Preface to the Princeton Landmarks in Biology Edition vii Preface xi Symbols Used xiii 1. The Importance of Islands 3 2. Area and Number of Speicies 8 3. Further Explanations of the Area-Diversity Pattern 19 4. The Strategy of Colonization 68 5. Invasibility and the Variable Niche 94 6. Stepping Stones and Biotic Exchange 123 7. Evolutionary Changes Following Colonization 145 8. Prospect 181 Glossary 185 References 193 Index 201

The Theory of Island Biogeography

■ Abstract The literature on effects of habitat fragmentation on biodiversity is huge. It is also very diverse, with different authors measuring fragmentation in different ways and, as a consequence, drawing different conclusions regarding both the magnitude and direction of its effects. Habitat fragmentation is usually defined as a landscape-scale process involving both habitat loss and the breaking apart of habitat. Results of empirical studies of habitat fragmentation are often difficult to interpret because (a) many researchers measure fragmentation at the patch scale, not the landscape scale and (b) most researchers measure fragmentation in ways that do not distinguish between habitat loss and habitat fragmentation per se, i.e., the breaking apart of habitat after controlling for habitat loss. Empirical studies to date suggest that habitat loss has large, consistently negative effects on biodiversity. Habitat fragmentation per se has much weaker effects on biodiversity that are at least as likely to be positive as negative. Therefore, to correctly interpret the influence of habitat fragmentation on biodiversity, the effects of these two components of fragmentation must be measured independently. More studies of the independent effects of habitat loss and fragmentation per se are needed to determine the factors that lead to positive versus negative effects of fragmentation per se. I suggest that the term “fragmentation” should be reserved for the breaking apart of habitat, independent of habitat loss.

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Effects of Habitat Fragmentation on Biodiversity

I. the origin of species by means of natural selection

Most empirical and theoretical studies of resource use and population dynamics treat conspecific individuals as ecologically equivalent. This simplification is only justified if interindividual niche variation is rare, weak, or has a trivial effect on ecological processes. This article reviews the incidence, degree, causes, and implications of individual-level niche variation to challenge these simplifications. Evidence for individual specialization is available for 93 species dis- tributed across a broad range of taxonomic groups. Although few studies have quantified the degree to which individuals are specialized relative to their population, between-individual variation can some- times comprise the majority of the population's niche width. The degree of individual specialization varies widely among species and among populations, reflecting a diverse array of physiological, be- havioral, and ecological mechanisms that can generate intrapopu- lation variation. Finally, individual specialization has potentially im- portant ecological, evolutionary, and conservation implications. Theory suggests that niche variation facilitates frequency-dependent interactions that can profoundly affect the population's stability, the amount of intraspecific competition, fitness-function shapes, and the population's capacity to diversify and speciate rapidly. Our collection of case studies suggests that individual specialization is a widespread but underappreciated phenomenon that poses many important but unanswered questions.

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The Ecology of Individuals: Incidence and Implications of Individual Specialization

In the Hamadryas baboon, males are substantially larger than females. A troop of baboons is subdivided into a number of ‘one-male groups’, consisting of one adult male and one or more females with their young. The male prevents any of ‘his’ females from moving too far from him. Kummer (1971) performed the following experiment. Two males, A and B, previously unknown to each other, were placed in a large enclosure. Male A was free to move about the enclosure, but male B was shut in a small cage, from which he could observe A but not interfere. A female, unknown to both males, was then placed in the enclosure. Within 20 minutes male A had persuaded the female to accept his ownership. Male B was then released into the open enclosure. Instead of challenging male A , B avoided any contact, accepting A’s ownership.

Evolution and the Theory of Games

Numerous studies reveal strong, positive skews in long-term breeding performance among free-living animals, yet few studies explore the mechanisms underlying such variation. We examine the results of a 12-yr study of a population of Western Gulls, Larus occidentalis. Of 112 pairs for which we have either long-term (≥5 yr) or lifetime reproductive output, 44% bred for only a single year, and an additional 25% bred for only 2–3 yr. A few pairs bred successfully for 6 –12 yr and showed higher average clutch size, hatching success, and fledging success within any single season than did less successful breeders. The principal trait influencing both survival and reproduction was individual diet, which consisted of a mix of human refuse and fish. A strong, positive relationship existed among the amount of fish taken, breeding life-span, and reproductive performance. Birds with short life-spans took little or no fish on an annual basis. Birds with breeding life-spans >10 yr and high breeding success took >60% fis...

