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Habitat Fragmentation and Pollination Dynamics

May 22, 2012

Photo credit: crustmania, Flickr Creative Commons. CC BY 2.0.

Plants provide immeasurable benefits to human communities by way of sustenance, shelter, and environmental services–not to mention that most people appreciate the trees and shrubs that line many of our streets and city blocks. But the rapid expansion of human development into natural habitats has not been equally kind to many plant communities. City growth, road construction, and deforestation all create discontinuities in habitats that can affect the behavior of, and interactions between, plants and animals (Pellissier et al. 2012); these changes, or fragmentation effects, can have serious consequences for organisms that aren’t well equipped to deal with them. Fragmented populations are also more susceptible to the negative effects of low genetic diversity and small population sizes, an especially serious concern for rare species that may remain only in limited areas (Rosas et al. 2011). Many studies are currently attempting to develop a better understanding of these effects, due in no small part to the obvious implications fragmentation has for biodiversity and conservation efforts (Aguilar et al. 2008). As such, we are presented with a fantastic opportunity to explore current research on the effects of fragmentation on pollination dynamics in this second of three posts on pollination ecology.

Why does population size and genetic diversity matter?
Before exploring how pollination can change in response to fragmentation, it is important to consider why this issue is of any concern to the science of ecology. One reason is that fragmented populations are often limited in gene flow and thus genetic diversity (Aguilar et al. 2008). When a group of individuals lacks a certain amount of genetic variation amongst themselves, they are less able to adapt to a variable environment; in other words, there is less of a chance that an individual will possess genes or traits that will allow it to persist if conditions change (Quinteros-Casaverde et al. 2012).

Population size is important because as populations shrink, they become more susceptible to suffering negative effects due to random events (Aguilar et al. 2008). For example, if a deer ate five plants in a population of ten, half of the existing population would be wiped out; conversely, five plants destroyed out of a population of 100 would only represent a loss of 5% of its individuals. In this case, the deer would eat five plants no matter what, but it just happens to feed on individuals in a small population. Since a certain level of pollination success (and thus reproductive success) is necessary to maintain genetic diversity and a viable population size, factors affecting pollination dynamics in fragments are of great interest to ecologists.

Herbivory can affect small populations more dramatically. Photo credit: Chalkie_CC, Flickr Creative Commons. CC BY-NC 2.0.

Effects on Wind-Pollinated Tree Species
Studies on fragmentation effects in plants often focus on the impacts of forests and their trees first, since they are home to high biodiversity and are relatively easy to study. Many findings have lead ecologists to consider long-lived, wind-pollinated plants as generally resistant to the losses of gene flow and diversity associated with fragmentation (Quinteros-Casaverde et al. 2012). This is because wind-pollinated trees can often deliver pollen over hundreds of feet to several miles, and have been shown to maintain adequate pollen flow between fragments in some cases (Rosas et al. 2011, Wang et al. 2011). However, these adaptations may not always be enough, such as in areas with extensive deforestation or among rare species that lack a necessary population density for success. In fact, even when many common tree species are able to overcome disturbances (Montoya et al. 2008), endangered species may not fare as well, as studies have found that rare trees in fragments may have significantly lower genetic diversity than more common varieties (Rosas et al. 2011). Species facing such risks may need help from concerted rehabilitation efforts in order to recover.

Trees may be more resilient to fragmentation effects, but they are not immune to them. Photo credit: *Micky, Flickr Creative Commons. CC BY 2.0.

Changes in Plant-Pollinator Dynamics
Compared to wind-pollinated plants, species that are dependent on animal pollinators and genetic outcrossing (i.e., those that can’t reproduce through self-pollination) can be more susceptible to the effects of habitat fragmentation because of their reliance on animal behavior to reproduce (Llorens et al. 2011). These effects are demonstrated well in a study by Annette Kolb (2008), which compared how often animals interacted with a perennial forest herb (Phyteuma spicatum, Campanulaceae; pictured below). In the study, the author observed the behavior of pollinators (mostly bumblebees) and inspected for plant damage from herbivores (primarily deer) among small and large populations with varying degrees of isolation (10 to 40 m from reference populations).

