Spatial variation and density-dependent dispersal in competitive coexistence

It is well known that dispersal from localities favourable to a species' growth and reproduction (sources) can prevent competitive exclusion in unfavourable localities (sinks). What is perhaps less well known is that too much emigration can undermine the viability of sources and cause regional competitive exclusion. Here, I investigate two biological mechanisms that reduce the cost of dispersal to source communities. The first involves increasing the spatial variation in the strength of competition such that sources can withstand high rates of emigration; the second involves reducing emigration from sources via density-dependent dispersal. I compare how different forms of spatial variation and modes of dispersal influence source viability, and hence source-sink coexistence, under dominance and pre-emptive competition. A key finding is that, while spatial variation substantially reduces dispersal costs under both types of competition, density-dependent dispersal does so only under dominance competition. For instance, when spatial variation in the strength of competition is high, coexistence is possible (regardless of the type of competition) even when sources experience high emigration rates; when spatial variation is low, coexistence is restricted even under low emigration rates. Under dominance competition, density-dependent dispersal has a strong effect on coexistence. For instance, when the emigration rate increases with density at an accelerating rate (Type III density-dependent dispersal), coexistence is possible even when spatial variation is quite low; when the emigration rate increases with density at a decelerating rate (Type II density-dependent dispersal), coexistence is restricted even when spatial variation is quite high. Under pre-emptive competition, density-dependent dispersal has only a marginal effect on coexistence. Thus, the diversity-reducing effects of high dispersal rates persist under pre-emptive competition even when dispersal is density dependent, but can be significantly mitigated under dominance competition if density-dependent dispersal is Type III rather than Type II. These results lead to testable predictions about source-sink coexistence under different regimes of competition, spatial variation and dispersal. They identify situations in which density-independent dispersal provides a reasonable approximation to species' dispersal patterns, and those under which consideration of density-dependent dispersal is crucial to predicting long-term coexistence.     

Microevolutionary patterns in the common caiman predict macroevolutionary trends across extant crocodilians

Both extinct and extant crocodilians have repeatedly diversified in skull shape along a continuum, from narrow‐snouted to broad‐snouted phenotypes. These patterns occur with striking regularity, although it is currently unknown whether these trends also apply to microevolutionary divergence during population differentiation or the early stages of speciation. Assessing patterns of intraspecific variation within a single taxon can potentially provide insight into the processes of macroevolutionary differentiation. For example, high levels of intraspecific variation along a narrow‐broad axis would be consistent with the view that cranial shapes can show predictable patterns of differentiation on relatively short timescales, and potentially scale up to explain broader macroevolutionary patterns. In the present study, we use geometric morphometric methods to characterize intraspecific cranial shape variation among groups within a single, widely distributed clade, Caiman crocodilus. We show that C. crocodilus skulls vary along a narrow/broad‐snouted continuum, with different subspecies strongly clustered at distinct ends of the continuum. We quantitatively compare these microevolutionary trends with patterns of diversity at macroevolutionary scales (among all extant crocodilians). We find that morphological differences among the subspecies of C. crocodilus parallel the patterns of morphological differentiation across extant crocodilians, with the primary axes of morphological diversity being highly correlated across the two scales. We find intraspecific cranial shape variation within C. crocodilus to span variation characterized by more than half of living species. We show the main axis of intraspecific phenotypic variation to align with the principal direction of macroevolutionary diversification in crocodilian cranial shape, suggesting that mechanisms of microevolutionary divergence within species may also explain broader patterns of diversification at higher taxonomic levels.     

