Vocabulary:

• Life History: a species’ typical lifetime pattern of growth, development, and reproduction. Takes into account:
  • Age and size at reproduction
  • Typical lifetime length
  • Number of offspring produced over the course of an individual’s lifetime
• Asexual reproduction: produces genetically identical offspring
• Sexual reproduction: produces genetically distinct offspring
• Extrinsic factors: environmental factors that influence life history strategies, such as age-specific mortality, survivorship
• Intrinsic factors: factors based on an individual’s genotype that influence life history strategies, including development, genetics, physiology
• Altricial: offspring are born less developed, requiring significant amounts of parental care
• Precocial: offspring are born more developed and independent. Require very little to no parental care
• Iteroparity: reproductive strategy in which an individual reproduces many times during their lifespan
• Semelparity: reproductive strategy in which an individual has only one reproductive cycling during their lifespan
• r-Strategist: a species with a small body size and short life span that produces lots of offspring earlier in life. Offspring have a low survival rate and low level of parental investment
• K-Strategist: a species with a large body size and long life span that produces fewer offspring later in life. Offspring have a high survival rate and high level of parental investment
• Intrasexual selection: competition within one gender to be able to mate (ex: males fighting over territory)
• Intersexual selection: mate choice, typically by females, based on desirable characteristics (ex: peacocks tail)
• Dioecious: plants that have either male or female reproductive organs
• Monecious: plants with both male and female reproductive organs
• Mating Systems:
  • Monogamy: one male, one female – largest time commitment
  • Polygamy: 
    • Polygyny: one male, multiple females
    • Polyandry: multiple males, one female
  • Promiscuity: multiple males, multiple females
  • Polygyny, polyandry, and promiscuity: individual defends limited resources

Outline of Notes:

Life History Strategies:

• Life History: A species’ typical lifetime pattern of growth, development, and reproduction
  • Takes into account age and size at reproduction, length of lifetime, and number of offspring produced over the course of an individual’s lifetime.

• An organism’s life history is informed by both intrinsic and extrinsic factors:
  • Extrinsic factors are based on the environment:
    • Age-specific mortality
    • Survivorship

• • Intrinsic factors are based on an individual’s genotype:
    • Time of reproduction in life
    • Number of offspring
    • Asexual or sexual reproduction
    • Timing/season of reproduction

• Species must make trade-offs in order to maximize the return on energetic investments. Trade-offs are inherent to different life histories:
  • Reproduce early in life, or delayed?
  • Many small offspring, or few and larger?
  • Asexual or Sexual reproduction?
  • Timing/season of reproduction?        

• Animals can be classified into two categories, based on their life histories:

1. r-selected species: Tend to have shorter lives, have smaller bodies, and many offspring

• • Reach carrying capacity quickly but have large amplitudes and variation around carrying capacity- boom and bust cycle  

2. K-selected species: Tend to have longer lives, larger bodies, and fewer offspring

• • Reach carrying capacity slower and have shallower amplitudes of population stability at carrying capacity

        → The R-K spectrum is often a generalization and cannot account for all of the factors different species face.

• Some species can change sex, this may be due to:
  • Change in population’s sex ratio
  • Availability of resources
  • Energy cost of producing genetic material

Energy Investment and Mating Systems

• Sexual reproduction is energy intensive, especially for females: reproductive effort
  • Gonad development
  • Movement to spawning area
  • Competition for mates
  • Nesting
  • Parental care

• Females need to balance the expense of growth versus offspring production

• • Example, Figure #: Female Sardinella can either grow faster or produce more eggs (b)

• Early investment to support reproduction later in life could cost you in the future:
  • Mortality rate increases dramatically after having offspring
  • Fecundity decreases with age and with reproduction
• Optimal allocation to reproduction maximizes parent survival:
  • Often at intermediate reproductive effort
  • Maximized total reproductive success over a lifetime
• Approaches to allocation:
  • Semelparous: Many offspring per reproductive event, low care, low survival rate
    • Usually precocial young
  • Iteroparous: Few offspring per reproductive event, high care, high survival rate
    • Usually altricial young
• Species engage in different mating systems, often depending on varying reproductive needs.
  • Monogamy: one male/female pair, often with long time commitment
  • Polygamy: multiple individuals within a system, competition over limited resources ensues
  • Types of Polygamy:
    • Polygyny: one male/multiple females
    • Polyandry: multiple males/one female
    • Promiscuity: multiple males/multiple females
• Sexual selection can depend on:
  • Morphology
  • Behavior
  • Social status
  • Desirable territory

