Tag Archives: functional groups

Prairie flowers at the Fort Worth Nature Center, Fort Worth, Texas

Ants in the Grassland: their importance and potential as indicators of ecosystem health

This is the presentation I made at the America’s Grassland Conference recently  in Manhattan, Kansas (August 2013).

I’ve posted the power point presentation below with a few additions and included the questions asked.

 

PRESENTATION SUMMARY

While we often think of ants as annoying pests, ants are important members of nearly every terrestrial ecosystem, except Antarctica. There are  perhaps 40,000 species worldwide and we have good species descriptions for about 14,000 of them – all of which are in the same family, the Formicidae.

Ants are incredibly diverse: they vary in their morphology, their behavior, their physiology, and their ecology. Ants may engineer ecosystems through their nesting and foraging habits – greatly shaping the physical landscape and thereby impacting a variety of other organisms including the plant community. Ants also have a diversity of relationships. They are important prey items as well as predators; they have important mutualistic relationships with plants, fungi, and other arthropods; they have their own parasites; they have commensals and parasites that live in their nests.

Because of all this ecological diversity, ants may be good indicators of habitats and ecosystem health. If there are changes in any of these relationships, for any of these organisms, this change may affect ant presence, activity, and abundance. Because ants are small and live on a small scale, they may detect such changes earlier than larger monitored species, such as vertebrates. Ants are also good candidates for indicators because they are easy to collect and do not have the problems of monitoring vertebrate populations which may be difficult to track, endangered or threatened species sensitive to handling, etc. The possibility for such utility has been shown in previous research.

I investigated the potential for grassland ant assemblages to be used as bio-indicators in prairies in the Fort Worth Nature Center and Wildlife Refuge in Fort Worth, Texas, including to discern habitat type and response to disturbance. I collected the ground active ants of 17 sites monthly from March – September 2012 using pitfall traps.  The 17 sites constituted a natural experiment: 3 were wooded sites and 14 were prairie sites. The sites were paired according to soil and ecological unit (from the Natural Resources Conservation Service) for wooded vs. non-wooded (3 replicates); mowed (and soil disturbance) vs. non-mowed (4 replicates); and low intensity burn vs. non-burned (2 replicates).

For each site, I measured environmental variables which are known or thought to be important to ants in choosing their nesting areas,  including depth of sand, soil penetration (compaction), depth to the restrictive layer, percent slope, drainage, percent bare ground, percent litter cover, percent standing plant cover, percent total cover, latitude, ecological units (from the NRCS), and soil type.

I used the program CANOCO to do ordination analyses: principle components analysis (PCA) on the environmental variables only and redundancy analysis (RDA) combining the environmental variables with species presence. Ant species were characterized by functional groups following Andersen (1997). The PCA confirmed that the variables chosen could be used to distinguish among sites. The RDA revealed that some of the ant species were aligned with habitats but disturbance did not matter. The sites grouped into three sets which aligned with soil types and ecological units. Some species did not align with their habitats but this may be explained by the foraging of those species into habitats other than where they nest. The RDA showed a strong relationship between the ants and the environmental variables with the interaction between percent litter cover and drainage, percent litter cover by itself and drainage by itself being significant factors. However, these factors combined did not explain more than 20% of the variation so either there are other significant factors or many factors account for the local presence of ants with none being particularly significant.

Andersen’s functional groups are problematic for these sites because some of the species placed in the groups do not have similar ecological roles as the Australian species upon which this work is based. Species richness by functional group did not vary significantly among the sites. And although the functional group designations are problematic, there is a  pattern in the composition of these assemblages with general myrmicines contributing most, followed by hot climate specialists, cryptic and opportunistic species, then tropical climate specialists and dominant species. This suggests a shape to the assemblages that may transcend individual species.

In conclusion, this project indicated weak support for these assemblages s as bioindicators and only two ants could be considered indicators of habitat: the carpenter ant species (Camponotus americanus and Camponotus pennsylvanicus) occurring in the woodlands and the Comanche harvester ant (Pogonomyrmex comanche) occurring in the Aquilla prairie.

