Michael D. Collins

On Testing the Competition‐Colonization Trade‐Off in a Multispecies Assemblage

The competition‐colonization trade‐off has long been considered an important mechanism explaining species coexistence in spatially structured environments, yet data supporting it remain ambiguous. Most competition‐colonization research examines plants and the dispersal‐linked traits of their seeds. However, colonization is more than just dispersal because rapid population growth is also an important component of colonization. We tested for the presence of competition‐colonization trade‐offs with a commonly used artificial assemblage consisting of protozoan and rotifer species, where colonization was the ability of a species to establish populations in patches. By ranking species according to their colonization abilities and their pairwise competitive interactions, we show that these species exhibit competition‐colonization trade‐offs. These results reveal that the competition‐colonization trade‐off exists within nonplant assemblages and that even in a laboratory setting, species are constrained to be either good competitors or colonizers but not both.

Rarefaction and nonrandom spatial dispersion patterns

Rarefaction estimates how many species are expected in a random sample of individuals from a larger collection and allows meaningful comparisons among collections of different sizes. It assumes random spatial dispersion. However, two common dispersion patterns, within-species clumping and segregation among species, can cause rarefaction to overestimate the species richness of a smaller continuous area. We use field studies and computer simulations to determine (1) how robust rarefaction is to nonrandom spatial dispersion and (2) whether simple measures of spatial autocorrelation can predict the bias in rarefaction estimates. Rarefaction does not estimate species richness accurately for many communities, especially at small sample sizes. Measures of spatial autocorrelation of the more abundant species do not reliably predict amount of bias. Survey sites should be standardized to equal-sized areas before sampling. When sites are of equal area but differ in number of individuals sampled, rarefaction can standardize collections. When communities are sampled from different-sized areas, the mean and confidence intervals of species accumulation curves allow more meaningful comparisons among sites.

Local host specialization, host-switching, and dispersal shape the regional distributions of avian haemosporidian parasites

Significance Within eastern North America, distributions of vector-transmitted haemosporidian blood parasites of birds, commonly known as “avian malaria parasites,” are associated with the distributions of their host species independently of direct effects of climate on potential vectors. Spatial analyses additionally indicated an absence of dispersal limitation for these parasites. Finally, host-breadth, ranging continuously from specialist to generalist, varies among parasite lineages and is dynamic within parasite assemblages over space and time. The distributions of avian haemosporidian parasites emphasize the ability of parasites to disperse across broad regions and to switch readily between hosts to become emerging infectious diseases. The drivers of regional parasite distributions are poorly understood, especially in comparison with those of free-living species. For vector-transmitted parasites, in particular, distributions might be influenced by host-switching and by parasite dispersal with primary hosts and vectors. We surveyed haemosporidian blood parasites (Plasmodium and Haemoproteus) of small land birds in eastern North America to characterize a regional parasite community. Distributions of parasite populations generally reflected distributions of their hosts across the region. However, when the interdependence between hosts and parasites was controlled statistically, local host assemblages were related to regional climatic gradients, but parasite assemblages were not. Moreover, because parasite assemblage similarity does not decrease with distance when controlling for host assemblages and climate, parasites evidently disperse readily within the distributions of their hosts. The degree of specialization on hosts varied in some parasite lineages over short periods and small geographic distances independently of the diversity of available hosts and potentially competing parasite lineages. Nonrandom spatial turnover was apparent in parasite lineages infecting one host species that was well-sampled within a single year across its range, plausibly reflecting localized adaptations of hosts and parasites. Overall, populations of avian hosts generally determine the geographic distributions of haemosporidian parasites. However, parasites are not dispersal-limited within their host distributions, and they may switch hosts readily.

Prevalence of avian haemosporidian parasites is positively related to the abundance of host species at multiple sites within a region.

Parasite prevalence is thought to be positively related to host population density owing to enhanced contagion. However, the relationship between prevalence and local abundance of multiple host species is underexplored. We surveyed birds and their haemosporidian parasites (genera Plasmodium and Haemoproteus) at multiple sites across eastern North America to test whether the prevalence of these parasites in a host species at a particular site is related to that host’s local abundance. Prevalence was positively related to host abundance within most sites, although the effect was stronger and more consistent for Plasmodium than for Haemoproteus. In contrast, prevalence was not related to variation in the abundance of most individual host species among sites across the region. These results suggest that parasite prevalence partly reflects the relative abundances of host species in local assemblages. However, three nonnative host species had low prevalence despite being relatively abundant at one site, as predicted by the enemy release hypothesis.

