi-arid scrub ecosystems are essential for evaluating shrub encroachment, conducting ecosystem-level studies of net primary productivity (NPP) and nutrient cycling, and examining effects of different fire regimes. In this study, we developed predictive equations to estimate total aboveground biomass and biomass of foliage and stems of the 8 most common shrubs in the semi-arid scrub (Monte) of central Argentina. We also examined the relationship between shrub size and proportions of these components for the dominant species, Larrea divaricata Cavanilles (creosotebush), and determined fuel characteristics (dead-to-live ratio, bulk density) of the 8 shrub species. Regression analyses were used to examine the relationships between aboveground biomass and 5 field measurements (diameter of the longest stem, shrub height, maximum crown width, crown width at right angles to maximum crown width, and crown volume). A natural log-log model based on a single variable best described this relationship in most cases. The easiest field measurement for 6 of the 8 species was diameter of the longest stem, and this measure was often the best predictor of shrub biomass. As L. divaricata increased in size, the proportional biomass of large stems increased, and bio mass of foliage and small stems decreased. This pattern suggests pro ductivity may decrease with shrub age. The mass of dead material was low in most shrub species. Bulk densities were comparable to those of shrubs in other semi-arid ecosystems. Equations developed here will allow rapid and accurate estimation of shrub biomass in the Monte of Argentina.