BIOS 3010: Ecology Lecture 20: Community Structure & Predation: Lecture summary: Effects of grazing herbivores. Effects of predators. Effects of parasites & disease. Variation in time. Disturbance & community patch dynamics: Dominance control. Founder control. Fig 7-10, Miller (2002) Essentials of Ecology Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 1 2. The effect of grazing herbivores: Herbivory, predation, disease and parasitism, competition, earthquakes, fire, rain, wind, temperature etc. can all disturb communities. Disturbance is: any relatively discrete event in time that removes organisms or otherwise disrupts the community by influencing the availability of space or food resources, or by changing the physical environment. A general consequence is likely to be the opening up of space, or freeing up of resources, that can be taken over by new individuals. Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 2 3. The effect of grazing herbivores: Disturbance (continued): For example, a predator, or herbivore, or lawn mower, or a strong wave, or a strong wind can open gaps in communities. Thus grazing by rabbits can strongly influence the structure of plant communities (Fig. 21.1). Intermediate grazing promotes most diversity through its influence on competition. exploiter-mediated coexistence - Fig. 21.2. Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 3 1
4. The effects of predators: Removal of predatory starfish by Paine (1966): Led to reduction in community species number from 15 to 8 (Fig. 21.3, 2 nd ed.): Because space made available by the predator helped competitively subordinate species and increased species diversity. Exploiter-mediated coexistence. Frequency-dependent effects can also influence community structure. e.g. switching according to prey density in fish (Fig. 21.3, 3 rd ed.). Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 4 5. The effects of parasites and disease: Exploiters can affect other interactions and facilitate coexistence of species (Fig. 21.4). Strongly negative effects on communities can also occur for highly pathogenic invading diseases: e.g. malaria and bird pox in Hawaii: may have exterminated 50% of the endemic bird species. Also destruction of chestnut and elm forests in North America by introduced pathogens. Effects are likely to be frequency dependent Influenced by the frequency of encounter in high density populations. Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 5 6. Community structure and temporal variation in conditions: Conditions constantly change in space and time: They have a marked impact on ecological processes and hence on species composition of communities (Fig. 21.5). Thus periodic, density independent reductions in population size may be expected to promote coexistence of competing species (Fig. 21.6). Frequency of disturbance also impacts diversity, as does growth rate (rate of increase) of species (Figs. 21.7 and 21.8). Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 6 2
7. Disturbances and patch dynamics: Includes the effects of movement among patches of resource available for colonization: Within patches are like Lotka-Volterra interactions. But extinctions can be reversed or created by immigration and emigration Like metapopulations. Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 7 8. Disturbances and patch dynamics: Dominance-controlled communities: Some species are competitively superior Gaps lead to succession-like changes in species composition. Disturbance knocks the community back to an earlier stage (Fig. 16.6 & Table 21.1): From pioneer species (p) to mid-succession species (m) to climax competitor species (c). Diversity is also greatest at intermediate levels of disturbance (Figs. 16.17 & 16.18). The intermediate disturbance hypothesis. Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 8 9. Disturbances and patch dynamics: Founder-controlled communities: All species are competitively similar. Thus succession is not expected. There is no competitive exclusion. Presence in the community becomes a lottery (Fig. 16.21). Some tropical reef fish communities may be like this and diversity may remain high. Species may be competing for living space in which larvae are the tickets, and the first arrival at the vacant space wins the site, matures quickly and holds the space for its lifetime. Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 9 3
Figure 21.1 (3 rd ed.): Effect of rabbit grazing on sand-dune plant species richness. Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 10 Figure 21.2 (3 rd ed.): Effect of Littorina littorea density on species richness (S) and species diversity (Shannon H) in (a) tide pools, (b) emergent rocks Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 11 Figure 21.3 (2 nd ed.): Paine s rocky shore community. Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 12 4
Figure 21.3 (3 rd ed.): Switching between planktonic and sediment waterfleas by roach. Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 13 Figure 21.4 (3 rd ed.): Effect of powdery mildew on inter-specific competition between barley & wheat Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 14 Figure 21.5 (3 rd ed.): Changes in community structure in a German pasture. Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 15 5
Figure 21.6 (3 rd ed.): Theoretical effect of density independent population reductions on competitive exclusion. Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 16 Figure 21.7 (3 rd ed.): Theoretical effect of frequency of population reduction on community diversity. Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 17 Figure 21.8 (3 rd ed.): Huston s model communities: Predicted relationships between diversity and (a) frequency of population reduction, (b) population growth rates, (c) contour of both. Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 18 6
Figure 16.6: Hypothetical mini-succession in a gap by pioneer (p i ), mid-successional (m i ) and climax (c i ) species. Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 19 (3rd ed.) Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 20 Figure 16.17: Rates of disturbance and effects on species richness Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 21 7
Figure 16.18: Insect species richness and disturbance in a New Zealand stream. Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 22 Figure 16.21: Hypothetical competitive lottery among species A-E for community gaps Dr. S. Malcolm BIOS 3010: Ecology Lecture 20: slide 23 8