The legacy of past human land use in current patterns of mammal distribution
Summary (3 min read)
Introduction
- Threatened species are unevenly distributed across the world, with remarkable differences among taxonomic groups (Grenyer et al. 2006).
- As a result, the relationship between environmental factors, overall species richness, and the number of threatened species is not straightforward and shows spatial heterogeneity (Orme et al.
- Since the beginning of sedentary human societies and the advent of agriculture, around B.C.8000-6000, the amount of land under human dominance has grown at an accelerating pace (Ellis 2011).
- The authors study aims to provide an understanding of how past human land use relates with current global mammalian biodiversity patterns.
Data sources and selection
- Data of mean proportion of land use per unit area at different time spans were obtained from Ellis et al. (2013; available at http://ecotope.org/products/datasets/used_planet/).
- Grid-cells with an emerged area smaller than 10,000 km2 were excluded to avoid comparing grid-cells with very unequal areas.
- For the purpose of the present work, it was represented as the value of human land use at c.A.D.2000, the most recent time break available on this data source.
- The proportion of threatened mammals was calculated by dividing the number of threatened mammals over the total mammal richness per grid-cell.
Statistical analyses
- To synthetize trajectory trends in longitudinal data of global land use the authors employed a clustering method that incorporates a k-means algorithm (Celeux and Govaert 1992) implemented in the kml package ('kml' function; Genolini et al. 2015) in R v.3.2.3 (R Core Team 2015).
- The obtained trajectory-clusters were used as a categorical past land-use predictor in global models to test differences among clusters in terms of mammalian diversity and vulnerability.
- Finally, from correlated pairs of two past land-use indicators the authors selected the past land-use indicator representing the oldest temporal span to increase the contrast with present land use (a control variable).
- To fit the models the authors used the function ‘gbm.step’, which calculates the optimal number of boosting trees using 10-fold cross validation, and it is included in the dismo package (Hijmans et al. 2013) in R.
- The explanatory power of each model was calculated as the percentage of deviance explained respect to a null model, defined as one without any splits –equivalent to an intercept only model in linear regression (Ferrier and Watson 1997).
Results
- All quality criteria supported the differentiation of three generalized trajectory-clusters describing global temporal patterns in past land use from c.B.C.6000 to c.A.D.2000 (Fig. S1.1).
- Recently-used areas largely correspond to territories of relatively modern human colonization and expansion, such as North America, Australia or southern and East Africa.
- While quality criteria supported three clusters, there were additional configurations that had partial support.
- These suggested further division of recently and steadilyused areas into more clusters with low-used areas always remaining as a single group (Fig. S1.2).
- Both total mammalian richness and proportion of threatened species distribution were mainly explained by environmental indicators; with the exception of the relevance of past land use (c.A.D.1000) on total richness (Table 1) and pre-industrial rate of land-use change on proportion of threatened mammals within recently-used areas (Table 3).
Discussion
- The authors results show that land-use history across the world can be broadly summarized into three trajectory-clusters: low-, recently- and steadily-used areas.
- There are disparities in the influence of different predictors in explaining mammal diversity metrics within each cluster, and in the shape of the relationship between predictors and responses.
- When disaggregating by trajectory-cluster, the authors detected descriptors of past landuse change as relevant.
- This signal of the past is most noticeable for recently-used areas and when assessing differences in total numbers of threatened mammals.
- Current land use or descriptors of remarkable past land-use changes were not identified as relevant in any model.
Low-used areas
- According to their results, more than 50% of the global land area analyzed (excluding Antarctica and most of Greenland) have followed a low-used trajectory.
- This high share of the global surface may explain why general trends in the effects of environmental and land-use predictors within these areas resemble the global trends.
- These areas broadly coincide with last-of-the-wild regions, traditionally seen as opportunities to preserve biodiversity given the relatively low human influence to which they are exposed (Sanderson et al. 2002).
- Lack of historical human pressure may be explained by two different reasons: low primary productivity associated to biomes in the northern hemisphere, such as the boreal forests, the Tundra, and deserts worldwide; and relative remoteness, limiting accessibility in some tropical forests, e.g. Amazon or Borneo (Fig. 1).
