Craig Guyer

Adaptive radiation and the topology of large phylogenies

The idea that some organisms possess adaptive features that make them more likely to speciate and/or less likely to go extinct than closely related groups, suggests that large phylogenetic trees should be unbalanced (more species should occur in the group possessing the adaptive features than in the sister group lacking such features). Several methods have been used to document this type of adaptive radiation. One problem with these attempts is that evolutionary biologists may overlook balanced phylogenies while focusing on a few impressively unbalanced ones. To overcome this potential bias, we sampled published large phylogenies without regard to tree shape. These were used to test whether or not such trees are consistently unbalanced. We used recently developed null models to demonstrate that the shapes of large phylogenetic trees: 1) are similar among angiosperms, insects, and tetrapods; 2) differ from those expected due to random selection of a phylogeny from the pool of all trees of similar size; and 3) are significantly more unbalanced than expected if species diverge at random, therefore, conforming to one prediction of adaptive radiation. This represents an important first step in documenting whether adaptive radiation has been a general feature of evolution.

Patterns of reproduction and habitat use in an assemblage of Neotropical hylid frogs

We censused pond-breeding hylid frogs in northeastern Costa Rica weekly for 15 months to deseribe patterns of reproduction, habitat use, and to establish baseline data on relative abundance for members of the assemblage. Reproduction in the Costa Rican assemblage was seasonal and occurred only during wet months. Some species called, but none reproduced, during the dry season. Three species (Agalychnis callidryas, Hyla ebraccata, and Scinax elaeochroa) accounted for more than 75% of the observations made during the study. The species overlapped broadly in time and space, but differed in substrate use and phenology. Two species of leaf-breeding frogs (A. callidryas and A. saltator) used perches that were significantly higher than those used by the other species. Some phenological differences were associated with different mating strategies. Explosive breeders (Scinax elaeochroa and Smilisca baudinii) were most common early in the wet seasons. Prolonged breeders (A. callidryas and H. ebraccata) were the most persistent members of this assemblage. Predation affects early and late life history stages of these hylids. Predation on arboreal egg masses by two snake species was observed. Ctenid spiders preyed on recently metamorphosed frogs and small adults. Our weekly samples were pooled into 21-day periods so that we could compare our results with those obtained for two communities of breeding anurans from South America. The patterns observed in the Costa Rican assemblage differed from those reported for South American pond-breeding frogs, but in all three assemblages reproduction was associated with wet periods.

Empirical evidence for an optimal body size in snakes.

Abstract The concept of optimal size has been invoked to explain patterns in body size of terrestrial mammals. However, the generality of this phenomenon has not been tested with similarly complete data from other taxonomic groups. In this study we describe three statistical patterns of body size in snakes, all of which indicate an optimal length of 1.0 m. First, a distribution of largest body lengths of 618 snake species had a single mode at 1.0 m. Second, we found a positive relationship between the size of the largest member of an island snake assemblage and island area and a negative relationship between the size of the smallest member of an island snake assemblage and island area. Best‐fit lines through these data cross at a point corresponding to 1.0 m in body length, the presumed optimal size for a one‐species island. Third, mainland snake species smaller than 1.0 m become larger on islands whereas those larger than 1.0 m become smaller on islands. The observation that all three analyses converge on a common body size is concordant with patterns observed in mammals and partial analyses of four other disparate animal clades. Because snakes differ so strikingly from mammals (ectotherms, gape‐limited predators, elongate body shape) the concordant patterns of these two groups provide strong evidence for the evolution of an optimal body size within independent monophyletic groups. However, snakes differ from other taxonomic groups that have been studied in exhibiting a body size distribution that is not obviously skewed in either direction. We suggest that idiosyncratic features of the natural history of ectotherms allow relatively unconstrained distributions of body size whereas physiological limitations of endotherms constrain distributions of body size to a right skew.

PATTERNS OF MOVEMENT AND BURROW USE IN A POPULATION OF GOPHER TORTOISES (GOPHERUS POLYPHEMUS)

We investigated movement and burrow use patterns from May 1997 through May 1998 in a gopher tortoise population (n = 123) within a 100-ha study site of high quality (old trees, ground cover intact, growing-season fire regime) longleaf pine-wiregrass habitat located in Baker County, Georgia, USA. Telemetered females moved more frequently in summer months (June–October) but traveled longer distances in September than during other months of the 1997 active season. Males exhibited a peak in movement during August and September 1997 that corresponded with mating activity. The longest distance moved between tracking locations did not differ between males and females, but mean distance per move, number of burrows used, and annual home range size were greater in males than in females. The maximum distance moved by an individual during an active season was negatively correlated with female body size but not male body size. No such relationship was found between body size and number of burrows used by either females or males. Similarly, no relationship was found between body size and annual home range area of adult females and males. Our study provides the most complete estimate of annual home range size of gopher tortoises. Because these data describe movements in an area that retains features of the ancestral habitat, these estimates are the best available for designing reserve areas for this threatened species.

