Defense Presentation Notes

Transcription

1 Defense Presentation Notes This is a summary of what I said slide by slide during my doctoral defense. There will be several corrections and changes based on the discussion I had after the presentation with my committee. Some of these I have added to the slides presentation and notes for clarity. Otherwise, I have noted that the presentation is preliminary and will be adjusted. I expect the dissertation to be available through the University of Texas at Arlington library by July Introduction The Spatial Ecology of the Comanche Harvester Ant, Pogonomyrmex comanche (Hymenoptera, Formicidae) 2. Overview: Here s what I will cover. I will present only part of the three projects of my dissertation. Introduction Ants Research Goal Study Sites Research Projects Colony spatial pattern Ground active ants Nestmate discrimination and neighborhood structure Conclusions Summary implications Future Work 3. Introduction Gay feather in the demonstration prairie in front of the Hardwicke Center, Fort Worth Nature Center, Fort Worth, Texas 4. Introduction Ants Animal spatial patterns Animals respond to environmental and other factors in their abundance and distribution patterns. These factors can be abiotic, biotic, intra- and inter-specific. Central place animals All their activities and distribution patterns are oriented around their central location (den, nest, etc.). The choice and procurement of a good nest site is essential. The central place often affords some protection from adverse environmental conditions and predators. These animals may be able to control the nest environment, especially for ground or cavity nesting species producing a more suitable and stable environment for raising young and surviving. These central place activities may affect other organisms. Thus, the spatial patterns of central place animals is influenced by environmental and other factors but also, by being localized in a central place, these animals can have profound effects on the spatial patterns of other organisms. Beyond such impacts, central place animals may be ecosystem engineers. Studying these patterns can be especially ecologically enlightening. Ant spatial patterns (colony) Diverse ecological roles: Ants have a great diversity of ecological roles, including as prey and predators; they may have parasitic, commensalistic, and symbiotic relationships. Ground nesting species are involved in bioturbation, the creation of soil and are important for soil health. Because of these roles, ants may be ecosystem engineers. Many species have been shown to engineer soil.

2 2 (I give a bit more explanation about ants as soil engineers on my blog, Ant Ecology and Other Adventures, along with selected literature.) Potential influences on these patterns regional: climate and geology local: soil characteristics (particle size, drainage, components), vegetation (shading temperature, food source, habitat structure), other organisms In this work, I am interested in the local level. Expectations of colony patterns: departing from random regular (overdispersed): hypothesized to be the result of competition and maintained by aggression (territoriality) clustered: often expected based on heterogeneity of environment in which animals cluster in optimal areas of the habitat. Pogonomyrmex species most studied: P. barbatus, P. rugosus, P. occidentalis Pogonomyrmex spp. are monodomous with fairly stable nest locations, though some do move their nests. They are seed harvesters with 60 80% of their diet consisting of seeds. This varies by species. They are known for aggression in low productive, arid habitats territoriality They are restricted to the New World (the Americas). ecosystem engineers known as agents of disturbance conspicuous nests, often with extensive cleared areas (nest yard) cleared trails significant bioturbation and soil production affecting soil chemical, physical, and biotic processes P. comanche unstudied 5. Research Goal To characterize the colony spatial distribution pattern, locally and investigate factors which may influence this pattern, including: other ant species interspecific interactions colony interactions 6. Study Sites: located in the Cross Timbers Ecoregion (post oak savannah) This is a transition from the forests of the Eastern US to the open prairies of the Southwest. consists of a mosaic of prairie and woods with oak motts and light gaps in constant transition dynamic interface historically maintained by large animals and fire perhaps helped by other animals of disturbance, like ants These prairies vary in their soil type and vegetation. Most do not have P. comanche colonies. 21 sites total: FWNC (Fort Worth, Texas) 4 sites with P. comanche 14 prairie sites 3 woods sites THAN (Fort Worth, Texas) no P. comanche 2 prairies: Tandy Hills Prairie and Stratford Prairie

