Disturbance Is an Important Driver of Clonal Richness in Tropical Seagrasses

Abstract

Clonality is common in many aquatic plant species, including seagrasses, where populations are maintained through a combination of asexual and sexual reproduction. One common measure used to describe the clonal structure of populations is clonal richness. Clonal richness is strongly dependent on the biological characteristics of the species, and how these interact with the environment but can also reflect evolutionary scale processes especially at the edge of species ranges. However, little is known about the spatial patterns and drivers of clonal richness in tropical seagrasses. This study assessed the spatial patterns of clonal richness in meadows of three tropical seagrass species, Thalassia hemprichii, Halodule uninervis, and Halophila ovalis, spanning a range of life-history strategies and spatial scales (2.5–4,711 km) in Indonesia and NW Australia. We further investigated the drivers of clonal richness using general additive mixed models for two of the species, H. uninervis and H. ovalis, over 8° latitude. No significant patterns were observed in clonal richness with latitude, yet disturbance combined with sea surface temperature strongly predicted spatial patterns of clonal richness. Sites with a high probability of cyclone disturbance had low clonal richness, whereas an intermediate probability of cyclone disturbance and the presence of dugong grazing combined with higher sea surface temperatures resulted in higher levels of clonal richness. We propose potential mechanisms for these patterns related to the recruitment and mortality rates of individuals as well as reproductive effort. Under a changing climate, increased severity of tropical cyclones and the decline in populations of mega-grazers have the potential to reduce clonal richness leading to less genetically diverse populations.

Conservation implications of ecological responses to extreme weather and climate events

“…We found a larger‐than‐expected number of positive or neutral responses to extreme events ( n = 144; 21% of all responses). These responses serve as a reminder that natural disturbances from extreme events often play a critical role in maintaining the structure and function of many ecosystems and life history strategies (Attiwill, ; McMahon et al, ; Whittaker, Willis, & Field, ). Some positive responses that were common yet temporary included cyclone events leading to increased abundances of bird species that prefer regenerating habitat (e.g., Brown, Sherry, & Harris, ; Freeman et al, ), and cyclones and floods leading to increased richness of fish and invertebrate species (e.g., Gerisch, Dziock, Schanowski, Ilg, & Henle, ; Horrocks, Cunningham, O'Dowd, Thomson, & Mac Nally, ; Kano et al, ).…”

Ecological association between seagrass and mangrove ecosystems increases seagrass population longevity in island ecosystem

“…The reproductive density at site 1 was higher than site 2 (Fig.2b), whereas the reproductive effort was higher at site 2, which indicates about the increased reproductive effort by T. hemprichii population to physical disturbances caused by the wave dynamics and sand erosion at this site. Higher sedimentation from wave breaking results in breakage of T. hemprichii fruits/flowers, leading to higher reproductive effort (McMahon et al, 2017), as the current sampling season (March 2019) is favourable for production of fruits in T. hemprichii (Tongkok et al, 2017). Secondly, intertidal exposure to high temperature and light plays a significant role in determining the phenology of seagrasses (Smith and Walker, 2002; McMahon et al, 2017), which in our case is the exposure of T. hemprichii plants to low tides twice in a day with high temperatures above 30°C, resulting in reduction of T. hemprichii fruit production.…”

“…Higher sedimentation from wave breaking results in breakage of T. hemprichii fruits/flowers, leading to higher reproductive effort (McMahon et al, 2017), as the current sampling season (March 2019) is favourable for production of fruits in T. hemprichii (Tongkok et al, 2017). Secondly, intertidal exposure to high temperature and light plays a significant role in determining the phenology of seagrasses (Smith and Walker, 2002; McMahon et al, 2017), which in our case is the exposure of T. hemprichii plants to low tides twice in a day with high temperatures above 30°C, resulting in reduction of T. hemprichii fruit production. Similar results of high reproductive effort and low fruit production were observed for T. hemprichii population of southern Thailand (Tongkok et al, 2017) and Indonesia (McMahon et al, 2017) in Andaman Sea.…”

The discernible and hidden effects of clonality on the genotypic and genetic states of populations: improving our estimation of clonal rates

“…We still face difficulties in inferring the rate of clonal (denoted c) versus sexual (1-c) reproduction or an approximate but consistent proxy for it (i.e., the "level of clonality"). These difficulties prevent access to the empirical information necessary to compare the ecological dynamics and evolutionary trajectories of partially clonal populations living in different environments (McMahon et al 2017). To understand the effect of PC on the fate of natural populations and species, the value of c should first be estimated.…”

“…Unfortunately, FIS is often neglected in ecological studies, possibly due to difficulties in disentangling the influence of technical shortcomings such as null alleles from non-random mating such as selfing in some studies. In the seagrass literature, for example, moderate levels of R have led some authors to propose that sexual reproduction has a high incidence and may thus contribute greatly to recombination and dispersal through seed production (McMahon et al 2017 (Hughes et al 2008;Hughes & Stachowicz, 2004;Reusch & Lampert, 2004; but see Massa et al 2013). Severe overestimation of genotypic diversities may thus have led to strongly misleading conclusions as to the resilience of the studied populations, enhanced by their supposedly high R value, as well as to their ability to rely on dispersal of seeds due to recurrent events of sexual reproduction (Kendrick et al , 2012McMahon et al 2017).…”