GCE-LTER Transformational Science

1. Salt Marsh Herbivores

The leaf beetle Ophraella notulata eats the salt marsh shrub Iva frutescens. Beetles are several times more abundant, and do several times more damage to shrubs, at low versus high latitudes. (photo courtesy of Jim Sheehan)

It is obvious to the most casual observer that natural communities are different in different parts of the world.  Early naturalists from Europe, for example, marveled at the diversity of life that they found on trips to the tropics.  Ever since, scientists have wondered how ecological processes might vary geographically.  Early workers suggested that interactions between species might be more intense at low latitudes, and that this might explain both the diversity of life and the elaborate defenses against consumers that species displayed in the tropics.  The logistical constraints involved in working at geographic scales, however, made it difficult to rigorously address this hypothesis.

Over the last decade, GCE investigators have systematically examined how interactions between salt marsh plants and their herbivores vary on multiple spatial scales along the Atlantic Coast of the U.S. This body of work represents one of the most thorough studies of latitudinal variation in species interactions in the literature.  It is remarkable for including large numbers of study sites at different latitudes, for sampling these sites repeatedly in different seasons and in different years, and for replicating the work across a parallel latitudinal gradient in Europe.

One key finding from this work was that herbivores are more abundant, and do more damage to plants, at low versus high latitudes.  Perhaps as an evolutionary response to intense consumption by herbivores, plants are better defended at low versus high latitudes. Low-latitude plants are tougher and have lower nitrogen levels than high-latitude plants. In addition, low-latitude plants have higher levels of “constitutive” (always present) defenses, and also a better ability to induce defenses following herbivore attack. Tolerance to herbivory, however, does not vary as a function of geography.

These latitudinal differences in plant quality are sufficient to produce latitudinal patterns in body size of herbivores, with higher-quality plants at high latitudes supporting larger herbivores. This last finding is of particular interest, because it could be a previously-unrecognized mechanism contributing to Bergmann’s rule, which states that animals are larger at high latitudes.  Bergmann’s rule was proposed over 160 years ago, but physiologists and ecologists have argued ever since about the mechanisms that might explain it.  The idea that animals are larger at high latitudes because they are eating higher-quality food is a novel hypothesis that may contribute to solving this long-standing biological puzzle.

Figure.  Variation between high and low latitudes in the densities of herbivores and in herbivore damage to the salt marsh shrub Iva frutescens. GCE-LTER scientists found that herbivores that eat leaves by chewing them (acridid grasshoppers, measured using two different methods; beetles; the omnivorous crab Armases) and herbivores that feed inside galls (a fly and a mite) were consistently more abundant at low versus high latitudes. In contrast, herbivores that feed by sucking plant juices (aphids) showed no pattern across latitude. Damage to leaves from chewing and gall-making herbivores was several times greater at low versus high latitudes. Similar results were found with other plant species studied.  (Source: Pennings et al. 2009)

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Dr. Steven C. Pennings