Guy N. Cameron

Operationally Defining Home Range: Temporal Dependence Exhibited by Hispid Cotton Rats

To determine whether cotton rats (Sigmodon hispidus) establish home ranges, and whether home—range size and shape vary temporally, we measured site fidelity at daily and multiday intervals. The concept of site fidelity can be used to provide a quantitative measure of the existence of a home range. Mean squared distance from the center of activity (MSD) and a linearity index (LI) were used to measure site fidelity of the hispid cotton rat. Significant differences for these metrics between actual and simulated, random movement showed that home ranges existed for both daily and multiday periods. Home—range characteristics varied with temporal scale. Daily home ranges were more concentrated (lower MSD), linear (higher LI), and elongate (higher eccentricity, ECC) than multiday home ranges. The effects of sex, age, and season on MSD, LI, and ECC differed for daily and multiday home ranges. The area of daily home range was not different between sexes or ages, or among seasons, but multiday home ranges was larger for males, for adults, and in winter and summer. Knowing that home ranges exist is a necessary prerequisite for ecology or behavioral interpretation of space—use patterns before comparisons of home—range characteristics are made, however, the temporal scale of measurement must be considered, because scale alone can yield differences in size and shape.

Rapid leaf decay and nutrient release in a chinese tallow forest

SummaryThe Chinese tallow tree, Sapium sebiferum, was introduced to the Texas Gulf Coast in the early 1900s and has spread into coastal prairie habitats. Since coastal prairie contains few deciduous trees, we studied leaf fall dynamics, rate of decomposition, and the quantity and rate of nutrient input from decomposing tallow leaves. Among-year leaf fall averaged 382.6 g·m-2·yr-1, similar to other south temperat deciduous forests and about as predicted by Meentemeyer et al.s (1982) AE-lignin model. Decay of tallow leaves (k=-4.33) was much more rapid than native black willow (k=-0.35) and than other temperate deciduous trees (k=-0.77). The ratio of lignin to initial nitrogen concentration, highly correlated with rate of decomposition for hardwood trees, was low for Chinese tallow and may contribute to rapid leaf decay. Taking AE and lignin content into account, Meentemeyers (1984) model predicted k=-1.39 for Chinese tallow and k=-0.88 for black willow. Decay of tallow was much faster but decay of willow was slower than predicted, suggesting that decay on the coastal prairie may be controlled by factors other than lignin content and climate. N, P, and K characteristically accumulate as leaves decay. However, these elements did not accumulate as tallow leaves decayed, possibly because high densities of Armadillidium vulgare, a detritivore, reduced immobilization of elements by microbes. This would result in increased turnover of these elements. Accumulation of Al, Fe, Zn, and S in decaying tallow litter may be related to flood-drain cycles on coastel prairie clay soils. Ca, N, K, Mg, and S were added to forest soil in greatest amounts from decaying tallow leaves. Concentrations of P, K, NO3−N, Zn, Mn, and Fe were significantly higher and Mg and Na were significantly lower in forest than in prairie soil, raising the possibility that Chinese tallow trees altered the distribution of nutrients in the soil profile. We conclude that the Chinese tallow tree may enhance productivity in ecosystems to which it has been introduced by addition of nutrients from rapid decay of leaves.

Effects of tannins on the decomposition of Chinese tallow leaves by terrestrial and aquatic invertebrates

SummaryThe hypothesis of this study was that tannins from Chinese tallow leaves have a negative effect upon terrestrial and aquatic reducer organisms and thereby may affect the overall rate of tallow litter decomposition. Species diversity and population size of aquatic reducers was lower in forest than adjacent grassland ponds; litter bags showed no difference in weight loss between bags which excluded reducers and those which did not. Differences in physical factors between habitats did not explain the paucity of reducers although rainfall permitted emigration of grassland organisms to forest ponds, yielding a temporary decrease in diversity.Tannin concentration in ephemeral ponds was altered by rainfall but leaching from leaves and soil continuously maintained tannin in ponds. Laboratory experiments showed that tannin was not directly toxic but inhibition of feeding caused high mortality in Asellus militaris and Crangonyx shoemackerii.Population density and reproduction of the terrestrial reducer (Armadillidium vulgare) was asynchronous with autumn leaf fall. Ground, leached leaves were consumed at much greater rate in laboratory experiments than unground, unleached leaves; in addition, mortality from starvation on the latter was high. These results suggest tallow leaves are not utilized by reducers until tannins are leached and the physical structure altered by rainfall and/or microbial action. Aquatic reducers are relatively unimportant in processing autumn leaf fall due to continual tannin leaching into ephemeral ponds from surrounding soil. Physical and microbial condition of leaves and leaching of tannin preceed spring and summer utilization by terrestrial isopods.

