Australia: The Land Where Time Began

A biography of the Australian continent 

Biotic Homogenisation is Promoted by Gains of Native Species Over 4 Decades in a Human-Dominated Landscape

The importance of anthropogenic activities in the reshaping of biodiversity is rapidly increasing. The interactive effects of climate changes, biological invasions and the replacement of species are all not well understood, particularly in megadiverse biomes and at large scales. The aim of this study was to assess the effects of climate change as a driver of spatio-temporal patterns of biodiversity and homogenisation of woody plants in the Atlantic Forest, Brazil, at multiple scales, in a species assemblage that was hyperdiverse. The expansion of current generalist and disturbance-tolerant species instead of the extinction and retraction of local endemics may lead to homogenisation of woody plants in the tropics, are the main conclusions of this study. In the Atlantic Forest the woody plant assemblage is prone to a structural reorganisation resulting from climate change, threatening the conservation of biodiversity and potentially leading to severe biotic homogenisation on a large scale in the future.

It is increasingly being recognised that climate change, biological invasion and replacement of species are pervasive components of the global biodiversity crisis (Domelas, Gotelli & McGill, 2014; Lewis & Maslin, 2105; Malhi, Gardner & Goldsmith, 2014; McGill, Domelas, Gotelli & Magurran, 2015). The interactive effects of such anthropogenic disturbances or reorganisation of biodiversity and the conservation of species are poorly understood (Bellard et al., 2014; Gardner et al., 2009; Olden, 2006). In tropical ecosystems this is particularly relevant, which are extraordinarily species rich, though they are facing massive impacts that threaten biodiversity and the wellbeing of humans (Cardinale, Duffy & Gonzalez, 2012; Gardner et al., 2009; MEA, 2005; Mittermeier et al., 2011). The top 5 global centres of diversity of vascular plants, e.g., are tropical forests which together cover 0.2% of the land surface of the Earth, but contains about 18,500 endemic species, which represents 6.2% of all vascular plants (Barthlott, Mutke, Rafiopoor, Kier & Kreft, 2005). About half of the original distribution of tropical forests have already been lost, and deforestation continues (Bellard et al., 2014; Early et al., 2016; Frehse, Braga, Nocera & Vitule, 2016; Wright, 2005).

The definition of biological invasion, from a broad perspective, is the rapid proliferation of and expansion of range of species after the disappearance of natural obstacles (Valéry, Fritz, Lefeuvre & Simberloff, 2008). Non-native species are most often seen to be the invaders, especially following disturbance (Simberloff, Souza, Nuñez, Barrios-Garcia & Bunn, 2012; Valéry, Fritz, Lefeuvre & Simberloff, 2009). As a general phenomenon, biological invasion is often enhanced by disturbance of habitat which may have detrimental effects on biodiversity (McGill et al., 2015; Simberloff & Vitule, 2014; Simberloff et al., 2103). Invasion and range-expansion of widespread species that are tolerant of disturbance at the expense of local endemics is leading to the simplification of the ecological communities affected, a phenomenon that has been termed Biotic homogenisation (BH) (McKinney & Lockwood, 1999).

Biotic homogenisation is a complex process during which local endemics are gradually replaced by widespread species, which decreases beta diversity over space and time (McKinney & Lockwood, 1999; Olden & Poff, 2003: Olden & Rooney, 2006). Extermination of local populations is a way in which homogenisation can occur, as is invasion of wide ranging non-native species,  range expansion of generalist native species, and the combination of all the above (Inderjit et al., 2017; Olden & Poff, 2003; Tabarelli, Peres & Melo, 2012). According to McCune & Vellend there is a growing consensus that anthropogenic destruction on a local scale and the introduction of non-native species are important drivers of biotic homogenisation; mounting current evidence also suggests, however, that the spread of native as well as non-native generalists may contribute strongly to homogenisation of ecological communities (Bellard et al., 2016; Inderjit et al., 2017; McCune & Vellend, 2013; Nasf &
Wulf, 2010; Pecl, Araújo & Bell, 2017; Tabarelli et al., 2012). There have been only a few studies that have assessed the contribution of the replacement of species to the biotic homogenisation across different scales in tropical forests (Lôbo, Leão, Melo, Santos & Tabarelli, 2011; Tabarelli et al., 2012), thereby restricting understanding of the process of biotic homogenisation and potential threats to biodiversity in these important ecosystems.

