Community-Level Conservation

1. Community-Level Conservation Much of conservation and much of this course has dealt with single species, but the very nature of the problem that we are facing dictates that we consider collections of interacting species

2. Conserving biodiversity ultimately means working at the community and ecosystem levels and dealing with the effects of species declines and losses on other species and on the potential reverberating effects of interactive processes

3. There has been a great deal of controversy over the nature (reality) of the community - Is the community an organized system of recurrent species or - A haphazard collection of populations with minimal integration?

4. Two extreme schools developed over this question early in the past century - On the one extreme are the views of F.E. Clements (1916) that the community is a superorganism or a quasi-organism - highly integrated This type of community organization is commonly called a closed community (species belonging to a community are closely associated with each other; the ecological limits of distribution of each species coincides with the distribution of the community as a whole)

5. Closed Community

6. Schools of Thought - At the other extreme is the individualistic view of H.A. Gleason (1926) that communities are not integrated units but collections of populations that require the same environmental conditions. In other words, the community is far from being a distinct unit like an organism but is merely a fortuitous association of organisms whose adaptations enable them to live together under the particular physical and biological conditions found at a particular location. Often referred to as an open community.

7. Open Community

8. The information available leans more toward the individualistic interpretation of the community. Communities are not discrete but grade continuously in space and in time, and species groups are not consistent from place to place

9. In spite of this continuous variation, communities can be classified, but this classification is for convenience and not a description of the fundamental structure of nature So, the consensus of ecologists is that communities are open associations without clear boundaries

10. Nonetheless, in spite of their general independence, species do adapt to the presence of other species - It is interactions that link species together into constantly changing biological communities and shape much of the life histories, physiologies and morphologies of organisms

11. Interspecific interactions in conservation have focused primarily on 4 kinds of ecological effects - Loss of certain species leading to an unraveling of the organization of communities & loss of species - Change in functioning of ecosystems - Invasion of alien species leading to loss of native species or local populations - Changes in environmental conditions, like forest fragmentation, have led to the formation of new interactions

12. Consequences of Species Losses - If we remove a species and all other species within that system persist, then we call that system “species deletion stable” - This definition was developed to investigate MacArthur’s 1955 argument that more complex systems were more stable - Within a web, species deletion stability varies widely: some species are in critical positions (trophic levels), where their removal will cause further losses; other species are in positions where their removal will have little effect

13. What experimental studies have been done, have shown that few, if any, natural communities are species deletion stable; removals cause further losses

14. The classic example:Paine, R.T. 1966. Food web complexity and species diversity. The American Naturalist 100:65-76

15. Paine performed a predator removal experiment along the rocky intertidal seacoast of the Olympic peninsula in Washington State When the major top predator, the sea star, Pisaster ochraceus, was removed, the number of species remaining was drastically reduced

16. - Control areas with Pisaster supported some 15 species of marine invertebrates, but the area without the starfish had only 8 species - The rocky intertidal is a space-limited system. In the absence of predation, more efficient occupiers of space, especially the bivalve Mytilus californianus (a mussel), dominated the area - By preying on Mytilus, Pisaster continually opens up new spaces that are rapidly colonized by fugitive species which are less efficient competitors for space

17. Such predators often have a powerful impact on community structure, and are therefore called keystone species - an important species that determines community structure

18. Top predators often have powerful effects on prey populations, but whether these direct effects propagate to the base of terrestrial food webs is debated Croll, D.A., J.L. Maron, J.A. Estes, E.M. Danner and G.V. Byrd. 2005. Introduced predators transform subarctic islands from grasslands to tundra. Science 307:1959-1961

19. They found that: Introduction of arctic foxes to the Aluetian archipelago induced strong shifts in plant productivity and community structure by a previously unknown pathway By preying on seabirds, foxes reduced nutrient transport from ocean to land, affecting soil fertility and transforming grassland to dwarf shrub/forb-dominated ecosystems

20. Following the collapse of the maritime fur trade in the late 19th and early 20th centuries, arctic foxes were introduced to >400 islands as an additional fur source