Contrasting species functional trait structuring of subarctic versus subtropical copepod communities

Abstract

40Classic niche theory assumes that species-level functional traits affect species relative 41 fitness and thus community structuring, but empirical tests of this assumption are 42 scarce. Moreover, recent evidence shows increasing functional over-redundancy 43 towards the tropics, suggesting that the extent to which functional traits confer 44 species' fitness and thus impact community structuring differs across latitudes. Here, 45we develop a new method: comparing the frequencies of trait categories in the 46 species-rank abundance distributions of local communities versus their frequencies in 47Main 55Research on species-level functional traits abounds due to the prevailing concept that 56 species traits play key roles in structuring communities and determining functioning 57 such as predator-prey interactions and trophic transfer 1-3 . Functional traits are defined 58 as the characteristics of an organism that affect its fitness, and subsequently, by 59 increasing the individual ability to grow, survive and reproduce, functional traits 60 affect species abundance (population size) in a community 3 . In general, communities 61 are usually made of few dominant and many rare species 4 . According to classic niche 62 theory, a species is selected to dominate because it possesses functional trait values 63 that favour its niche occupation and confer greater fitness compared to other species 64 in the community 5,6 . In addition, to explain the existence of many rare species, classic 65 niche theory predicts species coexistence by niche segregation: species avoid 66 competition by avoiding niche overlap 5,7 . However, modern niche theory adds that 67 species coexistence can still occur among species of overlapping niches (i.e., with 68 similar functional trait values) if these species have similar fitness 8 . Finally, 69 disregarding trait effects on community structuring, neutral theory argues that species 70 dominance occurs mainly due to stochastic processes such as drift and dispersal 9 . 71Which gives rise to an important question that we intend to assess: Can species traits 72 explain species dominance and coexistence in natural communities? 73 74 A myriad of trait-based indices have been proposed e.g., 2,12,13-15 , including various 75 facets of diversity (i.e., richness, evenness and divergence) 15,16 , albeit that many of 76 them are redundant 17 . The most popular indices are community-integrated measures 77 such as the community weighted mean (CWM), Convex Hull Volume 13 and Rao 78 quadratic entropy 12 . These indices have proven useful to characterise how 79 5 community traits change responding to environmental conditions e.g., 18,19 and affect 80 functioning e.g., 20 . Yet, community-integrated trait indices may not be suitable to 81 answer whether and how traits structure species communities. Community-integrated 82 indices assume that all of the selected traits have the same weight in community 83 structuring or functioning. However, functional traits should not be considered similar 84 in structu...