Robert E. Ricklefs

Community Diversity: Relative Roles of Local and Regional Processes

The species richness (diversity) of local plant and animal assemblages—biological communities—balances regional processes of species formation and geographic dispersal, which add species to communities, against processes of predation, competitive exclusion, adaptation, and stochastic variation, which may promote local extinction. During the past three decades, ecologists have sought to explain differences in local diversity by the influence of the physical environment on local interactions among species, interactions that are generally believed to limit the number of coexisting species. But diversity of the biological community often fails to converge under similar physical conditions, and local diversity bears a demonstrable dependence upon regional diversity. These observations suggest that regional and historical processes, as well as unique events and circumstances, profoundly influence local community structure. Ecologists must broaden their concepts of community processes and incorporate data from systematics, biogeography, and paleontology into analyses of ecological patterns and tests of community theory.

An analysis of nesting mortality in birds

Ricklefs, Robert E. An Analysis of Nesting Mortality in Birds. Smithsonian Contributions to Zoology, 9:1-48. 1969.—This study was initiated to evaluate nesting mortality of birds as a feature of the environment and as a selective force in the evolution of reproductive strategies. Representative nesting-success data from the literature for most groups of birds were transformed into daily mortality rates to eliminate differences among species in the length of the nest cycle. These data are presented by taxonomic groupings and for passerines by geographical region and nest construction and placement. The strength and pattern of various mortality factors are described in detail. Predation, starvation, desertion, hatching failure, and adverse weather are the most prevalent factors, but nestsite competition, brood parasitism, and arthropod infestation may be important in some species. It is demonstrated that the various mortality factors can be identified by characteristic patterns of nesting losses involving differences in mortality rates between the egg and nestling periods and the within-nest component of mortality rates. Among Temperate Zone passerines, field-nesting and marsh-nesting species have the highest mortality rates while those species nesting in trees, especially in cavities, enjoy higher success. Starvation is prevalent in marsh and field species but desertion is more restricted to tree-nesting species. In general, arctic species have lower mortality rates and tropical species higher rates, although there is a similar gradient from arid to humid regions within the tropics. The relative abundance of a species is related directly to its mortality rate in arctic regions, but is not in temperate and tropical regions. Birds of prey generally have low mortality rates although starvation is often a major factor. Nesting losses in seabirds are caused primarily by crowded conditions in colonies and loss of eggs due to inadequate nest construction. Chick deaths come about primarily through their wandering away from parental care which is most common in the semiprecocial Charadriiformes. Precocial shorebirds and water birds enjoy higher egg success than ground-nesting passerines but game birds exhibit similar mortality rates. Little is known of the survival of precocial chicks after hatching except that mortality rates may be initially quite high and decrease with age. The fate of altricial birds after fledging is also poorly documented. It is postulated that interspecific differences in mortality rates are determined by evolutionarily acceptable levels of adult risk to lower mortality rates of offspring through parental care, adult adaptations of morphology and behavior for foraging which result in limitations on nesting adaptations, environmental unpredictability which reduces the effectiveness of adaptations, and—most import—the diversity of predators to which a species must adapt. Official publication date is handstamped in a limited number of initial copies and is recorded in the Institutions annual report, Smithsonian Year. UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON : 1969 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 Price 55 cents (paper cover)

The physiology/life-history nexus

The rate of reproduction, age at maturity and longevity vary widely among species. Most of this life-history variation falls on a slow-fast continuum, with low reproductive rate, slow development and long life span at one end and the opposite traits at the other end. The absence of alternative combinations of these variables implies constraint on the diversification of life histories, but the nature of this constraint remains elusive. Here, we argue that individual and adaptive responses to different environments are limited by physiological mechanisms. Although energy and materials allocations are important results of physiological tradeoffs, endocrine control mechanisms can produce incompatible physiological states that restrict life histories to a single dominant axis of variation. To approach the problem of life-history variation properly, studies should integrate behavior and physiology within the environmental and demographic contexts of selection.

Adaptation and Diversification on Islands

Charles Darwins travels on HMS Beagle taught him that islands are an important source of evidence for evolution. Because many islands are young and have relatively few species, evolutionary adaptation and species proliferation are obvious and easy to study. In addition, the geographical isolation of many islands has allowed evolution to take its own course, free of influence from other areas, resulting in unusual faunas and floras, often unlike those found anywhere else. For these reasons, island research provides valuable insights into speciation and adaptive radiation, and into the relative importance of contingency and determinism in evolutionary diversification.

Early bursts of body size and shape evolution are rare in comparative data.

George Gaylord Simpson famously postulated that much of lifes diversity originated as adaptive radiations—more or less simultaneous divergences of numerous lines from a single ancestral adaptive type. However, identifying adaptive radiations has proven difficult due to a lack of broad‐scale comparative datasets. Here, we use phylogenetic comparative data on body size and shape in a diversity of animal clades to test a key model of adaptive radiation, in which initially rapid morphological evolution is followed by relative stasis. We compared the fit of this model to both single selective peak and random walk models. We found little support for the early‐burst model of adaptive radiation, whereas both other models, particularly that of selective peaks, were commonly supported. In addition, we found that the net rate of morphological evolution varied inversely with clade age. The youngest clades appear to evolve most rapidly because long‐term change typically does not attain the amount of divergence predicted from rates measured over short time scales. Across our entire analysis, the dominant pattern was one of constraints shaping evolution continually through time rather than rapid evolution followed by stasis. We suggest that the classical model of adaptive radiation, where morphological evolution is initially rapid and slows through time, may be rare in comparative data.

