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Ecology 18.docx

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Biology 2483A
Mark Moscicki

Ecology-Lecture 18 Nov 14, 2013 Introduction  Communities vary in the numbers and kinds of species they contain.  Species diversity differs among communities due to variation in regional species pools, abiotic conditions, and species interactions.  Species diversity at the local scale: We must ask two important questions.  What are the factors that control species diversity within communities?  What is the effect of species diversity on community function? Community Membership  Communities vary drastically in their species richness and composition. These species must all come together. We can explain this by considering the factors that control species membership.  Distribution and abundance of species in communities depends on: 1. Regional species pools and dispersal ability (species supply/what can get there). Species that can disperse to the community pass through the first filter. 2. Abiotic conditions. Species that can tolerate the abiotic conditions in the community pass through the second filter. 3. Species interactions. Species restricted by and/or dependent on particular species interactions in the community pass through the third filter.  These factors act as “filters,” which exclude species from (or include species in) particular communities. Species are lost at each "filter" so local communities only contain a fraction of the species in the regional pool. Regional Species Pools/Dispersal Ability  The regional species pool provides an upper limit on the number and types of species that can be present in a community. This is because the regional pool is what supplies the communities with species. Regions of high species richness tend to have communities of high species richness.  The importance of dispersal can be seen in cases of non-native species invasions.  Humans have greatly expanded regional species pools by serving as vectors of dispersal (methods of transport for non native species to new communities.  Aquatic species travel around the world in ballast water carried by ships. Seawater is pumped in and out of ballast tanks which serve to balance/stabilize cargo ships (organisms from the sea are taken up with it and released near ports). Ships are now larger and faster, so trans-ocean trips take less time— species are more likely to survive (thus non native species introduction via ballast water has increased).  Examples that have been introduced by ballast water are the zebra mussel (introduced to great lakes by ballast water. These became destructive invaders to the inland waterways of the U.S) and the release of the comb jelly Mnemiopsis Leidyi into the Black Sea. Abiotic Conditions  A species may be able to get to a community but be unable to tolerate the abiotic conditions.  Example: A lake might not support organisms that require fast-flowing water. In addition, lakes are good places for fish, aquatic plants, plankton etc. but not terrestrial plants.  Differences among abiotic environments are obvious constraints that determine where particular species can and cannot occur.  Humans transport many more species that can actually survive in the new location to which they are carried. For example, many species that are dispersed in ballast water can’t survive in a new habitat because of temperature, salinity, amount of light etc. As a result, many of these non native species die before they can become a threat. However, we can’t rely on physiological constraints to exclude invaders, as in the case of Caulerpa in the Mediterranean Sea. With multiple introductions, these invader species may adapt to the point where they are able to survive and reproduce.  Evidence shows that global climate warming is leading to alterations in the abiotic conditions of communities. It may facilitate the invasions of species that would be unable to survive in cooler conditions (Ascidians). Species Interactions  Coexistence with other species is also required for community membership  If a species depends on another for its growth, reproduction, and survival, those other species must be present.  A species may be excluded from a community by competition, predation, parasitism, disease.  For example, a lake may be a good habitat for many fish species but, but limited resources may prevent all of these species from living together in harmony. In this case, certain species may dominate leading to the extinction of weaker species.  Some non-native species do not become part of the new community. This is attributed to interactions with native species.  Biotic resistance occurs when interactions with the native species exclude or slow the population growth of the non native species/invader.  Example: Native herbivores can reduce the spread of non-native plants. Mortality of non native plants due to native species is quite high.  Not a lot is known about biotic resistance. This is because ecologists tend to focus on whether species spread ONCE they have been incorporated into a community, rather than looking at whether a species BECOMES a member of a community. This is also partly because failed introductions of non-native species tend to go undetected  However, many species are able to coexist in a single habitat. Numerous factors attribute to this.  Resource partitioning: Competing species coexist by using resources in different ways. It reduces competition and increases species richness. This has been shown with the Lotka Volterra competition model.  In a simple model, each species’ resource use falls on a spectrum of available resources. The resource spectrum represents different nutrients, prey sizes, habitat types. In the image to the right, each curve represents the resource use of a different species in the community. Species resource use lies somewhere along the spectrum, and we have overlapping of resource use by some of the species.  The more overlap of resource use, the more competition between species. An extreme of this would be complete overlap and competitive exclusion. The less overlap, the more specialized species have become (increased resource partitioning), and the less strongly they compete.  