2011 Field season review

More than five months have passed since my last posting suggested that the Heiberg amphibians were taking a liking to the newly constructed pools. Much has happened since then, both in terms of the ecological development of the pools and in our efforts to monitor them. I will attempt now to share the most significant happenings.

Egg mass surveys
In the previous blog post, I noted that colonization of newly constructed pools appeared to be successful, but wondered if that would come at the expense of reproductive investment in pre-existing pools. This did not turn out to be the case: wood frog and spotted salamader egg mass counts in pre-existing pools were comparable or in many cases greater in 2011 than in 2009 or 2010.

In the Microarray, 87 wood frog egg masses were deposited in 15 of 32 (47%) pools, and 100 spotted salamander egg masses were deposited in 12 pools (38%). Although most pools contained water in the early spring, both species demonstrated remarkable avoidance of pools that dried rapidly and offered no chance for successful larval development.

In the Hexagon Array, 158 wood frog egg masses were deposited in 29 of 39 (74%) pools, and 135 spotted salamander egg masses were deposited in 22 pools (56%). As described in the previous post, frogs colonized new pools more readily than salamanders. Interestingly, niether species avoided pools with green frog tadpoles that had successfully overwintered. More on this later...

Leaf litter decomposition experiment
In mid-May, I tied to a stake at the deepest point of each pool three onion bags, each containing a known mass (approximately 3 g) of red maple leaves collected during autumn of 2010. These were retrieved in late July, and I am in the process of taking final weights to determine decomposition rates for each pool.

Macroinvertebrate sampling
I conducted two rounds of aquatic macroinvertebrate sampling with nocturnal light traps developed by Professor Bruce Smith, the first in mid-May and the second in late June/early July. All Hexagon Array pools contained water and were trapped during both survey periods, but only 18 and 10 Microarry pools were inundated and trapped for the first and second surveys, respectively. Samples have been preserved and are waiting for the snow to fly and the pools to freeze before they will get the attention they deserve.



SUNY Buffalo PhD student Mike Habberfield deploys an aquatic light trap.

 Amphibian larval sampling
I had planned to conduct two rounds of sampling wood frog and spotted salamander larvae so as to measure population trends and individual growth rates. However, during the first survey period during the last week of May and the first week of June, salamanders had only begun emerging from eggs. During the second survey period (mid- to late June), salamanders had hatched, but wood frog tadpoles were dangerously close to being fully developed. Thus, a third round was conducted in mid-July so that two surveys were conducted between the period when the last embryo has hatched and the first individual has metamorphosed for each species.

Aside from additional work imposed by the different developmental schedules of wood frogs and spotted salamanders, by and large, the pipe sampling technique worked very well. We generally worked out a routine in which helpers would do the pipe sampling and bring me their quarry in numbered bins. I identified the animals, weighed amphibian larvae, and preserved a sample (n<30) of tadpoles for developmental staging in the lab since it was impractical to reliably stage tadpoles in the field.



SUNY ESF PhD student Jim Arrigoni plunges a pipe sampler into the water column to trap and sample amphibian larvae.

 


Field technician Tess Youker delivers pipe samples to Jim Arrigoni, who is hiding from the rain and wind in the F150 mobile lab.



Larval wood frogs and spotted salamanders captured with pipe sampler.

 Although I have only begun to assimilate this summer's data, a few interesting patterns were obvious on the ground. First, and no big surprise here, temperature had a huge influence on developmental rates. The pools in the field part of the Microarray drove our sampling schedule - the animals in these unshaded pools were the first to hatch and metamorphose. Second, density of wood frog tadpoles appeared to exert a strong inverse effect on developmental rates. Two adjacent pools that appeared identical in most respects had very different wood frog developmental rates when one had had substantially more egg masses deposited in it. (The lesson: more might not be better from a recruitment or fitness perspective). Third, wood frog tadpoles generally coexisted well with overwintered green frog tadpoles in the pools where the two species were sympatric. I noticed that the green frog tadpoles generally occupied the benthos in the shallow zone, whereas wood frog tadpoles tended to be more pelagic (we stratified pipe sampling accordingly). However, in two pools, green frog tadpoles consumed all of the wood frog embryos. Why this happened in only two pools (out of 10 in which they two species coexisted) is an open question. Finally, in two pools in the Hexagon Array, wood frog tadpole die-offs in late June suggested Ranavirus infections. This has subsequently been confirmed with PCR analysis by field technician Tess Youker with the help of Brooke Reeve, a recent MS graduate of SUNY ESF.

Hydrological monitoring
With help from spring semester interns Nathan Heath, Becca Landis and Tom Maigret, we created and installed guages to measure water levels in each pool. Water levels have been recorded at least every 10 days since installation following ice out.


Surface and substrate temperature monitoring apparatus (left) and water level gauge (right).

