Overview of Host-Parasite Ecology - Lou Gross Some terms General objectives and questions addressed Some people Basic concepts in parasitology Basics models of species interactions: Microparasites, macroparasites Survey of recent papers - topic coverage Papers to read: Marvier, M. A. 1996. Parasitic plant host interactions: plant performance and indirect effects on parasite feeding herbivores. Ecology 77: 1398-1409. Poulin, R. 1995. Phylogeny, ecology and the richness of parasite communities in vertebrates. Ecol. Monog. 65:283-295. References: Price, P. W. 1980. Evolutionary Biology of Parasites. Princeton Univ. Press. Esch, G. A. Bush and J. Aho. 1990. Parasite Communities: Patterns and Processes. Chapman and Hall. NY. Special Feature on Disease Ecology: Ecology 77: 989-1042 (June 1996) Terms: Ectoparasite - a parasite feeding on the internal tissues of a host, but having the greater poart of its body and reproductive structures on the surface. Endoparasite - a parasite living inside the body of its host Epiphyte - an organism attached to another with benefit to itself, but with no benefit to the organism it is attached to. Macroparasite - parasitic helminths Microparasite - parasitic bacteria, viruses, protozoa Parasitism: an internal or external relationship between two organisms which is detrimental to one (the host) and beneficial to the other (the parasite) Parasitoid - animal which is parasitic in one stage of the life history and subsequently free-living in the adult stage. (hymenopterans which lay eggs inside host and host is killed when eggs hatch and larvae grow) Pathogen - an organism (parasite, bareterium, virus, which causes disease. Social parasitism - a parasitic relationship between members of one species or with individuals of another species which involves exploiting aspecst of the hosts social behavior (e.g. nesting or hunting behavior - female cuckoo laying eggs in another bird's nest) General objectives of host-parasite ecology: 1. Understand the impacts of parasites on individual hosts physiology and behavior. 2. Understand how parasites may alter the structure of the population of their hosts in ecological and evolutionary time, and affect community structure as well. 3. Determine the differences between different parasitic types (e.g. micro and macroparasites) and the effects these may have on community structure. 4. Understand the population biology of parasites, both as a population on a given host as well as across hosts. Also how similar are parasite communities within similar hosts, and are there common processes involved? 5. Consider the evolutionary implications of parasites, including hypotheses for the prevalence of sex based upon parasites. What is a parasite? Some authors differentiate between pathogens and parasites, with pathogens causing disease and parasites not , others consider all pathogens as parasites. Anderson and May define parasites as those having a detrimnental effect on the intrinsic growth rate of the host population - others view parasitic actions asd only necessary at the individual ratrher than population level. Some authors would include blood sucking organisms such as mosquitos, others would not. since these have short impacts on hosts relative to ticks and organisms such as lice. Prevalence of parasites: Parasites are extremely common. Indeed this would include all viruses, many bacteria, protozoa, flatworms, nematodes and mites. All insects that feed in or on plants, including almost the entire order of Homoptera (aphids, scale insects, etc), the larvae of many Lepidopteran species feed and mature on a single host plant. There are > 3000 species of parasitic flowering plants, and these are widely dispersed taxonomically, including root parasites. Historically, there was very little attention paid to the ecology of parasites (excluding the entire field of human diseases, which could all be considered part of this). Price argues that this was due to both the fact that many parasites are small (no warm and fuzzies here), and because there was little understanding of just how common parasites were. Again, this is a case of anthropomorphic bias in what was being studied. Thus the study of parasite ecology has really only been extensively developed in the last several decades. Some key figures are: Roy Anderson and Bob May - developed much of the modern theory of disease Andy Dobson - macroparasites and their effect on community structure Peter Price- evolutionary population biology of parasites Mike Hassell - models for arthropod and parasitoid systems Jeremy Burden - disease and plant population biology Basic Concepts in Parasite Ecology 1. Since parasitism means that there is negative impact of individual on host, in many ways the host should be viewed as a resource for the parasite. But it isn't a static resource - it is an environment that acts defensively and individually over the short term to the impact of the parasite, as well as over a longer term as a population coevolves with the parasites. This can lead to a highly dynamic coevolutionary system and this often implies a very strong host specificity. Parasites cannot readily switch food resources or move to locations of high host densities, which are the common mechanisms invoked for population regulation and community stability. It was this host specificity that led many to argue that "parasites as a whole are worthy examplres of the inexhorable march of evolution into blind alleys" Noble and Noble 1976. Price argues that this is ridiculous - there are so many parasitic species that this can surely not be considered an evolutionary dead end. Price presents Ecological Concepts: 1. Parasites are adapted to exploit small discontinuous environments. With wide dispersion of hosts, this has led to (a) mass production of spores or eggs, (b) dispersal of inseminated females that form a high proportion of the population, (c) dispersal by attaching to a larger organism (phoresy), often the host, that makes host discovery easier. There is also dispersal in time by having very long resting stages. 