Species richness may increase as a result of resource partitioning. In the narrow spectrum, species show a high degree of specialization/partitioning (little overlap) in their resource use. More species can be packed into a community. This lower overlap may be due to the evolution of specialization or character displacement. In the broad spectrum, there are more kinds of resources available to support more species.  MacArthur (1958) studied resource partitioning in a community of warblers in New England forests. He wanted to see how they coexist in the face of similar resource needs. He helped understand how the principle "species that compete can coexist by using resources in different ways" can be applied to entire communities, where multiple interactions are occurring at once. He recorded feeding habits, nesting locations, and breeding territories. When he mapped the locations of warbler activity he found that the birds were using different parts of the habitat in different ways.  It was found that Warbler species partition resources by feeding in different parts of the trees. The shading in the below image indicates where on the tree each species of Warbler chooses to feed. It was also noticed that the nesting heights and locations of breeding territories varied as well. Although using the same habitat and resources, the Warbler community was able to partition the resources so that they could all coexist.  To explain how diatom species coexist in nature, Tilman proposed the resource ratio hypothesis: Species coexist by using resources in different proportions. Two diatom species (Cyclotella and Asterionella) were grown in media with different SiO 2PO r4tios.  Tilman found that Cyclotella dominated only when the ratio was low, Asterionella dominated when the ratio was high. Coexistence occurred only when SiO and PO were limiting to both species. 2 4  The above study works best outside of a laboratory setting when the natural resources vary naturally within an area. Robertson et al. (1988) mapped soil moisture and nitrogen concentration in an abandoned field and found variation over small spatial scales. If the two maps are overlapped, smaller patches corresponding to different proportions of these two resources emerge. This suggests that resource partitioning could occur in plants. Processes that Promote Coexistence  Processes such as disturbance, stress, predation, and positive interactions can mediate resource availability, thus promoting species coexistence and species diversity  Examples have shown that when the dominant competitor is unable to reach its own carrying capacity, competitive exclusion can’t occur, and coexistence will be maintained. This was shown in the case of sea palms and mussels that compete for space in the Rocky Intertidal Zone. Mussels are the dominant competitors and Sea Palms can only coexist when the mussels are frequently disturbed by waves. As long as the dominant competitor is unable to reach its own carrying capacity because of reductions in its abundance due to disturbance, stress, or predation, competitive exclusion cannot occur and coexistence will be maintained.  Charles Darwin showed that disturbance is a mechanism for the maintenance of species diversity (experiment in which he left a meadow unmowed except for one small patch. With mowing, dominant competitors were removed and weak competitors could thrive leading to increased richness). This supported the argument that nature applies limits to the tendency of species to increase in abundance and outcompete other species.  G. E. Hutchinson revived the idea in his paper “The Paradox of the Plankton” (1961). He provided one of the first mechanistic descriptions of how coexistence could be maintained under fluctuating environmental conditions. He focused on phytoplankton in a lake. These communities have very high diversity (30–40 species), all using the same limited resources, in a homogeneous environment (even distribution of resources). His explanation for coexistence was that conditions in the lake changed seasonally, which kept any one species from outcompeting the others. As long as conditions changed before the competitively superior species reached carrying capacity (thereby eliminating other species), coexistence would be possible.  Positive interactions may also have an effect on coexistence. For example, species that might normally be unable to tolerate stressful conditions can maintain viable populations under stressful conditions because of the facilitative effects of other species. Intermediate Disturbance Hypothesis  This hypothesis explains species diversity under variable conditions. It was proposed to explain how gradients in disturbance affect species diversity in communities.  The frequency and intensity of disturbance experienced by a particular community could have dramatic effects on the species diversity.  The intermediate disturbance hypothesis: Species diversity should be greatest at intermediate disturbance, and lowest at high and low disturbance. At low disturbance, competition determines diversity (dominant competitors can outcompete weak competitors) . At high disturbance, many species can not survive (high mortality rates). Intermediate disturbance allows for a nice balance between disruption of competition and mortality.  There have been many tests of this hypothesis.  Sousa studied communities on intertidal boulders in southern California that were overturned by waves. Small boulders were overturned frequently (disturbance), large boulders were overturned less often. Intermediate boulders were turned over at intermediate frequencies. After 2 years, it was determined that most of the small boulders had only one species (early successional species with high dominance). The large boulders had two species (for the most part) which were late successional species (most recent because early colonizers were killed off by high disturbance levels). The intermediate boulders had 4-7 species (mix of early, mid and late successional species)  Several people have elaborated on the intermediat
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