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Conservation and Restoration of Calcareous Grasslands: a Concise Review of the Effects of Fragmentation and Management on Plant Species

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Conservation and Restoration of Calcareous Grasslands: a Concise Review of the Effects of Fragmentation and Management on Plant Species
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  B A S E  Biotechnol. Agron. Soc. Environ. 2005 9 (2), 111–118 Conservation and restoration of calcareous grasslands: aconcise review of the effects of fragmentation andmanagement on plant species Jan Butaye, Dries Adriaens, Olivier Honnay Laboratory for Forest, Nature and Landscape Research. University of Leuven.Vital Decosterstraat 102.B–3000 Leuven(Belgium).E-mail: Jan.butaye@agr.kuleuven.ac.beWorldwide, the most species rich plant communities at small scale (<10 m 2 grid) are some types of temperate grasslandcommunities, and of these, the calcareous grasslands of temperate Europe are the most diverse. Calcareous grasslands wereonce widespread in the hilly calcareous regions of Western Europe but due to changes in agricultural practices (eitherintensification or abandonment of grazing by sheep and cattle, and the consequent succession towards forest) their extent hasdecreased dramatically and the remaining areas have become extremely fragmented. In this review we discuss the reportedeffects of decreasing habitat quality and habitat quantity on plant community composition of calcareous grasslands. Availableevidence reveals the strongly negative consequences of both of these processes on plant species, both at the community levelas at the population level. It seems, however, that especially the effects of fragmentation on plant species richness on the oneside and the effects of management and fragmentation on biotic interactions on the other are still poorly understood and thatmore research is necessary. Keywords. Community composition, population persistence, biotic interactions, ecological restoration. 1. INTRODUCTION Calcareous grasslands are known as the most speciesrich plant communities of northwestern Europe (up to8 0plant species/m 2 ) (Wa l l i s D e Vries e ta l . , 2002).Because of this extremely high biological diversity,which is also reflected by high arthropod diversity,these areas are integrated in the EU NATURA2000network and their communities are included in theappendix of the Habitat Directive (Semi-natural drygrassland and scrubland facies on calcareoussubstrates ( Festuco Brometalia )) (Romao, 1997).Historically these grasslands occurred on steep,calcareous outcrops in mountainous regions or on hillydomes, characterized by extreme environmentalconditions. Prehistorical human activities such asfelling of the primeval forest and later transhumancesand increased sheep flock migrations have lead to anincrease in area of this habitat (WallisDeVries etal. ,2002). At this time many species, often srcinatingfrom more southern regions, could establish in north-western European calcareous grassland regions, wherethey were earlier rather confined to natural calcareousopen habitats such as forest clearings on calcareousoutcrops or on steep slopes where forest developmentwas prevented.Intensification of agriculture and abandonment of traditional agricultural practices, often followed byafforestation, resulted in a tremendous decrease in thearea and habitat quality of the remaining calcareousgrasslands (Keymer, Leach 1990; Poschlod,Schumacher, 1998). As a consequence many often rarecalcareous grassland species went extinct or becameprone to extinction (WallisDeVries etal. , 2002).Habitat destruction and degradation have resultedin increased habitat fragmentation of the remainingcalcareous grasslands. Habitat fragmentationessentially encompasses three major components: –pure loss of habitat, –reduction of individual patch area and –increasing spatial isolation of the remnant patches(Andren, 1994).Since smaller patches generally support smallerpopulations, a decrease in patch area may lead toincreased extinction risk due to decreasing resistanceagainst stochastic extinction events, typically affectingsmall populations (Shaffer, 1981; Lande, 1988). Insmall patches that are also spatially isolated, extinctionprobabilities are expected to further increase through areduction of colonization events, hampering the‘ r e s c u e ’of the population by the inflow of new  individuals (Brown, Kodric-Brown, 1977) or therecolonization of the patches after the species wentextinct (Hanski, 1999).Simultaneously with the worldwide increasingdegree of habitat loss, the consciousness is growingamong conservationists that to conserve biodiversity,protecting