Alternative stable states and regional community structure

Many models of local species interactions predict the occurrence of priority effects due to alternative stable equilibria (ASE). However, few empirical examples of ASE have been shown. One possible explanation for the disparity is that local ASE are difficult to maintain regionally in patch dynamic models. Here we examine two possible mechanisms for regional coexistence of species engaged in local ASE. Biotically generated heterogeneity (e.g., habitat modification that favors further invasion by conspecifics) results in regional exclusion of one species at equilibrium. In contrast, abiotic heterogeneity due to spatial variation in resource supply ratios generates local-scale ASE and ensures regional coexistence with sufficiently broad environmental gradients. Abiotic heterogeneity can result in a species that is the dominant competitor over some of its range being excluded if the area where it is dominant is too small. Biotic heterogeneity can lead to alternative stable landscapes or regional priority effects, while abiotic heterogeneity results in regional determinism. Broad environmental gradients in resource supply favor regional coexistence of species that exhibit local ASE.     

Efficiency and establishment of three introduced parasitoids of the mealybug Paracoccus marginatus (Hemiptera: Pseudococcidae)

Three introduced parasitoids Acerophagus papayae, Anagyrus loecki, and Pseudleptomastix mexicana of the mealybug Paracoccus marginatus have been released in Miami-Dade and Broward counties (Florida) in 2003. Efficiency and establishment of these previously introduced parasitoids to control P. marginatus were measured in 2005 and 2006, at three locations in Homestead (Miami-Dade). Mealybug populations were initially established on three branches (per plant) of 10 hibiscus plants inside closed-sleeve cages. The three treatments, closed, open, and no-cage environments were applied to the three established mealybug populations on each plant. The number of mealybugs and natural enemies was monitored in all treatments. After 72 h, the number of P. marginatus in the open-sleeve cage was higher than in the no-cage treatment but both were lower than the number in the closed-sleeve cage. Efficiency of parasitoids was evaluated by their percentage parasitism. Percentage parasitism (=recovery) was also used as an indicator for parasitoid establishment. Two of the three previously released parasitoids (A. papayae and A. loecki) were recovered in this study. Acerophagus papayae had a higher percentage parasitism than A. loecki in both the open-sleeve cage (31.0% vs 2.3%) and the no-cage treatments (21.4% vs 1.6%) and caused the most mortality of P. marginatus. There was no recovery of P. mexicana in either of the treatments. Although both A. papayae and A. loecki were established in tested areas, A. papayae was more efficient in controlling P. marginatus than A. loecki. The reasons for not recovering P. mexicana in release areas is yet unknown.     

Host instar susceptibility and selection and interspecific competition of three introduced parasitoids of the mealybug Paracoccus marginatus (Hemiptera: Pseudococcidae)

Three previously introduced parasitoids (Acerophagus papayae Noyes and Schauff, Anagyrus loecki Noyes and Menezes, and Pseudleptomastix mexicana Noyes and Schauff [Hymenoptera: Encyrtidae]) of the mealybug Paracoccus marginatus Williams and Granara de Willink (Hemiptera: Pseudococcidae) were studied for their host instar susceptibility and sex ratio, host instar selection, and interspecific competition in the laboratory. All three parasitoids were able to develop in the second instars, third-instar females, and adult females of P. marginatus. No progeny emerged from first-instar mealybugs. The proportion of female emergence was increased with increasing host size. Parasitoids selected their host instars for oviposition when they had a choice. Between second- and third-instar hosts, A. papayae and P. mexicana had significantly higher parasitism in second-instar mealybugs, whereas A. loecki had higher parasitism in the third-instar mealybugs. When competed with either one or two parasitoid species, A. papayae was significantly more successful in second-instar hosts and A. loecki was significantly more successful in third-instar mealybugs. P. mexicana was significantly less competitive when with A. papayae in both second and third instars, with A. loecki in third instars and with both A. papayae and A. loecki in second and third instars. Overall, A. papayae provided a better control of the host, when present singly or with the other two parasitoids. This information is important in evaluating the efficiency of A. papayae, A. loecki, and P. mexicana and understanding the outcome of their recovery and establishment in field studies conducted in Florida.     