 Here is a hydrograph example from Microarray Pool 1:

This pattern was typical in many of the pools in the Hexagon Array: water levels lowered during summer, but like last summer, very few of the Hexagon Array pools dried completely. On the other hand, many of the pools in the Microarray dried prematurely. This situation will give perspective on the consequences of failure to replicate vernal pool hydrology on both ends of the hydroperiod gradient.

Temperature monitoring
Thermochron iButtons were deployed at the water surface and at the substrate in all pools in mid-November 2010, and recovered and replaced in late July 2011. Unfortunately, a large number of the units were corrupted, and I am uncertain whether this was due to exposure to harsh winter conditions or because I only double-coated units with Plasti-Dip, as opposed to triple-coating units that were deployed during the spring-to-autumn period of 2010, when zero failures occurred. Data from Hexagon 20 Pool E are shown below. Note the thermal inversions apparent in late November and mid-April.

Other happenings
With the support of an Edna Bailey Sussman Foundation internship, I sampled larval amphibians and macroinvertebrates in nine pools in southern NY on the property of Jim Curatolo of the Upper Susquehanna Coalition. Three pools were of natural origin, two were built in 2006, and four in 2008. The intent of this side-project was to assess development of invertebrate communities and production of amphibians at greater time scale than is permitted in the Heiberg pools, which were all constructed during 2010.

Shortcomings
Despite the successes described above, I was unable to deploy tiles for sampling periphyton. Although this certainly would have yielded interesting data in and of itself as well as in comparison to other biological data, it simply was not possible to undertake this component of the study along with the damands of other components.

Thanks!
I was fortunate this summer to have the help of three diligent, humorous and hardy people. Tess Youker of ESF was out at Heiberg more than anyone, and allowed me to frantically worry about many fewer things. As part-time interns, Shelby Persons of SUNY Cortland and Ryan Doherty of ESF did not feed quite as many mosquitos and deer flies as Tess, but their contributions of labor and cheer nonetheless are greatly appreciated.


Field technician Tess Youker steps carefully through the forest at night, intern Shelby Persons delivers more samples of larval amphibians, and intern Ryan Doherty admires his afternoon's work of waterproofing Thermochron iButtons

 -Jim Arrigoni, SUNY-ESF

Unexpected ecological function of smaller, ephemeral pools

A recent paper published in the journal Ecology adds to the evidence that bigger is not always preferable when it comes to wetlands, and also illustrates the importance of understanding the intricate details of natural history in complex ecological systems:

Altermatt F, Ebert D. 2010. Populations in small, ephemeral habitat patches may drive dynamics in a Daphnia magna metapopulation. Ecology 91: 2975-2982.

Daphnia are miniscule crustaceans that typically comprise a substantial fraction of the zooplankton in lentic freshwater systems throughout the world. They are a vital component of larval spotted salamander diet, particularly early in their development (see Joseph Freda's 1983 paper in Journal of Herpetology [vol. 17, pp. 177-179], "Diet of larval Ambystoma maculatum in New Jersey"). Daphnia are notable in that they employ multiple modes of reproduction, depending on their circumstances. When environmental conditions are favorable, females reproduce parthenogenetically -- they essentially clone themselves, which results in the production eggs that hatch into females. However, when environmental conditions take a turn for the worse (e.g., drawdown in a vernal pool), males develop from some of the eggs, and instead of reproducing asexually, females produce eggs that require fertilization, but are also encased in a hard structure that is resistant to drought and adverse conditions. This is called an ephippium. After adverse conditions have passed, and under appropriate conditions (e.g., inundation of a dry vernal pool basin), ephippia hatch with an endowment of increased and novel genetic variability that is otherwise constrained under stable, favorable conditions and parthenogenetic reproduction.

In addition to allowing Daphnia to persist through unfavorable conditions, it turns out that ephippia are well-suited as a dispersal mechanism since they are easily transported by wind, especially when they coat the basin of a dry pool. Thus, in the context of metapopulations, it is easy to comprehend the importance of smaller and more ephemeral pools as sources for colonizing new habitats and introducing new genotypes into established populations, as shown by Altermatt and Ebert. Larger and more permanent pools tend to sustain themselves nicely, but they do less in the way of interacting demographically or genetically with other pools. This contrasts with an assumption common to many metapopulation models in which bigger patches with larger populations tend to be the sources for dispersing individuals.

The authors admit that there is little in the way of conservation concern for Daphnia magna, which is not rare nor is the species perceived to be negatively affected by limits to dispersal or genetic impoverishment. So far as I know this is the case for our Daphnia spp. as well. Given the unique reproductive modes of Daphnia, the extent to which this pattern can be generalized to other taxa may be limited, although the authors suggest similar dynamics may be found under environmental conditions that negatively affect local survival but benefit dispersal. But I liked this paper because it bolsters the argument that a pool can still have substantial ecological value even if it is tiny and temporary and not churning out amphibian metamorphs.

-Jim Arrigoni, SUNY-ESF