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

Parasite communities are arranged into hierarchical levels of organization, covering various spatial and temporal scales. These range from all parasites within an individual host to all parasites exploiting a host species across its geographic range. This arrangement provides an opportunity for the study of patterns and structuring processes operating at different scales. Across the parasite faunas of various host species, several species-area relationships have been published, emphasizing the key role of factors such as host size or host geographical range in determining parasite species richness. When corrections are made for unequal sampling effort or phylogenetic influences, however, the strength of these relationships is greatly reduced, casting a doubt over their validity. Component parasite communities, or the parasites found in a host population, are subsets of the parasite fauna of the host species. They often form saturated communities, such that their richness is not always a reflection of that of the entire parasite fauna. The species richness of component communities is instead influenced by the local availability of parasite species and their probability of colonization. At the lowest level, infracommunities in individual hosts are subsets of the species occurring in the component community. Generally, their structure does not differ from that expected from a random assembly of available species, although comparisons with precise null models are still few. Overall studies of parasite communities suggest that the action of processes determining species richness of parasite assemblages becomes less detectable as focus shifts from parasite faunas to infracommunities.

SPECIES RICHNESS OF PARASITE ASSEMBLAGES: Evolution and Patterns

The dictionary definition of a law is: "Generalized formulation based on a series of events or processes observed to recur regularly under certain conditions; a widely observable tendency". I argue that ecology has numerous laws in this sense of the word, in the form of widespread, repeatable patterns in nature, but hardly any laws that are universally true. Typically, in other words, ecological patterns and the laws, rules and mechanisms that underpin them are contingent on the organisms involved, and their environment. This contingency is manageable at a relatively simple level of ecological organisation (for example the population dynamics of single and small numbers of species), and re-emerges also in a manageable form in large sets of species, over large spatial scales, or over long time periods, in the form of detail-free statistical patterns - recently called 'macroecology'. The contingency becomes overwhelmingly complicated at intermediate scales, characteristic of community ecology, where there are a large number of case histories, and very little other than weak, fuzzy generalisations. These arguments are illustrated by focusing on examples of typical studies in community ecology, and by way of contrast, on the macroecological relationship that emerges between local species richness and the size of the regional pool of species. The emergent pattern illustrated by local vs regional richness plots is extremely simple, despite the vast number of contingent processes and interactions involved in its generation. To discover general patterns, laws and rules in nature, ecology may need to pay less attention to the 'middle ground' of community ecology, relying less on reductionism and experimental manipulation, but increasing research efforts into macroecology.

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Are there general laws in ecology

Parasitism is the most common animal lifestyle, yet food webs rarely include parasites. The few earlier studies have indicated that including parasites leads to obvious increases in species richness, number of links, and food chain length. A less obvious result was that adding parasites slightly reduced connectance, a key metric considered to affect food web stability. However, reported reductions in connectance after the addition of parasites resulted from an inappropriate calculation. Two alternative corrective approaches applied to four published studies yield an opposite result: parasites increase connectance, sometimes dramatically. In addition, we find that parasites can greatly affect other food web statistics, such as nestedness (asymmetry of interactions), chain length, and linkage density. Furthermore, whereas most food webs find that top trophic levels are least vulnerable to natural enemies, the inclusion of parasites revealed that mid-trophic levels, not low trophic levels, suffered the highest vulnerability to natural enemies. These results show that food webs are very incomplete without parasites. Most notably, recognition of parasite links may have important consequences for ecosystem stability because they can increase connectance and nestedness.

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Parasites dominate food web links

Parasitism is one of the most successful modes of life displayed by living organisms, as measured by how often it evolved and how many parasitic species are presently in existence. Studying the diversity of parasites is particularly relevant because sympatric diversification may be important in some parasite taxa, and because of the opportunity for independent tests of evolutionary hypotheses in the many separate lineages in which parasitism evolved. Our incomplete knowledge of existing parasite species--the result of a range of phenomena that includes inadequate sampling effort or the lumping of different cryptic species under one name--is not always a major obstacle for the study of parasite diversity. Patterns in the diversity of parasites may be associated with either host or parasite characteristics. The distribution of parasite diversity among host taxa does not simply reflect the species diversity of the host taxa themselves; life history and ecological traits of hosts appear to play important roles. These may determine the likelihood that hosts are colonized by parasite species over evolutionary time. It is not yet clear whether some host traits also favor intrahost speciation and diversification of parasites, and the formation of new parasite species. Certain features of parasites may also be associated with speciation and diversification. Only parasite body size has received much attention; the patterns observed are not greatly different from those of free-living species, with small-bodied parasite taxa being more speciose than related large-bodied taxa. Epidemiological parameters such as the basic reproductive rate of parasites, or R0, can also generate predictions regarding the distribution or evolution of parasite diversity. For instance, parasite taxa characterized by high R0 values may be more speciose than related taxa with lower values of R0; such predictions remain untested. Large-scale biogeographical patterns of diversity have only been well studied for metazoan parasites of marine fish; for these parasites, latitudinal patterns can be explained by effects of temperature on speciation rates and epidemiological variables, though other causes are possible. The emphasis for future research must shift from pattern description to the elucidation of the processes responsible for the structure and diversity of parasite faunas. A better integration of ecological and historical (or phylogenetic) approaches to the study of parasite diversity should make this objective possible.

The diversity of parasites.

