bios 5970: plant-herbivore...

BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 12: Community Dynamics Slide - 1

BIOS 5970: Plant-Herbivore Interactions Dr. Stephen Malcolm, Department of Biological Sciences

•  D. POPULATION & COMMUNITY DYNAMICS

•  Week 12. Community Dynamics: – Lecture summary:

•  Community patterns •  East African grazing succession •  Keystone species •  White-sand forests

– Nutrient availability

•  Shifts through time


2. Community patterns: dependent or independent of population processes?: •  Howe & Westley ask:

– “To what degree do strong ecological interactions between pairs or guilds of plants and animals account for differences among natural communities?”

– “Are the immigrations, extinctions, and different reproductive successes of organisms that underlie differences in community composition dependent on interactions between species, or independent of them?”


3. Grazing Succession:

•  “East Africa now harbors the last extensive remnants of ecosystems that once covered much of North and South America, Asia and Australia.”

•  Fig. 10-9: kongoni in tall-grass savanna near Nairobi, Kenya and zebra in grazed grasslands of Serengeti-Mara plains in SW Kenya.


4. Succession and herbivory:

•  Wildebeest and zebra in grasslands of Tanzania. –  Plate 4, Begon et al.,

(2006). •  Herbivory is the

process most important to the structure of such communities.


5. Intensity of herbivory:

•  Herbivory is comparatively constant in these grassland communities & can be responsible for biomass losses of >90% a year.

•  In contrast, most plant communities are characterized by <10% biomass loss per year due to herbivory.


6. Effects of experimental manipulation of herbivory:

Fig. 1. Grass height when grazed (●) or fenced (○) in (a) short grassland, (b) medium grassland (McNaughton, 1984. Am. Nat. 124: 868).

Fig. 3a. Grass biomass inside and outside fences in short (○), medium (●) & tall (△) grasslands.


7. McNaughton cont’d:

Fig. 3b. Grass height inside and outside fences in short (○), medium (●) & tall (△) grasslands.

Fig. 3c. Max. biomass against max. height in fenced (●---) & grazed (○ ) grassland.


8. Species diversity

•  These grass communities support heavy grazing and a diversity of herbivore species because the large ungulate herbivores vary in their diets and distributions.

•  Large rumen, or intestinal volumes, allow buffalo, zebra, and large antelopes, like eland, to eat a wider range of plant species, than the smaller and more selective antelopes, such as gazelles.


9. Seasonal variation:

•  Large geographical and seasonal variability in plant community composition and growth also help to make these communities diverse and dynamic.

•  Grazing succession is the result of larger ungulate herbivores stimulating growth of plants that progressively smaller ungulates can exploit.


10. Effects of grazer size:

•  Different sized species tend to feed together to take advantage of the food resources made available: –  e.g. topi (90-140 Kg) with

eland (450-700 Kg) or Grant’s gazelles (42-68 Kg).

–  This succession is shown for wildebeest (200-228 Kg) and Thompson's gazelles (18-25 Kg) in Fig. 10-10.

Fenced-senescent

Unfenced-flushing

Dietary niche partitioning among large herbivores in east Africa

BIOS 5970: Plant-Herbivore Interactions - Dr. S. Malcolm. Week 12: Community Dynamics Slide - 11NMDS = nonmetric multidimensional scaling

grazers

non grazers


11. Are grazers mutualists?

•  Unlikely, because such compensatory growth cannot result in higher fitness than grasses that are protected with fences from herbivory. – Unless competition has a greater negative effect

than herbivory? •  Selection by intense herbivory could also

increase indirect competition because it would favor unpalatable plant species.


12. Keystone species:

•  “..animal or plant species with a pervasive influence on community composition.”

•  “Removal or extinction of keystone species profoundly changes the competitive relationships, and consequently the relative abundances, of other species in a community.” –  The most famous example is that of Paine (1966) who

showed in a marine community that removal of a predatory starfish freed mussels to outcompete 11 species of limpets, clams, and mussels and led to a much less diverse community.


13. Keystone mutualists:

•  Mutualists can also be keystone species: –  E.g. the uncommon

canopy tree, Casearia corymbosa, supports 6 spp. of fruit disperser in Costa Rica, including masked tityra and toucan (Fig. 10-11).


14. Casearia corymbosa:

•  This is a keystone mutualist because it produces fruits in December when other trees do not and so supports a community of bird fruit dispersers (including its primary disperser).

•  Loss of this tree could lead to a widening community impact with progressive loss of many tree species through time.


15. Tropical forest communities:

•  Influence of soil attributes on plant-animal interactions: –  Including leaf-feeders, flower-pollinators and

seed-dispersers. •  Lowland tropical forests have diverse &

dense vegetation, poor soils, high rainfall, & rapid ecological succession in gaps caused by soil exposure from treefalls, landslides and other disturbances: – bombs, napalm, floods, clearing etc.


16. White-Sand Forests:

•  Forest plant biomass can vary from 6 (white-sand forests) to 80 Kg/m2.

•  White-sand forests (caatinga & heath) are scrub forests with drought-adapted trees & <60% of the biomass in roots (Fig. 10-12).