2. Parasites represent the extreme in specialized resource exploitation. Their resource is coarse grained as opposed to fine-grained environments of predators. Due to these differences, predators are rarely host specific (e.g. have a very broad food base) relative to parasites which are much more specialized. Due to this specialization, there can be an extremely diverse parasite community in a given set of resources (e.g. hosts). 3. Parasites exist in non-equilibrium conditions. Some would say this is true of all species of course, but parasites live in a world of very small resource patches surrounded by other small patches with a very small chance of colonization between these without some mechanism to enhance it. The ephemeral nature of the resources is amplified by each additional host species required, leading to very close timing of life stages to guarantee transport to the appropriate host at the appropriate time. Evolutionary concepts: 1. Evolutionary and spoeciation rates can be very high. Get fractionation of gene pools readily occuring fostering rapid divergence, formation of genetic races and eventual speciation. 2. Adaptive radiation is extensive and its degree of development in each parasite taxon can be related to: (a) diversity of hosts in the taxa being exploited (b) the size of the host target (body size, population size) available to potential colonizers (c) the evolutionary time available for colonization (d) selective pressure for specialization We can add to the above several more recent concepts: 1. There can be extensive parasite mediation of host interactions. This occurs since parasites can impair host feeding and assimilation, cause diversion of nutrients in the host that might otherwise be used for reproduction, and lead to behavioral modifications of the host as well. There are indirect effects of parasites, such as mediating host vulnerability to predation. Thus there are effects across trophic levels of parasitism at one level. See Marvier paper as focus on tritrophic effects. 2. Microparasites can reduce host fitness and been implicated as causative factors in large oscillations of population size of the host, while macroparasites are not typically associated with major oscillations in host populations. 3. Parasites serve as a readily manipulated method to analyze community patterns since you can analyze the community of parasites within a given host and then see how this varies spatially with different hosts. This means you can look at the richness of the community within a given host as a function of host size diet, etc. This is called the infracommunity level. The parasite com,ponent community looks at the higher hierarchical level at the whole collection of paratite species across the whole host population. See Poulin's paper for how the richness of the component community depends upon the taxa of the host. 4. There is an entire field of disease ecology which has developed to address questions such as: What ecological and population processes account for the pattern and likelihood of disease emergence and invasion with ecosystems? What is the relationship between spatial structure of a population and disaese spread? What are the evolutionbary and genetic processes that account for the patterns of resistance and virulence among hosts and pathogens? How will diseases respond to global environmental changes? Whjat is the role of disease in ecosystem management? Basic Models : Microparasites - here there is the extension of the classical epidemiological models which track the structure of the host population from one state to another and the dynamics between these. This typically breaks down the host population into the susceptible and infected states and does not directly track the parasite at all, just it's presence in the host. Typical results here are that there are thresholds such that the spread ofm the pathogen will not occur unless the threshold is crossed. This is often expressed as the reproductive number R0 > 1. This says the mean number of new infections caused by a single infected individual must be greater than 1 for the disease to spread. A central question here is under what circumstances a pathogen can regulate a host population. Host-parasitoid interactions: Note that parasitoids are very common - some estimates are that greater than 20% of all insects are parasitoids and the parasitoids represent about 8% of the described insect species. The classic model here is due to Nicholson-Bailey (1935) and is a discrete time model for N(t) the hosts and P(t) the parasitoids in general written as N(t+1) = s N(t) f (N(t),P(t)) P(t+1) = c N(t) (1 - f(N(t),P(t)) where host can grow exponentially in absence of the parasitoid and f represents the fraction of host eggs that are not parasitized from one time to the next. Nicholson-Bailey assumes that f= exp (- a P(t)) which arises by assuming a Poisson distribution for number of parasites attacking a host. This model has a very unstable equilibrium, so it is typical to stabilize it using other assumptions including making the host density dependent, introducing interference between the parasitoids (rather than having them act completely independently of each other as they do in the N-B model), or adding host refuges. Macroparasitic Models: Here the emphasis is not on tracking just whether a host individaul is infected or not, but following the actual load of parasites within the host. This is particularly important for parasitic helminths in which parasite load can have very strong effects on individual health of the host. The approach is therefore to track the actual numbers of parasites in various life stages (free-living, or as adults within a host) as well as the host population. From this one can get the R0 and determine threshold number of hosts required to susteain the parasite population. Lots of mofication sof this have been made to follow more host species, more parasites, circumstances under which parasites can lead to coexistence of two hosts which otherwise would not coexist, etc. Survey of topics from 1990's Ecology journals on host-parasite Brood parasitism 1 Parasite-induced mortality 1 Communities and coexistence 8 Coexistence of parasites 1 Density dependence of host/parasites 4 Evolution of avirulence 1 Spatial spread 2 Tick-borne disease 1 Behavioral and social aspects 3 Population dynamics 3 Host size effects 1 Induced resistence 2 Patch dynamics 1 Total 27