Paleo-Myth Number 5: Cenozoic: The Age of Mammals?

Speak of the Cenozoic (New Life) and you implicitly speak of what is commonly referred to as "the Age of Mammals". Although mammals were around during the Mesozoic and were very diverse in small size guilds- its only in the Mesozoic do we see them expand into large and dominant carnivores, herbivores, and omnivores. We also see expansion into the marine realm and a certain aberrant, bipedal ape became especially dominant and widespread during the Holocene. Certainly when one looks across the breadth of mammalian diversity one is astounded by the myriad forms: from shrew to bat to giraffe to beaked whale they are all mammals.

And this post is not discounting the obvious success mammals have sustained during this time period. But what it is addressing is how this notion- how the Cenozoic became synonymous with "The Age of Mammals"- has distorted our view of the relative ecological importance of other vertebrates during this time period- namely birds.

Now some of you reading this might be well aware that the Cenozoic always represented a significant expansion of Aves and especially Neognathene passerines- this post and the Paleo-Myth series is not necessarily aimed at you but at those perhaps less conscious of the importance of birds in the Cenozoic.

Chances are wherever you are at on the planet right now as you read this, if you step outside and just start counting species you will see more of and greater diversity of birds than mammals.

Consider this:

Most estimates place the number of extant bird species at about 10,000. Mammals? roughly half the number of species as birds at about 5,500 species. And of the mammal species alive the vast majority are rodents, bats, and shrews (red, blue, and yellow in the graph below).

So basically about 75% of all mammals can be characterized as being small, primarily nocturnal, and insectivorous/omnivorous- they basically have not changed a bunch from the Mesozoic. And if you look at the largest order Rodentia it comprises a little less than 2,500 species- or roughly half the amount of species that compromise Passerines at roughly 5,000. This comparison is especially telling as rodents and passerines do share a lot in terms of ecological overlap. And this diversity among birds is not slowing down, if anything it is picking up!

Looking at species diversity offers us one perspective on the bird vs mammal question. It should be noted that the peculiar adaptations and specializations of many birds might have a way of inflating bird species counts compared to mammals. Also because birds can be quite mobile they are able to fly off to different areas and speciate- unlike rodents. But the sheer number of bird species, compared to mammals, can not be ignored.

But lets look at biomass, as this may offer another perspective on the issue.

Tsavo. (c) Antero Topp. The most abundant bird, red-billed Quelea, vs largest land mammal, African Elephant

Now biomass estimates are tenuous at best and it is hard to estimate the population of even people- but lets look at the most common bird in the world- Africas Red-Billed Quelea (Quelea quelea). Estimates for the population range from 3 billion to up to 10 billion- pretty respectable numbers. But here the mammals probably have the birds beat. Most experts believe the brown rat (Rattus norvegicus) to be the only land based vertebrate to come close to humans in terms of biomass. Some estimate New York City alone to have a population of rats at up to 100 million! Ok so mammals hold the #1 and #2 spot for terrestrial vertebrate biomass- who come in at #3? a bird, the common chicken (Gallus gallus domesticus).

Another point to be made with the rodent question is that birds of prey- owls, falcons, hawks- and also birds like corvids, herons, gulls, and shrikes are the most important predatory control on exponential rodent growth in many habitats. And, unlike mammal predators, can converge from long distance to exploit areas with rodent plagues. They thus serve as a last line of defense for rodent scourges. Another point for the birds.

Another avenue to explore in regards to the bird/mammal debate is extent of ecological niches. Both extant and extinct forms of birds show that they have made significant ecological inroads into niches of large terrestrial herbivore and carnivore traditionally occupied by mammals. Examples include modern flightless birds (ostriches, rheas etc) and in the past phorusrhachids, moas, and elephant birds. But mammals have not yet exploited many niches that are the exclusive domain of birds. There are no soaring mammals that scavenge carcasses (vultures, condors etc). There are no mammals that exploit shores/intertidal zones to the extent the myriad waders, shorebirds etc do. There are no filter feeding freshwater or brackish water mammals like ducks, flamingoes, anseriformes etc. Birds show a much greater diversity in their insectivorous forms than mammals. Although bats are important pollinators, seed dispersal agents in some areas they are less successful/diverse than the myriad birds that perform these same functions. And there are no bats that exploit marine resources, unlike the myriad diversity of marine birds.

In short birds can often do the same tasks that mammals do when given a chance- but mammals do not seem to succeed in the areas where birds do exceptionally well.

The Cenozoic: The Age of Mammals and Birds

(c) Flip Nicklin

Pertinencia

Along with Humans, Who Else is in the 7 Billion Club?

Global Diversity of Birds in Space and Time

Influence of Predation on Rodent Population

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