Environmental Heterogeneity and Plant Species Diversity: A HypothesisAuthor(s): Robert E. RicklefsReviewed work(s):Source: The American Naturalist, Vol. 111, No. 978 (Mar. - Apr., 1977), pp. 376-381Published by: The University of Chicago Press for The American Society of NaturalistsStable URL: http://www.jstor.org/stable/2460072 .Accessed: 08/05/2012 06:17

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376 THE AMERICAN NATURALIST

LITERATURE CITED

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Costlow, J. D., Jr., and C. G. Bookhout. 1957. Larval development of Balanus eburneus in the laboratory. Biol. Bull. Marine Lab., Woods Hole 112:313-324. 1958. Larval development of Balanus amphitrite var. denticulata Broch reared in the laboratory. Biol. Bull. Marine Lab., Woods Hole 114:284-295.

Dobkin, S. 1968. Abbreviated larval development in caridean shrimps and its significance in the artificial culture of the animals. FAO Fisheries Rep. 57:935-945.

Gurney, R. 1942. Larvae of decapod crustacea. Ray Society, London, 306 pp. Hubschmann, J. H., and A. C. Broad. 1974. The larval development of Palaeenonetes inter-

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Karande, H. A. 1974. Balanus variegatus Darwin: the laboratory reared larvae compared with Balanus amphitrite amphitrite Darwin (Cirripedia). Crustaceana 26:229-232.

Moyse, J. 1960. Mass rearing of barnacle cyprids in the laboratory. Nature 185: 120. Nyblade, C. F. 1974. Coexistence in sympatric hermit crabs. Ph.D. thesis. University of

Washington. Patel, B., and D. J. Crisp. 1960. Rates of development of the embryos of several species of

barnacles. Physiol. Zool. 33:104-119. Underwood, A. J. 1974. On models for reproductive strategy in marine benthic inverte-

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Natur. 107:339-352. 1974. Reply to Underwood. Amer. Natur. 108:879-880.

Williamson, D. I. 1968. The type of development of prawns as a factor determining suit- ability for farming. FAO Fisheries Rep. 57:77-84.

RICHARD R. STRATHMANN

FRIDAY HARBOR LABORATORIES

FRIDAY HARBOR, WASHINGTON 98250 DEPARTMENT OF ZOOLOGY

UNIVERSITY OF WASHINGTON

SEATTLE, WASHINGTON 98195 February 3, 1976

ENVIRONMENTAL HETEROGENEITY AND PLANT SPECIES DIVERSITY: A HYPOTHESIS

The number of species of trees in tropical forests is greater, on average, than in temperate forests. Whereas 1-hectare areas of temperate woods may have five to 15 species of trees (Braun 1950) similar areas of tropical forests may con- tain 50-100 species (Richards 1957). The maintenance of different levels of

LETTERS TO THE EDITORS ,-77

species diversity at different latitudes has been the subject of considerable dis- cussion in the ecological literature but the problem is far from being resolved, particularly for plants.

Animal ecologists have long recognized a relationship between the number of species in a community and the heterogeneity of the habitat (MacArthur 1965, 1969; Pianka 1966; Murdoch et al. 1972). For example, diversity in assemblages of birds has been linked to the variety of plant resources (Orians 1969; Karr 1971) and to the structural heterogeneity of the vegetation (MacArthur et al. 1966).

Whereas ecologists can point to plant diversity, both structural and taxo- nomnic, as a source of environmental heterogeneity to explain variation in the diversity of animals, variation in plant diversity is seemingly without similar basis. There is little reason to believe that topography, soil characteristics, and other environmental properties within small areas of forest are more varied in the tropics than in temperate regions.

Lacking hypotheses based on resource partitioning, ecologists have related plant species diversity to pressure exerted by herbivores on abundant or other- wise dominant species, which reduces their competitive ability and permits additional subordinate species to coexist (Doutt 1960; Gillett 1962; Janzen 1970). In one sense, this hypothesis also is based on environmental hetero- geneity inasmuch as herbivores add complexity to the environment of plants. The avoidance of efficient predators takes place within an escape space, which is partitioned among prey species through competition and specialization (Ricklefs and O'Rourke 1975).

