Detecting Trends in Species Composition Thomas E
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Iowa State University From the SelectedWorks of Philip Dixon 1998 Detecting trends in species composition Thomas E. Philippi, University of Georgia Philip Dixon, University of Georgia Barbara E. Taylor, University of Georgia Available at: https://works.bepress.com/philip-dixon/36/ 300 INVITED FEATURE Ecological Applications Vol. 8, No. 2 Ecological Applications, 8(2), 1998, pp. 300±308 q 1998 by the Ecological Society of America DETECTING TRENDS IN SPECIES COMPOSITION THOMAS E. PHILIPPI,PHILIP M. DIXON, AND BARBARA E. TAYLOR Savannah River Ecology Laboratory, Drawer E, Aiken, South Carolina 29802-0005 USA Abstract. Species composition re¯ects a combination of environmental and historical events at a site; hence, changes in species composition can provide a sensitive measure of ecologically relevant changes in the environment. Here, we consider the analysis of species composition when multiple sites are followed through time. Analyses of temporal trends in species composition either summarize species compo- sition into a few metrics (indices or axis scores) or analyze the similarity among sites. We develop and illustrate the similarity approach. Each pair of samples represents a pair of replicates, a pair from the same site at different times, a pair from different sites at the same time, or an unrelated pair. Differences among times can be estimated by comparing average temporal dissimilarity to average replicate dissimilarity. Temporal trends can be described by one of three statistics that measure progressive change, the correlation of temporal dissimilarity with the length of time between samples. These methods are illus- trated using data on changes in a South Carolina zooplankton assemblage following dis- turbance, and changes in bird species composition on Skokholm Island, Wales. It is dif®cult to de®ne and interpret temporal trends. Some de®nitions of interesting trends, like increasing divergence from another set of sample plots, place additional re- quirements on the sampling design. Including replicate samples or clustering sample plots and including ``control'' plots for comparison with sentinel sites would contribute to an understanding of changes in species composition. Key words: community change; dissimilarity; multidimensional scaling; ordination; randomiza- tion tests; species composition; temporal differences; trends. INTRODUCTION plankton assemblage and in the bird community of Skokholm Island, Wales. Similar sorts of questions and Most discussions of monitoring, including those in analyses are appropriate for many communities with this special feature, deal with scalar quantities: envi- multiple sources of variation, e.g., environmental stress ronmental conditions such as pH, or abundances of and spatial gradients (Vargo et al. 1982, Fausch et al. individual species. However, not all quantities that 1990, Clarke 1993). might be useful to monitor are scalars; species com- position at monitored sites is a vector, or multivariate WHY MONITOR SPECIES COMPOSITION? quantity. In general, species composition may be an- alyzed as vectors of 0's and 1's representing absence A need to monitor species composition may arise in or presence of species, vectors of abundances of each at least three ways: species composition may provide species, or vectors of proportional abundance. Al- a strong signal of the environmental factor of interest; though most of the principles and problems applicable it may be of direct ecological interest; or it may be the to detecting change or trends in scalars also apply to only feasible measurement. Because environmental species composition, the multivariate nature of species factors differentially affect species, species composi- composition presents additional problems. Change or tion can provide information about environmental fac- differences in composition among samples may be tors (Austin and Austin 1980, ter Braak and Prentice quanti®ed, but may be dif®cult to interpret. Trends are 1988). Indirect estimation of environmental character- much more dif®cult to de®ne, especially if species com- istics from species composition can be extremely useful position is responding to changes in more than one if it is dif®cult to de®ne or directly measure relevant stressor. Here, we review the forms of questions ad- characteristics of the environment. Two examples are dressed with species composition data, propose some paleoecological reconstruction of environmental con- methods using dissimilarity metrics, and then illustrate ditions, such as climate or pH, from historical species the methods using changes in a South Carolina zoo- composition and modern species±environment rela- tionships (Oksanen et al. 1988, Birks et al. 1990), and