3 3 SWNP (Arlington, Texas) 1 prairie with P. comanche 1 wooded site 7. Colony Spatial Pattern 8. Colony Spatial Pattern Hypothesis H1: P. comanche colony spatial pattern is regular or overdispersed. Regularity has been interpreted as a result of competition managed by aggression. Territoriality, aggression, and a regular spatial pattern of colonies appears typical of the genus, including P. maricopa which is in the same complex (closely related, similar habitat) as P. comanche. A regular pattern would support this conjecture from studies of other species in the genus. Together with nestmate discrimination via aggression (project 3). Typical external nest form and nest yard for P. comanche is a crater with a central entrance. The crater is on average 30 cm in diameter and may be as much as 50 cm. larger forms are probably typical of older colonies. The surrounding nest yard is an area the ants keep cleared of vegetation and may be an additional ½ to 1 m in diameter. Depending on colony abundance and density, the nests may amount to a significant area of bare ground in these prairies. 9. Methods: Colony mapping: FWNC map -- GL: 16 colonies; EP: 95 colonies; T1P: 150 colonies; T2P: 458 colonies GIS coordinates located colonies EP: GL: T1P: T2P: SP: Spatial analysis with ArcGIS program (Esri) Ripley s K function: This is a spatial point process analysis which finds the number of colonies at specified distance increments from each nest; then sums those colonies by increments to get an average colony number at each increment. The average number of colonies for each increment is then tested against a Poisson distribution (random). Thus, the pattern can be evaluated at scales according to the increments. I used 10 m increments up to 100 m. The pattern is then characterized as clustered or regular depending on the data departure from random (Poisson). I will be trying other increments that may more meaningfully represent the spatial scale the ants experience and to which they respond. 10. SP Map: 60 colonies + 6 in trails 11. Colony Spatial Pattern Results (> There is obviously a problem with ArcGIS performing this analysis. The graphs cannot be correct. This point came out in the discussion I had with my committee afterwards. I believe the pattern will still come out clustered on the scale that matters to the ants but I am currently working toward resolving the problems.) Ripley s K Diagrams interpretation: The red line is the actual data. The blue line is the expectation of a random pattern (used for comparison). The parallel gray lines are the 99% confidence interval. When the data (red line) are above the blue line, the pattern is clustered. When the data is below the blue, the pattern is regular. When the red line is outside of the gray lines, the data pattern is significant. The diagrams indicate a clustered pattern except SP which was random.

4 4 These are evaluations of all the colonies in the sites from GL is not included because the sample size was too small only 16 colonies when 30+ are suggested. 12. Conclusions Spatial pattern is clustered. With the exception of SP this is perhaps related to the different soil and isolated and small prairie It seems unlikely that P. comanche colony clustering is be due to heterogeneous habitat. There is no obvious heterogeneity for instance in soil or vegetation. Suggestion: new hypothesis Clustering is likely the result of queen dispersal and nest site choice. This has been shown in P. occidentalis and other ant species. 13. Ground Active Ants This clustering may involve conspecific attraction in which foundress queens choose nest sites near established colonies that indicate appropriate soil, etc. 14. Hypothesis: Ground active ants differ among sites. Background: Because of the diverse roles of ants and that ground nesting species are hypothesized to have specific nesting requirements, it is expected that ant assemblages vary with environmental variation (especially soil characteristics, shading/temperature, moisture/humidity; perhaps food resources depends on specialization of the ants). So, different habitats may have different ant assemblages in response to these factors. It is also thought that ants may be good indicator species they are expected to be sensitive to changes and differences in habitats and ecosystems and respond to these in terms of changes in their presence and abundance. There is evidence to support both of the above claims. I sought to determine the differences in ground active ants in prairie habitats especially with respect to P. comanche (occurrence/abundance) and to look for evidence that P. comanche was a dominant ant (the converse) affecting the ant assemblage when P. comanche is present. Part of this analysis could yield cooccurring species and indicator species. It has been, at least informally proposed, that P. comanche is an indicator of the Aquilla soil formation and other deep sandy soils. 15. Methods Field methods I collected ants in 21 sites once a month over 72 hours in June, July, and August with pitfall traps. Pitfall traps are an adequate way to get most ant species, but not all (There are trapability issues.). I measured some environmental variables each time I collected ants. I obtained other environmental variables through the local soil report and Natural Resources Conservation Service (NRCS). I had 13 environmental variables in all but found only two of these to be significant. I performed an ordination analysis, Redundancy analysis (RDA) to evaluate the presence and abundance of ant species in response to the environmental variables. The idea is that the environmental variables can be used to predict the ant species. 16. Results RDA (redundancy analysis) The RDA results were significant, both the first axis and all axes together. The significant

5 5 environmental factors were percent litter cover and soil drainage. In this model the first axis explains nearly 17% and the first two axes about 25%. Not bad for ecological data. There may be other significant factors or interactions that influence these patterns which I did not measure. 17. Results Attribute Plots These are diagrams for the RDA. It is the same RDA in each plot. The different plots are emphasizing a particular attribute. The relative size of the circles shows how much that attribute is important to that site (The labels are identifiers for the sites.). The sites where P. comanche is found are circled in orange. The orange box indicates the P. comanche species arrow. The axis are the same axis (1 and 2) in the preceding chart that together explain 25% of the variance. There are two arrows in red these are the significant environmental variables: percent litter cover and drainage. The smaller arrows are the species arrows the longer these arrows the more the RDA tells about that species. The labeled circles are the different sites. The locations of the sites and species arrows are based on their relationship to the environmental variables. When a species or site is positively related to a variable, it will be close to that arrow: the closer, the stronger the association. Species and sites located in the opposite direction of the variable arrows are negatively associated with that variable. Species Richness This is an attribute biplot from the RDA which shows species richness proportional to the circle size for each site. The lower sites where P. comanche is present, have the highest species diversity. Percent Litter Cover woods with the highest (expected) and the prairie sites are more variable Most Comanche prairies (sites with P. comanche) have a lower amount of cover. Drainage More consistency in the influence of this factor and arrangement of sites. Comanche prairies (sites with P. comanche) with greater drainage. P. comanche responded most strongly to drainage. 18. Conclusions There was no strong indication of distinctive ant assemblages among sites. P. comanche is not an indicator species and there were no co-occurring species. Prairies with P. comanche are more species rich but this is probably not due to P. comanche. 19. Nestmate Discrimination and Neighborhood Structure 20. Hypothesis: P. comanche distinguishes nestmates from non-nestmates with more, aggressive responses towards close neighbor colonies compared to farther away colonies (strangers). Background: Ants are hypothesized to distinguish nestmates from non-nestmates based on cuticular hydrocarbons which they detect with their antennae. These hydrocarbons vary based on several factors: internal nest environment, task/caste, external environment, food, the queen and how many times she has mated. Such distinction is used by many ants to recognize threats. They defend the nest and other areas from other colonies and other ant species based on this colony odor. Since ants are hypothesized to regulate and structure their populations and communities by aggression as a result of competition, it is expected that ants show more aggression to non-nestmates. Further, it has been found that some species show more aggression to other colonies based on distance between the two interacting colonies. In Pogonomyrmex species, the reverse of the dear enemy phenomena has been found that is, ants show more aggression to near neighbors with whom they are likely to have more encounters and more competition. Because of this phenomena, it is expected that ant colonies will space out to reduce the competition and therefore the aggressive encounters which may be costly, producing a regular or overdispersed pattern.