Climate change, in this sense, can potentially enhance biological invasions and biotic homogenisation by promoting the expansion of ranges of species that have broad environmental niches and large population sizes, i.e., generalists that have a high potential of invasion, and forcing the contraction of species that have narrow niches, i.e., specialists, thereby mixing the composition of biotas that were previously disparate (Bellard, et al., 2016; Hui et al., 2016; Inderjit et al., 2017; Lawler et al., 2013; McKinny & Lockwood, 1999; Rahel & Olden, 2008). Shifting climatic conditions have been attributed by palaeoecological evidence and long-term vegetation studies to both increases and decreases in richness of local plant species, and turnover of composition (Damschen, Harrison & Grace, 2010; Danby, Koh, Hik & Price, 2011; Savage & Vellend, 2013; Williams & Jackson, 2997). There has been intense scientific, social and political debate on possible mechanisms and threats imposed by climate change; there remains, however, considerable uncertainty of its contribution to homogenisation processes globally across multiple geographic regions and ecosystems (McGill et al., 2015; Olden et al., 2010). Studies that link climate change and biotic homogenisation pf Neotropical biomes that are hyperdiverse are therefore essential to understand and mitigate potential anthropogenic synergistic impacts on biodiversity (Brook, Sodhi & Bradshaw, 2008).

In this paper McCune & Vellend explored the patterns of diversity of woody plants in the Atlantic Forest biodiversity hotspot at the present and into the future (Mittermeier et al., 2011). They used ecological niche models (ENMs) to assess changes in geographic distributions and assemblages of species. They assumed there was a relationship between climatic niche and geographic distribution then investigated the effects of climate change as a driver of taxonomic homogenisation. More specifically, spatial patterns of the present and the future of species richness, size of range, and covariance in compositions across different ecoregions and climate change scenarios in the Atlantic Forest. They hypothesised that the invasion/expansion of widespread species, that are adapted to broad environmental conditions, at the expense of endemic species with a narrow distribution results in beta diversity of woody plants, increases in the mean covariance of species and size of range over time. Also, they explored and discussed potential changes in areas that are currently protected in the Atlantic Forest and their potential effects on efforts at conservation.

Discussion

Major threats to biodiversity as well as human wellbeing are constituted by biotic homogenisation, biological invasions and climate change, affecting organisms and species assemblages across a wide range of taxonomic functional groups (Lewthwaite, Debinski & Kerr, 2017; Pecl et al., 2017). The comparisons of assemblages of species that were obtained from ENMs allowed for the testing of the evidence for climate change being a driver of biological invasions and restructuring of biodiversity across ecoregions, time periods and emission scenarios. They suggest in this paper that shifts in the ranges of species that are induced by climate change will interact with biological invasions and replacements of native species, and thereby promote biotic homogenisation of woody plant diversity on a large scale in the Atlantic Forest hotspot.

The results of McCune and Vellend reinforce the notion that across the Atlantic Forest there is a tendency for species co-occurrence. McCune and Vellend estimate that 33% of shared species match closely recent results, about 31%, that were obtained by the comparison of forest inventories across different ecoregions in the Atlantic Forest (Eisenlohr & Oliveira-Filho, 2015). The ecoregions present unique species that characterise them as distinct forest types, in spite of having many species in common (Eisenlohr & Oliveira-Filho, 2015; Oliveira-Filho & Fontes, 2000). The expected singularities were supported by higher contribution of turnover to the total dissimilarity as evidenced in the RD plots, in which ecoregions could be distinguished as groups that were relatively cohesive that are well differentiated in richness of species and range size values. Similarities and singularities in composition of woody plant species, that as well as variation in environmental conditions are revealed by the results of McCune & Vellend (Marcilio-Silva et al., 2017; Marques et al., 2011; Oliveira-Filho & Fontes, 2000), give support to the recognition that the Atlantic Forest is a unique biome that is comprised of a series of ecosystems (Eisenlohr & Oliveira-Filho, 2015; Marcilio-Silva et al., 2017).