17. Nutrient cycling in white-sand forests:

•  Nitrogen and phosphorus shortages may be compensated for by catching leaf litter as it falls.

•  Dan Janzen (1974 Biotropica 6(2): 69-103): –  A great paper! –  Argued that resource-limited plants will not be able to

replace leaves easily and so should be evergreen during drought with obvious adaptations to reduce water loss.

•  How will this influence the white-sand community?


18. Janzen’s predictions:

•  1. Tough foliage heavily defended by chemicals.

•  2. Minimal herbivory. •  3. Low numbers and low biomass of

herbivores. •  4. Extremely rare carnivores at the top of

food chains: – Because herbivores are uncommon.


19. Janzen’s predictions:

•  These predictions are supported by: – Low species diversity in white-sand forests. – Black acidic rivers loaded with humic acids:

•  Tannins and other phenolic acids. – Undecomposed plant matter, suggesting very

high levels of plant secondary defenses.


20. Nutrient-poor forests continued:

•  Doyle McKey: – Ph.D. student of Janzen at Univ. Michigan. – Tested Janzen’s predictions.

•  Observed black colobus monkeys (Colobus satanas) as herbivores feeding on plants in the white-sand forests of Cameroon in west Africa.

•  Compared with C. badius (red colobus) and C. guereza (black-and-white colobus) feeding on leaves of trees in the richer soils of Uganda in central Africa.


21. Differences between soils in Cameroon and Uganda (Table 10-3):


22. McKey’s results:

•  Cameroon monkeys avoided most common trees and fed selectively on rare deciduous trees and uncommon herbaceous vines: – Not the common, well-defended evergreen

species. – 37% of food was leaf material, 53% seeds.

•  Ugandan monkeys had a 75% leaf diet from common trees and ate fewer seeds and the population was 10x larger than in Cameroon.


23. Janzen-McKey:

•  The rationale developed by Janzen (1974) and McKey et al. (1978, Science 202: 61-64) was later used by Bryant et al. (1983) and Coley et al. (1985) in their carbon:nutrient balance and resource availability hypotheses.

•  McKey et al. (1978) state: –  “Janzen (1) reasoned that the cost of replacing materials eaten by

herbivores would be greater in areas of nutrient-poor soils than for plants growing on sites richer in nutrients. He predicted that vegetation growing on impoverished white-sand soils would be found to contain greater concentrations of herbivore-deterrent toxic secondary compounds (such as tannins, saponins, and alkaloids) than would vegetation growing on more nutrient-rich soils.”

•  (1) D.H. Janzen (1974) Biotropica 6: 69-103.


24. Community, ecosystem, landscape and biome shifts through time:

•  As abiotic conditions change through time so their impact will shift patterns of species diversity and interactions within communities.

•  For example, increased aridity shifts vegetation from forests to savannas and steppes and so the herbivore community shifts from a predominance of browsers to mostly grazers.

•  Figs. 9-7, 9-8.


Figure 9-7: Increased aridity 19-5 million years ago (Miocene) generated shift from forest to savanna to steppe and change in herbivores from browsers to grazers.


Figure 9-8: Shift in North American, Miocene horse evolution from browsers to grazers.

References •  Begon, M., Townsend, C.R., and Harper, J.L. 2006. Ecology: From Individuals, to Ecosystems. 4th

edition. Blackwell Publishing Ltd., 738 pp •  Bryant, J.P., Chapin III, F.S., and Klein, D.R. 1983. Carbon/nutrient balance of boreal plants in relation

to vertebrate herbivory. Oikos 40(3): 357-368. •  Coley, P.D., Bryant, J.P., and Chapin, F.S.III 1985. Resource availability and plant antiherbivore

defense. Science 230: 895-899. •  Howe, H.F., and Westley, L.C. 1988. Ecological Relationships of Plants and Animals. New York:

Oxford University Press, 273 pp. •  Janzen, D.H. 1974. Tropical blackwater rivers, animals, and mast fruiting of the Dipterocarpaceae.

Biotropica 6(2): 69-103. •  Kartzinel, T.R., P.A. Chen, T.C. Coverdale, D.L. Erickson, W.J. Kress, M.L. Kuzmina, D.I. Rubinstein,

W. Wang, & R.M. Pringle. 2015. DNA metabarcoding illuminates dietary niche partitioning by African large herbivores. PNAS 112(26): 8019-8024.

•  McKey, D., Waterman, P.G., Mbi, C.N., Gartlan, J.S., and Struhsaker, T.T. 1978. Phenolic content of vegetation in two African rain forests: Ecological implications. Science 202: 61-64.

•  McKey, D.B., Gartlan, J.S., Waterman, P.G., and Choo, G.M. 1981. Food selection by black colobus monkeys (Colobus satanas) in relation to plant chemistry. Biol. J. Linn. Soc. 16: 115-146.

•  McNaughton, S.J. 1984. Grazing lawns: Animals in herds, plant form, and coevolution. Am. Nat. 124(6): 863-886.

•  Paine, R.T. 1966. Food Web Complexity and Species Diversity. Am Nat. 100(910): 65-75.


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