Food Supplementation in a Tropical Mainland Anole, Norops Humilis: Effects on Individuals

In this study I investigated how individuals of Norops humilis at a tropical lowland wet-forest site in Costa Rica used energy made available by supplemental feeding. I manipulated prey availability on three experimental plots by attracting arthropods to bits of rotting meat that were replenished weekly. Each experimental plot was paired with an unmanipulated control plot in a block design. Mark-recapture data were collected on all plots over a 7-mo test period during which I made 9910 observations on 1582 individuals. Home-range size was not affected by food supplementation. Instead, home-range over- lap was increased for all combinations of age-sex groups (juveniles, males, and females) except for females relative to other females and juveniles relative to females. Growth of females was not altered by supplemental food. For males, both the shape of the growth curve and the estimate of asymptotic largest size were altered by food supplementation. No difference in body mass relative to length was observed for either males or females. Rate of egg production of females on experimental plots was significantly more rapid than on control plots. Males appeared to shunt extra food energy into growth and females into reproduction. The responses of this mainland anole are similar to responses described for island anoles in that increased juvenile overlap, increased clumping of juveniles, and unchanged home-range sizes were observed. In contrast, the observations of unchanged relative body mass, unchanged female growth rate, and increased egg-production rates in supplementally fed N. humilis differ from observations of island anoles.

Sampling on private property to evaluate population status and effects of land use practices on the gopher tortoise, Gopherus polyphemus

Abstract Although private properties are predicted to play an increasingly significant role in conservation, surveys of species of special concern are rare on these lands. We created a template for a multi-county survey of randomly selected sites and sampled for burrows of the gopher tortoise (Gopherus polyphemus) in south-central Georgia, USA. Current land use was strongly correlated with tortoise population condition. The highest densities of tortoise burrows were found on lands with open-canopied pine stands that were managed with prescribed fire, a practice associated with types of selection forestry and/or wildlife management. Agricultural sites and unburned areas provided poor habitat and pine plantations were only slightly better. Our estimates of tortoise population densities indicated that the current landscape supports less than 20% of the animals present before implementation of modern land use practices. In addition, our estimate for density of active burrows was approximately one third of that projected for the entire state range 20 years ago by Auffenberg and Franz [Auffenberg, W., Franz, R., 1982. The status and distribution of the gopher tortoise (Gopherus polyphemus). In: Bury, R.B. (Ed.), North American Tortoises: Conservation and Ecology (US Fish and Wildlife Service Wildlife Research Report 12). pp. 95–126]. However, some good sites for gopher tortoises remain in south Georgia and our data also suggested that extraordinary conservation actions may not be required if ways can be developed to retain traditional land management practices on private property.

Burrow abandonment by gopher tortoises in slash pine plantations of the Conecuh National Forest

We investigated burrow dynamics and factors associated with abandonment of burrows hy gopher tortoises (Copherus polyphemus) in mature slash pine (Pinus elliottii) plantations of the Conecuh National Forest (CNF) in southcentral Alabama. Our objectives were to determine how frequently gopher tortoises abandoned burrows in pine plantations, if burrow abandonment was associated with changes in vegetation conditions, and how rapidly these changes occurred. Burrow survey data collected over 5 years indicated gopher tortoises abandoned burrows at an average rate of 22%/year. We believe burrow abandonment occurred frequently in pine plantations, primarily in association with a change in overstory structure that shaded active burrows. Abandoned gopher tortoise burrows had greater total basal area (P = 0.012), hardwood basal area (P = 0.016), and tree density (P = 0.003) than did active burrows. There was a significant positive correlation between age of active gopher tortoise burrows and canopy closure (total basal area: r 2 = 0.19, P = 0.007; pine basal area: r 2 = 0.20, P = 0.004; tree density: r 2 = 0.14, P = 0.018). We estimated that overstory conditions at newly active burrows changed to those observed at abandoned burrows in only 5-7 years. Increases in total basal area to 70 m 2 /ha and tree density to 1,400 trees/ha were associated with burrow abandonment. Active burrows had greater total plant cover (P = 0.009) and grass cover (P = 0.016) than did abandoned burrows. However, we found no correlation between age of active burrows and structure and composition of ground cover vegetation (r 2 values ranged from 0.00 to 0.07; P-values ranged from 0.096 to 0.987). Stand thinning to a basal area of 30 m 2 /ha and prescribed growing-season burns should improve habitat quality, thereby increasing burrow fidelity of gopher tortoises.