A multitude of indices may be used to summarise the spatial-temporal variation of diversity and similarity (Anderson, Crist & Chace, 2011; Tuomisto, 2010), though they are generated essentially by 2 distinct processes: spatial turnover of species composition, and gradient changes in the richness of species. There are only a few studies that have assessed the relative contribution of such processes to homogenisation of ecological communities (Baiser, Olden, Record, Lockwood & McKinny, 2012; McCune & Vellend, 2013; Okimura, Koide & Mori, 2016). It was observed by McCune & Vellend that in the Atlantic Forest the beta diversity of woody plant assemblages presented a systematic loss when current estimates were contrasted with future climatic scenarios. The increased similarity among sites, more importantly, was accentuated in climate change scenarios that were more severe, which was accompanied by an increase in the average richness and nestedness in the composition of species. This pattern resulted from different mechanisms. Species that had small ranges were more important when the conditions were severe because species that were more widely distributed had already expanded to the limits of their biome, though the expansion of species with large or intermediate ranges contributed mostly to the reduction of beta diversity in climate scenarios that were moderate. Biotic homogenisation was, thereby, driven mostly by expansion of the distributional limits of current species rather than the spatial turnover. Theoretical predictions and empirical evidence of increased richness of species on local scales and decreased beta diversity at larger scales is supported by the results of this study (Beauvais, Pellerin & Lavoie, 2016; Domelas et al., 2014; McCune & Vellend, 2013; McGill et al., 2015; Thomas, 2013; Vellend et al., 2017). This study also highlights the importance of changes in gradients of the richness of species, as opposed to spatial turnover, to explain the large-scale patterns of homogenisation (Baiser et al., 2012; McCune & Vellend, 2013).

Also, it was shown by the results of this study that the temporal variation of diversity changed across space and scale. Ecoregions experienced different intensity as well as the direction of variation, while most ecoregions presented a slight tendency of increasing richness, constant values or tendency to loss of species in future climatic conditions was presented by Bahia interior and coastal forests. Conversely, a constant tendency to higher values in the future was presented by mean proportional size of range and covariance. These results, combined, suggest that on a local scale some species may experience biotic homogenisation from turnover that was led invasion of species that are widely distributed that are adapted to the conditions in the region in synergy with local extinction of species that are narrowly distributed (Vallejos, Padial & Vitule, 2016; Vellend et al., 2017).

Conservation implications

Drastic declines are occurring in tropical forests that have importent consequences for the function of ecosystems and this affects the wellbeing of humans (Bellard et al., 2014; Cardinale et al., 2012). Protected areas play a crucial role in the conservation of remnants, protection of ecosystem services and refuge from non-native invasive species in such highly impacted landscapes (Gallardo et al., Isbell, Calcagno & Hector, 2011; Zwiener et al., 2017). The results obtained by McCune & Vellend, importantly, stress the potential of climate change to enhance biotic homogenisation within current protected areas, places where widespread native species could buffer the extinctions of local species and therefore maintain consistent levels of local richness at the expense of beta diversity. The apparent constancy or increase in local richness in tropical forests as well as other regions that are species rich, may mask the redistribution of biodiversity, such as turnover of species and Biotic homogenisation, that is driven by major anthropogenic impacts such as climate change (Baiser et al., 2012; Dornelas et al., 2014; Pecl et al., 2017; Sheldon, Yang & Tewksbury, 2011).