Host community similarity and geography shape the diversity and distribution of haemosporidian parasites in Amazonian birds

Identifying the mechanisms driving the distribution and diversity of parasitic organisms and characterizing the structure of parasite assemblages are critical to understanding host-parasite evolution, community dynamics, and disease transmission risk. Haemosporidian parasites of the genera Plasmodium and Haemoproteus are a diverse and cosmopolitan group of bird pathogens. Despite their global distribution, the ecological and historical factors shaping the diversity and distribution of these protozoan parasites across avian communities and geographic regions remains unclear. Here we used a region of the mitochondrial cytochrome b gene to characterize the diversity, biogeographical patterns and phylogenetic relationships of Plasmodium and Haemoproteus infecting Amazonian birds. Specifically, we asked whether, and how, host community similarity and geography (latitude and area of endemism) structure parasite assemblages across 15 avian communities in the Amazon Basin. We identified 265 lineages of haemosporidians recovered from 2661 sampled birds from 330 species. Infection prevalence varied widely among host species, avian communities, areas of endemism, and latitude. Composition analysis demonstrated that both malarial parasites and host communities differed across areas of endemism and as a function of latitude. Thus, areas with similar avian community composition were similar in their parasite communities. Our analyses, within a regional biogeographic context, imply that host switching is the main event promoting diversification in malarial parasites. Although dispersal of haemosporidian parasites was constrained across six areas of endemism, these pathogens are not dispersal-limited among communities within the same area of endemism. Our findings indicate that the distribution of malarial parasites in Amazonian birds is largely dependent on local ecological conditions and host evolutionary relationships. This article is protected by copyright. All rights reserved.

Habitat and Vegetation Variables Are Not Enough When Predicting Tick Populations in the Southeastern United States.

Two tick-borne diseases with expanding case and vector distributions are ehrlichiosis (transmitted by Amblyomma americanum) and rickettiosis (transmitted by A. maculatum and Dermacentor variabilis). There is a critical need to identify the specific habitats where each of these species is likely to be encountered to classify and pinpoint risk areas. Consequently, an in-depth tick prevalence study was conducted on the dominant ticks in the southeast. Vegetation, soil, and remote sensing data were used to test the hypothesis that habitat and vegetation variables can predict tick abundances. No variables were significant predictors of A. americanum adult and nymph tick abundance, and no clustering was evident because this species was found throughout the study area. For A. maculatum adult tick abundance was predicted by NDVI and by the interaction between habitat type and plant diversity; two significant population clusters were identified in a heterogeneous area suitable for quail habitat. For D. variabilis no environmental variables were significant predictors of adult abundance; however, D. variabilis collections clustered in three significant areas best described as agriculture areas with defined edges. This study identified few landscape and vegetation variables associated with tick presence. While some variables were significantly associated with tick populations, the amount of explained variation was not useful for predicting reliably where ticks occur; consequently, additional research that includes multiple sampling seasons and locations throughout the southeast are warranted. This low amount of explained variation may also be due to the use of hosts for dispersal, and potentially to other abiotic and biotic variables. Host species play a large role in the establishment, maintenance, and dispersal of a tick species, as well as the maintenance of disease cycles, dispersal to new areas, and identification of risk areas.

Geographic Patterns of Song Similarity in the Dickcissel (Spiza Americana)

Abstract Song sharing among neighboring males is a well-known, frequent outcome of song learning in oscine passerines and some other groups, but only limited investigations of the spatial scale of this phenomenon have been pursued. On the basis of recordings of 1,043 individuals, we investigated song sharing in Dickcissels (Spiza americana) at local and regional scales at sites from northern Kansas to northern Oklahoma. Classification of song elements revealed decreasing song similarity with increasing distances between individual birds at small to intermediate scales, to ~10 km. At the largest spatial scales (10–300 km between sites), there was very little similarity among sites and no obvious tendency for a decrease in similarity with increasing distances among our 30 sites. At our intensively sampled site, analyses of quantitative measurements showed that, at least for our most widely shared song element, frequency and duration were more similar in closer birds. Thus, distance between birds influences both quantitative and qualitative song similarity in Dickcissels. Variability existed among sites in the shape of the song-sharing decay curve, which indicates that other factors besides distance also govern song-sharing patterns. We found high repeatability of individual songs for both second-year (SY) and after-second-year (ASY) males throughout the season, and high conformity to the local song neighborhood in both SY and ASY males from their first recording soon after arrival in May. Returning ASY males sang the same song they had produced the previous breeding season.

