BIOL 4120: Principles of Ecology Lecture 16: Community Ecology Dafeng Hui Office: Harned Hall 320 Phone:

Outline (Chapter 17) Factors influence community structure 17.1 Fundamental niche constrains community structure 17.2 Species interactions are diffuse 17.3 Food web illustrate indirect interactions 17.4 Food web suggest controls of community structure 17.5 Species interactions along environmental gradients involve both stress tolerance and competition 17.6 Environmental heterogeneity influences community diversity 17.7 Resource availability can influences plant diversity within a community

17.1 Fundamental niche constrains community structure All organisms can live and reproduce over a specific range of environmental condition These conditions differ from organism to organism The conditions under which an organism function well are the consequence of physiological, morphological and behavioral adaptations. These same adaptations also limit its ability to perform equally well under different conditions. Plants adapted to shade, high nutrient The environmental conditions vary in time and space, and fundamental niche of species vary, thus fundamental niche difference among species help explore the processes that structure communities.

Fundamental niche constrains community structure Fundamental niches of hypothetical species along environmental gradient (e.g. T, Moisture, elevation) All species have bell- shaped niches Niche overlap Each species has limits beyond which it can’t survive For any given range of environment, only a subset of species can survive As environments change, abundance of species will change.

Distribution of three species Geographic distribution of three tree species that are part of the two forest communities presented in Tables 16.1 and Distribution of these three species overlap in West Virginia. As we move across eastern North America, the set of tree species whose distributions overlap will change, and therefore so will the species composition of the forest communities.

Null model Null model: assume that presence and abundance of the individual species found in a given community are a result of the independent responses of the individual species to the prevailing physical environment. Interactions among species have no significant influence on the structure Null model provides basis for experimental study Physically remove one species and examine the population response of the other (competition, predation, parasitism, mutualism) If no change, accept null model (no interaction). But many evidences show that species interactions do influence both the presence and abundance of species within communities. Example, competition influence realized niches. Mutualism enhances the presence and distribution.

17.2 Species interactions are diffuse Species interactions exist, but the importance are often underestimated. because such interactions are many and relatively diffuse Involve a number of species Competition Experiment by Norma Fowler (UT Austin) Usually the removal of a single species will have very limited effects Removal of group of species can have large effect this makes it difficult to determine the effect of any given species on another. But collectively, competition may be an important factor limiting the abundance of all species involved.

Species interactions are diffuse Pollination: A single plant species may be dependent on a variety of animals species for successful reproduction. Predator-prey: Predator species (lynx, coyote, horned owl etc.) and snowshoe hare Food web chart (on next slide), 11 of 12 predators prey on snowshoe hares. Any single predator species may have a limited effect on the snowshoe hare population; together, they regulate its population.

17.3 Food webs illustrate indirect interactions Food webs Provide information on indirect effects See lynx and white spruce Presence of lynx is good for white spruce due to survival of seedlings

Another example of predation in shaping structure of communities Starfish prey on mussels, barnacles, limpets, and chitons Remove starfish, what would happen? Species diversity increase or decrease? Why?

Keystone predation Predation can also influence outcome of interactions between prey species The starfish preys on many species of mussels, barnacles, limpets, etc Remove starfish from experimental plots and compare to normal situation Number of prey species in experimental plots was halved Diversity was decreased as better competitors excluded other species

Apparent competition In the absence of predator, the population of each prey is regulated by purely intraspecific density-dependent mechanisms Neither prey species compete, directly or indirectly, with each other Predator abundance depends on the total abundance of prey Under these conditions, the combined population abundance of two prey species will support a higher predator density.

Apparent competition Experimental supports: Nettle aphid, grass aphid and ladybug beetle (page 359, textbook) Brought nettle aphid plants to grass aphid plants together suppressed both population, as a results of larger ladybug beetle population. Combined populations of two prey species support a larger predator population neither can support alone. As a result, two prey populations reduced, gives outward appearance of interspecific competition.

Indirect commensalism Two species of herbivorous species Daphnia (water fleas) Two predators: Midge larva and Larval salamander Each predator prey on one species In a pond Where salamander larval were present, # of large Daphnia was low, small was high; where absent, small Daphnia were absent, midges could not survive Benefit midge larva, neutral to salamander

Indirect mutualism When indirect interaction is beneficial to both (predator) species.

Indirect interactions play an important role Removing top predators from community could have some unforeseen consequences (conservation and management) An Example: Predator control in Greater Yellowstone ecosystem (Joel Berger from UN Reno) Predator control, decrease in Grizzly bear and wolf   increase in moose population   decrease in willow and other woody species along riverline   decrease in birds (even local extinction for some species)

17.4 Food webs suggest controls of community structure Wealth of experimental evidence illustrates the importance of both direct and indirect interactions on community structure. How do you tell which ones are important in controlling community structure? Are all interactions important? Hypothesis one: all species interactions are important; remove any one of these species may have a cascading effect on all others. Hypothesis two: only a smaller subset of species interactions are controlling community structure. System stable until enough species are lost to make whole system collapse

Difficult to study (there are some dominant species like starfish, but majority is mystery) One approach is splitting species into functional groups Each group has a similar function and perhaps can replace each other Trophic levels 1.Primary producers 2.Herbivores 3.Carnivores

Bottom-up control Plant population control herbivore populations, which in turn control the diversity and population density of carnivore population Top-down control Predator (carnivore) populations control the diversity of prey species, and the prey of the prey, and so on. Bottom-up control is very common. Mostly, community structure is regulated by bottom-up control.

