Kenneth J. Locey

How species richness and total abundance constrain the distribution of abundance

The species abundance distribution (SAD) is one of the most intensively studied distributions in ecology and its hollow-curve shape is one of ecologys most general patterns. We examine the SAD in the context of all possible forms having the same richness (S) and total abundance (N), i.e. the feasible set. We find that feasible sets are dominated by similarly shaped hollow curves, most of which are highly correlated with empirical SADs (most R(2) values > 75%), revealing a strong influence of N and S on the form of the SAD and an a priori explanation for the ubiquitous hollow curve. Empirical SADs are often more hollow and less variable than the majority of the feasible set, revealing exceptional unevenness and relatively low natural variability among ecological communities. We discuss the importance of the feasible set in understanding how general constraints determine observable variation and influence the forms of predicted and empirical patterns.

A Process-Independent Explanation for the General Form of Taylor’s Law

Taylor’s law (TL) describes the scaling relationship between the mean and variance of populations as a power law. TL is widely observed in ecological systems across space and time, with exponents varying largely between 1 and 2. Many ecological explanations have been proposed for TL, but it is also commonly observed outside ecology. We propose that TL arises from the constraining influence of two primary variables: the number of individuals and the number of censuses or sites. We show that most possible configurations of individuals among censuses or sites produce the power-law form of TL, with exponents between 1 and 2. This “feasible set” approach suggests that TL is a statistical pattern driven by two constraints, providing an a priori explanation for this ubiquitous pattern. However, the exact form of any specific mean-variance relationship cannot be predicted in this way, that is, this approach does a poor job of predicting variation in the exponent, suggesting that TL may still contain ecological information.

Factors Affecting Range Expansion in the Introduced Mediterranean Gecko, Hemidactylus turcicus

Abstract Exotic species colonize new areas through the processes of jump dispersal, population establishment, and diffusion dispersal. From 1962 to 1997, Mediterranean Geckos, Hemidactylus turcicus, were intentionally introduced multiple times into a science building on the campus of the University of Central Oklahoma, Edmond, Oklahoma. In 2005, we recorded 365 captures of 305 individuals. The population size was estimated to be 1005 individuals with a density of 478 geckos/ha. The population has dispersed a maximum distance of 200 m from the point of introduction which translates to a maximum rate of diffusion dispersal of 20 m/yr. These results support the hypothesis that range expansion of introduced Mediterranean Geckos has occurred because of multiple jump dispersal events, despite slow diffusion dispersal.

A comparison of Mediterranean Gecko ( Hemidactylus turcicus ) populations in two temperate zone urban habitats

Urban ecosystems represent unique habitats that might be more or less suitable than natural systems for many species. The Mediterranean Gecko, Hemidactylus turcicus is a synanthropic species that has successfully colonized many urban and suburban areas outside its natural range. In the United States, H. turcicus has been found predominantly in southern states on exterior surfaces of buildings. We compared population density, demographics, and extent of dispersal of populations of H. turcicus at two anthropogenic sites in a temperate region of the United States. The University of Central Oklahoma (UCO) had higher building density and greater cover by anthropogenic surfaces and the Oklahoma City Zoo (ZOO) had lower building density and greater cover by vegetation. Mark and recapture sampling was conducted at UCO from 2006 to 2007 and during 2008 at ZOO. The sizes of the populations at the two sites were estimated using the Schnabel method and population densities were calculated using those estimates. Age class of captured animals, sex of adults, and gravid status of adult females were determined at both sites. We assumed that greater exterior wall surface area of individual buildings at UCO and greater land cover by anthropogenic surfaces might 1) reduce population density and 2) facilitate dispersal there relative to the ZOO. Population sizes and density in animals per ha of land and building cover and demographics were similar at the two sites. Population density per ha of building exterior wall area was higher at ZOO than at UCO. The higher density of animals on buildings, lower proportion of subadults in the population and lower maximum dispersal distance at ZOO suggest that successful diffusion dispersal might be limited at that site relative to UCO.

