Influence of density and salinity on larval development of salt-adapted and salt-naïve frog populations

Environmental change and habitat fragmentation will affect population
densities for many species. For those species that have locally adapted to
persist in changed or stressful habitats, it is uncertain how density
dependence will affect adaptive responses. Anurans (frogs and toads) are
typically freshwater organisms, but some coastal populations of green
treefrogs (Hyla cinerea) have adapted to brackish, coastal wetlands.
Tadpoles from coastal populations metamorphose sooner and demonstrate
faster growth rates than inland populations when reared solitarily.
Although saltwater exposure has adaptively reduced the duration of the
larval period for coastal populations, increases in densities during
larval development typically increase time to metamorphosis and
reduce rates of growth and survival. We test how combined stressors of
density and salinity affect larval development between salt-adapted
(“coastal”) and non-salt adapted (“inland”) populations by measuring
various developmental and metamorphic phenotypes. We found that increased
tadpole density strongly affected coastal and inland tadpole populations
similarly. In high-density treatments, both coastal and inland populations
had reduced growth rates, greater exponential decay of growth, a smaller
size at metamorphosis, took longer to reach metamorphosis, and had lower
survivorship at metamorphosis. Salinity only exaggerated the effects of
density on the time to reach metamorphosis and exponential decay of
growth. Location of origin affected length at metamorphosis, with coastal
tadpoles metamorphosing slightly longer than inland tadpoles across
densities and salinities. These findings confirm that density has a strong
and central influence on larval development even across divergent
populations and habitat types and may mitigate the expression (and
therefore detection) of locally adapted phenotypes.

环境变化与生境破碎化会对诸多物种的种群密度造成影响。对于那些已通过本地适应得以在改变后的或胁迫性生境中存续的物种而言，密度制约如何影响其适应性响应，目前仍不明确。
无尾目（Anurans，即蛙类与蟾蜍类）通常为淡水生物，但部分绿树蛙（Hyla cinerea）的沿海种群已适应了咸淡水海岸湿地生境。
单独饲养时，沿海种群的蝌蚪较内陆种群变态更早，生长速率也更快。
尽管盐水暴露已通过适应性演化缩短了沿海种群的幼虫期，但在幼虫发育阶段密度升高通常会延长变态所需时间，并降低生长与存活速率。
本研究通过测量多种发育与变态表型，探究了密度与盐度这两类协同胁迫因子，如何对盐适应型（“沿海种群”）与非盐适应型（“内陆种群”）的蝌蚪幼虫发育造成影响。
研究结果显示，蝌蚪密度升高对沿海与内陆种群的影响整体相似。
在高密度处理组中，两类种群均表现出生长速率下降、生长指数衰减加剧、变态时体型更小、变态所需时间更长，且变态存活率更低的特征。
盐度仅会加剧密度对变态时间与生长指数衰减的影响。
种群起源地会影响变态时的体长：在不同密度与盐度条件下，沿海种群蝌蚪的变态体长均略长于内陆种群。
上述研究结果证实，即便在不同种群与生境类型之间，密度对幼虫发育仍具有显著且核心的影响，且可能会抑制本地适应性表型的表达（进而影响其检测难度）。