Tree species co‐occurrence patterns change across grains: insights from a subtropical forest

Abstract

Co‐occurrence is a basic measure of spatial relationships between species. This commonly used measure has many benefits and limitations, yet a basic property that can strongly affect it has been overlooked. Co‐occurrence analysis is based on discrete sampling in space, and therefore, its grain size may affect the results and their interpretation, because species interactions and their environmental responses are scale‐dependent. We utilized a large dataset on tree species from a full‐stem mapped forest plot in China as a template for testing the effects of grain on species co‐occurrence patterns. We quantified co‐occurrence patterns for large trees and saplings in nested sampling plots with increasing radii and analyzed the effect of plot size on co‐occurrence. Co‐occurrence patterns varied greatly across grains. More than half of the species in large trees we analyzed had significantly non‐random co‐occurrence patterns at some grain. In contrast, saplings exhibited much fewer non‐random co‐occurrences. The proportion of segregated species pairs of large trees had a unimodal relationship with grain, whereas the proportion of aggregated species was positively related to grain. These patterns disappeared in saplings, suggesting that spatial interactions among trees are more prominent among larger individuals. Therefore, co‐occurrence patterns are scale‐dependent, and this scale dependency reflects a mixture of ecological (interspecific interactions, environmental responses) and statistical (sampling effects) processes. Our results suggest that insights from single‐grained studies cannot be generalized.

Testing the link between species interactions and species co‐occurrence in a trophic network

“…This suggests that the accuracy of inferring interactions is highest at finer scales. Second, similar to other recent findings (Araújo and Rozenfeld 2014, Belmaker et al 2015, Bar-Massada et al 2018, at finer spatial grains of observation (minimal connectivity among ponds), species were estimated to have more negative interactions, depending on the type of interaction, more than expected if ponds were randomly aggregated across the landscape. Only predator-prey pairs demonstrated a shift in associations across spatial scale, in line with our hypotheses.…”

“…Previous work has found that positive species interactions are more likely to be detected at large spatial grain sizes (Araújo and Rozenfeld 2014, Belmaker et al 2015, Bar-Massada et al 2018. However, this pattern may simply be a result of sampling: as more communities are aggregated on a landscape, any two species is more likely to be detected together.…”

Examining the link between competition and negative co‐occurrence patterns

“…Rather than inferring process from spatial patterns, we tested whether patterns of co-occurrence in a given year were predictive of specific changes in abundance across years. Our focus on small sample plots within a small plant community reduces the likelihood that the influence of competition on co-occurrence patterns was obscured by influences that primarily manifest at larger scales, such as dispersal limitation (D'Amen et al 2017, Bar-Massada et al 2018) and substrate variation (Tirado and Pugnaire 2005). As such, if competition were the primary driver of negative co-occurrence patterns and asymmetric abundance decline, this should have been clear in our results.…”

“…At the centre of our 1.5 × 1.5 m plots, we set out circular plots measuring 0.5 m in diameter. Recent research has identified that smaller plot sizes such as these are most appropriate for testing the influence of competition on co-occurrence patterns (McNickle et al 2018) while reducing the effects of abiotic variation and dispersal limitation (Bar-Massada et al 2018). While we do not have direct evidence that our study field is homogeneous with respect to abiotic resources, we present soil mapping here for undisturbed plots in an adjacent part of the field located 5 m away (70 plots, 0.5 m diameter).…”

Reconciling Darwin’s naturalization and pre‐adaptation hypotheses: An inference from phylogenetic fields of exotic plants in Japan

“…Lososová et al, ; Qian & Sandel, ), thus focusing on site‐specific invasibility of a recipient assemblage. However, site‐based approaches are affected by the spatial grain used for capturing species co‐occurrence patterns (Bar‐Massada, Yang, Shen, & Wang, ). Indeed, the debate on naturalization versus pre‐adaptation in biological invasion is often dependent on the spatial scale: the pre‐adaptation hypothesis is well supported at a larger grain size, whereas naturalization is supported at a smaller grain size (Cadotte, Hamilton, & Murray, ; Diez, Sullivan, Hulme, Edwards, & Duncan, ; Thuiller et al, ).…”