areas will not be sufficient in the long termand that also ecological restoration is very important.Agood understanding of the effects of bothfragmentation and management is a prerequisite forsuccessful ecological restoration of more or lessseverely altered areas (Hobbs, Norton, 1996; Young,2000).In this manuscript we aim at presenting a concisereview of the actual knowledge on the effects of habitat fragmentation and management on plantspecies composition, population persistence and bioticinteractions in calcareous grasslands. We discuss theseeffects both in a conservation and in an ecologicalrestoration context and illustrate them with somepreliminary results from a study on the Belgiancalcareous grasslands of the Viroin valley. 2. HABITATFRAGMENTATION ANDPOPULATION PERSISTENCE Studies monitoring the population persistence of individual species from calcareous grasslands overtime are rare (but see Matthies etal. , 2004). It istheoretically expected that small population sizeincreases the risk of erosion of genetic variation and of inter-population genetic divergence due to increasedrandom genetic drift, elevated inbreeding,accumulation of deleterious mutations, and reducedgene flow (Young etal. , 1996). In the short term, thesegenetic changes may result in a decline of individualplant fitness, lower reproduction and decreasedpopulation viability (Ellstrand, Elam, 1993; Young etal. , 1996).Empirical evidence for genetic erosion in smallpopulations is supported by the research of vanTreuren etal. (1991) on two calcareous grasslandspecies: Scabiosa columbaria and Salvia pratensis .For the latter species, Ouborg and van Treuren (1995)also investigated the relation between genetic erosionand fitness characteristics in relation to populationsize. It was demonstrated that small populations werein an early genetic erosion process where geneticdiversity had already decreased, but where geneticvariation underlying fitness traits had not yet beenaffected. Also for Gentianella germanica , a positiverelation between population size and genetic diversitywas reported (Fischer, Matthies, 1998a). Moreover, apositive relation between genetic diversity and seedproduction and germination was demonstrated.Other studies focused on the relation betweenpopulation size and reproductive success, quantifiedby seed weight, seed number, seed size, germinationsuccess and offspring performance. Generally, seedproduction and offspring performance decreased withdecreasing population size. This has been illustratedfor the calcareous grassland species S e n e c i ointegrifolius (Widén, 1993), Gentianella germanica (Fischer, Matthies, 1998b), Primula veris (Kéry etal. ,2000) and Orchis purpurea (Jacquemyn etal. , 2002),but not for Salvia pratensis (Ouborg, van Treuren,1995).Although a decrease in seed production may bedriven by inbreeding depression, several authors alsosuggest the importance of pollination limitation, whichmay differ between populations of different size,whereas small populations are less attractive topollinators than large populations (Sih, Baltus, 1987;Jennersten, 1988; Agren, 1996). To disentangle geneticeffects from effects of pollen-limitation, Fischer andMatthies (1998b) argued that experimental cross- andself-pollinations in populations of different size areneeded. This approach has been followed for O . purpurea , where a hand-pollination experiment clearlydemonstrated that in small populations fruitproduction was pollinator limited (Jacquemyn etal. ,2002). We further deal with pollination in a nextparagraph.Although studies dealing with plant populationgenetics and fitness characteristics revealed alreadyimportant results for plant conservation, none of themsucceeded to integrate them in a long-term, populationdemographic frame (Oostermeijer etal. , 2003). Thisapproach could lead to better insights in how reducedgenetic diversity is affecting different plant life stages. 3. HABITATFRAGMENTATION ANDCOMMUNITYCOMPOSITION The ultimate consequence of reduced populationviability is local extinction, leading to disappearanceof the species from the fragment and consequently todecreased species richness and altered communitycomposition. Often species-area relations are used todescribe species richness of fragmented habitats( Figure1 ).Apositive species-area relationship may resultfrom one of the two following ecological hypotheses.First, there is the habitat-heterogeneity hypothesiswhich predicts higher species numbers because of higher habitat heterogeneity in larger fragments. Thesecond hypothesis, the so called equilibriumhypothesis, considers increasing species numbers withincreasing patch area independent of habitatheterogeneity (Rosenzweig, 1995). In the latter case,small habitats contain fewer species than larg e r 112  Biotechnol.Agron.Soc. Environ. 