• Desired characteristics are usually indicators of good fitness. Examples include:
  • Plumage:
    • Color: Indicates the bird has enough surplus energy and nutrients to produce brightly colored feathers.
    • Length: Indicates that the bird is physically fit enough to evade predation even with long feathers that inhibit movement.
  • Song complexity: Can indicate a longer lifespan as many songs are learned behaviors

Plant Life History Strategies:

• Three main factors are theorized to contribute to plant life history strategies:

1. 1. Competition
   2. Disturbance: High levels select for high growth rate and large number of offspring
   3. Stress:  High levels select for large size and high “parental care”

→ Different combinations of these factors favor different life history strategies.

• Low stress, low disturbance: favors competitive species
  • Fast growth rate, low energy investment into producing seeds and “parental care”
• Low stress, high disturbance: favors ruderal species
  • Small size at maturity, fast growth rate, large number of offspring, low “parental care”
• High stress, low disturbance: favors stress-tolerant species
  • Large size at maturity, low growth rate, small seed bank, high “parental care”
• Plant version of parental care:
  • Seed size
  • Secondary compounds (tannins, toxification of environment)
• Plants can be either dioecious or monecious:
  • Dioecious plants: have either male or female reproductive organs
  • Monecious plants: have both male and female reproductive organs

Blog Style Summary:

Ever wondered how the characteristics that make you you (and bugs bugs, and wolves wolves, and fish fish, etc.) came to be? What I mean by this is, have you ever wondered why you are succeeding in life? Let me back up for a second. When I say succeed, I mean your fitness level, or your ability to survive in the world and reproduce. Organisms have an inherent set of life history strategies, or factors that allow them to go about living life (and succeeding). Each species has different strategies from the next, depending on factors such as their genetics and environment. For instance, let’s take a quick look at body size. When you’re walking down the street, you don’t see a bird the size of a human or a dog the size of a lizard, right? It would be really weird if you did… because different species have an optimal body size that they grow to (on the norm). A songbird doesn’t need to be the size of a human because it’s just fine being small. Its size is optimal for its energy intake and output, what it eats, and how it goes about living life.

Well, life histories are all about trade-offs, as you’ll see throughout this blog. Remember when we mentioned you don’t see a bird the size of a human? Well, it turns out they actually do exist. Meet the Harpy eagle.

Crazy, right? This bird is huge, weighing between 9-11 pounds. So naturally they require a LOT of energy intake for their body size. Harpies eat sloths, monkeys, and opossums, which can take a significant amount of energy to grab and lift. So, as a trade-off to conserve energy these birds are rarely seen flying about over the forest canopies where they live (which is generally in South America). They rarely fly long distances, and spend most of their time perched, waiting for meals to cross their path (“Largest Eagles in the World Are so Big That Their Talons Are Bigger than Bear Claws,” 2021). They also lay two eggs every 2-4 years, but ignore the second egg once the first one hatches. They focus all of their attention on protecting and raising their chick for about 6-7 months (Harpy Eagle Facts, 2011). As I said earlier, these birds require a lot of energy, and it’s all about supply and demand, so the parents focus their extra energy on the survival of only one fledgling.

Let’s talk some more about parental care; you know, the way your parents raise you. Believe it or not, some species of animals get kicked out of the house immediately (unlike that guy you know that’s 25 and still mooching off of his parents). Some species put more time and care into raising their young, and because of that have fewer offspring, like humans and elephants. Other animals have many offspring at once in the hopes that one or a few might make it, like rabbits.

A lot of that is based on whether you are a precocial or altricial species (there’s also semi-altricial and semi-precocial). The offspring of precocial species develop quickly and are capable of almost immediately surviving on their own. Altricial offspring are born helpless and develop much slower. To illustrate this point, below is an image of two different bird species’ hatchlings, with two different life history strategies. The chicks on the left (purple martins) are naked, blind, and helpless when first hatched, while the chick on the right (a killdeer) is feathered, fully sighted, and mobile upon hatching. 

