Trends and missing parts in the study of movement ecology

Abstract

Movement is important to all organisms, and accordingly it is addressed in a huge number of papers in the literature. Of nearly 26,000 papers referring to movement, an estimated 34% focused on movement by measuring it or testing hypotheses about it. This enormous amount of information is difficult to review and highlights the need to assess the collective completeness of movement studies and identify gaps. We surveyed 1,000 randomly selected papers from 496 journals and compared the facets of movement studied with a suggested framework for movement ecology, consisting of internal state (motivation, physiology), motion and navigation capacities, and external factors (both the physical environment and living organisms), and links among these components. Most studies simply measured and described the movement of organisms without reference to ecological or internal factors, and the most frequently studied part of the framework was the link between external factors and motion capacity. Few studies looked at the effects on movement of navigation capacity, or internal state, and those were mainly from vertebrates. For invertebrates and plants most studies were at the population level, whereas more vertebrate studies were conducted at the individual level. Consideration of only population-level averages promulgates neglect of between-individual variation in movement, potentially hindering the study of factors controlling movement. Terminology was found to be inconsistent among taxa and subdisciplines. The gaps identified in coverage of movement studies highlight research areas that should be addressed to fully understand the ecology of movement.

Fig. 1.

An estimate of the number of papers published per year referring to movement, based on a literature survey by using the ISI Web of Knowledge and search terms in Table 2, and as a percentage of all papers published in 496 journals. Numbers in italics are the total numbers of articles in any subject published in the 496 journals per year.

Fig. 2.

The percentage of studies using movement terms that focused on movement for various taxa. For bars with different letters above them, means differed at P < 0.05 in a generalized linear model with a logit link function, binomial distribution of sampling error, and weighted for sample size. Numbers above bars are sample sizes. Only taxa with n > 20 are shown. Herptiles are amphibians and reptiles.

Fig. 3.

The frequency of study of different components of the movement ecology framework. (Upper) Compared with the framework, published studies frequently omit feedbacks from movement path to internal state or external factors and links between internal and external factors. Arrow thickness represents the percent of focal studies in which each link was studied. Letters next to arrows label links and give percentages of cases. (Lower) The bar chart breaks the frequency of study into different taxa. *, the three taxa differed at P < 0.05; **, P < 0.005 in G tests. An overall G test for all links simultaneously (thereby protecting alpha) is given.

Fig. 4.

The frequency of usage of movement, migration, and dispersal in major taxonomic groups. Asterisks indicate differences (P < 0.05) from the all taxon averages in G tests. Statistics and sample sizes are given in Table 3.

Fig. 5.

The percentage of studies measuring movement, reporting results of measurement of movement, or using movement terminology at the individual level. The percentage considered only a binary classification of individual vs. population level. Unclear and ambiguous cases, which were 3% of papers, were excluded. Bars with symbols above them differed (at P < 0.05) from the overall all taxa average shown in χ² tests after Bonferroni correction for the overall number of comparisons. *, a difference for measurements; x, results; +, terminology. Numbers above bars are sample sizes.