To print this page, select File-> Print from your browser menu.

Conserving Genetic Diversity

Ellie M. Cohen


With the rapid decline of ecosystem function globally has come the homogenization of the natural world. Human activities are altering the very evolutionary processes on which all life depends, leading to a loss of genetic diversity and reducing nature's ability to adapt to environmental change.

Much effort has been made by the global environmental community to conserve the pattern of biodiversity, particularly highly threatened regions with high species richness (diversity) and large numbers of endemics (species found only in those regions), often called "biodiversity hotspots." In contrast, little attention has been focused on conserving the processes that generate and maintain biodiversity. Biologists now realize that, for conservation efforts to be successful, both pattern and process must be conserved.

Dr. Tom Smith, director of UCLA's Center for Tropical Research and former PRBO Board member, recently teamed up with Dr. Louis Bernatchez and others to convene an international summit of evolutionary biologists, conservation practitioners, and policy makers. They aimed to address human impacts on evolutionary processes, how to reduce those impacts, and the vital role of evolutionary science in conservation decision-making.

As Tom explained, "These evolutionary changes impact every corner of the globe and frequently pose serious health threats to humans. Habitat changes, deforestation, climate change, and other human-induced alterations have facilitated the evolutionary jump of animal diseases, such as avian influenza and simian immunodeficiency virus (the ancestor of HIV/AIDS), into human populations. The development of hatchery-raised and farmed salmon has resulted in the spread of poorly adaptive genes to the wild, diminishing the ability of wild populations to survive. Introductions of exotic species have impacted native species and limited their ability to adapt to climate warming."

Messages for Conservation

I was honored to be invited to participate on the policy panel at the conference. From interacting with some of the leading researchers in this arena, here are three key points I took away—and how these issues relate to PRBO's work.

1) Conservation policy can help increase the rate of species adaptation to rapid habitat and climate change by ensuring corridors for gene flow. Conserving large reserves, habitat gradients, and the corridors between them will be key to maximizing adaptive and genetic variation–and thus the chance that some populations will be adapted to future climate regimes.

For example, Dr. Victoria Sork (Department of Ecology and Evolutionary Biology, UCLA) presented her lab's research on California valley oaks that ring the Central Valley. A keystone member of the ecological community, this oak is one of the most threatened endemic species in the state, due to the spread of agriculture (historically) and development (more recently).
Caplifornia valley oaks. Photo courtesy Tom Gaman.

Her team found two locations where valley oaks possessed significantly higher genetic diversity than any others—in the Tehachapi Mountains (at the southern end of the Central Valley) and north of San Francisco. However, there has been little protection for these genetically diverse sites, and natural corridors to retain connectivity between populations through a changing climate are lacking. She recommended that these two valley oak "genetic hotspots" be considered a high priority for conservation efforts in the future.

This approach raises potentially new priorities for PRBO's work, such as including genetic measures in our data collection and in our selection of monitoring sites. Dr. Sork's results also highlight the importance of assessing what constitutes a corridor for species dispersal (as opposed to a barrier). PRBO scientists are addressing this need in tidal wetlands, riparian, and other habitats.

2) Population viability and biodiversity could be seriously threatened if the rate of micro-evolution does not match the rate of environmental change. While natural selection leads to adaptation, adaptive change is not occurring fast enough to keep up with the rate of climate warming . For example, researchers in the Netherlands showed that for the migratory Pied Flycatcher (Ficedula hypoleuca), peak food availability (caterpillars) is advancing faster than the arrival dates of birds returning in the spring. As a consequence of this mismatch between predator and prey, some populations of Pied Flycatcher have declined by as much as 90%.1 PRBO is currently involved in collaborative studies, using our long-term bird ecology data sets, to evaluate the relationship between changes in climate and bird timing (phenology)—e.g., changes in spring arrival dates of breeding songbirds, egg laying dates of seabirds and songbirds, and fall arrivals of over-wintering songbirds. Our ultimate goal is to evaluate the consequences, for the long-term viability of populations, of changes in phenology.

3) Early and complete eradication of invasive species is the most cost-effective solution to protecting native genetic diversity and resilience in plants and animals. PRBO recently assessed the impact of non-native cordgrass (Spartina alterniflora) on shorebirds in San Francisco Bay. The bay holds 70% of California's mudflats and provides habitat to more wintering and migratory shorebirds than any other wetland along the Pacific coast of the contiguous U.S. However, the quickly spreading, non-native cordgrass can render large mudflat areas effectively unavailable to shorebirds for finding food. Led by Landscape Ecologist Diana Stralberg, we developed several scenarios that showed possible habitat loss ranging from 9% to 80%. We identified areas of highest value to shorebirds in the South Bay: the upper mudflats, due to their greater exposure time; and the east- and south-shore mudflats, due to the high numbers of birds detected there. These areas also coincide with the areas of greatest potential for Spartina invasion and therefore are the highest priorities for early eradication to prevent further spread.
Ellie M. Cohen.

The gap between conservation in practice and evolutionary biology is significant. Adding genetic and adaptive diversity into the decision matrix for prioritizing conservation is urgently needed, in order to help ensure that species can adapt to quickly changing environments. Putting this into practice, however, may require some novel and perhaps controversial actions, such as "assisted migration"—the intentional moving of species from one site to another to overcome dispersal barriers. It may prove to be more cost-efficient to determine what constitutes a dispersal barrier and then reduce those barriers.

It seems that PRBO and the conservation community at large need to expand our collective vision—to steward our natural world through rapid habitat and climate change by understanding and maintaining the processes that support habitat and species diversity and also adaptive and genetic diversity.

Many thanks to Tom Smith for his contributions to this perspective and for his leadership in bringing evolutionary ecology to conservation practice. Thanks also to PRBO's Nadav Nur for contributing to this piece.