1 Patterns and processes in helminth parasite communities: an overview.- 2 Host populations as resources defining parasite community organization.- 3 Spatial scale and the processes structuring a guild of larval trematode parasites.- 4 Guild structure of larval trematodes in molluscan hosts: prevalence, dominance and significance in competition.- 5 Helminth communities in marine fishes.- 6 Helminth communities in freshwater fish: structured communities of stochastic assemblages?.- 7 Helminth communities of amphibians and reptiles: comparative approaches to understanding patterns and processes.- 8 Helminth communities in avian hosts: determinants of pattern.- 9 Helminth community of mammalian hosts: concepts at the infracommunity, component and compound community levels.- 10 Models for multi-species parasite-host communities.- 11 Free-living communities and alimentary tract helminths: hypotheses and pattern analyses.- 12 Concluding remarks.

Parasite communities: patterns and processes.

We examine patterns of community richness among intestinal parasitic helminth communities in fishes, herptiles, birds and mammals with respect to the comparative number of component species in a host population. We show that terrestrial hosts have, on average, fewer component species than aquatic hosts. We also show that the mean number of component species in aquatic hosts increases from fishes through herptiles to birds before declining slightly in mammals. For terrestrial hosts, the mean number of component species increases from herptiles, through birds, reaching a maximum in mammals. We conclude that: (i) habitat of the host is more important in determining community richness than is host phylogeny; (ii) the phenomenon of ‘host capture’ may be largely responsible for increased species richness in some host groups; (iii) aquatic birds harbour the richest intestinal helminth communities; and (iv) as we interpret them, our data refute the time hypothesis, which would predict that fishes as the oldest lifestyle should have the richest helminth communities.

Ecological versus phylogenetic determinants of helminth parasite community richness

Recently, some authors (Kennedy, 1981; Price & Clancy, 1983) have argued that there are fundamental differences between the communities of helminths in fish and bird hosts. Such differences are foreshadowed by the work of Dogiel (1964) and are apparent from survey data (e.g. Threlfall, 1967; Bakke, 1972; Hair & Holmes, 1975 on birds, and compare Chubb, 1963; Mishra & Chubb, 1969; Wootten, 1973; Ingham & Dronen, 1980 on fish). Questions still remain, however, as to whether the distinctions are truly justified and whether the differences are really fundamental. In this paper, we address these questions by examining helminth diversity in a series of hosts. We then discuss and provide explanations for the observed differences.

Patterns in helminth communities: why are birds and fish different?

Examples of the apparently stochastic nature of freshwater fish helminth communities illustrating the erratic and unpredictable occurrence and distribution of many species are provided for six species of fish from several localities throughout Britain. By focussing on parasite colonization strategies two categories of helminths are recognized: autogenic species which mature in fish and allogenic species which mature in vertebrates other than fish and have a greater colonization potential and ability. Three groups of fish are distinguished: salmonids, in which helminth communities are generally dominated by autogenic species which are also responsible for most of the similarity within and between localities; cyprinids, in which they are dominated by allogenic species which are also responsible for most of the similarity within and between localities; and anguillids, whose helminth communities exhibit intermediate features with neither category consistently dominating nor providing a clear pattern of similarity. Recognition and appreciation of the different colonization strategies of autogenic and allogenic helminths in respect of host vagility and ability to cross land or sea barriers and break down habitat isolation, and their period of residence in a locality, whether transient or permanent, provides an understanding of, and explanation for, the observed patchy spatial distribution of many helminths. Comparison with other parts of the world indicates that colonization is a major determinant of helminth community structure.

Patterns in helminth communities in freshwater fish in Great Britain: alternative strategies for colonization

Problems of pattern and scale are considered in relation to helminth communities of freshwater fish by examining them at different hierarchical taxonomic and spatial scales, with a view to seeking generalizations of heuristic value, assessing the importance of phylogenetic and ecological determinants of community structure and improving understanding of unpredictable communities. Initially, communities were analysed at the level of salmonid genera, focusing on Oncorhynchus, in its heartland in Canada: then in O. mykiss throughout its global range and finally in individual localities to which it has been introduced in Britain. In the heartland, communities are dominated by salmonid specialist helminths, forming a phylogenetic element: the minority ecological element comprises broad generalists and non-salmonid specialists. Most species except generic specialists are shared between host genera. As the distance to which O. mykiss was translocated from its heartland increases, so generic specialists disappear first and then salmonid specialists decline. The community is thus increasingly composed of generalists and it also becomes increasingly poor. Helminths may be acquired from native salmonids and/or unrelated hosts, depending on availability. This same pattern is paralleled in individual localities in a restricted region: the phylogenetic element reflects the native salmonid species present and the ecological element the presence of other genera of fish, i.e. a supply-side situation. The change of scale in analysis has thus enabled the recognition of generalizations and patterns of heuristic value and improved the understanding of unpredictable communities by interpreting local variation as ecological ‘noise’ that often obscures fundamental patterns. In this and other taxa of fish, phylogenetic elements dominate helminth communities in the heartlands, but ecological elements dominate as the host increasingly becomes a stranger in a strange land.

Albert O. Bush

Papers

Parasite communities: patterns and processes.

Ecological versus phylogenetic determinants of helminth parasite community richness

Patterns in helminth communities: why are birds and fish different?

Patterns in helminth communities in freshwater fish in Great Britain: alternative strategies for colonization

The relationship between pattern and scale in parasite communities: a stranger in a strange land.