2005 9 (2), 111–118ButayeJ., AdriaensD., HonnayO.  habitats because population size in small habitats isrestricted and because small populations are moreextinction prone.Both hypotheses have been tested for 31calcareousgrassland fragments in Germany (Krauss e ta l . ,2004a). These authors found evidence for the habitatheterogeneity hypothesis, but not for the equilibriumhypothesis. It was further shown that this was true forboth habitat specialist and generalist species. Theauthors attributed the lack of a relation betweenfragment size and extinction probability to the specificplant traits of many calcareous grassland plants. Manycalcareous grassland species are long-lived perennialsthat form remnant populations by prolonged clonalgrowth (Sammul etal. , 2003), possibly exhibiting alowered susceptibility to habitat fragmentation. Theresult of this persistence is that current patchoccupancy patterns of most clonally propagatingspecies may be not in equilibrium with the presentdegree of habitat fragmentation (Eriksson, Ehrlén,2001). This means that the time period since habitatfragmentation (in most studies c. 50years) may be notsufficient to monitor extinction and that we are dealingwith a so called extinction debt (Tilman e ta l . , 1994) inour fragmented grasslands. In this case current patchoccupancy is reflecting the historical distribution of habitat fragments rather than the actual distribution.Very recently, Lindborg and Eriksson (2004) foundevidence for an extinction debt in Swedish semi-natural grasslands.However, in a preliminary analysis of the relationbetween average plant species richness of 1 m 2 plots ina grassland fragment and the area of that fragment wefound a positive species-area relation. This mayindicate that large fragments contain more speciesbecause of lower extinction risks of the populations( Figure2 ). The relation remained significant whencontrolled for the cover of   Brachipodium pinnatum .The role of specific traits in the persistence of plantspecies was confirmed by Fischer and Stöcklin (1997)who demonstrated that species with short life cycles,high habitat specificity and short living seeds weremore prone to extinction. Also Maurer etal. (2003)showed that species persistence related characteristics,such as onset of flowering, life form, mass per seedand duration of flowering, explained more of thespecies frequency of occurrence, then other traits suchas those affecting dispersal.Besides patch area, spatial isolation or the degreeof connectivity between patches is another importantfeature of fragmented habitats. Whereas in literaturemany threshold values for habitat connectivity can befound for fragmented forests and forest plant species(e.g. Butaye etal. , 2001; Honnay etal. , 2002), to ourknowledge, no information is available with respect tothe effects of spatial isolation on the distribution of plant species from calcareous grasslands. We alsofound no studies that have attempted to integrateindividual plant species traits in a landscape ecologicalcontext, as has recently been performed forfragmented forest and heathland habitat (e.g. Dupré,Ehrlén, 2002; Piessens etal. , 2004). Yet, this approachcould elucidate the complex relationship betweenpatch area and connectivity on the one side, and patchoccupancy of individual calcareous grassland specieson the other. Conservation and restoration of calcareous grasslands 113 140120100806040200Area (square meter) Figure1. Species-area relationship for the plant species in73 calcareous grassland fragments in the Viroin region (S-Belgium). Relation: S = -13 + 25*Log(Area), R 2 = 0.41,p<0.001. 353025201510500.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8log (area) (ha) F i g u re2 . Relation between the average plant speciesrichness per 1 m 2 quadrat in a grassland fragment and thearea of that fragment. R 2 =0.18, p<0.01. Data from thecalcareous grasslands in the Viroin region in Southern-Belgium.  4. HABITATFRAGMENTATION AND BIOTICINTERACTIONS With the growing consciousness that habitatfragmentation seriously affects plant fitness andreproductive success, the hypothesis that plant-pollinator interactions may be disrupted by habitatfragmentation, became a major topic in conservationbiology (Wilcock, Neiland, 2002; Goverde e ta l . ,2002). Pollination in particular can limit seedproduction –in habitats that became more isolated than theforaging distance of the pollinator; –in habitat patches that are too small to support aviable pollinator population or –when pollinators avoid small relic populations(Kearns etal. , 1998; Kwak etal. , 1998).Steffan-Dewenter and Tscharntke (1999) showedfor Sinapsis arv e n s i s