Insect herbivory on C3 and C4 grasses

SummaryThis study tested the hypothesis that grasses with the C4 photosynthetic pathway are avoided as a food source by insect herbivores in natural communities. Insects were sampled from ten pairs of C3−C4 grasses and their distributions analyzed by paired comparisons tests. Results showed no statistically significant differences in herbivore utilization of C3−C4 species. However, there was a trend towards heavier utilization of C3 species when means for both plant groups were compared. In particular, Homoptera and Diptera showed heavier usage of C3 plants. Significant correlations between insect abundances and plant protein levels suggest that herbivores respond to the higher protein content of C3 grasses. δ13C values for six of the most common grasshopper species in the study area indicated that three species fed on C3 plants, two species fed on C4 plants, and one species consumed a mixture of C3 and C4 tissue.

Influence of pocket gopher mounds on a Texas coastal prairie

SummaryEffects of pocket gopher (Geomys attwateri) mound-building activity on plant community composition and soil nutrient concentrations were investigated in south Texas on both burned and unburned coastal prairie sites. Pocket gophers deposited large amounts of soil which were lower in nutrient content than randomly-collected samples. Above-ground plant biomass was greater around mounds than in random samples mainly because of increased dicots around mounds on the burned area when compared with random samples on the same area. Pocket gophers may have concentrated their activities (and therefore, mounds) in areas with higher dicot biomass on the burned area since they prefer perennial dicots as food, or the presence of mounds may have ameliorated the apparent negative effect of fire on dicots.

Growth and reproduction of hispid cotton rats (Sigmodon hispidus) in response to naturally occurring levels of dietary protein

Hispid cotton rats ( Sigmodon hispidus ) ingest both monocotyledon (monocot) and dicotyledon (dicot) plants, ostensibly to balance intake of nutrients. On the Texas coastal prairie, monocots are more abundant, require less handling time, and have more fiber and soluble carbohydrates than dicots, which contain more protein, lipid, water, energy, and minerals. Because availability of protein may be limiting here, cotton rats were raised on artificial diets that mimicked protein content of either monocots (4%) or dicots (11%). Body mass, growth rate, and length of hind foot were lower and liver mass was higher in individuals fed a monocot-protein diet. Tissue protein, but not lipid, content was higher in individuals fed a dicot-protein diet. First estrus occurred at the same body mass on both diets, but slower growth on a monocot-protein diet resulted in females being 50% older at first estrus. Cotton rats must ingest dicots to obtain sufficient protein for growth and reproduction because monocots are not a complete food source.

Activity and Burrow Structure of Attwater's Pocket Gopher (Geomys attwateri)

Activity of Attwaters pocket gopher ( Geomys attwateri ) was monitored by radio-telemetry and related to burrow structure and temperature at the Welder Wildlife Refuge, Texas. In May 1984, radio fixes were taken hourly over a 2-day period on 10 individuals and each position was marked aboveground. After completion of radiotracking, entire burrow systems were excavated and mapped. Architecture of burrow systems did not reflect local variation in total above-ground biomass. Average area covered by the burrow systems was 202 m2, mean total length of burrows in a system was 91 m, and mean minimum distance moved between fixes was 4.3 m with no difference between sexes. Distance moved was less in more convoluted burrow systems and was correlated positively with length of burrow systems. Body mass was not correlated with burrow length or distance between fixes. More than 60% of the fixes were localized in the vicinity of a nest or refuge and neither sex exhibited a diel activity period. There was little daily variation in temperature in the burrow because of insulation by the soil. We conclude that architecture of burrow systems was not adaptive to resource availability, and that burrow size and configuration affected underground movement, but not frequency of activity. The looping burrow structure of G. attwateri contrasts with the linear, branched systems reported for other geomyids and may be an adaptation to low primary production or social interactions in southern Texas.

