Does Dispersal Limitation Explain the Latitudinal Richness Gradient?

Introduction

In a field with very few laws (Lawton, 2007), the latitudinal gradient of species richness is one of the most fundamental and well-studied relationships in ecology (Ricklefs, 2004). As you move from the poles toward the equator the number of species increases. This pattern is ubiquitous, and is found across a range of taxonomic groups and is present in both the Eastern and Western Hemispheres.

There are many proposed reasons for the richness gradient (Willig et al. 2003). One family of ideas is that the increase in thermal energy that low latitudes experience supports a higher diversity of species. For example, the tropics have higher primary productivity, and thus have a potential for more alternative stable equilibria in community composition than cooler regions. An alternate family of hypotheses is that species richness is higher at the equator because of historical events. That region of the Earth has been climatically stable over the past several ice ages. That stability has prevented organisms from going extinct and has also allowed more time for speciation. Much of today’s temperate ecosystems were buried under ice 10,000 years ago and may be fully recolonized by species that survived the last ice age.

The historical mechanism for the latitudinal richness gradient has some support in the literature. Work on several taxonomic groups including trees has found that dispersal limitation and historical causes may be responsible for current range distributions (Svenning & Scov, 2007). Furthermore, studies on metacommunity dynamics at sub-continental scales have found that dispersal limitation is a major driver of species' current distributions within regions (De Bie et al. 2012). This provides support for the idea that current distributions of species are not necessarily representative of their fundamental niche. With the exception of micro-organisms, who truly are able to disperse globally, it is important that species current ranges be evaluated to determine if they are representative of their potential ranges.

To address the important question of whether latitudinal richness gradients are a result of dispersal limitation, I propose to measure dispersal ability in beetles and measure the relationship between dispersal ability and the slope of the latitudinal richness gradient.

Objectives

1) Accurately measure dispersal ability for a large sample of coleopterans native to Europe.

2) Determine whether dispersal ability is a good predictor of the relationship between species richness and latitude. I predict that dispersal ability will be negatively corrected with the slope of the latitudinal richness gradient for European beetles.

Methods

Species and study region

The biological range data sets I will use include 4078 species of beetle in the superfamilies Caraboidea, Staphylinoidea, Scarabaeoidea, Chrysomeloidea, and Curculionoideae. The study area will include all of continental Europe from 11oW – 60oE longitude and 36oN to 72oN latitude, following the methods of Baselga et al. (2012).

Species Trait Measurements

I propose to turn to museum and university collections of preserved beetles found throughout Europe to measure morphological traits that contribute to dispersal ability. Specifically, I intend to measure leg length, body length (front of the head to the back of the abdomen), wing length, wing width, body width, and body depth. I’ll measure at least 4 typical adult individuals per species of as many species as can be located within 18 months. Averages of each trait will be recorded and ratios such as leg length to body length will also be recorded. All of these data will be put into a public database after the work is complete so that other interested researchers can access our data and add it to the database.

Species Range Distributions

Species ranges will be developed following the methods of Baselga et al. 2012. I will determine ranges by a combination of existing inventories developed by Lobl & Smetana (2003, 2004, 2006), the Fauna Europea Web Service ( Fauna Europaea version 1.1, available online at http://www.faunaeur.org), and literature review. The zones used in Baselga et al were coarse, so if the data are available, I intend to increase the spatial resolution of the grid on which I’ll base my analyses (2012).

Statistical Analysis

Species that are measured will be assigned a continuous value for each trait that is relevant to dispersal ability. For example, the ratio of leg length to body length is indicative of running ability. The presence and size of wings relative to the overall body size are also traits that will be used in the score. Species with large wings have stronger flight potential than small winged and flightless beetles. The scores will be weighted based upon a literature review to develop a dispersal metric that can be applied to all measured taxa.

Taxa will then be binned into groups based on dispersal ability alone (as opposed to assigning dispersal ability scores to pre-existing taxonomic groups). This will allow me to directly measure the effect of dispersal, in insolation from confounding traits for a particular taxonomic group. For each dispersal category, the richness of species in the category will be regressed against latitude. The slope of the relationships will then be regressed against the dispersal score for each group to determine if dispersal ability is a strong predictor of latitudinal richness gradient.

Significance

The relationship between species richness and latitude is one of the strongest patterns in ecology, and yet we still don’t understand what causes it (Willig et al., 2003). This transformative research will definitively test one of the leading hypotheses for the phenomenon, that latitudinal richness gradients are an artifact of historical glaciation events and dispersal limitation. In addition to the intellectual merit of the work, there are significant applied aspects of the proposed research.