Any factor purported to explain a geographical trend in diversity must itself exhibit a similar geographical gradient that ultimately can be related to physical variables. Herbivores possibly would be able to regulate plant populations more effectively in the tropics than in temperate regions if the physical environment were more constant, and if constancy enhanced herbivore specialization and proficiency. Whether environments are more constant in the tropics than in temperate regions, in any way that is critical to herbivore populations, remains to be seen. Furthermore, the ecological pathways through which small, perhaps two- or threefold, differences in physical environments (precipitation, insola- tion, constancy) are translated into 10-fold differences in diversity are not evident.

In this paper, I suggest that local heterogeneity in soil properties and surface microenvironment, caused by the influence of physical factors within gaps in the forest canopy, may underlie a part of geographical differences in tree species diversity. The hypothesis is based on latitudinal trends in the angle and in- tensity of incident solar radiation, precipitation, temperature, and the distribu- tion of nutrients between the soil and vegetation. The action of physical factors within gaps in the forest canopy alters the environment near the ground surface, creating a gradient of conditions between sites under the unbroken forest canopy and in the center of canopy openings and clearings formed by branch and tree falls. If species compete during early stages of seedling establishment, specializa- tion could occur with respect to conditions along the temporary environmental

378 THE AMERICAN NATURALIST

gradients established between the forest floor and clearings (e.g., Williamson 1975). I argue that this gradient is much broader in the tropics than in temperate regions.

The forest-clearing gradient in the tropics could be wider than it is in temper- ate regions owing to several factors: (1) The greater biomass of tropical forest vegetation modifies light levels, humidity, temperature, and environmental constancy at the forest floor to a greater degree than in temperate forests. (2) The greater ratio of nutrients in vegetation to nutrients in the soil in tropical forests (Ovington 1965; Kira and Shidei 1967) suggests that the influx of nutri- ents to the soil from decaying vegetation in tree falls compared to nutrient levels in soils under closed canopies may be greater in the tropics. (3) More rapid de- composition of leaf litter and other organic detritus in lowland tropical regions, at perhaps 10 times the rate in temperate areas (Jenny et al. 1949; Olson 1963; Hopkins 1966), would speed the release of mineral nutrients and organic detritus from fallen trees and increase the flush of nutrients in the soil. (4) Even if nutri- ent levels in the soils of temperate and tropical forests were similar (Buol 1973; Sanchez and Buol 1975), the humus content of soils is certainly lower in tropical forests than in temperate forests (Jenny et al. 1949). Humus content influences the retention of soil moisture and the stability of other soil properties in the face of exposure to physical factors within gaps in the forest canopy. (5) The greater rainfall of tropical areas enhances the leaching of readily movable ions from exposed soils. Differential leaching of ions leading to changes in relative con- centrations of ions in the soil would further enhance gradients created by forest openings. (6) Because the sun is nearly overhead in the tropical sky for much of the day, its light strikes the soil in a forest opening more directly and for a longer period of each day than in temperate regions. The drying and heating effect of the sun on the tropical soil and surface microclimate further distin- guishes environments at the center of the clearing and under the forest canopy.

Conditions in large forest openings may be so different from the conditions under closed canopies that such clearings cannot be invaded by "climax" species, but only by successional forms (Richards 1957). To a lesser degree, we can regard most elements of the forest as occurring in a mosaic of successional stages in a pattern of continual turnover and replacement with few truly climax species (Aubreville 1938, cited in Richards 1957; Jones 1945; Watt 1947). Similarly, what is often regarded as the alpha or local diversity of a habitat could as well be viewed as the expression of beta or regional diversity, incorpor- ating the turnover of species between habitats, on a fine geographical scale.