Nestedness of ectoparasite-vertebrate host networks.

Determining the structure of ectoparasite-host networks will enable disease ecologists to better understand and predict the spread of vector-borne diseases. If these networks have consistent properties, then studying the structure of well-understood networks could lead to extrapolation of these properties to others, including those that support emerging pathogens. Borrowing a quantitative measure of network structure from studies of mutualistic relationships between plants and their pollinators, we analyzed 29 ectoparasite-vertebrate host networks—including three derived from molecular bloodmeal analysis of mosquito feeding patterns—using measures of nestedness to identify non-random interactions among species. We found significant nestedness in ectoparasite-vertebrate host lists for habitats ranging from tropical rainforests to polar environments. These networks showed non-random patterns of nesting, and did not differ significantly from published estimates of nestedness from mutualistic networks. Mutualistic and antagonistic networks appear to be organized similarly, with generalized ectoparasites interacting with hosts that attract many ectoparasites and more specialized ectoparasites usually interacting with these same “generalized” hosts. This finding has implications for understanding the network dynamics of vector-born pathogens. We suggest that nestedness (rather than random ectoparasite-host associations) can allow rapid transfer of pathogens throughout a network, and expand upon such concepts as the dilution effect, bridge vectors, and host switching in the context of nested ectoparasite-vertebrate host networks.

Bird assemblage response to restoration of fire‐suppressed longleaf pine sandhills

The ecological restoration of fire-suppressed habitats may require a multifaceted approach. Removal of hardwood trees together with reintroduction of fire has been suggested as a method of restoring fire-suppressed longleaf pine (Pinus palustris) forests; however, this strategy, although widespread, has not been evaluated on large spatial and temporal scales. We used a landscape-scale experimental design to examine how bird assemblages in fire-suppressed longleaf pine sandhills responded to fire alone or fire following mechanical removal or herbicide application to reduce hardwood levels. Individual treatments were compared to fire-suppressed controls and reference sites. After initial treatment, all sites were managed with prescribed fire, on an approximately two- to three-year interval, for over a decade. Nonmetric multidimensional scaling ordinations suggested that avian assemblages on sites that experienced any form of hardwood removal differed from assemblages on both fire-suppressed sites and reference sites 3-4 years after treatment (i.e., early posttreatment). After >10 years of prescribed burning on all sites (i.e., late posttreatment), only assemblages at sites treated with herbicide were indistinguishable from assemblages at reference sites. By the end of the study, individual species that were once indicators of reference sites no longer contributed to making reference sites unique. Occupancy modeling of these indicator species also demonstrated increasing similarity across treatments over time. Overall, although we documented long-term and variable assemblage-level change, our results indicate occupancy for birds considered longleaf pine specialists was similar at treatment and reference sites after over a decade of prescribed burning, regardless of initial method of hardwood removal. In other words, based on the response of species highly associated with the habitat, we found no justification for the added cost and effort of fire surrogates; fire alone was sufficient to restore these species.

Landscape-level influences of terrestrial snake occupancy within the southeastern United States.

Habitat loss and degradation are thought to be the primary drivers of species extirpations, but for many species we have little information regarding specific habitats that influence occupancy. Snakes are of conservation concern throughout North America, but effective management and conservation are hindered by a lack of basic natural history information and the small number of large-scale studies designed to assess general population trends. To address this information gap, we compiled detection/nondetection data for 13 large terrestrial species from 449 traps located across the southeastern United States, and we characterized the land cover surrounding each trap at multiple spatial scales (250-, 500-, and 1000-m buffers). We used occupancy modeling, while accounting for heterogeneity in detection probability, to identify habitat variables that were influential in determining the presence of a particular species. We evaluated 12 competing models for each species, representing various hypotheses pertaining to important habitat features for terrestrial snakes. Overall, considerable interspecific variation existed in important habitat variables and relevant spatial scales. For example, kingsnakes (Lampropeltis getula) were negatively associated with evergreen forests, whereas Louisiana pinesnake (Pituophis ruthveni) occupancy increased with increasing coverage of this forest type. Some species were positively associated with grassland and scrub/shrub (e.g., Slowinskis cornsnake, Elaphe slowinskii) whereas others, (e.g., copperhead, Agkistrodon contortrix, and eastern diamond-backed rattlesnake, Crotalus adamanteus) were positively associated with forested habitats. Although the species that we studied may persist in varied landscapes other than those we identified as important, our data were collected in relatively undeveloped areas. Thus, our findings may be relevant when generating conservation plans or restoration goals. Maintaining or restoring landscapes that are most consistent with the ancestral habitat preferences of terrestrial snake assemblages will require a diverse habitat matrix over large spatial scales.