        Personally, I’d much rather have a baby that didn’t whine and cry every time it needed something. So at first glance, I wonder what benefit a species would gain from having their young born basically helpless? Well, let’s not forget that life history strategies are all about trade-offs. Generally, the altricial species spend their time gathering sufficient food for their helpless offspring, while the females of precocial species have to eat twice as many calories as the altricial species to ensure they produce energy-rich eggs. Precocial animals are often born with big brains, but their adult brain is small in relation to their body size. On the other hand, altricial young are born small-brained but have significant postnatal brain growth and end up with proportionally larger brains as adults than precocial species (Precocial and Altricial Young, 1984).

Of course, you can’t have life without reproduction, and there are many trade-offs in this area too. First, there is the choice between asexual and sexual reproduction. Asexual reproduction is more energy efficient and only requires one organism, but reduces genetic variation as all offspring produced are genetically identical. Sexual reproduction is more energy intensive but results in more genetic variation, which can result in better fitness. This extra effort disproportionately falls on female members of species, as they are the ones producing eggs/milk.

It may seem like this is not actually a choice as we cannot just switch between these two options. However, many plants can do just this through a process called vegetative propagation. Almost all plants retain totipotent cells–those with the ability to grow into many different cells–throughout their life, allowing for asexual reproduction to be possible. You have probably seen many examples of vegetative propagation in your life. For example, have you ever left a potato too long in your fridge and seen many shoots coming out of it? Since potatoes are not the plant’s fruit, this cannot be sexual reproduction. In fact, these are fully-fledged, genetically identical plants growing from a modified root. Aren’t plants neat?

How can we summarize these different approaches? Well, living things generally follow two life history strategies. r-strategists have lots of offspring but do not put much effort into raising them. K-strategists, on the other hand, delay having offspring and have fewer offspring, but provide more parental care. You can see how these overall strategies mesh with more specific life history traits. r-strategists would benefit from precocial offspring, as they can survive without much parental care. Of course, we humans are the ultimate K-strategists and our babies are pretty useless. I don’t think any other animal cares for its offspring for 18+ years.

Life history traits are important as they are key to understanding how and why a species’ population has and will change. As we humans drive more and more species towards extinction, life history will help us understand which species are the most vulnerable and how to best preserve our planet.

Spotlight on NC:

The Carolina gopher frog is an endangered resident of North Carolina’s coastal plains and sandhills [1]. Gopher frogs (Rana capito) are a member of the “true frog” family, Ranidae, even though their dark, bumpy skin makes them look a lot like a toad. Their native range coincides with the historic distribution of longleaf pine savannas across the southeastern United States, from North Carolina to western Florida [2]; However, widespread deforestation (up to 98%) of longleaf pine forests [3] has resulted in severe habitat fragmentation and thus isolated gopher frog populations.

The gopher frog is a specialist with strict habitat requirements; they spend most of their life in burrows, but emerge yearly to breed in wetlands. Gopher frogs are considered iteroparous because they can reproduce multiple times throughout their lifetime. Due to their independent, underground lifestyle, not much is known about the life history of gopher frogs besides their breeding biology. For example, data on gopher frog longevity is incomplete, but one gopher frog is known to have lived 9 years in captivity [4]. In North Carolina, gopher frogs’ dens are often found in the stump holes of fallen pines [5]; In their more southern range, gopher frogs are a commensal species, relying heavily on the burrows of gopher tortoises for refuge [6]. To mate, gopher frogs require access to nearby wetland habitats containing grassy vegetation and an open forest canopy [7]. Gopher frogs in North Carolina have been observed to travel up to 3.5 km from their den site to access suitable breeding ponds [5]. Only 7 populations of gopher frogs remain across the state of North Carolina (Figure 1).

The Breeding season for Carolina gopher frogs lasts from January to April, with mating occurring immediately after a heavy rain event. They select ephemeral (temporary) ponds in the coastal plain which are absent of fish. This means their main predators during the breeding season are caddisfly and dragonfly larvae, diving beetles, and turtles. The males stay in these ephemeral ponds for around a month, calling for females. The female frogs stay for less than a week, usually laying one cluster of thousands of eggs, which are then externally fertilized by the males. This means the Carolina gopher frog is polygynous! It may surprise some that the larval stage for this species ranges from 87 to 225 days and they reach sexual maturity at 1.5-2 years old, but these are relatively typical timelines for a lot of frogs [8]. Furthermore, like most other amphibians, these frogs’ reproductive strategy is r-selection instead of K-selection. This means gopher frogs invest more energy into producing a high quantity of offspring versus high-quality offspring that they have to take care of for a long time. This concept is demonstrated in Figure 9 below.

