can landscape-scale characteristics be used to predict plant invasions along rivers?

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  • Can landscape-scale characteristics be usedto predict plant invasions along rivers?G. S. Campbell1*, P. G. Blackwell2 and F. I. Woodward1 1Department of Animal and Plant

    Sciences, University of Sheffield, Sheffield, UK, and 2Department of Probability and Statistics,

    University of Sheffield, Sheffield, UK

    Abstract

    Aim To determine whether the invasions of hydrochorus plants, that is those which canmake use of rivers to transport their propagules, can be predicted using informationderived at the landscape scale. This is desirable to avoid the need for the difficult tomeasure parameters required by detailed invasion models.

    Methods A model for plant propagule dispersal was developed that simulated bothlocal dispersal (autochory) and aided dispersal along river corridors (hydrochory). Thisprovided the simulated invasion behaviour that was to be predicted by the simpleanalytical method. This latter was based on readily available river network character-istics. The analytical summary was then tested for its ability to predict the results of aseries of simulation experiments.

    Results Predicted dispersal rates derived from the analytical summary method werestrongly correlated (R2 of 0.8941) to the mean seed displacement simulated by the plantdispersal model.

    Main conclusion The simple analytical summary of the river networks provides a goodprobabilistic description of the simulated invasion process. This means that readilyavailable information might be able to be used to predict real invasions by alien plantspecies. This method should now be tested against observed invasions by alien plants.

    Keywords

    Hydrochory, invasion, simulation, landscape.

    INTRODUCTION

    This paper describes the development of a model for theinvasion of the UK by alien plant species, specifically thosethat are associated with riparian habitats. There are manywell-documented examples of invasion by alien plant speciesin the UK (see for example Ellis, 1993; Beerling & Palmer,1994; Dawson, 1994; Perrins et al., 1993). In the ecologicalcontext an invasion is the dispersal of a non-endemic speciesand its subsequent spread into a new environment (Cousens& Mortimer, 1995). The term dispersal is used here to meanthe movement of propagules away from the parent (Speller-berg & Sawyer, 2000) and migration is used for the advanceof a species from one area to another (Allaby, 1992). An alienis a species, which has arrived because of human activity(Clement & Foster, 1994) and in the British context may be

    defined as a species which was not present in the area nowforming the British Isles, at the end of the last Ice age (Ellis,1993). Modelling such invasions is made difficult by thediscrepancy between the scale at which the mechanisms ofinvasion occur, i.e. at the scale of individual plants and that atwhich its progress is recorded nationally, generally at aresolution of 10 km. This paper reports an approach to themodelling of plant invasions that attempts to reconcile thehighly random nature of the processes involved with theavailable spatial and autecological data.

    Much work has been aimed at developing predictivemodels of the spatial response of plants to natural andanthropogenic environmental disturbances (see Higgins &Richardson, 1996 for a review). Such disturbances includeclimate change, edaphic alteration and, perhaps mostdramatically, the introduction of plants to novel environ-ments. The reactiondiffusion approach adopted in theclassic work by Skellam (1951) assumed that the rate ofspecies spread was a constant given that the asymptotic rate

    *Correspondence: Department of Plant and Soil Sciences, University of

    Aberdeen, Aberdeen AB24 3UU, UK. E-mail: gary.campbell@abdn.ac.uk

    Journal of Biogeography, 29, 535543

    2002 Blackwell Science Ltd

  • of spread has been reached. The central assumption in thismodel that dispersal is effectively a random process exhib-iting Brownian motion, does not allow for a non-uniformlandscape or for more than one mode of dispersal. In fact, itwas the underestimation of the post-glacial migration byoak trees in England, using this model which lead Skellam(1951) to the conclusion that aided dispersal in the form ofornithochory (dispersal by birds) must have played a part inthe migration (Higgins & Richardson, 1996). Similarpalaeoecological evidence for hazel (Corylus avellana L.)(Huntley, 1993) and alder (Alnus glutinosa L.) (Chambers& Elliot, 1989) is indicative of a post-glacial migration rategreater than that, which would be expected from autochorusdispersal alone.

