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
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