Demographic and genetic contributions from nonspecific immigrants tend to reduce ex- tinction rates of insular populations. The MacArthur-Wilson model of island biogeography is modified to provide for this effect of immigration on extinction, which we call the rescue effect. This new model predicts that when immigration rates are high relative to extinction rates, turnover rate is directly related to the distance between an island and the source of colonizing species. A field study of the distribution of arthropods among isolated plants supports the model.

Turnover Rates in Insular Biogeography: Effect of Immigration on Extinction

Regional differences in species number have puzzled naturalists ince the early 1800's, and explanations account for a large part of modern ecological research. Two venerable observations form the cornerstone of our knowledge on the subject: The number of species within a taxonomic group tends to increase with decreasing latitude (see Fischer 1960; Pianka 1966); and the number of species within a taxonomic group tends to increase with increasing area (see Preston 1960, 1962; Williams 1964; MacArthur and Wilson 1967; Simberloff 1972). Despite early research on the latter trend (the species-area relationship), ecologists have studied it intensely only in the last 50 yr. The relationship was originally envisioned as an empirical tool and used in three principle ways: (1) to determine optimal sample size and sample number, (2) to determine the minimum area of a \"community,\" and (3) to predict the number of species in areas larger than those sampled. All three uses are discussed by Kilburn (1966). More recently interest in the species-area relationship has focused on mechanistic explanations, its precise mathematical descriptions, and interpretations of parameters derived from these mathematical descriptions. Williams (1964) and Preston (1960, 1962) have proposed that the exponential and power function models (\"exponential model\" throughout his paper also refers to the species/log area transformation, and \"power function\" also refers to the log species/log area transformation) of the species-area relationship result from the way in which individuals are distributed among species. Williams' (1964) exponential model, which emphasizes habitat heterogeneity, was considered important by many plant ecologists but is now largely ignored. Preston's (1960, 1962) power function model was based on the assumption of a dynamic equilibrium of species exchanges between islands in an archipelago. This assumption led to the equation of the power function model with the idea of a dynamic equilibrium as expounded by MacArthur and Wilson (1963, 1967), such that an adequate fit of this model to observed species numbers has been viewed as support of the equilibrium hypothesis (Grant 1970; Diamond 1973; Simpson 1974). The interplay of the equilibrium hypothesis and the power function model of the species-area relationship has led to interpretation of the slope and intercept of the power function model exclusively in the context of the equilibrium hypothesis. In particular, specific values of the slope of the power function are often construed to * Order of authorship determined by the toss of a coin. t Present address: Department of Biology, University of South Florida, Tampa, Florida 33620. Am. Nat. 1979. Vol. 113, pp. 791-833. c) 1979 by The University of Chicago. 0003-0147/79/1306-0002$03.26

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The statistics and biology of the species-area relationship

119 Small invertebrates are functionally important in many terrestrial and aquatic ecosystems (Wilson 1992, Freckman et al. 1997, Palmer et al. 1997, Postel and Carpenter 1997). In freshwater sediments, benthic invertebrates are diverse and abundant, but they are often patchily distributed and relatively difficult to sample, especially when they live in deep subsurface sediments. Thus, the species richness and functional importance of freshwater benthic invertebrates generally go unnoticed until unexpected changes occur in ecosystems. Unanticipated changes in freshwater ecosystems are often due to alterations in the complex connections among sediment-dwelling species and associated food webs (e.g., Goedkoop and Johnson 1996, Lodge et al. 1998b, Stockley et al. 1998) or to disturbances, such as floods or drought (e.g., Covich 1993, Power 1995, Johnson et al. 1998), that alter the species composition of the benthos. In addition, benthic species can themselves constitute a disturbance, such as when they transmit diseases. For example, certain benthic invertebrate species (e.g., Tubifex tubifex) serve as parasite-transmitting vectors; if these invertebrates increase in abundance in stream sediments, they may spread a lethal disease to trout, causing trout populations to decline (Brinkhurst 1997). Fish kills may also occur because of increased accumulation of nutrients, which cause formation of toxic algal blooms, deoxygenation of deeper, density-stratified waters, and high concentrations of ammonia or hydrogen sulfide (Covich 1993). The bottom muds of lakes and streams may at first glance appear to be uniform and, therefore, unlikely habitats for high biodiversity. However, physical, chemical, and biological processes create significant horizontal and vertical heterogeneities in the substrata (Figure 1) that provide a physical template for distinct niches (Hutchinson 1993). These sedimentary processes include changes in direction and rates of flows, differential deposition of sediment grain sizes and dead organisms, growth and death of roots, burrowing and sediment reworking, and fecal production by benthic consumers. Microhabitats are also created by chemical gradients and microzonation in concentrations of dissolved oxygen, hydrogen sulfide, ammonia, phosphorus, and other critical chemicals (Groffman and Bohlen 1999). Colwell (1998) emphasizes that such “biocomplexity” of habitats and biological relationships is an important aspect of biodiversity. Bioturbation and other biotic interactions create extensive biocomplexity in freshwater sediments (Charbonneau and Hare 1998). These biocomplexities must be better understood if clean drinking water and recreational uses of fresh waters are to be maintained. Science-based policies require an ecosystem perspective on the multiple roles of many diverse benthic species. Previous studies have often dealt with the “goods” produced by benthic species, such as the quantity of prey items consumed by fish. These goods are clearly important components of food webs, but how their functional relationships respond to changes in species composition are also important. In this article, we highlight examples of how some species have a disproportionately large impact on food-web dynamics and how particular species provide essential ecosystem services. These ecosystem functions include sediment mixing, nutrient cycling, and energy flow through food webs. The Role of Benthic Invertebrate Species in Freshwater Ecosystems