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Long-term reproductive output in western gulls: consequences of alternate tactics in diet choice

The breeding biology of the Herring Gull (Larus argentatus) was studied on Great Island, Newfoundland for part of the 1976 breeding season, and for two succeeding complete breeding seasons (1977 and 1978). Gulls nesting in three different habitats (designated puffin, rocky, and mead- ow) were compared with regard to several reproductive parameters. More pairs were found to nest in rocky habitat, and fewer pairs were found to nest in puffin habitat, than would be expected from a random distribution. In 1977, a year of low food availability, pairs in rocky habitat laid and hatched eggs significantly earlier than in the other two habitats. In 1978, when food was more abundant, gulls in rocky habitat laid heavier eggs than their counterparts in the other habitats. In both 1977 and 1978, chicks from rocky habitat grew at the fastest rate and were heavier than chicks in the other habitats. Finally, results of an experiment to test the egg-production capacity of females demonstrated that female gulls in rocky habitat were capable of producing significantly more eggs than their counterparts in meadow and puffin habitats. Despite the apparently better condition of gulls in rocky habitat, however, Herring Gull pairs in puffin habitat fledged as many chicks per nest as pairs in rocky habitat in 1977 and 1978, and even fledged more chicks in 1976. In all 3 yr of study, gulls in meadow habitat fledged the fewest chicks per nest. Within habitats, chick survival was strongly correlated with early laying dates and high rates of growth. However, there was also a large residual effect which was probably due to differences in the habitats. These differences are probably due to crowding and socially induced mortality in rocky habitat, and to the presence of predatory Great Black-backed Gulls in meadow habitat. The results support the theoretical models for habitat selection and dispersion developed by Fretwell and Lucas (1970), which suggest that increasing density in a preferred habitat can create a situation whereby fitness may actually be greater in a less-preferred habitat.

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Habitat selection and its effect on reproductive output in the herring gull in newfoundland

There has been considerable debate in the study of hybrid zones as to whether hybrids may be superior to parental types within the area of contact (bounded hybrid superiority). In birds, naturally occurring hybridization is relatively common, and hybridization within this group always involves mate choice. If hybrids are superior, females choosing heterospecific mates should be expected to show higher fitness under the conditions prevalent in the hybrid zone. Hybrid superiority under these circumstances would reduce reinforcement and thereby help to maintain the hybrid zone. To examine this issue, we studied reproductive performances of hybrids and parental species of gulls (Larus occidentalis and Larus glaucescens) at two colonies within a linear hybrid zone along the west coast of the United States. This hybrid zone contains predominantly gulls of intermediate phenotype. Previous studies indicated that hybrids were superior to one or both parental types, but provided no data on possible mechanisms that underlie this hybrid superiority. Using a hybrid index designed specifically for these species, we identified to phenotype more than 300 individuals associated with nests, including both individual males and females within 73 pairs in the central portion of the hybrid zone and 74 pairs in the northern portion of the hybrid zone. There was little evidence of assortative mating, and what little there was resulted solely because of pairings within intergrades. In the central hybrid zone, females paired with hybrid males produced larger clutches and hatched and fledged more chicks compared with females paired to western gull males. This was a result of heavy predation on eggs in sand habitat, where male western gulls established territories. In contrast, many hybrid males established territories in vegetated cover that was less vulnerable to predation. In the northern part of the hybrid zone, clutch size did not differ among pair categories, however, there were differences in hatching and fledging success, with females paired to hybrid males showing better success compared to females paired to glaucous-winged gull males. Hybrids showed better hatching and fledging success in the north because hybrids are more like western gulls than glaucous-winged gulls in foraging behavior, taking a higher percentage of fish in their diet, which enhances chick growth and survival. This is believed to be the first documentation of bounded hybrid superiority that delineates the mechanisms that underlie hybrid superiority.

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Bounded hybrid superiority in an avian hybrid zone: effects of mate, diet, and habitat choice

Spite and Altruism in Gulls

We compared the density and spatial distribution of four small mammal species (Microtus ochrogaster, Peromyscus maniculatus, Sigmodon hispidus, and P. leucopus) along with general measures of an old field plant community across two successional phases (1984–1986 and 1994–1996) of an experimental study of fragmentation in eastern Kansas. During the early phase the plant community was characterized by little spatial or temporal variance across patch size, consistent with spatially neutral models of succession. In contrast, there was a strong, species-specific effect of patch size on small mammal species distribution and abundance. The lack of variance in vegetation structure across patch size during the early seres suggests that small mammal distributions were responding in large part to features of the system other than variance in vegetation structure and composition across patch size. As succession proceeded, the colonization of the system by woody plant species precipitated a series of patch size effect...

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Raymond Pierotti

Papers

Long-term reproductive output in western gulls: consequences of alternate tactics in diet choice

Habitat selection and its effect on reproductive output in the herring gull in newfoundland

Bounded hybrid superiority in an avian hybrid zone: effects of mate, diet, and habitat choice

Spite and Altruism in Gulls

The interaction of habitat fragmentation, plant, and small mammal succession in an old field