and  Raphanus sativus , twoannual crucifers, that increasing isolation of experimentally established small calcareous grasslandfragments resulted in decreasing numbers of pollinators and lower seed set of the two crucifers.M o r e o v e r, since species of different trophic levelsare differently affected by habitat fragmentation it islikely that also other biotic interactions are altered( S t e ff a n - D e w e n t e r, Tscharntke, 2002). Generally thetrophic-level hypothesis of island biogeographystates that species of higher trophic levels are moreprone to the effects of fragmentation than species of lower levels (Holt e ta l . , 1999). Empirical evidencefor this hypothesis was found by Kéry e ta l . ( 2 0 0 1 ) ,who showed that reduced reproduction of  G e n t i a n ac ru c i a t a in small and isolated calcareous grasslandfragments was offset by decreased herbivory by itsspecialised herbivore,  Maculinea re b e l i . Colling andMatthies (2004) came to similar conclusions anda rgued that habitat fragmentation may release plantsfrom parasites and pathogens, which may mask thedirect negative effects of habitat fragmentation onplant fitness. Krauss e ta l . (2004b) studied therelation between the occurrence of  Cupido minimus and its larval food plant  Anthyllis vulneraria .Herbivore population size was positively related tothe cover of its host plant, but was independent of patch size and isolation. Unfortunately, the effect of herbivory on the host plant reproduction was notquantified. Groppe e ta l . (2001), finally, investigatedthe relation between  B romus ere c t u s and thepathogen,  Epichloë bro m i c o l a , with respect to small-scale habitat fragmentation. These authors relatedincreased choke disease at the plant level of the hostplant and/or the fungus to a higher degree of habitatfragmentation. 5.CONSERVATION AND RESTORATION OFCALCAREOUS GRASSLANDS Abandonment of calcareous grassland managementhas resulted in a sharp decline in suitable habitat areaand patch connectivity. Conservation and restorationof the remaining, often strongly degraded calcareousgrasslands, is therefore of utmost importance (Kahmen etal. , 2002, Mortimer etal. , 2002). Since calcareousgrasslands in northern and central Europe are a semi-natural vegetation, their long-term conservationrequires an appropriate management.Abandonment of extensive agriculturalmanagement primarily results in a decrease in speciesrichness due to increasing dominance of   Brachypodium pinnatum (Willems, 1983; Bobbink,Willems, 1987; 1988; 1991, f i g u re3 ), which isattributed to changes in soil nutrient availability(specifically increased availability of nitrogen),increased litter deposition and reduced lighta v a i l a b i l i t y. However, it is not clear whetherincreasing dominance of   Brachypodium pinnatum is aresult of the increased nitrogen levels or whetherincreased nitrogen levels are a result of increased litterdeposition of   Brachypodium pinnatum (Bobbink etal. ,1988; Hurst, John, 1999).Irrespective of the answer to this question,increasing nutrient availability and decreasing lightavailability may be partially offset by grazing ormowing. Jacquemyn etal. (2003) showed that in theshort-term, grazing and mowing of formerlyabandoned grasslands resulted in species accumulationcurves with higher accumulation rates and saturationpoints. Moreover, grazing proved to be more efficientthan mowing in countering the effects of increasednitrogen levels. Whether grazing or mowing is themost appropriate regular management for high quality 114  Biotechnol.Agron.Soc. Environ. 2005 9 (2), 111–118ButayeJ., AdriaensD., HonnayO. 16141210864200 10 20 30 40 50 60 70Cover Brachypodium pinnatum  (%) Figure3. Relation between the  Brachypodium pinnatum cover in a 1m 2 quadrat and the number of rare species. Rarespecies are species occurring in less then 20% of the 4km 2 squares of the Belgian plant species distribution atlas. R 2 =0.16, p<0.05. Xerophilous fragments were omitted for thisanalysis. Data from the calcareous grasslands in the Viroinregion in Southern Belgium.  