Maintenance of plant species diversity in forest ecosystems.

Abstract

Abstract One of the major questions in ecology is how plant species coexist and thus how diversity is maintained. While there are many theories to explain the maintenance of plant species diversity, compelling empirical support exists for very few of them. Here we summarize four major putative theories to explain the maintenance of forest plant species diversity, each of which has ample empirical support. These theories are: (1) niche differentiation; (2) negative density dependence; (3) disturbance; and (4) neutral dynamics with respect to competition. We also present a literature review comprising 51 studies that explicitly examined the maintenance of plant species diversity, published between 2000 and 2015. In the literature review, we include only studies that stated, either as part of their introduction or in clearly stated objectives, that the hypothesis was the maintenance of diversity mechanism. We found that there has been a huge amount of progress in the research on the maintenance of species diversity. An overwhelming majority (all but three) found significant evidence for the mechanism that they tested. The large majority (64%) of the studies were conducted in tropical regions (27% in Panama alone). Trees were the main focal group, comprising 75% of the studies, 14% focused on shrubs, and 7% on lianas. Only 4% of the studies focused on non-woody plants. Seedlings were also a main focus, and 61% of the studies focused explicitly on the seedling life stage. Our findings lead us to conclude that the study of the maintenance of species diversity has progressed rapidly over the past 15 years. However, there is still a substantial amount of additional research to be done. Future studies may benefit from explicitly examining key theoretical requirements of the mechanism that they are investigating. The use of spatial and temporal heterogeneity may be critical and, in many cases, would improve the precision of the study. Most tests have focused on single plant groups and single mechanisms for the maintenance of species diversity, but moving into the future, a focus on multi-mechanism models for the maintenance of diversity using plant groups that differ in multiple axes of life-history strategy will be crucial to understanding how species coexist.