Here the Comanche harvester ant (Pogonomyrmex comanche) forages  Pecan Sandies at a bait station in the Fort Worth Nature Center, Fort Worth, Texas

A Consideration of Andersen’s Functional Groups for North American Ant Assemblages

(These thoughts came out of an email exchange with noted myrmecologists Dr. James Trager of the Missouri Botanical Garden and Dr. Terry McGlynn of California State University Dominguez Hill).

I understand the concern that we not naively appropriate Andersen’s functional groups and the taxons that go into them – I think I have been on the verge of doing that. I obviously need to be exposed to more ant species.

My understanding is that Andersen tested dominance of the ants with bait stations and looked for aggressive interactions. Terry McGlynn’s questioning of this — that bait stations are artificial situations and do not reflect real ant behaviors/interactions well —  I think is well placed. I had thought to show that Pogonomyrmex comanche was a dominant ant in her habitat based on several observations but Dr. Walter Tschinkel, who is now on my committee, raised questions about this: what does dominance mean? I think this is a stickier issue than some believe.

For instance, there appears to be a trade-off between foragers who come early to baits and then leave and those who come later but seem to “dominate” the baits once they arrive. Are these later arrivals really dominant? I question this interpretation because of observations I have
made such as the interaction between P. comanche and a foraging grasshopper over a dead grasshopper (I have a  video) in which Comanche dealt with the competition by cutting up the grasshopper and foraging faster only at the last, stinging the offending grasshopper. When I see
Comanche dominate bait stations (photo above), I do not see the ants directly interact.

Here’s a photo of fire ant (Solenopsis sp.) at a food resource (though this was a cookie dropped in a parking lot). The fire ants seem to get to food resources quickly and to “swarm” on them effectively dominating them. I have seen fire ants overwhelm bait stations in the Fort Worth Nature Center in a similar manner. It seems a typical ant behavioral response to a desirable food resource is to dominate it indirectly by quick removal of it to the nest rather than more direct aggressive interactions which are probably more costly.

Fire ants (Solenopsis invicta or xyloni) opportunistically foraging on an Oreo cookie. I have seen this kind of "swarming" behavior on bait stations in the field as well.

Fire ants (Solenopsis invicta or xyloni) opportunistically foraging on an Oreo cookie. I have seen this kind of “swarming” behavior on bait stations in the field as well.

In the nestmate discrimination trials I am currently completing, I do not see Comanche often acting aggressively. I do see some gaster wagging in these trials. Gaster wagging is interpreted as am aggressive behavior but I am not so sure that it is always aggressive. In some situations, one ant grabs another and the one grabbed does the gaster wagging which appears more submissive. Other times, it is obvious the ants are curling their abdomens to sting. But mostly, the gaster wagging gives the impression of wafting some pheromone for some kind of communication which I am not able to discern (I need ant antennae, I guess.). Perhaps it is like arm or finger wrestling.

I think these situations are more nuanced than just dominance and perhaps many things are still ill defined.

Dr. Trager raised some important concerns about climate specialists and yet we know that climate contributes significantly to  ant occurrence at regional and continental levels. I think Andersen’s work my fit for these larger scales but not so well for more local scales at which I am working. (Scale is a significant concern and challenge in ecological studies. ) I think the scale issue is a part of the local spatial pattern of Comanche as well – it looks like regionally, they are found in sandy soiled prairies associated with oak forest, etc. but locally, Comanche spatial pattern and location may be a matter of queen dispersal. At first perhaps, new queens are attracted to locations with other colonies (conspecific attraction) but after landing, queens have to move farther away.  Later, negative interactions between colonies may come into play as well. Comanche colonies do move — and apparently, not in response to shading, like Pogonomyrmex badius or P. barbatus. But this is just my hunch based on observation and a few preliminary tests.

I also wonder about this: ant species appear to partition habitat seasonally, spatially, and temporarily in terms of activity patterns and so co-exist. These patterns add a level of
complexity which is a challenge.