Baselga et al. recently found support for the theory that dispersal limitation may be the true causal factor for the latitudinal richness gradient using beetles (2012). Despite the evidence, Baselga et al. did not directly measure dispersal (2012). Instead, they used longitudinal spatial turnover of species within Europe as a proxy measurement for dispersal ability. Prior to the analysis, they did find a relationship between longitudinal turnover and dispersal traits, but their measure misses 38% of the variation attributable to dispersal traits and is also confounded by environmental variation along the longitudinal gradient.

Species able to persist in a greater range of habitats will have lower turnover, similar to species with excellent dispersal. This is problematic because species having greater latitudinal ranges could also be explained by them being able to persist in a greater range of environmental conditions. Given this fact, it’s difficult to disentangle the actual influence of dispersal ability on the relationship Baselga et al. found between longitudinal turnover and the slope of the latitudinal – species richness gradient (2012).

Issues with using proxies to measure dispersal are common in the literature. For example, two recent works used body size as a proxy for dispersal ability and found completely different patterns. Borthagaray et al. were looking at desert communities in South America and found body size was a positive predictor of dispersal because larger-bodied individuals were capable of traversing the hot sands between vegetated habitats (2012). De Bie et al. found an opposing pattern with freshwater species because larger species had a harder time making out of water trips (2012). My proposed research will address the short-comings of this body of literature by directly measuring traits that contribute to dispersal rather than using proxies.

Earth is currently undergoing a powerful climate transformation. Anthropogenic climate change is altering the current distribution of habitats and biomes at a rate not seen since the end of the last ice age. Rainfall patterns and temperature regimes are changing all over the world. Some rare types of climate may disappear completely, other climate zones are shifting, and some areas may become completely novel climates (Williams, 2007). An important unanswered question is whether the biota of Earth will be able to respond to these shifts at rates that are fast enough to prevent extinction. If they cannot then whole groups of species may face extinction from global climate change unless we intervene by assisting their dispersal.

If this research demonstrates that dispersal limitation is responsible for the latitudinal richness gradient, then it will also demonstrate that our study organism, beetles, will be unable to track the changing climate resulting from anthropogenic climate change. If we find dispersal limitation is an issue with beetles, a fairly mobile group, then it stands to reason that these results will be applicable to a range of other taxa as well. Thus not only is this important research contributing to theoretical ecology, but it is also contributing to the future conservation of the species of Earth.

References

Baselga, A., Lobo, J.M., Svenning, J., Aragon, P., & Araugo, M.B. (2012). Dispersal ability modulates the strength of the latitudinal richness gradient in European beetles. Global Ecology and Biogeography, 21, 1106-1113.

Borthagaray, A.I., Arim, M., & Marquet, P.A. (2012). Connecting landscape structure and patterns in body size distributions. Oikos, 121, 697-710.

De Bie, T., De Meester, L., Brendonck, L., Martens, K., Goddeeris, B., Ercken, D., Hampel, H., Denys, L., Vanhecke, L., Van der Gucht, K., Wichelen, J.V., Vyverman, W., & Declerck, S.A.J. (2012). Body size and dispersal mode as key traits determining metacommunity structure of aquatic organisms. Ecology Letters, 15, 740-747.

Lawton, J.H. (2007). Are there general laws in ecology? Oikos, 84, 177- 192

Lobl, I. & Smetana, A. (2003). Catalogue of Palaearctic Coleoptera. Vo1. 1: Archostemata-Myxophaga-Adephaga, 1st edn. Apollo Books, Stenstrup, Denmark.

Lobl, I. & Smetana, A. (2004). Catalogue of Palaearctic Coleoptera. Vo1. 2: Hydrophiloidea-Staphylinoidea, 1st edn. Apollo Books, Stenstrup, Denmark.

Lobl, I. & Smetana, A. (2006). Catalogue of Palaearctic Coleoptera. Vo1. 1: Scarabaeoidea, Scirtoidea, Dascilloidea, Buprestoidea, and Byrrhoidea, 1st edn. Apollo Books, Stenstrup, Denmark.

Ricklefs, R.E. (2004). A comprehensive framework for global patterns of biodiversity. Ecology Letters, 7, 1-15.

Svenning, J.-C. & Scov, F. (2007). Ice age legacies in the geographical distribution of tree species richness in Europe. Global Ecology and Biogeography, 16, 234-245.

Williams, J.W., Jackson, S.T., & J.E. Kutzbach. (2007). Projected distributions of novel and disappearing climates by 2100 AD. Proceedings of the National Academy of Science, 104, 5738 – 5742.

Willig, M.R., Kaufman, D.M., & Stevens, R.D. (2003), Latitudinal gradients of biodiversity: pattern, process, scale, and synthesis. Annual Review of Ecology, Evolution, and Systematics, 34, 273-309.