MARINE BENTHIC SUCCESSION

Transcription

1 MARINE BENTHIC SUCCESSION Lab 10 Reminders! Memory Stick Key Concepts Primary & secondary succession; Climax Intermediate disturbance hypothesis Keystone predator hypothesis r & K selected species Student Learning Outcomes After Lab 10 students will be able to: 1. describe the process and different stages of a succession in biological communities. 2. explain the relationship between the frequency of disturbance and species diversity during a succession. 3. recognize the colonization characteristics of different species and the role each plays during a succession. Book Chapters Campbell: mainly Chp. 54.3, but also Chp and Chp succession also applies to animal communities. For example, temperate rocky intertidal zones are first colonized by fast growing algae, then barnacles, then slower growing algae, and then mussels. After the onset of succession, a community may go through a number of seral stages where different flora and fauna dominate. A sere is the sequence of communities encountered in a particular location over time. Eventually, a stage may be reached where community structure will remain relatively constant in the absence of a major disturbance. This equilibrium community state is called the climax community. The species composition of a climax community is dependent on the local environmental conditions (e.g., temperature, rainfall, and soil type). Other factors affecting the pattern of succession and composition of the climax community include the rate and sequence of colonization of different species. I. SUCCESSION Succession is a progressive, often predictable change in species composition (and abundance) in a community over time. With time, certain species may become rare or even disappear, while others increase in abundance. Primary succession begins when a previously uninhabited area becomes available for colonization. For example, primary succession occurs when retreating glaciers expose soil that has been sterile for thousands of years. On the island of Hawai i, primary succession occurs on fresh lava from volcanic flows. Secondary succession follows the disturbance of a previously inhabited area. A forest fire or hurricane that damages a forest leads to secondary succession as a new set of plants initially begin to replace the fallen trees. Although these examples involve plants, the concept of Time Figure 1. Simplified example of a successional sere involving three plant communities. The concept of a monotypic climax, in which there is a single climax community for a given environment, has given way to the more dynamic concept of the pattern-climax. The patternclimax concept recognizes that most forests are comprised of a patchwork of climax communities and different seral stages (due to patches of disturbance). Some communities never reach a climax state and are maintained in a state of subclimax. This occurs in communities that are regularly disturbed. This disturbance can come in a variety of forms, such as herbivory, wind, fire, flood, drought, ice scour, or wave action. Fall

2 Mechanisms of Succession Three primary mechanisms have been proposed to act during succession: facilitation, inhibition, and tolerance (Connell & Slayter, 1977). In facilitation, early successional species modify the environment, making the environment more suitable for germination and growth of later successional species. For example, a nitrogenfixing plant may add nitrogen to nutrient-poor soil, allowing nitrogen-loving plants to colonize. Facilitation is extremely important during the early stages of primary succession, but it can also be important in secondary succession. The inhibition model states that once colonizing species become established, they resist invasion simply by taking up space (inhibiting later successional species). When these established individuals die or are damaged by disturbance, new, later successional species can replace the earlier species, assuming that seeds or propagules are present. This process of replacement following death of the established species eventually leads to domination by long-lived species, since longlived species can hold on to a space for a very long time. Inhibition is one factor that maintains a climax community. Species that dominate climax communities tend to be very long lived, providing few opportunities for species replacement. span, and low competitive ability over the long term. Species possessing this particular set of traits are said to be r selected. At the other end of the spectrum, later successional species are often characterized by lower reproductive output, higher maternal investment per offspring, high competitive ability, long life span, and slow growth rate. Species possessing this set of traits are said to be K selected. The r and the K refer to the parameters of the logistic growth equation (also see Figure 2). Recall the K is the carrying capacity. A K selected species typically does well in a crowded environment (near carrying capacity). In contrast, r is the intrinsic rate of increase (growth rate when population density is low). r-selected species are often found colonizing open areas associated with frequent disturbance. Of course, most species do not fit perfectly into either of these groups. These categories represent the extreme ends of a continuum, and many species exhibit some r and some K selected characteristics. The tolerance model proposes that free resources become scarce in later successional stages, and those species that can tolerate the lowest resource conditions (e.g. low light, low nutrients) will survive better and eventually out-compete those species requiring more resources. The tolerance model predicts that over time, the fast-growing, high resource-demanding species will reduce resource levels, leading to their replacement with species that are more tolerant of low resource conditions. Tolerance, inhibition, and facilitation are not mutually exclusive and may act together during a successional series. r and K Selected Species The first species to colonize a newly disturbed area are often termed pioneer species, and these species are usually characterized by high reproductive output, high growth rate, short life Figure 2. Relationship between poplations size and rate of population increase for r- and K-strategists. Diversity and Succession Your first impression might be that diversity should be lowest at the onset of a successional series (sere) and highest at the climax. In reality, maximal diversity usually occurs before the climax community. In a community that experiences very frequent disturbance, only a few, rapid growing species can survive and reproduce. Likewise, in a climax community experiencing little or no disturbance, climax species outcompete most other organisms, leading to lower species diversity. Intermediate disturbance will tend to maintain a patchwork of low, mid, and late seral stages (including some species typical of Fall