Ontogenetic Factors Affecting Diffusion Dispersal in the Introduced Mediterranean Gecko, Hemidactylus turcicus

Abstract Dispersal occurs when an individual leaves its natal area or home range, travels through adjacent areas, and then establishes a new home range where it will attempt to reproduce. Previous studies have suggested that subadults (i.e., hatchlings and juveniles) are the main agents of dispersal among introduced populations of Mediterranean Geckos, Hemidactylus turcicus. We derived a testable hypothesis for subadult dispersal in Mediterranean Geckos. In September 2006, we conducted tethering experiments to test the prediction that adults act aggressively toward subadults. From June to October 2006, we conducted 18 mark-recapture samples to test predictions of perch height occupancy and relative presence of adults and subadults at buildings and areas between buildings (i.e., transient habitats). As predicted, subadults occupied lower regions of buildings than adults, and subadults were disproportionately abundant in transient habitats. Apparently because of small sample size, there was no difference in the relative presence of adults and subadults at recently occupied buildings (i.e., arrival habitats). Our results supported the hypothesis that hatchlings were the main agents of diffusion dispersal within our study population and suggested that hatchling dispersal may be motivated by hostile encounters with adults. It was also clear that Mediterranean Geckos attempt to disperse from buildings more frequently than previously suggested.

Terrestrial Flight Response: A New Context for Terrestrial Activity in Sonoran Mud Turtles

Abstract The risk allocation hypothesis predicts that animals should be most averse to risk during brief and infrequent high risk situations. For animals in shallow aquatic habitats, encounters with terrestrial predators may represent such a situation. Terrestrial flight responses, acute movements from water onto land following a disturbance, may be a viable escape strategy during encounters with terrestrial predators foraging in shallow aquatic habitats. Sonoran mud turtles (Kinosternon sonoriense) occur in intermittent aquatic habitats in the Peloncillo Mountains, New Mexico. When Sonoran mud turtles were captured by hand in shallow pools or shallow stock tanks they frequently underwent terrestrial flight responses. In five of six trials in shallow stock tanks, and three of seven trials in shallow pools, turtles left the water within 20 min of being released, often climbing out of sight up relatively steep slopes. This behavior was not observed in four trials in which turtles were captured with hoop nets in deep stock tanks. Turtles that underwent terrestrial flight responses were significantly larger than non-responders. Terrestrial flight responses appear to be a risk aversion strategy in Sonoran mud turtles occupying shallow aquatic habitats. This unusual behavior may be widespread among animals that inhabit shallow aquatic habitats.

Simple Structural Differences between Coding and Noncoding DNA

Background The study of large-scale genome structure has revealed patterns suggesting the influence of evolutionary constraints on genome evolution. However, the results of these studies can be difficult to interpret due to the conceptual complexity of the analyses. This makes it difficult to understand how observed statistical patterns relate to the physical distribution of genomic elements. We use a simpler and more intuitive approach to evaluate patterns of genome structure. Methodology/Principal Findings We used randomization tests based on Morisitas Index of aggregation to examine average differences in the distribution of purines and pyrimidines among coding and noncoding regions of 261 chromosomes from 223 microbial genomes representing 21 phylum level groups. Purines and pyrimidines were aggregated in the noncoding DNA of 86% of genomes, but were only aggregated in the coding regions of 52% of genomes. Coding and noncoding DNA differed in aggregation in 94% of genomes. Noncoding regions were more aggregated than coding regions in 91% of these genomes. Genome length appears to limit aggregation, but chromosome length does not. Chromosomes from the same species are similarly aggregated despite substantial differences in length. Aggregation differed among taxonomic groups, revealing support for a previously reported pattern relating genome structure to environmental conditions. Conclusions/Significance Our approach revealed several patterns of genome structure among different types of DNA, different chromosomes of the same genome, and among different taxonomic groups. Similarity in aggregation among chromosomes of varying length from the same genome suggests that individual chromosome structure has not evolved independently of the general constraints on genome structure as a whole. These patterns were detected using simple and readily interpretable methods commonly used in other areas of biology.