https://www.jstor.org/stable/10.1525/bisi.1999.49.2.119

The Role of Benthic Invertebrate Species in Freshwater Ecosystems: Zoobenthic species influence energy flows and nutrient cycling

The distribution of native, chaparral-requiring bird species was determined for 37 isolated fragments of canyon habitat ranging in size from 0.4 to 104 hectares in coasta4 urban San Diego County, California The area of chaparral habitat and canyon age (time since isolation of the habitat fragment) explains most of the variation in the number of chaparral-requiring bird species. In additiolz, the distribution of native predators may influence species num- ber. There is statistical evidence that coyotes control thepop- ulations of smallerpredators such as foxes and domestic cats. The absence of coyotes may lead to higher levels of predation by a process of mesopredator release. The distance of canyons from otherpatches of chaparral habitat does not add significantly to the explained variance in chaparral- requiring species number-probably because of the virtual inability of most chaparral-requiring species to disperse through developed areas and nonscrub habitats. These re- sults and other lines of evidence suggest that chaparral- requiring birds in isolated canyons have very high rates of extinction, in part because of their low vagility. The best predictors of vulnerability of the individual species are their abundances (densities) in undisturbed habitat and their

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Reconstructed Dynamics of Rapid Extinctions of Chaparral‐Requiring Birds in Urban Habitat Islands

One of the ways that animals are thought to coexist is by differences in their body sizes. Very simply, different sized animals eat different sized foods or otherwise utilize different resources, until at some point there is enough nonoverlap to allow coexistence. This paper inspects the role of body size in competition, using MacArthur's (1972) theoretical framework. As well as describing patterns of convergence and divergence, the model developed here unifies two seemingly contradictory concepts that have become established over the past decade: (1) the concept that differences in body size promote a \"niche difference\" (Hutchinson 1959; Brown and Wilson 1956; Schoener 1965, 1967, 1970, 1974a and b; Grant 1968, 1972), and (2) the concept that differences in body size set up a competitive gradient whereby the larger can exclude the smaller (Brooks and Dodson 1965; Galbraith 1967; Hall et al. 1971; but see Dodson 1974 and Zaret 1975 for alternative view).

The Adequacy of Body Size as a Niche Difference

Holsinger, John R., and Glenn Longley. The Subterranean Amphipod Crustacean Fauna of an Artesian Well in Texas. Smithsonian Contributions to Zoology, number 308, 62 pages, 27 figures, 1980.—Subterranean amphipod crustaceans of the suborder Gammaridea have been sampled on a regular basis from the artesian well in San Marcos, Texas, since late 1973. Water in the artesian well comes from the extensive, subterranean Edwards Aquifer, which is associated with the Balcones Escarpment and Fault Zone of south-central Texas. The taxonomically diverse amphipod fauna of the well is believed to be one of the richest of its kind in the world and is composed of five families, six genera, and 10 species. One family, four genera, and six species are newly described herein. Four distinct phylogenetic lineages are represented in the amphipod fauna: crangonyctids, hadzioids, bogidielloids, and sebids. The zoogeographic and ecological implications of the artesian well amphipod fauna are profound. Approximately 80 jiercent of the species have obvious affinities with marine or brackish water species and are presumably derivatives of marine ancestors that colonized newly opened freshwater habitats during recession of epeiric seas in the Late Cretaceous. The high taxonomic diversity of the amphipod fauna is probably explained by: (1) exposure of south-central Texas to an extensive marine embayment during the Cretaceous period, (2) complex geological structure of the Balcones Fault Zone and large size of the Edwards Aquifer, and (3) ecological complexity of aquatic communities in the Edwards Aquifer. OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the Institution's annual report. Smithsonian Year. SERIES COVER DESIGN: The coral Montastrea cavernosa (Linnaeus). Library of Congress Cataloging in Publication Data Holsinger, John R. The subterranean amphipod crustacean fauna of an artesian well in Texas. (Smithsonian contributions to zoology ; no. 308) Bibliography: p. 1. Amphipod—Texas—San Marcos. 2. Artesian wells—Texas—San Marcos. 3. Crustacea—TexasSan Marcos. I. Longley, Glenn, joint author. II. Title. III. Series: Smithsonian Institution. Smithsonian contributions to zoology ; no. 308. QLl.S54no.S08 [QL444.M315] 591'.08s [595'.371] 79-18366