Spiked rampion (Phyteuma spicatum) can’t self-pollinate, and relies on insects to spread its pollen and reproduce. Photo credit: col&tasha, Flickr Creative Commons. CC BY 2.0.

What she found is not entirely surprising: small populations showed fewer overall pollinator visits, lower seed numbers per pollinated flower, and greater proportional herbivore damage than large populations, just like the deer example from earlier. A somewhat unexpected result, however, was that the amount of visits per flower was not higher among any particular population size; however, in small populations, bumblebees tended to travel back and forth between flowers they had already visited rather than continue on randomly to new flowers like they did in larger populations. This suggests that fragmentation can force pollinators to travel in ways that reduce the quality of pollination services, and could explain the low seed counts found in smaller populations in this study. There is also evidence that the shape and connectivity of fragments plays a role in how successful pollen dispersal can be. For example, linear fragments (such as trees or shrubs planted in rows) can limit pollinator foraging patterns to essentially two planes of movement (Llorens et al. 2008), whereas natural and interconnected distributions of plants in an area could promote more efficient pollinator movement. Given the popularity of unnatural distributions in landscaping practices, these findings have important implications for creating viable plant populations within human communities.

Bumblebees are important pollinators for many plants, but their efficiency may be limited in small or unnaturally shaped fragments. Photo credit: jonboy mitchell, Flickr Creative Commons. CC BY 2.0

Habitat fragmentation is still a relatively young area of scientific concern, but the literature already suggests some serious long-term negative effects on plant and animal biodiversity in response to human development. Given what we have learned in a previous post about environmental stability and its role in promoting plant and pollinator diversity, it is clear that disturbing habitats can not only reduce the potential for new species to develop but also limit the ability of existing species to persist. While it is not an ideal area of study for ecology (we’d rather see biodiversity flourishing), there is much yet to be learned about the  effects our society has and will have on nature, and creates many opportunities for investigation. Hence the next post on pollination ecology will address the concerns of accelerated climate change and its effect on the relationships between plants, their pollinators, and a changing environment.

This is the second of three posts on pollination dynamics. View others:
1. Plant-Pollinator Relationships
3. Pollination Dynamics in a Changing Climate

References

1. Aguilar, R., Quesada, M., Ashworth, L., Herrerias-Diego, Y., & Lobo, J. 2008. Genetic consequences of habitat fragmentation in plant populations: Susceptible signals in plant traits and methodological approaches. Molecular Ecology 17, 5177-5188.

2. Kolb, A. 2008. Habitat fragmentation reduces plant fitness by disturbing pollination and modifying response to herbivory. Biological Conservation 141(10), 2540-2549.

3. Llorens, T.M., Byrne, M, Yates, C.J., Nistelberger, H.M., Coates, D.J. 2012. Evaluating the influence of different aspects of habitat fragmentation on mating patterns and pollen dispersal in the bird-pollinated banksia sphaerocarpa var. caesia. Molecular Ecology 21(2):314-28.

4. Montoya, D., Zavala, M. A., Rodriguez, M., & Purves, D. W. 2008. Animal versus wind dispersal and the robustness of tree species to deforestation. Science 320 (5882) Pp.1502-1504.

5. Pellissier, V., Muratet, A., Verfaillie, F., Machon, N. 2012. Pollination success of lotus corniculatus (L.) in an urban context. Acta Oecol 39:94-100.

6. Quinteros-Casaverde, N., Flores-Negron, C., Williams, D.A. 2012. Low genetic diversity and fragmentation effects in a wind-pollinated tree, polylepis multijuga plige (rosaceae) in the high Andes. Conservation Genetics 13(2):593-603.

7. Rosas, F., Quesada, M., Sork, V.L., Lobo, J.A. 2011. Effects of habitat fragmentation on pollen flow and genetic diversity of the endangered tropical tree swietenia humilis (meliaceae). Biol Conserv 144(12):3082-8.

8. Wang, R., Chen, X., Compton, S.G. 2011. Fragmentation can increase spatial genetic structure without decreasing pollen-mediated gene flow in a wind-pollinated tree. Mol Ecol 20(21):4421-32.

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