Pollen assemblage richness does not reflect regional plant species richness: a cautionary tale

Abstract

Summary1. Palaeoecological records of species richness spanning time intervals over which climate variables have shifted relative to one another can help reduce issues of colinearity that might affect our understanding of patterns of species richness.2. Fossil pollen assemblages have the potential to serve as a proxy for past plant richness because they record the presence of plant taxa. However, pollen assemblages are typically limited by low taxonomic resolution and taphonomic processes (pollen production, transport, deposition and preservation), which may degrade the degree to which pollen accurately represents vegetation communities.3. We combined pollen assemblages from modern lake sediments (n = 546; n = 167 in British Columbia, Canada) in the Pacific Northwest, with a detailed data base (n = 16 071) of plant presence across the province and a published record of gamma richness to test the accuracy of pollen assemblages as a proxy for regional patterns of plant richness. 4.A generalized linear model using plant richness resolved at multiple taxonomic levels suggests taxonomic differences between plant and pollen taxa may reduce the ability of pollen richness to predict plant richness at the site level, but that this relationship is still recoverable, albeit with broad confidence intervals. 5.Spatially explicit analysis using a generalized additive model shows that predicted plant richness has no relationship with raw pollen assemblage richness at all taxonomic levels.6. The taxonomic composition of the region (i.e. the ratio of wind-pollinated to insect-pollinated species) and/ or the morphological specificity of the dominant pollen types in the region may play a role in limiting the reconstruction of plant richness from pollen richness. Nonetheless, we believe this study is the first to empirically test the relationship between plant and pollen richness, and fails to find a significant relationship.7. Synthesis. Palynological richness in itself cannot be considered a universally reliable proxy for inferring plant richness; however, broad spatial and temporal patterns of change in richness have been reported in the literature. Our findings suggest that more work is needed to understand previously reported patterns of pollen assemblage richness through time and in space. We suggest the use of functional diversity or phylogenetically based analysis may help link pollen richness to plant community richness.

Pollen-vegetation richness and diversity relationships in the tropics

“…However, comparison of richness between high diversity (tropical) and low diversity (temperate) systems does seem to demonstrate that pollen richness does track vegetation diversity at a global scale. The range of palynological richness captured in the tropical traps presented here is 86-144 pollen types (Table 1), compared with richness values from temperate regions which are typically lower; for example 3-27 pollen types in > 500 modern lake samples in North America (Goring et al 2013), and 9-45 pollen types in fossil records from a wide variety of temperate regions across the globe (Flenley 2005).…”

“…The absence of a clear relationship between the pollen and vegetation richness (Fig. 1b) suggests that, perhaps unsurprisingly, factors other than the plot (local) vegetation richness are playing an important role in determining palynological richness (Goring et al 2013;Birks et al 2016). Important factors likely to be influencing the pollen rain, and consequently the richness signal, are variations in flower structure and pollen dispersal mechanism (Bush 1995;Bush andRivera 1998, 2001).…”

“…The link between pollen and vegetation diversity is known to be complex due to the different taxonomic resolution possible, the variation in pollen production and dispersal between plants, and the accuracy with which the pollen sum captures the diversity of the sample (Goring et al 2013;Birks et al 2016). Furthermore, the relationship between pollen (palynological) diversity and its parent vegetation from the most biodiverse region of the world, the tropics, remains under-studied.…”

Time continuum and true long-term ecology: from theory to practice

“…However, as macrofossils are random parts of the whole organism, diversity estimation is always problematic. Some studies have shown that pollen diversity and vegetation diversity do not coincide (Goring et al, 2013), while others show that their trends can be consistent, and it may be possible to derive community diversity changes in time from fossil diversity tendencies (van der Knaap, 2009). Diversity estimation using transfer functions has been rarely attempted and might be potentially useful.…”

Reducción de la incertidumbre de los modelos de distribución de especies con datos ecológicos y paleoecológicos.

“…El tamaño de los restos y su resistencia a la descomposición y la idoneidad de los depósitos sedimentarios para conservar material orgánico (temperatura, pH y falta de oxígeno) limitan el proceso de fosilización. Asimismo los procesos tafonómicos (incluyendo la producción, el transporte y su deposición) limitan la correspondencia entre el registro fósil y la vegetación en el pasado (Goring et al, 2013). Por ello, el registro fósil está sesgado hacia determinados taxones, períodos de tiempo y sitios que contienen aquellas condiciones favorables para su preservación (Behrensmeyer et al, 2000y Varela et al, 2011.…”