Can Parasites Indicate Host Biodiversity

Can Parasites Indicate Host Biodiversity?

Outline of an Ecology Learning Module - Liz Harp and Kate Huyvaert

Learning objectives:

After participating in this learning module, students will:

a) understand and be able to calculate measures of biodiversity including species richness, abundance, and heterogeneity;

b) realize the importance of ‘indicator species’ in the study of biodiversity;

c) understand what parasites are including parasites’ dependence on ‘host’ species to complete their life cycles;

d) appreciate the value of parasites as indicator species and as critical components in the maintenance of healthy host populations, communities and ecosystems;

e) be able to navigate straightforward data tables, develop short tables of their own, and develop line graphs of two sets of values.

Module structure:

This module is meant to accompany class material covering biodiversity and one or more of the following areas: community or ecosystem ecology, host-parasite interactions, or disease ecology. We anticipate that class material covering biodiversity will have been presented around when the module is used. In class, students will need about ten minutes to ‘collect’ data needed for the module. Additional data, the learning exercise, and the assessment assignment will be provided on the Web. Outside of class, the learning module and assessment assignment should require about 1 to 1.5 hours for students to complete. The instructor or teaching assistant will need approximately one hour before the exercise to prepare for in-class data collection and s/he will need another half hour for data table compilation after the in-class data collection.

Background information:

Biodiversity is an important gauge of a healthy, functioning ecosystem. However, measuring biodiversity is extremely difficult and sometimes impossible. Scientists and resource managers often use 'indicator species' to estimate the species richness of other taxa. Often, indicator species are those that provide resources or habitat for other species, although the presence of a particular resource or habitat does not require the actual presence of the species that uses that resource.

Parasites are organisms that live in or on another organism (the host) from which it obtains nutrients and habitat. Some parasites have direct life cycles requiring only one host species, while others have indirect life cycles requiring two or more host species. For example, hookworms, which are parasitic nematodes (roundworms), require only humans to complete their life cycle. In contrast, trematodes (flatworms) require at least two different host species to complete their life cycle. Because trematodes require at least two species for their life cycle, they make an excellent choice as an indicator species of the diversity of one or more of their hosts.

Many trematodes use birds, fish, or benthic invertebrates for one part of their life cycle, and snails for another part of their life cycle. In these cases, the bird, fish or benthic invertebrate is considered the definitive host, while the snail is the intermediate host. It is often difficult and expensive to estimate abundance and diversity of birds. However, it is very easy to collect snails. Thus, in this system, the variety of trematodes that infect snails and then birds might be effective indicators of the diversity of birds in a community or ecosystem.

In fact, some researchers have shown support for the hypothesis that trematode species diversity can be used to accurately and effectively (i.e., cheaply) predict bird species diversity. Your mission is to examine this hypothesis in a model ecosystem meant to mimic a natural system.

Learning Activity:

In this activity, we will work as a class to collect data necessary to test the idea that the trematode species diversity found in one species of snail is positively correlated with (and accurately predicts) species diversity in a coastal wetland ecosystem. But because we can’t all travel to a coastal wetland to study this question, you will use a model of the snail/trematode system- jelly bean trematodes inside plastic egg snails- to collect data. Each color of plastic egg represents a different study site. Further, bird species diversity data needed to examine the hypothesized correlation will be provided on the Web exercise.

In class:

a) Everybody receives one plastic egg (snail) containing variable numbers and colors of jelly beans (trematodes).

b) Fill out species information on forms provided in class (TA or instructor will tally number of each species of jelly bean and post on the module Website or provide through RamCT; see example, Table 1).

Outside of class:

Collaborate with a partner to calculate measures of biodiversity (species richness, evenness, abundance, and heterogeneity) using the following equations and the data provided by your instructor. After completing your calculations, follow the instructions on the module website to complete your written report.

Species Richness is the number of species (either birds or trematodes in our case), and is usually represented in equations by the letter S.

Abundance is the average number of individuals per some unit such as area or host, in this case it is the average number of trematodes per snail (i.e., the average number of jelly beans in a plastic egg).

Evenness describes the variability in species abundances. One measure of evenness is the Shannon Evenness Measure (J′):

Measures of heterogeneity combine species richness (S) and evenness (J′). The resulting statistic is often referred to as a “diversity index” because it combines measures of both species richness and species evenness. One such index is the Shannon Index (H′):

Where pi is the proportion of total individuals found in the ith species (calculated by dividing the number of individuals of species i by the total number of individuals of all species).

Assessment:

Students will write a one-page report responding to the following:

(1) Using the jelly bean raw data provided by your instructor or TA, report your estimates of trematode overall abundance, species richness, and heterogeneity for each study site in table format.

(2) ‘Evenness’ is another measure of biodiversity that describes how equally distributed species are numerically. By inspecting the raw data table and your calculations of abundance, which trematode species (jelly bean color) is most evenly distributed among the sites?

(3) Compare your values of trematode species richness with the species richness values for birds at each site. Develop a line graph that shows your comparison.

Hint: You should have as many points on your line graph as there are sites in the study.

(4) In their paper, Hechinger and Lafferty (2005) show that bird species richness and trematode species richness are positively correlated across the sites that they studied in nature. Do your findings support this hypothesized

relationship? Why or why not? What are two assumptions that our study makes that the natural study does not make?

Table 1. Example compilation of raw data collected by students in-class for use in studying parasites as indicators of host biodiversity.

Literature Cited:

Hechinger, RF, and KD Lafferty. 2005. Host diversity begets parasite diversity: bird final hosts and trematodes in snail intermediate hosts. Proceedings of the Royal Society B. 272: 1059-1066.