Dietary Choice of a Generalist Grassland Herbivore, Sigmodon hispidus

Dietary choice as well as the nutritional and perceptible characteristics of known dietary items were determined for cotton rats (Sigmodon hispidus) on the Texas coastal prairie. The nutritional landscape of the coastal prairie is dynamic : monocots are abundant, dispersed in clumps, and exhibit relatively little seasonal variability ; by contrast, dicots are highly variable both spatially and temporally. The lower green stems of four species of grasses constitute the bulk of the cotton rat diet ; however, some plant parts of dicots are always ingested. Parts of plants with high soluble-carbohydrate content and short-handling times were selected by cotton rats ; however, these same parts of plants typically were low in protein and phosphorus content

Experimental Species Removal: Demographic Responses by Sigmodon hispidus and Reithrodontomys fulvescens

Experimental species removal was conducted for three years to assess the demographic consequences of interspecific interactions between cotton rats ( Sigmodon hispidus ) and fulvous harvest mice ( Reithrodontomys fulvescens ). Six 1.6-hectare (ha) plots were monitored monthly by live-trapping; two control, two Sigmodon -only, and two Reithrodontomys -only plots were maintained by appropriate species removal. Frequent trapping and habitat discontinuities between plots minimized the effect of interplot movement and maintained the experimental perturbation. Seasonal vegetation analyses were conducted on each field. Density, percent reproduction, and expectation of further life but not survival were significantly lower for both Sigmodon and Reithrodontomys on experimental plots (codominant species removed). Lower density for both species and lower reproduction for Reithrodontomys were significantly correlated to lower abundance of preferred habitat on these plots. Lower expectation of further life reflected increased mobility ( AvD p ) of Sigmodon on experimental plots, thereby contributing to lower densities and decreased reproduction through lower mate contact. Cotton rat mean individual body weight was significantly lower and male harvest mouse mean body weight significantly higher on experimental plots; lower cotton rat body weight reflected immigration onto experimental plots in response to a density gradient. Sigmodon sex ratio was skewed toward males on control plots, whereas Reithrodontomys sex ratio was skewed toward males on both plots; differential movement or trappability by either sex may be responsible for this result. The significantly lower proportion of adults and young adults on Sigmodon experimental plots was due to emigration; there was no between-plot age class difference for Reithrodontomys . Most demographic differences between control and experimental plots are explained by differential movement, survival, or abundance of preferred habitat. However, increased mean body weight of male Reithrodontomys and earlier appearance of Reithrodontomys juveniles on experimental plots remain unexplained. These results suggest interspecific interactions and/or behavioral interactions may be of secondary importance in determining the demographic structure of rodents in this system.

Assessment of space-use patterns in the hispid cotton rat (Sigmodon hispidus)

SummaryEcological interpretation of space use patterns often suffers from two methodological problems: inadequate number of captures per individual and pooling of data over time intervals. Insufficient sample size biases the computation of spatial areas, while pooling data over time intervals may mask shifts in space use due to changes in resource abundance. Radiotelemetry was used to alleviate these problems in an analysis of space use by the hispid cotton rat (Sigmodon hispidus). Home range area was greater for males than females, was largest during summer and winter months, was positively correlated with body hass, and was negatively correlated with population dencity. Exclusivity of home range revealed a high degree of ntolerance (41% exclusivity) and was positively correlated with body mass for males. In addition, like-sex categories (male-male, female-female) were more exclusive than unlike sex categories (male-female).Habitat composition of home ranges of females was significantly different from that of males and from that available. This result suggested home ranges of females were responsive to habitat composition (and quality), while males may respond more to female occurrence than resource availability.Space-use patterns of the hispid cotton rat indicated a solitary existence with greater tolerance of individuals of the opposite sex. Home range size decreased as population size increased, whereas home range overlaps were not affected by population density. These results reinforced the view of a dominance hierarchy in this species and suggested the existence of a polygynous mating system.