Examples support top-down control Large-mouth bass experiment by Mary Power at OU Bass (Predator) prey on minnows (herbivore), minnows graze on algae Remove bass vs control Pools with bass had low minnow population and a luxuriant growth of algae Pools with bass removed had high minnow populations and low populations (biomass) of algae Top predator control plant population indirectly through their direct control on herbivores. “The world is green”: predators will keep herbivores in control

17.5 Species interactions along environmental gradients involve both stress tolerance and competition Biological structure of a community is constrained by environmental tolerances of the species (fundamental niche). Those tolerances are often modified through both direct and indirect interactions with other species (realized niche). Competitors and predators can function to restrict a species in a community and mutualists can function to facilitate a species’s presence and abundance within a community. Results: a pattern of species distribution across an environmental gradient This is due to trade-off Some species that can grow well are at an advantage when resource at best Some species than can survive when resource is at worst can survive where previous species cannot.

General pattern of trade- off between a species’ ability to survive and grow under low resource and maximum growth achieved under high resource availability. The resulting outcome of competition will be a pattern of zonation.

Salt provides stress Level of salt varies with distance from low water mark Lower boundary is determined by stress Upper boundary is determined by competition Note also that nutrition can change situation Increase fertilization results in change in species at boundary Tradeoff Competitive ability Tolerance of stress Classic examples of zonation occur in salt marshes

17.6 Environmental heterogeneity influences community diversity Biological structure of a community reflects both the ability of the component species to survive and grow in the prevailing environmental conditions and their interactions. As environmental changes from location to location, so will the species and their interaction. But how does local environmental heterogeneity within a community influence patterns of diversity?

Relationship between bird species diversity and foliage height diversity for deciduous forest communities in eastern North America. (x is not height) Late Robert MacArthur 13 communities in northeastern US Bird species diversity Foliage height diversity

17.7 Resource availability can influence plant diversity within a community High nutrients will support high rates of photosynthesis, plant growth, and a high density of plants. How does nutrient availability influence plant diversity in communities?

Why? Michael Huston, ORNL, TN (Texas State Uni.) Relationship of tree species richness to a simple index of soil fertility for 46 forest communities in Costa Rica.

Hypothesis Inverse relationship results from reduced competitive displacement under low nutrient availability Low nutrient availability reduces growth rates and supports a lower density and biomass of vegetation. Species that might dominate under higher nutrient availability cannot realize their potential growth rates and biomass and as a result are unable to displace slower growing, less competitive species. Supported by many other experiments (Rothamsted Experimental Station in Great Britain, 1859-) Increasing nutrient availability has been to decrease diversity

What processes cause the decrease in diversity with increasing nutrient availability? What processes cause the decrease in diversity with increasing nutrient availability? J. Cahill (University of Alberta, Canada) examined how competition in grassland communities shifts along a gradient of nutrient availability J. Cahill (University of Alberta, Canada) examined how competition in grassland communities shifts along a gradient of nutrient availability A shift in the importance of belowground and aboveground competition and the nature of their interaction under varying levels of nutrient availabilityA shift in the importance of belowground and aboveground competition and the nature of their interaction under varying levels of nutrient availability

Competition for belowground and aboveground resources differs in an important way Competition for belowground and aboveground resources differs in an important way Competition for belowground resources is size symmetric because nutrient uptake is proportional to the plant’s root size Competition for belowground resources is size symmetric because nutrient uptake is proportional to the plant’s root size Symmetric competition occurs when individuals compete in proportion to their sizeSymmetric competition occurs when individuals compete in proportion to their size Competition for aboveground resources is size asymmetric — larger plants have a disproportionate advantage by shading smaller ones. Competition for aboveground resources is size asymmetric — larger plants have a disproportionate advantage by shading smaller ones.

Under low nutrient availability, plant growth rate, size, and density are low for all species Under low nutrient availability, plant growth rate, size, and density are low for all species Competition primarily occurs belowground  symmetricCompetition primarily occurs belowground  symmetric Growth rate, size, and density increase as nutrient availability increases Growth rate, size, and density increase as nutrient availability increases As faster-growing species overtop the others, creating a disparity in light availability  asymmetric competitionAs faster-growing species overtop the others, creating a disparity in light availability  asymmetric competition

Fertilization results in an increase in the species richness of autotrophs in both freshwater and marine communities Fertilization results in an increase in the species richness of autotrophs in both freshwater and marine communities Why the difference between patterns in terrestrial versus aquatic communities? Why the difference between patterns in terrestrial versus aquatic communities? Differences in the role of competitionDifferences in the role of competition Reduced competition results from fertilization Reduced competition results from fertilization Limited by more than one nutrient, no single spp has a competitive advantage. Limited by more than one nutrient, no single spp has a competitive advantage. Freshwater and marine communities

END

14 circular mescosms, 1.6 m in diameter, 1.5 m height Three years