Re-evaluating the volume-species relationship in biodiversity

The species-area relationship (SAC) is a long-time considered pattern in ecology and is discussed in most of academic Ecology books. Its implications are relevant for many ecological, evolutionary, conservation and biogeographic purposes. Conversely, the associated volume-species relation has been almost ignored. 

Nevertheless, according to a new study published in the journal Plant Ecology (Cazzolla Gatti R. , Di Paola A., Bombelli A., Noce S., Valentini R. Exploring the relationship between canopy height and terrestrial plant diversity. Plant Ecology, 2017 doi:10.1007/s11258-017-0738-6) this relationship may play a fundamental role in ecology and it is relevant for many different ecological applications, such as the estimation of minimum viable populations, species ranges, protected areas, etc. 

In this global-scale study, Roberto Cazzolla Gatti, Ph.D., Associate Professor at Biological Institute of the Tomsk State University (Russia) and his Italian colleagues from the Euro-Mediterranean Center on Climate Change (CMCC) in Viterbo, investigated such new perspective looking at canopy height as a proxy of ecosystem volume (“biospace”), which influences plant richness in forest ecosystems.

Some decades ago, the Italian forest ecologist Lucio Susmel developed the idea of “biospace” and stated that “the features of multi-aged forest are a function of the aboveground biospace, modified by plants and animals living in a physical environment”. The Italian ecologist suggested that “[Biospace may be defined as a] protected space within which it is possible to play all physiological, biological and evolutionary processes of a community […] the parameter most appropriate for assessing the biospace is the volume of the system that can be measured by the average height of the dominant trees”.

Up to date, the difficulty to detect tree’s height by ground and the lack of a comprehensive global flora census has impeded to define a general pattern of forest ecosystem volume and species diversity. Only the recent developments of new powerful technologies, such as Light Detection and Ranging (LiDAR), allowed to map forest vertical structure globally. Together with the availability of new accurate botanical data, such as the NASA Canopy Height Global Map, which is a novelty in satellite technology combined with field analyses, this opens incredible opportunities in ecology. 

“We investigated a possible global relation between species richness and canopy height by comparing the high resolution Global Map of Canopy Height provided by NASA with the Plant Diversity Map produced by Barthlott and colleagues in 2007,” explained Prof. Roberto Cazzolla Gatti.

Eventually, the results of this study show that higher canopies account for more plant species: there is a third dimension fully exploitable by the species. This is certainly because larger volumes can contain larger numbers of species, but it is not only a matter of the available space.

“I came across this idea – revealed Prof. Cazzolla Gatti – working for many years in tropical, temperate and boreal forests. I wondered why higher canopies are almost always positively associated with higher level of biodiversity. If the reason were only the climate, which is able to increase both trees height and biodiversity, we would have found within the same climatic strip around the world similar level of diversity in forests with different canopy heights. This wasn’t the case: although climatic conditions are the same biodiversity increases when canopy rises up”.

The study suggest that this positive correlation between biodiversity and canopy height is due to the increased biospace: the larger the volume of a forest ecosystem, the more layers and ecological conditions (light, humidity, food resources, water availability, climbing opportunity for lianas, presence of epiphytes, ferns, etc.) that diversify the environment. This is also offers an empirical proof to the hypotheses developed during the last years by Prof. Cazzolla Gatti about the emergence of new niches biodiversity-related, i.e. the idea that biodiversity begets biodiversity (Cazzolla Gatti 2011 in Theoretical Biology Forum; Cazzolla Gatti 2016 in Biologia; Cazzolla Gatti et al. 2017 in Ecological Modelling).

The relationship between biodiversity and canopy height is particularly evident in tropical regions. In fact, according to the latitudinal gradient theory, tropical rainforests are, on average, taller than temperate ones, and therefore offer more space for physiological, biological and evolutionary processes of the community. This feature allows species with distinct traits to coexist and begets the emergence of new niches that increases the richness of a more stratified ecosystem.

However, because both climatic and latitudinal gradients could veiled and correlate with forest height and diversity, Cazzolla Gatti and colleagues disentangled their hypothesis of a pure canopy height-diversity relation by analysing it within different macroclimate zones according to the Koppen-Geiger climate classification. This classification reflects a latitudinal zonation and allows to remove the climatic influence from the canopy height-diversity relationship. However, the relationship was observed in each of the three main climate zone and this confirmed that canopy height influences species diversity irrespective of other factors such as precipitation and temperature (i.e. climate).

Roberto Cazzolla Gatti commented: “The relation between canopy height and biodiversity has been poorly considered to date, even if it can play a significant role in ecology. Indeed, the vertical dimension of ecosystems, as a proxy of the biospace, should be considered together with the well-known species-area relationship. Moreover, the relation we discovered is fundamental for setting protected areas extensions, when the bi-dimensional available environment should not be considered alone, but associated with the three-dimensional ecological volume. In this era of climatic changes and anthropogenic pressures, high rates of species are exposed to an extinction risk. Under the urgent need of species conservation and climate mitigation, 3D is not only a fun or an innovative technological approach, but a new way to look at the natural dynamics to better plan the study and protection of ecosystems.”

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