3 early succession and others typical of climax communities), thus maintaining maximal diversity. The intermediate disturbance hypothesis (Connell, 1978) proposes that diversity is highest at intermediate levels of community disturbance (Figure 3). It should be noted that the intermediate disturbance hypothesis is based on the assumption that competition is an important regulator of community diversity. Therefore, even if there is no or minimal disturbance occurring in a community, highest species diversity will be present at intermediate stages of a succession (i.e. before the climax) because there is a mix of primary and secondary succession species present competing with each other. community, leading to very low species diversity. In the natural community, the seastar acted as an agent of disturbance by eating the mussels and preventing their competitive dominance. Although not reported in Paine s study, if sea stars become too abundant they will start eating the mussels competitors (along with everything else) and diversity may decrease. These findings again emphasize the importance of intermediate levels of disturbance in maximizing diversity. Rocky shores provide another good example supporting the intermediate disturbance hypothesis (Sousa, 1979). Small rocks, which are frequently overturned by pounding waves, are colonized by few or no species because they tend to be crushed as the rocks roll. In contrast, very large rocks are rarely moved by wave action, and they are usually covered by one or only a few encrusting (permanently attached) species. Medium sized rocks, which are moved by waves at an intermediate frequency, harbor the most species. Figure 3. The intermediate disturbance hypothesis according to Connell Predators can play an important role in reducing competition among prey, thereby increasing diversity. The results of Paine s (1966) study of an intertidal community in Washington led to the keystone predator hypothesis, a hypothesis that predation by certain keystone species is important in maintaining community diversity. This experiment also supported the intermediate disturbance hypothesis, since you can consider predation to be a disturbance. When the top predator, a seastar Pisaster sp., was removed from the intertidal community, mussels dominated the Experimental Studies of Succession Studying succession of a forest is often very difficult because of the length of time involved. Many successional experiments, therefore, are based on smaller scale systems. Studying the succession of plankton communities is relatively easy because the experiments can be conducted in a controlled environment and results can be obtained quickly. Trees may live for 1000 years, but phytoplankton and zooplankton have much shorter life spans. Phytoplankton experiments can be easily replicated and don t take up much space. Experimental studies of aquatic successions have been conducted in containers ranging in size from a beaker (microcosm) to an above ground swimming pool (mesocosm) (Duffy & Hay, 2000). Studies on succession in marine benthic communities have also been popular. A benthic community is an aquatic community that is associated with the bottom, whether it is sand, mud, or rock. Coral reefs are famous benthic communities and were used by Connell and Fall