- The fact that more threatened species tend to occur in areas more rapidly transformed during the period c.A.D. 0-1750 suggests the existence of a land-use legacy on these parts of the Earth, where mammals remain negatively influenced by past human impacts and local extinctions have not yet occurred given the low magnitude of land-use changes (Bürgi et al. 2017).
Recently-used areas
- Around 32% of global land is classified as a recently-used trajectory, which coincide with areas humanized after the great colonization events of the 15th century onwards.
- Many of these regions are located within highly developed countries, such as the United States or Australia (Fig. 1).
- Today, these areas do not present particularly high species richness or accumulations of threatened mammals (Table S2.1), thus they are not generally considered a global priority (Brooks et al. 2006).
- As for the land-use-history variables of interest, total mammalian richness is lower where the proportion of land use c.A.D.1000 had been relatively low (Fig. 4), opposite to what the authors expected under a time-lag effect (lower species richness predicted for areas heavily modified in the past).
- Therefore, there is not a lagged effect of past land-use changes on current total richness, which remains relatively high within these areas (Table S2.1).
Steadily-used areas
- Less than 16% of global land belongs to this trajectory-cluster, which is characterized by a relatively high and long-lasting human land use covering different tropical and temperate regions.
- In these areas, steep changes in use were rare, but their average level of human appropriation of land by A.D.0 was already higher than levels observed today in low-used areas (Table S2.1).
- Furthermore, areas with more threatened mammals are also characterized by rapid, very low, or even negative preindustrial changes (i.e. where human land use decreased; Fig.4).
- On the other hand, in areas like Eastern Europe and African regions from Guinea to Chad, changes were pronounced and their prediction of species not having had time to recover from past human pressures may hold (Fig. S4.18).
- Nevertheless, it is important to note that, within these areas, environmental factors have the greatest relevance in explaining diversity, therefore, time-lagged effects must be cautiously interpreted (Table 2).
Conclusions
- To their knowledge, the use of temporal trajectory-clusters' delineation has never been applied in biogeographical or ecological studies.
- In particular, the rate of change during the period c.A.D.0-1750 repeatedly appears as relevant in tested models, highlighting the importance of this time-span.
- Extinction debts at regional scales have been found to reflect more recent habitat modifications and particularly affect sensitive species, agreeing with their global results (Uezu and Metzger 2016).
- Overall, their study shows current biodiversity patterns reflect a signal of environmental drivers but also of past land use.
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...The mammalian distributions that we know today are not only a reflection of the most recent human actions but also those exerted during the last few millennia (Faurby and Svenning, 2015; Polaina et al., 2020)....
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References
272,030 citations
"The legacy of past human land use i..." refers methods in this paper
...To synthetize trajectory trends in longitudinal data of global land use we employed a clustering method that incorporates a k-means algorithm (Celeux and Govaert 1992) implemented in the kml package ('kml' function; Genolini et al. 2015) in R v.3.2.3 (R Core Team 2015)....
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"The legacy of past human land use i..." refers background or methods in this paper
...Present environmental conditions were synthesized for each 110 km grid-cell in terms of annual mean actual evapotranspiration (AET), obtained from Zhang et al. (2010); annual mean temperature and precipitation, obtained from WorldClim2 (Fick and Hijmans 2017); and mean elevation, extracted from the global digital elevation model GTOPO30 (LP DAAC 2004; Table S2....
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...…grid-cell in terms of annual mean actual evapotranspiration (AET), obtained from Zhang et al. (2010); annual mean temperature and precipitation, obtained from WorldClim2 (Fick and Hijmans 2017); and mean elevation, extracted from the global digital elevation model GTOPO30 (LP DAAC 2004; Table S2....
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...…AET mm, accumulated 2000 1 degree month Zhang et al. (2010, 2015) Mean annual temperature Temperature ºC, average 1970-2000 10 arc minutes month Fick & Hijmans (2017) Mean annual precipitation Precipitation mm, average 1970-2000 10 arc minutes month Fick & Hijmans (2017) Global digital…...