    Another approach to modelling plant invasions, which hasproved partially successful, is spatio-phenomenologicalmodelling, that is the use of historical data to determinemathematical relationships between elapsed time and areainvaded (see for example Perrins et al., 1993; Pysek &Prach, 1993). However for some species, such modelsproduce estimates of the mean rate of invasion thatunderestimates the observed invasion rate by an order ofmagnitude (Andow et al., 1990). An example of such aspecies is Himalayan Balsam (I. Glandulifera), which has anobserved maximum invasion rate 38 km year)1, whereas thedehiscent (explosive seed pod) dispersal alone should give aninvasion rate of 23 m year)1 (Williamson, 1996). As withthe post-glacial migration of tree species this discrepancy inpredicted and observed invasion rates is attributed to thephenomenon of aided dispersal (Higgins & Richardson,1996) whereby organisms make use of secondary dispersalvectors to augment autochorus mechanisms. A furtherweakness of spatio-phenomenological models is that theydo not consider the heterogeneous nature of the landscapewhen estimating dispersal rates.

    A contrasting and in many ways complementary approachis that taken in spatio-mechanistic models, which useempirically derived growth and dispersal functions todeterministically model the behaviour of plant populations.Unlike spatio-phenomenological models, spatio-mechanisticmodels such as MIGRATE (Collingham et al., 1997) canallow for biological differences between species. The spa-tially explicit, spatio-mechanistic model allows for landscapeheterogeneity and can incorporate functions for modellingmultiple dispersal vectors, including occasional long-dis-tance dispersal of propagules. These models perform well atcertain landscape scales, providing a platform for sensitivityanalysis and invasion rate prediction (see for example Williset al., 1997). However, such models deal with the fate ofindividual propagules in a probabilistic, albeit biologicallyrealistic manner, consequently the resulting populationinvasion predictions, whilst being entirely feasible, repre-sents just one of the many possible scenarios. The resultsfrom such models need careful interpretation as the effect ofone or more rare and long-distance dispersal events on theoverall pattern of propagule dispersal can be substantial(Perrins et al., 1993).

    An alternative modelling approach is not to attempt tomodel the fine scale mechanisms of the invasion process butrather to use analytical methods to model the process at alandscape and population scale. This approach parallels thatused by Yves et al. (1998), to investigate the ability of simpleanalytical summaries of landscape characteristics (spatialarrangements of habitats) to predict population dynamics.The predictions of a simple analytical description of habitatsuitability were regressed against the results from a demo-graphic simulation to test the predictive ability of the former.The strong relationship between the values predicted by theanalytical method and those simulated by the demographicmodel (R2 0.88, where R2 is the proportion of variation inthe observed values explained by the model) suggest thatsuch analytical approaches could be used to study landscape-scale population dynamics in the absence of detailedecological and geographical data. This type of analyticalmodel was developed to predict the invasion of hydrochorusalien plants in the UK and is described here.

    For species that are able to use rivers as agents of aideddispersal it was hypothesized that the contribution toinvasion rate made by hydrochorus dispersal is such thatfor certain plants, predictions based on this alone willexplain the observed pattern of invasion (Campbell, 2001).To assess the potential of such an analytical summarymethod the following approach was adopted.

    1. Construct a model of plant propagule dispersal, whichsimulates local, plant-mediated (autochorus) and long-range aided dispersal by rivers (hydrochory).

    2. Carry out a series of simulation experiments to developrelationships between a relevant landscape characteristic(river network distribution) and simulated rates ofdispersal.

    3. Assess the capacity of the analytical summary method topredict the dispersal rate of an independent series ofsimulation experiments.

    4. If step 3 is successful, apply the analytical summarymodel to historical plant invasions in the UK and assessits predictive ability in real world situations.

    The first three stages are designed to develop a relationshipbetween river network characteristics and plant dispersalrates and to assess the ability of the analytical method toexplain the behaviour of a simulated invasion using onlyvery limited input data. The important point about this inputdata is that unlike the multiple parameters required fordetailed mechanistic models, it is readily available at thenational scale for the UK. The fourth step, as describedabove, is designed to see if this model can predict realinvasions using only this minimal input data. This paperreports on the first three stages of this process, the modeldevelopment and testing against simulated invasions.

    LANDSCAPE GENERATION PROGRAM

    The experimental landscapes were gen