The principal causes of the current declines of biodiversity are considered to be the fragmentation of habitats and degradation (Brooks, Mittermeier & Mittermeier, 2002; Newbold et al., 2016; Silva & Tabarelli, 2000). This study was limited by the reduction of beta diversity obtained considering only variations of climate; McCune & Vellend indicate, however, that habitat alteration that is human-induced may act in synergy with climate change to homogenise biodiversity more severely than anticipated. E.g., local loss of species and proliferation of native generalists due to changes in landscape have been implicated as drivers of biotic homogenisation of birds and plants (Lôbo et al., 2011; Vallejos et al., 2016). Given that the inability of plant species that are narrowly distributed to track their climatic niche in the future may be limited by edaphic factors, interspecific competition, and loss of pollinators and seed dispersers, (Corlett, 2009; Opdam & Wascher, 2004), local endemics may be jointly reduced by climate changes and habitat conversion may favour the same generalist species (Frishkoff et al., 2016). The predictions based on this study are important within this context to prevent a potential severe homogenisation framework in the near future. McCune & Vellend therefore highlight the role of climate change in biotic homogenisation and emphasising the need for integrative approaches, such as the assessment of changes in land use, combined with long term standardised monitoring across different ecosystems and taxa, in order to reveal and potentially counteract mechanisms that affect biotic homogenisation (Inderjit et al., 2017; Vellend et al., 2017).

Future Patterns and homogenisation

A multitude of indices may be used to summarise the spatial-temporal variation of diversity and similarity (Anderson, Crist and Chase, 2011; Tuomisto, 2010) but are generated by 2 distinct processes; spatial turnover in the species composition and changes in the species richness gradients. The relative contributions of such processes to homogenisation of ecological communities have been assessed by only a few studies (Baiser, Olden Lockwood & McKinny, 2012; McCune & Vellend, 2013; Okimura, Koide & Mori; 2016). It was observed in this study that the overall beta diversity of the woody plant assemblages of the Atlantic Forest presented systematic loss when current estimates were contrasted with future climatic scenarios. McCune & Vellend suggest it is more important that increased similarity among sites was accentuated in climate change scenarios that are more severe, which was accompanied by average richness increases and nestedness in species composition. Different mechanisms produced this pattern. Though the reduction of beta diversity in climate change scenarios that are moderate was contributed to by the expansion of species with large and intermediate ranges, while under severe conditions species with small ranges were more important and widely distributed species had already expanded to the limits of the biome. Biotic homogenisation was, therefore, driven mostly by expansion of the limits of the distribution of current species rather than spatial turnover. Theoretical predictions and empirical evidence of increased species richness at local scales and at larger scales decreased beta diversity are supported by the results of this study (Beauvais, Pellerin & Lavoie, 2016; Domelas et al., 2014; McCune & Vellend, 2013; McGill et al., 2015; Thomas, 2013; Vellend et al., 2017). The relative importance of changes in gradient of species richness, as opposed to spatial turnover, to explain homogenisation on large scales (Baiser et al., 2012; McCune & Vellend, 2013).

Also, temporal variation of diversity was found by this study to have changed across space and scale. Ecoregions experience different intensity and even different variation direction on regional scales. The Bahia interior and coastal forests presented constant values of tendency to loss of species under climatic conditions in the future, though most ecoregions presented a slight tendency of richness increase. Mean proportional range size and covariance presented, conversely, a consistent tendency to higher values in the future. It is suggested by these results in combination at local scales that some regions may experience biotic homogenisation from turnover that was led by invasion of widely distributed species that were adapted to the conditions in the region in synergy with local extinction of species that were narrowly distributed (Vallejos, Padial & Vitule, 2016; Vellend et al., 2017).

Implications for conservation

At present tropical forests are undergoing drastic biodiversity declines that have important consequences for the functioning of ecosystems and human wellbeing (Bellard et al., 2014; Cardinale et al., 2012). Protected areas play a crucial role in conservation of remnants in such landscapes that are highly impacted, provision of ecosystem services and refuge from non-native invasive species (Gallardo et al., 2017; Isbell, Calcagno & Hector, 2011; Zwiener et al., 2017). The results of this study, importantly, stress the potential of climate change to enhance biotic homogenisation within areas that are currently protected, where invasion of widespread native species could buffer extinction of local species and thereby maintain constant levels of local richness at the expense of beta diversity. In tropical forests, as well as other regions that are species-rich, the apparent constancy or even increasing local richness may mask redistribution of biodiversity, such as turnover of species and biotic homogenisation, that are driven by major anthropogenic impacts such as climate change (Baiser et al., 2012; Dornelas et al., 2014; Pecl et al., 2017; Sheldon, yang & Tewksbury, 2011).