The Checkered History of Checkerboard Distributions: Reply

Diamond et al. (2015) raise three criticisms of Connor et al. (2013). The first is that by analyzing each archipelago separately rather than analyzing species pairs using their entire or global geographic ranges, Connor et al. (2013) have misinterpreted the factors that affect the geographic ranges of congeneric species pairs. The second is that Connor et al. (2013) did not plot the geographic ranges of species pairs. Finally, Connor et al. (2013) did not include information on vagrancy. The checkered history of checkerboard distributions is characterized by its pioneer (Diamond 1975) and subsequent followers (Diamond and Gilpin 1982, Gilpin and Diamond 1982, 1984, Sanderson et al. 2009) examining the pairwise geographical distributions of species pairs within archipelagos. Connor et al. (2013), as in previous work (Connor and Simberloff 1979, 1983, 1984, Simberloff and Collins 2010, Collins et al. 2011), followed this convention since it appeared to be part of the definition of and the tradition for inferring competitively determined checkerboard distributions. It is conceivable that one could attempt to analyze rigorously the global pairwise distributions of species, but Diamond et al. (2015) have not done so. Furthermore, such an analysis would raise new issues. For example, how should patchy distributions within larger islands like New Guinea be treated when one scores checkerboard distributions? How should the barriers to dispersal among island groups within archipelagos, as proposed by Mayr and Diamond (2001), inform the analysis? Diamond et al. (2015) marshal only a single example to support their contention that, by analyzing the entire or global distributions of species, one would detect many pairs of species that display checkerboard distributions because of competition. Furthermore, their critique is based on the simple inspection of a map, which is tantamount to Diamond’s (1975) original basis for inferring that competition had affected the geographical distribution of species: that a checkerboard distribution is prima facie evidence for competitive interactions shaping geographical distributions; in essence, checkerboards arise only because of competition. They claim that merely by visually examining the ranges of Macropygia mackinlayi andM. nigrirostris they can tell that the distribution of these two species requires an explanation involving interspecific competition—a clear case of déjà vu all over again. However, Mayr and Diamond (2001) provided compelling evidence for the existence of barriers to dispersal within archipelagoes, and barriers likely exist between archipelagoes as well. Any analysis would need to account for potential dispersal limitation both within and between archipelagoes. Connor et al. (2013) motivated the three attributes that they claim define a ‘‘true checkerboard,’’ a species pair that would have geographical distributions consistent with competitive interactions: (1) the pair would have exclusive island-by-island distributions, (2) their geographic ranges would overlap more than expected were they independently determined, and (3) the pair would share one or more of the island groups defined by Mayr and Diamond (2001) and mapped by Simberloff and Collins (2010) and Collins et al. (2011) for the Solomon Islands and the Bismarck Archipelago, respectively. These three criteria were intended to provide an operational definition of a ‘‘checkerboard’’ distribution sensu Diamond (1975) and Mayr and Diamond (2001). Diamond et al. (2015) do not object to this definition, yet as mentioned above they feel confident that their visual inspection of the ranges satisfies the second criterion. In the analysis conducted by Connor et al. (2013), the pair of Cuckoo Doves in question met Manuscript received 10 June 2015; revised 6 July 2015; accepted 7 July 2015. Corresponding Editor: P. de Valpine. 1Department of Biology, San Francisco State University, 1600 Holloway Avenue, San Francisco, California 94132 USA 2Department of Biology, Rhodes College, Memphis, Tennessee 38112 USA 3Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee 37996 USA 4 E-mail: efc@sfsu.edu

Neotropical Migrants Exhibit Variable Body-Size Changes Over Time and Space

Abstract Recent changes in the Earths climate have been linked to changes in phenology, geographic distributions, and morphology of species, and warming temperatures associated with climate change have been predicted to result in decreases in avian body sizes. We examined changes in wing length and fat-free mass of 34,844 fall migrants from 31 neotropical migratory species captured at Patuxent Wildlife Research Center in Maryland between 1980 and 2012. Body size changes varied across species, but wing length and fat-free mass increased significantly over time in the pooled sample of all species. Magnitudes of change were small and similar to other studies, with mean wing length increasing 0.55% and mean fat-free mass increasing 1.30% across all species. General morphological changes at our site differed from those at a banding station located 235 km away. Across species, changes in wing length were weakly correlated between stations, and changes in fat-free mass were uncorrelated. Populations of some species showed opposite morphological changes, demonstrating that morphological changes can vary regionally. Over short time scales, factors other than climate might drive observed changes in body size of neotropical migrants, and alternative hypotheses for body size changes should be considered.