There are a number of ecological challenges that gopher frogs face. If you are familiar with the idea of the “Sixth Mass Extinction” you’ll know that amphibians are some of the most at-risk species. One of the ecological factors that is affecting gopher frogs in particular is the deforestation of their habitat. The longleaf pine forests that gopher frogs call home are in fact endangered themselves. Longleaf pine forests have been heavily deforested and have a difficult time recovering because they are dependent on seasonal burns for reproduction. The reduction in this habitat has affected the gopher frog population in NC. The government is taking some steps to address the ecological challenges that gopher frogs are facing: the Department of Defense is implementing new strategies for conservation for bases that include gopher frogs and their habitat [9]. In 2022, the Biden administration reinstated the “Endangered” status to gopher frogs, overturning a decision made by the previous administration. This will allow more of the gopher frog’s natural habitat to be protected for conservation [10].
 
Habitat fragmentation is one factor that has been contributing to the species’ decline. The construction of roads, towns, and cities have directly destroyed their habitat but has also reduced gene flow between subpopulations. As roads are built through longleaf pine forests they cut through frog habitats, creating a barrier separating populations. When mating season comes, frogs will either have to risk crossing the road and potentially dying, or they must mate with those closest to them. This leads to reduced genetic diversity over generations. Another ecological challenge that gopher frogs are facing is the intensifying drought in the southeast United States (https://www.drought.gov/drought-status-updates/drought-status-update-southeast-us). Drought can disrupt the reproduction cycle and development of gopher frogs in such a way that reduces their populations, as they rely on the ephemeral pools produced by heavy rainfall [8].

 Due to the fact that habitat loss is the main challenge faced by the species, a large amount of future conservation efforts rely on the conservation and restoration of longleaf pine forests. One of the main reasons that longleaf pine forests have been in decline for the past century is due to the fact that they require frequent, low-intensity fires [11]. For much of the 20th century, the US government has had an extreme anti-wildfire campaign [12]. Forest managers believed that all forest fires were dangerous or bad for the health and growth of forests. However, longleaf pines require fire to eliminate competitors and allow their saplings to grow [11]. Nutrient-poor soils are also common in this ecosystem [12], meaning that longleaf and many of the other plant species the gopher frog relies on are considered stress-tolerant. The suppression of the fires, along with logging and habitat loss due to development, are the reasons that this ecosystem has been on the decline for so long [12]. In order for species like the gopher frog to have a chance at recovery, proper management of the remaining longleaf pine forest, along with restoration efforts to return longleaf pines to their native range, are required. The NC Wildlife Resource Commission has strategized that the best course of action for the conservation and recovery of gopher frogs is to use prescribed fire to create more breeding habitats [4]. Essentially, if there is more habitat, wildlife managers can attempt to establish new populations. Thankfully, there are many organizations working to conserve and restore longleaf pine forests across the southeast. Organizations like the Longleaf Alliance have listed the recovery of the gopher frog (among other wildlife species) as one of the main benefits of supporting longleaf restoration. If we hope to better understand the interesting life history of the gopher frog, it is important that we work to conserve the species for future generations to study.

Featured Ecologists:

Dr. Diana Wall, PhD

Diana Wall is a soil ecologist who studies soil biodiversity. Her research aims to determine how soil biodiversity contributes to ecosystem services and how climate change will impact this diversity. She conducts her research within Antarctica’s dry valleys, where climate change impacts are amplified to understand these questions. Her interest in soil microbes and nematodes has better uncovered the life histories of species that survive in these harsh conditions. Namely, she discovered Scottnema lindsayae, a soil nematode species, to have a surprisingly broad distribution and high abundance in dry, nutrient-deficient soils. Diana Wall’s research within these habitats can be used to determine how species and ecosystems may respond to the increasing pressures from climate change.

Student contributors:

Note Outline: Adrian Chamberlin, Noel Griffin, James Holmes, Emily Usinger, Shelby Williford

Blog Style Summary: Kimmy Gebbia, Madison Allen, Landon Wang

Spotlight on NC: Ian Warr, Kelly Koehler, Jackson Wimpy, Sophie Meng

Featured Ecologists: Madison Shrader