Environmental Heterogeneity and Plant Species Diversity: A Hypothesis

Barnes, H., and LI. Barnes. 1965. Egg size, nauplius size, and their variation with local, geographical, and specific factors in some common cirripedes. J. Anim. Ecol. 34: 39 1-402. Batham, E. J. 1946. Pollicipes spinosus Quoy and Gaimard. II. Embryonic and larval development. Trans. Roy. Soc. New Zeal. 75:405-418. Blake, J. A. 1969. Reproduction and larval development of Polydora from northern New England (Polychaeta: Spionidae). Ophelia 7:1-63. 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 intermedius Holthius, 1949 (Decapoda, Palaemonidae) reared in the laboratory. Crustaceans 26:89-103. 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 invertebrates. Amer. Natur. 108:874-878. Vance, R. R. 1973. On reproductive strategies in marine benthic invertebrates. Amer. 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 suitability for farming. FAO Fisheries Rep. 57:77-84.

Disintegration of the Ecological Community

In this essay, I argue that the seemingly indestructible concept of the community as a local, interacting assemblage of species has hindered progress toward understanding species richness at local to regional scales. I suggest that the distributions of species within a region reveal more about the processes that generate diversity patterns than does the co‐occurrence of species at any given point. The local community is an epiphenomenon that has relatively little explanatory power in ecology and evolutionary biology. Local coexistence cannot provide insight into the ecogeographic distributions of species within a region, from which local assemblages of species derive, nor can local communities be used to test hypotheses concerning the origin, maintenance, and regulation of species richness, either locally or regionally. Ecologists are moving toward a community concept based on interactions between populations over a continuum of spatial and temporal scales within entire regions, including the population and evolutionary processes that produce new species.

Host specificity of Lepidoptera in tropical and temperate forests

For numerous taxa, species richness is much higher in tropical than in temperate zone habitats. A major challenge in community ecology and evolutionary biogeography is to reveal the mechanisms underlying these differences. For herbivorous insects, one such mechanism leading to an increased number of species in a given locale could be increased ecological specialization, resulting in a greater proportion of insect species occupying narrow niches within a community. We tested this hypothesis by comparing host specialization in larval Lepidoptera (moths and butterflies) at eight different New World forest sites ranging in latitude from 15° S to 55° N. Here we show that larval diets of tropical Lepidoptera are more specialized than those of their temperate forest counterparts: tropical species on average feed on fewer plant species, genera and families than do temperate caterpillars. This result holds true whether calculated per lepidopteran family or for a caterpillar assemblage as a whole. As a result, there is greater turnover in caterpillar species composition (greater β diversity) between tree species in tropical faunas than in temperate faunas. We suggest that greater specialization in tropical faunas is the result of differences in trophic interactions; for example, there are more distinct plant secondary chemical profiles from one tree species to the next in tropical forests than in temperate forests as well as more diverse and chronic pressures from natural enemy communities.

Diversification and host switching in avian malaria parasites

The switching of parasitic organisms to novel hosts, in which they may cause the emergence of new diseases, is of great concern to human health and the management of wild and domesticated populations of animals. We used a phylogenetic approach to develop a better statistical assessment of host switching in a large sample of vector–borne malaria parasites of birds (Plasmodium and Haemoproteus) over their history of parasite-host relations. Even with sparse sampling, the number of parasite lineages was almost equal to the number of avian hosts. We found that strongly supported sister lineages of parasites, averaging 1.2% sequence divergence, exhibited highly significant host and geographical fidelity. Event–based matching of host and parasite phylogenetic trees revealed significant cospeciation. However, the accumulated effects of host switching and long distance dispersal cause these signals to disappear before 4% sequence divergence is achieved. Mitochondrial DNA nucleotide substitution appears to occur about three times faster in hosts than in parasites, contrary to findings on other parasite–host systems. Using this mutual calibration, the phylogenies of the parasites and their hosts appear to be similar in age, suggesting that avian malaria parasites diversified along with their modern avian hosts. Although host switching has been a prominent feature over the evolutionary history of avian malaria parasites, it is infrequent and unpredictable on time scales germane to public health and wildlife management.

Global patterns of tree species richness in moist forests: energy-diversity theory does not account for variation in species richness

Energy-diversity theory has gained currency as an explanation for global patterns of species richness. We examine the suggestion of Currie and Paquin (1987; Nature 329: 326-327) that variation in evapotranspiration - a function of moisture availability and temperature that is directly related to plant production - predicts tree species richness in global comparisons. We present contrary evidence: the number of tree species on 26 large (17-7401 kmz) areas of moist temperate forest show continental differences unrelated to geographical patterns in evapotranspiration. Tree species richness of 128 samples of ca 1ha within moist-forest biomes of the world reveal patterns of variation amone continents. and with latitude. that likewise cannot be attributed to eeoeraohi