The subterranean amphipod crustacean fauna of an artesian well in Texas

Systematics, speciation, and distribution of the subterranean amphipod genus Stygonectes (Gammaridae)

The comparative data from studies of two ‘superfamily’ groups with large numbers of subterranean taxa, the exclusively freshwater Crangonyctoidea and the predominately marine Hadzioidea, support the hypothesis that distributional patterns and evolutionary processes of stygobiont amphipods are closely linked and that the former can be a useful indicator of the latter. Three major biogeographic patterns are indicated by the distribution of subterranean species in these groups, each apparently reflecting a particular mode of origin: (1) freshwater stygobionts (limnostygobionts) derived from epigean freshwater ancestors through colonizations probably influenced by adaptive shifts, or assisted by stream capture and spring failure; (2) freshwater stygobionts derived from marine/brackish water ancestors by stranding during regression of marine embayments; and (3) marine/brackish water stygobionts (thalassostygobionts) derived from epigean marine/brackish water ancestors through adaptive shifts possibly in concert with fluctuating sea levels.

Pattern and process in the biogeography of subterranean amphipods

The theory of island biogeography developed by MacArthur and Wilson (1963, 1967) is playing an increasingly important role in our understanding of habitats other than oceanic islands. MacArthur and Wilson pointed out that many habitats are discontinuous, that the environment for many species consists of habitable patches separated by uninhabitable areas, and that island biogeography theory may be relevant for these situations. But the theory of island biogeography is more than a convenient conceptual framework for considering a certain class of questions: the theory can be directly tested (Simberloff and Wilson, 1969); and in addition, different island-like communities show different patterns that help us better understand the forces that determine the distribution of species (Vuilleumier, 1970; Brown, 1971; Culver, 1970). In this paper we will examine the islandlike patterns of aquatic and terrestrial cave-limited species in the Greenbrier Valley karst area in West Virginia. Caves are limited to limestone in the study area (Davies, 1965), and the exposed limestone which bears caves in the study area has an island-like pattern (Fig. 1). Each continuous area of exposed limestone will itself contain many caves, and each of these caves could be viewed as an island within the continuous region of exposed limestone (Culver, 1970). However, there are good reasons for believing that an analogy between karst areas separated from other

Toward a predictive cave biogeography: the greenbrier valley as a case study.

Holsinger, John R. Systematics of the Subterranean Amphipod Genus Stygobromus (Crangonyctidae), Part II: Species of the Eastern United States. Smithsonian Contributions to Zoology, number 266, 144 pages, 77 figures, 1978.—The amphipod genus Stygobromus is widespread in the subterranean waters of the United States and is recorded from a variety of groundwater habitats. A revised diagnosis of the genus is given and a tentative division into three subgeneric groups is proposed. The recently proposed synonymy of Stygonectes and Apocrangonyx with Stygobromus is reaffirmed. Forty-eight described species are herein recognized from the Appalachian and Coastal Plain regions of the eastern United States, of which 30 are newly described, three are completely redescribed, and new systematic data are given for the remainder. In addition, six populations are provisionally recognized as distinct species but are not described because of lack of adequate material. On the basis of morphology and geographic distribution, 12 species groups are recognized in the eastern United States, eight of these being newly established herein. Ancestral stygobromids are now believed to have occurred earlier in the freshwater environment than originally suggested. Species of Stygobromus occupy a diversity of groundwater and groundwater-related habitats in the eastern United States, including primarily cave pools and streams, phreatic reservoirs and the hypotelminorheic media of seeps and small springs. Habitat selection is based on niche breadth and movement through shallow groundwater, both of which are believed to be significant factors in dispersal and geographic distribution. A number of species occur syntopically and/or sympatrically but most closely allied species have allopatric ranges. The possibility that several species are glacial relicts of the Pleistocene is raised. OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the Institution's annual report, Smithsonian Year. SERIES COVER DESIGN: The coral Montastrea cavernosa (Linnaeus). Library of Congress Cataloging in Publication Data Holsinger, John R. Systematics of the subterranean amphipod genus Stygobromus (Gammaridae). (Smithsonian contributions to zoology, no. 160, 266) Vol. 2 has title: Systematics of the Subterranean amphipod genus Stygobromus (Crangonyctidae) Bibliography: v. 1, p. 62-63; v. 2, p.

John R. Holsinger

Papers

The subterranean amphipod crustacean fauna of an artesian well in Texas

Systematics, speciation, and distribution of the subterranean amphipod genus Stygonectes (Gammaridae)

Pattern and process in the biogeography of subterranean amphipods

Toward a predictive cave biogeography: the greenbrier valley as a case study.

Systematics of the subterranean amphipod genus Stygobromus (Crangonyctidae) : Part II. Species of the eastern United States