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...…Mean annual temperature Temperature ºC, average 1970-2000 10 arc minutes month Fick & Hijmans (2017) Mean annual precipitation Precipitation mm, average 1970-2000 10 arc minutes month Fick & Hijmans (2017) Global digital elevation model Elevation m 1996 30 arc seconds - LP DAAC (2004) Figure S2....
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"The legacy of past human land use i..." refers background in this paper
...…notable transitions, such as the European invasions in the 15th century, the 19th century Industrial Revolution with its productive, technological and demographic shifts and, more recently, the Green Revolution that triggered the so-called 'great acceleration' (c.A.D.1950, Steffen et al. 2015)....
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"The legacy of past human land use i..." refers methods in this paper
...…was defined using five non-parametric quality indices according to different criteria: three variants of the Calinski & Harabanz criterion, the Ray-Turi criterion and the Davies-Bouldin criterion (Calinski and Harabasz 1972, Davies and Bouldin 1979, Ray and Turi 1999, Kryszczuk and Hurley 2010)....
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...…different quality tests, 1(black): Calinski-Harabatz (Calinski & Harabasz 1972); 2 (red): Calinski-Harabatz2, Kryszczuk variant (Kryszczuk & Hurley 2010); 3 (green): Calinski-Harabatz3, Genolini variant; 4 (blue): Ray-Turi (Ray & Turi 1999); 5 (cyan): Davies-Bouldin (Davies & Bouldin 1979)....
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...The optimal number of clusters was defined using five non-parametric quality indices according to different criteria: three variants of the Calinski & Harabanz criterion, the Ray-Turi criterion and the Davies-Bouldin criterion (Calinski and Harabasz 1972, Davies and Bouldin 1979, Ray and Turi 1999, Kryszczuk and Hurley 2010)....
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...Different numbers and colors represent different quality tests, 1(black): Calinski-Harabatz (Calinski & Harabasz 1972); 2 (red): Calinski-Harabatz2, Kryszczuk variant (Kryszczuk & Hurley 2010); 3 (green): Calinski-Harabatz3, Genolini variant; 4 (blue): Ray-Turi (Ray & Turi 1999); 5 (cyan): Davies-Bouldin (Davies & Bouldin 1979)....
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Frequently Asked Questions (7)
Q2. What have the authors stated for future works in "The legacy of past human land use in current patterns of mammal distribution" ?
The fact that past land-use metrics are more relevant in explaining total and especially threatened mammalian richness distributions than present land use also has implications for future studies given the widespread practice of including present land use, but not past use, as a predictor of numbers of threatened species ( Lenzen et al. 2009, Koh and Ghazoul 2010, Brum et al. 2013 ). Deeper impacts on species biodiversity may have already occurred, but they may only be apparent to the conservation biologists of the future. On the other hand, what the authors call current mammalian biodiversity patterns may be an optimistic picture of what actually remains, since rapid changes may not be captured by global biodiversity databases and, although their resolution of 1° is recommended at the global scale, IUCN range maps are known to overestimate species geographic ranges ( Hurlbert and Jetz 2007 ). Recentlyused areas, which have been rarely prioritized for conservation up to now, are likely most affected by extinction debt which may be best addressed via restoration initiatives.
Q3. How much land belongs to this trajectory-cluster?
Less than 16% of global land belongs to this trajectory-cluster, which is characterized by a relatively high and long-lasting human land use covering different tropical and temperate regions.
Q4. How much land is classified as a recently-used trajectory?
Around 32% of global land is classified as a recently-used trajectory, which coincide with areas humanized after the great colonization events of the 15th century onwards.
Q5. How many areas of the world have followed a low-used trajectory?
According to their results, more than 50% of the global land area analyzed (excluding Antarctica and most of Greenland) have followed a low-used trajectory.
Q6. What is the relationship between threats and threatened species?
It seems clear that whereas the most threatening activities are normally related to lower total species richness, the relationship between threats and threatened species likely varies across regions and spatial scales in more complex ways.
Q7. What was the effect of the global model on the distribution of threatened mammals?
Within low-used areas, effects directions were the same as described for the global modeland, additionally, more threatened mammals were found where pre-industrial land-use change was relatively faster (Fig. 4).