The relationship outlined here between environmental heterogeneity and plant species diversity suggests the following predictions: (1) Gradients of soil and surface environment characteristics between intact forests and forest openings should be greatest, relative to the niche breadths of seedlings, in regions of high plant diversity. Depending upon how rapidly forest soil conditions are restored in the tropics, soil conditions under closed canopies may or may not be more heterogeneous in regions of high diversity. (2) Seedlings and young indi- viduals of each tree species should exhibit distinct distributions according to

LETTERS TO THE EDITORS 379

TABLE 1

NUMBER OF SPECIES OF PLANTS IN TEMPERATE AND TROPICAL GRASSLAND HABITATS

No. OF SPECIES

LOCALITY Regional* Localtj REFERENCE

Temperate: North Dakota ........... ... 22-37 Redmann 1972 North Dakota ........... 268 24-82 Dix and Smeins 1967 North Dakota ........... 173 11-46 Hadley and Buccos 1967 Minnesota .............. .. 13-39 Smeins and Olsen 1970 N.D.-Minn . ............ ... 15-31 Wali et al. 1973

Tropical: Nigeria ................. 93 ca.30 Hopkins 1962 Trinidad................ 243 24-35 Richardson 1963

* Study areas of several hundred hectares to several hundred km2 of similar habitat type. t Study sites generally less than 0.1 hectare.

soil and microenvironmental conditions and distinct responses to experimentally produced gradients of conditions. (3) Because plant communities dominated by herbaceous or shrubby species influence soil conditions to a lesser degree than forest communities, and because the death of individual herbs or shrubs disturbs the structural integrity of the community less than the death of a tree, geo- graphical gradients of plant diversity should be less steep for herb and shrub communities than for forest communities. The similar numbers of species of plants in temperate and tropical grasslands (table 1) are consistent with this prediction.

The hypothesis outlined here suggests a simple explanation for geographical patterns in the diversity of species of trees: namely, that latitudinal gradients in physical factors interacting with openings in the canopies of forests result in greater local heterogeneity of environmental conditions for seedling establish- ment in the tropics than in temperate areas. The hypothesis is not incompatible with others based on herbivore pressure, environmental constancy, speciation rate, geological age, and so on. Environmental heterogeneity provides the basis for resource partitioning and coexistence of competing species. The environ- ments of trees clearly have dimensions other than the edaphic and micro- climatological conditions of the early development period. Furthermore, levels of competition are set as much by the invasion of communities by new species as by opportunities for reducing competition within communities through specialization. But local heterogeneity of soils, and its causes, nonetheless warrant further study.

ACKNOWLEDGMENT

I am grateful to H. Hespenheide, J. Hickman, D. Janzen, E. Leigh, P. A. Sanchez, A. P. Smith, D. Sprugel, M. Wali, G. B. Williamson, and R. I. Yeaton for helpful comments and discussion. The study was supported by NSF GB 42661.

380 THE AMERICAN NATURALIST

LITERATURE CITED

Aubreville, A. 1938. La fort coloniale: les forts de lAfrique occidentale francaise. Ann. Acad. Sci. Colonialle, Paris 9:1-245.

Braun, E. L. 1950. Deciduous forests of eastern North America. Hafner, New York. Buol, S. W. 1973. Soil genesis, morphology and classification. Pages 1-38 in P. A. Sanchez,

ed. A review of soils research in tropical Latin America. North Carolina Agr. Exp. Sta., Tech. Bull. No. 219.

Dix, R. L., and F. E. Smeins. 1967. The prairie, meadow, and marsh vegetation of Nelson County, North Dakota. Can. J. Bot. 45:21-58.

Doutt, R. L. 1960. Natural enemies and insect speciation. Pan-Pacific Entomol. 36:1-13. Gillett, J. B. 1962. Pest pressure, an underestimated factor in evolution. Syst. Ass. 4:37-46. Hadley, E. B., and R. P. Buccos. 1967. Plant community composition and net primary

production within a native eastern North Dakota prairie. Amer. Midland Natur. 77:116-127.

Hopkins, B. 1962. Vegetation of the Olokemeji Forest Reserve, Nigeria. I. General features of the reserve and the research sites. J. Ecol. 50:559-598. 1966. Vegetation of the Olokemeji Forest Reserve, Nigeria. IV. The litter and soil with special reference to their seasonal changes. J. Ecol. 54:687-703.

Janzen, D. H. 1970. Herbivores and the number of tree species in tropical forests. Amer. Natur. 104:501-528.