*********************************************

Alan Andersen and his colleagues have conducted  many studies of ant assemblages with respect to disturbance (mining and deforestation in paricular) and functional groups especially in Australia. An important paper in consideration of these ideas in North America is:

Andersen, A. N. 1997. Functional groups and patterns of organization in North American ant communities: a comparison with Australia. Journal of Biogeography 24 (4): 433-460.

Ant Functional Groups

I have been reading papers by Alan Andersen in Australia and others who follow his work. He has been very interested in ants and ant assemblages as bioindicators and has made a good case for this in Australia. He also did some work in Arizona, USA and got similar results.

I believe considering the ant species in terms of functional groups may be more enlightening than considering species richness or niche space of individual species because the functional group designations have to do with the ecological functions. Ecological functions are where the ants matter in terms of being bioindicators — where they are sensitive and where shifts in ant presence will matter to the overall function/health of the system.

So, I worked through Andersen’s functional groups and assigned my genera in accordance with his work. However, I had some reservations — but since I am still learning ant ecology for most of these species, I went with Andersen’s designations. I ran this by a myrmecological expert, Dr. James C. Trager (Missouri Botanical Garden, where he is a restoration biologist and specializes in ants) who questioned me on some of this and asked me to think some more. I agree with Trager’s criticisms but here is my first attempt basically to match my ant assemblages with an uncritical engagement of Andersen’s work. I note the difficulties in parentheses.

Ant Functional Groups: 17 genera; 34 species over all my sites (17 sites: 3 woodland and 14 prairie)

1. Dominant Dolichonderinae

Forelius (Are they dominate?)

2. Subordinate Camponotini

Camponotus (Are they subordinate?)

3. Hot Climate Specialists

              Pogonomyrmex

             Solenopsis (Solenopsis – fire ants)

4. Cold Climate Specialists

              Formica (pallidefulva group — Probably not cold specialists)

              Prenolepis

5. Cryptic Species

              Brachymyrmex

Solenopsis (Diplorhoptrum)

              Temnothorax (probably not cryptic)

6. Opportunistic Species

              Aphaenogaster

              Dorymyrmex

              Nylanderia

7. Generalized Myrmicinae

              Crematogaster

              Monomorium

              Pheidole

8. Specialist Predators

              Strumigenys

9. Tropical Climate Specialists

              Labidus

              Trachymyrmex

 

Species Occurrence by Functional Group

Site Dom Dol Sub Camp Hot Cl Cold Cl Cryp Opp GM Sp Pr Trop
EP-1F

1

0

3

0

3

2

1

0

1

EP-1P

1

0

3

0

3

3

5

1

2

EP-1W

1

1

3

0

1

4

5

0

2

EP-1T

1

0

3

0

1

4

5

1

1

GL

1

0

3

0

3

4

6

1

2

EP-2

1

0

2

0

1

2

5

0

2

EP-3

1

0

2

1

1

1

4

0

0

TI-1P

1

0

2

0

3

2

4

0

2

TI-1N

2

0

3

0

1

3

4

0

2

TI-1W

0

2

3

1

3

0

4

0

1

TI-2P

2

0

1

0

1

3

7

0

2

TI-2W

1

0

2

0

1

4

3

0

1

AA

1

1

2

0

2

2

4

0

1

AAB

1

0

2

0

1

1

3

0

1

HC

1

0

1

0

2

2

5

0

1

BR

1

0

2

1

1

2

4

0

0

GH

1

0

3

0

3

1

5

0

2

 A Few Andersen Papers

Andersen, A. N. 1997. Functional groups and patterns of organization in North American ant communities: A comparison with Australia. Journal of Biogeography 24: 433 – 460.

Andersen, A. N., Hoffmann, B. D., Muller, W. J., and Griffiths, A. D. 2002. Using ants as bioindicators in land management: simplifying assessment of ant community responses. Journal of applied ecology 39(1): 8 – 17.

Hoffmann, B.D. and Andersen, A. N. 2003. Responses of ants to disturbance in Australia, with particular reference to functional groups. Austral ecology 28 (4): 444-464.