6 6 Other species in the same complex as P. comanche show such behaviors and Hölldobler suggested and showed that for individual foraging species (that is, species that do not use trails), the spatial pattern of such colonies was regular or overdispersed (notably, P. maricopa). 21. Methods I conducted a test of nestmate discrimination by observing aggression between P. comanche foragers. Introductions were made of two ants in plastic arenas and also of one ant on a nest mound so in a neutral context and in a nest mound context. Ants were collected from colonies in plastic tubes and allowed to calm for 20 minutes and then introduced into the arenas or on nest mounds. Ants were introduced to their own colony (as a control) and to colonies from different distances. Arena interactions were recorded for 1 minute, coded, and viewed later for aggressive behavior. Mound interactions were observed until the introduced ant left the mound these were recorded as field notes since I did not have access to a recorder at this time. Specific distances were grouped into distance categories for analysis. 22. Results There was no significant difference in the occurrence of aggression among encounters in the arenas. There was a significant difference in the occurrence of aggression among encounters on the nest mounds. 23. Results I followed this test up with post hoc tests to determine at which distances there were differences in aggression for nest mound encounters. The post hoc tests consisted of all pairwise comparisons with Bonferroni correction. There was a difference in aggression between all control or self introductions and all other neighbors with more aggression towards neighbors. Further, there was a significant increase in aggression between colonies within about 11 m of one another. Suggesting that colonies distinguish near neighbors from farther ones with more aggression expressed towards near neighbors, similar to what Gordon has found in P. barbatus. 24. Conclusions From this data, I constructed a hypothesized neighborhood structure for the GL site. The circles are the 11 m area in which colonies are likely to be more aggression with one another so colonies located at or near this line are likely to have aggressive encounters with the colony at the center, kind of a radius of aggression. Notice, that even with this 11 m area, the areas overlap a great deal. I observed no colony moves during this experiment although the new colony (just founded at the beginning of this experiment) did not make it through the summer I do not know why. The queen may simply have died. So, aggression may be structuring the interactions among colonies but it does not appear to be a significant factor in the colony spatial pattern. P. comanche does not exhibit aggression as several other members of the genus do Aggression appears to be a much reduced trait, though still used, in this species. I have called it a mild territoriality. The difference from these other species is probably related to the greater productivity of this habitat and therefore lessened competition, among colonies. 25. Final Conclusions 26. P. comanche colonies are probably clustered at the habitat scale which matters to the ants. P. comanche colonies are most responsive to drainage.

7 7 P. comanche colonies do discriminate nestmates and non-nestmates by distance. But this aggression seems rather weak and may be structuring colony interactions but contributing little to the nest spatial pattern. 27. Important Future Work There are many unanswered questions related to colony spatial pattern: It is unclear how frequently colonies move. It is unclear what the colony survival rate is. Movement and survival may be confounding. I know some colonies have been in these sites for at least 7 years. I do not know how old colonies are when they produce the first alates (reproductive ants) nor what their colony size is. Some basic colony and population information is needed. I believe that queen dispersal and nest site choice may be the most significant factors for colony nest spatial patterns. P. comanche is visually orienting as opposed to chemically orienting in foraging. Queens may use vision to locate nest sites based on seeing the obvious crater structure of other, particularly older and more established, colonies a kind of conspecific attraction. Because P. comanche is a subdominant ant and has obvious nest forms and activity, it would be interesting to pursue a better definition of their niche and role in the ecosystem. How much do they impact plant populations with their seed harvesting? How much do they circulate materials between the forest and prairie very important to the Cross Timbers? How much between above and below ground? What is their impact on soil? And what happens when a colony dies is this now a good area for plant colonization, etc? Are they soil engineers? 28. Acknowledgements. Colin McLeod runs an on-line resource, GIS in Ecology. He studies marine mammals and has written many books about using GIS in ecological work. I recommend his site, interacting with him, and his books.