calcareous grasslands is less straightforward. In Dutchcalcareous grasslands, During and Willems (1984)found that compared to mowing, grazing resulted in ahigher species richness and species diversity. Besidesthe difference between grazing and mowing, alsograzing intensity can be extremely variable. Sincegrazing intensity affects species selectively the highbiodiversity of calcareous grasslands can impossiblybe conserved by only one type management (Dolek,Geyer, 2002).Although most studies recommend grazing as themost appropriate management for calcareousgrasslands, Fischer and Wipf (2002) found that in theupper sub-alpine region, calcareous grasslands thathave been traditionally mown, were favored bymowing, rather than by grazing. Based on a long term(25years) experiment Kahmen etal. (2002) found thatburning and mulching both were less desirablemanagement regimes for calcareous grasslandhabitats. When defining appropriate managementregimes, history and nature of the community must beconsidered as very important variables (Grime etal. ,2000; Britton etal. , 2001).Although there may be time-lag of at least20years, cessation of management may also result in adecrease in bryophyte diversity. Particularly thenumber of annual bryophytes which are of highconservation interest decreased due to litteraccumulation and the almost complete cover of largepleurocarps (Vanderpoorten etal. , 2004). Moreover ahigh cover of bryophytes also has a negative impact onshort-lived forbs in terms of predation, emergence andestablishement of seedlings (Keizer etal. , 1985; vanTooren, 1988; Zamfir, 2000).When calcareous grasslands have been abandonedfor some time and succession towards forest hasseverely altered community composition, thesuccessful restoration of the calcareous grassland willnot only depend on an appropriate managements t r a t e g y, but also on propagule availability andfavorable germination conditions (Pärtel etal. , 1998).Thereafter, a good management will further enhancethe development towards mature, seed producingindividuals (Willems, Bik, 1998).Apotential propagule source for restorationpurposes that frequently has been investigated is thesoil seed bank (Akinola etal. , 1998; Davies, Waite,1998; Kalamees, Zobel, 1998). Most of these studiesagree that typical calcareous grassland species arebadly represented in the persistent seed bank,suggesting that seeds of species of open grasslands areunable to persist under developing scrub in the absenceof a continuous seed rain (but see Kalamees, Zobel,1997). Moreover, Davies and Waite (1998) showedthat the size of the viable seed bank decreased withdistance into the scrub and with the age of the scrub.For all these reasons, the significance of the seedbank for restoration of calcareous grasslandcommunities may be rather low.Additional nearbypropagule sources are therefore needed to restorespecies rich calcareous grassland communities,although the importance of natural dispersalmechanisms may not be overestimated. Verkaar etal. (1983) showed that the majority of the seeds incalcareous grasslands were found to reach the groundwithin 0.5m of the maternal plant, indicating that seeddispersal of most species is rather low. Poschlod etal. (1998) confirmed this statement and concluded thatspecies’own dispersal capacities are very low, exceptfor some well wind-dispersed species.Besides these mechanisms, several authors havepointed towards the importance of migrating sheep andcattle for plant species dispersal (Fischer etal. , 1996;Poschlod etal. , 1998). Especially sheep are known asimportant dispersal vectors for most calcareousgrassland species. Based on an extended database of 1 7 0calcareous grassland species and theircharacteristics, Poschlod e ta l . (1998) predicted aspecies richness of respectively 38, 68 and >120species for sites that have been clear-cut, mown orgrazed by sheep. The importance of seed dispersal bysheep was also demonstrated by Fischer etal. (1996)who showed that during one season half of the speciesnumber of a calcareous grassland were transported byone sheep and that it was independent from seed orfruit morphology. This indicates that routinelysubdividing species by linking dispersal syndromesaccording to seed or fruit morphologicalcharacteristics as has been done by many investigatorsis a huge oversimplification of the real plant dispersalmechanisms in grazed calcareous grasslands, as wasalso shown by Higgins et al. (2003). 6. CONCLUSIONS After abandonment of extensive agricultural grazingpractices, both conservation and restoration of calcareous grasslands seems to be seriously hamperedby increasing effects of fragmentation and decreasinghabitat quality. Most processes, however, remainpoorly understood.Although efforts should be increased to increasethe extent of calcareous grassland area and to improvehabitat connectivity, it must be noticed that the latter isoften difficult to realize in most northwesternEuropean landscapes, particularly due to a lack of financial and human means. Moreover, since manycalcareous grassland species have a high habitatspecificity, do not form a persistent seed bank, andhave limited dispersal capacities of their own, thereintroduction of grazing by sheep and thetranslocation of flocks between spatially isolated Conservation and restoration of calcareous grasslands 115
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