Jenny, H., S. P. Gessel, and F. T. Bingham. 1949. Comparative study of decomposition rates of organic matter in temperate and tropical regions. Soil Sci. 68:419-432.

Jones, E. W. 1945. The structure and reproduction of the virgin forest of the north-temper- ate zone. New Phytol. 44:130-148.

Karr, J. R. 1971. Structure of avian communities in selected Panama and Illinois habitats. Ecol. Monogr. 41:207-233.

Kira, T., and T. Shidei. 1967. Primary production and turnover of organic matter in different forest ecosystems of the western Pacific. Jap. J. Ecol. 17:70-87.

MacArthur, R. H. 1965. Patterns of species diversity. Biol. Rev. 40:510-533. . 1969. Patterns of communities in the tropics. Biol. J. Linnaean Soc. 1: 19-30.

MacArthur, R. H., H. Recher, and M. Cody. 1966. On the relation between habitat selection and species diversity. Amer. Natur. 100:319-332.

Murdoch, W. W., F. C. Evans, and C. H. Peterson. 1972. Diversity and pattern in plants and insects. Ecology 53:819-829.

Olson, J. S. 1963. Energy storage and the balance of producers and decomposers in ecol- ogical systems. Ecology 44:322-331.

Orians, G. H. 1969. The number of bird species in some tropical forests. Ecology 50: 783-801. Ovington, J. D. 1965. Organic production, turnover, and mineral cycling in woodlands. Biol.

Rev. 40:295-336. Pianka, E. R. 1966. Latitudinal gradients in species diversity: a review of concepts. Amer.

Natur. 100:33-46. Redmann, R. E. 1972. Plant communities and soils of an eastern North Dakota prairie.

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comparison. Evolution 29:313-324. Sanchez, P. A., and S. W. Buol. 1975. Soils of the tropics and the world food crisis. Science

188:598-603. Smeins, F. E., and I). E. Olsen. 1970. Species composition and production of a native

northwestern Minnesota tall grass prairie. Amer. Midland Natur. 84:398-410. Wali, M. K., G. W. Dewald, and S. M. Jalal. 1973. Ecological aspects of some bluestem com-

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LETTERS To THE EDITORS 381

WV'att, A. 8. 1947. Pattern and process in the plant community. J. Ecol. 35: 1--22.

Willi,,amsoi., G. B. 1 975. Pattern and seral composition in an old-growth beech-maple forest. Ecology 56:727-731.

ROBERT F. RICKLEFS

DEPARTMENT OF BIOLOGY

UNIVERSITY OF PENNSYLVANIA

PHILADELPHIA, PENNSYLVANIA 19174 February 3, 1976

A MORE FUNCTIONAL RESPONSE TO PREDATOR-PREY STABILITY

Oaten and Murdoch (1975) investigated the effect of the predator functional response upon the stability of a predator-prey system, basing their conclusions on the equations

dH _dP - = aH - Pf(H), - = -cP + dPf(H), (1) dt cdt

in which H and P are the densities of the prey and predator populations; a, b, c, and d are constants; and f (H) is the consumption rate of prey by an individual predator (the "functional response"). In an extension to multiple prey species, the assumption that (I/P) dP/dt is independent of P is maintained.

When the assumption that dP/dt is proportional to P is relaxed, some inter- esting new phenomena appear. This is particularly true when the spatial dis- tributions of populations are considered, for then different notions of stability become essential. Consider the more general set of equations

dH dP - - aH - Pf(H), -= -PC(P) + dPf(H), (2) dt dt

in which the function C(P) replaces the constant c; system (1) is thus clearly a special case of (2). This model retains all of the salient features of (1) except linearity; that the conclusions which derive from (1) are not robust in the con- text of (2) is evidence that those conclusions may be artificial.

Equilibria for (2) satisfy P = aH/f(H),

where C[aH/f (H)] = df(H).

Stability is governed by the eigenvalues of the matrix

f-PH[f (H)/H]' -f (H) l dPf'(H) -PC'(P)S

at P = P, H = H. When C(P) is constant, as in Oaten and Murdoch (1975), a necessary condition for stability is that the functional response be "stabilizing"