4 Slatyer (1977) in developing their ideas about the mechanisms of succession. Marine benthic communities make good study subjects because the organisms are fairly small and succession occurs rapidly compared to terrestrial forests. Benthic communities have also received a lot of attention because they grow on ship hulls, costing millions of dollars in extra fuel and carrying invasive species from one location to another as ships move to new regions. Benthic succession is often studied by putting a bare substrate (settling plates) into the water and photographing or collecting them periodically. Successional patterns are studied by analyzing the composition of replicated settling plates over time. If there is minimal disturbance, you can typically expect to see a climax community on the settling plates within a year or two. II. METHODS Today we will be studying primary succession of a Hawaiian marine benthic community. To initiate a successional series, PVC settlement plates were deployed at Coconut Island in Kaneohe Bay. The 8cm x 8cm (64cm 2 /side) plates were sanded to provide a favorable settling surface and hung beneath a floating dock about a foot below the water s surface. Ten plates were deployed monthly for the last year, and all plates were collected last weekend. During this laboratory, you will work in small groups to analyze the community that has developed on the settling plates. Your TA will assign you a specific time period in the successional sere. Towards the end of the period, data from the entire class will be compiled to provide a picture of how the community on the settling plates changed over time. A. Taxa Identification and Percent Cover 1. Your first task is to describe the communities on all of the settlement plates (page 10-7, Datasheet 1). Settlements are placed in big Petri dishes under a dissecting scope. Keep the plates submerged in seawater at all times. Identify all of the taxa on the settlement plates. Identification books and close-up photos of invertebrates and algae will be available. Your TA will let you know when you can switch to the next dissecting scope with a settlement plate of a different age. 2. Once everybody described all the settlement plates, your TA will assign you a specific time period s settlement plate to examine. You will also be given a 6 by 6 quadrat grid. 3. You will randomly sample ten quadrats on each side of the plate. Handle your settlement plate very carefully because other students will be sharing your plate. 4. On page 10-8 is a worksheet to help you keep track of your data (Datasheet 2). There is also a file with the same worksheet on Laulima. This file also has a random number generator for choosing your quadrat coordinates. 5. For each quadrat, identify the taxa present and estimate the percent cover of each taxon. Enter these data in the excel worksheet. If your random number generator gives you repeat quadrat coordinates, select a different one. Note: zooming in too close will only cause more confusion zoom back out to estimate community structure and cover! Use your best judgment with the taxon IDs. Once you have identified the taxa in your quadrat, you should spend no more than about seconds to estimate the cover of each taxon. Fall

5 6. Competition for space may be a major factor influencing succession in benthic communities. Within each quadrat where you had estimated % cover, record whether you see any evidence of taxa overgrowing each other (enter 1 for Yes or 0 for No on your excel worksheet). 7. Using the life history attributes provided (Table I), take note of the taxa that can be classified roughly as r selected or K selected based on their known life histories. Remember, not all species can be neatly classified as r or K selected. Table I. Life History Attributes Organisms Growth Age to Body Size Rate Maturity Hydroides Bivalvia B. Sharing Data 8. Once you have completed all the steps, enter your data into your section s google doc. Your TA will upload the compiled data as an excel file on Laulima. 9. You will use the complete data set from your section to complete your assignments. From your section s data, the following will be calculated and posted on the web: Average total community cover per plate Relative % cover for each taxon encountered Frequency (average number of quadrats per plate) of observed evidence of overgrowth (will be somewhere between 0 and 25). Species (taxon) richness per plate Simpson s index of diversity for each plate (see lab 9). C. Clean up When you are done, return your settlement plate to the tank or where your TA instructs you. Return the seawater from your Petri dish to the tank and rinse the Petri dish and your quadrat grid as well as any other equipment you used (e.g. forceps) in fresh water. Make sure there is no saltwater on the scope, wipe off if needed. III. ASSIGNMENTS Do not leave lab early. Complete as many graphs as possible in class, and save the written questions for later. Be sure to label your axes and include a brief description of each graph in a caption. Graphs can be shared among classmates, but each person must individually answer the questions below. 1. Plot a graph of total % cover (all taxa combined) vs. duration of submersion. (1 pt.) a. How does total % cover relate to the duration of time that the settling plates were in the water? (1 pt.) Is this what you expected? Explain. (2 pts.) b. Does total % cover seem to reach equilibrium? (1 pt.) If yes, is this (if not, would it be) good evidence that a climax community has been reached? (5 sentences max) (3 pts.) 2. Plot two graphs: Species richness vs. duration of submersion. (1pt.) Species diversity vs. duration of submersion. (1 pt.) Describe the patterns in the graphs in a cohesive paragraph including the following questions and explain your reasoning: - How does species richness and diversity relate to the duration of time the settlement plates were in the water? continued on next page Fall

