FSD 2010 - Cousens

5th International Symposium-Workshop on Frugivores and Seed Dispersal (1985-2010)

Roger David Cousens

Department of Resource Management & Geography,
The University of Melbourne, Victoria 3010,
Australia

rcousens(a)unimelb.edu.au

Activities

I am fascinated by patterns of abundance, both in time and space, and their drivers. The study of pattern draws upon many techniques, combining the fields of modelling, statistics and experimentation. I have become a jack of all these trades, master of none. I work hard, I am able to model (a bit), I can apply the methods of others (if they are simple), I know how to ask questions and, most importantly, I think a lot. Thus I sit amongst the majority of ecologists: I have had no Eureka moments of novel ecological insight, I have not invented any wonderful new techniques, I struggle to get research funding and I have not attracted a throng of disciples (with the exception of Iranian weed scientists, for some reason!). But I am fascinated by ecology and have plenty to say.

I am particularly in awe of the wealth of tools available to the latest generation of ecologists. We are capable of finding out so much! So I get really mad when I see methods misused or misinterpreted! My major contribution to ecology, particularly relating to agricultural weeds, has thus been to initiate debates on methodology, to point out common errors, and to remind people of the potency of the combination of good logic and simple methods. I also react against vague concepts, pointless jargon and ecological dogma (like the referee who said that LDD was the only valid reason for studying dispersal).

I have never ignored dispersal (I was a co-author of one of the first stratified seed dispersal “kernels”: Howard et al., 1991) but like most Harper-inspired plant ecologists from the 1970/80s I tended to simplify issues by assuming that immigration and emigration balance in the middle of populations. Realising how much I was missing, I have recently been exploring short-distance dispersal: so un-appealing as a topic (the failure to disperse long distances) but oh so important to population and community structure. Several of my recent students have digitised and analysed plant architectures – spatial pattern in the release points for seed trajectories – and the patterns of heavy fruits on the ground post-dispersal. In the future, I would like to be part of a team developing truly mechanistic models for animal-dispersal of seeds, but the Australian Research Council apparently does not agree!

As a result of seeing very inconsistent, and thus confusing, use of terminology in the ecological literature, and discovering errors that this led to in spatial modelling, I decided to write a book on dispersal from a population perspective, to try to straighten the literature out for similar ecologists. Two colleagues were kind enough to help me out. It is not a book about the entirety of dispersal ecology – just the bits that will impact on patterns in space and time. It is at a level that (hopefully) general ecologists like me can understand. It may not inspire (except perhaps Iranian weed scientists), but we hope it will be useful!

Abstract

What on earth is a dispersal kernel? Why loose terminology confuses everyone and proliferates mistakes.

Modellers coined the term "kernel". It sounds scientific, is embedded in theoretical work, so now everyone is using it. But the one word is being used to mean two very different things. Many papers confuse the frequency of seeds dispersing a given distance with the density of seeds arriving at a particular point. These are quite different, and have very different shapes, but both are being called kernels. Modellers usually model "in one dimension" but parameterise their models with data from two dimensions. The result is that mistakes are being made, interpretations of results are inappropriate, and in many papers it is unclear what the authors have done. In this paper, I will yet again plead for a consistent use of nomenclature. I will explain the difference between the two types of dispersal "curve", review the use of the term "kernel" by modellers, give examples of where nomenclature is ambiguous and examples of where errors have been made in predicting dispersal patterns.

Related references

Cousens, R. & Mortimer, M. (1995). Dynamics of Weed Populations. Cambridge University Press.

Cousens, R., Dytham, C. & Law, R. (2008) Dispersal in Plants: A Population Perspective. Oxford University Press.

Woolcock, J. L. & Cousens, R. D. (2000) A mathematical analysis of factors affecting the rate of spread of a weed within an arable field. Weed Science, 48, 27-34.

Cousens, R. D. & Rawlinson, A. A. (2001) When will plant morphology affect the shapes of seed dispersal “kernels”. Journal of Theoretical Biology 211: 229-238.

Cousens, R.D., Wiegand, T. & Taghizadeh, M. S. (2008) Small-scale spatial structure within patterns of seed dispersal. Oecologia 158, 437-448.

Taghizadeh MS, Crawford S, Nicolas ME & Cousens RD (2009) Water deficits change the anatomy of the fruit abscission zone in Raphanus raphanistrum (Brassicaceae). Australian Journal of Botany 57: 708-714.

Howard, C.L., Mortimer, A.M., Gould, P., Putwain, P.D., Cousens, R. and Cussans, G.W. (1991). The dispersal of weeds - seed movement in arable agriculture. Proceedings of the Brighton Crop Protection Conference - Weeds - 1991, pp.664-673.

Cousens, R. D. & Woolcock, J. L. (1997) Spatial dynamics of Weeds: An Overview. Proceedings of the Brighton Crop Protection Conference - Weeds - 1997, pp. 613-8.

Niknam, S. R., Moerkerk, M. & Cousens, R. (2003) Weed seed contamination in cereal and pulse crops. In: (Spafford Jacod, H., Dodd, J. & Moore, J. H., eds) Papers and Proceedings of the 13^th Australian Weeds Conference: Weeds “Threats now and forever?”. pp. 59-62.

Taghizadeh, M. S. & Cousens, R. D. (2006) Is seed dispersal density-dependent? Proceedings of the 15^th Australian Weeds Conference, Adelaide. Pp.155-158.