American Society of Naturalists

A membership society whose goal is to advance and to diffuse knowledge of organic evolution and other broad biological principles so as to enhance the conceptual unification of the biological sciences.

“Pollen on stigmas of herbarium specimens: a window into the impacts of a century of environmental disturbance on pollen transfer”

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Anna L. Johnson, María Rebolleda-Gómez, and Tia-Lynn Ashman (Sep 2019)

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A century of anthropogenic change in Hawaiʻi leaves a pollen ‘fingerprint’ on flower stigmas

Stigmas and anthers with visible pollen on an herbarium specimen of <i>Metrosideros polymorpha</i>, viewed through a dissecting scope.<br />(Credit: Anna L. Johnson)
Stigmas and anthers with visible pollen on an herbarium specimen of Metrosideros polymorpha, viewed through a dissecting scope.
(Credit: Anna L. Johnson)

Biological collections provide a window into the past, and are the foundation of much of our knowledge about the diversity, range, and evolution of life. Billions of these specimens are preserved in natural history museums around the world. Recently, especially with the spread of new technologies such as genetic sequencing, mass digitization, and high resolution imaging, these collections are being revisited, and are yielding many exciting new insights into how global change is impacting life on earth.

In this study, three University of Pittsburgh researchers, Anna Johnson, Maria Rebolleda-Gomez, and Tia-Lynn Ashman applied a novel method for gathering data from herbarium specimens to examine how pollination interactions have changed over longer ecological time scales than can usually be documented through long-term field studies. They collected pollen from herbarium specimen stigmas collected in the dry tropical forests of Hawaiʻi, an ecosystem that has experienced rapid habitat loss and disturbance over the last century. After counting and identifying the pollen grains to species, they compared the quantity and diversity of pollen which native dry forest plant species interacted with prior to 1950, and post 1950. They found that while the amount of pollen which these species received did not change dramatically, the identity of the pollen grains observed on herbarium stigmas was very different between the two time periods. This suggests that for plant species to survive in a rapidly changing world, they must be robust to shifts in species interactions, even for pollination, a key reproductive mutualism. The techniques demonstrated in this study hold promise for uncovering the history of pollination in other systems, especially those for which we currently lack an understanding of the types of interactions that species historically engaged in prior to widespread anthropogenic disturbances.

<i>Metrosideros polymorpha</i> being visited by a honeybee in the field.<br />(Credit: Anna L. Johnson)
Metrosideros polymorpha being visited by a honeybee in the field.
(Credit: Anna L. Johnson)


Pollination is necessary for plant reproduction, but often highly susceptible to disruption, e.g., by habitat fragmentation and climate change. Here, we indirectly evaluated on a century time scale pollination interactions for species in one of the historically most disturbed habitats on earth—tropical dry forests of Hawaiʻi. We employed a novel method for acquiring a historical perspective on temporal change in pollination by characterizing pollen on stigmas of herbarium specimens from six remnant native species collected from 1909-2002. We determine whether temporal shifts occurred in 1) pollination quantity and quality, or 2) the composition of species interacting via pollen transfer. While pollen quantity remained constant, these remnant species interact with different species in modern times via pollen transfer than they did nearly 100 years ago. Species that are resilient to long-term environmental change may also be the ones subject to changes in pollination interactions.