Fragmentation and degradation of habitats are considered to be the principal cause of declines of biodiversity (Brooks, Mittermeier & Mittermeier, 2002; Newbold et al., 2016; Silva & Tabarelli, 2000). The reduction of beta diversity was obtained by considering only climate variation was a limitation of this study; McCune & Vellend advocate, however, that habitat alteration that is human induced may act in synergy with climate change to homogenise biodiversity more severely than has been anticipated. E.g., loss of local species and proliferation of native generalists that result from landscape changes have been implicated as biotic homogenisation of birds and plants (Lôbo et al., 2011; Vallejos et al., 2016). Given that edaphic factors, interspecific competition, and loss of pollinators and seed dispersers may limit the ability of plant species that are narrowly distributed to track their climatic niche in the future (Corlett, 2009; Opdam & Wascher, 2004), climate change and the conversion of habitat together may reduce local endemics and favour the same generalist species (Frishkoff et al., 2016). The predictions of this study are, within this context, important to prevent severe homogenisation framework in the near future. This study therefore highlights the role of climate as a driver for biotic homogenisation, emphasising the need for integrative approaches, such as land use change assessment combined with standardised monitoring across ecosystems and taxa in the long term, to reveal and potentially counteract mechanisms affecting biotic homogenisation (Inderjit et al., 2017; Vellend et al., 2017).

Final considerations

According to McCune & Vellend inferences concerning the consequences of climate change in hyperdiverse tropical assemblages at large spatial scale has remained mostly theoretical (Malhi et al., 2014; McGill et al., 2105), especially for plant assemblages for which the lack of long-term standardised field data and biodiversity complexity make difficult substantial empirical validation (Franklin et al., 2917; Lima et al., 2015). In spite of limitations that are inherent in the estimation of community composition with individual ENMs (Aranda & Lobo, 2011; Thuiller et al., 2008), inferences are allowed to be made over large spatial and temporal scales by this approach, and may still capture the imprint of assembly processes across scales (Distler et al., 2015; Thuiller, Pollock, Gueguen & Münkemüller, 2015). This study used a comprehensive database of ENMs that were carefully built for 2,255 species that are native to the Atlantic Forest to infer the potential changes in biodiversity in the future in one of the most important hotspots in the world (Mittermeier et al., 2011). It is shown clearly by this study that in spite of increases in or invariability of species richness, it is likely that the Atlantic Forest will experience biotic homogenisation over time and space.

It has increasingly been reported that for many taxa and ecosystems biotic homogenisation has been occurring (Arroyo-Rodríguez et al., 2013; Baiser et al., 2012; Lôbo et al., 2011; McCune & Vellend, 2013; Solar, Barlow & Ferreira, 2015; Vellend et al., 2017; Vitule, Skóra & Abilhoa, 2012). There is a growing realisation that biotic homogenisation will be intensified by changes in climate and land use and that the integrity and functioning of ecosystems may be compromised by such influences, stresses the urgency for mitigating actions to be taken that consider the many threats to biodiversity and human wellbeing. The success of conservation strategies is dependent on explicit recognition of patterns as well as drivers of the biodiversity crises across spatial and temporal scales (McGill et al., 2015). It is shown by this study that the Atlantic Forest is prone to structural reorganisation and homogenisation on a large scale as a result of impacts that are human driven, suggesting the efforts at conservation should extend beyond the establishment of new protected areas, while also accounting for potential effects of changes of climate and land use, restore endemic specialists and manage widespread invaders in order to optimise the conservation of biodiversity.

Sources & Further reading

1. McCune, J. L. and M. Vellend (2013). "Gains in native species promote biotic homogenization over four decades in a human-dominated landscape." Journal of Ecology 101(6): 1542-1551.