Determinants of sequence-specificity within human AID and APOBEC3G

Human APOBEC3G (A3G) and activation-induced deaminase (AID) belong to a family of DNA-cytosine deaminases. While A3G targets the last C in a run of C's, AID targets C in the consensus sequence WRC (W is A or T and R is a purine). Guided by the structures of the A3G carboxyl-terminal catalytic domain (A3G-CTD), we identified two potential regions (region 1 and region 2) that may interact with DNA and swapped the corresponding regions between a variant of A3G-CTD and AID. The resulting hybrids were expressed in Escherichia coli and two different genetic assays and a biochemical assay were used to determine the sequence selectivity of the hybrids in promoting C to T mutations. The results show that while the 10 amino acid region 2 of A3G was its principal sequence-specificity determinant, region 1 of A3G enhanced the target cytosine preference conferred by region 2. In contrast, neither of the two regions in AID individually or in combination were sufficient to confer the DNA sequence preference of this protein upon A3G. Instead, introduction of AID sequences in A3G relaxed the sequence-specificity of the latter protein. Our results show that the sequence selectivity of APOBEC family of enzymes is determined by at least two separate sequence segments and there may be additional regions of the protein involved in DNA sequence recognition.     

Spatial population structure in the banner-tailed kangaroo rat,Dipodomys spectabilis

I attempted to characterize spatial units of local dynamics and dispersal in banner-tailed kangaroo rats (Dipodomys spectabilis), to determine if spatial structure influenced population dynamics in the way predicted by current metapopulation models. D. spectabilis exhibited a hierarchical spatial structure. Local populations that appeared as discrete entities on a scale of kilometers were subdivided into clusters of mounds on a scale of meters. This structure, however, cannot be characyerized in terms of the discrete habitat patches envisioned by the metapopulation models. Occupied areas were statistically distinguishable from the surrounding matrix, but this difference was only quantitative. There were no discrete boundaries between occupied areas and the matrix. Habitat within occupied areas was heterogeneous, and occupied areas in different locations were statistically distinguishable from each other. High heterogeneity within occupied areas, and high contrast among them, make it difficult to define what is a suitable habitat patch for D. spectabilis. On a smaller spatial scale, there was significant aggregation of resident mounds within occupied areas. These aggregations, however, do not correspond to discrete habitat patches. Rather, they appear to result from an interaction between fine-scale habitat heterogeneity and limited dispersal due to natal philopatry and low adult vagility. These complications make it difficult to identify habitat patches independent of the species' distribution. For species like D. spectabilis that are patchily distributed but do not occupy discrete habitat patches, a patch occupancy approach does not seem appropriate for describing spatial structure. Hierarchical spatial structure underscores the need for a framework that incorporates multiple scales of spatial structure, rather than one that pre-imposes a single spatial scale as being important for population dynamics. A framework that (i) considers patchiness as a combination of both habitat heterogeneity, and life-history and behavioral characteristics, and (ii) incorporates hierarchical spatial structure, appears to be the most suitable for conceptualizing spatial dynamics of behaviorally complex vertebrates such as D. spectabilis.     

Spatial dynamics of communities with intraguild predation: the role of dispersal strategies

I investigate the influence of dispersal strategies on intraguild prey and predators (competing species that prey on each other). I find an asymmetry between the intraguild prey and predator in their responses to each other's dispersal. The intraguild predator's dispersal strategy and dispersal behavior have strong effects on the intraguild prey's abundance pattern, but the intraguild prey's dispersal strategy and behavior have little or no effect on the intraguild predator's abundance pattern. This asymmetry arises from the different constraints faced by the two species: the intraguild prey has to acquire resources while avoiding predation, but the intraguild predator only has to acquire resources. It leads to puzzling distribution patterns: when the intraguild prey and predator both move away from areas of high density, they become aggregated to high-density habitats, but when they both move toward areas of high resource productivity, they become segregated to resource-poor and resource-rich habitats. Aggregation is more likely when dispersal is random or less optimal, and segregation is more likely as dispersal becomes more optimal. The crucial implication is that trophic constraints dictate the fitness benefits of using dispersal strategies to sample environmental heterogeneity. A strategy that affords greater benefits to an intraguild predator can lead to a more optimal outcome for both the intraguild predator and prey than a strategy that affords greater benefits to an intraguild prey.