Manuscript received 6 January 1997; accepted 15 October using changes in the shrubby and herbaceous vegeta- 1997. For reprints of this Invited Feature, see footnote 1, p. 225. tion to monitor changes in site capability for timber 300 Monday Sep 28 01:17 PM ecap 8 221 Mp 301 Allen Press x DTPro File # 21sc May 1998 MEASURING ECOLOGICAL TRENDS 301 production, which might take decades to measure di- a spatial pair from different sites at the same time, or rectly (Korstian 1919, Leak 1992). Infrequent impacts a mixed pair from different sites at different times. (e.g., occasional discharges) may be dif®cult to detect We divide questions addressed with species com- directly without continuous monitoring, but species position into three temporal categories. Composition composition may effectively and appropriately inte- may be compared among groups of sites at a single grate such acute impacts over time, providing a means time, the amounts and directions of change in com- of detection and an estimate of severity or importance. position between two dates may be compared among Fish species composition has been used as an index of groups of sites, and trends or progressive change across pollution in rivers and streams (Fausch et al. 1990), a series of dates may be compared among sites. Each and changes in marine benthic invertebrates have been class of change corresponds to a comparison of dis- used as measures of environmental impact (Warwick similarity between types of pairs (e.g., temporal pairs and Clarke 1991). to replicate pairs). Species composition itself may be the relevant en- COMPARISON AMONG SITES AT A SINGLE TIME vironmental end point, especially in an environmental impact context where the concern is that there be no If species composition changes between impacted change in species composition due to operation of some and unimpacted sites, then the pairwise dissimilarities facility (e.g., Gray et al. 1990, Environment Canada among impacted sites and among unimpacted sites will 1995). Finally, percent species composition may be the be smaller than the pairwise dissimilarities between only feasible measurement (e.g., in palynological stud- impacted and unimpacted sites. These dissimilarities ies). Such values are not independent among species; can be summarized either by the mean or median pair- a species can have a low percent abundance in a par- wise intergroup dissimilarity, or by a test statistic such ticular sample, either because it has low absolute abun- as a Mann-Whitney U or Clarke's (1993) R that com- dance in that sample, or because another species has pares between-group to within-group dissimilarities. extraordinarily high abundance in that sample (e.g., Clarke's R is computed by ranking all n(n 2 1)/2 pair- wise dissimilarities between the n sites, and then com- Hellawell 1977). Therefore, it can be quite misleading puting to treat such data as re¯ecting patterns in individual species. For some taxa (e.g., soil microorganisms), it 4(r 2 r ) R 5 bw is only possible to detect presence or absence; thus, an n(n 2 1) individual species provides little information. Changes in species composition may be of interest where rb is the average rank of impacted±unimpacted for both retrospective and prospective monitoring. Al- (between-group) pairs, and rw is the average rank of though compositional change may be an ``effect'' rel- within-group pairs. If the smallest difference between ative to some other primary stressor, it may, at the same groups is larger than every difference within groups, time, be a ``stressor'' predictive of future changes in then R 5 1, but if differences between groups are sim- ilar to those among groups, R will be close to 0. ecosystem properties such as biomass and nutrient cy- Randomization tests are required to evaluate whether cling, and thus may allow earlier detection and inter- dissimilarities between groups are larger than expected vention. Underwood (1994) suggested that, although by random chance (Field et al. 1982, Smith et al. 1990, univariate chemical or indicator species measures may Clarke and Warwick 1994). Traditional signi®cance respond rapidly with strong signals to other stressors tests are invalid because dissimilarities are computed and, thus, may be preferable to assemblage data, anal- from all pairs of samples. Even if each of the n sites ysis of species or assemblage composition generally is is considered to be independent, the n(n 2 1)/2 possible required if the goal is to predict community or eco- pairwise dissimilarities are not. In each randomization, system consequences of a change. labels (e.g., impacted or unimpacted) are randomly In a typical monitoring design, samples are collected reassigned (without replacement) to sites, and the mea- or measured at multiple sites monitored over time. Ide- sure