ENDEMISM AND RANGE LIMITS IN SOUTHEASTERN PLANTS
John Hall Memorial Fund (2012-2014)
Climate change is expected to be one of the top threats to biodiversity in the 21st century. Changing climate regimes are predicted to cause major shifts in the geographic distribution of climate envelopes currently occupied by individual species. In particular, suitable habitat is projected to shift strongly northward and up in elevation requiring dispersal and colonization of new areas. At the same time, these climate-change driven range dynamics are likely to cause population declines or regional extinctions at the southern distributional limits of species unless populations can rapidly adapt to novel environmental conditions.
Climate change models predict that 18-37% of species are threatened with extinction because of an inability to migrate quickly enough to track the movement of suitable habitat. Global hotspots of endemism are especially at risk because many of these contain high concentrations of narrowly endemic species in small geographic areas. One standard approach to species conservation – preserving habitat where threatened species are concentrated – may fail if suitable habitat in the future lies outside of current areas of occupation. The extent to which dispersal limitation may affect species’ responses to climate change remains poorly understood. This problem is particularly relevant for plants because dispersal limitation is common at both local and regional spatial scales.
Species’ responses to historical climate change provide a window into the problem. It is well known that species’ ranges shifted dramatically and that extinctions were caused by historical fluctuations in climate. Temperate Deciduous Forests (TDF), the subject of this investigation, were widespread across North America and Eurasia during the late Cretaceous and Tertiary including high latitude areas now occupied by boreal forest and tundra. Cooling and drying during the Pliocene and glaciation during the Pleistocene forced species southward in a series of climate-driven range contractions. While vagile species such as trees and animals rapidly recolonized northern areas following glacial retreat, many herbaceous species appear to have failed to do so. The southeastern United States and southern Europe contain a great diversity of narrowly endemic herbaceous plants, many of which are very closely related to widespread species.
Jesse Bellemare (Smith College) and I conducted a biogeographic analysis of the distribution of narrow-ranged forest herbs in eastern North America, in an effort to (1) identify biodiversity hotspots and (2) understand the relationship between species richness and the location of the last glacial maximum. In the figure below, you can see a very high concentration of small-ranged, endemic forest herbs at the southern tail of the Appalachians, in the panhandle of Florida, and in the Ozarks of southern Missouri and Arkansas. There is also a strong correspondence between the concentration of small-ranged plants and the location of the last glacial maximum, with very few of those species occurring north of the boundary. This analysis appears in a book chapter (Bellemare and Moeller 2014) in The Herbaceous Layer in Forests of Eastern North America (2nd edition).
Little is understood about the causes of endemism of TDF plants in the Southeastern United States. That is, are hotspots of endemism in TDF plants the result of high in situ diversification (ecological endemism) or because they served as glacial refugia and limited dispersal has hampered subsequent northward range expansion? Answering these questions has important applied implications since climate change models predict that these regions off narrow endemism may become unsuitable under future climates. Specifically, the potential to recolonize northern areas with a managed relocation conservation strategy remains largely unexplored in terms of empirical studies.
In collaboration with Jesse Bellemare (Smith College), we have pursued focused research on two plant species that are narrowly endemic to the high elevations of the Southern Appalachians. Beginning ~ 10 years ago, we initiated an experiment on Diphylleia cymosa (Berberidaceae) that involved transplanting seeds into 3 sites in its native range (North Carolina) and 5 sites north of its current distribution – these sites span from Virginia to Massachusetts. We have followed these experiments from germination to flowering. After ~ 10 years, plants are just beginning to flower!!
Diphylleia cymosa fruits and flowers
Stephanie Erlandson, a graduate student in the lab, has undertaken a study of Phacelia fimbriata, an annual plant with a distribution similarly as narrow as Diphylleia. She has conducted transplant experiments both at a local scale — from inside to outside of natural population boundaries — and at a continental scale — from inside the native range to far north of it (as far north as PA). She has also developed species distribution models (SDMs) that predict the distribution of suitable habitat in the current climate as well as in a range of future climates. This work collectively evaluates the hypothesis that northern range limits are set by limited dispersal vs. limited adaptation, and examines the consequences of climate change for the distribution and persistence of the species.
Bellemare, J. & D.A. Moeller. 2014. Climate change and the herbaceous layer of temperate deciduous forests. in. F.S. Gilliam, ed., The Herbaceous Layer in Forests of Eastern North America (2nd ed.), pp. 460-480. Oxford University Press.