6 - Are the relationships similar? Is this what you expected? (4 pts.) 3. Discuss the following in a cohesive paragraph: - Which taxon seems to be dominant in A) the first third of the time series, B) the second third of the time series, and C) the last third of the time series? - Is it the same taxon? - Is this what you expected? (4 pts.) 4. Plot the mean number of quadrats showing evidence of overgrowth versus duration of settlement plate submersion. (1 pt.) Describe the pattern and include if competition appears to be frequent in this community? (5 sentences max) (3 pts.) 5. Choose one r- selected and one K- selected taxon and describe (in a cohesive paragraph) how their cover changes with increasing duration of submersion. Discuss if these compare to patterns theoretically predicted for r and K selected species and if the patterns for the r and K species seem to relate to the patterns observed in the overgrowth graph (question #4). (4 pts.) 6. Write a draft for the Discussion section of your final paper. Your discussion should start with some introductory sentences relating back to your introduction and setting a framework for your hypotheses you tested in the results section. This is the section where you interpret your results in a biological/ecological context. Make sense of the data and the outcome of your tests from the results section in the context of community ecology on Wa ahila ridge. Refer to other referenced studies if possible to put your study in a bigger context. Once you explain your results in a biological context you should address any methodological issues, which could have affected your data, or things that you would improve in future studies. End with some concluding sentences that put your main results into a bigger context and leaves the reader a simple take home message. (6 pts.) NOTE: Even though this is a draft it will be graded as part of your homework assignment. The better your draft, the more feedback your TA is able to give you and the better your final paper will be (worth 20% of your final Lab grade) 7. Make a graphical representation of your overall Ridge project, so that anybody looking at it can get the whole story in a few minutes. This should help you putting all the pieces together and tell a story around your two hypotheses. You can do this on paper (and include a picture in your homework) or in digital format. There are no strict guidelines, other than it has to fit on one letter sized sheet, needs to be legible, include all parts of your ridge project, and not be continuous text. Your TA will show you some examples. (3 pts.) IV. REFERENCES Connell, J.H Diversity in tropical rain forests and coral reefs. Science 199: Connell, J.H. & R.O. Slatyer Mechanisms of succession in natural communities and their role in community stability and organization. American Naturalist 111: Duffy, J.E. & M.E. Hay Strong impacts of grazing amphipods on the organization of a benthic community. Ecological Monographs 70: Paine, R.T Food web complexity and species diversity. American Naturalist 100: Sousa, W.P Disturbance in marine intertidal boulder fields: The nonequillibrium maintenance of species diversity. Ecology 60: Fall

7 Student Name: Date: Section/TA Name: No. months submerged 2 dominant species other species Observations Fall

8 Name: Month# First Side Quadrat # Coordinates (x,y) Taxon Phylum enter percent cover for each of your taxa average Cyanophyta (blue-greens) Cyanophyta (Peyssonnellia) (crustose coralline) (Hypoglossum) (unknown) (Ulva) (Cladophora) (unknown) Sponge Porifera Anemone/polyp Cnidaria Polychaeta (Hydroides) Annelida Polychaeta (Spirorbis) Annelida Bryozoan (encrusting) Bryozoa Bryozoan (branching) Bryozoa Bivalvia (Dendostrea) Mollusca Cirripedia/barnacles Arthropoda Tunicate Chordata Does quadrat show overgrowth? (1=yes, 0=no) 0 =To Second Side Quadrat # Coordinates (x,y) Taxon Phylum enter percent cover for each of your taxa average Cyanophyta (blue-greens) Cyanophyta (Peyssonnellia) (crustose coralline) (Hypoglossum) (unknown) (Ulva) (Cladophora) (unknown) Sponge Porifera Anemone/polyp Cnidaria Polychaeta (Hydroides) Annelida Polychaeta (Spirorbis) Annelida Bryozoan (encrusting) Bryozoa Bryozoan (branching) Bryozoa Bivalvia (Dendostrea) Mollusca Cirripedia/barnacles Arthropoda Tunicate Chordata Does quadrat show overgrowth? (1=yes, 0=no) 0 =To Fall